Exchange rates of CO2 and H2O and metabolism of hydrogen peroxide have been measured in leaves of alfalfa ev. Aragón) under drought stress. The inhibitory effect of drought upon photosynthesis depended on the severity of the stress treatment. Leaf water potential (Ψleaf) down to,-2.8 MPa reduced CO2 availability due to stomatal closure and inhibited the rate of photosynthesis. Leaf water potential lower than,-2.8 MPa directly affected CO2 fixation, although CO2 was not limiting. Transpiration was more affected by stornatal closure than photosynthesis, which led to am apparent improvement in WUE (water use efficiency). Alfalfa leaves with Ψleaf lower than,-2.0 MPa had an increased quantum requirement, probably due to the severe stress effect on photoenergetic reactions.
Ethylene evolution from alfalfa leaves increased when they were subjected to Ψleaf of,- 1.6 MPa. Under more severe stress, the leaves showed low or almost no ethylene production. In parallel with the increase in ethyiene production, alfalfa leaves exhibited an increased membrane lipid peroxidation index (maloridialdehyde content) and an increased peroxide content. Superoxide disinutase activity (SOD; EC 22.214.171.124) was not affected by drought stress. Catalase (EC 126.96.36.199) was inhibited at slight stress, but significantly increased at a Ψleaf of -2.0 MPa. Peroxidase (EC 188.8.131.52) was progressively inhibited as drought stress developed. The possible implication of reactive O2 intermediates in drought stress-induced senescence of alfalfa leaves is discussed in the light of the pattern of enzymatic scavenging systems.