Modal Logics of Space

Abstract

Net CO2 assimilation rates (A), stomatal conductance to water vapour (gw), photosynthetic O2 evolution in 5% CO2 (Amax) and photochemical efficiency of photosystem II were monitored in leaves of young oak (Quercus petraea) saplings in response to increasing drought. Both A and gw declined rapidly as soon as predawn leaf water potential dropped below -1.0 MPa. The calculated intercellular concentration of CO2 (ci) 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 CO2 were involved in the drought-induced reductions in A and gw. However, this assumption was not supported by observations that, with increasing drought, the decline in Amax was limited, and the photochemistry of photosystem II and the quantum yield of light-driven electron transport remained stable. Autoradiograms of 14CO2-fed leaves revealed non-uniform assimilation rates during water stress. It is concluded that, under low light conditions, water deficits imposed rapidly (during a few days) limit photosynthesis by reducing the supply of CO2 to chloroplasts as a result of reductions in stomatal conductance in response to soil drying. The use of calculated ci to support the hypothesis of non-structural limitation to net CO2 assimilation may be misleading because of stomatal 'patchiness', or changes in mesophyll conductance, or both.