Water status and ABA content of floral organs in drought-stressed wheat

Chemical signals from roots have been shown to mediate the response of vegetative shoots to drought. Our objective was to test whether root signals such as abscisic acid (ABA) affect grain set in wheat. Uniculm wheat was grown in a controlled environment and exposed to a water deficit from pollen mother cell meiosis to late boot stage-a period of reproductive development very sensitive to drought. The water deficit decreased grain numbers per spike up to 70%. As soil moisture was depleted, leaf, glume, ovary and anther water potential (Psi(W)) decreased with leaf Psi(W). Turgor decreased in the leaves, but remained at or above control levels in all floral organs examined. Free ABA content of leaves increased 30-fold as leaf turgor declined, while ABA in floral organs increased 10-15-fold. To separate the effects of shoot and root water status on grain set, plants were pressurised to maintain leaf Psi(W) at control levels as the soil dried. Pressurisation increased flowers and grains per spike over that of droughted plants at comparable soil water suctions, but not to control levels. Free ABA content in leaves and floral organs increased only about 3-fold when leaves were maintained at high Psi(W). Shoot water status had a greater effect on grain set than did soil water status. In both pressurised and unpressurised plants, grains per spike and percentage grain set decreased with increasing ABA content in ovaries and anthers. The results indicate that maintenance of a high shoot water status reduces the effect of soil water deficit on grain set by reducing the accumulation of ABA.