*4.1. Plant Materials and Drought Treatments*

*Verbena bonariensis* L. with the age of one month were used in this study. The experimental materials were incubated in greenhouse (25 ◦C, 16 h photoperiod, 50% RH), soil texture for the medium loam, the maximum soil moisture content of 80%. A total of 90 strains of Verbena were randomly divided into two groups T01 (control group) and T02 (drought experiment group), 1 seeding in each pot and 15 in one duplicated group, each of the T01 and T02 contained 3 duplicated groups named T1–T3 and T4–T6, respectively. Determination of soil moisture content was measured by the oven drying method [54]. The soil water content of T01 was kept at 80% of saturated soil moisture all the time, while watering of the T02 was stopped until the soil water content had reduced to 25% of the soil saturated water content. After that, the soil moisture content was measured every 2 days to replenish the amount of deficiency and maintained this drought status for 15 days.

At the 5th, 10th and 15th day, the mature leaves (3rd to 8th functional leaf) were selected randomly from the plants of T01 and T02 for the determination of physiological indexes. After the drought had been maintained for 15 days, samples were collected, rapidly frozen with liquid nitrogen, stored at −80 ◦C, and finally sent to Biomarker Technologies Co., Ltd. (Beijing, China) for whole-transcriptome sequencing.
