MECHANISMS OF DROUGHT RESISTANCE IN GRAIN II:.STOMATAL REGULATION AND ROOT GROWTH

Abstract

Earlier research works conducted to identify mechanisms of drought resistance in grain legumes under soil water stress of -0.6 MPa showed that drought resistance (maintenace of turgor) in chick pea was due to a significant decrease in osmotic potential (osmotic adjustment) while in common bean it was due to maintenance of high leaf water potential, but not due to osmotic adjustment. Green house experiments were conducted in the University of Hohenheim, Germany to determine whether maintenance of high plant water potential in common bean under stress was the function of stomatal regulation and/or root growth. Seven days mild drought (-0.15 MPa ) decreased the water potential to -0.35 and -0.89 MPa, accompanied by a dry matter decrease of 25 and 15% in common bean and chick pea, respectively. Higher dry matter decrease in common bean was due to reduced CO2-fixation, whereby photosythesis was reduced by 75% in common bean but only by 20% in chickpea. Significant decrease in the rate of photosynthesis decreased the sugar reserve (glucose, fructose and sucrose) of commont bean significantly, which could be responsible for the reduced biomass synthesis. Decrease in the rate of photosythesis was attributed to significant decrease in stomatal conductance. However, water use efficiency was significantly higher in common bean than in chick pea regardless of water regimes. A rhizotrone experiment showed that root length density of common bean was higher than that of chick pea (by a factor of two), accompanied by higher root weight. It was concluded that high plant water potential of common beans under stress was not due to osmotic adjustment but it was the function of effective stomatal regulation and robust root system.