Effects Of Uniconazole Application On Lodging Resistance, Antioxidant Defense System, And Grain Filling Of Maize (Zea Mays L.)

Lodging is an important factor affecting crop yield and quality.In addition to lodging,the deficit and unpredictable precipitation in semi-arid areas negatively affect crop growth.Water deficit leads to accumulation of reactive oxygen species（ROS）and thus degrading photosynthetic pigments,inhibiting photosynthesis,accelerating leaf senescence,and reducing yield.Plant growth regulators have an important impact on crop growth and lodging resistance.In order to study the effects of plant growth regulators on lodging resistance,antioxidant defense system,grain-filling characteristics,and yield of summer maize,a two-year field experiment was carried out and seeds were soaked in uniconazole at concentrations of 0（SCK₀）,25(SU₂₅),50(SU₅₀),and 75(SU₇₅)mg kg^–1 and was applied to the foliage at concentrations of 0（FCK₀）,25(FU₂₅),50(FU₅₀),and 75(FU₇₅)mg L^–1 at the eight-leaf stage.Additional experiments were conducted in order to further study the effects of uniconazole foliar applied alone or in combination with ethephon on antioxidant defense system and grain-filling characteristics on later growth stages.Uniconazole was applied to the foliage at the twelve-leaf stage at the concentration of 0（CK）,25(U₂₅),50(U₅₀),and 75(U₇₅)mg L^–11 while in the second experiment uniconazole was foliar sprayed at the twelve-leaf stage and ethephon at ten days after silking stage at concentrations of 0（CK）,25 mg L^–1 uniconazole+100 mg L^–1 ethephon(U₂₅+E₁₀₀),50 mg L^–1 uniconazole+200 mg L^–1 ethephon(U₅₀+E₂₀₀)and 75 mg L^–1 uniconazole+300 mg L^–1 ethephon(U₇₅+E₃₀₀),respectively.The main findings of our experimental study are as follows:1.Uniconazole is a gibberellin biosynthesis inhibitor that controls crop vegetative growth,shortens internode length,enhances lignin metabolism,and improves crop resistance to lodging and stresses.The morphological characteristics,rind penetration strength,lignin content,lodging rate,and grain yield of summer maize were studied by soaking seeds with different concentrations of uniconazole and spraying at the eight-leaf stage to obtain the best concentration of uniconazole.The results showed that uniconazole significantly reduced the lodging rate by enhancing lignin metabolism and optimizing the culm morphological characteristics.SU₂₅ and FU₂₅ treatments significantly increased the lignin content and activities of lignin-related enzymes such as phenylalanine ammonia-lyase,cinnamyl alcohol dehydrogenase,tyrosine ammonia-lyase,and peroxidase.The grain yield and ear characteristics increased but then decreased as the uniconazole concentration increased,where treatments SU₂₅ and FU₂₅5 obtained the highest grain yield and ear characteristics.Seed soaking significantly enhanced lignin accumulation and alleviated lodging stress compared with foliar application.Our results suggest that the lignin content is closely related to lodging resistance in maize and higher lignin accumulation in the third basal internode improved lodging resistance.Uniconazole application significantly reduced the risk of lodging stress and improved the grain yield of maize,where the most effective treatments were soaking seeds at 25 mg kg^–1 or foliar application of 25 mg L^–1 at the eight-leaf stage.2.The degradation of photosynthetic pigments leads to decrease in the photosynthetic capacity of the leaves,which in turn accelerates the senescence process of the leaves,resulting in crop yield reduction.The results showed that uniconazole seed soaking and foliar application significantly increased the chlorophyll content,soluble protein content,and net photosynthetic rate,where their maximum values were obtained with the SU₂₅,FU₂₅,and U₅₀treatments.Uniconazole significantly improved the activities of antioxidant enzymes comprising superoxide dismutase,peroxidase,and catalase,and reduced that of malondialdehyde（MDA）and the accumulation of ROS during the leaf senescence process.Treatments SU₂₅,FU₂₅,and U₅₀0 had higher antioxidant enzymes activities but reduced MDA,superoxide radical,and hydrogen peroxide contents.Uniconazole significantly reduced leaf senescence in leaves in the bottom layer,while also increasing the middle layer leaf area and decreasing the top layer leaf area.The degradation of photosynthetic pigments was reduced by uniconazole because the enhanced antioxidant activities of enzymes protected plants from harmful ROS.Uniconazole treatments in different stages significantly improved the photosynthetic characteristics,antioxidant defense system,and grain yield of maize.Uniconazole application in combination with ethephon resulted in decreased in the chlorophyll content,soluble protein content,and activities of antioxidant enzymes while increased MDA and ROS accumulations in later growth stages.The most effective treatments were SU₂₅,FU₂₅,U_50,and U₅₀+E₂₀₀.3.Grain-filling is the key stage of yield formation.Improvement in grain-filling rate can result in higher grain yield.The results indicated that uniconazole significantly increased the grain-filling rate of maize by regulating the endogenous hormone contents in grains.Lower concentration of uniconazole as seed soaking or foliar application at the eight leaf stage significantly improved the maximum grain weight,mean grain-filling rate,and maximum grain-filling rate.Uniconazole medium concentration applied alone or in combination with ethephon on later growth stages significantly improved the maximum grain weight,mean grain-filling rate,and maximum grain-filling rate.Treatments SU₂₅,FU₂₅,and U₅₀significantly improved the abscisic acid（ABA）and zeatin（Z）+zeatin riboside（ZR）contents while reduced the gibberellic acid（GAs）contents in grains.With the increase of uniconazole concentration gradual decreased in ABA,Z+ZR,and GAs contents were recorded.Application of uniconazole in combination with ethephon resulted in decreased in the ABA,Z+ZR,and GAs contents in grains.The ABA and Z+ZR contents were significantly positively correlated with maximum grain weight,maximum grain-filling rate,and grain-filling rate while the GAs content was not significantly correlated.The higher ABA and Z+ZR contents and lower GAs contents significantly improved the grain-filling rate.Based on the results of the present study,the effects of the treatments SU₂₅,FU₂₅,U_50,and U₅₀+E₂₀₀ on grain-filling rate were most significant.