Distribution of nitrate reductase in ageing bean seedlings

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Abstract
The in vivo activity of nitrate reductase (NR, E.C. 1.6.6.1) in the roots, stem and leaves of bean (Phaseolus vulgaris L.) was measured at different ages of seedlings. The leaves always had higher levels of the enzyme than the roots or stem. The level of the enzyme in the very young leaves were low, increasing to a maximum by day 10 to 11 of seedling growth at 26°C, after which it start to decline. The level of the enzyme in 7 dayold decotyledonized leaves was about 2.5 times higher than that in leaves from intact seedlings. A supply of exogenous nitrate caused a considerable increase in the total organic nitrogen in the leaf only after day 9, when the nitrogen supply from the cotyledons presumably is low.