Stem water relations and net CO2 uptake for a hemiepiphytic cactus during short-term drought

Hylocereus undatus is widely distributed naturally and is currently cultivated in 19 countries for fruit. Because of its relatively thin stems, H. undatus was hypothesized to respond to drought more rapidly than other cacti. Stem water potential, water content and thickness were monitored during drought to provide easily measured parameters to be correlated with net CO2 uptake ability, allowing the development of irrigation schedules to optimize water-use efficiency. H. undatus exhibited Crassulacean acid metabolism, as maximal stomatal opening and net CO2 uptake occurred at night. Although the soil water potential decreased to −4.2 MPa during 12 days without watering, the stem water status parameters remained near their values under wet conditions (Ψstem of −0.67 MPa, water content of 90.8%, thickness of 4.48 mm). The drought was accompanied by a 63% decrease in the maximal water vapor conductance and a 57% decrease in the maximal net CO2 uptake rate, but when the roots were excised for plants under wet conditions, neither parameter decreased appreciably over a comparable time period. Injection of 100 μM abscisic acid into attached stems and placing cut ends of detached stems in such a solution substantially reduced gas exchange 1 day later; at 2 days after injecting the hormone, the maximal water vapor conductance was similar to the minimal daytime values under wet conditions and the net CO2 uptake rate was inhibited by 97%. Abscisic acid produced in the roots apparently leads to stomatal closure for this hemiepiphyte—whose roots can occur in very limited soil volumes—as soon as the water supply starts to deplete rather than after a large fraction of its stem water is transpired. [Copyright &y& Elsevier]