INTERPRETATIONS OF GRADIENTS IN DELTA-C-13 VALUE IN THICK PHOTOSYNTHETIC TISSUES OF PLANTS WITH CRASSULACEAN ACID METABOLISM

In Ceropegia dichotoma, Crassula argentea, Esheveria colorata, Kalanch oe beharensis, Opuntia ficus-indica, Sansveria stuckyi and Opuntia ine rmis the carbon-isotope ratio (deltaC-13) of tissues close to the epid ermis is 2 4.3 parts per thousand more negative than those in the cent re of the leaf or cladode. The greatest change in deltaC-13 value occu rs between the epidermal layer and the layer of mesophyll tissue immed iately underneath. Analysis of major metabolic and structural componen ts in successive layers of Crassula argentea grown under controlled en vironmental conditions conducive to Crassulacean acid metabolism confi rmed that much of the variation in deltaC-13 values of bulk carbon is caused by differences in chemical composition. Thus the steep gradient in deltaC-13 value at the epidermis reflects, in part, the contributi on of more-negative deltaC-13 values of lipids in these tissues. Moreo ver, during nocturnal CO2 fixation the amount of malic acid synthesise d decreases with depth and the deltaC-13 value of the methanol-soluble fraction is less negative with distance away from the upper epidermis . These results are consistent with diffusion limitation to CO2 uptake in these thick leaf tissues, which also contributes to the observed g radients in deltaC-13 value.