The activity of nitrate reductase and the pool sizes of some amino acids and some sugars in water-stressed maize leaves.

The activity of nitrate reductase and the pool sizes of some amino acids and some sugars were measured in relation to the leaf water potential (ψ) of maize leaves. The activity of nitrate reductase was severely inhibited in water-stressed maize leaves. This was not due to substrate shortage or the presence of an inhibitor at reduced leaf water potential. While the typical proteinogenic amino acids valine, tyrosine, leucine and isoleucine were almost undetectable in the leaves of the control plants, their concentrations markedly increased with declining ψ, thus indicating protein degradation. The concentrations of serine, glycine and glutamate increased upon water stress, their total amount in severely stressed leaves ranging 5- to 6-fold higher than the total amount of valine, tyrosine, leucine and isoleucine at this stage of water deficit. The pool sizes of glucose, fructose and sucrose decreased in relation to decreasing ψ. The total amount of organic solutes remained almost constant at least up to a ψ of approx.-1.0 MPa and then dropped to about 50% when ψ reached -1.25 MPa.