Attributes of the nitrogen uptake systems of maize (Zea mays L.): maximal suppression by exposure to both nitrate and ammonium

summary Higher plants arc commonly exposed to rapid changes in the amounts and proportions of ammonium and nitrate in their root environment. Some of the potential consequences of such changes have been determined by examining the suppressive effects of growing maize (Zea mays L.) seedlings with a high concentration (3 mM) of nitrogen on the subsequent uptake of nitrogen from a more dilute (0.2 mM) solution. To document the interactive role of nitrate and ammonium supply in suppression, ratios of these ions in the growth solution were maintained at 3:0, 2:1, 1:2 and 0:3. Rates of nitrate and ammonium uptake were then measured during initial (0–1 h) exposure to 0.2 mM KNO3 or NH4NO3 and hourly during a subsequent 7 h period of adaptation to the dilute solutions. Maximal suppression of nitrate uptake occurred only when both nitrate and ammonium were present (2: 1 and 1:2) during prior growth. Nitrate uptake rates increased two‐ to three‐fold as the seedlings adapted to the dilute KNO3 solution, but the presence of ammonium with nitrate in the prior growth solution restricted the rate and extent of recovery from suppression. Recovery was further restricted when ambient ammonium was present during the adaptation period. Neither the magnitude of suppression nor the rate and extent of recovery was readily explained by (a) the proportion of nitrate and ammonium present during prior growth, (b) carbohydrate concentrations in root and shoot tissues, (c) concentrations of nitrate and ammonium in the root tissue, or (d) indirect effects of ammonium on potassium uptake. Total nitrate reduction by the entire plant in the 8 h adaptation period decreased as the proportion of ammonium in the prior growth solution increased. Ambient ammonium restricted nitrate reduction only in seedlings previously grown entirely with ammonium. Thus the effect of ambient ammonium on nitrate reduction by the whole plant differed substantially from that on nitrate uptake. Since nearly all the ammonium taken up by the roots was assimilated, as well as that generated by nitrate reduction, it appears that carbohydrate supply did not limit the observed uptake of ammonium and nitrate. The accumulation in roots of both nitrate and specific metabolites of ammonium serving as negative electors could account for most of the observations.