Controlled drought stress affects the chilling-hardening capacity of tomato seedlings as indicated by changes in phenol metabolisms, antioxidant enzymes activity, osmolytes concentration and abscisic acid accumulation

To elucidate the physiological, biochemical and hormonal mechanisms of chilling stress mitigated by drought pretreatment, tomato seedlings (Lycopersicon esculennon cv. C.H Falat) were pretreated by 0, 10 and 20% Polyethylene glycol (PEG) for 7 days followed by subjecting to chilling stress at 3 degrees C for 6 days and 6 h per day. Results showed that PEG-induced drought improved growth rate of tomato seedling subjected to chilling stress and enhanced their antioxidant enzyme activity, abscisic acid (ABA), anthocyanin accumulation, potassium (K+) and proline content compared with the control (0% PEG) at the end of chilling stress period. PEG pretreatment provided significant protection against chilling stress and reduced poly phenol oxidase (PPO) activity, electrolyte leakage (EL) and H2O2 content in root and leaf of chilled seedlings. Moreover, when PEG pretreatment was applied with chilling stress, we observed an alleviation in the growth impeding and a decrease in chilling symptoms as compared to control seedlings. The highest chilling tolerance was induced by application 20% of PEG. In general, the results indicate that PEG-induced drought, by altering in some tolerant responses, could be effectively used to protect tomato's seedling from the adverse effects of low temperatures stress.