THE INFLUENCE OF NO3- AND NH4+ NUTRITION ON THE GAS-EXCHANGE CHARACTERISTICS OF THE ROOTS OF WHEAT (TRITICUM-AESTIVUM) AND MAIZE (ZEA-MAYS) PLANTS

Respiratory oxygen consumption by roots was 1-4- and 1'6-fold larger in NH^-fed than in NO^-fed wheat;(Triticum aestivumL.) and maize (ZeamaysL.) plants respectively. Higher root oxygen consumption in NHLfed plants thanin NOs-fed plants was associated with higher total nitrogen contents in NH+-fed plants. Root oxygen consumption;was, however, not correlated with growth rates or shoot:root ratios. Carbon dioxide release was 1-4- and I-2-foldlarger in N03-fed than in NH(-fed wheat and maize plants respectively. Differences in oxygen and carbon dioxide gas exchange rates resulted in the gas exchange quotients of NH^-fed plants (wheat, 0-5; maize, 0.6) being greatly;reduced compared with those of N03-fed plants (wheat, b0; maize, 1-1). Measured rates of HC03;assimilation by;PEPc in roots were considerably larger in 4;iumNH^-fed than in 4;mM;NO201C;-fed 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 HC03;loss during NOj uptake is also important in determining carbon dioxide fluxes.Thus reduced root extension in NHJ-fed compared with NO201C;-fed wheat plants could not be ascribed to differences in carbon dioxide losses from roots. © 1993 Annals of Botany Company.