effects of salt stress on photosystem ii of canola plant (barassica napus, l.) probing by chlorophyll a fluorescence measurements

the changes of chl a fluorescence induction curve in response to different salinity levels in canola (barassica napus, l.) plants were investigated. the canola seedlings were exposed to salt stress (100 mm and 200 mm nacl) for 7 days and chl a fluorescence induction kinetics was analysed. by using the ojip-test, various parameters like vj, area, n, φpo, φeo, ψo, rc/abs and piabs were calculated and compared at different salinity levels and compared with the control plants. the results showed that the parameters related to the donor site of electron in psii [fv/fo, φpo and φpo /(1-φpo)] were constant and unaffected by salt stress. in contrast, the parameters dependent on the acceptor site of the electron such as vj, area, n, φeo, ψo and ψo /(1- ψo) significantly decreased with an increase in salt concentration. also, the multi parametric index piabs (performance index) showed enoughsensitivity to salt stress as a useful parameter to screen the activity of photosynthetic apparatus in canola plant. based on our results, we suggest that in response to salt stress the electron acceptor site of psii, especially at fractions of the transfer of electron from qa to qb and qa redox turnovers is more affected. in conclusion, this fraction of electron transport chain of psii is possibly the first and major target of salt stress at photosynthetic apparatus in canola plant.