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Shading requirements under the conditions of the northern Negev desert of Israel, and the effect of elevated CO2 on CO2 uptake were studied for two promising fruit crops, Selenicereus megalanthus and Hylocereus polyrhizus. Both are hemiepiphytic cacti of shady habitats. Stem length, dry matter and morphological traits were determined in plants cultivated under different shade levels in two orchards; one in Beer-Sheva (30, 60, 90% shade) and the other in Besor (30 and 60% shade). Growth occurred during the warm season, and highest biomass was obtained at 30% shade. S. megalanthus was found to be better adapted to deep shade; this was expressed in a lower reduction in stem biomass with increased shading. Several morphological adaptations to shade were observed: branches tended to grow horizontally, ribs became shallower, and the root/stem ratio decreased. Both species showed CAM behavior with CO2 uptake occurring mainly during the night. For both species daily CO2 uptake was 30% higher under conditions of 1000 ppm CO2 than under ambient conditions.