Evolution of C₃ and C₄ plants along an environmental moisture gradient: patterns of photosynthetic differentiation in Hawaiian Scaevola and Euphorbia species.

Light-saturated photosynthetic capacities range from 12.0n24.7 mu mol CO₂ m^-2 s^-1 in the Scaevola species and from 18.2-51.4 mu mol CO₂ m^-2s^-1 in the Euphorbia species. Within each genus, differences in light-saturated photosynthetic capacity are paralleled by differences in mesophyll and leaf conductances to CO₂. Within each habitat, the C₄ Euphorbia species exhibits a significantly higher photosynthetic capacity and a significantly higher mesophyll conductance than the corresponding C₃, Scaevola species, differences greatest in the dry scrub habitat and least in the wet forest habitat. One photosynthetic characteristic that exhibits little variation among the species within each genus, yet that exhibits a consistently large difference between the species within each habitat, is photosynthetic water-use efficiency. The C₄ Euphorbia species possess water-use efficiencies that are 2-3.5 times as high as those of the C₃ Scaevola species. -from Authors