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 C3photosynthesis. The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO2uptake 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 CO2uptake, 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 CO2(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 CO2), a CAM bromeliad with little water‐storage parenchyma that is restricted to wetter, cooler regions of Trinidad. Gas‐exchange rates of C3bromeliads were found to be similar to those of the CAM bromeliads, with CO2uptake from 1 to 3 μmol m−2s−1and stomatal conductances generally up to 100 mmol m−2s−1. The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO2uptake of the C3bromeliad Guzmania lingulata was saturated at around 500 μmol m−2s−1PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade‐tolerant rather than shade‐demanding. Transpiration ratios (TR) during CO2fixation in CAM (Phase I and IV) and C3bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO2, TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO2uptake in CAM (42 ± 41, n= 12), Phase IV of CAM (69 ± 36, n= 3) and for C3photosynthesis (99 ± 73, n= 4) in these plants. It appears that recycling of respired CO2by CAM bromeliads and efficient use of water in all phases of CO2uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics. Copyright © 1986, Wiley Blackwell. All rights reserved