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Selenicereus megalanthus and Hylocereus polyrhizus, hemiepiphytic cacti of shady habitats, have recently been introduced as fruit crops to the Negev Desert of Israel. Since they become bleached and die when they are grown in full sunlight, the shading responses of these cacti were studied. Both species showed typical CAM CO2 fixation, namely CO2 was fixed during the night, followed by acid accumulation. Nocturnal acid accumulation and the concentration of photosynthetic pigments (chlorophyll and carotenoids) were determined in short term experiments in the summer under full sunlight or in net houses with 30, 60 or 90% shade. The effect of shading on morphological traits, growth and fruiting was determined in long term experiments, in net-houses only, since plants usually degenerate under full sunlight. S. megalanthus was found to be more sensitive to high light flux density and better adapted to deep shade than H. polyrhizus, as judged from the more marked decrease in both nocturnal acid accumulation and in the concentrations of chlorophyll and carotenoids under full sunlight in the former species. In addition, under 90% shade the stem biomass of S. megalanthus was reduced less than that of H. polyrhizus. In response to the shade, stem ribs became shallow, root dry weight decreased and shoot water content increased in both species. The most favorable conditions for growth and fruit production were found to be 30% shade for H. polyrhizus while for S. megalanthus 60% shade seemed to be preferable, since it facilitated higher acid accumulation, carotenoid and chlorophyll content, and better stem appearance. H. polyrhizus was distinguished by its higher fruit yield, estimated at 16 ton ha(-1) vs. 3.8 ton ha(-1) for S. megalanthus in the second year after planting when both species were growing under optimal conditions. The differences between the species in terms of light adaptation may be partially related to the wax layer coating the stem and sunken stomata of H. polyrhizus compared to S. megalanthus, which had neither wax nor sunken stomata. (C) 1998 Elsevier Science B.V.