Effects Of Exogenous Melatonin On Physiological Response Of Cucumber Seedlings Under High Temperature Stress

Taking the cucumber cultivar'Jinchun 4'as test material, we studied the effect of foliar application of exogenous melatonin on cucumber physiological response according to determine the changes of photosynthesis and chlorophyll fluorescence parameters, active oxygen metabolism and accorbate metabolism under high temperature stress. The results showed:1. Superoxide radical (O2?-) production rate, hydrogen peroxide (H2O2), malonaldehyde (MDA) content and cell membrane permeability (electrolyte leakage) in cucumber seedlings decreased under high temperature stress, but superoxide radical (SOD), peroxidase (POD), catalase (CAT) activities, AsA, GSH and soluble proteins contents increased when the seedlings were treated with exogenous MT, which illustrated MT could inhibit production of ROS, raise the activity of anti-oxidative enzymes and the content of antioxidants, decrease the level of membrane lipid peroxidation and the leakage of electrolyte to protect the integrity of lipid membranes, therefore it alleviated the damage degree caused by high temperature stress.2. The results showed 24 h 40±1℃high temperature stress declined net photosynthetic rate (Pn), but increased stomatal conductance, intercellular CO2 concentration (Ci) and transpiration rate. This note the reduction of Pn was mainly due to non-stomatal factors. Test result showed initial fluorescence (F0), maximal photochemical efficiency of PSII (Fv/Fm), PSII potential activity (Fv/F0), electron transport rate (ETR), PSII quantum yield of electron transport (ΦPSII) , photochemical quenching coefficient (qP), Chl·a concentration and Chl·a/b ratio decreased significantly are the main reason for the decline of Pn after the high temperature stress. Handled seedlings with MT can inhibit the decreasing of plant Pn, Chl·a, Chl·b content, F0, Fv/Fm, Fv/F0, ETR and qP and the increasing qN. Meanwhile, Chl·a/b ratio can be increased. These showed MT had the ability to alleviate the heat stress on photosynthetic organ damage and improve photosynthetic capacity of cucumber seedlings.3. H2O2 and MDA content were significantly increased under high temperature stress. Ascorbate (AsA) and Glutathione (GSH) contents decreased intensively while Dehydroascorbate (DHA) and Oxidized glutathione (GSSG) contents largely increased, so AsA/DHA and GSH/GSSG ratio decreased. Concomitantly, Ascorbate peroxidase (APx), Monodehydroascorbate reductase (MDHAR), Dehydroascorbate reductase (DHAR) and Glutathione reductase (GR) were increased under high temperature stress and reached maximum after 12 h stress, then decreased slightly. The levels of H2O2 were positively and significantly correlated with MDA while H2O2 and MDA were negatively and obviously correlated with AsA and GSH. In contrast with control cucumber seedlings, MT treatment could significantly increase the activities of APx, MDHAR, DHAR and GR, so as to raise levels of AsA and GSH, which could enhance the ability of scavenging H2O2 and inhibit the accumulation of MDA to maintain the cell membrane stability, alleviate the damage degree caused by high temperature stress.Exogenous MT could improve the photosynthesis of cucumber seedlings under high temperature stress, enhance the activity of antioxidant enzymes in plants to increase the level of antioxidants, inhibit the generation of reactive oxygen species or scavenge the ROS, alleviate the adverse of the stress on the plant and enchance the stress resistance.