SEASONAL-CHANGES IN PHOTOSYNTHESIS AND STOMATAL CONDUCTANCE OF 5 PLANT-SPECIES FROM A SEMIARID ECOSYSTEM

In order to determine whether stomatal closure alone regulates photosy nthesis during drought under natural conditions, seasonal changes in l eaf gas exchange were studied in plants of five species differing in l ife form and carbon fixation pathway growing in a thorn scrub in Venez uela. The species were: Ipomoea carnea, Jatropha gossypifolia, (C3 dec iduous shrubs), Alternanthera crucis (C4 deciduous herb), and Prosopis juliflora and Capparis odoratissima (evergreen phreatophytic trees). Xylem water potential (Psi) of all species followed very roughly the p recipitation pattern, being more closely governed by soil water conten t in I. carnea and A. crucis. Maximum rate of photosynthesis, P-max, d ecreased with Psi in I. carnea, J. gossypifolia, and A. crucis. In I. carnea and J. gossypifolia stomatal closure was responsible for a 90 % decline in net photosynthetic rate (P-N) as Psi decreased from -0.3 t o -2.0 MPa, since stomatal conductance (g(s)) was sensitive to water s tress, and stomatal limitation on P-N increased with drought. In A. cr ucis, P-N decreased by 90 % at a much lower Psi (-9.3 MPa), and g(s) w as relatively less sensitive to Psi. In P. juliflora and C. odoratissi ma, P-max, g(s), and intercellular CO2 concentration (C-i) were indepe ndent of soil water content. In the C3 shrubs stomatal closure was app arently the main constraint on photosynthesis during drought, C-i decl ining with Psi in I. carnea. In the C4 herb, C-i was constant along th e range of Psi values, which suggested a coordinated decrease in both g(s) and mesophyll capacity. In P. juliflora C-i showed a slow decreas e with Psi which may have been due to seasonal leaf developmental chan ges, rather than to soil water availability.