Matching canola phenology to the environment

What is phenology 

When studying plants, distinct stages of growth have been identified and these have been formalised into keys that are often used in both plant physiology and agronomy (Figure 1). The description of crop growth stages is called the phenology of the plant. The most common and easily recognised canola stages are emergence, green bud, flowering, podding and maturity.

Figure 1. Growth stages for canola highlighting the different stages and the dominant environmental signals that influence growth in that stage.

Plants respond to environmental signals such as temperatures, to determine when they move from one growth stage to another. At the biochemical level, this is caused by specific temperatures inducing the production of specific plant hormones until a critical concentration triggers the change within the plant. A simpler way to think of this is as a biological clock that accumulates average daily temperatures (day degrees) until a specific target (thermal time target) is achieved. 

Why would we want to know this? 

Understanding how the environment affects the growth of a plant can assist in crop management. Many management decisions are time critical, that is, for optimum results the intervention (spray application, defoliation, cease grazing, fertilisation) needs to occur before a plant reaches a particular growth stage. Identifying these stages can be difficult, for example, floral initiation can occur well before any visible sign appears in the plant. If the crops are grazed, or stressed during this floral initiation period, then a yield penalty can occur. Knowing what stage a plant is at can often help prevent yield loss or ensure untimely management does not occur.

In many environments, it is important that canola flowers within a particular window, to avoid frost on the one hand and high temperature heat stress on the other. If you farm in a region that generally has sowing rain within a particular month, you can match your variety’s maturity to ensure  flowering inside the optimum window every year. However, a sowing rain in many areas is unpredictable and may occur too early or too late. An understanding of phenology for different varieties allows specific variety selection to ensure flowering occurs at the optimum time, and the risk of crop loss is reduced. An example of this for wheat was presented by Richards et al. 2014 and has been reproduced here with its original caption to highlight the value of understanding the mechanisms of plant development.

Figure 2. Reproduction of Richards et al. 2014, Figure 9. 

There are a number of projects that are currently being funded by GRDC to improve our understanding of plant phenology and the components to describe them and ways to use this information to better manage the farming system. 

For the remainder of this paper, we will describe how canola development can be measured and present data from our current research work to illustrate the different mechanisms. 

The data is from a detailed phenology trial that was conducted in Canberra (ACT) and Gatton (QLD). The trial was a time of sowing trial with 3-4 sowing dates between April and June over two years. At the May sowing, two additional day length treatments were included by using lights to create 14, 13 and 16 hour days, in addition to the ambient 10.5 hour day. Twenty-five commercial canola cultivars were included in the trial. 

What are day degrees and thermal time targets?

Day degrees are the units of a plant’s biological clock. They are a way of combining time and temperature into a single number. In its simplest form, it is the average temperature recorded during a day (Figure 2). To calculate the thermal time target for a plant’s development stage, you accumulate the day degrees until a specific target is reached, for example, variety X accumulates 500 degree days between emergence and flowering. 

Figure 3. Calculation for simple day degrees (average daily temperature) and how the day degrees can be accumulated over time to calculate a thermal time target to move from one plant growth stage to another.

This example is the simplest form and assumes that the plant has a base temperature of 0°C, and that no growth or development will occur below this temperature. Likewise it assumes that growth and development will continue at high temperatures (>35°C). This is not always the case. 

The simple day degree calculation can be made more complex by identifying those temperatures where plant growth and development occur and only calculating day degree temperatures when they are within these temperatures.

Figure 4. A representation of the cardinal temperatures for canola (describing the upper and lower limits for plant growth) for growth and development of a plant that will accumulate day degrees when the temperature is above 0°C and below 35°C. 

Leaf appearance in canola

 
Leaf appearance in canola is a temperature driven response so can be easily described by day degrees. If the rate of leaf appearance is plotted against day of year, the lines appear roughly linear but the later plantings take longer for each leaf to occur due to the colder days as you move into winter (Figure 5a). Using day degrees as the unit instead of calendar days, a simple linear relationship can describe each cultivar (Figure 5b). In general, the thermal target for leaf appearance ranged from 45-53 degree days for the 29 cultivars examined. 

Figure 5a. Leaf appearance data for a canola cultivar collected in Canberra during 2016. When the data is plotted against calendar days, the time between leaves changes as the days get colder causing a slight curve. b. Combined leaf appearance data for a canola cultivar collected at Gatton and Canberra during 2016. When data is plotted against thermal time,  a linear line can describe the rate of leaf appearance. 

Growth and development in crops like canola is complicated.   In addition to accumulating thermal time, they have two other mechanisms that can influence the time it takes to move between growth stages. These are vernalisation and photoperiod response. The combination and interaction of the three responses complicate the process of estimating when crops will flower. 

Vernalisation

Vernalisation can be described as a low temperature promotion of flowering (Salisbury and Ross, 1978). To measure vernalisation, we use a similar approach to a degree day, however, a vernal day is a measure of days when the average temperature is below a specific temperature threshold. For canola if the average temperature is 2 degrees, then one vernal day is accumulated — no vernal days are accumulated if the average temperature is below 0 or greater than 15. Between 2 degrees and 15 degrees, only a proportion of a vernal day is accumulated. 

There are two types of vernalisation — obligate and facultative.

Obligate vernalisation

Obligate vernalisation is the need for a plant to accumulate cold days before the day degree calculation can begin. One way of thinking about this is that once a plant has emerged, it remains vegetative and grows leaves until the vernal target has been achieved, then as if a switch has been turned, it starts accumulating thermal time to reach flowering (Figure 6). Such cultivars can be planted from late spring to autumn in southern NSW and not flower before winter. 

Figure 6. Accumulated thermal time to floral initiation against accumulated vernal time for the winter type canola Hyola^®971. The vertical line (grey dashed) indicates obligate vernalisation (13 vernal days required before floral initiation occurred. The sloped line (black solid) indicates facultative vernalisation. The more vernal time accumulated, the less time required to reach floral initiation. 

Facultative vernalisation 

Facultative vernalisation occurs in both spring and winter type canola. It simply means the more cold days the plant accumulates between sowing and floral initiation (stage before green bud), the lower the thermal time target required. The sloped section of Figure 6 shows a good example of facultative vernalisation. The lower points which all come from the Canberra trial required less thermal time to reach initiation, because they had experienced more cold. Generally, vernalisation is saturated after the accumulation of around 22 vernal days. 

Winter wheat and winter canola are extreme examples of plants with an obligate vernal requirement. However, many spring wheats and canola varieties can also display a vernal response. These varieties when grown in Victoria can be short season, but if moved to Queensland or planted earlier than usual become longer seasoned.   This is due to the difference in rate of vernal day accumulation. It is not uncommon for varying degrees of vernalisation, thermal time and day length sensitivity to occur within different varieties of a crop, so every variety is different and needs to be measured. 

Photoperiod (day length)

Photoperiodism is the response of plants to increasing or shortening day lengths. Long day plants (canola) respond to increasing day length. As we move from winter to summer, the days lengthen and the crop requires fewer day degrees to move between growth stages so flowers earlier. For example, if we think about a canola crop with no day length sensitivity, it would flower after accumulating the same number of day degrees no matter when or where you planted it (if it has no vernal response). If you planted it in April, it would take the same number of day degrees to flower as if you planted it in June (Figure 7). However, if the plant was sensitive to day length, then as the days became longer, the plant’s thermal target would be reduced requiring fewer day degrees to be accumulated and reducing the time to flowering. This complicates growth calculations because day length is not only seasonal, it is also dependent on your position on the earth surface. For example, in winter, Narrabri has a minimum day length of about 11 hours while Melbourne has about 10.5 hours. If you grew a day length sensitive crop somewhere near Melbourne that had exactly the same growth temperatures as Narrabri, it would flower earlier in Narrabri due to day length effects.

This is why recommending a time to flowering for a variety in calendar days after sowing is meaningless, unless the date of sowing and location are presented. Identifying when a plant is sensitive to photoperiod can be difficult; however, our results suggest that the day length at green bud could explain our observed data the best. 

Figure 7. How day length can reduce the flowering thermal time target, that is, the number of accumulated degree days required to reach flowering.

Conclusion

An understanding of crop phenology enables the crop’s behaviour in different environments to be predicted. The description and results presented can be used in crop models to predict when a crop will flower. If using a model like Yield Prophet® during the current season, the model will help identify and predict when each growth stage will occur. Alternatively, models such as the Agricultural Production Systems simulator (APSIM) can be used to look at historical records and identify the variety by sowing date combinations that maximise yield production. In addition to the work described here that aims to better understand canola phenology and describe modern cultivars, we are looking at identifying the optimal period to target flowering in different regions across the cropping zone. Our eventual aim is the creation of simple web based phenology models to allow growers to assess the risks of planting different cultivars within their region at different times and help match crop genotypes to their environment. 

References

Richards, R.A., Hunt, J.R., Kirkegaard, J.A., 2014. Yield improvement and adaptation of wheat to water-limited environments in Australia—a case study. Crop Pasture Sci. 65, 676-689. doi:10.1071/CP13426

Acknowledgements

This work is a component of the 'Optimised Canola Profitability' project (CSP00187), a collaboration between CSIRO, NSW DPI and GRDC, in partnership with SARDI, Charles Sturt University (CSU), Mallee Sustainable Farming (MSF) and Birchip Cropping Group (BCG).

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the author would like to thank them for their continued support. 

Contact details 

Jeremy Whish
CSIRO, Toowoomba Qld
07 45713215
Jeremy.Whish@CSIRO.au