Effects of Ca2 + on Respira tory Metabolism in Roots of Cucumber Seedlingsunder Root-zone Hypoxia Stress Effects of Ca2 + on Respira tory Metabolism in Roots of Cucumber Seedlingsunder Root-zone Hypoxia Stress Effects of exogenous γ-aminobutyric acid on polyamine metabolism of melon seedlings under hypoxia stress. Changes of Polyamines Metabolism in Roots of Cucumber Seedlings underRoot-zone Hypoxia Stress Changes of Polyamines Metabolism in Roots of Cucumber Seedlings underRoot-zone Hypoxia Stress