Rapid determination of the damage to photosynthesis caused by salt and osmotic stresses using delayed fluorescence of chloroplasts

Chloroplasts are one of the most susceptible systems to salt and osmotic stresses. Based on quantitative measurements of delayed fluorescence (DF) of the chloroplasts, we have investigated the damage to photosynthesis caused by these two kinds of stresses in Arabidopsis seedlings by using a custom-built multi-channel biosensor. Results showed that the DF intensity and net photosynthesis rate (Pn) decreased in a similar way with increasing NaCl or sorbitol concentration. Incubation of the seedlings in 200 mM NaCl induced a rapid and reversible decline and subsequent slow and irreversible loss in both the DF intensity and Pn. The rapid decline was dominantly related to osmotic stress, whereas the slow declines in the DF intensity and Pn were specific to ionic stress and could be reversed to a similar extent by a Na+- channel blocker. The DF intensity and Pn also exhibited a similar response to irradiation light under NaCl or sorbitol stress. All results indicated that the DF intensity correlated well with Pn under salt and osmotic stresses. We thus conclude that DF is an excellent marker for detecting the damage to photosynthesis caused by these two stresses. The mechanism of the correlation between the DF intensity and Pn under salt and osmotic stresses was also analyzed in theory and investigated with experiments by measuring intercellular CO2 concetration (Ci), stomatal conductance (Gs), chlorophyll fluorescence parameter, and chlorophyll content. This proposed DF technique holds the potential to be a useful means for analyzing the dynamics of salt and osmotic stresses in vivo and elucidating the mechanism by which plants respond to stress.