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In this paper, responses of germination physiology of pigeon pea (Cajanus cajan) seeds to drought stress in karst water environment and non-karst (allogenic) water environment were studied to explore the adaptability of pigeon pea to karst environment. The results showed that: (i) Under drought stress of 20% PEG-6000, the germination rate, vigor index, germination index and biomass of pigeon pea seeds on day 7 cultivated with karst water were all greater than that of the allogenic water treatment group, while the seed germination stress index was significantly smaller than that of the allogenic water treatment group, suggesting that karst water environment was more favorable to pigeon pea seed germination, (ii) Without drought stress, the malondialdehyde (MDA) and superoxide dismutase (SOD) activities of pigeon pea seeds cultivated with karst water were all smaller than that of the allogenic water group. However, under drought stress, the SOD activity was significantly higher than that of allogenic water group, suggesting pigeon pea SOD in karst water was able to more rapidly respond to external drought stress, and increase its own activity to reduce the damage to the plants. And (iii) with and without drought stress, the soluble protein level of the karst water group was higher than that of the allogenic water group, while the free amino acid level was lower than that of the allogenic water group. This difference was more significant with the presence of drought stress, suggesting that the karst water environment was more favorable to the accumulation of soluble proteins and thus produced larger biomass. Hence, pigeon pea is a tree species that is adapted to high-calcium, alkaline environments in karst areas, and is of great significance for the revegetation and rocky desertification control in mountainous karst areas.
