Leaf phenology determines the response of poplar genotypes to O3 through mesophyll conductance

Tropospheric ozone (O3) is a phytotoxic air pollutant that impairs photosynthesis. The mechanisms of O3-induced reduction of mesophyll conductance (gm) are not clear. We investigated the interaction of O3 and leaf age on gm by using structural equation modelling (SEM) for two poplar clones (I-214 and Oxford) exposed to three O3 levels (ambient air, AA; 1.5 × AA; 2.0 × AA) in a free-air controlled experiment. Clone-specific phenological responses to elevated O3 were found: I-214 showed a rapid leaf turnover and formed new productive leaves, whereas Oxford was more ‘conservative’ maintaining old or injured leaves. In the I-214 clone with fast leaf turnover, gm was reduced due to increasing cell wall thickness in new leaves, a possible reaction to increase its resistance against O3 damage. As I-214 leaves aged, a decrease in the fraction of the mesophyll surface area unoccupied by chloroplasts was observed at 2.0 × AA prior to a reduction in photosynthesis. In the Oxford clone with slow leaf turnover, gm was mainly affected by physiological rather than structural factors: in particular, a marked reduction of gm caused by abscisic acid (ABA) was noticed. As photosynthesis is limited by diffusional barriers, O3 effects on gm will be key for carbon sequestration modelling of O3 pollution and climate change.