Photosynthesis of oak leaves under water stress: maintenance of high photochemical efficiency of photosystem II and occurrence of non-uniform CO(2) assimilation.

Net CO(2) assimilation rates (A), stomatal conductance to water vapor (g(w)), photosynthetic O(2) evolution in 5% CO(2) (A(max)) and photochemical efficiency of photosystem II were monitored in leaves of young oak saplings (Quercus petraea Matt. Liebl.) in response to increasing drought. Both A and g(w) declined rapidly as soon as predawn leaf water potential dropped below -1.0 MPa. The calculated intercellular concentration of CO(2) first declined and then increased again as drought intensity increased, suggesting that both stomatal closure and a decreased ability of mesophyll chloroplasts to fix available CO(2) were involved in the drought-induced reductions in A and g(w). However, this assumption was not supported by the observations that, with increasing drought, the decline in A(max) was limited, and the photochemistry of photosystem II and the quantum yield of light-driven electron transport remained stable. Autoradiograms of (14)CO(2)-fed leaves revealed non-uniform assimilation rates during water stress. The consequences of a potential artifact induced by this patchiness on the calculation of intercellular CO(2) concentration are discussed.