The effect of temperature on inflorescence initiation and the Growth and
Development of Broccoli (Brassica oleracea)
Jonathan. G. Kingsbury
Abstract
This study performed on four cultivars of broccoli (Brassica oleracea) Marathon, Green King,
Summer King and Southern Comet. DNA extraction, PCR and gel electrophoresis was performed on
all four cultivars of broccoli indicated that the FLC2 gene was non – functional in all the broccoli
plants. The FLC2 gene is a gene responsible for a major part of the requirement for vernalisation in
broccoli (Okazaki, 2007). Hence vernalisation was not performed on the broccoli plants at all during
any stage of development, in order to stimulate the climate in Port Morseby. Climate controlled
cabinets were used to simulate varying climates in sub – tropical and tropical – environments, with
temperature regimes of 20/15, 25/20 and 3/25 – day/night temperatures o C. Four replicates of each
variety were randomly placed into each cabinet and leaf number, length of plant and the
developmental stage of the crop/flower. SPPS was used to statistically analyse the data. The results
suggest that a temperature regime of 20/15 provided the best growing conditions to produce a crop.
However the crop formed was not of marketable quality, therefore the FLC2 gene may not be the only
gene influencing the vernalisation requirement of broccoli (Brassica oleracea).
Introduction
Port Moresby, the capital city of Papua New Guinea is a growing centre for industry and development
and markets to supply development need to expand to supply the increasing population. The
increasing population had shifted the preferences of food, including an increased demand for
temperate vegetables. The ACIAR have done research into diversifying the vegetable industry to
achieve constant supply to the local markets. Broccoli was seen as a vegetable that may have good
potential in the area, with fertile ferrosols and high rainfall. Trials commenced on the Sogeri Plateau
in Papua New Guinea, seen as a location for broccoli production that was located generally close to
the city and was easily accessible by road, making it a good location for broccoli production and
supply to Port Moresby. The trial conducted looked at the ability of varieties Marathon, Shogun,
Green King, Southern Comet, Southern.Star to produce market\able heads. Early maturing varieties
Southern Comet, Southern Star and Green King produced head that were small in size and flat in
shape, and were of a non – marketable quality. Excessive sunlight and high temperatures affected the
quality of the broccoli crop especially during curd development. Poor soil conditions and vandalism
had negative implications and contributed to the poor set of results.
Broccoli (Brassica oleracea) are not well adapted to high temperatures, especially those experienced
in tropical and sub – tropical climates (Heather et al,1992). Hence the production of broccoli is
generally restricted to maritime climates (Heather et al, 1992). The effect of temperature on broccoli
growth and development is key in uncovering the plants ability to produce in variable climates,
especially those that experience extreme weather conditions such as Port Moresby.
Broccoli has a requirement for vernalisation, a property of many plant species in which cold treatment
is required for floral initiation and further production of a crop 2. In Port Morseby vernalisation is not
going to be possible since the average temperatures Port Morseby rarely dip below 22 oC 1 during the
winter.
In Brassica oleracea that have the functional FLC2 gene vernalisation is required for floral initiation
(Okazaki et al, 2007). However the results of the study admit that the FLC2 gene may only contribute
to the flowering time of Brassica oleracea, hence other FLC genes may contribute to the vernalisation
requirement in Brassica oleracea.
Materials and Methods
DNA extraction and PCR were performed on four cultivars of broccoli enabling gel electrophoresis to
take place. Gel electrophoresis tested for the functionality of FLC2 gene.
Four different cultivars of broccoli were used in this experiment. Marathon, Green King, Summer
King and Southern Comet. The plants were grown up from seed in seedling trays (5 x 5 cm) and
stored at 20 oC. The seeds were provided with 12 hours of light per day. Two weeks after the seeds
were sown (weeks after planting = WAP) the seedlings were placed into 6 inch pots (diameter).
Potting mix was used and contained a mixture of 80% (volume), composted Pine Bark and 20%
(volume), coarse Sand. Dolomite (4Kg/m3), FeSO4 (0.75Kg/m3), Wettasol granules (0.75Kg/m3),
and Osmocote Plus (3Kg/m3) were added. The Osmocote Plus contained:Nitrogen (16.0%),
Phosphorus (3.5%), Potassium (10.0%), Sulfur (2.4%), Magnesium (1.2%), Boron (0.02%), Copper
(0.05%), Iron (0.4%), Manganese (0.06%), Molybdenum (0.02%), Zinc (0.01%) and Organic resin
coating (8.0%).
During the experiment water was not a limiting factor.
Four replicates of each cultivar were placed into each cabinet. The cabinets were set at temperature
regimes of (30 / 25) = unit 4, (25 / 20) = unit 3, (20 / 15) = unit 2 – (day / night) oC for 12 hours.
The plants were grown until 26 WAP, number of leaves, length of plants and crop or flower
development was measured.
SPSS was used to analyse the data.
Results
Figure 2: The average change in broccoli
Figure 1: The average number of leaves
plant growth in length over time for each
across all 3 temperature regimes over time.
temperature regime.
Figure 3: Gel electrophoresis was performed on
Marathon, Green King, Summer King and Southern
Comet. Testing for the functionality of the FLC2 gene.
Figure 3: Gel electrophoresis of the
four cultivars of broccoli.
Figure 4: Average change in diameter of the broccoli
crop/flower over time for each temperature regime.
Figure 5: The Average crown size at the end of the
trial for each temperature regime and variety of broccoli.
\Discussion
All three temperature regimes indicate that there was a general trend of increasing number of leaves
over time, with no significant difference between the units, indicated in (fig 1). The length of the
broccoli plants over time indicated in figure 2 show that there was a general trend of increasing height
over time across the temperature regimes with no significant difference early on in development,
however there was a huge jump in height of the broccoli plants in unit 2, this might possibly be due to
the plants reaching the flowering stage and going to seed. However the measurements should not have
increased so dramatically in such a short time frame, so we expect that the measurements may have
been made to the top of the flower instead of to the meristem for there to be such as dramatic
inbcrease in average height.
The broccoli plants stored in the temperature regime of 20/15 started to crop 5 weeks before a crop
was seen in the 25/20 climate controlled cabinet, indicated in fig 4. This indicates that the broccoli
plants may well have required vernalisation treatment for floral initiation to occur and that broccoli
plants have not well adapted to warmer climate (Heather et al, 1992). The hastened floral initiation by
the broccoli plants in unit 2 are supported by literature as within the range for optimum growth
(Strange et al, 2010). High temperature exposure of broccoli plants was found to cause detrimental
injury to the broccoli crop through a study done by Bjo¨rkman and Pearson.
The development of the crowns of all four cultivars was highest in unit 2 at the end of the trial and the
trend indicates that at higher temperatures the crown reduced in diameter. The head diameter from
this experiment was not at a marketable size in comparison to commercial plantings, where broccoli
(Brassica oleracea) is generally grown between (7.5 cm – 20 cm) in diameter (Strange et al, 2010),
This further amplifies the need for vernalisation of all the broccoli plants whether grown at lower
temperatures or higher temperatures.
The gel electrophoresis performed on all the broccoli cultivars indicates that the FLC2 gene was not
functional at the time of planting. The FLC2 gene has been proven to have a major function in the
need of vernalisation by some crop species in order to initiate floral parts (Okazaki, 2007).
Conclusion:
In conclusion, broccoli plants (Brassica oleracea) are more adapted to maritime climates and that is
further exacerbated through the time taken for floral initiation to occur in higher temperature climates
that have been simulated in this experiment.
However the growth rate of the broccoli plants over all three temperature regimes was not
significantly different, it was simply that vernalisation was required by the broccoli plants to hasten
the initiation of floral parts.
The FLC2 gene that was not functional in the broccoli plants at the time of planting may not be the
only gene responsible for vernalisation in Brassica oleracea. Further research into the effect of FLC1,
FLC3 and FLC5 genes and there effect on vernalisation requirement for Brassica oleracea at higher
temperatures is required to better understand vernalisation of Brassica oleracea and the potential for
growth in tropical and sub – tropical climates.
Acknowledgements: Alisatait Gracie, Marek Matuszek
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