Development of Nitrate Reductase Activity in Green Tissues of Soybean Seedlings (Glycine max. L. Merr.)

Abstract

The pattern of in vivo activity of nitrate reductase was determined in chlorophyllous tissues of soybean seedlings cultivated under constant temperature and continuous light. Enzyme activity was detected in the cotyledons after approximately 20 hours of light exposure, the values being proportional to the concentration of nitrate in the irrigating solutions. In cotyledons watered with 15 mM of nitrate the peak of activity was reached after 120 hours of illumination. Chlorophyll accumulation preceded and approximately paralleled nitrate reductase activity in the cotyledons indicating that nitrate reduction is associated with the development of anabolic processes. Nitrate reductase activity in light-grown cotyledons was inhibited by cycloheximide when the inhibitor was added at the beginning of the illumination period. The effect progressively disappeared as the tissues were treated at later stages after having being exposed to light. On the other hand, actinomycin D did not affect enzyme activity, suggesting that the process does depend in part on de novo protein synthesis but does not require RNA synthesis. The appearance of enzyme activity in the primary leaves coincided with the time of maximum activity in the cotyledons. By the time nitrate reductase activity started decreasing in the primary leaves, the first trifoliate leaf showed increasing enzyme activity. The patterns determined in leaves showed low values of activity in the early stages of development, reached a peak when the leaves were almost expanded and declined thereafter. Maximum nitrate reductase activities were calculated as 2.3, 3.2 and 2.2 times higher in leaves infiltrated with nitrate than in the controls without exogenous nitrate, in the cotyledons, primary and first trifoliolate leaves, respectively.