Slow development of leaf photosynthesis in an evergreen broad‐leaved tree, Castanopsis sieboldii: relationships between leaf anatomical characteristics and photosynthetic rate
Changes in net photosynthetic rate on a leaf area basis and anatomical properties during leaf development were studied in an evergreen broad-leaved tree, Castanopsis sieboldii and an annual herb, Phaseolus vulgaris. In C. sieboldii, surface area of mesophyll cells facing the intercellular air spaces on a leaf area basis (S mes ) was already considerable at the time of full leaf area expansion (FLE). However, surface area of chloroplasts facing the intercellular air spaces on a leaf area basis (S c ), and chlorophyll and Rubisco contents on a leaf area basis increased to attain their maximal values 15-40 d after FLE. In contrast, in P. vulgaris, chloroplast number on a leaf area basis, S c and S mes at 10 d before FLE were two to three times greater than the steady-state levels attained at around FLE. In C. sieboldii, the internal CO 2 transfer conductance (g i ) slightly increased for 10 d after FLE but then decreased toward the later stages. Limitation of photosynthesis by g i was only about 10% at FLE, but then increased to about 30% at around 40 d after FLE. The large limitation after FLE by g i was probably due to the decrease in CO 2 concentration in the chloroplast caused by the increases in thickness of mesophyll cell walls and in Rubisco content per chloroplast surface area. These results clearly showed that: (1) in C. sieboldii, chloroplast development proceeded more slowly than mesophyll cell expansion and continued well after FLE, whereas in P. vulgaris these processes proceeded synchronously and were completed by FLE; (2) after FLE, photosynthesis in leaves of C. sieboldii was markedly limited by g i . From these results, it is suggested that, in the evergreen broad-leaved trees, mechanical protection of mesophyll cells has priority over the efficient CO 2 transfer and quick construction of the chloroplasts.