Overview
The report provides background on vine responses to water deficit, detailing the root-to-leaf hormone signalling pathway, particularly ABA metabolism-catabolism and signalling. It links rootstock differences in ABA regulation to water use efficiency, offering context for understanding vine drought responses. More recent research extends knowledge and management techniques on this subject.
In order to study the root to leaf hormone signalling pathway, it is crucial to obtain samples that accurately represent the composition of the xylem sap and we tested a number of methods for sampling xylem sap from grapevines so that we could be confident that our methods are the most appropriate.
Our studies have centred on the substance abscisic acid (ABA) which is universally present in plant tissues and is primarily responsible for regulating vine water loss through the stomata. Using a leaf pressure bomb to expel petiole sap is the most commonly used method for the routine assessment of xylem-mobile bio-active molecules in field vines. It is important to recognise the factors which may influence their concentration in expressed sap and to gain an understanding of their role in determining vine response to soil water deficits and the environment.
The concentration of ABA and its metabolites is higher in petiole sap collected with a leaf pressure chamber than their concentration in sap expelled from the distal half of the same petiole through the application of pneumatic pressure to the roots, suggesting that apoplastic sources contribute to solutes in petiole sap. Even though petiole sap may not quantitatively reflect the sap as it leaves the roots it may be a reasonable representation of the solution that is actually in contact with the stomatal guard cells which ultimately control leaf transpiration.
Petiole sap collected at a pressure within 0.2 MPa of the leaf water potential appears to be minimally affected by applied pressure. The concentration of ABA in sap from unstressed vines is not affected at all by the collection pressure but if the vines have been subject to water deficit, the sap collected at a pressure near the water potential has an elevated ABA content but as the pressure is increased the sap becomes progressively diluted so that the ABA content falls. Sap collected from stressed vines at a pressure as close as possible to the leaf water potential will therefore give the best result.
Abstract
In Australian viticulture most vines are periodically exposed to varying degrees of soil water deficit, that is less water than they can potentially use. This is either done deliberately, as in the application of RDI or PRD or as result of undesired reductions in soil water due to water supply restrictions.
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