CO₂ uptake and fluorescence responses for a shade‐tolerant cactus <i>Hylocereus undatus</i> under current and doubled CO₂ concentrations

Abstract

Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO 2 mol −1 air showed a Crassulacean acid metabolism (CAM) pattern of CO 2 uptake, with 34% more total daily CO 2 uptake under the doubled CO 2 concentration and most of the increase occurring in the late afternoon. For both CO 2 concentrations, 90% of the maximal daily CO 2 uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m −2 day −1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO 2 uptake by doubling the CO 2 concentration was greater under the highest PPFD (30 mol m −2 day −1 ) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO 2 uptake under 370 μmol CO 2 mol −1 was 25% of that under 740 μmol CO 2 mol −1 . The ratio of variable to maximal chlorophyll fluorescence (F y /F m ) decreased as the PPFD was raised above 5 mol m −2 day −1 , at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F v /F m was higher under the doubled CO 2 concentration, indicating that the current CO 2 concentration was apparently limiting for photosynthesis. Thus net CO 2 uptake by the shade‐tolerant H. undatus , the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO 2 concentration, especially under stressful environmental conditions.