Abstract
Bulbophyllum involutum (Ochidaceae) grow in areas with outcrops and high light heterogeneity. We considered that this intermediate C3-CAM species should have more expressive CAM metabolism, and leaf succulence (mainly in tissues/structures related to water storage or economy) in individuals that have grown exposed to high light intensities. Here, we investigated (1) if the leaves of B. involutum undergo significant morphological and anatomical changes when growing under different light regimes; (2) which leaf structural features were most influenced by different light levels; and (3) what is the interplay between such variation and the photosynthetic metabolism, and its relation to the ecophysiology of B. involutum. For that, the plants were selected along a transect (across the outcrop) into three classes according to the light condition under which they grown: sun (receiving above 35molm−2day−1 of photosynthetically active radiation [PAR]); partially shaded (between 14 and 23molm−2day−1 of PAR); and shaded (less than 8molm−2day−1 of PAR). The leaves of plants growing in direct sunlight were smaller, thicker and showed high leaf specific mass. The dry mass of leaf per unit area increased during the dry season, indicating that some carbohydrates play important role in the acid metabolism (PEP synthesis). They could help to reduce cell water potential in leaves, facilitating water flux and the maintenance of leaf succulence throughout the year. The leaf succulence increases the possibility of organic acid storage during the night by the C3-CAM photosynthetic pathway. The low flexibility of C3-CAM metabolism in B. involutum leaves didn't show high expression of CAM in dry season, but seems to be helpful to these plants' establishment at variable light environments.
Highlights
Crassulacean acid metabolism (CAM) is a highly variable process, and can show different physiotypes depending on environmental factors such as CO2 concentrations, water and light availability, as well as temperature (Dodd et al, 2002; Lüttge, 2004)
Some CAM species open their stomata during the day to assimilate direct atmospheric CO2 via rubisco, and this CO2 is fixed as organic acids via PEP-carboxylase at night (Lüttge, 2004); other species though exhibit C3 photosynthesis under favorable environmental conditions, while expressing CAM metabolism under stress circumstances (Lüttge, 2004)
In the present study we addressed the following questions: does the occupation of sites characterized by different levels of photosynthetically active radiation (PAR) imply significant leaf morphological and anatomical differences? If this variation exists, which leaf structural features are most influenced by the different light levels? What is the interplay between such variation and the photosynthetic metabolism, and its relation to the ecophysiology of B. involutum?
Summary
Crassulacean acid metabolism (CAM) is a highly variable process, and can show different physiotypes depending on environmental factors such as CO2 concentrations, water and light availability, as well as temperature (Dodd et al, 2002; Lüttge, 2004). Some organisms keep their stomata closed during most of the day and night, while fixing CO2 produced from the metabolism of organic acids. Some CAM species open their stomata during the day to assimilate direct atmospheric CO2 via rubisco, and this CO2 is fixed as organic acids via PEP-carboxylase at night (Lüttge, 2004); other species though exhibit C3 photosynthesis under favorable environmental conditions, while expressing CAM metabolism under stress circumstances (such as those produced through water deficits) (Lüttge, 2004). The environmental factors that affect CAM expression, may affect leaf structure
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