Carbon-Isotope Composition of Biochemical Fractions and the Regulation of Carbon Balance in Leaves of the C3-Crassulacean Acid Metabolism Intermediate Clusia minor L. Growing in Trinidad

Abstract

Carbon-isotope ratios (PCs) were measured for various biochemical fractions quantitatively extracted from naturally exposed and shaded leaves of the C3-Crassulacean acid metabolism (CAM) intermediate C h i a 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 13C by approximately 3.5 to 4%0 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 13C and the second a transport pool depleted in 13C. 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%0. The 613C of structural material did not change significantly between wet and dry seasons, indicating that most of the carbon used in growth was derived from C3 carboxylation.Clusia minor L. is a tropical dicotyledonous tree that shows extreme flexibility in regulating the relative amounts of C 0 2 fixed during the day and night in response to changes in the environment (Franco et al., 1990Borland et al., 1992Borland et al., , 1993Winter et al., 1992;Zotz and Winter, 1993). Under natural conditions, short-term changes in on-line, instantaneous A have illustrated how shifts in C3 and C4 carboxylation occur on a daily basis in C. minor, reflecting changes in environmental parameters (Borland et al., 1993). These variations in carboxylation pathway may in the first instance

Oscillatory Nature of Metabolism and Carbon Isotope Distribution in Photosynthesizing Cells

“…Indeed, the consideration of labile carbohydrates (oxygenase G6P), accumulated in the light, as substrate for dark synthesis of organic acids (Borland et al, 1994), allows concluding the following way of the conversion . At first glucose splits into two triose molecules.…”

“…Table 4. Carbon isotope composition of biochemical fractions isolated from leaves of Clusia minor under different environmental conditions (Borland et al, 1994). Samples were taken at dawn and dusk.…”

A model for the partitioning of photosynthetically fixed carbon during the C3‐CAM transition in Sedum telephium

“…The extraction procedure was based on that of Borland et al (1994). The midribs were removed, the leaves powdered in liquid nitrogen then placed in 2 X 15 cm^' of 80 % methanol and heated at 80 °C for 30 min.…”

“…4) of organic acid and carbohydrate pools overnight, the isotopic signature of each ' mobile' carbon pool can be estimated using mass balance calculations (Deleens & GarnierDardart, 1977;Borland et al, 1994). As shown in Table 2, carbon lost from the starch pool at night was enriched in ^^C by some 3-6%o compared with carbon lost from the soluble sugar pool, indicating once more that diel starch turnover was intimately involved in the CAM cycle, whilst soluble sugars depleted at night were more likely to be derived in the main from daytime Cg carboxylation.…”

“…Carbohydrates synthesized directly from Cg photosynthesis are depleted in ^^C compared with those arising from the daytime decarboxylation of malate (Deleens & Gamier-Dardart, 1977). Therefore, analyses of the S^^Q composition of organic acid and carbohydrate fractions during the day-night cycle can illustrate how products of Cg and C^ carboxylation contribute to the diel carbon budget (Borland et al, 1994). A model has been constructed to use this data in a quantitative description of the partitioning of carbon in leaves of S. telephium showing different degrees of CAM-cycling and to assess the extent to which CAM is induced or enhanced at the expense of growth.…”