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Environmental and Stress Physiology

Carbon Dioxide and Light Responses of Photosynthesis in Cowpea and Pigeonpea during Water Deficit and Recovery ¹

Department of Plant Science and Biochemistry, University of the West Indies, St. Augustine, Trinidad, Department of Agronomy, Cornell University, Ithaca, New York 14853

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 CO₂ exchange rate (CER), leaf diffusive conductance to CO₂ (g_L), and CO₂ concentration in the leaf intercellular air space (C_i) were determined at various CO₂ 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 (_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 _w was –1.9 megapascal. After 8 days of withholding water, _w at midafternoon declined to –0.9 and –2.4 megapascals in cowpea and pigeonpea, respectively. The solute component of water potential (_s) decreased substantially less in cowpea than pigeonpea. Photosynthetic CER at saturation photon flux density (PFD) and ambient external CO₂ concentration (360 microliters per liter) on day 5 of withholding decreased by 83 and 55% in cowpea and pigeonpea, respectively. When measured at external, CO₂ 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.

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