CO2 UPTAKE AND FLUORESCENCE RESPONSES FOR A SHADE-TOLERANT CACTUS HYLOCEREUS-UNDATUS UNDER CURRENT AND DOUBLED CO2 CONCENTRATIONS

Hylocereus undatus (Haworth) Britton and Rose growing in controlled en vironment chambers at 370 and 740 mu mol CO2 mol(-1) air showed a Cras sulacean acid metabolism (CAM) pattern of CO2 uptake, with 34% more to tal daily CO2 uptake under the doubled CO2 concentration and most of t he increase occurring in the late afternoon. For both CO2 concentratio ns, 90% of the maximal daily CO2 uptake occurred at a total daily phot osynthetic photon flux density (PPFD) of only 10 mol m(-2) day(-1) and the best day/night air temperatures were 25/15 degrees C. Enhancement of the daily net CO2 uptake by doubling the CO2 concentration was gre ater under the highest PPFD (30 mol m(-2) day(-1)) and extreme day/nig ht air temperatures (15/5 and 45/35 degrees C). After 24 days of droug ht, daily CO2 uptake under 370 mu mol CO2 mol(-1) was 25% of that unde r 740 mu mol CO2 mol(-1). The ratio of variable to maximal chlorophyll fluorescence (F-v/F-m) decreased as the PPFD was raised above 5 mol m (-2) day(-1), at extreme day/night temperatures and during drought, su ggesting that stress occurred under these conditions. F-v/F-m was high er under the doubled CO2 concentration, indicating that the current CO 2 concentration was apparently limiting for photosynthesis. Thus net C O2 uptake by the shade-tolerant H. undatus, the photosynthetic efficie ncy of which was greatest at low PPFDs, showed a positive response to doubling the CO2 concentration, especially under stressful environment al conditions.