Daily changes in stem thickness and related gas exchange patterns for the hemiepiphytic cactus Hylocereus undatus

Pairs of linearly variable differential transducers, with a sensitivity of 1 mm, were used to compare daily changes in stem thickness with transpiration and net CO2 uptake for the hemiepiphyte Hylocereus undatus and to examine the possible effects of hormonal signals from its roots. Two daily patterns of transpiration were observed for well-watered H. undatus: solely nocturnal stomatal opening, which can occur for obligate crassulacean acid metabolism ( CAM), and CAM with substantial daytime stomatal opening associated with C-3 photosynthesis. Each caused different daily patterns of stem thickness, which could be affected by changes in relative humidity within 1 min. Removing possible root signals by severing distal stem segments or by rapidly drying the root system increased the maximal transpiration rate over 4 d, indicating that root hormonal signals influence stomatal opening, even under well-watered conditions. Rewatering plants that had their root systems air dried for 4 d resulted in rapid water uptake, reduced the magnitude of daily thickness changes, and led to the recovery of original stem thickness in ca. 4 d. Stem injections of abscisic acid equivalent to a 1 mu M average tissue concentration halved transpiration and eliminated daily net CO2 uptake. Measurements of stem thickness can help interpret patterns of water movement and the influence of hormonal signals on gas exchange for this vinelike cactus.