2.1 Experimental design
The experiment was conducted in the greenhouse at Hebei Agricultural University (38°85’N, 115°30’E) in Baoding Hebei Province, China. Two cotton varieties, the drought-tolerant type ”Guoxin 02” (DT type) and the drought-sensitive type ”Ji 228” (DS type), were selected for this study (Guo et al., 2022). After sterilization with 75% ethanol, the seeds were rinsed 5-8 times repeatedly with distilled water, then soaked in distilled water for 8 hours and placed on a towel to germinate. The radicle was transferred to a hydroponic pot at 2 cm. Once the cotyledons were fully expanded, the samples were transferred to an incubator (485*355*245 mm) for continued hydroponic growth in the Hoagland nutrient solution. Each incubator contained 24 seedlings and 50 L of Hoagland’s nutrient solution.
Cotton plants were grown to the two true leaf stages, at which point water treatments were initiated. The treatments included no-stress (NS, 0% PEG6000) and drought stress (DS, 8% PEG6000) for 15 days, and each treatment had 24 replications. The pH of the nutrient solution was adjusted to 6.0 ± 0.1, and the solution was replaced every 3 days. The growth parameters included an average daily temperature of approximately (28 ± 2) / (25 ± 2) °C (day/night), a photoperiod of 14/10 hours of light/darkness, relative humidity of 40%-50%, and a light intensity of 600 µM. Oxygen was supplied twice daily, with each session lasting 30 minutes.