Evaluation of broccoli performance under Papua New Guinea growing
conditions
Aaron Thomason
School of Agricultural Science, The University of Tasmania, Private Bag 54, Hobart TAS 7001, www.utas.edu.au Email
act@utas.edu.au
Abstract
Four varieties of broccoli (Brassica oleracea var. italica) were grown under three different temperature
treatments, those being 15/20ᵒC, 20/25ᵒC and 25/30ᵒC, with the aim of evaluating the potential of broccoli as
a commercial crop in Sogeri, Papua New Guinea. Physical growth and phenological development were
recorded weekly over the duration of the experiment. Increased temperature caused delay or prevention of
floral initiation across all cultivars, and all performed poorly under simulated PNG growing conditions. It
was concluded that the cultivars evaluated would not be suitable for commercial production in Sogeri, PNG.
Key Words
Broccoli, vernalisation, heat stress, Papua New Guinea, food security
Introduction
Papua New Guinea is Australia’s closest geographic neighbour and the two nations have a close and
mutually beneficial relationship in many respects. PNG is a developing nation, with around 40% of the
population living in poverty (Department of Foreign Affairs & Trading 2013). Australia is the single largest
donor of aid money for socioeconomic development in PNG, and this includes funding for ACIAR projects.
The capital of PNG, Port Moresby, is a large city with a rapidly growing population due to rural – urban
migration. This has placed added pressure upon food markets and supply cannot meet demand for fresh food
products (Birch et al, date unknown). The marketplace is also shifting with an emerging middle class and
growing number of expatriate workers in the capital due to mining related industries, driving demand for
western fruits and vegetables. One ACIAR project was concerned with the identification of key weak points
in Papua New Guinea’s fresh produce supply chains, and one major issue identified was the lack of transport
infrastructure between Port Moresby and current vegetable growing regions. To this end, other regions closer
to the capital city were explored for their suitability as a location for vegetable cropping (Birch et al, date
unknown). One such location was the Sogeri plateau, a mid-altitude region of eastern PNG sitting between
500m and 1000m above sea level. The combination of high year-round rainfall and relatively fertile Ferrosol
soils in the region, and its proximity to the capital and quality of road access, made it the chief region
identified for cropping potential (Birch et al, date unknown). One of the crops investigated in this region for
market potential was broccoli, and a field trial was commenced using six current commercial cultivars Marathon, Shogun, Green King, Summer King, Southern Comet and Southern Star. Of these six, Southern
Comet, Southern Star and Green King were early maturing and produced heads, but none of the heads were
marketable. The poor performance of the field trial was compounded by vandalism, with a number of plants
being stolen prior to head initiation (Gracie A, pers. Comm 2013). As the results of the trial did not give any
clear direction for recommendations to be given, it was decided that a new trial would be carried out using
temperature controlled conditions in the UTAS horticulture glasshouses at Sandy Bay, Tasmania.
Broccoli traditionally is grown as a temperate vegetable crop, with most modern cultivars having some
vernalisation requirement to initiate inflorescence development. The cultivars chosen in this experiment were
analysed for the FLC2 gene, which gives broccoli its vernalisation requirement, and found the gene to be
inactive in each of the four cultivars, implying that there was no genetic requirement for vernalisation prior
to inflorescence (Gracie A, pers. Comm 2013). The marketable part of the crop is the “head”, which is the
immature floral structure of the plant. Existing literature generally agrees that time to harvest is also closely
correlated to temperature, and a temperature of around 16ᵒC is generally agreed upon as the cardinal
temperature for the most rapid progression of the phenological development of the crop (Wurr et al 1995).
Methods & Materials
Four different cultivars of broccoli were selected to be used in this experiment, they were – Marathon, Green
King, Summer King and Southern Comet. All plants were grown from seed sourced from PNG. Plants were
grown from seed in non-limiting conditions with respect to water and nutrients. A photoperiod regime of 12
hours light/12 hours dark was given to all plants. Each variety was screened for dormancy of the FLC2 gene
which controls vernalisation requirement prior to inflorescence development. Screening was performed using
PEA genomic DNA extraction and PCR.
Two weeks after planting (WAP), seedlings were transplanted into 15cm pots, and subsequently four
different experiments were conducted to simulate varying environmental conditions which may be
experienced in Papua New Guinea.
The experiment in this report investigated broccoli growth and development under different day/night
temperatures. Four replicates of each cultivar were placed in three climate controlled cabinets with different
day/night temperature regimes, those being 30/25ᵒC, 25/20ᵒC, and 20/15ᵒC.
For the duration of each of the experiment, plants were monitored regularly to ensure water supply was
adequate, and measurements were taken weekly to monitor plant development. Measurements were taken
weekly for the experiment’s duration, from 18/04/2013 until 19/09/2013 inclusive. Measurements included
cumulative leaf number since germination, plant height from soil surface to meristem, phenological stage
and head/flower diameter in the cases where the plant had produced a head or flower. Data analysis was
undertaken in Microsoft Excel and IBM SPSS software.
Results
Table 1: Days after planting (DAP), number of leaves > 2cm, and height from the soil to the inflorescence
butt at buttoning. Values shown are means ± one standard deviation
Temperature
Regime
20/15
25/20
30/25
Cultivar
DAP
Number of Leaves
Height to Inflorescence
butt (cm)
Green King
97.5 ± 5.7
14.3 ± 2.7
7.0 ± 0.9
Southern Comet
84.0 ± 0.0
11.3 ± 0.5
5.3 ± 0.3
Marathon
104.8 ± 3.1
16.5 ± 0.6
7.9 ± 0.6
Summer King
110.0 ± 4.6
17.8 ± 1.8
7.1 ± 0.5
Green King
140.0 ± 7.6
31.5 ± 1.7
16.5 ± 1.7
Southern Comet
156.3 ± 4.7
32.3 ± 0.9
15.2 ± 1.1
Marathon
145.3 ± 8.3
33.3 ± 4.0
13.4 ± 1.6
Summer King
168.0 ± 0.0
46.0 ± 0.0
17.5 ± 0.0
Green King
168.0
35.0
20.5
Southern Comet
161.0
29.0
19.8
Marathon
-
-
-
Summer King
-
-
-
Plants with
Inflorescences
100%
Figure 2: Graph of mean growth stage of the plants at end of experiment (19/9/2013). Growth stage 1 is
vegetative, 2 is cropping, 3 is flowering and 4 is seedpod production.
100%
100%
100%
100%
75%
100%
25%
50%
25%
0%
0%
In Figure 1 the most apparent difference in the different temperature treatments is the increase in time
required for floral initiation to commence for all varieties. Southern Comet was the quickest variety to
transition to floral initiation in the 15⁰C/20⁰C treatment, which is matched by a corresponding shorter stem
length and fewer leaves at that time. Green King also transitions to floral initiation earlier than Marathon or
Summer King, with less leaves than Marathon or Summer King. In the 20⁰C/25⁰C unit we see that the time
to FI increases by between 35 and 70 days depending on the cultivar, and the standard error also increases,
meaning greater variation in the time to FI. In the 25⁰C/30⁰C unit none of the Marathon or Summer King
broccoli had initiated reproductive development by the end of the experiment, and the Green King and
Southern Comet cultivars showed only 50% and 25% of plants transitioned to floral development
respectively, at the end of the experiment.
In Figure 2 we can see that by the end of the experiment, 100% of the plants in the 15/20⁰C cabinet had
completed their floral development and had produced seedpods. In cabinet 3 we see that Green King and
Marathon were the most advanced in terms of growth stage, with most plants being in flower production.
Southern Comet was less developed, showing only some head development. In cabinet 4 we see that all of
the plants remained in the vegetative state of development except for 1 Southern Comet plant (and 2 Green
King plants, not shown).
Figure 3: Graph of average leaf count at the end of the experiment (19/0/2013) for all cabinets and varieties.
Unit 2 is the 15/20⁰C treatment, unit 3 is 20/25⁰C and unit 4 is 25/30⁰C.
In figure 3 we can see that Green King and Southern Comet had significantly less leaves than Marathon and
Summer King in all temperature treatments, especially so in the lowest temperature cabinet. All plants had
the highest leaf count in the 20/25⁰C treatment.
Discussion
The results of this experiment show that temperature is closely correlated to time required until FI, a stage
crucial to the marketability of the crop. This result is supported by Wurr et al (1995), who found that the
meristem apex showed delayed increase in apex diameter, which corresponds to floral initiation. The earlier
floral initiation observed for Green King and Summer Comet in the 15/20ᵒC cabinet is because they are early
maturing cultivars (Known-you seeds Taiwan). As the head size of pot-grown plants does not translate to
field head size, this data was ignored.
The total leaf number in response to temperature changes mirrors results by Tan et al (2000), whose results
show that for Marathon, leaf count at FI ranges from 21 at an average temperature of 14ºC, to 27 at an
average temperature of 23ºC. The lower leaf count in the 25/30ᵒC treatment indicates potential heat stress, a
hypothesis supported by Bjorkman & Pearson (1998). In this study, the temperature difference between units
2 & 3 translated to a 35 – 70 day increase in time taken to initiate reproductive growth. This is despite the
20/25ºC treatment being a more ideal temperature for plant growth, as literature reports between 20-25ᵒC to
be optimum (Tan et al 2000). This implies a vernalisation requirement in the plants despite being nonfunctional in the FLC2 gene. This can be seen clearly in Figure 2, where all plants had completed their life
cycle by the end date of the experiment. The middle cabinet showed reduced FI. In the literature, work has
been done on the characterisation of genes inducing a vernalisation requirement in broccoli, and it has been
found that there are a number of genes which are homologous to vernalisation genes in Arabidopsis and
other brassica species such as Brassica rapa (Okazaki et al 2007). The genes FLC1, FLC3 and FLC5 are
also present in broccoli in highly conserved forms, but have not been linked to quantitative trait loci inducing
vernalisation. However, these genes have been shown to be involved in vernalisation for Brassica rapa
(Okazaki et al 2007), and therefore may be involved in modulation of flowering time in a more subtle way
than FLC2.
With respect to the actual temperatures required for vernalisation in broccoli to occur, Wurr et al (1997)
suggest an absolute upper limit for vernalisation to occur in broccoli to be 23.6ᵒC as determined by their
vernalisation model. This is supported by the results of this experiment, with the plants experiencing an
average temperature of 22.5ºC being greatly delayed in FI, and the plants experiencing average temperature
of 27.5ºC, with zero time below 23.6ᵒC, showing very little transition to FI at all, suggesting that these
temperatures simply will not support the reproductive development of broccoli. Huth et al (2009) observed
that less vernal time experienced by the crop corresponds to a linear increase in thermal time requirement for
the emergence to FI phase, consistently across three cultivars including Marathon.
Another factor contributing to the poor performance at high temperatures is the heat sensitivity which is
characteristic of Brassica oleracea. The literature describes broccoli as a heat sensitive crop, and says that
floral initiation can be arrested by high temperature stress, resulting in irreversible damage to the plant in
some cases. The deformity can be seen as uneven size of budding in the meristem at the time of transitioning,
and this was observed when meristems from the high temp treatment were analysed microscopically
(Bjorkman & Pearson 1998).
The Sogeri plateau in PNG has average temperatures of around 19.3ᵒC to 27.8ᵒC (Birch et al, date unknown),
and because of this, it is unlikely that these cultivars would be commercially successful grown in the region.
However, a number of hybrid cultivars are in development in the US as reported by Farnham & Bjorkman
(2011), and these cultivars have been successful growing at higher temperatures, in the 25 – 30ᵒC region. If
field trials were to be continued in Sogeri PNG, more success may be achieved using these novel cultivars, a
potentially increasing food security and welfare of the people of Papua New Guinea.
Acknowledgements
Thanks are extended to Alastair Gracie for experimental design, supplying background information and
methods and materials for the experiment. Thanks are also extended to Marek Matuszek and Mirjam
Storcken for taking measurements & collating data.
References
Birch C, Sparrow L, Woruba M, Kapal D, Maino G, Kambuou R, Bonney L, Doyle R, ‘Future vegetable
farming in Papua New Guinea – responding to resource constraints and population in a developing
country: a case study,’ Australian Centre for International Agricultural Research, Canberra, ACT.
Bjorkman T & Pearson K J (1998), ‘High temperature arrest of inflorescence development in broccoli
(Brassica oleracea var. italica).’ Journal of Experimental Botany, 49, 101-106.
Department of Foreign Affairs and Trading (2013), Papua New Guinea country brief, Australian
Government, <http://www.dfat.gov.au/geo/png/png_brief.html>
Farnham Mw, Bjorkman T (2011). ‘Evaluation of experimental broccoli hybrids developed for
summer production in the eastern United States,’ Hortscience, 46, 858-863.
Okazaki K, Sakamoto K, Kikuchi R, Saito A, Togashi E, Kuginuki Y, Matsumoto S, Hirai M (2007),
‘Mapping and characterization of FLC homologs and QTL analysis of flowering time in Brassica
oleracea.’ Theoretical and Applied Genetics, 114, 595-608.
Tan DKY, Birch CJ, Wearing AH, Rickert KG (2000), ‘Predicting broccoli development - I.
Development is predominantly determined by temperature rather than photoperiod,’ Scientia
Horticulturae,84, 227-243.
Wurr D, Fellows J, Phelps K, Reader R (1995), ‘Vernalization in calabrese (Brassica oleracea var. italica) a model for apex development,’ Journal of Experimental Botany, Vol. 46, No. 291, pp. 1487-1496