One-year old sweet almond (Prunus dulcis) seedlings were submitted to four levels of salt stress induced by NaCl, namely 0.3, 0.5, 0.7, and 1.0 S m −1 . Effects of salt stress on a range of chlorophyll (Chl) fluorescence parameters (Chl FPs) and Chl contents were investigated in order to establish an eco-physiological characterization of P. dulcis to salinity. Salt stress promoted an increase in F 0 , F s , and F 0 /F m and a decrease in F m , F' m , F v /F m , q P , ΔF/F' m , F v /F 0 , and UQF (rel) , in almost all Chl fluorescence yields (FY) and FPs due to its adverse effect on activity of photosystem 2. No significant changes were observed for quenchings q N , NPQ, and q N(rel) . The contents of Chl a and b and their ratio were also significantly reduced at increased salt stress. In general, adverse salinity effects became significant when the electric conductivity of the nutrient solution (EC n ) exceeded 0.3 S m −1 . The most sensitive salt stress indicators were F v /F 0 and Chl a content, and they are thus best used for early salt detection in P. dulcis. Monitoring of a simple Chl FY, such as F 0 , also gave a good indication of induced salt stress due to the significant correlations observed between the different Chl FYs and FPs. Even essential Chl FYs, like F 0 , F m , F' m , and F s , and mutually independent Chl FPs, like F v /F 0 and q P , were strongly correlated with each other.Additional key words: fluorescence quenching.
