Combining carbon and oxygen isotopic signatures to identify ozone-induced declines in tree water-use efficiency

Ground-level ozone (O-3) pollution affects the plant carbon and water balance, but the relative contributions of impaired photosynthesis and the loss of stomatal functioning to the O-3-induced reductions in water-use efficiency (WUE) remain unclear. We combined the leaf stable dual isotopic signatures of carbon (delta C-13) and oxygen (delta O-18) with related instantaneous gas exchange performance to determine the effects of O-3 dose on the net photosynthetic rate (A(n)), stomatal conductance (g(s)) and intrinsic WUE (iWUE = A(n)/g(s)) in four tree species (one being a hybrid) exposed to five O-3 levels. The iWUE declined for each step increase in O-3 level, reflecting progressive loss of the coupling between leaf carbon gain and water loss. In ambient compared with charcoal-filtered air, the decreased iWUE was associated with reductions in both A(n) and g(s) (i.e., decreased delta C-13 and increased delta O-18). In elevated O-3 treatments, however, the iWUE declines were caused by reduced A(n) at constant or increased g(s). The results show that the dual isotope approach provides a robust way to gather time-integrated information on how O-3 pollution affects leaf gas exchange. Our study highlights that O-3-induced decoupling between photosynthesis and stomatal regulation causes large and progressive declines in the WUE of forest trees, demonstrating the need for incorporating this hitherto unaccounted for effect into vegetation models.

Automated summary

Ground-level ozone pollution affects plant carbon and water balance, with a decrease in water-use efficiency mainly attributed to reduced photosynthesis under elevated O-3 levels. The study highlights the importance of incorporating the decoupling effect between photosynthesis and stomatal regulation into vegetation models to accurately predict the impact of O-3 pollution on forest ecosystems.