Recent reports have indicated that leaf growth of plants in drying soil can be regulated by chemical signals originating from the roots. A potential signal is the plant hormone abscisic acid, ABA. This thesis aimed to identity sources of variation in the sensitivity of cereal leaf growth to ABA using a detached shoot leaf elongation assay. Preliminary experiments showed that altering the nutrient composition or pH of the feeding solution had no effect on sensitivity. Assay variability resulted in considerable uncertainty over whether observed genotypic differences in sensitivity were real, or reproducible. Increasing the air temperature around the detached shoots greatly increased the sensitivity of leaf growth to ABA. Increased concentrations of ABA in the elongation zone ( [EZ-ABA] ) of detached shoots seemed to account for this increased sensitivity. When detached shoots were fed sap from droughted plants, sap ABA concentration could not explain the growth inhibitory activity. Measurement of [EZ-ABA] accounted for this "unexplained" growth inhibition. When intact plants were subjected to a slowly developing drought, growth was reduced by 35 % without any increases in [EZ-ABA]. Measurement of leaf water relations and xylem sap pH similarly failed to account for the growth inhibition. The diurnal growth pattern of droughted plants suggested an interaction of water relations with chemical signals. Use of a coleoptile growth assay showed that individual application of mild osmotic stresses or ABA did not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants. It also accounts for the relative (compared to droughted plants) insensitivity of leaf elongation to ABA in the leaf elongation assay.