Environmental characteristics, field water relations, and photosynthetic responses of C Hawaiian Euphorbia species from contrasting habitats.

Four endemic Hawaiian Euphorbia species range in habitat from open arid coastal strand to shaded mesic forest and in growth-form from small prostrate shrubs to trees. As shown in the present study, these large differences in habitat and growth-form are paralleled by equally large differences in maximal photosynthetic rate (13.7 to 37.1 μmol CO ms), dark respiration rate (0.7 to 4.1 μmol CO ms), light level for saturation of photosynthesis (0.9 to over 2.0 mmol ms), light compensation point (0.01 to 0.11 mmol ms), leaf conductance to CO (1.7 to 4.9 mm s), and mesophyll conductance to CO (3.7 to 8.5 mm s). A principal consequence of this differentiation is that the capacity for photosynthesis at high light levels is higher in open site species, such as E. celastroides and E. degeneri, and at low light levels is higher in shade species, such as E. forbesii. E. hillebrandii, a species from intermediate semiopen habitats, exhibits an intermediate photosynthetic capacity at both high and low light levels. Despite this remarkable diversity, all four species exhibit the distinguishing physiological features of C photosynthesis.