Variations in leaf anatomical characteristics drive the decrease of mesophyll conductance in poplar under elevated ozone

Decline in mesophyll conductance (g(m)) plays a key role in limiting photosynthesis in plants exposed to elevated ozone (O-3). Leaf anatomical traits are known to influence g(m), but the potential effects of O-3-induced changes in leaf anatomy on g(m) have not yet been clarified. Here, two poplar clones were exposed to elevated O-3. The effects of O-3 on the photosynthetic capacity and anatomical characteristics were assessed to investigate the leaf anatomical properties that potentially affect g(m). We also conducted global meta-analysis to explore the general response patterns of g(m) and leaf anatomy to O-3 exposure. We found that the O-3-induced reduction in g(m) was critical in limiting leaf photosynthesis. Changes in liquid-phase conductance rather than gas-phase conductance drive the decline in g(m) under elevated O-3,O- and this effect was associated with thicker cell walls and smaller chloroplast sizes. The effects of O-3 on palisade and spongy mesophyll cell traits and their contributions to g(m) were highly genotype-dependent. Our results suggest that, while anatomical adjustments under elevated O-3 may contribute to defense against O-3 stress, they also cause declines in g(m) and photosynthesis. These results provide the first evidence of anatomical constraints on g(m) under elevated O-3.

Automated summary

The decline in mesophyll conductance (g(m)) is crucial in limiting photosynthesis in plants exposed to elevated ozone (O-3) levels. Leaf anatomical traits have been known to impact g(m), but the potential effects of O-3-induced changes in leaf anatomy on g(m) are still unclear. In this study, two poplar clones were exposed to elevated O-3 levels. The impacts of O-3 on photosynthetic capacity and anatomical characteristics were assessed to investigate the leaf anatomical properties that potentially affect g(m). Additionally, a global meta-analysis was conducted to explore the general response patterns of g(m) and leaf anatomy to O-3 exposure. The findings suggest that O-3-induced reduction in g(m) is critical in limiting leaf photosynthesis, and the changes in liquid-phase conductance, cell wall thickness, and chloroplast size contribute to this decline under elevated O-3 conditions.