This illustrates why the use of only isotopic measurements to
determine the type of photosynthetic metabolism tends to underestimate the number of
species capable of expressing CAM photosynthesis. Therefore, other parameters, such as
day/night fluctuations in titratable acidity, activity of enzymes of CAM and diurnal patterns
of gas exchange, are needed to determine the photosynthetic pathway.
Other problems that researchers need to face when determining the photosynthetic pathway
of orchid plants is the fact that many experiments are planned using only some parts of the
plant, normally leaves, and other organs such as roots, pseudobulbs and flowers are often
not included in the study analysis. Moreover, few studies give attention to the fact that some
orchids have plasticity in switching between C3 and CAM photosynthesis in response of
changes in environmental conditions.
Despite the existence of technical difficulties in studying the CAM features, there is no
doubt that CAM plants have sparked the curiosity of many researchers around the world
for decades. Nowadays, it is well known that the majority of CAM plants have been found
living in the epiphytic habitat along with many species of Orchidaceae. The proportion of
CAM epiphytic orchid flora is completely associated with the degree of water availability in
the ecosystems. The occurrence of CAM orchid species increases from wet tropical rainforest
and moist tropical forests to dry forests, and steadily declines with increasing altitude,
which is entirely related with the increase of mean annual precipitation, and, within a single
site, the percentage of CAM epiphytic orchids increases with canopy height. The scarcity of
water is arguably the most important and severe abiotic stress in the epiphytic habitat. The
epiphytic orchids need to adjust water-balance mechanisms in all plant tissues rapidly and
constantly, via appropriate stomatal and photosynthetic responses, to avoid irreversible
drought injuries and maintain water storage. The drought endurance observed in the
majority of epiphytic orchids is provided by a strong CAM photosynthetic behavior, which
promotes a very favorable water economy.
The CAM features also appear to be linked with succulence in orchid plant tissues. The
existence of a thicker chlorenchyma tissue, which was detected in some strong CAM
orchids, can be important to increase the capacity of organic acid storage. Interestingly, the
thickness of the chlorenchyma is not entirely associated with leaf thickness. In the majority
of cases the succulence is due to the presence of a thick hydrenchyma, which does not
contribute to CAM in terms of night-produced organic acid storage capacity. Therefore,
there are epiphytic orchid species with thin leaves performing CAM, while some species
with thick leaves display typical C3 photosynthesis.