Abstract:

Calcium has been found to be a second-messenger in plants involved in the perception and regulation of responses and adaptations of plants to the environment. During this period, polyamines (PAs) accumulation, the activities of diamine oxidase (DAO) and polyamine oxidase (PAO) play an important role in increasing stress-resistance of plants. However, little is known about the effect of Ca²⁺ on polyamines metabolism of seedlings under hypoxia stress. In this experiment, muskmelon cultivar `Xiyu 1' (Cucumis melo var. raticulalus) seedlings with less hypoxia-resistance under hydroponic cultivation were used to study the relationship between Ca²⁺ and PAs metabolism under hypoxia stress. When seedlings grew to 3 euphylla, the seedlings were transferred from quartz sand to Hoagland nutrient solutions for preculture. After 2 days, the seedlings were divided into four sets. The first and second sets of seedlings were all cultured in normal Hoagland nutrient solutions containing 4 mmol·L^-1 Ca²⁺. The third set of seedlings was cultured in a calcium sufficient Hoagland nutrient solution which contained 10 mmol·L^-1 Ca²⁺, and the fourth set of seedlings was cultured in a calcium deficient Hoagland nutrient solution with no Ca²⁺ (0 mmol·L^-1 Ca²⁺). The first set of seedlings was exposed to normal dissolved oxygen (DO) level by using vigorous aeration (30 min·h^-1) to keep DO level at 8-8.5 mg·L^-1, and seedlings in the other treatments were exposed to hypoxia stress by using DO analyzer (QUANTUM-25) to maintain DO level at (2±0.2) mg·L^-1. Hence, the four treatments were control, hypoxia stress, Ca²⁺ sufficient-hypoxia stress treatment, and Ca²⁺ deficient-hypoxia stress treatment. After 4 d of hypoxia stress, the growth of seedlings, PAs content, DAO and PAO activities, and H₂O₂ content were all assayed. The results showed that the fresh weight and dry weight of seedlings declined significantly after 4 d of hypoxia stress, the content of three polyamines (Put, Spd, Spm) of different forms (free, conjugated, bound), DAO and PAO activities, and H₂O₂ content in roots and leaves of seedlings all increased significantly with hypoxia stress treatment compared to the control. In the calcium sufficient-hypoxia stress treatments, seedling growth under hypoxia stress was not inhibited, PAs (Put, Spd, Spm) and their three different forms in roots and leaves were significantly higher in the Ca²⁺ sufficient-hypoxia stress than in the hypoxia stress treatment, but DAO and PAO activity and H₂O₂ contents were lower. In the Ca²⁺ deficient treatment, seedlings incurred the greatest damage due to hypoxia stress, and PAs metabolism indices under Ca²⁺ deficiency showed opposite trends as compared to seedlings in the Ca²⁺ sufficient treatment. We suggest that Ca²⁺ might be involved in the regulation of polyamines metabolism and play an important role in enhancing hypoxia tolerance in muskmelon seedlings.

Key words: Calcium, Hypoxia stress, Muskmelon (Cucumis melo var. raticulalus), Polyamines, Polyamine oxidase, H₂O₂