The nitrogen balance of Raphanus sativus X raphanistrum plants. I. Daily nitrogen use under high nitrate supply

Abstract Growth‐chamber cultivated Raphanus plants 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⁻³ m⁻³ concentration of NO⁻₃ in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO⁻₃ uptake, NO⁻₃ fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by an in vivo test. NO⁻₃ uptake by roots and mesophyll cells was greater in the light than in the dark. The NO⁻₃ concentration 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⁻₃ was 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⁻₃ influx was turned into organic nitrogen, with the remaining NO⁻₃ accumulating in both the light and the dark.