*2.1. Plant Material*

In the present study, the tomato cultivar "Fenli" was collected from Minghao seed store, Hainan, China. The germinated seeds were sown in vermiculite-filled trays with a 50-cell plug in a controlled growth chamber. After the development of the second-trueleaf, equal size tomato seedlings were transferred into the vermiculite-filled black plastic pots (with height = 8.5 cm, top and bottom diameter = 10 and 7 cm, respectively), having one emerged seedling/pot. Subsequently, all the pots were placed in a greenhouse with specific conditions (25 ◦C ± 5 temperature; 16/8 h dark/light period; and 55–90% relative humidity). The pre-cultivation period was 10 days, with the aim of allowing seedlings get adapted to new conditions, with watering (80 mL per plant) at two days' interval using Hoagland's nutrient solution (pH 5.8 ± 0.1).

#### *2.2. Experimental Design*

The treatment plan was implemented after ten days, when the seedlings were attained at the four-true-leaf stage. The treatments are followed in the current study including: (1) CK (control) treatment included the seedlings with full water application during the complete span of the experiment, (2) DR (drought) treatment included the plants which were first given water fully for eight days, followed by up to two weeks' water-withholding, (3) ME + DR (melatonin + drought) was the treatment in which ME pretreatment was given to seedlings with 100 μM solution of ME (80 mL per plant) [33]. This pretreatment continued four times, with a 2-days' interval, followed by up to two weeks' water-withholding. Each treatment comprised of 3 replicates, with 8 plants per replicate. Further, 0 day was the day when irrigation-withholding was started. After two-weeks of DR treatment, collection of leaf samples was done to perform different analysis, including both biochemical and physiological tests.
