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

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