The Light Dependence of Mesophyll Conductance and Relative Limitations on Photosynthesis in Evergreen Sclerophyllous Rhododendron Species.

Mesophyll conductance (g(m)) limits CO2 diffusion from sub-stomatal internal cavities to the sites of RuBP carboxylation. However, the response of g(m) to light intensity remains controversial. Furthermore, little is known about the light response of relative mesophyll conductance limitation (l(m)) and its effect on photosynthesis. In this study, we measured chlorophyll fluorescence and gas exchange in nine evergreen sclerophyllous Rhododendron species. g(m) was maintained stable across light intensities from 300 to 1500 mu mol photons m(-2) s(-1) in all these species, indicating that g(m) did not respond to the change in illumination in them. With an increase in light intensity, l(m) gradually increased, making g(m) the major limiting factor for area-based photosynthesis (A(N)) under saturating light. A strong negative relationship between l(m) and A(N) was found at 300 mu mol photons m(-2) s(-1) but disappeared at 1500 mu mol photons m(-2) s(-1), suggesting an important role for l(m) in determining A(N) at sub-saturating light. Furthermore, the light-dependent increase in l(m) led to a decrease in chloroplast CO2 concentration (C-c), inducing the gradual increase of photorespiration. A higher l(m) under saturating light made A(N) more limited by RuBP carboxylation. These results indicate that the light response of l(m) plays significant roles in determining C-c, photorespiration, and the rate-limiting step of A(N).