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 prop erties during leaf development were studied in an evergreen broad-leaved tr ee, Castanopsis sieboldii and an annual herb, Phaseolus vulgaris. In C. sie boldii, 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 R ubisco contents on a leaf area basis increased to attain their maximal valu es 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 g reater 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 photosy nthesis 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 probab ly due to the decrease in CO2 concentration in the chloroplast caused by th e increases in thickness of mesophyll cell walls and in Rubisco content per chloroplast surface area. These results clearly showed that: (1) in C. sie boldii, chloroplast development proceeded more slowly than mesophyll cell e xpansion and continued well after FLE, whereas in P. vulgaris these process es proceeded synchronously and were completed by FLE; (2) after FLE, photos ynthesis in leaves of C. sieboldii was markedly limited by g(i). From these results, it is suggested that, in the evergreen broad-leaved trees, mechan ical protection of mesophyll cells has priority over the efficient CO2 tran sfer and quick construction of the chloroplasts.