Abstract
Crassulacean acid metabolism (CAM) is a photosynthetic pathway that conserves water by restricting stomatal opening, hence water loss, to the night. Thus, it is not surprising to find that floras of arid regions include a large number of CAM plants. In addition, CAM is common among epiphytes in tropical and subtropical environments, where water is plentiful. Despite the latter, evidence exists that comparatively short periods of drought, coupled with the unusual morphology and/or microhabitat of such epiphytes, comprise stress substantial enough that the CAM pathway proves highly adaptive.
It is more difficult to explain the adaptive significance of CAM in tropical and subtropical rain forests with exceedingly high annual rainfall, i.e., in excess of 4 meters per year. This review explores a variety of hypotheses proffered as explanations for this apparent conundrum. Such hypotheses include:
1. CAM allows an epiphytic CAM plant to capitalize on the high concentrations of atmospheric CO2 at night in the canopies of its host trees;
2. even in such rain forests, rainless periods are long enough and/or frequent enough that CAM proves advantageous as a water conservation adaptation.
3. The ability of many CAM plants to assimilate CO2 during both the day and night is advantageous in such environments.
4. The increase in daytime CO2 levels in CAM photosynthetic tissue prevents photoinhibition and minimizes CO2 losses during the day.
5. The high acid content of CAM photosynthetic tissue, at least early in the day, deters herbivory.
6. The stimulation of CO2 uptake following wetting of the leaves reported for at least one epiphytic CAM plant is advantageous in such high-rainfall environments.
7. The low demand for essential elements observed in at least one CAM plant increases the nutrient efficiency of such plants, which is adaptive in rain forests with excessive rainfall and, hence, high levels of nutrient leaching from the leaf tissues.
8. Guttation resulting from osmotically-driven high tissue water contents benefits CAM plants several ways.
9. Epiphytic CAM plants are well-adapted to shady microhabitats, comprising a valuable adaptation to the cloudy environment of high-rainfall rain forests.
10. CAM is not beneficial for such plants in these environments for at least 3 possible reasons.
All of the hypotheses have only rarely been examined in past studies. Thus, all are in need of further investigation.