Influx and Efflux of Nitrate and Ammonium in Italian Ryegrass and White Clover Roots: Comparisons Between Effects of Darkness and Defoliation

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Seedlings of Italian ryegrass (Lolium multiflorum Lam. cv. RVP) and clonal stolon cuttings of white clover (Trifolium repens L. cv. Blanca) were grown for 19 d in flowing solution culture, with N supplied as either 250 mmol m ~3 NO3 or NH4 . Rates of net uptake, influx and translocation of NO3 and NH4 were then determined using 15N and 13N labelling techniques: between 3-5 h into the photoperiod following 8 h darkness for white clover (CL), and for ryegrass plants that were either entire (IL) or with shoots excised 90 min prior to 13N influx (IC); and 75 min into the photoperiod following 37-39 h darkness for ryegrass (ID). Rates of net uptake, influx and efflux of NH4 exceeded those of NO3 in IL and IC ryegrass plants: the opposite occurred in white clover (CL). The decrease in net uptake following defoliation of ryegrass was greater for NH4 (62%) than NO3 (40%). For NH4 this was associated with a large decrease in influx from 110 to 6 0 /.i.mol h1 g-1 root fr. wt.; but for NOJ, influx only decreased from 4-2 to 3-7 /xmol h1 g-1. Prolonged exposure to darkness (ID plants) also lowered net uptake of NO3 and NH4 by, respectively, 86% and 95% of IL levels. For NH4 this was characterized by a large decrease in influx and a small decrease in efflux; whilst for NO3 the effect of a large decrease in influx was reinforced by a smaller increase in efflux. The data were used to estimate the translocatory fluxes of NO J (0-3-2-0 jumol h_1 g1) and NH4 (0-03-0-4 |U.mol h 1 g-1), assimilation in the roots of NO3 (0-2-2-6/xmol h_1 g_1) and NH4 (0-5-8-9 /xmol h~ 1 g-1), and the concentrations of NOj (9-15 mol m-3) in the cytoplasmic compartment of the roots. The relevance of variable influx and efflux to models for the regulation of N uptake is discussed.