Carbon dioxide exchange characteristics of C4 Hawaiian Euphorbia species native to diverse habitats.
The characteristics of the photosynthetic apparatus of 11 Hawaiian Euphorbia species, all of which possess C4 photosynthesis but range from arid habitat, drought-deciduous shrubs to mesic or wet forest evergreen trees and shrubs, were investigated under uniform greenhouse conditions. Nine species exhibited CO2 response curves typical of C4 plants, but differed markedly in photosynthetic capacity. Light-saturated CO2 uptake rates ranged from 48 to 52 μmol m-2 s-1 in arid habitat species to 18 to 20 μmol m-2 s-1 in mesic and wet forest species. Two possessed unusual CO2 response curves in which photosynthesis was not saturated above intercellular CO2 pressures [p(CO2)] of 10 to 15 Pa, as typically occurs in C4 plants.Both leaf (g'1) and mesophyll (g'm) conductances to CO2 varied widely between species. At an atmospheric p(CO2) of 32 Pa, g'1 regulated intercellular p(CO2) at 12-15 Pa in most species, which supported nearly maximum CO2 uptake rates, but did not result in excessive transpiration. Intercellular p(CO2) was higher in the two species with unusual CO2 response curves. This was especially apparent in E. remyi, which is native to a bog habitat. The regulation of g'1 and intercellular p(CO2) yielded high photosynthetic water use efficiencies (P/E) in the species with typical CO2 response curves, whereas P/E was much lower in E. remyi.Photosynthetic capacity was closely related to leaf nitrogen content, whereas correlations with leaf morphological characteristics and leaf cell surface area were not significant. Thus, differences in photosynthetic capacity may be determined primarily by investment in the biochemical components of the photosynthetic apparatus rather than by differences in diffusion limitations. The lower photosynthetic capacities in the wet habitat species may reflect the lower light availability. However, other factors, such as reduced nutrient availability, may also be important.