Comparison of changes in water status and photosynthetic parameters in wild type and abscisic acid-deficient sitiens mutant of tomato (Solanum lycopersicum cv. Rheinlands Ruhm) exposed to sublethal and lethal salt stress.

Abscisic acid (ABA) regulates many salt stress-related processes of plants such as water balance, osmotic stress tolerance and photosynthesis. In this study we investigated the responses of wild type (WT) and the ABA-deficient sitiens mutant of tomato (Solanum lycopersicum cv. Rheinlands Ruhm) to sublethal and lethal salt stress elicited by 100 mM and 250 mM NaCl, respectively. Sitiens mutants displayed much higher decrease in water potential, stomatal conductance and net CO2 assimilation rate under high salinity, especially at lethal salt stress, than the WT. However, ABA deficiency in sitiens caused more severe osmotic stress and more moderate ionic stress, higher K+/Na+ ratio, in leaf tissues of plants exposed to salt stress. The higher salt concentration caused irreversible damage to Photosystem II (PSII) reaction centres, severe reduction in the linear photosynthetic electron transport rate and in the effective quantum yields of PSII and PSI in sitiens plants. The cyclic electron transport (CET) around PSI, which is an effective defence mechanism against the damage caused by photoinhibition in PSI, decreased in sitiens mutants, while WT plants were able to increase CET under salt stress. This suggests that the activation of CET needs active ABA synthesis and/or signalling. In spite of ABA deficiency, proline accumulation could alleviate the stress injury at sublethal salt stress in the mutants but its accumulation was not sufficient at lethal salt stress.