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 CO2 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 CO2 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 CO2 transfer and quick construction of the chloroplasts.