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...

A custom oxygen analyzer in conjunction with an infrared carbon dioxide analyzer and humidity sensors permitted simultaneous measurements of oxygen, carbon dioxide, and water vapor fluxes from the shoots of intact barley plants (Hordeum vulgare L. cv Steptoe). The oxygen analyzer is based on a calciazirconium sensor and can resolve concentration differences to within 2 microliters per liter against the normal background of 210,000 microliters per liter. In wild-type plants receiving ammonium as their sole nitrogen source or in nitrate reductase-deficient mutants, photosynthetic and respiratory fluxes of oxygen equaled those of carbon dioxide. By contrast, wild-type plants exposed to nitrate had unequal oxygen and carbon dioxide fluxes: oxygen evolution at high light exceeded carbon dioxide consumption by 26% and carbon dioxide evolution in the dark exceeded oxygen consumption by 25%. These results indicate that a substantial portion of photosynthetic electron transport or respiration generates reductant for nitrate assimilation rather than for carbon fixation or mitochondrial electron transport.The influence of NO3-assimilation upon photosynthesis and respiration has been the subject of much speculation (9,17,20,25). To provide energy for NO3-assimilation, a portion of the electrons that are usually transferred to CO2 during photosynthesis or to 02 during respiration may be instead transferred to 19 (8, 26). Plant material is exposed to an atmosphere highly enriched with the heavy isotope 180. Changes in the levels of 160 and 180 indicate rates of oxygen production and uptake, respectively. These systems cannot monitor water fluxes and, therefore, cannot estimate intercellular CO2 concentrations. (c) Paramagnetic analyzers can resolve small 02 concentration differences (1 1), but demonstrate a high sensitivity to gas flow rate and vibration ( 16). (d) The signal drift and noise of polarographic 02 sensors require that relatively large (_300 ML L-') 02 depletions be obtained (13,14). (e) Commercial instruments based on ceramic electrolytic cells have had adequate sensitivity and stability only at very low ambient 02 concentrations (_2000 AL L-') (4,15

The widespread planting of corn genetically modified to produce Bacillus thuringiensis endotoxin has led to speculation that pollen from these fields might adversely affect nearby nontarget lepidopterans. A previous study of Bt corn engineered with Monsanto event 810 failed to detect an effect of pollen exposure on the black swallowtail, Papilio polyxenes, in either the field or the laboratory. Here, we report results of a field study investigating the impact of exposure to pollen from a Bt corn hybrid containing Novartis event 176 on two species of Lepidoptera, black swallowtails and monarch butterflies, Danaus plexippus. Nearly half of the 600 monarch larvae died within the first 24 h; this and subsequent mortality was not associated with proximity to Bt corn and may have been due in part to predation. Survivorship of black swallowtails was much higher than that of the monarchs and was also independent of proximity to the transgenic corn. However, despite five rainfall events that removed much of the pollen from the leaves of their host plants during the experiment, we observed a significant reduction in growth rates of black swallowtail larvae that was likely caused by pollen exposure. These results suggest that Bt corn incorporating event 176 can have adverse sublethal effects on black swallowtails in the field and underscore the importance of event selection in reducing environmental impacts of transgenic plants.

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The short-lived radiotracer 13N was used to study feedback regulation of nitrate influx through the inducible high-affinity transport system of barley (Hordeum vurgare 1. cv Steptoe) roots. 60th wild-type plants and the mutant line Az12:Az70 (genotype narla;nar7w), which is deficient in the NADH-specific and into reduced products is greatly reduced, indicated that nitrate itself was capable of exercising feedback regulation upon its own influx. This conclusion was supported by the observation that feedback in wild-type plants occurred in both the presence and absence of i-methionine sulfoximine, an inhibitor of ammonium assimilation. Nitrite and ammonium were also found to be capable of exerting feedback inhibition upon nitrate influx, although it was not determined whether these ions themselves or subsequent metabolites were responsible for the effect. It is suggested that feedback regulation of nitrate influx i s potentially mediated through several nitrogen pools, including that of nitrate itself.