Differential Light Induction of Nitrate Reductases in Greening and Photobleached Soybean Seedlings

Abstract

Soybean (G. max (L.) Merr.) seeds were imbibed and germinated with or without NO3-, tungstate and norflurazon (San 9789). Norflurazon is a herbicide which causes photobleaching of chlorophyll by inhibiting carotenoid synthesis and which impairs normal chloroplast development. After 3 days in the dark, seedlings were placed in white light to induce extractable nitrate reductase activity. The induction of maximal nitrate reductase activity in greening cotyledons did not require NO3- and was not inhibited by tungstate. Induction of nitrate reductase activity in norflurazon-treated cotyledons had an absolute requirement for NO3- and was completely inhibited by tungstate. Nitrate was not detected in seeds or seedlings which had not been treated with NO3-. The optimum pH for cotyledon nitrate reductase activity from norflurazon-treated seedlings was at pH 7.5, and near that for root nitrate reductase activity, whereas the optimum pH for nitrate reductase activity from greening cotyledons was pH 6.5. Induction of root nitrate reductase activity was also inhibited by tungstate and was dependent on the presence of NO3-, further indicating that the isoform of nitrate reductase induced in norflurazon-treated cotyledons is the same or similar to that found in roots. Nitrate reductases with and without a NO3- requirement for light induction appear to be present in developing leaves. In vivo kinetics (light induction and dark decay rates) and in vitro kinetics (Arrhenius energies of activation and NADH:NADPH specificities) of nitrate reductases with and without a NO3- requirement for induction were quite different. Km values for NO3- were indentical for both nitrate reductases.