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

Lawlor, D. W., Boyle, F. A., Kendall, A. C. and Keys, A. J.1987. Nitrate nutrition and temperature effects on wheat: Enzymecomposition, nitrate and total amino acid content of leaves.—J.exp. Bot. 38: 378–392. Wheat plants were grown in controlled environments in two temperatureregimes with two rates of nitrate fertilization. In some experimentstwo light intensities were combined with the nitrogen and temperaturetreatments. The composition of the third leaf was studied fromsoon after emergence until early senescence. The amounts ofchlorophyll, soluble protein, ribulose bisphosphate carboxylase-oxygenase(RuBPc-o) protein, nitrate, and total amino acids were measuredtogether with the activities of RuBPc-o, fructose- 1,6-bisphosphatase,glycolate oxidase, carbonic anhydrase, nitrate reductase, glutaminesynthetase and serine- and glutamate-glyoxylate aminotransferases.Additional nitrate supply increased the amounts, per unit leafarea, of chlorophyll, total soluble protein and RuBPc-o proteinand the activities of all the enzymes. The ratio of RuBP carboxylaseto RuBP oxygenase activity, when measured at constant CO²/O²ratio and temperature, was unaffected by growth conditions orleaf age. Leaves grown at the lower temperature, especiallywith more nitrate, contained much more soluble protein, nitratereductase, fructose bisphosphatase and free amino acids perunit area than the plants grown in the warmer conditions. However,young leaves grown in the warm contained more nitrate than thosegrown in the cool. Amounts of protein, amino acids and chlorophylland most enzyme activities reached maxima near full leaf expansionand decreased with age; additional nitrate slowed the decreaseand senescence was delayed. Nitrate content and nitrate reductaseactivities were highest in leaves before full expansion andthen fell rapidly after full expansion. Increased light intensityincreased the content of RuBPc-o protein at the higher rateof nitrate supply. Chloroplast components and, to a lesser extent,peroxisomal enzymes associated with photosynthetic nitrogenassimilation changed in proportion with different treatmentsbut nitrate reductase activity was not closely related to chloroplastenzymes. Control of tissue composition in relation to environmentalconditions is discussed. Key words: Nitrate nutrition, temperature, wheat, enzyme, amino acid, leaves, ribulose bisphosphate carboxylase oxygenase, nitrate reductase