2.2 Experimental design
A two-factor (seed source of region × soil moisture treatment) experiment was carried out at Nankai University. The experiment was established in plastic pots (14.5 cm in diameter and 12.5 cm in height), with a mixture of vermiculite and nutrient soil (1:1) as culture substrate. Seed source of region included the eastern, middle and western regions of S. krylovii distribution area, with three populations per region (Table 1). Soil moisture treatment was quantified by volumetric water content (VWC) which was estimated by an ECH2O Check (Decagon Devices, Pullman, WA, USA). Considering that the water retention of nutrient soil used in this study and the corresponding soil water potential in their local habitats in the Inner Mongolia Steppe, we set VWC to 20% for control treatment, 15% for light drought treatment and 8% for heavy drought treatment. Each treatment included 18 pots (6 pots per population), and each pot included 4 six-month healthy seedlings and each seedling was similar in size (about 10 tillers, uniform height (10 cm), and root length (12 cm)). In total, there were 162 pots (3 seed source of regions × 3 soil moisture treatments × 18 replications). To exclude precipitation, all pots were randomly placed under a five-meter-height rain-proof shed, and their positions were changed every week to avoid position effects. The experiment lasted 72 days from June 20 to September 14, 2022.