Light-acclimation of cladode photosynthetic potentials in Casuarina glauca: trade-offs between physiological and structural investments.

Many arid and saline habitat species possess sparse canopies with cylindrical foliage that is considered relatively invariable along environmental gradients. However, even in sparse canopies strong gradients of light develop between the canopy top and bottom. We studied structural and photosynthetic acclimation to within-canopy light gradient in Casuarina glauca Sieb. ex Spreng., the photosynthetic organs of which are cylindrical cladodes. Seasonal average integrated quantum flux density (Q) varied 25-fold between the canopy top and the canopy bottom. Cladode cross-sectional shape was unaffected by irradiance, but cladode dry mass per unit total area (M) varied 2-fold within the canopy light gradient. This resulted primarily from light-dependent changes in cladode thickness (volume to total area ratio,V / A) and to a lesser extent from changes in cladode density (D, M = DV / A). Nitrogen content, and the volume of mesophyll per unit surface area increased with increasing Q and V / A, resulting in positive scaling of foliage photosynthetic potential (capacity of photosynthetic electron transport and maximum Rubisco carboxylase activity per unit area) with light. However, nitrogen content per unit dry mass and the volume fraction of mesophyll decreased with increasing irradiance. This was explained by greater fractional investment in mechanical tissues in cladodes with greater volume to surface area ratio. This trade-off between photosynthetic and support investments reduced the cladode photosynthetic plasticity. Our study demonstrates a significant acclimation potential of species with cylindrical foliage that should be included in larger-scale carbon balance estimations of arid and saline communities.