Physiological aspects of stomatal regulation and water use in wheat (Triticum aestivum L.) under terminal drought

[Truncated] Wheat grown in Mediterranean-type environments is often exposed to end-of-season drought (terminal drought). As terminal drought develops, soil dries in the top layers of the soil profile, exposing the upper part of the root system to soil water deficit while roots below may still have access to deeper soil water. It is hypothesised that the part of the root system exposed to drying soil induces abscisic acid (ABA) production and the corresponding increase in leaf ABA concentration partially closes stomata, which regulates the use of available water at depth, allowing completion of grain filling. The aim of this research was to examine if ABA accumulation and regulation of stomatal conductance in response to drying soil, would improve water use and grain yield under conditions of terminal drought. Four experiments were conducted from 2011 to 2014 in controlled environmental conditions.

In the first experiment described in Chapter 3, four wheat genotypes were grown in a split-root system and terminal drought was induced in half of the vertically-split root system, while the other half was kept well watered. Genotypes were selected for their putative adaptation to contrasting dryland environments in Australia. The aim of this experiment was to identify genotypes with contrasting stomatal responses and leaf ABA concentration. The two most contrasting genotypes identified were the cultivar Drysdale and the breeding line IGW-3262. When one half of the root system was exposed to drying soil, leaf water potential decreased and Drysdale had lower stomatal conductance and higher leaf ABA concentration. Leaf water potential remained unchanged in IGW-3262 but stomatal conductance decreased without concomitant increase in leaf ABA concentration.