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