Effect of abscisic acid and epibrassinolide on physiological and hormonal responses of tomato plants subjected to water stress

In this study, the effect of abscisic acid (ABA; 150 μmol) or epibrassinolide (EBL, 3 μmol) in mitigating the adverse drought conditions was evaluated in tomato plants (Solanum lycopersicum L. cv. Vilma). Potted plants were subjected to two 6-day periods separated by a one-time rehydration. Results showed that water deficit increased the content of superoxide radical (O2*-), malondialdehyde (MDA), proline, ABA and its metabolites. On the other hand, the studied cytokinins showed a rather opposite trend. ABA application maintained and later reduced the O2*- content. At the same time, the MDA level was lower but later increased, while the proline content was reduced compared to untreated plants. This indicates that ABA helps the plants cope with the initial stress phase. In addition, ABA-activated signalling pathways showed increased levels of ABA, auxins, salicylic acid or jasmonic acid. EBL even more increased O2*- and proline content. At the same time, EBL increased the content of auxins, jasmonic acid and later ABA. In contrast, a decrease in salicylic acid and cytokinins was monitored. These findings indicate that ABA contributed to improved stress responses through early phytohormone-mediated signalling and reduction of stress markers, whereas EBL appeared less effective under our experimental conditions.