High transpiration rates were found to affect the photosynthetic capacity ofXanthium strumariumL. leaves in a manner analagous to that of low soil water potential. The effect was also looked for and found inGossypium hirsutumL.,Agathis robusta(C. Moore ex Muell.) Bailey,Eucalyptus microcarpaMaiden,Larrea divaricataCav., the wiltyflaccatomato mutant (Lycopersicon esculentum(L.) Mill.) andScrophularia desertorum(Munz) Shaw. Two methods were used to distinguish between effects on stomatal conductance, which can lower assimilation by reducing CO2availability, and effects on the photosynthetic capacity of the mesophyll. First, the response of assimilation to intercellular CO2pressure (Ci) was compared under conditions of high and low transpiration. Second, in addition to estimatingCiusing the usual Ohm's law analogy,Ciwas measured directly using the closed-loop technique of T.D. Sharkey, K. Imai, G.D. Farquhar and I.R. Cowan (1982, Plant Physiol,60, 657–659). Transpiration stress responses ofXanthium strumariumwere compared with soil drought effects. Both stresses reduced photosynthesis at highCibut not at lowCi; transpiration stress increased the quantum requirement of photosynthesis. Transpiration stress could be induced in small sections of leaves. Total transpiration from the plant did not influence the photosynthetic capacity of a leaf kept under constant conditions, indicating that water deficits develop over small areas within the leaf. The effect of high transpiration on photosynthesis was reversed approximately half-way by returning the plants to low-transpiration conditions. This reversal occurred as fast as measurements could be made (5 min), but little further recovery was observed in subsequent hour
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