Comparison of Inhibitory Effects on Leaf Photosynthesis in Cotton Seedlings between Drought and Salinity Stress

In order to investigate the similarities and differences in photosynthetic inhibition between drought and salinity stress,seedlings at three-leaf stage of a commercial Bt cotton cultivar,SCRC 16,was grown and treated with either iso-osmotic drought or NaCl in a greenhouse,and the photosynthetic rate(Pn),chlorophyll fluorescence parameters,peroxidase(POD) activity,malondialdehyde(MDA) and free praline(Pro) contents in seedling leaves were examined at 15 d after being treated.The results showed that either drought or salinity significantly reduced Pn,transpiration rate(Tr),stomatal conductance(Gs),chlorophyll and carotenoid contents.Either of the two stresses increased minimum chlorophyll florescence(F0),and decreased maximum chlorophyll fluorescence(Fm),PSⅡprimary chemical efficiency(Fv/Fm)and PSⅡpotential activity(Fv/F0),but did not significantly result in variation in quantum yield of PSⅡ linear electron(ΦPSⅡ) relative to the un-stressed control.POD activity,MDA and Pro contents were significantly raised by drought or salinity.Moderately quantitive differences in chlorophyll and Pro contents,and POD activity and Gs in cotton leaves between drought and salinity were also detected.It suggested that there existed considerable similarities and moderate differences in mechanisms of photosynthetic inhibition between the two kinds of stress.