Response of Photosynthetic Characteristics and Antioxidant Enzyme Activities in Poplar Seedlings to Drought Stress

Poplar( Populus בLulin- 1') seedlings grown in pots were subjected to four different water treatments( normal irrigation,light drought,moderate drought and severe drought). The relative water content,gas exchange, chlorophyll fluorescence parameters and protective enzyme activities in leaves of poplar were tested. The results indicated that with the extension of drought stress time,the relative water content,net photosynthetic rate( Pn) and stomatal conductance( gs) in leaves of poplar decreased gradually under light drought stress,while decreased rapidly under both moderate and severe drought stress. Furthermore,the intercellular CO2concentration( Ci) decreased tardily under light drought stress,increased after an initial decrease under moderate stress and increased all the time under severe stress. This suggested that reductions in Pnresulted from stomatal limitations under light drought stress,from both stomatal and non-stomatal limitations under moderate drought stress and from non-stomatal limitations under the severe drought stress. At the same time,the maximal photochemical efficiency of PSII( Fv/ Fm) and photochemical quenching coefficient( qP) in leaves of poplar declined slowly under light stress,however,declined quickly under both moderate and severe stress. The non-photo chemical quenching coefficient( NPQ) increased significantly under the light drought stress,but had a trend of increasing firstly and decreasing then under either moderate or severe drought stress. Additionally,the activities of superoxide dismutase( SOD) and peroxidase( POD) in leaves decreased after an initial increase. As a result,under the light drought stress the decreased Pnwere mainly owing to stomatal limitations through stomatal regulation,the enhance of thermal dissipation and antioxidant enzyme jointly; under the moderate and severe drought stress major nonstomatal limitations responsible for reduction in Pnwere associated with the impairment of PSII and antioxidant enzyme system.