The influence of NO3- and NH4+ nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants
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- Cramer, M.D. & Lewis, O.A.M. Plant Soil (1993) 154: 289. doi:10.1007/BF00012534
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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 14C or 15N techniques. Greater allocation of 14C to amino-N fractions occurred at the expense of allocation of 14C to carbohydrate fractions in NH4+-compared to NO3--fed plants. The [14C]carbohydrate:[14C]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 [14C]carbohydrate:[14C]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 NH4+-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 14N and 15N within separate sets of plants confirmed that NH4--fed plants accumulated more amino-N compounds than NO3--fed plants. Wheat roots supplied with 15NH4+ for 8 h were found to accumulate 15NH4+ (8.5 μg 15N g-1 h-1) whereas in maize roots very little 15NH4+ accumulated (1.5 μg 15N g-1 h-1)
It is proposed that the observed accumulation of 15NH4+ 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 NH4+-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO3- and NH4+ nutrition.