Rubisco is a large enzyme with a molecular mass of approximately 550 kD. The maximum rate of CO 2 fixation (i.e. ribulose-1,5-bisphosphate [RuBP] carboxylation) at CO 2 saturation is only 15 to 30 mol CO 2 mol 21 Rubisco protein s 21 at 25°C. Affinity to CO 2 is also low, and the K m , K c , at 25°C in the absence of oxygen is comparable to the CO 2 concentration in water equilibrated with air containing 39 Pa CO 2 (approximately 390 mL L 21 ), 13 mM. Moreover, RuBP carboxylation is competitively inhibited by RuBP oxygenation, which is the primary step of the energy-wasting process, photorespiration. If the CO 2 concentration in the chloroplast stroma is low, the carboxylation rate will decrease while the oxygenation rate will increase. Under such conditions, light energy and other resources, including nitrogen and water, are all wasted, eventually leading to a decrement of fitness of the plants. From these data, we may consider that structural features of the leaf contributing to the maintenance of the high CO 2 concentration in the chloroplast stroma may have been selected during evolution.In this Update, we focus on the key structural features that affect CO 2 concentration in the chloroplast stroma. First, we analyze the conductance for CO 2 diffusion from the substomatal cavity to the chloroplast stroma (mesophyll conductance [g m ], also called internal conductance). Because the low g m limits photosynthesis,

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.