Effects of water stress on the gas exchange, the activities of some enzymes of carbon and nitrogen metabolism, and on the pool sizes of some organic acids in maize leaves.

Water-stressed maize (Zea mays L.) leaves showed a large decrease in leaf conductance during photosynthesis. Net CO2 uptake and evaporation declined fast at mild stress (ψ=-0.6 to -1.0 MPa) and slower at more severe stress (ψ=-1.0 to -1.2 MPa), whereas the CO2 concentration in the intercellular spaces (Ci) did not drop to the CO2 compensation point. The activities of the enzymes of photosynthetic carbon metabolism tested in this study dropped by approx. 30% at ψ=-1.2 MPa. Glutamine synthetase activity was unaffected by water stress, whereas the activity of nitrate reductase was almost completely inhibited. The decline of enzyme activities in relation to ψ was correlated with a concomitant decrease in the content of total soluble protein of the stressed leaves. The total leaf pools of malate, pyruvate and oxaloacetate decreased almost linearly in relation to ψ, thus obviously contradicting the almost constant Ci. In comparison to the controls (ψ=0.6 MPa) the content of citrate and isocitrate increaed markedly at ψ=-0.9 MPa and decreased again at ψ=-1.2 MPa.