Carbon Dioxide and Light Responses of Photosynthesis in Cowpea and Pigeonpea during Water Deficit and Recovery

Abstract

Greenhouse-grown pigeonpea (Cajanus cajan, [L.] Millsp.; cultivar UW-10) and cowpea (Vigna unguiculata, [L.] Walp.; cultivar California No. 5) were well-watered (control) or subjected to low water potential by withholding water to compare their modes of adaptation to water-limited conditions. Leaf CO(2) exchange rate (CER), leaf diffusive conductance to CO(2) (g(l)), and CO(2) concentration in the leaf intercellular air space (C(i)) were determined at various CO(2) concentrations and photon flux densities (PFD) of photosynthetically active radiation (400 to 700 nanometer). In cowpea, g(l) declined to less than 15% of controls and total water potential (psi(w)) at midafternoon declined to -0.8 megapascal after 5 days of withholding water, whereas g(l) in pigeonpea was about 40% of controls even though midafternoon psi(w) was -1.9 megapascal. After 8 days of withholding water, psi(w) at midafternoon declined to -0.9 and -2.4 megapascals in cowpea and pigeonpea, respectively. The solute component of water potential (psi(s)) decreased substantially less in cowpea than pigeonpea. Photosynthetic CER at saturation photon flux density (PFD) and ambient external CO(2) concentration (360 microliters per liter) on day 5 of withholding decreased by 83 and 55% in cowpea and pigeonpea, respectively. When measured at external, CO(2) concentration in bulk air of 360 microliters per liter, the CER of cowpea had fully recovered to control levels 3 days after rewatering; however, at 970 microliters per liter the PFD-saturated CERs of both species were substantially lower than in controls, indicating residual impairment. In stressed plants of both species the CER responses to C(i) from 250 to 600 microliters per liter indicated that a substantial nonstomatal inhibition of CER had occurred. Although the sensitivity of g(l) to water limitation in cowpea suggested a dehydration avoidance response, parallel measurements of CER at various C(i) and PFD indicated that photosynthetic activity of cowpea mesophyll was substantially inhibited by the water-limited treatment.