NITRATE REDUCTASE AND NITRITE REDUCTASE-ACTIVITY IN NITRITE-STRESSED AND CHLORATE-STRESSED RICE SEEDLINGS

Nitrite is a natural intermediate of nitrate assimilation in higher pl ants. However, because of the concerted action of nitrate reductase (N R, EC 1.6.6.1) and nitrite reductase (NIR, EC 1.7.7.1), normally it do es not appear in high concentration in the cytoplasm. An internal accu mulation of nitrite (caused by an external increase, or by herbicides) is toxic to plants, leading to extensive morphological and metabolic changes. In the reported experiments the effects of nitrite and chlora te (converted in vivo to chlorite, a slowly metabolizable, indirect mo del compound for nitrite) on the growth parameters and on the ill vivo measured NR and in vitro measured NiR activities were compared in 14- day-old seedlings of rice (Oryza sativa L., cv. Oryzella) to confirm t he use of the chlorate model in nitrite stress studies. The growth par ameters (fresh weight, root and shoot lengths), and the NR and NiR act ivities were tested in hydroponic cultures, chlorate or nitrite being applied through the roots, in a whole-plant system. The effect of chlo rate proved to be very similar to that of nitrite, but the higher mobi lity of chlorate in the xylem caused a more pronounced inhibition in t he shoots. Unlike nitrite toxicity the chlorate/chlorite effect is har dly alleviated by NIR. Both compounds decreased the NR activity, but i n the case of chlorate no induction was observed in the low concentrat ion range. NiR was insensitive to both ions.