The Influence of Nitrate and Ammonium Nutrition on the Growth of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

The effects of NO-3 and NH+4 nutrition on hydroponically grown wheat (Triticum aestivum L.) and maize (Zea mays L.) were assessed from measurements of growth, gas exchange and xylem sap nitrogen contents. Biomass accumulation and shoot moisture contents of wheat and maize were lower with NH+4 than with NO-3 nutrition. The shoot:root ratios of wheat plants were increased with NH+4 compared to NO-3 nutrition, while those of maize were unaffected by the nitrogen source. Differences between NO-3 and NH+4-fed plant biomasses were apparent soon after introduction of the nitrogen into the root medium of both wheat and maize, and these differences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected by the form of nitrogen supplied whereas those of 12 mM NH+4-fed wheat plants were reduced to 85% of those 12 mM NO-3-fed wheat plants. In maize supplied with 4 and 12 mM NH+4 the photosynthetic rates were 87 and 82% respectively of those of NO-3-fed plants. Reduced photosynthetic rates of NH+4 compared to NO-3-fed wheat and maize plants may thus partially explain reduced biomass accumulation in plants supplied with NH+4 compared to NO-3 nutrition. Differences in the partitioning of biomass between the shoots and roots of NO-3-and NH+4-fed plants may also, however, arise from xylem translocation of carbon from the root to the shoot in the form of amino compounds. The organic nitrogen content of xylem sap was found to be considerably higher in NH+4- than in NO-3-fed plants. This may result in depletion of root carbohydrate resources through translocation of amino compounds to the shoot in NH+4-fed wheat plants. The concentration of carbon associated with organic nitrogen in the xylem sap of maize was considerably higher than that in wheat. This may indicate that the shoot and root components of maize share a common carbon pool and thus differences induced by different forms of inorganic nitrogen are manifested as altered overall growth rather than changes in the shoot:root ratios.