GAS AND ION EXCHANGES IN WHEAT ROOTS AFTER NITROGEN SUPPLY

Wheat (T. aestivum L. cv. Sicco) was grown for 10 days on CaSO4 (0.5 mmol dm-3) and then exposed for 2 days to various nitrogenous salts in an apparatus designed to measure the exchange of O2 and CO2, at constant pH and pNO3-. Nitrate salts (KNO3 at 0.5 and Ca(NO3)2 at 0.25 and 1 mmol dm-3) caused a transient increase (40-50%) in both O2 uptake and CO2 release by the roots. The rate of gas exchange was nearly doubled by (NH4)2SO4 (0.25 mmol dm-3). Respiration was constant in roots kept on CaSO4 or given KCl. In CaSO4 the content of water-soluble sugars in roots fell by about 15% day-1. The pletion of soluble sugars was higher with NO3- and NH4+ (40 and 30% day-1, respectively). At most 10-20% of the released CO2 is involved in HCO3- NO3- exchange and this fraction represents at most 10% of the total C imported or 30% of the net C gain by the roots. The contribution of the non-phosphorylating alternative route to total root respiration was 15% in CaSO4 and over 40% with NH4+. In NO3- the roots respired exclusively via the cytochrome route. Increased respiration at decreased efficiency in roots of NH4+ plants may be due to an overproduction of NADH. Excess NADH as a by-product of the formation of carboxylates in the citrate cycle can be disposed of in an alternative respiratory pathway during NH4+ nutrition.