Potassium Mitigation Effect of Salinity on Photosynthesis and
Water Relations in Maize (Zea mays L.) Hybrids
*M. Akram
University College of Agriculture and Environmental Sciences, Islamia University of
Bahawalpur, Pakistan.
*Corresponding author e-mail: akramcp@gmail.com
Running title: Maize grown under salinity and potassium
ABSTRACT
An experiment was conducted on two maize (Zea mays L.) hybrids viz.,
Pioneer32B33 and Dekalb979 grown in pots subjected at three level of salinity stress
under four potassium levels to determine the role of potassium under saline conditions.
This study aimed to reveal changes in physiological characters of maize plants in
response to salinity stress and role of potassium nutrition. Salinity stress caused a
decrease in gas exchange characteristics and water relation components, while potassium
application enhanced the physiological and water related parameters of both maize
hybrids under salinity stress. It could be concluded that potassium is very helpful in
alleviating the harmful effect of salinity stress and the maize hybrid Pioneer32B33 might
perform better than Dekalb979 under saline conditions when sufficient potassium is
applied in the rooting medium.
Key words: Zea mays L, gas exchange parameters, water relation, salinity, plant
nutrition, potassium
INTRODUCTION
Salinity is one of the most significant abiotic factor limiting crop productivity
(Munns, 2002). At the present time about 20% of the world’s cultivated land and
approximately half of all irrigated land is affected by salinity (Zhu, 2001). Effects of
salinity are more obvious in arid and semiarid regions where limited rainfall, high
evapotranspiration, and high temperature associated with poor water and soil management
practices are the major contributing factors (Azevedo Neto et al., 2006). The evaporation rate
is generally high and exceeds that of precipitation in such regions. Thus, the insufficient
rainfall together with high evaporative demand and shallow ground water in most locations
1
enhances the movement of salts to the soil surface (Shannon, 1998). It is quite clear that
salinity affects crop production and agricultural sustainability in many regions of the
world mainly by reducing the value and productivity of the affected land (Mohammed et
al., 1998).
Salinity has three potential effects on plants: lowering of the water potential,
specific ion toxicity (sodium and chloride) and interference with the uptake of essential
nutrients. (Flowers and Flowers, 2005). Osmotic stress, ion imbalances, particularly with
K+ and the direct toxic effects of ions on the metabolic process are the most important
and widely studied physiological impairments caused by salt stress (Munns et al., 2006).
Plant growth and productivity is normally addressed (Al-Karaki, 2000) as a result of
several physiological modifications in ion balance, water status, stomatal behaviour and
photosynthetic efficiency (Munns, 1993). More specifically salinity hinders plant growth
either through direct effects of ion toxicity or indirect effects of saline ions on soil water
potential, which cause soil/ plant osmotic imbalances (Hasegawa et al., 1986). Root zone
salinity reduces soil water potential resulting in less availability of water to plants (Lloyd
et al., 1989). As a result reduction in the water content leading to dehydration at cellular
level and osmotic stress is observed.
Photosynthesis is the most important process that is affected in plants, growing
under saline conditions. Reduced photosynthesis under salinity is not only attributed to
stomata closure leading to a reduction of intercellular CO2 concentration, but also to nonstomata factors. Salt stress affects photosynthetic enzymes and chlorophylls contents
(Stepien and Klobus, 2006). Reduced photosynthesis under salinity is not only attributed
to stomatal closure leading to a reduction of intercellular CO2 concentration, but also to
non-stomatal factors. There is increasing evidence that salt affects photosynthetic
enzymes, chlorophylls and carotenoids (Misra et al., 1997). Salt stress also induces a
decrease in stomatal conductance and transpiration. Under saline conditions, stomatal
closure helps to maintain higher leaf water content, however; this leads to a decrease in
the leaf CO2 assimilation rate (Parida et al., 2004).
Maize is one of the major food crops in most of the countries where salinity
problems exist or may develop and it occupies a key position as one of the most
important cereals both for human and animal consumption. In Pakistan, maize is the third
2
most important cereal after wheat and rice and is grown through out the country in a wide
range of climatic conditions. Being an important “Kharif” crop, maize is grown on about
one million hectares with a total yield of about 4 million tones and an average yield of
3610 kg ha-1 (Govt. of Pakistan, 2008-09). In view of its increasing importance,
improvement in physiological characteristics of maize has received considerable attention
in Pakistan (Mehdi and Ahsan, 2000). Because salinity lowers the soil water potential,
similar physiological mechanisms such as the water deficit or osmotic effect in plants
might explain the reduction in plant growth. Therefore, considerable attention has been
focused on comparing the differential responses of plant growth under salinity that are
mediated by the lowered soil water potential (Munns, 2002).
Potassium is ubiquitous in all higher plants and plays a vital role in a wide range
of biochemical and biophysical processes. It is the most important inorganic cation in
plant tissues and in physiological and biochemical processes and an important nutrient in
photosynthesis and maintenance of turgidity in plant cells (Carroll et al., 1994) and as
such makes a major contribution to the low osmotic potential in the stele of the roots that
is a prerequisite for turgor-pressure driven solute transport in the xylem and the water
balance of plants (Marschner, 1995). Potassium also plays a major part in the enzyme
system that controls the metabolism of photosynthesis (Cakmak, 1994). The maintenance
of high cytoplasmic levels of K is therefore essential for plant survival in saline habitats
(Chow et al., 1990). Adequate supply of K to plants growing in saline and drought stress
environments is believed to have an important role in inducing tolerance (Beaton and
Sekhon, 1985).
This experiment was therefore carried out to investigate the response of maize to
potassium application under saline growing condition. The objectives of the present study
were to assess the influence of salt stress on the photosynthesis and water relations of
maize crop and compare and discuss the physiological responses and adaptive strategies
to salt stress.
MATERIAL AND METHODS
The experiment was conducted under natural conditions in the wire house of Plant
Stress Physiology Laboratory of Nuclear Institute for Agriculture and Biology (NIAB),
Faisalabad, Pakistan. Maize (Zea mays L.) hybrids Pioneer 32B33 (salt tolerant) and
3
Dekalb 979 (salt sensitive) was grown in the pots of 24.5 cm diameter and 28 cm deep.
The pots were filled with 12 kg of sandy loam soil having a bulk density of 1 500 kg m-3,
ECe 1.09 dS m-1, pH 7.70, SAR 10.9. The field capacity of the soil was 16.35 % and the
saturation percentage was 35%. After germination, four plants were maintained in each
pot and recommended doses of fertilizers were applied at appropriate time. Three salinity
(NaCl) levels, S0 (Control i.e 1.09 dS m-1) S1 (5 dS m-1) and S2 (10 dS m-1) was
developed by adding calculated amount of NaCl in the soil. Four potassium levels i.e. K0,
K1, K2 and K3 (with out application, 75, 125 and 175 kg K ha-1) were applied in the
present study. Salinity level i.e 5 dS m-1, was developed before sowing the seed by
mixing the calculated amount of salt (NaCl) with the soil and latter on 5 dS m-1 was
applied 25 days after sowing. Nitrogen was applied to the plants in splits i.e. 1/3 at
sowing, 1/3 at knee height and 1/3 at tasseling stage. The nitrogen and phosphorus were
applied at recommended rate i.e. 225 kg ha-1, 125 kg ha-1 respectively. The whole
phosphorus was applied at sowing time. The nitrogen, phosphorus and potassium were
used in the form of ammonium nitrate, single super phosphate (SSP) and sulphate of
potash (SOP). The experiment was laid out in a three factors completely randomized
design (CRD). There were three replicates for each treatment. Plants irrigated with
normal water as and when required. Observations were recorded on fully expanded top
third leaves at tasseling stage.
Gas exchange parameter such as net photosynthetic rate (PN), transpiration rate (E) and
stomatal conductance (gs) were measured by open system LCA-4 ADC portable infrared
gas analyzer (Analytical Development Company, Hoddesdon, England). The fully
expanded leaf from top was enclosed in the assimilation chamber and the PN was
monitored while CO2 concentration changed over a definite time interval. The systems
automatically calculated PN on the basis of preloaded flow and leaf area. E and C were
also measured directly by CID-301 on the same leaf. All these measurements were taken
from 10:00 to 11:30 a.m. (Pakistani time) in triplicates with the following
specifications/adjustments: leaf surface area 11.35 cm2, ambient CO2 concentration (Cref)
342.12 µmol mol-1, temperature of leaf chamber (Tch) varied from 36.2 to 42.9oC, leaf
chamber volume gas flow rate (v) 396 mL min-1, leaf chamber molar gas flow rate (U)
4
251 µmol s-1, ambient pressure (P) 99.95 kPa, molar flow of air per unit leaf area (Us)
221.06 mol m-2 s-1, PAR (Q leaf) at leaf surface was maximum up to 1030 µmol m-2 s-1.
Water use efficiency (WUE) was calculated was estimated using the relationship PN/E
(Nieva et al., 1999).
Leaf relative water contents (%)
Fully expanded leaf from top was taken from each treatment. Fresh weight of
each sample was taken. Leaves were dipped in the water for 14-16 hours. Then they were
wiped with tissue paper and turgid weight was taken. They were dried at 65 ± 20C then
dry weight of each sample was taken. For each sample relative water content was
calculated by using the formula (Karrou and Maranville, 1995) given below:
Fresh weight – Dry weight
Relative water contents =
x 100
Turgid weight – Dry weight
Leaf water potential (-MPa)
The third leaf from top (fully expanded youngest leaf) was excised at 6:30 a.m. to
8:30 a.m. to determine the leaf water potential with a Scholander type pressure chamber
(Arimad-2-Japan, ELE international).
Leaf osmotic potential (-MPa)
The same leaf, as used for water potential measurement was frozen in a freezer
below -20oC for more than seven days, after which, the frozen leaf material was thawed
and the sap was extracted by pressing the material with a glass rod. The sap was used
directly for the determination of osmotic potential in a vapor pressure osmometer (Vapro,
5520).
Leaf turgor potential (MPa)
The turgor potential was calculated as the difference between osmotic potential
(ψs) and water potential (ψw) values as following (Nobel, 1991).
ψp = ψw - ψ s
5
Statistical analysis
Three factor completely randomized design (Analysis of variance technique) of
the data was computed for all attributes by using the MSTAT Computer Program
(MSTAT Development Team 1989). The treatment’s means were compared using least
significant difference test at 5% probability level (Steel et al., 1997).
RESULTS
Net rate of photosynthesis (µ mol CO2 m-2 S-1)
Maize hybrids differed significantly with respect to rate of photosynthesis. There
was a significant effect of salt stress on net rate of photosynthesis, whereas, the
application of potassium improved this variable. Interaction between maize hybrids and
salinity stress indicate that in Pioneer32B33 the highest rate of photosynthesis was
recorded at 5 dS m-1 which was closely followed by control. However, high reduction in
net rate of photosynthesis was observed at the highest salinity level (Fig. 1). The
interaction among salinity x hybrids x potassium indicate that with the increase in salinity
the rate of photosynthesis decreased. However, the application of potassium significantly
improved the net rate of photosynthesis in both the hybrids. Maximum rate of
photosynthesis was recorded at the highest potassium level in all the salinity levels in
both hybrids. Maize hybrid Pioneer32B33 showed highly significant improvement in net
rate of photosynthesis with the application of potassium (Fig. 1).
Overall, from the results it can be concluded that potassium application was effective in
enhancing net rate of photosynthesis.
Transpiration rate (m mol H2O m-2 S-1)
Maize hybrid Pioneer32B33 maintained significantly higher transpiration rate
(1.15 m mol H2O m-2 S-1) than Dekalb979 (1.07 m mol H2O m-2 S-1). Transpiration rate
was significantly affected due to NaCl salinity, however; highest dose of potassium
application improved the transpiration rate significantly. Interaction between maize
hybrids and different salinity levels show that in Pioneer32B33 there was no significant
difference in transpiration rate in S1 (5 dS m-1) with control. High reduction in
transpiration rate was observed due to salinity in both the hybrids (Fig. 2a). Interactive
effect of maize hybrids and different potassium levels show that transpiration rate
increased with the application of potassium but the effect was more pronounced in hybrid
6
Pioneer32B33 than Dekalb979 at all the salinity levels (Fig. 2b). The interaction between
salinity and potassium levels reveal that with increase in salinity stress the rate of
transpiration decreased. However, the application of potassium significantly improved
the rate of transpiration in both the hybrids. Maximum rate of transpiration was recorded
at the highest potassium level under all the salinity levels in both hybrids. Pioneer32B33
showed highly significant improvement in transpiration with the application of potassium
(Fig. 3).
Stomatal conductance (m mol m-2 S-1)
The results for stomatal conductance reveal that Pioneer32B33 maintained
significantly higher stomatal conductance than that of Dekalb979. Salinity stress at 5 dS
m-1 did not influence the stomatal conductance, while, significant reduction was noted at
high salinity level (10 dS m-1). Application of potassium increased the stomatal
conductance. Interactive effect of maize hybrids and salinity levels show that in
Pioneer32B33 highest stomatal conductance was recorded at 5 dS m-1, which was closely
followed by control. However, the high salinity level significantly decreased the stomatal
conductance in both the hybrids (Fig. 4). Interaction between the maize hybrids and
potassium levels (Fig. 5a) reveal that in Pioneer32B33 there was significant improvement
in stomatal conductance under all the potassium levels, whereas, in Dekalb979 no
significant difference in K0 (no application) K1 (75 kg K2O ha-1) and K2 (125 kg K2O ha1
) levels was observed but there was significant improvement at supra-optimal level.
Interactive effect of salinity and potassium levels show that salinity stress caused a
decrease, while applications of potassium improved the stomatal conductance (Fig. 5b).
Water use Efficiency (µ mol/m mol)
Water use efficiency (WUE) is an important index with which to measure a plant
ability to maintain water equilibrium. The water use efficiency was significantly modified
as a result of the reported changes in the net photosynthetic rate and transpiration rate.
Hybrids differ significantly (P>0.01) with respect to WUE, however; there was no
significant difference between salinity treatments. With the application of potassium the
WUE increases. Maximum WUE was recorded when nitrogen was applied @ 175 kg ha-1
which was at par with K2 (125 kg ha-1) and K1 (75 kg ha-1) Interactive effect of hybrids
and salinity (Fig. 6) indicate that in Pioneer 32B33 maximum WUE was noted at lower
7
salinity level (5 dS m-1) and it was reduced at the highest salinity level (10 dS m-1). In
Dekalb979 maximum WUE was recorded in control (1.09 dS m-1) and it decreased under
salinity stress conditions.
Leaf relative water content (%)
Analyzed data presented clearly indicate that relative water content (RWC) varied
significantly in both the hybrids and was also influenced by the salinity and potassium
levels. The Pioneer32B33 maintained significantly higher relative water content
(85.12%) than that of Dekalb979 (78.65%). Salinity stress significantly reduced the
relative water content. Maximum relative water content was recorded for the plants
growing under normal conditions which were closely followed by the plants treated with
5 dS m-1 salinity, while the minimum relative water content was recorded at the highest
salinity level (10 dS m-1). The application of potassium significantly improved the
relative water content. Maximum relative water content was recorded at the highest
potassium level i.e. (175 kg K2O ha-1) whereas minimum in control (no application)
treatment. Relative water content differed significantly at all the potassium levels. Fig. 7a
shows that interaction between maize hybrids and salinity levels was significant which
clearly indicate that tolerant hybrid maintained higher relative water content than
sensitive one under all the salinity levels. Similarly, the interaction between hybrid and
potassium indicated that relative water content increased in both the hybrids with increase
in the potassium levels. However, Pioneer32B33 possessed higher leaf relative water
content than that of Dekalb979 (Fig. 7b).
Leaf water potential (-MPa)
A significant difference for leaf water potential was observed between maize
hybrids. Salinity levels significantly decreased (more negative values) the leaf water
potential, whereas, it increased with increase in potassium levels in the growth medium.
Interactive effect of maize hybrids and salinity levels showed that water potential was
decreased (become more negative) with increase in the salinity levels in both the hybrids
(Fig. 8).
Leaf osmotic potential (-MPa)
The difference of osmotic potential in maize hybrids was significant. Salinity
stress significantly decreased (more negative value) the leaf osmotic potential. A
8
progressive decrease in the osmotic potential was observed with increase in the salinity
levels. Potassium effect on osmotic potential was significant and with increase in the
application of potassium level, the osmotic potential became more negative. Interactive
effect of maize hybrids and different potassium levels (Fig. 9) on leaf osmotic potential
indicate that with the application of potassium the osmotic potential became more
negative in both the hybrids. Salinity and potassium interaction on osmotic potential
reveal that potassium application significantly improved the leaf osmotic potential in
plants under all the salinity levels (Fig. 10). Potassium application was very effective in
maintaining low leaf osmotic potential and helpful in osmotic adjustment in plants.
Leaf turgor potential (MPa)
Turgidity is an important phenomenon in the plant growth and development;
maintenance of turgor potential is the major requirement to cope with stress conditions
especially in salinity stress. Data pertaining to leaf turgor potential reveal that plant turgor
potential was decreased due to salinity stress. Turgor potential was highly significant
between maize hybrids and among different salinity stress levels. Interaction between
maize hybrids and different salinity levels on leaf turgor potential indicate that salinity
stress significantly decreased turgor potential, while the application of potassium
improved the turgor potential in both the hybrids (Fig. 11, 12).
Correlation: The Correlation coefficients (r) between different physiological attributes
(Table 1) clearly show a significant positive relationship (r = 0.60, p>0.001) between net
CO2 assimilation rate of the two maize hybrids under saline conditions (Table 1). There
was a non significant correlation between each photosynthetic indices and water relation
parameters except relative water contents and turgor potential (Table 1). Correlation
parameters between water use efficiency and water relation parameters were non
significant. The correlations between osmotic potential and turgor potential were
significantly positive.
DISCUSSION
Salinity is a common abiotic stress affecting seriously crop production in different
regions of the world. It greatly reduces the crop productivity of glycophytic plants, which
usually tolerate only relatively low salt concentrations. Among the cereal species, maize
9
seems to be sensitive to salt stress (Maas and Hoffman, 1997). In the present study, salt
stress reduced the physiological, water relation, biochemical and yield parameters of
maize hybrids. Salinity strongly decreased stomata conductivity (gs), which reduced
transpiration rate (E). Since transpiration rate followed the same trend as that in
photosynthesis, it is clear that the reduction in photosynthesis has the same effects on
both stomata and transpiration as the three are integral elements of the photosynthetic
apparatus of plants (Gamma et al., 2007). Generally, transpiration rate tended to decline
with increasing salinity. This may be due to the fact that lowered water potentials in the
root can trigger a signal from root to shoot (such as abscisic acid, which has been
suggested to be the operating mechanism (Zhang and Davies, 1991). However an
alternative hypothesis could be that the inhibition of photosynthesis caused by salt
accumulation in the mesophyll produces an increase in intercellular CO2 concentration,
which reduces the stomatal aperture (Josefa et al., 2003).
Plant growth is the result of integrated and regulated physiological processes
which are affected by number of environmental factors determining the response of
plants to stress. Limitation of plant growth by environmental factors can not be assigned
to a single physiological process. The salt stress causes an earlier decrease in
photosynthesis by restriction in stomatal and mesophyll conductance (Delfine et al.,
1999). Present studies on maize confirmed that photosynthesis and stomatal conductance
decreased by salinity stress however, the applications of potassium improved both of
these attributes which ultimately increased the plant productivity. Photosynthesis was
severely reduced in salt sensitive poplar with lower capacity to exclude salt from the
chloroplasts (Wang et al., 2007). Decrease in rate of photosynthesis is due to several
factors: (1) salt toxicity (2) reduction of CO2 supply because of closure of stomata (3 )
enhanced senescence induced by salinity (4) changes in enzymatic activity (6) negative
feed back by reduced sink activity (Iyengar and Reddy, 1996). The decrease in
photosynthesis caused by salt stress is mainly associated with decrease in stomatal
conductance (Centritto et al., 2003). However, salt tolerant plant species are thought to
have mechanisms that allow them to maintain photosynthesis in the presence of high salt
levels.
10
Potassium is essential for cell expansion, osmoregulation and cellular and whole
plant homeostasis. High stomatal K+ requirement is reported for optimum photosynthesis
(Chow et al., 1990). The role of K in response to salt stress is also well documented,
where Na+ and K+ exchange during salt uptake. In the present study, significant decrease
in K+ content in plant with salinity suggests that Na+ decreased the K+ uptake. The
exchange of K+ for Na+ by the cells in the stele of the roots in the vascular bundles in the
stems is considered as one type of control to the transport of salt to leaves or growing
tissues. Salinity reduces the ability of plants to utilize water and causes a reduction in
growth rate, as well as changes in plant metabolic processes (Munns, 2002). The water
use efficiency was significantly modified as a result of the reported changes in the net
photosynthetic rate and transpiration rate. Brougnoly and Lauteri (1991) also indicated
that reduced photosynthetic carbon assimilation was attributed to reduced stomata
conductance. This tendency, with respect to the water use efficiency, observed in the
leaves suggests that the non-stomata factors, in addition to the stomata ones, affected
photosynthesis. Potassium application improves the water use efficiency and alleviates
the negative effects of salinity and it has been observed in the present investigation by the
change in the physiological parameters.
Water status is highly sensitive to salinity and therefore is dominant in
determining the plant responses to stress (Stepien and Klobus, 2006). Salinity decrease
the relative water content (RWC) in maize hybrids and that decrease in RWC were
greater in saline conditions than that under control condition However, application of
potassium increased the RWC. Therefore, potassium played an important role in maize
hybrids water relations under saline condition. Ghoulam et al. (2002) reported that sugar
beet high NaCl concentrations caused a great reduction in growth parameters such as leaf
area and fresh and dry weight of leaves and root. These changes were associated with a
decrease in relative water content and K+ concentration. The decreased RWC under saline
condition was also reported in different crops viz. Sayed and Gadallah (2002) in
sunflower; Serraj and Drevon (1998) in alfalfa; Nandwal et al. (2000) in mungbean.
Subbarao et al. (2000) reported that leaf RWC and osmotic potential were significantly
decreased at low potassium content in red beat. Potassium in soil helped to absorb more
water to reach turgidity under saline condition.
11
Water potential, solute potential and turgor potential are inter-related in plant cells
and are markedly affected when plants are exposed to salt stress. In the present study, salt
stress also had an adverse effect on water relation parameters of the two maize hybrids.
Leaf water potential became more negative due to salinity; however, the application of
potassium markedly improved the leaf water potential, leaf turgor potential and leaf
osmotic potential in the control as well as salt stressed plants. These results are in
harmony with those obtained by Mengel and Header (1977), who concluded that
potassium promotes the loading of the sieve tube companion cell complex with,
assimilates thus decreasing the water potential in the phloem sap and inducing an
increased water uptake to the sieve cell of companion cell components. Abd-Ellah and
Shalaby (1993) also reported that the beneficial effects in cotton might be due to
improved plant water status, as well as the status of better mineral nutrient relationship. It
is well known that plants under saline conditions can maintain their turgor by osmotic
adjustment, which is one of the common mechanisms of salinity resistance in crop plants
(Hernandez and Almansa, 2002; Chaparzadeh et al., 2003).
It has been observed that potassium represents the main cation in plant cells and is
an important component of cell osmotic potential, which is involved in almost all the
physiological and biochemical processes including photosynthesis and maintenance of
turgidity in plants exposed to salt stress conditions (Chow et al., 1990; Carroll et al.,
1994). Thus potassium plays an important role in plant water relations particularly when
water is limiting factor like under saline conditions. Increased water potential due to
potassium application under both control and saline conditions has confirmed its role in
mitigating the effects of water stress due to salinity.
Based on all the physiological attributes measured in this study, it can be
concluded that the maize hybrids differing in salt tolerance showed a differing response
to salt stress with respect to water relations and various gas exchange characteristics.
High salt tolerance of Pioneer32B33 was positively associated with water relation
parameters and net CO2 assimilation rate or Pn/E.
Acknowledgements
The authors are so grateful to the Higher Education Commission Islamabad,
Pakistan for funding this research as part of a PhD study.
12
REFERENCES
Abd-Ellah, M.K. and Shalaby, E.E. 1993. Cotton response to salinity and different
potassium-sodium ratio in irrigation water. Journal of agronomy and Crop
Science, 170: 25-31.
Al-Karaki, G.N. 2000. Growth, water use efficiency, and sodium and potassium
acquisition by tomato cultivars grown under salt stress. Journal of Plant Nutrition,
23: 1-8.
Azevedo Neto, A.D., Prisco, J.T., Eneas-Filho, J., Abreu, C.E.B. and Filho, E.G. 2006.
Effect of salt stress on antioxidative enzymes and lipid peroxidation in leaves and
roots of salt-tolerant and salt-sensitive maize genotypes. Environmental and
Experimental Botany, 56: 87-94.
Beaton, J.D. and Sekhun, G.S. 1985. Potassium nutrition of wheat and other small grains.
Potassium in Agriculture, 701-752.
Brougnoly, E. and Lauteri, M. 1991. Effects of salinity on stomatal conductance,
photosynthetic capacity, and carbon isotope discrimination in salt-tolerant
(Gossypium hirsutum) and salt-sensitive (Phaseolus vulgaris) C3 non-halophytes.
Plant Physiology, 95: 628-635.
Cakmak I. 1994. Activity of ascorbate-dependent H2O2-scavenging enzymes and leaf
chlorosis are enhanced in magnesium and potassium deficient leaves, but not in
phosphorous deficient leaves. Journal of Experimental Botany, 45: 1259-1266.
Carroll, M.J., Slaughter, L.M. and Krouse, J.M. 1994. Turgor potential and osmotic
constituents of kentucky blue grass leaves supplied with four levels of potassium.
Agronomy Journal, 86: 1079-1083.
Centritto, M., Loreto, F. and Chartzoulakis, K. 2003. The use of low CO2 to estimate
diffusion and non diffusional limitations of photosynthetic capacity of salt
stressed olive saplings. Plant Cell Environment, 26: 585-594.
Chaparzadeh, N., Khavari-Nejad, R.A., Navari-Izzo, F. and Izzo, A. 2003. Water
relations and ionic balance Calendulaa oficinalis L. under salinity conditions.
Agrochimica, 47: 69-79.
13
Chow, W.S., Ball, M.C. and Anderson, J.M. 1990. Growth and photosynthetic responses
of spinach to salinity: implications of K nutrition for salt tolerance. Australian
Journal of Plants Physiology, 17: 563-578.
Delfine, S., Alvino, A., Villani, M.C. and Loreto, F. 1999. Restriction to carbon dioxide
conductance and photosynthesis in spinach leaves recovering from salt stress.
Plant Physiology, 119: 1101-1106.
Flowers, T.J. and Flowers, S.A. 2005. Why does salinity pose such a difficult problem for
plant breeders? Agriculture water management, 78: 15-24.
Gamma, P.B., Inanaga, S., Tanaka, K. and Nakazawa, R. 2007. Physiological response of
common bean (Phaseolus Vulg. L.) seedlings to salinity stress. African J.
Biotechnology, 6: 79-88.
Ghoulam, C., Foursy, A. and Fares, K. 2002. Effect of salt stress on growth, inorganic
ions and proline accumulation in relation to osmotic adjustment in five sugar beet
cultivars. Environmental and Experimental Botany, 47: 39-50.
Government of Pakistan. 2008. Pakistan Economic Survey. (2008-09). Finance Divisions,
Economic Advisor’s Wing, Islamabad. p. 22.
Hasegawa, P.M., Bressan, R.A. and Handa, A.K. 1986. Cellular Mechanisms of salinity
tolerance. Horticulture Science, 21: 1317-1324.
Hernandez, J.A. and Almansa, M.S. 2002. Short-term effects of salt stress on antioxidant
systems and leaf water relations of pea leaves. Physiologia Plantarum, 115: 251257.
Iyengar, E.R.R. and Reddy, M.P. 1996. Photosynthesis in highly salt tolerant plants. In:
Pessarakli, M. (Ed.), Handbook of photosynthesis. Marshal Dekar, Baten Rose,
USA, pp. 897-909.
Josefa, M.N., Gariddo, C., Martinez, V. and Carvajal, M. 2003. Water relations and
xylem transport of nutrients in pepper plants grown under two different salt stress
regimes. Plant Growth Regulation, 41: 237-245.
Karrou, M. and Maranville, J.W. 1995. Response of wheat cultivars to different soil
nitrogen and moisture regime: II. Leaf water content, stomatal conductance and
photosynthesis. Journal of Plant Nutrition, 8: 777-791.
14
Lloyd, J., Kriedmann, P.E. and Aspinall, D. 1989. Comparative sensitivity of ‘Prior
Lisbon’ lemon and ‘Valencia’ orange trees to foliar sodium and chloride
concentrations. Plant Cell and Environment, 12: 529-540.
Maas, E.V. and Hoffman, G.J. 1997. Crop salt tolerance, current assessment. Journal of
Irrigation and Drainage Division. ASCE. 103: 115-134.
Marschner, H. 1995. Mineral nutrition of higher plants. 2nd edn. London: Academic
Press.
Mehdi, S.S. and Ahsan, M. 2000. Coefficient of variation, inter-relationships and
heritability estimates for some seedlings traits in maize in recurrent selection
cycle. Pakistan Journal of Biological Science, 3: 181-182.
Mengel, K. and Header, H.E. 1977. Effect of potassium supply on the rate of phloem sap
exudation and the composition of phloem sap of Ricinus communis. Plant
Physiology, 59: 282-284.
Misra, A.N., Sahu, S.M., Misra, M., Singh, P., Meera, I., Das, N., Kar, M. and Shau, P.
1997. Sodium chloride induced changes in leaf growth, and pigment and protein
contents in two rice cultivars. Biologia Plantarum, 39: 257-262.
Mohammed, M., Shibli, R., Ajlouni, M. and Nimri, L. 1998. Tomato root and shoot
responses to salt stress under different levels of phosphorus nutrition. Journal of
Plant Nutrition, 21: 1667-1680.
Mstat Development Team. 1989. MSTAT user’ guide: A Microcomputer Programme for
the Design Management and Analysis of Agronomic Research Experiments,
Michigan State Univ. East Lansing, USA.
Munns, R. 1993. Physiological processes limiting plant growth in saline soils: some
dogmas and hypotheses. Plant Cell and Environment, 16: 15-24.
Munns, R. 2002. Comparative physiology of salt and water stress. Plant Cell and
Environment, 25: 239-250.
Munns, R., James, R.A. and Lauchli, A. 2006. Approaches to increasing the salt tolerance
of wheat and other cereals. Journal of Experimental Botany, 57: 1025-1043.
Nandwal, A.S., Godara, M., Kamboj, D.V., Kundu, B.S., Mann, A., Kumar, B. and
Sharma, S.K. 2000. Nodule functioning in trifoliate and pentafoliate mungbean
genotypes as influenced by salinity. Biologia Plantarum, 43: 459-462.
15
Nieva, F.J.J., Castellanos, E.M., Figueroa, M.E. and Gil, F. 1999. Gas exchange and
chlorophyll fluorescence of C3 and C4 salt marsh species. Photosynthetica, 36:
397-406.
Nobel, P.S. 1991. Physicochemical and Environmental Plant Physiology. Academic
Press, San Diego.
Parida, A.K., Das, A.B. and Mittra, B. 2004. Effect of salt on growth, ion accumulation,
photosynthesis and leaf anatomy of the mangrove, Bruguiera parviflora. TreesStructure and Function, 18: 167-174.
Sayed, S.A. and Gadallah, M. A.A. 2002. Effects of shoot and root application of thiamin
on salt stressed sunflower plants. Plant Growth Regulation, 36: 71-80.
Serraj, R. and Drevon, J.J. 1998. Effect of salinity and nitrogen source on growth and
nitrogen fixation in alfalfa. Journal of Plant Nutrition, 21: 1805-1818.
Shannon, M.C. 1998. Adaptation of plants to salinity. Advance Agronomy, 60: 75-119.
Steel, R.G.D., Torrie, J.H. and Deekey, D.A. 1997. Principles and Procedures of
Statistics: A Biometrical Approach, (3rd ed). McGraw Hill Book Co. Inc. New
York. pp. 400-428.
Stepien, P. and Klobus, G. 2006. Water relations and photosynthesis in Cucumis sativus
L. leaves under salt stress. Biologia Plantarum, 50: 610- 616.
Subbarao, G.V., Wheeler, R.M., Stutte, G.W and Levine, L.H. 2000. Low potassium
enhances sodium uptake in red beet under moderate saline conditions. Journal of
Plant Nutrition, 23: 1449-1470.
Wang, R., Chen, S., Deng, L., Fritz, E., Huttermann, A., and Polle, A. 2007. Leaf
photosynthesis, florescence response to salinity and the relevance to chloroplast
salt compartmentation and anti-oxidative stress in two poplars. Trees-Structure
and Function, 21: 581-591.
Zhang, J. and Davies, W.J. 1991. Antitranspirant activity in xylem sap of maize plants.
Journal of Experimental Botany, 42: 317-321.
Zhu, J.K. 2001. Plant salt tolerance. Trends Plant Science, 6: 66-72.
16
Table 1: Correlations coefficients (r) of physiological attributes, water use
efficiency and water relation of maize hybrids grown under various NaCl and
potassium treatments.
Pn
E
gs
WUE
RWC
ψw
ψs
ψp
Pn
-
0.93**
0.96**
0.57 NS
0.73*
-0.73NS
0.35NS
0.71*
E
-
-
0.94**
0.25 NS
0.65*
-0.78NS
0.22NS
0.62*
gs
-
-
-
0.42 NS
0.67*
-0.71NS
0.33NS
0.68*
WUE
-
-
-
-
0.55NS
-0.24NS
0.46NS
0.55NS
RWC
-
-
-
-
-
-0.50NS
0.67NS
0.88*
ψw
-
-
-
-
-
-
0.12NS
-0.43NS
ψs
-
-
-
-
-
-
-
0.84*
*, **, *** = significant at 0.05, 0.01, and 0.001 levels, respectively; NS = nonsignificant
Pn = Net rate of photosynthesis E = Transpiration rate gs = Stomatal conductance
WUE = Water use efficiency RWC = Relative water content ψw = Water potential
ψs = Osmotic potential ψp = Turgor potential
17
Pn (µ mol CO2 m-2 s -1)
25
K0: Control
K1: 75 kg ha-1
K2: 125 kg ha-1
K3: 175 kg ha-1
20
15
10
5
0
Pioneer 32B33
Dekalb979
Pioneer 32B33
Control
Dekalb979
5 dS m
-1
Pioneer 32B33
Dekalb979
10 dS m
-1
Salinity levels
Fig. 1: Interactive effect of maize hybrids, salinity and potassium on
net rate of photosynthesis (Pn) in maize hybrids ± SE
Control
5 dS m -1
10 dS m -1
1.6
E (m mol H2O m-2 s-1)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Pioneer 32B33
Dekalb979
Fig.2: Interactive effect of maize hybrids and salinity on
transpiration rate (E) in maize hybrids ± SE
18
1.6
K0: Control
K1: 75 kg ha-1
K2: 125 kg ha-1
K3: 175 kg ha-1
E (m mol H2O m-2 s-1)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Pioneer 32B33
Dekalb 979
Fig. 3: Interactive effect of maize hybrids and potassium on transpiration
rate (E) ± SE
250
Control
5 dS m -1
10 dS m -1
C (m mol m-2 s-1)
200
150
100
50
0
Pioneer 32B33
Dekalb979
Fig. 4: Interactive effect of maize hybrids and different salinity levels on
stomatal conductance (C) ± SE
19
250
K0: Control
K1: 75 kg ha-1
K2: 125 kg ha-1
K3: 175 kg ha-1
C (m mol m-2 s -1)
200
150
100
50
0
Pioneer 32B33
Dekalb 979
Fig. 5: Interactive effect of maize hybrids and potassium on stomatal
conductance (C) ± SE
14
Control
5 dS m-1
10 dS m-1
WUE (µ mol/m mol)
12
10
8
6
4
2
0
Pioneer 32B33
Dekalb979
Fig. 6: Interactive effect of maize hybrids and different salinity levels on
water use efficiency (WUE) ± SE
20
Control
5 dS m-1
10 dS m-1
Relative water contents (%)
90
85
80
75
70
65
Pioneer 32B33
Dekalb979
Fig. 7: Interactive effect of maize hybrids and salinity on leaf relative water
contents ± SE
Relative water contents (%)
100
K0: Control
K1: 75 kg ha-1
K2: 125 kg ha-1
K3: 175 kg ha-1
90
80
70
60
50
40
Pioneer 32B33
Dekalb979
Fig. 8: Interactive effect of maize hybrids and different potassium levels on
leaf relative water contents ± SE
21
Control
5 dS m-1
10 dS m-1
Leaf water potential (-MPa)
1.6
1.4
1.2
1
0.8
0.6
0.4
Pioneer 32B33
Dekalb 979
Fig. 9: Interactive effect of maize hybrids and different salinity levels on leaf
water potential ± SE
Leaf osmotic potential (-MPa)
K0: Control
K2: 125 kg ha-1
K1: 75 kg ha-1
K3: 175 kg ha-1
3
2.5
2
1.5
1
0.5
0
Pioneer 32B33
Dekalb 979
Fig. 10: Interactive effect of maize hybrids and different salinity levels on
leaf osmotic potential ± SE
22
K0: Control
K2: 125 kg ha-1
Leaf osmotic potential (-MPa)
3
K1: 75 kg ha-1
K3: 175 kg ha-1
2.5
2
1.5
1
0.5
0
5 dS m -1
Control
10 dS m -1
Salinity levels
Fig. 11: Interactive effect of different salinity and potassium levels on
leaf osmotic potential ± SE
1.4
Control
5 dS m -1
10 dS m -1
Turgor potential (MPa)
1.2
1
0.8
0.6
0.4
0.2
0
Pioneer 32B33
Dekalb 979
Fig. 12: Interactive effect of maize hybrids and different salinity levels
on leaf turgor potential ± SE
K0: Control
K1: 75 kg ha-1
K2: 125 kg ha-1
K3: 175 kg ha-1
1.6
Turgor potential (MPa)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Pioneer 32B33
Dekalb 979
Fig. 13: Interactive effect of maize hybrids and different potassium
levels on leaf turgor potential ± SE
23