INFLUX AND EFFLUX OF NITRATE AND AMMONIUM IN ITALIAN RYEGRASS AND WHITE CLOVER ROOTS - COMPARISONS BETWEEN EFFECTS OF DARKNESS AND DEFOLIATION

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 N-15 and N-13 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 N-13 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 11.0 to 6.0-mu-mol h-1 g-1 root fr. wt.; but for NO3-, influx only decreased from 4.2 to 3.7-mu-mol h-1 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 NO3- (0.3-2.0-mu-mol h-1 g-1) and NH4+ (0.03-0.4-mu-mol h-1 g-1), assimilation in the roots of NO3- (0.2-2.6-mu-mol h-1 g-1) and NH4+ (0.5-8.9-mu-mol h-1 g-1), and the concentrations of NO3-(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.