Irradiance and phenotype: comparative eco-development of sun and shade leaves in relation to photosynthetic CO₂ diffusion

The subject of this paper, sun leaves are thicker and show higher photosynthetic rates than the shade leaves, is approached in two ways. The first seeks to answer the question: why are sun leaves thicker than shade leaves? To do this, CO₂ diffusion within a leaf is examined first. Because affinity of Rubisco for CO₂ is low, the carboxylation of ribulose 1,5-bisphosphate is competitively inhibited by O₂, and the oxygenation of ribulose 1,5-bisphosphate leads to energy-consuming photorespiration, it is essential for C₃ plants to maintain the CO₂ concentration in the chloroplast as high as possible. Since the internal conductance for CO₂ diffusion from the intercellular space to the chloroplast stroma is finite and relatively small, C₃ leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO₂ dissolution and transport. This explains why sun leaves are thicker. The second approach is mechanistic or 'how-oriented'. Mechanisms are discussed as to how sun leaves become thicker than shade leaves, in particular, the long-distance signal transduction from mature leaves to leaf primordia inducing the periclinal division of the palisade tissue cells. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells. Issues of cell size are discussed to understand plasticity in leaf thickness.