Nitrate Nutrition and Temperature Effects on Wheat: Enzyme Composition, Nitrate and Total Amino Acid Content of Leaves

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Abstract
Lawlor, D. W., Boyle, F. A., Kendall, A. C. and Keys, A. J. 1987. Nitrate nutrition and temperature effects on wheat: Enzyme composition, nitrate and total amino acid content of leaves.—J. exp. Bot. 38: 378–392. Wheat plants were grown in controlled environments in two temperature regimes with two rates of nitrate fertilization. In some experiments two light intensities were combined with the nitrogen and temperature treatments. The composition of the third leaf was studied from soon after emergence until early senescence. The amounts of chlorophyll, soluble protein, ribulose bisphosphate carboxylase-oxygenase (RuBPc-o) protein, nitrate, and total amino acids were measured together with the activities of RuBPc-o, fructose- 1,6-bisphosphatase, glycolate oxidase, carbonic anhydrase, nitrate reductase, glutamine synthetase and serine- and glutamate-glyoxylate aminotransferases. Additional nitrate supply increased the amounts, per unit leaf area, of chlorophyll, total soluble protein and RuBPc-o protein and the activities of all the enzymes. The ratio of RuBP carboxylase to RuBP oxygenase activity, when measured at constant CO2/O2 ratio and temperature, was unaffected by growth conditions or leaf age. Leaves grown at the lower temperature, especially with more nitrate, contained much more soluble protein, nitrate reductase, fructose bisphosphatase and free amino acids per unit area than the plants grown in the warmer conditions. However, young leaves grown in the warm contained more nitrate than those grown in the cool. Amounts of protein, amino acids and chlorophyll and most enzyme activities reached maxima near full leaf expansion and decreased with age; additional nitrate slowed the decrease and senescence was delayed. Nitrate content and nitrate reductase activities were highest in leaves before full expansion and then fell rapidly after full expansion. Increased light intensity increased the content of RuBPc-o protein at the higher rate of nitrate supply. Chloroplast components and, to a lesser extent, peroxisomal enzymes associated with photosynthetic nitrogen assimilation changed in proportion with different treatments but nitrate reductase activity was not closely related to chloroplast enzymes. Control of tissue composition in relation to environmental conditions is discussed.