Influence of some internal and external conditions on the induction of nitrate reductase in rice seedlings

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Induction of nitrate reductase (NR) in 7-day-old rice seedlings was depressed when endosperms were removed. NR activity in the seedlings from which endosperms were removed (deendospermed seedlings), reached a maximum 6 hr after supplying NaNO_3 and then gradually decreased. That in intact seedlings continued to increase for 12 hr, and then decreased fairly rapidly. Sucrose (30 mM concentration) supplied exogenously to deendospermed seedlings raised NR activity to the level of the intact seedlings. Macronutrients added exogenously did not show such an effect. NR activity in deendospermed seedlings placed in the dark was extremely low. However, in the presence of exogenous sucrose, the activity was raised to the same level as that in the light in the absence of exogenous sucrose. This suggests that sucrose substitutes for light in the induction of NR in deendospermed seedlings. Protein inhibitors suppressed NR induction when the plants were fed continuously with nitrate solution containing the inhibitor. In cases where the plant roots were immersed in inhibitor solutions for 2 hr before transfer to nitrate solution, only chloramphenicol promoted and the others inhibited NR induction. NR induction was also suppressed by respiratory inhibitors, of which sodium azide was very potent.
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