The Response of the C3-CAM Tree,Clusia rosea, to Light and Water Stress

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Abstract
Gas exchange measurements were undertaken on 2-year-old plants of Clusia rosea. The plants were shown to have the ability to switch from C3-photosynthesis to CAM and vice versa regardless of leaf age and, under some conditions, CO2 was taken up continuously, throughout the day and night. The light response was saturated by 120 μmol m−2 s−1 typical of a shade plant. Gas exchange patterns in response to light, water and VPD were examined. All combinations of daytime and night-time CO2 uptake were observed, with rates of CO2 uptake ranging from 2 to 11 μmol m−2 s−1 depending upon water status and light. Categorization of this plant asC3, CAM or an intermediate is impossible. Differing VPD affected the magnitude of changes from CAM to C3-photosynthesis (0 to 0.5 and 0 to 6.0 μmol m−2 s−1 CO2, respectively) when plants were watered. Under well-watered conditions, but not under water stress, high PPFD elicited changes from CAM to C3 gas exchange. This is unusual not only for a shade plant but also for a plant with CAM. It is of ecological importance for C. rosea, which may spend the early years of its life as an epiphyte or in the forest understorey, to be able to maximize photosynthesis with minimal water loss.