CO2 uptake by the cultivated hemiepiphytic cactus, Hylocereus undatus

The climate of the native tropical forest habitats of Hylocereus undatus, a hemiepiphytic cactus cultivated in 20 countries for its fruit, can help explain the response of its net CO2 uptake to environmental factors. Under wet conditions, about 85% of the total daily net CO2 uptake occurs at night via Crassulacean acid metabolism, leading to a high water-use efficiency. Total daily net CO2 uptake is reduced 57% by only 10 days of drought, possibly involving stomatal closure induced by abscisic acid produced in the roots, which typically occupy a small substrate volume. Total daily net CO2 uptake for H. undatus is maximal at day/night air temperatures of 30/20degreesC, optimal temperatures that are higher than those for desert cacti but representative of ambient temperatures in the tropics; its total daily net CO2 uptake becomes zero at day/night air temperatures of 42/32degreesC. Stem damage occurs at 45degreesC for H. undatus, whose photosynthetic cells show little acclimation to high temperatures compared with other cacti and are also sensitive to low temperatures, -1.5degreesC killing half of these cells. Consistent with its shaded habitat, total daily net CO2 uptake is appreciable at a total daily PPF of only 2 mol m(-2) day(-1) and is maximal at 20 mol m(-2) day(-1), above which photoinhibition reduces net CO2 uptake. Net CO2 uptake ability, which is highly correlated with stem nitrogen and chlorophyll contents, changes only gradually (halftimes of 2-3 months) as the concentration of applied N is changed. Doubling the atmospheric CO2 concentration raises the total daily net CO2 uptake of H. undatus by 34% under optimal conditions and by even larger percentages under adverse environmental conditions.