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 (g_m) are not clear. We investigated the interaction of O₃ and leaf age on g_m 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, g_m 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, g_m was mainly affected by physiological rather than structural factors: in particular, a marked reduction of g_m caused by abscisic acid (ABA) was noticed. As photosynthesis is limited by diffusional barriers, O₃ effects on g_m will be key for carbon sequestration modelling of O₃ pollution and climate change.