The respiratory energy requirements involved in nocturnal carbohydrate export from starch-storing mature source leaves and their contribution to leaf dark respiration

The present study explores the potential contribution of the energy requirements associated with nocturnal carbohydrate export to (1) the fraction of dark respira­ tion correlating with leaf nitrogen concentration and (2) the dark respiration of mature source leaves. To this end, we determined the nocturnal carbohydrate-export rates from leaves with an optimal nitrogen supply, and the correlation between the nitrogen concentration and the dark respiration of leaves. The specific energy costs of carbohydrate export from starch-storing source leaves were determined both experimentally and theor­ etically. The present estimate of the specific energy cost involved in carbohydrate export as obtained by linear regression (0. 70 mol C02 (mol sucrose) - 1 ), agrees well with both literature data obtained by different methods (0.4 7 to 1.26 mol C02 (mol sucrose) - 1 ) and the theoretically calculated range for starch-storing species (0.40 to 1.20 mol C02 (mol sucrose) - 1 ). The conversion of starch in the chloroplast to sucrose in the cytosol is a major energy-requiring process. Maximally 42 to '107'% of the slope of the relationship between respiration rate and organic nitrogen concentration of primary bean leaves, may be ascribed to the energy costs associated with nocturnal export of carbohyd­ rates. Total energy costs associated with export were derived from the product of the specific costs of carbo- hydrate export and the export rates, either measured on full-grown (primary) leaves of potato and bean or derived from the literature. These export costs account, on average, for 29% of the dark respiration rate in starch-storing species. We conclude that nocturnal carbohydrate export is a major energy-requiring pro­ cess in starch-storing species.