Hormone Metabolism and Substance Accumulation in Cucumber Plants: Downy Mildew Infection and Potassium Stress

In order to understand the effects of biotic stress and abiotic stress on hormone metabolism and substance accumulation in cucumber, two different infection levels of downy mildew were set, B0 (diseased) and B1 (disease-free), and T1 (K-50%: 50% of normal potassium application), T2 (K-100%: normal potassium application), and T3 (K-150%: 150% of normal potassium application). The results show that the hormone metabolism in the cucumber plants was measured on the 15th day after transplanting. Compared with the B1T2 treatment, the cane sugar contents in the B0T1, B0T2, B0T3, B1T1, and B1T3 treatments decreased by 35.87%, 23.29%, 25.41%, 12.98%, and 6.05%; the soluble sugar contents in the B0T1, B0T2, B0T3, B1T1, and B1T3 treatments decreased by 36.16%, 27.13%, 31.97%, 9.89%, and 7.84%; the hydrogen peroxide (H2O2) contents in the B0T1, B0T2, B0T3, and B1T1 treatments increased by 30.59%, 21.08%, 14.74%, and 9.2%; the catalase (CAT) contents in the B0T1, B0T2, B0T3, B1T1, and B1T3 treatments decreased by 53.78%, 36.57%, 47.18%, 34.75%, and 16.39%; the superoxide dismutase (SOD) contents in the B0T1, B0T2, B0T3, and B1T1 treatments increased by 30.28%, 22.59%, 12.9%, and 13.17%; and the polyphenol oxidase (PPO) contents in the B0T1, B0T2, B0T3, and B1T1 treatments increased by 39.38%, 25.27%, 4.49%, and 23.05%, respectively. Compared with the B1T2 treatment, the malondialdehyde (MDA) contents in the B0T1, B0T2, B0T3, B1T1, and B1T3 treatments increased by 82.78%, 23.07%, 60.21%, 74.55%, and 25.12%. Compared with the B1T2 treatment, the hydrogen peroxide (H2O2), superoxide dismutase (SOD), and polyphenol oxidase (PPO) contents in the B1T3 treatment decreased by 16.11%, 4.36%, and 4.43%. On the 20th day after transplanting the cucumber plants, downy mildew infection and potassium stress had significant effects on the dry and fresh weights of the leaves, stems, and roots (p < 0.05). The B0T1 treatment had the greatest effect on the dry and fresh weights of the leaves, stems, and roots of the cucumber plants. The B1T3 treatment had the least effect on the dry and fresh weights of the leaves, stems, and roots of the cucumber plants. This research can provide a theoretical basis for the study and cultivation of cucumber stress resistance.