Root Respiration Associated with Ammonium and Nitrate Absorption and Assimilation by Barley

Abstract

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...

Fluorescência e teores de clorofilas em abacaxizeiro cv. pérola submetido a diferentes concentrações de sulfato de amônio

“…A reação no processo de assimilação do amônio requer redutor biológico -NAD(P)H, ou ferrodoxina provenientes da fase fotoquímica e de transferência de elétrons (fluxo linear de elétrons a partir da água através do PSII e do PSI até os receptores) da fotossíntese (TAIZ;ZEIGER, 2008). Na raiz, o suprimento de redutor é via processo respiratório (BLOOM et al, 1992).…”

The effects of light on induction, time courses, and kinetic patterns of net nitrate uptake in barley

“…These authors concluded that nitrate uptake was regulated by the flows of carbohydrates from the shoots to the roots. However, Bloom, Sukrapanna & Warner (1992) calculated that only 5% of root carbon catabolism was coupled to nitrate uptake. In the present investigations the plants were supplied very well with light in a 16 h photoperiod in precultivation compared with other greenhouse plants.…”

“…Subsequent steps in nitrate metabolism, nitrate reduction and ammonium assimilation, are also inducible and energy dependent since reducing power, ATP and C-skeletons are needed. The uptake and assimilation of ammonium and nitrate require a significant amount of root respiration (Bloom, Sukrapanna & Warner 1992). Hence in an intact plant, nitrate uptake, N metabolism, C metabolism and ultimately photosynthesis are all linked.…”

Root growth and function of three Mojave Desert grasses in response to elevated atmospheric CO2 concentration

“…Root uptake and assimilation of NO $ and NH % are energy-requiring processes (Bloom et al, 1992 ;Lambers, 1996), and because elevated CO # can enhance the supply of root respiratory substrates (Tschaplinski et al, 1993 ;BassiriRad et al, 1996b), metabolically regulated processes such as root N uptake are expected to be stimulated under elevated CO # . However, recent studies of NO $ and NH % uptake in response to elevated atmospheric CO # have shown surprising and sometimes conflicting results.…”