Diurnal Extension Rates of Wheat Leaves in Relation to Temperatures and Carbohydrate Concentrations of the Extension Zone

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Abstract
The objective of this experiment was to determine how diurnal variations in rates of leaf extension of wheat plants in an irrigated field crop were related to temperatures and carbohydrate concentrations of the extension zone. Leaves 3, 4. 5 of the main shoot were studied as each emerged from the encircling sheath. The carbohydrates in the extension zone of the emerging leaf were analysed by converting them to glucose-6-phosphate and then measuring the reduction of NADP in the presence of glucose-6-phosphate dehydrogenase. Average hexose concentrations (glucose and fructose) increased each day from 4 up to 5 mg g−1 fr. wt. and sucrose from 3 up to 7 mg g−1 fr. wt. with the maximum in mid-afternoon; there were no differences among the three leaves. Concentrations of fructans were constant throughout the day for leaves 3 and 4 but showed a mid-afternoon rise in leaf 5. The average concentrations of fructans in the extension zones increased from 0 to 5 to 11 mg g−1 fr. wt. for leaves 3, 4, and 5 respectively and was consistent with the conclusion that there was an increasing over-supply of carbohydrates for growth of the shoot as the plant increased in size. Rates of leaf extension were correlated with temperature but not with hexose concentrations. We concluded that the supply of carbohydrates did not limit the growth of leaves under field conditions but their utilization in leaf growth was limited by temperature. The rates of leaf extension increased exponentially with temperature and the relationship was described by the Arrhenius equation. The Q10 at 15 °C for leaf extension was 2.7 for leaves 4 and 5 and 3.2 for leaf 3.