Control of Photosynthesis and Stomatal Conductance in Ricinus communis L. (Castor Bean) by Leaf to Air Vapor Pressure Deficit

Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO₂ assimilation, stomatal conductance (g_s), quantum yield of photosystem II (_II), and nonphotochemical quenching of chlorophyll fluorescence (q_N) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO₂ assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g_s initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40°C, CO₂ assimilation and g_s were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g_s. The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO₂ and O₂) versus nonphotorespiring (700 microbars CO₂ and 2% O₂) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O₂ increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO₂ in the leaf (C_i) where C_i was estimated from gas exchange measurements. However, estimates of C_i from measurements of _II and q_N suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C_i. The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C_i from gas exchange measurements. Under low VPD, 30°C, high light, and saturating CO₂, castor bean (C₃ tropical shrub) has a rate of photosynthesis (61 micromoles CO₂ per square meter per second) that is about 50% higher than that of tobacco (C₃) or maize (C₄) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.