The nitrogen balance ofRaphanus sativusXraphanistrumplants. I. Daily nitrogen use under high nitrate supply

Abstract

AbstractGrowth‐chamber cultivatedRaphanusplants accumulate nitrate during their vegetative growth. After 25 days of growth at a constant supply to the roots of 1 mol m−3(NO−3) in a balanced nutrient solution, the oldest leaves (eight‐leaf stage) accumulated 2.5% NO−3‐nitrogen (NO3‐N) in their lamina, and almost 5% NO3‐N in their petioles on a dry weight basis. This is equivalent to approximately 190 and 400 mol−3m−3concentration of NO−3in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO−3uptake, NO−3fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by anin vivotest. NO−3uptake by roots and mesophyll cells was greater in the light than in the dark. The NO−3concentration in the xylem fluid was constant with leaf age, but showed a distinct daily variation as a result of the independent fluxes of root uptake, transpiration and mesophyll uptake. NO−3was reduced in the leaf at a higher rate in the light than in the dark. The reduction was inhibited at the high concentrations calculated to exist in the mesophyll vacuoles, but reduction continued at a low rate, even when there was no supply from the incubation medium. Sixty‐four per cent of the NO−3influx was turned into organic nitrogen, with the remaining NO−3accumulating in both the light and the dark.