Relationship between nitrate reductase activity and level of soluble carbohydrates under prolonged darkness in MM.106 apple leaves

Abstract

Changes of specific soluble carbohydrates, nitrate reductase activity (NRA), and nitrate levels were monitored under prolonged darkness in MM.106 apple leaves (Malus dom- estica Borkh.). The prolonged darkness significantly reduced the level of total soluble carbohydrates, mainly due to the decrease in sorbitol. The prolonged darkness also greatly reduced NRA and consequently resulted in the accumulation of nitrate. The NRA correlated well with nitrate, total soluble carbohydrates, and sorbitol. The addition of 100 mM sorbitol, glucose or 50 mM sucrose to the in vivo nitrate reductase (NR) assay medium significantly increased the NRA of dark-pretreated leaves by 30 to 39%. These observations are consistent with the hypothesis that soluble carbohydrates produced photosyn- thetically provide the reductant and carbon skeletons required for the reduction of nitrate. However, the rate of decrease in NRA was much greater than that of decrease in the level of soluble carbohydrates and the NRA of dark pretreated leaves was not fully recovered to the level of the leaves, which had been subjected to 16 h light and 8 h dark, by the addition of soluble carbohydrates to the in vivo NR assay medium. It thus appears that there are other possible factors, in addition to the soluble carbohydrates, responsible for the decrease in NRA under the prolonged darkness.