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 theenergy requirements associated with nocturnal carbohydrate exportto (1) the fraction of dark respiration correlating with leafnitrogen concentration and (2) the dark respiration of maturesource leaves. To this end, we determined the nocturnal carbohydrate-exportrates from leaves with an optimal nitrogen supply, and the correlationbetween the nitrogen concentration and the dark respirationof leaves. The specific energy costs of carbohydrate exportfrom starch-storing source leaves were determined both experimentallyand theoretically. The present estimate of the specific energycost involved in carbohydrate export as obtained by linear regression(0.70 mol CO₂ mol sucrose]^–1), agrees well with bothliterature data obtained by different methods (0.47 to 1.26mol CO₂ mol sucrose]^–1) and the theoretically calculatedrange for starch-storing species (0.40 to 1.20 mol CO₂ molsucrose]^–1). The conversion of starch in the chloroplastto sucrose in the cytosol is a major energy-requiring process.Maximally 42 to ‘107’% of the slope of the relationshipbetween respiration rate and organic nitrogen concentrationof primary bean leaves, may be ascribed to the energy costsassociated with nocturnal export of carbohydrates. Total energycosts associated with export were derived from the product ofthe specific costs of carbohydrate export and the export rates,either measured on full-grown (primary) leaves of potato andbean or derived from the literature. These export costs account,on average, for 29% of the dark respiration rate in starch-storingspecies. We conclude that nocturnal carbohydrate export is amajor energy-requiring process in starch-storing species Key words: Carbohydrate export, leaf dark respiration, nitrogen concentration, respiratory costs, specific energy cost