No abstract

Growth and development of tomato is restricted by numerous abiotic stress factors especially drought. Crop genetic improvement through traditional methods is time-consuming and conducted under uncontrolled conditions; thus, in vitro selection offers an efficient alternative. This research was carried out at the National Commission for Biotechnology (NCBT) and the General Commission for Scientific Agricultural Research (GCSAR), Syria during 2018 and 2019. The main objective was to select callus tolerant to drought stress and regenerate tolerant somaclonal variant plants in four local tomato varieties. Callus (formed from root explants on MS medium supplemented with 2 mg/L BAP and 0.2 mg/L NAA) was subjected to four different levels of drought stress induced by PEG 6000 (2%, 4%, 6%, 8% in addition to the non-treated control). After 8 weeks, cell viability, relative growth rate (RGR), proline content, and tolerance index (TI) were determined. Callus was then transferred to regeneration medium (MS supplemented with 2 mg/L zeatin, 0.2 mg/L GA3 and the sub-lethal concentration of PEG (4% for Daher-Aljabal, 6% for Brieh and 6%, 8% for Daraa variety). The regenerated somaclonal variant plants were evaluated, based on morphological characters, for drought tolerance compared to mother plant. Results showed that RGR, TI and cell viability decreased while free proline content increased in response to increasing PEG concentration. Based on the studied parameters, Daraa was the most tolerant variety, followed by Daher-Aljabal, Brieh and Baskanta. A single regenerated plantlet was obtained from Daher-Aljabal variety named DH1 and another plantlet from Brieh variety named BR1, while two regenerated plantlets were obtained from Daraa variety named DA1 and DA2. All of them showed better tolerance to drought stress than mother plants at 8% PEG concentration in terms of root length, plant height, root-shoot ratio and dry weight. In conclusion, a successful callus formation and plant regeneration protocol has been developed under drought stress treatments induced by PEG 6000. Four somaclonal variant plants were successfully developed and they were different from mother plants based on the studied parameters.