The Sclerophyllous Eucalyptus camaldulensis and Herbaceous Nicotiana tabacum Have Different Mechanisms to Maintain High Rates of Photosynthesis

It is believed that high levels of mesophyll conductance (g(m)) largely contribute to the high rates of photosynthesis in herbaceous C(3) plants. However, some sclerophyllous C(3) plants that display low levels of g(m) have high rates of photosynthesis, and the underlying mechanisms responsible for high photosynthetic rates in sclerophyllous C(3) plants are unclear. In the present study, we examined photosynthetic characteristics in two high-photosynthesis plants (the sclerophyllous Eucalyptus camaldulensis and the herbaceous Nicotiana tabacum) using measurements of gas exchange and chlorophyll fluorescence. Under saturating light intensities, both species had similar rates of CO(2) assimilation at 400 μmol mol(−1) CO(2) (A(400)). However, E. camaldulensis exhibited significantly lower g(m) and chloroplast CO(2) concentration (C(c)) than N. tabacum. A quantitative analysis revealed that, in E. camaldulensis, the g(m) limitation was the most constraining factor for photosynthesis. By comparison, in N. tabacum, the biochemical limitation was the strongest, followed by g(m) and g(s) limitations. In conjunction with a lower C(c), E. camaldulensis up-regulated the capacities of photorespiratory pathway and alternative electron flow. Furthermore, the rate of alternative electron flow was positively correlated with the rates of photorespiration and ATP supply from other flexible mechanisms, suggesting the important roles of photorespiratory pathway, and alternative electron flow in sustaining high rate of photosynthesis in E. camaldulensis. These results highlight the different mechanisms used to maintain high rates of photosynthesis in the sclerophyllous E. camaldulensis and the herbaceous N. tabacum.