THE INFLUENCE OF NO3(+) AND NH-4(+) NUTRITION ON THE CARBON AND NITROGEN PARTITIONING CHARACTERISTICS OF WHEAT (TRITICUM-AESTIVUM L) AND MAIZE (ZEA-MAYS L) PLANTS

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO3- or NH4+ nutrition were investigated using either C-14 or N-15 techniques. Greater allocation of C-14 to amino-N fractions occurred at the expense of allocation of C-14 to carbohydrate fractions in NH4+- compared to NO3--fed plants. The [C-14]carbohydrate: [C-14]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO3--compared to 12 mM NH4+-fed wheat. In both 4 mM and 12 mM N-fed maize the [C-14]carbohydrate: [C-14]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO3--compared to NH4+-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture With one root-half in NO3-- and the other in NH+-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of N-14 and N-15 within separate sets of plants confirmed that NH+-fed plants accumulated more amino-N compounds than NO3--fed plants. Wheat roots supplied with (NH4+)-N-15 for 8 h were found to accumulate (NH4+)-N-15 (8.5 mug N-15 g-1 h-1 whereas maize roots very little (NH4+)-N-15 accumulated (1.5 mug N-15 g-1 h-1). It is proposed that the observed accumulation of (NH4+)-N-15 in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH4+, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C4 maize plants ensure greater carbon availability to the root than in the C3 wheat plants. These differences in carbon and nitrogen partitioning between NO3-- and NH+-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO3- and NH4+ nutrition.