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.
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Michael D. Cramer (1993) studied this question.