Seasonal changes in photosynthesis and stomatal conductance of five plant species from a semiarid ecosystem

In order to determine whether stomatal closure alone regulates photosynthesis during drought under natural conditions, seasonal changes in leaf gas exchange were studied in plants of five species differing in life form and carbon fixation pathway growing in a thorn scrub in Venezuela. The species were: Ipomoea carnea, Jatropha gossypifolia, (C3 deciduous 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 precipitation pattern, being more closely governed by soil water content in I. carnea and A. crucis. Maximum rate of photosynthesis, P-max, decreased 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 to -2.0 MPa, since stomatal conductance (g(s)) was sensitive to water stress, and stomatal limitation on P-N increased with drought. In A. crucis, P-N decreased by 90 % at a much lower Psi (-9.3 MPa), and g(s) was relatively less sensitive to Psi. In P. juliflora and C. odoratissima, P-max, g(s), and intercellular CO2 concentration (C-i) were independent of soil water content. In the C3 shrubs stomatal closure was apparently the main constraint on photosynthesis during drought, C-i declining with Psi in I. carnea. In the C4 herb, C-i was constant along the range of Psi values, which suggested a coordinated decrease in both g(s) and mesophyll capacity. In P. juliflora C-i showed a slow decrease with Psi which may have been due to seasonal leaf developmental changes, rather than to soil water availability.