Exogenous glucose regulates activities of antioxidant enzyme, soluble acid invertase and neutral invertase and alleviates dehydration stress of cucumber seedlings

To investigate proposed alleviative effects of exogenous glucose on dehydration stress and the physiological mechanisms behind, seedlings of Cucumis sativus L. cv. Jinchun no. 4 were pretreated with glucose for 3 days and then were exposed to dehydration conditions induced by 10% polyethylene glycol (PEG) 6000 for 2 days. Based on the results of our initial experiments, 20 mM glucose was chosen since it mitigated growth inhibition caused by dehydration and as well resulted in the lower levels of malondialdehyde, superoxide radical (O2−) and hydrogen peroxide (H2O2) under dehydration stress than other concentrations of glucose. After three days of exposure to 20 mM glucose, the levels of reduced glutathione, ascorbate, proline, soluble sugars, glucose and fructose in leaves were increased, and the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), glutathione reductase (EC 1.6.4.2), soluble acid invertase (SAI, EC 3.2.1.26) and neutral invertase (NI, EC 3.2.1.27) were enhanced. When the glucose-pretreated seedlings were exposed to dehydration stress for 2 days, the levels of reduced glutathione, ascorbate, proline, soluble sugars, glucose and fructose and the activities of superoxide dismutase, catalase, glutathione reductase, SAI and NI were changed further and were higher than PEG treatment alone, which was in accordance with the increased transcript levels of copper/zinc superoxide dismutase, manganese superoxide dismutase and catalase genes. Meanwhile, we observed the mitigated growth inhibition in glucose + PEG treatment in comparison to PEG treatment. We showed that pretreatment with 20 mM glucose induced antioxidants, SAI, NI, proline and soluble sugars in leaves, and it thus can protect cucumber seedlings from dehydration stress. These indicate that exogenous glucose may have the application possibility for a future practical trial of stress reduction.