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 re quirements associated with nocturnal carbohydrate export to (1) the fr action of dark respiration correlating with leaf nitrogen concentratio n and (2) the dark respiration of mature source leaves. To this end, w e determined the nocturnal carbohydrate-export rates from leaves with an optimal nitrogen supply, and the correlation between the nitrogen c oncentration and the dark respiration of leaves. The specific energy c osts of carbohydrate export from starch-storing source leaves were det ermined both experimentally and theoretically. The present estimate of the specific energy cost involved in carbohydrate export as obtained by linear regression (0.70 mol CO2 [mol sucrose](-1)), agrees well wit h both literature data obtained by different methods (0.47 to 1.26 mol CO2 [mol sucrose](-1)) and the theoretically calculated range for sta rch-storing species (0.40 to 1.20 mol CO2 [mel sucrose](-1)). The conv ersion of starch in the chloroplast to sucrose in the cytosol is a maj or energy-requiring process. Maximally 42 to '107'% of the slope of th e relationship between respiration rate and organic nitrogen concentra tion of primary bean leaves, may be ascribed to the energy costs assoc iated with nocturnal export of carbohydrates. Total energy costs assoc iated with export were derived from the product of the specific costs of carbohydrate export and the export rates, either measured on full-g rown (primary) leaves of potato and bean or derived from the literatur e. These export costs account, on average, for 29% of the dark respira tion rate in starch-storing species. We conclude that nocturnal carboh ydrate export is a major energy-requiring process in starch-storing sp ecies.