This study aas conducted to determine whether diurnal fluctuations of nitrate reductase activity in two cotton species, Pima (Gossypium barbadense L.) and Acala (Gossypium hirsutum L.), are regulated by NO3- uptake or leaf NO3- concentration. The two species differ greatly in their N use efficiency. The seedlings were grown for 25 d in nutrient solutions containing 0.05, 0.10 or 1.0 mM NO3- under a 14-h light/10-h dark cycle at 30 degreesC/20 degreesC. Uptake rates were measured by following NO3- depletion from the uptake solutions at 20 degreesC or 30 degreesC. Nitrate reductase activity (NRA) in fully expanded first true leaves was determined by an anaerobic method in vivo. Upon illumination uptake rates for both species, measured at 30 degreesC, increased until they plateaued in about 5 h and were maintained at that level for the remainder of the light period and the following dark cycle. When measured at 20 degreesC in the dark, however, the uptake rates decreased 25 to 30% for the first 3 h and then remained constant. Leaf NRA increased rapidly during the first hour of illumination, reached a plateau and then decreased after 4 to 6 h of illumination. The decline was more rapid when NRA was assayed in the absence of NO3-. Tn darkness, NRA levels were low and did not fluctuate, Upon illumination, leaf NO3- concentration also increased for about 7 h and then decreased gradually The results indicate that (i) the diurnal rhythm of NO3- uptake is modulated by temperature rather than by light/ dark transitions, and (ii) while the increase in NRA upon illumination may he regulated by NO3- flux from roots to shoots, the decrease in NRA under prolonged illumination is independent of that flux.
