Effects of norflurazon (San 9789) on light‐increased extractable nitrate reductase activity in soybean [Glycine max (L.) Merr.]seedlings

The effects of norflurazon (San 9789) on light‐increased extractable NADH nitrate reductase activity (NRA) in soybean seedlings were studied. Continuous white light (W) increased NRA steadily in root and cotyledonary tissues over a 5 d period. Morflurazon, a pyridazinone herbicide which causes chlorophyll bleaching in W, reduced the initial NRA induction rate in roots and cotyledons. However, in cotyledons of norfiurazon‐treated plants NRA increased at a more rapid rate than in the control after 24 h of W, with activity levels reaching three times those of control seedlings after 5 d. NRA induced by W in control and norflurazon‐treated cotyledons was fluence‐rate dependent. Continuous FR induced equal amounts of NRA in control and norflurazontreated tissues, suggesting that the superinduceable NRA of norflurazon‐treated plants under W is not phytochrome induced. The FR‐induced NRA of control and norflurazon‐treated cotyledons had pH optima of 6.6, but during development under W the pH optimum of control cotyledons changed from 6.3 to between 6.6 and 7.1. The pH optimum of the norflurazon‐induced NRA of the cotyledon under W was about 7.5. The NADH/NADPH NRA ratio after 4 d of W was 1.3 in control and 2.5 in norflurazontreated cotyledons. These data indicate that photosynthelic pigments are involved only secondarily in light‐induction of NRA in this system.