Anatomy of the photosynthetic tissue in ten taxa of Opuntia established tothe mediterranean arid zone of Chile

The anatomy of the photosynthetic tissue and the effect of cladode exposure to radiation was studied in ten taxa of Opuntia. plants with crassulacean acid metabolism (CAM), established in the dry lands of the IV Region of Chi le. At the epidermic level the cuticle thickness, stomatal frequency, and d imension of guard cells were evaluated. In the photosynthetic tissue, cell dimensions were determined in order to estimate the area of cell walls expo sed to gaseous exchange per unit of photosynthetic tissue (A (mes)/ A). The results indicate large cell dimensions and low stomata frequency which res ults in a reduced number of stomata per unit area in relation to species wi th other metabolic pathways (C3 and C4), The comparison between taxa shows significant differences in the relation of A(mes)/A areas, a parameter clos ely associated to the photosynthetic capacity and water use efficiency. The above anatomic characteristics of these plants in combination with pathway CAM contribute to their adaptation to limited rainfall conditions.