Biotechnology Advances 24 (2006) 389 – 409 www.elsevier.com/locate/biotechadv Research review paper The use of 1-methylcyclopropene (1-MCP) on fruits and vegetables Chris B. Watkins ∗ Department of Horticulture, Cornell University, Ithaca, NY 14853, USA Available online 10 March 2006 Abstract The recent availability of the inhibitor of ethylene perception, 1-methylcyclopropene (1-MCP), has resulted in an explosion of research on its effects on fruits and vegetables, both as a tool to further investigate the role of ethylene in ripening and senescence, and as a commercial technology to improve maintenance of product quality. The commercialization of 1-MCP was followed by rapid adoption by many apple industries around the world, and strengths and weaknesses of the new technology have been identified. However, use of 1-MCP remains limited for other products, and therefore it is still necessary to speculate on its commercial potential for most fruits and vegetables. In this review, the effects of 1-MCP on fruits and vegetables are considered from two aspects. First, a selected number of fruit (apple, avocado, banana, pear, peaches and nectarines, plums and tomato) are used to illustrate the range of responses to 1-MCP, and indicate possible benefits and limitations for commercialization of 1-MCPbased technology. Second, an outline of general physiological and biochemical responses of fruits and vegetables to the chemical is provided to illustrate the potential for use of 1-MCP to better understand the role of ethylene in ripening and senescence processes. © 2006 Elsevier Inc. All rights reserved. Keywords: Fruit; Vegetables; Ripening; Senescence; Ethylene; Softening; Respiration; Flavor; Antioxidants Contents 1. 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Responses of selected fruit and vegetables to 1-MCP . . . . . . . . . . . . 2.1. Apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] 2.1.1. Ripening physiology and quality. . . . . . . . . . . . . . 2.1.2. Commercial application of 1-MCP. . . . . . . . . . . . . 2.2. Avocado. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Ripening physiology and quality. . . . . . . . . . . . . . 2.2.2. Factors affecting commercial application of 1-MCP . . . . . 2.3. Banana (Musa sp., AAA group, Cavendish subgroup) . . . . . . . . 2.3.1. Ripening physiology and quality. . . . . . . . . . . . . . 2.3.2. Factors affecting commercial application of 1-MCP . . . . . 2.4. Pear (Pyrus communis L.). . . . . . . . . . . . . . . . . . . . . . 2.4.1. Ripening physiology and quality. . . . . . . . . . . . . . 2.4.2. Factors affecting commercial application of 1-MCP . . . . . ⁎ Tel.: +1 607 255 1784. E-mail address: cbw3@cornell.edu. 0734-9750/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.biotechadv.2006.01.005 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 392 392 392 393 394 394 394 394 394 395 395 395 395 390 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 2.5. Peach and nectarine (Prunus persica L.) . . . . . . . . . . . . . . . . . . . . 2.5.1. Ripening physiology and quality . . . . . . . . . . . . . . . . . . . . 2.5.2. Factors affecting commercial application of 1-MCP . . . . . . . . . . . 2.6. Plum (Prunus domestica L. and Prunus salicina L.) . . . . . . . . . . . . . . 2.6.1. Ripening physiology and quality . . . . . . . . . . . . . . . . . . . . 2.6.2. Factors affecting commercial application of 1-MCP . . . . . . . . . . . 2.7. Tomato (Solanum esculentum Mill) . . . . . . . . . . . . . . . . . . . . . . . 2.7.1. Ripening physiology and quality . . . . . . . . . . . . . . . . . . . . 2.7.2. Factors affecting commercial application of 1-MCP . . . . . . . . . . . 3. Physiological and biochemical responses of fruits and vegetables to 1-MCP . . . . . . 3.1. Ethylene metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Respiration rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Pigment metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Cell wall metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Volatile compound metabolism . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Nutritional quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Physiological storage disorders . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.1. Apple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7.2. Other fruits and vegetables . . . . . . . . . . . . . . . . . . . . . . . 3.7.3. A physiological basis for disorder development in response to 1-MCP 3.8. Pathological storage disorders . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction Ethylene is one of several plant growth regulators that affect growth and developmental processes including ripening and senescence (Abeles et al., 1992). It is a simple hydrocarbon that can diffuse into and out of plant tissues from both endogenous and exogenous (non-biological and biological) sources (Saltveit, 1999) and has been the subject of extensive research on its biosynthesis and action (Lelievre et al., 1998; Saltveit, 1999; Giovannoni, 2001; Watkins, 2002; Adams-Phillips et al., 2004). Ethylene can profoundly affect quality of harvested products. These effects can be beneficial or deleterious depending on the product, its ripening stage, and its desired use (Saltveit, 1999). Endogenous ethylene production is an essential part of ripening of climacteric fruit and probably acts as rheostat for ethylene-dependent processes (Theologis, 1992). Exogenous ethylene application is routinely used to initiate uniform ripening for fruit such as banana. Most commonly, however, commercial strategies for horticultural products are based on avoiding exposure to ethylene and/or attempting to minimize ethylene production and action during ripening, harvest, storage, transport and handling by temperature and atmosphere control (Watkins, 2002). An exciting new strategy for controlling ethylene production and thus ripening and senescence of fruit, especially climacteric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 396 396 396 396 397 397 397 397 398 398 400 400 400 401 402 402 402 403 403 404 404 405 ones, as well as senescence of vegetative tissues, has emerged with the discovery and commercialization of the inhibitor of ethylene perception, 1-methylcyclopropene (1-MCP). 1-MCP is thought to interact with ethylene receptors and thereby prevent ethylene-dependent responses (Sisler and Blankenship, 1996; Sisler and Serek, 1997, 2003). The use of cyclopropenes to inhibit ethylene action was patented by Sisler and Blankenship (1996). A commercial breakthrough in 1MCP application technology resulted from the formulation of 1-MCP as a stable powder in which it is complexed with γ-cyclodextrin, so that 1-MCP is easily released as a gas when the powder is dissolved in water. 1-MCP was approved by the Environmental Protection Agency (EPA) in 1999 for use on ornamentals, and was marketed as EthylBloc® by Floralife, Inc. (Walterboro, SC). AgroFresh, Inc., a subsidiary of Rohm and Haas (Springhouse, PA), subsequently developed 1-MCP under the trade name SmartFresh™ and have global use rights for edible horticultural products. 1-MCP has a non-toxic mode of action, negligible residue and is active at very low concentrations (E.P.A., 2002), and by 2005 food use registration for the chemical had been obtained in Argentina, Australia, Austria, Brazil, Canada, Chile, Costa Rica, France, Guatemala and Honduras, Israel, Mexico, the Netherlands, New Zealand, South Africa, Switzerland, Turkey, UK, and the US. Registered Table 1 Climacteric and non-climacteric fruit and vegetables for which responses to 1-MCP have been investigated from references available at Watkins and Miller (2005a) Fruit (climacteric) Banana (Musa L.) Blueberry, highbush (Vaccinium corymbosum L.) Chinese bayberry (Myrica rubra Siebold and Zuccarni) Chinese jujube (Zizyphus jujube M.) Custard apple (Annona squamosa L.) Figs (Ficus carica L.) Guava (Psidium guajava L.) Kiwifruit (Actinidia deliciosa (A. Chev) C.F. Liang et A.R. Ferguson var. deliciosa Lychee (Litchi chinensis) Mamey sapote (Pouteria sapote (Jacq.) H.E. Moore and Stearn) Mango (Mangifera indica L.) Vegetables Melon (Cucumis melo L.) Cherry (Prunus avium L.) Broccoli (Brassica oleracea L.) Mountain papaya (Vasconcellea pubescens) Nectarine (Prunus persica Lindl.) Clementine mandarin (Citrus reticulata L.) Cucumber (Cucumis sativus L.) Papaya (Carica papaya L.) Peach (Prunus persica L. Batsch) Grape (Vitis vinifera L.) Grapefuit (Citrus paradisi Macf.) Carrot (Daucus carota L.) Chinese cabbage (Brassica campestris L. spp. pekinensis (Lour) Olsson) Chinese mustard (Brassica juncea var. foliosa) Choysum (Brassica rapa var. parachinensis) Pear (Pyrus communis L.) Lime (Citrus latifolia Tanaka) Chrysanthemum, garland (Chrysanthemum coronarium) Pear (Pyrus pyrifolia Nakai) Persimmon (Diospyros khaki L.) Plum (Prunus salicina L.; Prunus x domestica L.) Tomato (Solanum esculentum Mill) Orange (Citrus sinensis L. Osbeck) Pepper (Capsicum frutescens L.) Pineapple (Ananas comosus L.) Coriander (Coriandrum sativum L.) Lettuce (Lactuca sativa L.) Mibuna (Brassica rapa var. nipposinica) Strawberry (Fragaria × ananassa Duch.) Watermelon (Citrullus lanatus) Mizuna (Brassica rapa var. nipposinica) Pak choy (Brassica rapa) Parsley (Petroselinum crispum Mill.) Potato (Solanum tuberosum) C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 Apple [Malus sylvestris (L) Mill. var. domestica (Borkh.) Mansf.] Apricot (Prunus armeniaca L.) Avocado (Persea americana Mill.) Fruit (non-climacteric) Tatsoi (Brassica rapa var. rosularis) 391 392 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 crops, which are specific to countries, include apple, apricot, avocado, kiwifruit, mango, melon, nectarine, papaya, peach, pear, pepper, persimmon, pineapple, plantain, plum, squash, tomatoes and tulip bulbs. Registration for use on various fruit and vegetables is expected soon for other countries. The impact of 1-MCP on postharvest science and technology has been two-fold. First, it provides the potential to maintain fruit and vegetable quality after harvest. Second, 1-MCP provides a powerful tool to gain insight into the fundamental processes that are involved in ripening and senescence. A large literature on the responses of both whole and fresh cut fruit and vegetables (as well as for ornamental products) to 1-MCP is developing, with 40 papers published in 2004 and 73 papers in 2003, compared with 36 in 2002, 19 in 2000, and a total of 16 published by 1998 (Watkins and Miller, 2005a). Recent reviews on the effects of 1-MCP on horticultural products include Blankenship and Dole (2003), Sisler and Serek (2003), Watkins (2002), Watkins and Miller (2003), Watkins and Ekman (2005) and Watkins and Miller (2005b). A website bhttp://www.hort. cornell.edu/mcp/ N that categorizes the physiological and biochemical responses for each product as decreased or delayed, increased, or unaffected, was initiated in 2001 and is regularly updated (Watkins and Miller, 2005a); research results are available for 35 fruit and 14 vegetables (Table 1). In this review, the effect of 1-MCP on fruit and vegetables is considered from two aspects. The first considers the effects on factors that influence product quality using several fruit that have received the most attention in the literature, and that highlight some of the challenges that exist in commercialization of 1MCP-based technology. While the responses of a wide range of vegetables have been examined (Table 1); most of these investigations have required supplementary treatment with exogenous ethylene to show effects of 1-MCP and future commercial utilization is uncertain. Therefore, individual vegetables are not considered in detail here. Commercial development of 1-MCP has largely centered on apple fruit. While semicommercial trials have been carried out with a wide variety of other registered crops, much of this work is proprietary, and access to commercial information is less easily obtained. The second aspect takes information from the literature to provide an overview about the physiological and biochemical responses of fruit and vegetables to 1-MCP to illustrate its potential to better understand the role of ethylene in ripening and senescence processes. 2. Responses of selected fruit and vegetables to 1-MCP 2.1. Apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] 2.1.1. Ripening physiology and quality 1-MCP dramatically inhibits ripening of apple fruit. The increases in ethylene production and internal ethylene concentrations (IECs) associated with the climacteric ripening stage are prevented or delayed by 1-MCP treatment, the extent of inhibition being related to cultivar, storage type and length of storage (Fan et al., 1999; Fan and Mattheis, 1999a; Rupasinghe et al., 2000; Watkins et al., 2000; Dauny and Joyce, 2002; Jiang and Joyce, 2002; Pre-Aymard et al., 2003; Saftner et al., 2003; Defilippi et al., 2004; Arquiza et al., 2005; Bai et al., 2005; Kondo et al., 2005; Mattheis et al., 2005; Moran and McManus, 2005; Pechous et al., 2005; Toivonen and Lu, 2005; Watkins and Nock, 2005). Respiration rates in treated fruit have been less commonly reported but are also inhibited by 1-MCP (Fan et al., 1999; Fan and Mattheis, 1999a, 2001; Jiang and Joyce, 2002; Pre-Aymard et al., 2003; Saftner et al., 2003; Defilippi et al., 2004; Mattheis et al., 2005; Toivonen and Lu, 2005). However, respiration rates are not reduced by 1-MCP below pre-climacteric levels (Mir and Beaudry, 2002). Softening is prevented or delayed by 1-MCP, the effects of treatment often closely associated with ethylene production (Fan et al., 1999; Rupasinghe et al., 2000; Watkins et al., 2000; Mir et al., 2001; Dauny and Joyce, 2002; Pre-Aymard et al., 2003; Saftner et al., 2003; Zanella, 2003; Defilippi et al., 2004; DeLong et al., 2004; Jayanty et al., 2004; Bai et al., 2005; Mattheis et al., 2005; Moran and McManus, 2005; Toivonen and Lu, 2005). The components of texture that are affected by 1-MCP have not been adequately investigated but tissue toughness is greater in 1-MCPtreated fruit than untreated fruit (Baritelle et al., 2001). Firmness retention can also be excellent in fruit kept at high temperatures (20–24°C) after treatment (Fan et al., 1999; Mir et al., 2001), and interestingly, Toivonen and Lu (2005) found that effects of 1-MCP on an early ripening summer apple cultivar were lost at storage temperatures below 15 °C. Loss of greenness of the background or ground color of the fruit skin, usually considered as a negative attribute in commercial conditions, is inhibited (Fan and Mattheis, 1999a, 2001; Dauny and Joyce, 2002; Jiang and Joyce, 2002; Pre-Aymard et al., 2003; Saftner et al., 2003; Zanella, 2003), although no effect of 1-MCP on color was detected by Dauny and Joyce (2002). C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 In general, 1-MCP delays loss of titratable acidity (TA) concentrations (Fan et al., 1999; Fan and Mattheis, 1999a, 2001; Watkins et al., 2000; Pre-Aymard et al., 2003, 2005; Saftner et al., 2003; Zanella, 2003; Defilippi et al., 2004; Bai et al., 2005; Toivonen and Lu, 2005). However, absence of treatment effects (Mir et al., 2001) or mixed responses depending on storage type (air or CA), on TA have been reported (Watkins et al., 2000). Soluble solid concentrations (SSC) in 1-MCP treated fruit can be higher, lower or the same as those in untreated fruit (Fan et al., 1999; Watkins et al., 2000; Dauny and Joyce, 2002; DeEll et al., 2002; Saftner et al., 2003; Zanella, 2003; Bai et al., 2005; Moran and McManus, 2005; Pre-Aymard et al., 2005). Total volatile contents are reduced by 1-MCP treatment, although individual volatiles are affected differentially (Rupasinghe et al., 2000; Lurie et al., 2002; Saftner et al., 2003; Defilippi et al., 2004; Bai et al., 2005; Kondo et al., 2005). These effects are discussed in detail in Section 3.5. Apples are susceptible to a wide variety of physiological and pathological disorders, and the impact of 1-MCP on susceptibility of fruit to these disorders is discussed in Sections 3.7 and 3.8. Little formal sensory research is available on the effects of 1-MCP on consumer acceptance of apples, partly because recent registration has precluded consumer testing. The rapidly ripening summer apple ‘Anna’ treated with 1-MCP that had less fruity, ripe and overall aromas, and were firmer, crisper, juicier and less mealy, were more preferred in sensory analyses than untreated fruit (Lurie et al., 2002; Pre-Aymard et al., 2005). 2.1.2. Commercial application of 1-MCP The apple fruit provides an important example of a horticultural product to illustrate the opportunities and limitations of 1-MCP-based technologies. The apple was the first crop that received registration for 1-MCP use, and 1-MCP use has been incorporated rapidly by industries around the world. The feature of 1-MCP technology that has made it widely accepted by apple industries is that treated apple fruit maintain texture after removal from storage; in contrast, while CA storage can also maintain quality of apples during storage, these benefits are often lost during the subsequent shipping and marketing periods resulting in overly ripe and soft apple at the retail level. Nevertheless, the rapid commercial adoption of 1MCP has not surprisingly resulted in a number of challenges to growers and storage operators. The 393 “apple” is a fruit with a wide variety of cultivars with different ripening rates and susceptibility to various physiological and pathological disorders, and depending on cultivar, may be stored for up to a year from harvest (Watkins, 2003). Also, preferences for fruit attributes in the marketplace vary greatly with cultivar, and include those with firm and acid characteristics as well as those that are soft and aromatic. 1-MCP-treated fruit have to meet varietal requirements. The research described in Section 2.1.1 has catalogued a wide variety of apple fruit responses to 1MCP, but many of these results have been obtained under ideal laboratory conditions, often when fruit are treated on the day of harvest. Under commercial conditions fruit are usually placed into cold storage and may be accumulated over several days before treatment with 1-MCP. Research that examines the effects of various harvest and handling procedures is emerging in the literature, and is resulting in development of protocols to ensure that maximum benefits of the technology are realized by the industry. The relationship between 1-MCP concentration and storage period is affected by cultivar and storage type (Rupasinghe et al., 2000; Watkins et al., 2000; PreAymard et al., 2003). Longer exposure periods are required as the treatment temperature decreases (DeEll et al., 2002), although little difference in responses of fruit treated with 1-MCP for 24h on the day of harvest at 20 °C or after cooling overnight were detected (Dauny and Joyce, 2002; Watkins and Nock, 2005). In addition, 1-MCP may lead to modification of CA recommendations, for example, it may reduce the requirement for CO2 during storage to maintain firmness (DeEll et al., 2005). Ethylene production by apple fruit occurs both on and off the tree when the climacteric is initiated, and therefore the effectiveness of 1-MCP is affected both by the maturity/ripening stage at harvest and by the period of time that the fruit are kept in cold storage before treatment. These two factors are inter-related as more mature fruit at harvest produce autocatalytic ethylene sooner than earlier harvested fruit. Cultivar effects are also important; Mir et al. (2001) found relatively small effects of harvest maturity for ‘Delicious’, but large effects were shown for ‘Empire’ (Watkins and Nock, 2005). Higher 1-MCP concentrations result in better responses in fruit of advanced maturity (Watkins, unpublished data). However, these may exceed the maximum treatment concentrations of 1-MCP, which are established by regulation for each country, for example, 1μl l− 1 for apples in the US. 394 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 Watkins and Nock (2005) found that the effect of delays between harvest and application of 1-MCP is affected by cultivar, storage type and storage length. The suppliers of the commercial product, SmartFresh™, provide recommendations for maximum delays between harvest and treatment, but these times are variable depending on fruit physiological condition, and a rule of thumb is that the delay times must be as short as possible if longer storage periods are desired by the storage operator. It may be possible to store treated fruit at warmer than normal cold storage temperatures if decay and loss of other quality attributes such as TA is not unacceptably high (Mir et al., 2001; Jayanty et al., 2004). CA can prolong the impact of 1-MCP on both physical and sensory responses of apple fruit (Rupasinghe et al., 2000; Watkins et al., 2000; Mattheis et al., 2005). It is possible that 1-MCP can be an alternative to CA storage, but the two technologies generally are more effective when used in combination (Rupasinghe et al., 2000; Watkins et al., 2000). Importantly, variation in response of different fruit lots to 1-MCP can occur under commercial conditions, and CA use as a supplement can become more critical. Fruit from orchard lots within a room that do not respond to 1MCP continue to deteriorate as storage times increase resulting in unacceptable commercial losses (Watkins, unpublished data). Nevertheless, 1-MCP may be an excellent replacement for CA storage for short term air storage, especially for maintaining quality of summer apples (Toivonen and Lu, 2005) and of other cultivars that deteriorate after only 2–3 months of storage (Watkins et al., 2000; Bai et al., 2005), especially for smaller local market operations that do not have CA facilities. The availability of 1-MCP for the apple is allowing the rapid identification of commercial opportunities and limitations of the technology. The success of 1-MCP for apple fruit appears to be based greatly on the maintenance of texture, especially because apple, unlike many other fruit types, do not soften markedly after harvest, and to date, at least the minimum demands for flavor have been met for most treated cultivars. An extensive literature describing many aspects of the responses of apple fruit can be expected in the next several years. 2.2. Avocado 2.2.1. Ripening physiology and quality Ripening of avocado fruit is not initiated until they are harvested, and the fruit then softens to an edible texture and with skin color changes appropriate to the cultivar. Fruit responses to 1-MCP are ‘concentration × exposure time’-dependent in most (Feng et al., 2000; Jeong et al., 2002), but not all (Woolf et al., 2005), studies. Both the timing of the peak of ethylene production and the respiratory climacteric are delayed (Feng et al., 2000, 2004; Jeong et al., 2002, 2003; Hershkovitz et al., 2005). Maximum rates of ethylene and carbon dioxide were usually lower, but the magnitude can be higher in some cultivars (Jeong et al., 2003; Hershkovitz et al., 2005). Treated fruit are firmer, slower to soften, and slower to change skin color (Feng et al., 2000, 2004; Hofman et al., 2001; Jeong et al., 2002; Adkins et al., 2005; Hershkovitz et al., 2005; Woolf et al., 2005). 1-MCP treated fruit have lower weight loss than untreated fruit (Jeong et al., 2002). 1-MCP treatment combined with waxing does not extend the shelf life more than 1-MCP alone, but results in reduced weight loss and better maintenance of green color (Jeong et al., 2003). 2.2.2. Factors affecting commercial application of 1-MCP Successful commercial utilization of 1-MCP for avocado is dependent on an appropriate balance between excessively delayed ripening that can increase decay development, especially after removal from storage, and desirable increases in storage potential with reduced internal disorders (Woolf et al., 2005). Adkins et al. (2005) concluded that 1-MCP could extend marketing periods, but that treated fruit must have low disease inoculum load or be more resistant to infection to avoid increased decay potential. Diffuse flesh discoloration, a symptom of internal chilling injury, was reduced in 1-MCP-treated fruit, although external chilling injury (skin blackening) in ‘Hass’ was not affected by treatment (Pesis et al., 2002; Woolf et al., 2005). Therefore, 1-MCP could allow storage of fruit at lower temperatures. 1-MCP delayed ripening after ethylene treatment if fruit softening was not initiated (Jeong and Huber, 2004). Ethylene treatment could overcome inhibition of ripening by 1-MCP at lower treatment concentrations, but ethylene recovery of 1-MCP ripening inhibition was only partial and differed by ripening factor (Jeong and Huber, 2004). 2.3. Banana (Musa sp., AAA group, Cavendish subgroup) 2.3.1. Ripening physiology and quality The banana fruit are typically harvested at the mature green stage of maturity, transported, and then ripened artificially with ethylene before being sent to market. Banana had increased ‘green life’ when treated with C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 1-MCP, responses being ‘concentration × exposure time’ dependent (Jiang et al., 1999b; Harris et al., 2000; Bagnato et al., 2003). Ethylene production and respiration rates were lower in 1-MCP-treated fruit than in untreated fruit (Golding et al., 1998, 1999; Pathak et al., 2003; Pelayo et al., 2003; Lohani et al., 2004). Softening of fruit was inhibited by 1-MCP treatment (Jiang et al., 1999a,b; Macnish et al., 2000; Botrel et al., 2002; Pelayo et al., 2003; Lohani et al., 2004). Color changes were also delayed in 1-MCP treated fruit (Botrel et al., 2002), and may have been associated with disrupted or incomplete and uneven yellowing, even in the presence of propylene (Golding et al., 1998; Harris et al., 2000; Macnish et al., 2000). Total volatile production of fruit was inhibited by 1-MCP treatment, and quantitatively, ester concentrations were lower, while those of alcohols were higher, in treated fruit (Golding et al., 1998). Little is known about the effect of 1-MCP on other compositional changes of banana fruit, although sugar content was not affected by treatment (Golding et al., 1998). 2.3.2. Factors affecting commercial application of 1-MCP The limitation for commercial application of 1-MCP for bananas may be the partial disruption of ripening events in treated fruit (Golding et al., 1998) that highlight the integrative role of ethylene in ripening (Golding et al., 1999). Harris et al. (2000) concluded that 1-MCP had limited commercial potential because of the uneven color development, and that this problem was exacerbated because of the range of maturities present in a commercial consignment. In contrast, however, Bagnato et al. (2003) found that while ripening was inhibited and decay increased at high 1MCP concentrations, a lower concentration of 300nl l− 1 resulted in delayed ripening but firmness, color, SSC and aroma profiles were similar with those of untreated fruit when compared at the same ripening stages. One strategy for use of 1-MCP on fruit such as banana for which ripening is artificially initiated with ethylene treatment could be to apply 1-MCP after this treatment. Pelayo et al. (2003) concluded, however, that variability in responses of partially ripe bananas was too variable for commercial application. Also, if 1-MCP was applied to fruit 24 h after propylene treatment to induce ripening, then the onset of ethylene and respiration rates was not affected but color and volatile production was inhibited (Golding et al., 1998). Treatment of pre-climacteric fruit, or earlier 1-MCP treatment, resulted in inhibition of all processes. 395 2.4. Pear (Pyrus communis L.) 2.4.1. Ripening physiology and quality The effects of 1-MCP have been investigated using summer, autumn and winter pears. High quality pear fruit have a buttery texture, with color change appropriate to the cultivar, and development of characteristic taste and aroma associated with sugar and acid contents and volatile production (Kappel et al., 1995; Ma et al., 2000). Pears require exposure to chilling temperatures to ripen properly, with winter pears requiring as long as 8 weeks. Ethylene production was inhibited by 1-MCP treatment (Argenta et al., 2003; Hiwasa et al., 2003; Kubo et al., 2003; Ekman et al., 2004; Larrigaudiere et al., 2004; Trinchero et al., 2004; Mwaniki et al., 2005). Respiration rates were also lower in 1-MCP-treated fruit (Argenta et al., 2003; Kubo et al., 2003; Ekman et al., 2004). 1-MCP delayed or prevented fruit softening, the degree of response depending on the cultivar and 1-MCP concentration applied (Baritelle et al., 2001; Argenta et al., 2003; Hiwasa et al., 2003; Kubo et al., 2003; Calvo and Sozzi, 2004; Ekman et al., 2004; Trinchero et al., 2004). Loss of greenness or yellowing but not SSC, was inhibited by 1-MCP (Calvo and Sozzi, 2004; Larrigaudiere et al., 2004; Trinchero et al., 2004). Treated fruit had higher TA in one study (Argenta et al., 2003) but no effect of 1-MCP was found by others (Calvo and Sozzi, 2004; Larrigaudiere et al., 2004; Trinchero et al., 2004). 2.4.2. Factors affecting commercial application of 1-MCP ‘1-MCP concentration × time’ relationships for delays of ripening have been shown (Argenta et al., 2003), and the 1-MCP concentrations that delay, but do not ultimately prevent, normal ripening are variable. Application of 0.2 μl l− 1 1-MCP resulted in normal ripening with reduction in over-ripening problems (Calvo and Sozzi, 2004), while concentrations as high as 10 and 100 μl l− 1 to fruit in which ripening was initiated by chilling temperatures resulted in maintenance of optimal eating firmness for extended periods and prevention of senescent breakdown development after storage (Kubo et al., 2003). Repeated 1-MCP applications can further slow down ripening (Ekman et al., 2004), but sensitivity to second applications are affected if ethylene production by fruit has been initiated during storage (Trinchero et al., 2004). The efficacy of post-storage ethylene treatments to initiate ripening of 1-MCP-treated fruit after storage depends on 1-MCP concentration and storage duration (Argenta et al., 2003; Calvo and Sozzi, 2004; Ekman et al., 2004). 396 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 Both positive and negative aspects of 1-MCP that may be commercially important for pear fruit have been identified. 1-MCP reduces susceptibility of fruit to skin browning, and vibration and impact bruising, and therefore its use may permit greater flexibility during grading, packaging and transport operations (Calvo and Sozzi, 2004; Ekman et al., 2004). However, increased storage periods of 1-MCP-treated fruit may result in greater weight loss (Calvo and Sozzi, 2004). Ethylene appears to be required for both initiation and progression of ripening of pears (Hiwasa et al., 2003). Ekman et al. (2004) found that skin color and firmness was dissociated and suggested that this could problematic at the retail level. Variation of individual fruit treated with 1-MCP could result in the need for post-treatment sorting of fruit (Trinchero et al., 2004). 2.5. Peach and nectarine (Prunus persica L.) 2.5.1. Ripening physiology and quality Ethylene production is inhibited by 1-MCP treatment of peaches and nectarines (Mathooko et al., 2001; Fan et al., 2002; Rasori et al., 2002; Liguori et al., 2004; Bregoli et al., 2005; Girardi et al., 2005), although transient increases in production were detected immediately after treatment in one study (Fan et al., 2002). Ethylene production of 1-MCP treated fruit was unaffected at the time of treatment, but was reduced after storage (Dong et al., 2001b). Recovery from 1-MCP-induced ripening inhibition resulted in greater production of ethylene than observed in untreated fruit (Rasori et al., 2002). Respiration rates of treated fruit were lower (Fan et al., 2002), or not affected (Dong et al., 2001b; Liguori et al., 2004) by 1-MCP treatment. Softening of fruit was delayed when fruit were kept at about 20 °C (Dong et al., 2001b; Mathooko et al., 2001; Fan et al., 2002; Rasori et al., 2002; Bregoli et al., 2005; Liu et al., 2005), and in fruit from early and late harvest (Liguori et al., 2004). However, Fan et al. (2002) found that the effects of fruit maturity were greater than those of 1-MCP treatment, with early harvested fruit showing little response to treatment. Also, beneficial effects of 1-MCP were lost at 4°C (Bregoli et al., 2005). 1-MCP treatment either did not affect SSC of fruit (Liguori et al., 2004), resulted in lower SSC (Fan et al., 2002; Bregoli et al., 2005), or the increase of SSC during ripening was delayed (Liu et al., 2005). Loss of TA was reduced in high acid (Fan et al., 2002; Liguori et al., 2004; Bregoli et al., 2005; Liu et al., 2005), but not in low acid cultivars (Liguori et al., 2004). 2.5.2. Factors affecting commercial application of 1-MCP Responses of fruit to 1-MCP are affected by concentration and exposure period, but not treatment temperature (Liguori et al., 2004). Optimal 1-MCP concentrations vary greatly from as low as 0.4μl l− 1 (Liu et al., 2005) to 5 μl l− 1 (Liguori et al., 2004), the latter concentration being higher than that registered for use. Inhibition of fruit ripening is transitory in all published studies, but repeated 1-MCP applications helps maintain suppression of ripening (Liu et al., 2005). The transitory effect of 1-MCP is not related to diffusion limitations within the flesh (Hayama et al., 2005). 1MCP treatment at ambient temperatures could allow commercially significant extension of fruit shelf life. However, decreased 1-MCP effects in fruit stored at 4 o C (Bregoli et al., 2005), and a greater incidence of chilling-related disorders in treated fruit (Dong et al., 2001b; Fan et al., 2002; Girardi et al., 2005) suggests that this technology is limited for extending the storage life of peaches and nectarines in cold storage. 2.6. Plum (Prunus domestica L. and Prunus salicina L.) 2.6.1. Ripening physiology and quality 1-MCP prevented or delayed the climacteric increase in ethylene production of plums (Abdi et al., 1998; Dong et al., 2001a, 2002; Martinez-Romero et al., 2003; Salvador et al., 2003; Valero et al., 2003, 2004), even when fruit were harvested close to the climacteric peak (Salvador et al., 2003). Respiration rates were also decreased or the climacteric increase delayed (Dong et al., 2002; Martinez-Romero et al., 2003; Salvador et al., 2003; Valero et al., 2003), but no effect of 1-MCP treatment was detected by Dong et al. (2001a). Softening of the fruit was delayed by 1-MCP treatment (Dong et al., 2001a, 2002; Skog et al., 2001; MartinezRomero et al., 2003; Salvador et al., 2003; Valero et al., 2003, 2004; Menniti et al., 2004). Skin color changes were delayed by 1-MCP (Dong et al., 2002; MartinezRomero et al., 2003; Salvador et al., 2003; Valero et al., 2003, 2004; Menniti et al., 2004), and weight loss during and after storage decreased (Martinez-Romero et al., 2003; Valero et al., 2003). Responses of SSC and TA to 1-MCP treatment are variable. The SSC was not affected by 1-MCP treatment (Dong et al., 2002; Salvador et al., 2003; Menniti et al., 2004), but its ripening-associated increase was (Valero et al., 2004). While TAwas not affected by 1-MCP treatment (Menniti et al., 2004), loss of acidity was reduced in other studies (Dong et al., 2002; Salvador et al., 2003). The SSC to TA ratio was lower in 1-MCP-treated than untreated fruit (Martinez-Romero et al., 2003; Valero et al., 2003). C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 Although plums are categorized as climacteric fruit, suppressed-climacteric cultivars are available (Abdi et al., 1998; Martinez-Romero et al., 2003). The ‘suppressed’ cultivars did not ripen when treated with 1MCP unless they were subsequently treated with propylene (Abdi et al., 1998), and the effects of 1MCP were not dose dependent, maximum responses occurring at the lowest 1-MCP concentration applied (Martinez-Romero et al., 2003). In contrast, dose dependent responses were shown for a normal ethylene-producing cultivar. 2.6.2. Factors affecting commercial application of 1-MCP Postharvest softening and susceptibility to mechanical injury and pathogens are major factors limiting the shipping, storage and shelf life of plums. The degree of response to 1-MCP varies greatly by cultivar and harvest maturity (Abdi et al., 1998; Skog et al., 2001; Martinez-Romero et al., 2003) but reports of extension of storage periods from 1 week for untreated fruit to 4 weeks for 1-MCP-treated fruit plus 7 days at 20°C without negative effects indicate that 1-MCP may be a very useful technology for this fruit. Moreover, 1MCP is effective at later stages of maturity when better quality characteristics have developed (Salvador et al., 2003; Valero et al., 2003). Studies on climacteric and suppressed-climacteric cultivars indicate however, that 1-MCP concentrations will need to be calibrated for cultivars with different ethylene production (Abdi et al., 1998; Martinez-Romero et al., 2003). No differences were noted between treatment at 0 and 20 °C (Menniti et al., 2004), although Valero et al. (2003) suggested that treatment at cold storage temperatures resulted in better control of ethylene. 1-MCP treatment prior to simulated mechanical harvest decreased fruit losses due to bruising (Lippert and Blanke, 2004). In one of the few studies that have investigated possible interactions between 1-MCP treatment and postharvest handling operations, Valero et al. (2004) found that 1-MCP treatment of fruit packed in small ventilated cardboard boxes resulted in better ripening control than for bulked fruit. 2.7. Tomato (Solanum esculentum Mill) 2.7.1. Ripening physiology and quality 1-MCP treatments markedly affected ripening of tomato fruit by inhibiting ethylene production (Hoeberichts et al., 2002; Wills and Ku, 2002; Krammes et al., 2003; Opiyo and Ying, 2005; Tassoni et al., in press). Other ripening processes that are inhibited 397 include respiration rates (Wills and Ku, 2002; Colelli et al., 2003; Krammes et al., 2003), color change and softening (Sisler et al., 1996; Hoeberichts et al., 2002; Colelli et al., 2003; Mostofi et al., 2003; Mir et al., 2004; Opiyo and Ying, 2005; Tassoni et al., in press). TA was higher in treated fruit (Moretti et al., 2002; Wills and Ku, 2002; Krammes et al., 2003; Opiyo and Ying, 2005), but SSC were not affected by 1-MCP treatment (Moretti et al., 2002; Wills and Ku, 2002; Colelli et al., 2003; Krammes et al., 2003; Mir et al., 2004; Opiyo and Ying, 2005). Weight loss from fruit was not affected by 1-MCP treatment (Wills and Ku, 2002; Colelli et al., 2003). Only small effects of treatment on aroma volatiles were detected by Mir et al. (2004), although changes tended towards those volatiles that were associated with harvest of fruit at earlier ripening stages. 2.7.2. Factors affecting commercial application of 1-MCP The desirable response of fresh market tomato fruit to 1-MCP is a delay of ripening, but then ripening to redness, desired softness, and flavor development. Another possible commercial benefit of 1-MCP is inhibition of abscission of cherry tomatoes from vines (Beno-Moualem et al., 2004). The extent of ripening inhibition of tomato fruit is affected by 1-MCP concentration, exposure time and ripening stage (Sisler et al., 1996; Hoeberichts et al., 2002; Moretti et al., 2002; Wills and Ku, 2002; Mir et al., 2004; Opiyo and Ying, 2005), and optimal treatment concentrations are also affected by cultivar (Krammes et al., 2003). Fruit recover capacity to ripen after treatment, but second applications further delay ripening (Hoeberichts et al., 2002; Mir et al., 2004). However, the maturity stage of the fruit at the time of 1-MCP treatment affects the ability of the fruit to recover; mature green and breaker stage fruit shriveled and developed decay before ripening, although fruit treated at pink and light red stages ripened properly after a delay (Hurr et al., 2005). The presence of ethylene was required throughout for ripening even when 1-MCP was applied at advanced ripening stages (Hoeberichts et al., 2002). Mostofi et al. (2003) also found that treatment of mature green fruit resulted in impaired color development, and that the ripening temperature was important; whereas ripening of fruit treated at the breaker stage was coordinated at 15, 20 and 25 °C, development of color was out of phase with softening at 25°C. The reddening of locular tissues was later than that of pericarp tissues after treatment of mature green fruit, which could affect appearance of fresh cut slices (Mir et al., 2004). 398 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 Formal sensory studies on the effects of 1-MCP on consumer acceptability are not available. However, Wills and Ku (2002) concluded that MCP-treated fruit should be of superior quality because of improved SSC / TA ratios. 1-MCP had little effect on aroma volatiles in the study of Mir et al., (2004), but negatively and irreversibly affected them in informal sensory analyses reported by Hurr et al. (2005). 3. Physiological and biochemical responses of fruits and vegetables to 1-MCP Generalizations regarding the effects of 1-MCP on physiological and biochemical responses of fruits and vegetables are shown in Table 2. Specific examples are provided here, but fuller documentation of these effects is described on a website bhttp://www.hort. cornell.edu/ mcp/N (Watkins and Miller, 2005a). 3.1. Ethylene metabolism The action of 1-MCP is mediated through the inhibition of ethylene perception of plant tissues by interacting with the receptor and competing with ethylene for binding sites (Sisler et al., 1996; Sisler and Serek, 1997, 2003). Therefore, the effectiveness of inhibition of ripening and/or senescence of fruit and vegetables is a function of the 1-MCP concentration applied, up to saturation of the binding sites. Depending on the product it can be desirable for the inhibition of ethylene-mediated responses to persist indefinitely, especially in the case of leafy vegetables, but for fruit, recovery from 1-MCP-induced inhibition of ripening is often essential in order to provide a ripened product that is acceptable to the consumer. The extent and longevity of 1-MCP action is affected by species, cultivar, tissue and mode of ethylene biosynthesis induction. A ‘concentration × time’ effect is apparent with longer exposure periods required for lower 1-MCP concentrations to obtain the same physiological effects (Sisler and Serek, 1997). Some products such as pea require higher concentrations (40 nl l − 1 ) than carnations (0.5 nl l− 1) and banana (0.7 nl l− 1), suggesting that new receptors are produced in growing tissues or that a low affinity form of the receptors is present (Sisler and Serek, 2003). Synthesis of new binding sites may be affected by temperature; in banana, temperatures between 30 and 40°C results in faster recovery of ripening, while application of 1-MCP at 2.5 °C is less effective than at 15 and 20 °C suggesting that binding of 1-MCP at low temperatures was incomplete (Jiang et al., 2002b, 2004c). Research with yeast has shown that ETR1 and ERS1, genes encoding the ethylene binding proteins, show equal sensitivity to 1-MCP (Hall et al., 2000), but little research on expression of these genes in fruit and vegetables is yet available. Accumulation of transcripts for the genes encoding ERS decreased in treated apple (Defilippi et al., 2005). Rasori et al. (2002) found that 1-MCP did not affect transcription of the gene PP-ETR1, but downregulated that of PP-ERS1. Recovery from 1-MCP inhibition was associated with increased accumulation of PP-ERS1 transcripts. In tomato, recovery from ripening inhibition was associated with increased gene expression for both ETR1 and ERS1 (Tassoni et al., in press). Ethylene production of fruit is usually inhibited by 1-MCP treatment, but the persistence of the inhibition can be variable (Fan et al., 1999; Fan and Mattheis, 1999a; Dong et al., 2002; Jeong et al., 2002; Ergun et al., 2005). Ethylene production is not always inhibited. The decline in ethylene production was slower over time in 1-MCP treated pineapples than untreated fruit (Selvarajah et al., 2001), and greater ethylene production has been observed in fruit and vegetables depending on cultivar, maturity, stage or ripening or 1-MCP concentration, including treated avocado (Jeong et al., 2003; Hershkovitz et al., 2005), banana (Golding et al., 1998), grapefruit (Mullins et al., 2000), strawberry (Tian et al., 2000), Chinese cabbage (Porter et al., 2005), coriander (Jiang et al., 2002a), and parsley (Ella et al., 2003). In some cases the increased ethylene does not appear to affect other senescence processes, e.g., 1-MCP citrus fruit remained green despite higher ethylene production (Mullins et al., 2000), but increased ethylene production can accelerate senescence in other tissues, e.g., parsley (Ella et al., 2003). It is likely that stimulated ethylene production is due to loss of negative feedback regulation of ethylene biosynthesis. The genes encoding two key enzymes of the ethylene biosynthetic pathway, 1-aminocyclopropane carboxylic acid oxidase (ACO) and 1-aminocyclopropane carboxylic acid synthase (ACS), and their respective enzyme activities have been studied in several crops. In apple, banana, melon and pear fruit, inhibition of ethylene production by 1-MCP was accompanied by lower expression of these genes (Lelievre et al., 1997; de Wild et al., 1999; Defilippi et al., 2005), and lower activities of ACS and ACO (Dong et al., 2001b; Pathak et al., 2003; Defilippi et al., 2005). In peach, inhibited ethylene production was associated with reduced activity of ACO and a reduction in PP-ACO1 and PP-ACO2 transcript accumulation, but 1-aminocyclopropane carboxylic acid (ACC) accumulated in treated fruit, and PP-ACS1 expression and ACS activity was not 399 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 Table 2 Generalizations regarding the effects of 1-MCP on metabolism of fruit and vegetables Attribute or process affected Enzyme activity or associated gene expression Increased (↑), decreased (↓), or unchanged (↔) Ethylene metabolism Ethylene perception Ethylene production ↓↔ ↓↑ ACC synthase (ACS) expression and ↓ activity ACC oxidase (ACO) expression and ↓ activity ETR1, ERS1 Respiratory metabolism Pigments Respiration rate SSC TA Chlorophyll degradation Lycopene accumulation Anthocyanin accumulation Chlorophyllase activity ↓ ↑↔ ↓ ↑↔ ↓ ↑↔ ↓ ↓ ↓ ↓ Phenolic metabolism Total phenolic content ↓ Phenylalanine ammonia lyase (PAL) ↓ activity Polyphenol oxidase (PPO) activity ↓ Cell wall metabolism Soluble polyuronide content Polygalacturonase (PG) activity Pectin methylesterase (PME) Endo-β-1,4-glucanase (EGase) Glycosidases ↓ ↓ ↓ ↓ ↓↔ Volatile compound metabolism ↓ ↔ ↔ ↓↑ Esters Aldehydes Terpenoid biosynthesis Acetaldehyde and ethanol accumulation Alcohol acyl transferase activity Alcohol dehydrogenase activity Nutritional Vitamin C loss Anthocyanin contents Phenolic contents Antioxidant activity loss ↓ ↔ ↓ ↓ ↓↔ ↓ Physiological disorders Senescent disorders Chilling injury Superficial scald (apples and pears) Ethylene-induced disorders Controlled atmosphereinduced Abscission ↓ ↓↑ ↓ Susceptibility to pathogens Fungal growth ↑↓↔ ↓↔ ↓ ↑ ↓ Pathological disorders affected by treatment (Mathooko et al., 2001). 1-MCP treated nectarines had lower ACS, ACO1 and ACO2 transcript accumulations than untreated fruit at ambient but not cold storage temperatures (Bregoli et al., 2005). Application of 1-MCP to avocados at the pre-climacteric stage and at the onset of the climacteric inhibited ACS and ACO activities, and the transcription of PA-ACS1, and suppressed PA-ACO and PA-ERS1 mRNAs to trace 400 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 levels (Owino et al., 2002). Discontinuation of 1-MCP action resulted in super-induction of these genes in the fruit. In citrus, 1-MCP-induced increases of ethylene production were associated with greater ACC accumulation and higher ACS transcript accumulation and enzyme activity (Mullins et al., 2000). In tomato, transcript accumulations of ACO1, phytoene synthase 1 (PSY1) and expansin 1 (EXP1), used as indicators of treatment effects on ethylene biosynthesis, color and softening, respectively, were decreased by 1-MCP (Hoeberichts et al., 2002). Increases in transcript abundance of LE-ACS2, LE-ACS4 and LE-ACO1 mRNAs in ripening fruit were inhibited by 1-MCP, but ethylene production, ACC content and ACS and ACO activities were not inhibited to the expected levels suggesting involvement of negatively regulated genes in ethylene biosynthesis (Nakatsuka et al., 1997). Reduced ethylene production of fruit that were treated at the turning and pink stages was accompanied by inhibited ACS2, ACS4, ACO1 and ACO4 transcript accumulation, while several other ACS genes were not affected (Nakatsuka et al., 1998). 1-MCP abolished expression of E4 mRNA in wounded tomato fruit, but not expression profiles of LEACS2, LE-ACS6 and LE-ACO1 (Yokotani et al., 2004). Itai et al. (2003) showed that α-L-arabinofuranosidase (LeARF1) expression was negatively regulated by ethylene, while two B-D-xylosidase genes were independent of ethylene action. NR transcripts were immediately suppressed by 1-MCP treatment but as fruit ripened, they recovered from 1-MCP in a similar pattern to that of the ACS and ACO genes (Tassoni et al., in press). 3.2. Respiration rate As described for several fruit in Section 2, the respiration rates of most treated products decreased or were delayed, especially in climacteric fruit where increases accompany increases of ethylene production. The peak respiration rates at the climacteric were reduced by 1-MCP (Jeong et al., 2002, 2003). Enhanced respiration rates of ethylene-treated strawberry fruit were reduced by 1-MCP at earlier harvests, but not in fruit from later harvests (Tian et al., 2000). Bower et al. (2003) found higher respiration rates in 1-MCP-treated strawberry fruit that may have been associated with earlier onset of decay. Starch degradation is sometimes delayed in 1-MCPtreated fruit (Fan et al., 1999). SSC in treated products might be expected to be higher than in untreated products because of lower respiration rates, but can be higher, lower or the same as in untreated fruit depending on the product and the storage conditions (Fan et al., 1999; Watkins et al., 2000; Benassi et al., 2003). The sugar content of banana was not affected by 1-MCP treatment (Golding et al., 1998), but lower SSCs in 1MCP treated fruit were associated with lower sucrose concentrations (Defilippi et al., 2004). 3.3. Pigment metabolism Loss of greenness, or yellowing, in most products is inhibited by 1-MCP. For many products, especially leafy vegetables and certain fruit such as apple, maintenance of green color is desirable in the marketplace as yellowness is regarded as a sign of senescence. However, for many fruit loss of chlorophyll and development, or unmasking, of colored pigments is an essential aspect of ripening (Kays, 1997). Therefore, successful 1-MCP use requires a delay, but not irreversible inhibition, of the processes involved in pigment metabolism. 1-MCP inhibited anthocyanin increases in strawberry fruit (Jiang et al., 2001), but loss of chlorophylls and development of colors (anthocyanins, lycopene) eventually reached desirable levels of those of untreated fruit. However, yellowing of banana fruit could be disrupted or incomplete and uneven, even in the presence of propylene (Golding et al., 1998; Harris et al., 2000; Macnish et al., 2000), and there was a separation of color changes from other ripening attributes in pears (Ekman et al., 2004). Ethylene-induced degreening of 1-MCP-treated cucumbers was totally inhibited or occurred with development of uneven yellowing (Nilsson, 2005). Marty et al. (2005) used 1-MCP treatments to investigate ethylene regulation of carotenoid accumulation and carotenogenic gene expression in orange and white apricot cultivars. Both cultivars accumulated the colorless phytoene and phytofluene pigments, but β-carotene accumulated only in the orange fruit. While1-MCP inhibited ethylene production of both cultivars, effects on pigment accumulations were small. Chlorophyll fluorescence changes were delayed in 1-MCP-treated fruit kept at ambient temperatures (Mir et al., 2001; Jayanty et al., 2004). Little is known about the effects of 1-MCP on pigment metabolism. However, Gong and Mattheis (2003) and Hershkovitz et al. (2005) have found that chlorophyllase activity was reduced in 1-MCP-treated broccoli florets and avocado fruit, respectively. 3.4. Cell wall metabolism For most products, delayed rather than complete inhibition of softening, is desirable (Section 2). For non- C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 climacteric fruit such as strawberries and oranges, the effects of 1-MCP on softening were usually not detectable (Porat et al., 1999; Tian et al., 2000), but softening was enhanced under some treatment combinations with ethylene (Tian et al., 2000). Studies of 1-MCP on cell wall changes of treated fruit are limited, but a number of investigations on cell wall enzymes are available. Decreased softening in 1-MCP-treated bananas is associated with lower expression of an ethylene induced expansin (MaExp1) gene (Trivedi and Nath, 2004), and lower activities of pectin methylesterase (PME), polygalacturonase (PG), endo-β-1,4-glucanase (EGase) and pectate lyase activities (Lohani et al., 2004). Effects of 1-MCP on softening of pears were associated with decreased β-galactosidase activity and differential effects on expression of its genes (Mwaniki et al., 2005), lower glycosidase activities (Trinchero et al., 2004), and transcript accumulation of genes for PG1 and PG2, but not EGase (Hiwasa et al., 2003). Delayed softening of peaches was associated with delayed increases in soluble pectin concentrations (Liu et al., 2005). Activities of exo-PG and EGase were lower in 1-MCP treated plums than in untreated fruit, but treatment did not affect activities of endo-PG and pectin esterase (PE). Alterations of cell wall enzyme associated with enhanced susceptibility of peach fruit to a chilling injury expressed as woolliness is discussed in Section 3.7.2. Delayed softening of 1-MCP-treated avocado fruit is reflected in similar patterns of delayed solubilization and degradation of polyuronides (Jeong et al., 2002; Jeong and Huber, 2004). Activities of PG and EGase were lower in 1-MCP-treated fruit (Feng et al., 2000). Interestingly PG activity did not recover even though fruit softened to those comparable with control fruit indicating that PG is not required for extensive softening (Jeong et al., 2002). Changes of PME, αand β-galactosidase and EGase activities were delayed but essentially followed patterns of increase or decline of the untreated fruit (Jeong and Huber, 2004). Lower activities of β-galactosidase, α-arabinofuranosidase and β-xylosidase were associated with delayed softening of 1-MCP treated kiwifruit (Boquete et al., 2004). 1-MCP affected gene expression of cell wallrelated genes of the non-climacteric strawberry, but specific effects depended on fruit ripening stage (Balogh et al., 2005). 1-MCP treatment resulted in up-regulation of a putative EGase in green fruit, but its down-regulation in red fruit, up-regulation of a ripening-repressed β-galactosidase, and down-regulation of pectic lyase gene. 401 3.5. Volatile compound metabolism Flavor is a composite of taste and odor, and volatile production can be greatly affected by ethylene. Therefore, decreased and/or altered volatile production in 1-MCP compared with untreated fruits may impact product acceptability by consumers. However, sensory analyses are limited to few products because of the relatively recent registration of 1-MCP, and absence of registration for many fruits and vegetables. In apple fruit, ester production was inhibited by 1-MCP, but results for other components were variable, perhaps reflecting different cultivars and storage conditions. Mattheis et al. (2005) found that production of esters, alcohols, aldehydes, acetic acid and 1-methoxy4-(2-propenyl)benzene was inhibited by 1-MCP, but their production increased when kept at room temperatures after long term storage. Esters and alcohols were lower in 1-MCP-treated fruit than untreated fruit (Defilippi et al., 2004; Kondo et al., 2005) but aldehydes were not affected (Defilippi et al., 2004). Esters were reduced but the proportions of alcohols and the aldehyde 2-hexenal in treated fruit were higher (Lurie et al., 2002). CA storage also inhibits volatile production of apple fruit. This inhibition was enhanced in 1-MCP-treated fruit, but the dynamics of volatile change over time were different in response to the two treatments (Mattheis et al., 2005); the effects of CA increased, while those of 1-MCP decreased, over time. The concentrations of branched but not straight-chained esters recovered in 1-MCP-treated fruit, though not to those of untreated fruit (Mattheis et al., 2005) confirming earlier indications that sensitivity to ethylene action varies among the pathways of ester production (Fan and Mattheis, 1999b). Ethylene production clearly has some direct effects on volatile production of fruit, and indeed continuous ethylene action is required for maximum volatile production (Fan et al., 1998; Fan and Mattheis, 1999b). While the mechanisms of 1-MCP action on volatiles are not known, Defilippi et al. (2005) found that alcohol dehydrogenase (ADH) activity was not affected by 1-MCP, but alcohol acyl transferase (AAT) transcripts and activity were lower in treated apple fruit suggesting that modulation of this step in volatile synthesis by ethylene may be responsible for lower ester production. Expression of four AAT genes were suppressed by 1-MCP treatment of melons (El-Sharkawy et al., 2005). In pear fruit, increases of ester and alcohol production were delayed by 1-MCP, with quantitative differences between long chain and branched esters, but volatiles were similar in treated and untreated pear fruit when equally ripe (Argenta et al., 2003). Total volatile production of 402 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 bananas was inhibited by 1-MCP treatment, and quantitatively, ester concentrations were decreased, while those of alcohols were increased in treated fruit (Golding et al., 1998). Abdi et al. (1998) found that aroma volatile production of plums was ethylene-dependent or -independent, depending on the cultivar. 1-MCP increased acetaldehyde and ethanol accumulation in citrus resulting in off-flavor development (Porat et al., 1999), but delayed accumulation of these compounds in plums (Salvador et al., 2003). Where reduced aroma is associated with 1-MCP treatment, the commercial implications are likely to vary by product type, being more critical for products and cultivars where aroma is a quality characteristic expected by the consumer. For some products, certain aromas are associated with over-ripening and therefore their inhibition is desirable, or aroma concentrations may be less important than texture and acid/sugar levels. 3.6. Nutritional quality The effect of 1-MCP on nutritional quality has not been studied thoroughly. However, 1-MCP slows vitamin C loss in Chinese jujube (Jiang et al., 2004b), peaches (Liu et al., 2005), pineapples (Selvarajah et al., 2001), and minimally processed lettuce and pineapple (Budu and Joyce, 2003; Tay and Perera, 2004). Although phenolic contents of apples were not affected by 1-MCP treatment (Defilippi et al., 2004), 1-MCP treated ‘Delicious’ and ‘Empire’ apples maintained higher watersoluble antioxidant activity than untreated fruit after cold storage (MacLean et al., 2003). Lower phenolic and anthocyanin contents in strawberry fruit treated with high 1-MCP concentrations (Jiang et al., 2001) could potentially reduce antioxidative activity. 3.7. Physiological storage disorders 3.7.1. Apple The literature on physiological storage disorders is greatest for apple fruit. 1-MCP can reduce senescent breakdown (Watkins et al., 2000; DeLong et al., 2004; Moran and McManus, 2005), brown core (syn. coreflush) (Fan and Mattheis, 1999a; Zanella, 2003), core browning (DeLong et al., 2004), coreline browning (Moran and McManus, 2005) and soft scald (Fan and Mattheis, 1999a), which are variously disorders associated with senescence and cold storage. Dissipation of watercore from fruit is reduced by 1-MCP treatment (Watkins, unpublished data). Susceptibility of some disorders appears to be increased by 1-MCP treatment, although little research on these responses has yet been published. An exception is external carbon dioxide injury which is higher in 1-MCP-treated fruit than untreated fruit (DeEll et al., 2003; Zanella, 2003; Watkins and Nock, 2004). The disorder is associated with early harvested fruit and early exposure to carbon dioxide in the storage atmosphere (Watkins et al., 1997; FernandezTrujillo et al., 2001) and the effect of 1-MCP is consistent with maintenance of the fruit in a less ripe state and thus more susceptible to injury. Risk of injury is eliminated by use of the antioxidant diphenylamine (DPA), registered to control superficial scald, or reduced by maintaining very low carbon dioxide concentrations in the storage atmosphere for the first few weeks of storage (Watkins and Nock, 2004). Some interactions of 1-MCP with other technologies have been investigated: Fan and Mattheis (2001) found that irradiation damage was aggravated in 1-MCP-treated fruit when they were kept at 20 °C. Lu and Toivonen (2003) showed a synergistic benefit of 1-MCP and 35% CO2 on quality of ‘Gala’ apples, without injury. Most research focus of 1-MCP and disorders of apple has been on superficial scald (syn. storage scald) because of the interaction between ethylene production and that of α-farnesene, and early reports that 1-MCP inhibited disorder development (Fan and Mattheis, 1999a,b; Rupasinghe et al., 2000; Watkins et al., 2000). Superficial scald is a physiological storage disorder of susceptible apple and pear cultivars that is manifested as browning or blackening of the skin resulting from necrosis of the hypodermal cells (Bain and Mercer, 1963). Development of the disorder occurs during low temperature storage and it is thought to be a chilling injury (Watkins et al., 1995). Injury to cells probably results from free radical reactions associated with the oxidation of α-farnesene to conjugated trienols (CTols), predominantly 9E and 9Z isomers of 2,6,10-trimethyldodeca-2,7,9,11-tetraen-6-ol (Rowan et al., 1995; Whitaker et al., 1997). Accumulation of α-farnesene in the skin is associated with ethylene (Du and Bramlage, 1994; Watkins et al., 1995; Whitaker et al., 2000). Development of scald can be reduced or prevented by inhibiting α-farnesene production or its accumulation, e.g., by ventilation, use of oil wraps, or by inhibiting its oxidation to CTols by DPA (Huelin, 1968; Lurie et al., 1989). It has been well demonstrated that inhibition of scald by 1-MCP is associated with inhibition of αfarnesene accumulation that restricts substrate available for oxidation (Fan and Mattheis, 1999a; Rupasinghe et al., 2000; Watkins et al., 2000; Shaham et al., 2003; Arquiza et al., 2005; Pechous et al., 2005). Although it may not be considered a storage disorder sensu stricto, an undesirable feature of some apple cultivars is development of greasiness, or slipperiness to C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 touch, which is disliked by consumers. Greasiness is associated with changes in the wax and oil fractions in the skin (Morice and Shorland, 1973), and in susceptible cultivars, its development can occur in more mature fruit and with longer storage periods (Leake et al., 1989a,b). 1-MCP inhibited development of greasiness (Fan and Mattheis, 1999a; Watkins and Nock, 2005) suggesting that it may be a useful tool to manage the problem. 3.7.2. Other fruits and vegetables Development of superficial scald in pear fruit was also controlled by 1-MCP, but developed as fruit were released from inhibition of ripening (Ekman et al., 2004). Thus, total control of the disorder only occurred in fruit that did not ripen. 1-MCP inhibited production of αfarnesene and its oxidation product, MHO, in pears (Argenta et al., 2003). Other disorders of pears that are inhibited by 1-MCP include senescent scald and core browning (Argenta et al., 2003), watery and core browning (Calvo and Sozzi, 2004), senescent breakdown (Kubo et al., 2003), internal breakdown (Ekman et al., 2004) and decay (Argenta et al., 2003). Incidences of low temperature disorders of a number of fruit were reduced by 1-MCP treatment, including internal flesh browning in avocado (Pesis et al., 2002; Hershkovitz et al., 2005; Woolf et al., 2005), and pineapple (Selvarajah et al., 2001), and chilling injury of citrus fruit (Dou et al., 2005). Reduced browning in avocados was associated with reduced polyphenol oxidase (PPO) and peroxidase (POD) activities (Pesis et al., 2002; Hershkovitz et al., 2005). In contrast, the chilling injuries, internal browning, flesh woolliness and reddening were increased by 1-MCP treatment in peaches and nectarines (Dong et al., 2001b; Fan et al., 2002; Girardi et al., 2005). Development of woolliness was associated with lower accumulations of transcripts for ACO, and associated decreases of those encoding PG, PE, while that of EGase was increased. 1-MCP enhanced these effects suggesting that a certain level of ethylene production by fruit is required for normal ripening after storage (Dong et al., 2001b). A slight increase in internal reddening occurred in 1-MCP-treated plums (Dong et al., 2002), but no effects of 1-MCP on internal browning and flesh gelling, have been reported. 1-MCP-treated ‘Shamouti’ oranges (Porat et al., 1999) and banana fruit (Jiang et al., 2004a) are also more sensitive to chilling injury. In untreated banana fruit stored at chilling-inducing temperatures, the development of injury was associated with decreased ethylene binding. 1-MCP treatment further decreased ethylene binding, and Jiang et al. (2004a) suggested that enhanced chilling injury of bananas was 403 associated with decreased responses of the fruit to ethylene. 1-MCP treatment prevents ethylene induced development browning and chemical changes in lettuce and carrots. 1-MCP delayed ethylene-induced russet spotting of whole and minimally processed lettuce, and browning of the cut surfaces of shredded lettuce (Wills et al., 2002; Saltveit, 2004; Tay and Perera, 2004). Interestingly, while these disorders are associated with induction of phenypropanoid metabolism and greater accumulation of phenolic compounds, 1-MCP did not interfere with wound-induced browning (Saltveit, 2004). 1-MCP-treated carrots had inhibited accumulation of 8-hydroxy-3-methoxy-3,4-dihydro-isocoumarin, associated with bitter flavors and the phytoalexin, 6methoymellin (Fan and Mattheis, 2000; Fan et al., 2000). 1-MCP prevented ethylene-induced water-soaking of watermelon and the associated increase in phospholipid (phosphatidylcholine and phosphatidylinositol) degradation and activities of phospholipases C, phospholipases D and lipoxygenase (Mao et al., 2004). Postharvest pitting of citrus fruit was inhibited by 1MCP (Dou et al., 2005). 3.7.3. A physiological basis for disorder development in response to 1-MCP The effects of 1-MCP on disorders of fruit and vegetables can be categorized into several types: a. Disorders that are associated with senescence and therefore are prevented by inhibition of ethylene production. These include disorders such as senescent breakdown of apple fruit, which are reduced by 1-MCP-induced slowing of senescence processes. b. Chilling-related disorders that are inhibited when ethylene production is prevented. Examples include superficial scald of apples, and internal flesh browning of avocado and pineapple. c. Chilling-related disorders that are increased by inhibition of ethylene production. Examples include woolliness (mealiness) and internal breakdown in peach and nectarines, and chilling injury in citrus, suggesting that these disorders are aggravated if normal ripening processes mediated by ethylene are prevented. d. Ethylene-induced disorders that are inhibited by 1-MCP application. These include browning responses in lettuce, increases in undesirable chemical changes such as isocoumarin accumulation in carrots, and water-soaking of watermelon. e. Disorders associated with CA storage, e.g., CO2 injury of apple fruit, which is enhanced by 1-MCP. 404 C.B. Watkins / Biotechnology Advances 24 (2006) 389–409 3.8. Pathological storage disorders 1-MCP increased disease susceptibility of avocado (Hofman et al., 2001; Adkins et al., 2005; Woolf et al., 2005), custard apple, mango and papaya (Hofman et al., 2001). Treatment of strawberry fruit with higher 1-MCP concentrations also increased decay (Ku et al., 1999). In citrus fruit, 1-MCP inhibited decay at low concentrations, but enhanced it at high concentrations (Dou et al., 2005). Also, in citrus, 1-MCP increased mold rots caused by Penicillium digitatum and P. italicum (Porat et al., 1999; Marcos et al., 2005) and stem rots caused by Diplodia natalensis (Porat et al., 1999). In contrast, Mullins et al. (2000) found no effect of 1-MCP on progression of P. digitatum growth on innoculated grapefruit. Susceptibility of apples to bitter rot (Colletotrichum acutatum) and blue mold (P. expansum) was higher in 1-MCP treated than untreated fruit (Janisiewicz et al., 2003). 1-MCP slightly increased severity of decay in fruit inoculated with P. expansum, especially when used in conjunction with other stress treatments such as prestorage heat (Leverentz et al., 2003). Saftner et al. (2003) found that 1-MCP with or without pre-storage heat treatment reduced decay due to wound-inoculation by P. expansum, Botrytis cinerea, C. acutatum at the time of harvest and after CA storage, probably by maintaining firmness and thereby resistance to infection. Decay incidence of peaches after inoculation with P. expansum was slightly reduced by 1-MCP treatment, and it was suggested that resistance in these fruit was related to higher activities of PAL, PPO and POD (Liu et al., 2005). Reduced decay caused by brown rot, Monilinia laxa, was found in 1-MCP-treated plums (Menniti et al., 2004). In strawberry, decay was more rapid in 1-MCP treated fruit (Bower et al., 2003), while Ku et al. (1999) found inhibited decay at low 1-MCP concentrations and enhanced decay with treatments of 500–1000nl l− 1. Increased disease susceptibility in fruit treated with high 1-MCP concentrations may be associated with lower phenylalanine ammonia lyase (PAL) activity and lower phenolic contents (Jiang et al., 2001). Relatively little about 1-MCP effects on disease incidence is known, but it is likely to become an important factor in the less ideal environments that exist in the commercial world compared with laboratory-based experimental systems. Factors that influence the effects of 1-MCP on disease development are likely to be specific to the product and its interaction with the specific pathogen and the environment. Delayed ripening associated with reduced ethylene production may increase product resistance to infection and lesion devel- opment. However, sensitivity can be beneficial against some pathogens but deleterious to resistance against other pathogens. Small amounts of endogenous ethylene may be necessary to maintain basic levels of resistance to environmental and pathological stress because of its involvement in regulation of plant defense genes (Marcos et al., 2005). 4. Summary The discovery and subsequent commercialization of 1-MCP has provided exciting opportunities for postharvest scientists to gain insight into the fundamental processes that are involved in ripening and senescence of fruit and vegetables. Prospects for commercialization of 1-MCP for several products appear high. For products such as most vegetables and perhaps non-climacteric fruit, where further senescence (e.g., yellowing) will decrease product value, 1-MCP applications that prevent any change are desirable. However, for products such as many climacteric fruit, success will be based on delaying rather than preventing ripening, in order to provide a product that meets consumer requirements. Data obtained thus far for the apple show that the issues associated with commercialization are not trivial and provide an exciting era for postharvest researchers as they aid horticultural industries to realize the full potential for 1-MCP. Under commercial conditions, handling practices will need to take into account many factors including commodity type, cultivar, maturity or ripeness stage, time between harvest and treatment, treatment temperature, and desired effects on quality. Each of these factors can affect the 1-MCP concentrations that should be applied to each product. Whether 1-MCP concentrations that will allow beneficial delays in ripening, while still allowing positive aspects of ethylene action to occur, can be optimized to meet conditions associated with pre- and post-harvest variations of some products is still uncertain. Commercial utilization of this technology will also be a function of the cost of 1-MCP application relative to its benefits for each product. The cost / benefit ratio will be affected by many factors including the product response, especially in relation to quality as perceived by the consumer, how successfully 1-MCP use can be incorporated into handling, storage and transport systems, the scale of the industry involved, competition in the marketplace, and whether it provides access to markets that are not available using current technologies. 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