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 meta bolism (CAM) plant developing a characteristic rosette habit, leaves b eing produced in the middle from a short stem. The resulting habit has a characteristic pattern of leaf age increasing from the centre to th e periphery of the rosette. As a consequence light climate changes con tinuously during the leaf life span, both due to changes in leaf incli nation and to shading produced by the new leaves produced in the centr e. Increasing leaf age also determines strong variations in lipid and nitrogen contents. 2. This study reports the pattern of variation of C AM activity of the leaves as measured by deltaC-13 values of whole tis sue in pineapple plantations (A. comosus cv. Panare and Brecheche) gro wing under contrasting light conditions in 'morichales' along the Orin oco river. Leaf nitrogen and lipid contents are higher in sun plants a nd in green leaf blades compared to shade plants or white leaf bases a nd stems. The deltaC-13 Values of leaf blades are more negative than t hose of white leaf bases and stems. The actual values of green blades become more negative from young to older leaves. The range of variatio n of deltaC-13 values is more pronounced in sun plants of both cultiva rs. These changes are only partially associated with the differences i n 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 intensi ty and nitrogen contents leading to lower CAM activity both in older l eaves and in shade plants as a whole. 4. The calculated contribution o f 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 reduc ed by 25% if discrimination during CAM phase I is negligible.