Export of carbon from chloroplasts at night

Hexose export from chloroplasts at night has been inferred in previous stud ies of mutant and transgenic plants. We have tested whether hexose export i s the normal route of carbon export from chloroplasts at night. We used nuc lear magnetic resonance to distinguish glucose (Glc) made from hexose expor t and Glc made from triose export. Glc synthesized in vitro from fructose-6 -phosphate in the presence of deuterium-labeled water had deuterium incorpo rated at C-2, whereas synthesis from triose phosphates caused C-2 through C -5 to become deuterated. In both tomato (Lycopersicon esculentum L.) and be an (Phaseolus vulgaris L.), Glc from sucrose made at night in the presence of deuterium-enriched water was deuterated only in the C-2 position, indica ting that >75% of carbon is exported as hexoses at night. In darkness the p hosphate in the cytosol was 28 mM, whereas that in the chloroplasts was 5 m M, but hexose phosphates were 10-fold higher in the cytosol than in the chl oroplasts. Therefore, hexose phosphates would not move out of chloroplasts without the input of energy. We conclude that most carbon leaves chloroplas ts at night as Glc, maltose, or higher maltodextrins under normal condition s.