Carbon Assimilation, Water Use Efficiency And Photochemical Activity In Xerophytes From Semiarid Ecosystems Of Venezuela

This work compares, with values already published and others taken for this purpose, the ecophysiological characteristics (water potential, Psi, photosynthetic rate, A; stomatal conductance, g(s); water use efficiency, WUE; integrated daily carbon gain, A(D); and PSII relative quantum efficiency, Phi(PSII)) in xerophytes of different life forms and photosynthetic metabolisms in four northern coastal locations of Venezuela in the rainy and dry seasons. Water potential of all species varied between -0.3 and -4.8 MPa. The highest A (22 mu mol m(-2) s(-1)) and g s (0.4 mol m(-2) s(-1)) were found during the rainy season in Calotropis procera while the lowest (1 mu mol m(-2) s(-1), A and 0.03 mol m(-2) s(-1), g(s)) corresponded to Cissus trifoliata during the dry season. In all life forms A was highly correlated to g(s). Daily carbon gain in most of the studied species may be estimated from maximum photosynthetic rate (A(max)). In deciduous life forms, WUE was reduced with drought, whereas it did not change in evergreen trees and shrubs and decreased in herbs. Drought reduced F PSII while non-photochemical quenching (NPQ) increased, suggesting down-regulation of the photochemical system.