Photosynthesis and photosystem 2 efficiency of two salt-adapted halophytic seashore Cakile maritima ecotypes

The effects of salinity (0-400 mM NaCl, marked S-0, S-100, S-200, and S-400) on growth, photosynthesis, photosystem 2 (PS2) efficiency, ion relations, and pigment contents were studied in two seashore Cakile maritima ecotypes (Tabarka and Jerba, respectively, sampled from humid and arid bioclimatic areas). Growth of Jerba plants was improved at S-100 as compared to S-0. Tabarka growth was inhibited by salinity at all NaCl concentrations. Leaf sodium and chloride concentrations increased with medium salinity and were higher in Jerba than in Tabarka plants. Chlorophyll content, net photosynthetic rate, stomatal conductance (g(s)), and intracellular CO2 concentration were stimulated at moderate salinity (S-100) in Jerba plants and inhibited at higher salt concentrations in both ecotypes: g(s) was the most reduced parameter. The maximum quantum efficiency of PS2 (F-v/F-m), quantum yield, linear electron transport rate, and efficiency of excitation energy capture by open PS2 reaction centres showed no significant changes with increasingsalt concentration in Jerba plant and were decreased in Tabarka subjected to S-400. However, the efficiency of dissipation of excess photon energy in the PS2 antenna was maintained in Jerba and was increased in Tabarka plants challenged with S-400. Hence the relative salt tolerance of Jerba was associated with a better ability to use Na(+)supercript stop and Cl- for osmotic adjustment, the absence of pigment degradation, and the concomitant PS2 protection from photodamage.