The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Abstract

Respiratory oxygen consumption by roots was 1·4- and 1·6-fold larger in NH+4-fed than in NO-3-fed wheat (Triticum aestivum L.) and maize (Zea mays L.) plants respectively. Higher root oxygen consumption in NH+4-fed plants than in NO-3-fed plants was associated with higher total nitrogen contents in NH+4-fed plants. Root oxygen consumption was, however, not correlated with growth rates or shoot:root ratios. Carbon dioxide release was 1·4- and 1·2-fold larger in NO+3-fed than in NH+4-fed wheat and maize plants respectively. Differences in oxygen and carbon dioxide gas exchange rates resulted in the gas exchange quotients of NH-4-fed plants (wheat, 0·5; maize, 0·6) being greatly reduced compared with those of NO-3-fed plants (wheat, 1·0; maize, 1·1). Measured rates of HCO-3 assimilation by PEPc in roots were considerably larger in 4 mM NH+4-fed than in 4 NO-3 plants (wheat, 2·6-fold; maize, 8·3-fold). These differences were, however, insufficient to account for the observed differences in root carbon dioxide flux and it is probable that HCO-3 uptake is also important in determining carbon dioxide fluxes.Thus reduced root extension in NH+4-fed compared with NO-3-fed wheat plants could not be ascribed to differences in carbon dioxide losses from roots.