Photosynthetic characteristics and chloroplast ultrastructure of C3 and C 4 tree species grown in high- and low-light environments.

Plants of the C4 tree species, Euphorbia forbesii, Sherff and the C3 tree species, Claoxylon sandwicense Muell-Arg., were grown in a full sun and a shade environment designed to simulate the understory of their native Hawiian forest habitat. When grown under shade conditions, both species exhibited a photosynthetic light response typical of shade plants with low light compensation points and low dark respiration rates. E. forbesii, however, exhibited greater acclimation of light saturated photosynthetic rates and no evidence of photoinhibition in high light. In contrast, quantum yields for CO2 uptake and chlorophyll contents were reduced in the high-light as compared to the low-light grown C. sandwicense plants. Both species exhibited similar changes in the intercellular CO2 response curves and chloroplast whole-chain electron transport capacities, suggesting that the underlying mechanisms of light acclimation are similar. Chloroplasts of E. forbesii exhibited large changes in ultrastructure, with much greater thylakoid membrane development in low than high light. In contrast, C. sandwicense exhibited different starch contents, but otherwise similar membrane development in high and low light. The results show that E. forbesii possesses a very flexible photosynthetic apparatus which may account for its ability to survive in the understory of shaded forests.