EFFECT OF HEAT AND DROUGHT STRESS ON SORGHUM (SORGHUM-BICOLOR) .2. GRAIN-YIELD

Three early and four later flowering lines of sorghum were subjected to three drought stress treatments (early, late and early plus late stress) in the field during the dry season at Hyderabad in India. Mean diurnal temperature and evaporation rate were uniformly high throughout the experiment. The late and early plus late stress conditions were severe, while the early stress was mild. Grain yield was affected by both the timing and the severity of the stress. The largest reduction (87%) in grain yield resulted from stress imposed during booting and flowering (late stress) in the early flowering lines; the same stress treatment on vegetative plants had no effect on grain yield. Increasing the duration of severe stress on vegetative plants (early plus late stress) reduced grain yield by 50-60%. Grain yield was strongly and positively correlated with the number of grains m-2. Variation in grain yield was associated with variation in total dry matter rather than with harvest index, which was only reduced when stress occurred at flowering. Treatment effects on thermal growth rates (g m-2 degrees-Cd-1) during the phase from booting to flowering confirmed that growth during this phase is the major determinant of yield potential (number of grains). The importance of phenology in research into drought resistance is discussed.