TIME-COURSE OF NITRATE UPTAKE AND NITRATE REDUCTASE-ACTIVITY IN NITROGEN-DEPLETED DWARF BEAN

The rate of nitrate uptake by N‐depleted French dwarf bean (Phaseolus vulgaris L. cv. Witte Krombek) increased steadily during the first 6 h after addition of NO3 ‐After this initial phase the rale remained constant for many hours. Detached root systems showed the same time‐course of uptake as roots of intact plants. In vivo nitrate reductase activity (NRA) was assayed with or without exogenous NO3‐ in the incubation medium and the result ing activities were denoted potential and actual level, respectively. In roots the difference between actual and potential NRA disappeared within 15 min after addition of nitrate, and NRA increased for about 15 h. Both potential and actual NRA were initially very low. In leaves, however, potential NRA was initially very high and was not affected by ambient nitrate (0.1–5 mol m‐3) for about 10 h. Actual and potential leaf NRA became equal after the same period of time. In the course of nitrate nutrition, the two nitrate reductase activities in leaves were differentially inhibited by cycloheximide (3.6 mmol m‐3) and tungstate (1 mol m‐3). We suggest that initial potential NRA reflects the activity of pre‐existing enzyme, whereas actual NRA depends on enzyme assembly during NO3‐ supply. Apparent induction of nitrate uptake and most (85%) of the actual in vivo NRA occurred in the root system during the first 6 h of nitrate utilization by dwarf bean. Copyright © 1979, Wiley Blackwell. All rights reserved