The effect of plant size on wheat response to agents of drought stress .1. Root drying

Plant size has long been implicated in plant response to drought stress. This study is a first 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 cv. Bersee). Top-root drying is a common stress condition when the top soil dries in the field. In this experiment wheat was grown in hydroponics system in long PVC tubes. Stress was applied by allowing the top (40 cm) roots to dry throughout most of the growing season while the remaining roots were immersed in the nutrient solution. Average seasonal top-root water potential was reduced from -0.097 MPa in the controls to -1.93 MPa under stress. This stress condition caused a reduction in shoot biomass while it increased total root length. There was a general progressive increase in leaf diffusive resistance under the treatment as plants became larger. This stomatal closure could not be accounted for by reduction in leaf water potential. In fact, leaf turgor increased as stomatal diffusive resistance increased with increasing plant size, leading us to conclude that stomatal closure was the primary response to top-root drying, followed by turgor maintenance possibly as a result of a non-hydraulic signal produced by the drying top-roots. Smaller plants were affected relatively less than were larger plants by this stress condition in terms of stomatal closure, plant height, tillering and shoot biomass reduction. Grain yield per plant was actually increased by stress in the smallest plants while it was unaffected by stress in the larger plants. It is concluded that the smallest plants were the most resistant to top-root drying but absolute biomass and yield under this stress condition was the greatest in the largest plants because of their relatively greater potential.