Physiological and ecophysiological responses of Humulus scandens seedlings to chromium stress

The ecophysiological characteristics and photosynthesis of Humulus scandens seedlings under chromium stress(0,50,200,300,and 500mmol/L Cr3+)were studied.Under 50mmol/L stress,the net photosynthetic rate(Pn)and instant water use efficiency(WUE)increased significantly compared with the CK butstomatal conductance(Gs),transpiration rate(Tr),stomatal limitation(Ls),intercellular CO2concentration(Ci)and chlorophyll contents were not significantly different.Under 200-300mmol/L stress,actual maximal fluorescence(Fm′),actual minimum fluorescence(Fo′),dark fluorescence yield(Fo),maximal fluorescence yield(Fm),variable fluorescence(Fv),potential activity of PSⅡ(Fv/Fo),maximum quantum yield of PSⅡ(Fv/Fm)and proportion yield of PSⅡ(qP)gradually decreased with an increase of Cr3+stress,while Lsand WUEiinitially increased but then decreased,whereas Ciinitially decreased but then increased.The opposite side of electron transport rate(ETR)and malonaldehyde(MDA)showed a tendency to increase,while the coefficient of non-photochemical quenching(qN)showed irregular changes.In particular,the Pn,Tr,Ls,WUEi,content of MDA and qNdeclined significantly in 500mmol/L Cr3+stress,but the Ciand other chlorophyll fluorescence parameters suddenly increased.In summer,there was a clear difference of gas change,content of MDA and chlorophyll and chlorophyll fluorescence parameters.Although some physiological indexes declined in the H.scandens seedlings with increasing Cr3+stress,but showed a positive response,keep certain photosynthesis under high Cr3+stress,and its tolerance to Cr3+stress was the strongest.