Potential improvement of canopy management in oilseed rape by exploiting advances in root to shoot signalling (PhD)

ABSTRACT

The aim of this project was to determine the potential for alkaline pH sprays to be used as a leaf growth regulator. Triazole growth regulators are applied in spring if oilseed rape canopies grow above the optimum leaf area index of 3.5 because sparser canopies can give greater yield. Alkaline foliar sprays for use as growth regulators in oilseed rape could potentially reduce the use of expensive Triazoles growth regulators or replace them if prohibited under the impending EU legislation. During this project pH-signalling in oilseed rape and the effect of alkaline pH on leaf expansion was investigated.

Oilseed rape stem (xylem) sap pH of glasshouse grown plants was found to be slightly acidic with an overall mean of pH 6.29. There was a tendency for a diurnal rhythm with the xylem sap pH becoming more acidic during the morning and more alkaline during the late afternoon. When glasshouse-grown plants were subjected to controlled water stress the xylem sap pH significantly increased compared to controls, but leaf area growth rate was not affected suggesting that the leaf buffered the pH to prevent changes in leaf growth. Plants subjected to natural soil drying did not significantly increase xylem sap pH but leaf area was reduced. Based on the results it was suggested that during water stress oilseed rape controls shoot processes by hydraulic signals rather than pH signals.

Feeding detached oilseed rape shoots with alkaline pH reduced leaf area over a 4 day period. There was an increase in stem xylem sap pH for alkaline fed shoots and the leaf buffering capacity was possibly over-ridden by the alkaline pH resulting in a reduction in leaf area. This assumption was supported by an increase in leaf pH visualised with fluorescence microscopy. The leaf growth response to different alkaline buffers applied as foliar sprays was inconsistent possibly due to penetration difficulties, as oilseed rape has a thick cuticle. Sodium bicarbonate however showed potential for regulating leaf growth. It was concluded that alkaline pH could reduce leaf expansion but further research is needed to fully exploit the pH signal in oilseed rape.