INFLUENCE OF CALCIUM ON ANTIOXIDANT SYSTEM AND NITROGEN METABOLISM OF MUSKMELON SEEDLINGS UNDER NUTRIENT SOLUTION HYPOXIA

Two varieties of muskmelon were cultivated in hydroponics culture using the following [Ca²⁺] in nutrient solution under hypoxia with 4mmol•L^-1 Ca²⁺ (hypoxia), hypoxia with 10mmol•L^-1 Ca²⁺ (hypoxia+Ca²⁺), hypoxia with 0mmol•L^-1 Ca²⁺ (hypoxia-Ca²⁺), and normal dissolved oxygen with 4mmol•L^-1 Ca²⁺ was the control.　Leaf samples were collected at 0, 2, 4, 6 and 8d from all treatments for fresh weight, reactive oxygen species (ROS) and nitrogen metabolism analyses. The results showed that compared to control treatment, the fresh weights of both varieties were decreased and activities of antioxidant enzymes, productive rate of ROS, contents of MDA and NH₄⁺-N, NO₃^--N were increased under hypoxia. Calcium application treatment alleviated the harmful effects of hypoxia to seedlings, as manifested by the higher levels of fresh weights, activities of antioxidant enzyme, contents of NH₄⁺-N, NO₃^--N under hypoxia, while the ROS productive rate and MDA content were lower. The calcium-deficient treatment aggravated the harmful effects of hypoxia, which were reversed by calcium application. The different levels of response by the two varieties show differences in tolerance to hypoxia.