RESPONSES OF 2 HEMIEPIPHYTIC FRUIT CROP CACTI TO DIFFERENT DEGREES OFSHADE

Selenicereus megalanthus and Hylocereus polyrhizus, hemiepiphytic cact i of shady habitats, have recently been introduced as fruit crops to t he Negev Desert of Israel. Since they become bleached and die when the y are grown in full sunlight, the shading responses of these cacti wer e studied. Both species showed typical CAM CO2 fixation, namely CO2 wa s fixed during the night, followed by acid accumulation. Nocturnal aci d accumulation and the concentration of photosynthetic pigments (chlor ophyll and carotenoids) were determined in short term experiments in t he summer under full sunlight or in net houses with 30, 60 or 90% shad e. The effect of shading on morphological traits, growth and fruiting was determined in long term experiments, in net-houses only, since pla nts usually degenerate under full sunlight. S. megalanthus was found t o be more sensitive to high light flux density and better adapted to d eep shade than H. polyrhizus, as judged from the more marked decrease in both nocturnal acid accumulation and in the concentrations of chlor ophyll and carotenoids under full sunlight in the former species. In a ddition, under 90% shade the stem biomass of S. megalanthus was reduce d less than that of H. polyrhizus. In response to the shade, stem ribs became shallow, root dry weight decreased and shoot water content inc reased in both species. The most favorable conditions for growth and f ruit production were found to be 30% shade for H. polyrhizus while for S. megalanthus 60% shade seemed to be preferable, since it facilitate d higher acid accumulation, carotenoid and chlorophyll content, and be tter stem appearance. H. polyrhizus was distinguished by its higher fr uit yield, estimated at 16 ton ha(-1) vs. 3.8 ton ha(-1) for S. megala nthus in the second year after planting when both species were growing under optimal conditions. The differences between the species in term s of light adaptation may be partially related to the wax layer coatin g the stem and sunken stomata of H. polyrhizus compared to S. megalant hus, which had neither wax nor sunken stomata. (C) 1998 Elsevier Scien ce B.V.