The highly drought-tolerant pitaya (Hylocereus undatus) is a non-facultative CAM plant under both well-watered and drought conditions

The effects of water deficits on the growth and physiology of potted pitaya plants were examined in this study. Soil water decreased from 80% to 15% (severe deficit) over 16 weeks in non-irrigated pots, while stem water decreased from 90% to 80% (small deficit). Drought decreased final stem growth, chlorophylls, and F_v/F_m by 18% to 30% compared to the control. The stomata were mainly open from 1600 to 2400 h, with drought decreasing the percentage of stomata that were open and their aperture size. Starch grains accumulated in the chloroplasts during the day, with fewer starch grains under drought. Sugars and starch accumulated in the stem throughout the day and peaked at 2000 or 2400 h, while acids were lowest at 1600 h in the control. They were all significantly lower and relatively stable under drought. RUBPC activity peaked at 1200 h and PEPC activity at 2000 to 2400 h in the controls. Both were relatively stable with PEPC activity always lower under drought. Pitaya is a non-facultative CAM plant with strong ability to hold water under severe drought. Drought reduces short-term carbon production but does not appear to have a large effect on stem growth which possibly uses stored reserves.