[Effect of exogenous melatonin on physiological and biochemical characteristics of cucumber radicles under p-hydroxybenzoic acid.]

'Jinyan No. 4'cucumber was used as experimental material. The growth of cucumber radicles treated with different concentrations of p-hydroxybenzoic acid (0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20 mmol·L ^-1 ) was investigated firstly, and 10 mmol·L ^-1 p-hydroxybenzoic acid was chosen for the further experiment. To investigate if melatonin alleviate p-hydroxybenzoic acid stress on cucumber radical elongation, 0, 1, 10, 25, 50, 75 and 100 μmol·L ^-1 melatonin were used to pretreat cucumber seeds for 24 h before 10 mmol·L ^-1 p-hydroxybenzoic acid treatment. The results suggested that exogenous melatonin could alleviate the radicle growth inhibition induced by p-hydroxybenzoic acid, and 75 μmol·L ^-1 melatonin showed the best effect. The growth of cucumber radical was significantly inhibited with an increase in p-hydroxybenzoic acid concentration, which was accompanied with lower activity of amylase. The melatonin pretreatment could significantly increase the activities of amylase and antioxidant enzymes in cucumber seeds resulting in lower accumulation of O ₂ ^-· , H ₂ O ₂ and MDA. Exogenous melatonin effectively reduced the PHBA stress on cucumber germination, which might be due to its function in decreasing oxidative stress and promoting starch catabolism.