Potential of exploiting pH-signalling to increase yield in oilseed rape (Brassica napus L.)

Study to explore the potential of using pH-signalling to increase yield in oilseed rape.

Year of Publication2008

When plants are subjected to drought they generally close stomata and reduce leaf expansion to conserve water for survival. It has been accepted that this response to reduced soil water availability can occur via chemical signals and not by a change in shoot water status. The search to identify the chemical signal is not completed but a strong candidate is the plant growth hormone abscisic acid (ABA) which can promote stomatal closure (Jones and Mansfield, 1970) and inhibit shoot growth (Wilkinson and Davies, 2002). Another chemical signal that may act in conjunction with the ABA is an increase in xylem sap and apoplastic leaf pH. An increase in xylem sap pH can close stomata (Wilkinson and Davies, 1997), and leaf expansion in detached shoots of barley and forsythia have been reduced when fed alkaline pH (Bacon et al., 1998; Wilkinson and Davies, 2008). Furthermore, leaf expansion of intact plants has been reduced after a foliar application of slightly acidic buffer sprays. Growth regulators are applied in spring if oilseed rape canopies grow above the optimum leaf area index of 3.5. It has been suggested that sparser canopies can yield higher (Lunn et al., 2001) with a lower requirement of nitrogen for growth which will lower input costs. Alkaline foliar sprays for use as growth regulators in oilseed rape could potentially reduce the use of other potentially environmentally harmful chemical growth regulators. Oilseed rape xylem sap pH was found to significantly increase in drought-stressed plants compared to controls, but stomatal conductance and leaf area growth rates were not affected. It was concluded that there may be a pH-signal but that hydraulic signals had more effect on shoot physiology. 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 apoplastic 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 apoplastic leaf pH visualised with fluorescence microscopy. The leaf growth response to different alkaline buffers (e.g. potassium phosphate) applied as foliar sprays was inconsistent possibly due to penetration difficulties as oilseed rape has a thick cuticle. Sodium bicarbonate showed however 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.

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