INFLUENCE OF SOLAR-RADIATION AND VAPOR-PRESSURE DEFICIT ON TRANSPIRATION EFFICIENCY OF RAIN-FED SORGHUM

Besides assimilation, plant water relations are important aspects of p hysiological basis of productivity of crops in water limited environme nt. The relationships of photosynthesis rate, transpiration rate, leaf water potential and stomatal conductance with photosynthetically acti ve radiation (PAR) and vapour pressure deficit (VPD) during pre-flower ing (panicle initiation to ear emergence) and grain filling (from anth esis to maturity) stages of a sorghum hybrid (cv. CSH-6) grown under r ainfed conditions were studied. Photosynthesis rate declined when PAR was above 1300 mumol m-2-s-1 during both the growth stages. Higher tra nspiration rate during grain filling stage at higher PAR caused the tr anspiration efficiency to be lower than during pre-flowering stage whe n PAR was above 1200 mumol m-2 s-1.Leaf water potential and stomatal c onductance decreased with increase in PAR. Leaf water potential was hi gher during pre-flowering than during grain filling stage but maximum photosynthesis rate was similar during both the growth stages. Changes in VPD did not qualitatively alter the relationships of the physiolog ical variables with PAR. Decreasing photosynthesis rate and LWP at hig h PAR suggest that photosynthesis rate was limited by low leaf water p otential when PAR was optimal, and by low PAR even when leaf water pot ential was high in rainfed sorghum during rainy season.