Diurnal rhythm in nitrate reductase activity of Coffea arabica L. shoots and roots

Abstract

The distribution of nitrate reductase activity in the shoots and roots of coffee plants was studied according to the time of the day associated with the availability of nitrate and carbohydrates in the tissues. In the leaves, the enzyme rate declined gradually in the light, fol-lowed by an increase during the night period. The diurnal rhythm corresponded to the variations in the nitrate concentration in the xylem sap during the photoperiod. The nitrate reductase activity in the roots showed a little increase during the day, coinciding with increase in the levels of sol-uble sugars and nitrate in the organic nitrogen concentration in the xylem sap. The greatest con-tribution of the roots to the nitrate reduction occurred in the light while the leaves showed a greater contribution during the night. However, in a 24-hour cycle, the total average contribution was approximately identical both for roots and for leaves.