While quantitative census data for CAM diversity and biomass are largely missing, intuition suggests that the larger CAM domains are those systems which are governed by a network of interacting stress factors requiring versatile responses and not systems where a single stress factor strongly prevails. CAM is noted to be a strategy for variable, flexible and plastic niche occupation rather than lush productivity. 'Physiological syn-ecology' reveals that phenotypic plasticity constitutes the ecophysiological advantage of CAM.

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The Chenopodiaceae Suaeda salsa L. was grown under different salt concentrations and under osmotic stress. The fresh weight was markedly stimulated by 0.1 M NaCl, 0.4 M NaCl and 0.1 M KCl and reduced by osmotic stress (PEG iso-osmotic to 0.1 M NaCl). Treatment with 0.4 M KCl severely damaged the plants. Membrane vesicle fractions containing tonoplast vesicles were isolated by sucrose gradient from leaves of the S. salsa plants and modulations of V-ATPase and V-PPase depending on the growth conditions were determined. Western blot analysis revealed that V-ATPase of S. salsa consists of at least nine subunits (apparent molecular masses 66, 55, 52, 48, 36, 35, 29, 18, and 16 kDa). This polypeptide pattern did not depend on culture conditions. V-PPase is composed of a single polypeptide (69 kDa). An additional polypeptide (54 kDa) was detected in the fractions of NaCl-, KCl- and PEG-treated plants. It turned out that the main strategy of salt-tolerance of S. salsa seems to be an up-regulation of V-ATPase activity, which is required to energize the tonoplast for ion uptake into the vacuole, while V-PPase plays only a minor role. The increase in V-ATPase activity is not obtained by structural changes of the enzyme, but by an increase in V-ATPase protein amount.

Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to eompare carbon assimilation with water use in CAM and C3 photosynthesis.The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO2 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 direetly to humidity or temperature. Total night-time CO2 uptake, when compared with malic-acid formation (measured as the dawn-dusk difference in acidity, AH^), could only account for 10-40% of the total AH"" accumulated. The remaining malic acid must have been derived from the refixation of respired CO2 (recycling). Within the genus Aechtnea (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 AH"^ derived from respired CO2), a CAM bromeliad with little water-storage parenchyma that is restricted to wetter, cooler regions of Trinidad.Gas-exehange rates of C3 brotneliads were found to be similar to those of the CAM bromeliads, with CO2 uptake from 1 to 3 ^mol m"^ s~' and stomatal conductances generally up to 100 mmol m~^ s"T he midday depression of photosynthesis oeeurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthesis in shaded habitats. CO2 uptake of the C3 bromeliad Guzmatiia lingulata was saturated at around 500/miol m"^ s~' PAR, suggesting that epiphytie plants found in the Abbreviations: CAM, erassulacean acid metabolism; PAR, photosynthetically active radiation.shaded forest understorey are shade-tolerant rather than shade-demanding.Transpiration ratios (TR) during CO2 fixation in CAM (Phase 1 and IV) and C3 bromeliads were cotnpared at different sites iti order to assess the eflicieney of water utilization. For the epiphytes displaying marked uptake of CO2, TR were found to be lower than many previously pubfished values. In addition, the average TR values were very similar for dark COj uptake in CAM (42 + 41, n = 12), Phase IV of CAM (69 ±36, 0 = 3) and for C3 photosynthesis (99 ±73, /i = 4) in these plants. It appears that recycling of respired CO2 by CAM brotneliads and efilcient use of water in all phases of CO2 uptake are physiological adaptations of brotneliads to arid microclimates in the humid tropics.