Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: II. Regulation by Nitrate Flux at Low Leaf Water Potential.

Experiments were conducted to determine whether the nitrate flux to the leaves or the nitrate content of the leaves regulated the nitrate reductase activity (NRA) in leaves of intact maize (Zea mays L.) seedlings having low water potentials (psi(w)) when other environmental and endogenous factors were constant. In seedlings that were desiccated slowly, the nitrate flux, leaf nitrate content, and NRA decreased as psi(w) decreased. The decrease in nitrate flux was caused by a decrease in both the rate of transpiration and the rate of nitrate delivery to the transpiration stream. Upon rewatering, the recovery in NRA was correlated with the nitrate flux but not the leaf nitrate content.Recovery depended on protein synthesis, since recovery could be prevented in excised leaves if an inhibitor of protein synthesis was present. However, it also depended on a high nitrate flux, since recovery could be prevented if there was no nitrate flux, despite a relatively high, constant leaf nitrate content, a high psi(w), and the absence of an inhibitor of protein synthesis.The synthesis of NRA could be increased at low psi(w) if seedlings were desiccated in the presence of additional nitrate, which increased the nitrate flux to the leaves. Since the decrease in NRA at low psi(w) could be relieved by increasing the nitrate flux and recovery also depended on nitrate flux, the inhibition of NRA at low psi(3) was not controlled by a direct effect of psi(w) on protein synthesis nor by alterations in the leaf nitrate content, but rather by a decrease in the nitrate flux that in turn regulated the synthesis of the enzyme.