Comparative ecophysiology of CAM and C<inf>3</inf> bromeliads. III. Environmental influences on CO<inf>2</inf> assimilation and transpiration
GRIFFITHS H., LÜTTGE U., STIMMEL K., CROOK CE., GRIFFITHS NM., SMITH JAC.
Abstract Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to compare carbon assimilation with water use in CAM and C 3 photosynthesis. The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO 2 uptake was restricted to the end of the dark period in plants found at drier habitats, and stomatal conductance in two CAM species was found to respond directly to humidity or temperature. Total night‐time CO 2 uptake, when compared with malic‐acid formation (measured as the dawn‐dusk difference in acidity, ΔH + ), could only account for 10–40% of the total ΔH + accumulated. The remaining malic acid must have been derived from the refixation of respired CO 2 (recycling). Within the genus Aechmea (12 samples from four species), recycling was significantly correlated with night temperature at the six sample sites. Recycling was lowest in A. fendleri (54% of ΔH + derived from respired CO 2 ), a CAM bromeliad with little water‐storage parenchyma that is restricted to wetter, cooler regions of Trinidad. Gas‐exchange rates of C 3 bromeliads were found to be similar to those of the CAM bromeliads, with CO 2 uptake from 1 to 3 μmol m −2 s −1 and stomatal conductances generally up to 100 mmol m −2 s −1 . The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO 2 uptake of the C 3 bromeliad Guzmania lingulata was saturated at around 500 μmol m −2 s −1 PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade‐tolerant rather than shade‐demanding. Transpiration ratios (TR) during CO 2 fixation in CAM (Phase I and IV) and C 3 bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO 2 , TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO 2 uptake in CAM (42 ± 41, n= 12), Phase IV of CAM (69 ± 36, n= 3) and for C 3 photosynthesis (99 ± 73, n= 4) in these plants. It appears that recycling of respired CO 2 by CAM bromeliads and efficient use of water in all phases of CO 2 uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics. Copyright © 1986, Wiley Blackwell. All rights reserved