We examined nitrate assimilation and root gas fluxes in a wildtype barley (Hordeum vulgare L. cv Steptoe), a mutant (narla) deficient in NADH nitrate reductase, and a mutant (narla;nar7w) deficient in both NADH and NAD(P)H nitrate reductases. Estimates of in vivo nitrate assimilation from excised roots and whole plants indicated that the narla mutation influences assimilation only in the shoot and that exposure to N03-induced shoot nitrate reduction more slowly than root nitrate reduction in all three genotypes.When plants that had been deprived of nitrogen for several days were exposed to ammonium, root carbon dioxide evolution and oxygen consumption increased markedly, but respiratory quotient-the ratio of carbon dioxide evolved to oxygen consumed did not change. A shift from ammonium to nitrate nutrition stimulated root carbon dioxide evolution slightly and inhibited oxygen consumption in the wild type and narla mutant, but had negligible effects on root gas fluxes in the narla;nar7w mutant. These results indicate that, under NH4' nutrition, 14% of root carbon catabolism is coupled to NH4' absorption and assimilation and that, under N03-nutrition, 5% of root carbon catabolism is coupled to N03-absorption, 15% to N03-assimilation, and 3% to NH4+ assimilation. The additional energy requirements of N03-assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH4' and N03-absorption and assimilation constitute a significant portion of root respiration.Nitrogen assimilation is among the most energy-intensive processes in plants, requiring the transfer of two electrons per N03 converted to NO2, six electrons per N02 converted to NH4', and two electrons and one ATP per NH4' converted to glutamate. To provide sufficient electrons for these reactions, plants may divert reductant from mitochondrial electron transport. During dark N03-assimilation, shoots of a higher plant (8) and algae (18, 29) evolved CO2 significantly faster than they consumed O2, presumably because the TCA cycle or the oxidative pentose phosphate pathway catabolized substrates and transferred some electrons to N03-and N02-rather than to O2. These results indicate that, in the dark, shoots expend up to 25% of their respiratory energy on nitrogen assimilation (8).Plants from 5 to 95% of the N03-absorbed from the rhizosphere (1, 20). Estimates of root nitrogen acquisition and the associated energy transfers have been limited (2, 4, 5, 10, 12, 14-16, 23, 26, 27), and these could not distinguish among expenditures for tissue maintenance, root growth, NH4' and NO3-absorption, and NH4' and N03-assimilation. Root respiration is usually determined from net 02 uptake, yet N03-, N02-, and NH4' can substitute for 02 as electron acceptors during nitrogen assimilation. Root carbon catabolism might be a more pertinent measure, but analysis of dissolved CO2 has required discontinuous sampling (24,29) or elevated CO2 concentrations (17). The present study employed an instrumentation system that simultaneously mo...

Studies that quantify plant δ15N often assume that fractionation during nitrogen uptake and intra‐plant variation in δ15N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetum Mill. cv. T‐5) at NH4+ or NO−3 concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole‐plant δ15N was not significantly different from source δ15 N for plants grown on either nitrogen form. No intra‐plant variation in δ15N was observed for plants grown with NH+4. In contrast. δ15N of leaves was as much as 5.8% greater than that of roots for plants grown with NO−3. The contrasting patterns of intra‐plant variation are probably caused by different assimilation patterns. NH+4 is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO−3 assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ15N of NO−3 to become enriched relative to organic nitrogen; the δ15N of NO−3 was 11.1 and 12.9% greater than the δ15N of organic nitrogen in leaves and roots, respectively. Leaf δ15N may therefore be greater than that of roots because the NO−3 available for assimilation in leaves originates from a NO−3 pool that was previously exposed to nitrate assimilation in the root.

In barley (Hordeum vulgare L. cv Steptoe) seedlings, the time course for induction of root nitrate absorption varied significantly with pretreatment. Net nitrate uptake of nitrogen-deprived plants more than doubled during the 12 hours after first exposure to nitrate. For these plants, gentle physical disturbance of the roots inhibited net nitrate absorption for more than 6 hours and potassium absorption for 2 hours. Pretreatment with ammonium appeared sufficient to induce nitrate absorption; plants either grown for 2 weeks on or exposed for only 10 hours to a medium containing ammonium as a sole nitrogen source showed high rates of net nitrate uptake when first shifted to a medium containing nitrate. Gentle physical manipulation of these plants inhibited nitrate absorption for 2 hours and potassium absorption for more than 12 hours. These results indicate (a) that experimental protocols should avoid physical manipulation of the roots whenever possible and (b) that ammonium or a product of ammonium assimilation can induce nitrate absorption.The presence of NO3-in the rhizosphere induces root absorption of NO3-(10, 11, 14, for reviews). For example, the barley cultivar Steptoe (Hordeum vulgare L.) showed a sixfold increase in the rate of net NO3-uptake during the 12 h after first exposure to ical manipulation diminishes calcium absorption (23), phosphate absorption and energy charge (13), or root pressure and exudation (20). To minimize transplant shock, we placed the roots of an intact plant in a cuvette at least 10 h before measurements began and shifted from one medium to another by changing the solution that flowed through the cuvette (3). Most studies of NO3-induction of NO3-absorption have initiated the N03--induction period by physically transferring plants from one solution to another, a procedure that might produce transplant shock. In this study, we examined whether transplant shock might influence the induction of NO3-absorption.The absorption of counterions such as K+ and NH4' affect the induction of NO3-absorption (18, 24). For example, absorption of K+ moderated the inhibitory effects of NH4' on this process (24). We thus monitored K+ and NO3-absorption concurrently during the following experiments. MATERIALS AND METHODSBarley seeds (Hordeum vulgare L. cv Steptoe) were surfaced sterilized for 1 min in 2.6% NaClO, thoroughly washed with distilled water, and germinated on wet toweling. Six 3-d-old seedlings were suspended above light-tight root boxes holding 2 L of an aerated nutrient solution. The solution consisted of 2.5 mm CaSO4, 10 mm KH2PO4, 5.0 mM MgSO4, micronutrients and iron as described previously (3), and contained either 0.6 mM NH4NO3 for the NH4' and NO3-absorption kinetic experiments, 0.6 mm KNO3 for the control treatment, or 0.6 mM NH4H2PO4 for the treatments designated as NH4+-grown and NH4+-tested. The pH of the solution was adjusted to 6.0 with H2SO4 or KOH. Each day, the root box was replaced with one that had been sterilized with isopropyl alcohol and filled with fresh nu...