LIGHT CONDITIONS DURING GROWTH AS REVEALED BY DELTA-C-13 VALUES OF LEAVES OF PRIMITIVE CULTIVARS OF ANANAS-COMOSUS, AN OBLIGATE CAM SPECIES

1. Pineapple, Ananas comosus, is a constitutive Crassulacean acid metabolism (CAM) plant developing a characteristic rosette habit, leaves being produced in the middle from a short stem. The resulting habit has a characteristic pattern of leaf age increasing from the centre to the periphery of the rosette. As a consequence light climate changes continuously during the leaf life span, both due to changes in leaf inclination and to shading produced by the new leaves produced in the centre. Increasing leaf age also determines strong variations in lipid and nitrogen contents. 2. This study reports the pattern of variation of CAM activity of the leaves as measured by deltaC-13 values of whole tissue in pineapple plantations (A. comosus cv. Panare and Brecheche) growing under contrasting light conditions in 'morichales' along the Orinoco river. Leaf nitrogen and lipid contents are higher in sun plants and in green leaf blades compared to shade plants or white leaf bases and stems. The deltaC-13 Values of leaf blades are more negative than those of white leaf bases and stems. The actual values of green blades become more negative from young to older leaves. The range of variation of deltaC-13 values is more pronounced in sun plants of both cultivars. These changes are only partially associated with the differences in lipid content. 3. The main cause for more negative deltaC-13 values as the leaf ages has been assumed to be the reduction in light intensity and nitrogen contents leading to lower CAM activity both in older leaves and in shade plants as a whole. 4. The calculated contribution of carbon fixed during CAM phase I to the deltaC-13 value of the green leaf tissues ranged from 98.5% in adult leaves of sun plants to 84% in old leaves of both sun and shade plants. These values should be reduced by 25% if discrimination during CAM phase I is negligible.