How important is woody tissue photosynthesis in EuCahetus dunnii Maiden and Osmanthus fragrans (Thunb.) Lour. under O stress?

Numerous studies have demonstrated the negative effects of elevated O on leaf photosynthesis. Within trees, a portion of respired CO is assimilated by woody tissue photosynthesis, but its response to elevated O remains unclear. Saplings of two evergreen tree species, EuCahetus dunnii Maiden (E. dunnii) and Osmanthus fragrans (Thunb.) Lour. (O. fragrans), were exposed to non-filtered air (NF), 100 nmol mol O air (E1) and 150 nmol mol O air (E2) in open-top chambers from May 5 to September 5, 2016 (8 h a day; 7 days a week) in subtropical China. In this study, O fumigation significantly reduced leaf net photosynthesis rate in both two tree species on most measurements. However, compared with leaf net photosynthesis rate, woody tissue gross photosynthesis rate showed less negative response to O fumigation and was even stimulated to increase. Refixation rate reflects the utilization efficiency of the respired CO by woody tissue photosynthesis. During the experiment period, E1 and E2 both increased refixation rate in O. fragrans compared with NF. Whereas for E. dunnii, E1 increased refixation rate until 81 days after starting of fumigation and then decreased it, and E2 decreased it all the time. Refixation rate had a significant positive correlation with woody tissue chlorophyll contents, indicating that the response of refixation rate to elevated O may relate to chlorophyll contents. All these suggested that under O fumigation, when atmospheric CO uptake and fixation by leaf is limited, woody tissue photosynthesis can contribute more to the total carbon assimilation in trees. The findings help to understand the significance of woody tissue photosynthesis under elevated O conditions.