Crassulacean acid metabolism under severe light limitation: a matter of plasticity in the shadows?

Despite the increased energetic costs of CAM in comparison to C3 photosynthesis, it is hypothesized that the inherent photosynthetic plasticity of CAM allows successful acclimation to light limiting conditions. The present work sought to determine if CAM presented any constraints to short and longer term acclimation to light limitation and to establish if and how metabolic and photosynthetic plasticity in the deployment of the 4 phases of CAM might facilitate acclimation to conditions of deep shade. Measurements of leaf gas exchange, organic acids, starch and soluble sugar (glucose, fructose and sucrose) contents were made in the leaves of the constitutive CAM bromeliad Aechmea ‘Maya’ over a 3 month period under severe light limitation. A. `Maya` was not particularly tolerant of severe light limitation in the short term. A complete absence of net CO2 uptake and fluctuations in key metabolites (i.e. malate, starch or soluble sugars) indicated a dampened metabolism whilst cell death in the most photosynthetically active leaves was attributed to an over-acidification of the cytoplasm . However in the longer term, plasticity in the use of the different phases of gas exchange and different storage carbohydrate pools ensured a positive carbon balance for this CAM species under extremely low levels of irradiance. As such, coordinated plasticity in the use of C3 and C4 carboxylases and different carbohydrate pools together with an increase in the abundance of light harvesting complexes appear to underpin the adaptive radiation of the energetically costly CAM pathway within light-limiting environments such as wet cloud forests and shaded understories of tropical forests.status: publishe