Diurnal Fluctuations of Nitrate Uptake and In Vivo Nitrate Reductase Activity in Pima and Acala Cotton

This study was 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 NO − 3 uptake or leaf NO − 3 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 m M NO − 3 under a 14‐h light/10‐h dark cycle at 30°C/20°C. Uptake rates were measured by following NO − 3 depletion from the uptake solutions at 20°C or 30°C. 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°C, 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°C 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 NO − 3 In darkness, NRA levels were low and did not fluctuate. Upon illumination, leaf NO − 3 concentration also increased for about 7 h and then decreased gradually. The results indicate that (i) the diurnal rhythm of NO − 3 uptake is modulated by temperature rather than by light/dark transitions, and (ii) while the increase in NRA upon illumination may be regulated by NO − 3 flux from roots to shoots, the decrease in NRA under prolonged illumination is independent of that flux.