Canola production in Western Australia is vulnerable to large variations in rainfall and frequent droughts. Recent poor seasons and price volatility have contributed to farmers’ perceptions of risk with canola and it can be observed that the area of canola grown often declines after periods of drought. Nevertheless, the total area of canola grown in Western Australia has still increased over the last five years and the importance of the crop has become even more apparent. An increase in the area of potassium deficient agricultural sandy soils and the prediction of drier years in the Western Australian wheat belt has been identified. Therefore, more work is needed to enhance the current knowledge on plant physiological responses to potassium deprivation and drought stress particularly for canola.
The first objective was to measure drought tolerance of canola under different potassium treatments. The second objective was to understand the mechanisms controlling drought tolerance of canola by comparing the response of varying potassium treatments during a drying cycle as well as monitoring the ability to recover after the drought stress. The third objective was to observe any morphological differences of canola plants exposed to different potassium levels and water stress. In order to achieve these objectives, a range of climate chamber, glasshouse and field experiments were conducted growing Trigold canola plants in sandy soils and hydroponically, mixed with increasing potassium concentrations, which were subjected to drought stress.
This project increased our understanding of the effect of potassium nutrition on the drought response of canola. A mild potassium deficiency (40 mg kg-1 soil potassium) only decreases the dry weight of canola significantly if plants are exposed to a long-term potassium deprivation. The ability to maintain a similar size over a range of applied potassium levels could be due to a morphological mechanism that might have evolved in canola to manipulate root growth towards a smaller shoot:root ratio. The low K treatments had increased transpiration and stomatal conductance and reduced relative turgor pressure compared with high K, under drought stress. This was due to the stomata remaining open in the low k and closed in the high K. Canola with a higher supply of potassium also had a higher rate of photosynthesis under water stress than canola with a low level of applied potassium. The opening of the stomata and associated higher transpiration rate and stomatal conductance may be caused by an interaction effect of the plant hormones abscisic acid and ethylene depending on the level of potassium and water stress that the canola plant is exposed to.
Thus, further work should focus on measuring abscisic acid and ethylene levels under varying water and potassium treatments in order to understand the mechanisms behind this response and how best to mitigate the effects of the stress. In addition to that, research should be done on the interaction between potassium and drought by studying the canola plants grown continually at different soil moisture and potassium levels in particular focusing on the 40-60 mg kg-1 soil potassium level where most changes have been observed in order to determine any effects on plant size, shoot:root ratio, relative turgor pressure and transpiration per leaf area. Especially to monitor any changes in shoot:root ratio, which may be a useful morphological trait for selection of drought tolerance in canola. The interaction between nitrogen and potassium on canola response to drought also needs further investigation because it may be necessary to apply higher amounts of potassium fertiliser when applying relatively high nitrogen rates.