Photosynthesis and Stomatal Conductance of Selected Root Crops as Related to Leaf Age

Single leaf net photosynthesis (Pn), canopy photosynthesis (CPn) and stomatal conductance of sweet potato (lpomoea batatasL.), cassava (Manihot esculentaCrantz) and yam (Dioscorea alata) were determined under field conditions in full sunlight (above 1500 gmol m−2s−1) between 1100 and 1500 h EST during 1983 and 1984. Canopy photosynthesis was determined with a portable plexiglass photosynthesis chamber in a closed system under ambient conditions. Single leaf net photosynthesis was determined at 30± 2 °C by enclosing attached fully mature leaves in an acrylic plastic chamber in an open system. The mean Pn for sweet potato, cassava, and yam were 1.10, 0.70, and 0,30 mg CO2m−2s −1, respectively. Mean CPn rates, expressed on leaf area basis, for sweet potato, cassava, and yam were 0.18, 0.38, and 0.17 mg CO2 m−2s−2, respectively. Abaxial stomatal conductances for cassava and sweet potato were almost similar, but both were three to five times higher than for yam. Adaxial stomatal conductance ranged from 0.007 to 0.010 m s&1for sweet potato and was less than 0.001 m s&for both yam and cassava, indicating the lack of stomata or their poor development on the adaxial surface. With increase in leaf age from about 20 to 60 d, both Pn and stomatal conductance declined faster in sweet potato than in cassava and yam. Single leaf photosynthetic rates were higher for sweet potato than cassava. However, CPn comparisons involving complete plant foliage indicated that photosynthetic efficiency of cassava was superior to that of both sweet potato and yam. Cassava also partitioned more photosynthate to storage roots than the other two species. The lesser decline in cassava Pn with aging (and consequently of CPn) than sweet potato partially explains why cassava productivity is similar to sweet potato even when sweet potato Pn is 50.0% more than that of cassava.