Effects Of Elevated CO₂ And O₃ Alone And In Combination On Photosynthetic Mechanism Of Ginkgo Biloba L.

Ginkgo biloba L. seedlings are exposed to elevated CO₂ exposure(700±20μmol/mol)and elevated O₃ exposure (80±8 nmol/mol),singly and in combination,and control (naturalCO₂ and O₃ concentration in chamber) in open-top chambers for a growing season in thestudy.The effects of different treatments on photosynthesis of Ginkgo biloba L. are studied toanalysis photosynthetic response mechanism of Ginkgo biloba L. to elevated CO₂ and O₃concentration and predict the feasibility of Ginkgo biloba L. as one composing tree species ofShenyang urban forest under future climate change.Elevated CO₂ exposure induces a significant increase in net photosynthetic rate(Pn) andintercellular CO₂ concentration(Ci),a reduction in stomatal conductance(Gs) and transpirationrate(Tr).Carboxylation rate(CE) is enhanced by elevated CO₂ exposure during the earlygrowing season and then turned to be decreased.Chl(a+b),Chla/b and carotenoid content areall increased by elevated CO₂ exposure.The activities of Hill reaction and Ca²⁺/Mg²⁺-ATPasein chloroplast are increased,accompanied by an increase in contents of soluble protein,solublesugar, and starch under elevated CO₂ exposure.In addition,these phenomena are detected underelevated CO₂ exposure:stomatal number of Ginkgo biloba L.leaves reduces,stomatal aperturedeclines,chloroplast in leaf cells develops well,volume of starch grains in chloroplast largenevidently, chloroplast grana lamellae is kept integrated in order and slightly thickens,and leafcell wall also thickens slightly.Elevated O₃ exposure shows the opposite effects,which results in lower netphotosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate(Tr) and intercellular CO₂concentration(Ci).Carboxylation rate(CE) is higher earlier during the growing season and islower later.Chl(a+b) is decreased,Chla/b and carotenoid content are increased in the earliergrowing season and decreased later by elevated O₃ exposure.The activities of Hill reaction andCa²⁺/Mg²⁺-ATPase in chloroplast as well as contents of soluble protein,soluble sugar, andstarch are decreased under elevated O₃ exposure.Another results suggest that stomatal shapein Ginkgo biloba L. leaves is changed from rounded to elliptic,some stomata are closed,chloroplast and its grana lamellae in leaf cells are disorganized,the amount and volume ofstarch grains in chloroplast are decreased,leaf cell wall thickens evidently and becomescoarse.In combination,the negative effects of elevated O₃ exposure on photosynthesis of Ginkgobiloba L. are alleviated by elevated CO₂.Under this treatment net photosynthetic rate(Pn) and intercellular CO₂ concentration(Ci) are increased,stomatal conductance(Gs) and transpirationrate(Tr) are decreased,whereas carboxylation rate(CE) is increased earlier during the growingseason and then turned to be decreased.Chl(a+b),Chla/b and carotenoid content all have thesimilar variations to carboxylation rate,and Chl(a+b) is slightly reduced at whole level.Incombination of elevated CO₂ and O₃,Hill reation activity is increased,Ca²⁺-ATPase activity ishigher early and lower afterward,whereas Mg²⁺-ATPase activity shows an opposite varyingtrend, meanwhile the contents of soluble protein,soluble sugar, and starch are increased.Stomatal number of Ginkgo biloba L. leaves is reduced,some stomata are closed,but stomatalshape has no significant change under combined treatment.In addition,chloroplast in leaf cellsdevelops well,the amount of starch grains in which slightly reduces,whose volumes increasesignificantly,some of chloroplast grana lamellae begin to disorgnize,but some still is intact.Leaf cell wall becomes coarse,too.Moreover, relative conductance efficiency and proline content are all higher underdifferent treatments,whose whole change trend is elevated O₃ exposure＞combination ofelevated CO₂ and O₃＞elevated CO₂ exposure＞control,suggests that elevated CO₂ and O₃singly and in combination result in injuries of Ginkgo biloba L. leaves at some extent,but thedamages resulted from elevated O₃ exposure are partly counteracted by elevated CO₂exposure.