Photosynthetic Physiological Response To Ozone Stress Of A Few Typical Subtropical Forest Plants

In recent years, with the increase in tropospheric ozone concentration and range, ozone pollution has become an issue of great concern both by researchers and the public. Elevated O3in the air has done great harm to farmland, forest and human health.The effects of ozone on photosynthesis on such five broadleaved tree species, as Machiluspauhoi Kanehira, Lindera setchuenensis Gamble, Phoebe bournei, Phoebe chekiangensis andMachilus thunbergii,were carried out by using12open-top chambers (OTCs) with differentlevels of ozone at200ppb,140ppb,80ppb and natural atmosphere，respectively, and the netphotosynthetic rate (Pn), stomatal conductance (Cond) and transpiration rate (Tr) weredetermined by Li-6400. The results showed that the variation trend of photosynthesis of alltested plants was changed, but ozone exposure did not necessarily always play an inhibitoryrole on photosynthesis, whereas it changed with ozone concentration, experimental period,season and specific species. The exposure to ozone could even help promote most of testedplants to reach its peak value of photosynthesis in the shorter term, which was bigger ascompared with the control. Moreover, the remarkable enhancement was observed on Machilusthunbergii and Phoebe chekiangensis with ozone treatments at lower ozone level. In addition,the Pn peak value was advanced due to the ozone treatments, and the correlation of Pn an Condwas broken, but the positive correlation of Tr and Cond remained. Machilus thunbergii wasevaluated as the most resistant one to ozone, followed by Phoebe bournei in the second place,whereas Phoebe chekiangensis, Machilus pauhoi Kanehira and Lindera setchuenensis Gamblewere more sensitive. Ozone can increase stomatal conductance and the transpiration ofMachilus thunbergii within a certain range of concentration and accumulation time exposed toO3.On the contrary, the stomatal conductance and transpiration of phoebe chekiangensis andMachilus pauhoi Kanehira were increased while their photosynthesis was restrained. Therewas no obvious correlation between Cond and Pn of Phoebe chekiangensis, and ozone resultedin the increase in Cond of Machilus pauhoi Kanehira and Phoebe chekiangensis, the enhancement of Tr and the inhibition of Pn, but without more inhibition when accompanied byhigher ozone concentration; As to Lindera setchuenensis Gamble and Phoebe bournei, thehigher the ozone concentration, the lower the Cond, the lower the Tr, and the weaker the Pn.