Nitrate Uptake and Assimilation following Nitrate Deprivation

Upon first exposure to NO3-⁠, the NO3- uptake and reduction capacities of dark-grown corn (Zea mays L.) roots are initially low, but increase markedly within 6 h. The development of the accelerated NO3- uptake rate appears to be substrate ‘induced’ as is NO3- reductase (NR), the first enzyme in the NO3- assimilatory pathway. However, the ‘induction’ of NO3- uptake is independent of NR induction. The effect of NO3- deprivation was studied to determine the role of endogenous NO3-14 on subsequent NO3-15 uptake and reduction. Corn roots were ‘induced’ for 24 h in 0–5 mol m−3NO3-14 nutrient solution and then exposed for 0 to 32 h to NO3--free nutrient solution. Uptake and reduction of NO3- were determined periodically by exposing sets of roots to a 1 h pulse of 0.5 mol m−3NO3-15⁠. Neither NO3-15 uptake (4.57 μmol root−1 h−1) nor the percentage of absorbed NO3-15 reduced (27%) was changed significantly (P⩽ 0.05) by exogenous NO3- deprivation. However, the estimated ‘induced’ component of NO3-15 uptake decreased significantly (50% after 32 h). Concurrently, the ‘non-induced’ basal component of NO3- uptake increased. Previously accumulated NO3-14 decreased from 23 to 4.5 μmol root−1 after 32 h of exogenous NO3- deprivation. Nearly equivalent quantities of endogenous NO3-14 were used for translocation and reduction during NO3- deprivation. During each 1 h NO3-15 pulse, the amounts of NO3-14 translocation and net efflux of NO3-14 to the uptake solution were similar. Net efflux of NO3-14 was strongly correlated (r = 0.991) to the amount of endogenous NO3-14⁠. The remaining endogenous NO3-14 and its rate of utilization were apparently sufficient to minimize a rapid decline or complete loss in both the ‘induced’ NO3- uptake state and the rate of in vivoNO3- assimilation.