Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: I. Regulation by Nitrate Flux

The roles that leaf nitrate content and nitrate flux play in regulating the levels of nitrate reductase activity (NRA) were investigated in 8- to 14-day old maize (Zea mays L.) plants containing high nitrate levels while other environmental and endogenous factors were constant. The nitrate flux of intact plants was measured from the product of the transpiration rate and the concentration of nitrate in the xylem. NRA decreased when the seedlings were deprived of nitrate. The nitrate flux and the leaf nitrate content also decreased. When nitrate was resupplied to the roots, all three parameters increased.Attempts to alter the nitrate flux by varying transpiration rates were unsuccessful due to a relatively constant rate of delivery of nitrate to the xylem as transpiration rates fell. However, cooling the roots resulted in a decrease in the nitrate flux. Plants with a lower nitrate flux rapidly lost NRA, although the leaf nitrate content was initially unaffected. If the roots remained cool for a long enough time, the leaf nitrate content eventually decreased. Rewarming the roots increased the nitrate flux, leaf nitrate content, and NRA to control levels. When the nitrate flux in excised shoots was varied in three separate ways, decreasing the nitrate flux to the leaves resulted in a rapid decrease in NRA, although leaf nitrate contents were unchanged.These experiments show that the nitrate flux to the leaves from the roots plays a much larger regulatory role than the leaf nitrate content in controlling the level of NRA in intact plants.