The effect of plant size on wheat response to agents of drought stress .2. Water deficit, heat and ABA

Plant size has long been implicated in plant response to drought stress. This study is the second in a series of two intended to examine the effect of plant size on plant performance under the effect of various agents of drought stress. Variable plant size (in terms of plant height and shoot biomass) independent of genetic background effects was experimentally achieved using rht (tallest), Rht(1) and Rht(2) (medium) and Rht(3) (shortest) homozygous height isogenic lines of spring wheat (Triticum aestivum) cultivars Bersee and April-Bearded. Plants were grown in hydroponic culture in the growth chamber. In the first experiment, juvenile plants were challenged by osmotic stress using polyethylene glycol (PEG) in the nutrient solution giving a water potential of -0.55 MPa. The control nutrient solution was at -0.05 MPa. Plant growth, shoot biomass, leaf area, relative water content (RWC) and osmotic adjustment (OA) were measured. In the second experiment, effects on growth rate of chronic heat stress and abscisic acid (ABA) in the root medium of juvenile plants were evaluated. Potential plant size as determined by shoot biomass in the controls at 25 days after emergence was greatest in rht, medium in Rht(1) and Rht(2), and smallest in Rht(3) genotypes. Potential growth rate and leaf area were greater in plants of larger potential biomass (rht) than in plants of smaller potential biomass (Rht(3)). Growth reduction by osmotic stress was inversely related to plant size, while the extent of osmotic adjustment during osmotic stress was directly related to plant size. RWC did not vary with plant size. Relative growth reduction by heat stress and by ABA also decreased in smaller plants. ABA did not alleviate the depressing effect of heat on growth. Despite the greater stress tolerance of smaller (Rht(3)) plants, the absolute growth and biomass of large (rht) plants under stress conditions was always better than that of smaller plants. The results of these series of experiments suggest that greater stress tolerance of small plants is derived from their relatively smaller size and slower growth rate. Consequently, we conclude that growth under stress is sustained by potential growth rate and plant size of the genotype when stress is mild and by plant tolerance (even at the expense of potential growth rate and size) when stress is more severe.