The Growth Rate of Wheat Leaves in Relation to the Extension Zone Sugar Concentration Manipulated by Shading

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Abstract
The main objective of this study was to determine the relationship between the relative rate of growth of emerging wheat leaves and the hexose sugar concentration of the extension zone. Short periods of intense shading (to 20 or 5% of full sun for up to 14 d) were used to decrease hexose concentrations. Shading decreased hexose concentrations to a fraction of that of controls and also resulted in thin and narrow leaves that were less in dry weight than control leaves of the same length. Shading did however increase the length of the zone of extending tissue at the leaf base by 30%. The effect of hexose concentrations on the relative rate of leaf growth was evaluated by determining the ratio between growth rates of shaded and control leaves. This ratio declined as hexose concentrations declined and the relationship was described by a rectangular hyperbola (r > 0.95, P < 0.01). Combined data from many leaves on the main shoot and its tillers from two irrigated wheat crops all conformed to the same relationship. The hexose concentrations where the ratio of growth rates was half the maximum rate were 0.42 mg g−1 fr. wt. for extension growth and 1.74 mg g−1 fr. wt. for dry weight growth. These values were significantly (P < 0.01) different. These results were compared with data from emerging leaves of field crops and it was concluded that hexose concentrations had not limited leaf growth rates, the lowest values recorded being 2.5–3.0 mg g−1 fr. wt. It was further suggested as unlikely that leaf growth rates of wheat crops in the field would be limited by hexose concentrations.