CARBON-ISOTOPE COMPOSITION OF BIOCHEMICAL FRACTIONS AND THE REGULATION OF CARBON BALANCE IN LEAVES OF THE C-3-CRASSULACEAN ACID METABOLISM INTERMEDIATE CLUSIA-MINOR L GROWING IN TRINIDAD

Carbon-isotope ratios (delta(13)Cs) were measured for various biochemical fractions quantitatively extracted from naturally exposed and shaded leaves of the C-3-Crassulacean acid metabolism (CAM) intermediate Clusia minor, sampled at dawn and dusk on days during the wet and dry seasons in Trinidad. As the activity of CAM increased in response to decreased availability of water and higher photon flux density, organic acids and soluble sugars were enriched in C-13 by approximately 3.5 to 4 parts per thousand compared to plants sampled during the wet season. The induction of CAM was accompanied by a doubling in size of the reserve carbohydrate pools. Moreover, stoichiometric measurements indicated that degradation of both chloroplastic reserves and soluble sugars were necessary to supply phosphoenolpyruvate for the synthesis of organic acids at night. Results also suggest that two pools of soluble sugars exist in leaves of C. minor that perform CAM, one a vacuolar pool enriched in C-13 and the second a transport pool depleted in C-13. Estimates of carbon-isotope discrimination expressed during CAM, derived from the trafficking among inorganic carbon, organic acids, and carbohydrate pools overnight, ranged from 0.9 to 3.1 parts per thousand. The delta(13)C of structural material did not change significantly between wet and dry seasons, indicating that most of the carbon used in growth was derived from C-3 carboxylation.