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

We examined nitrate assimilation and root gas fluxes in a wild-type barley (Hordeum vulgare L. cv Steptoe), a mutant (nar1a) deficient in NADH nitrate reductase, and a mutant (nar1a;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 nar1a mutation influences assimilation only in the shoot and that exposure to NO(3) (-) 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 nar1a mutant, but had negligible effects on root gas fluxes in the nar1a;nar7w mutant. These results indicate that, under NH(4) (+) nutrition, 14% of root carbon catabolism is coupled to NH(4) (+) absorption and assimilation and that, under NO(3) (-) nutrition, 5% of root carbon catabolism is coupled to NO(3) (-) absorption, 15% to NO(3) (-) assimilation, and 3% to NH(4) (+) assimilation. The additional energy requirements of NO(3) (-) assimilation appear to diminish root mitochondrial electron transport. Thus, the energy requirements of NH(4) (+) and NO(3) (-) absorption and assimilation constitute a significant portion of root respiration.