PLANT PHYSIOLOGY , Vol 106, Issue 2 493-501, Copyright © 1994 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
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
A. M. Borland, H. Griffiths, MSJ. Broadmeadow, M. C. Fordham and C. Maxwell
Department of Agricultural and Environmental Science, Ridley Building, The University, Newcastle Upon Tyne, NE1 7RU, United Kingdom
Carbon-isotope ratios ([delta]13Cs) were measured for various bio-chemical
fractions quantitatively extracted from naturally exposed and shaded leaves
of the C3-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 13C by approximately 3.5 to 4%[per mille
(thousand) sign] 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%[per mille (thousand) sign]. The
[delta]13C 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.
