Physiological Mechanism Of Polyglutamic Acid In Regulating The Growth And Fruit Quality Of Strawberry

To clarify the physiological mechanism of poly-γ-glutamic acid（PGA）in regulating strawberry growth and fruit quality,pot experiments were conducted to investigate the effects of different concentrations of PGA on strawberry growth index,photosynthetic physiology parameters,chlorophyll fluorescence parameters,fruit mass,and associated metabolic enzyme activity by using strawberry variety’Zhangji’as the material.The application methods of PGA included foliar spray and root irrigation.Three concentration gradients were set for PGA solution treatment,including 50mg/L,100mg/L and 200mg/L.At seedling stage and bud stage,the SPAD value,photosynthetic physiological parameters（transpiration rate,net photosynthetic rate,intercellular CO₂mole fraction,stomatal conductance,stomatal limitation value and water use efficiency）and chlorophyll fluorescence parameters（PSII actual photochemical efficiency,PSII maximum photochemical quantum yield,PSII effective photochemical quantum yield,apparent electron transport rate,non-photochemical quenching coefficient and photochemical quenching coefficient）of strawberry leaves were measured.The aboveground growth indexes（plant height,transverse width,longitudinal width,basal diameter,maximum leaf length,maximum leaf width and aboveground biomass）,root growth indexes（total root length,root surface area,average root diameter,total root volume and underground biomass）,total biomass and root-shoot ratio were measured at bud stage.Fruit quality indexes（fruit shape index,color parameters,soluble solids,protein,total phenol,anthocyanin,citric acid,malic acid,total acid,Vc,soluble sugar,fructose,sucrose and glucose）were measured at mature stage.The activities of Vc metabolism-related enzymes（ascorbate peroxidase,glutathione reductase,dehydroascorbate oxidase,monodehydroascorbate oxidase and L-galactose-1,4-lactone dehydrogenase）,glucose metabolism-related enzymes（sucrose phosphate synthase,sucrose synthase,neutral invertase and acid invertase）,acid metabolism-related enzymes（citric acid synthase,NAD-malic enzyme,NADP-malic enzyme,NAD-malic dehydrogenase and NADP-malic dehydrogenase）and the contents of related metabolites were determined at color-changing stage and mature stage.Through above studies,the aim of our study was to elucidate the physiological mechanisms of PGA in regulating the growth and fruit quality of strawberry,which can provide a reference for the application of PGA in strawberry production and culture.Statistical analysis of the data obtained from the study led to the following conclusions:1.PGA treatment increased chlorophyll SPAD value of strawberry leaves at seedling stage and bud stage.For foliar spraying and root irrigation,chlorophyll SPAD value reached the maximum value under 100mg/L PGA treatment,but there was no significant difference between two application methods.At the same time,PGA treatment could increase the net photosynthetic rate（Pn）,transpiration rate（Tr）,intercellular CO₂mole fraction（Ci）,stomatal conductance（Gs）and water use efficiency（WUE）of leaves.Among different concentrations of PGA,100mg/L PGA treatment had the most significant effect under foliar spraying and root irrigation.PGA could also reduce stomatal limitation value（Ls）of strawberry leaves at seedling stage and bud stage.For foliar spraying and root irrigation,the effect of 100 mg/L PGA treatment was the most significant,but there was no significant difference between two application methods.In addition,PGA treatment could also increase the actual photochemical efficiency of chlorophyll fluorescence parameter PSII（Y（II））,the maximum photochemical quantum yield of PSII（Fv/Fm）,the effective photochemical quantum yield of PSII（Fv’/Fm’）,the apparent electron transfer rate（ETR）and the photochemical quenching coefficient（q P）,and reduce the non-photochemical quenching coefficient（NPQ）.These results indicated that PGA under both foliar spraying and root irrigation could improve PSII light conversion efficiency and utilization rate of strawberry leaves,reduce non-photochemical energy consumption,and thus improve photosynthetic performance and promote the accumulation of photosynthetic products.For two application methods,the effect of 100mg/L PGA treatment was the most significant,and the effect of foliar spraying was better than that of root irrigation.2.PGA treatment promoted the growth of strawberry plants,especially for the growth of plant transverse and longitudinal.At the same time,above-ground biomass,subsurface biomass and total biomass of strawberry plants were increased,but root-shoot ratio was not significantly affected.In addition,PGA treatment also promoted the growth of plant roots and significantly increased total root length,but had no significant effect on average root diameter.For two application methods,100mg/L PGA treatment had the best effect on strawberry growth index,and the effect of foliar spraying was better than that of root irrigation.3.PGA treatment increased fruit shape index,color parameters,soluble solids,protein,total phenols,anthocyanin,Vc,soluble sugar,fructose,sucrose and glucose content in mature fruit,and reduce citric acid,malic acid,total acid content,thereby improving fruit quality.These results indicated that PGA treatment promoted the appearance quality and nutritional quality of strawberry fruit.For two application methods,100mg/L PGA treatment had the best effect,and the effect of foliar spraying was better than that of root irrigation.4.PGA treatment increased the activities of ascorbate peroxidase（APX）,glutathione reductase（GR）,dehydroascorbate oxidase（DHAR）,monodehydroascorbate oxidase（MDHAR）and L-galactono-1,4-lactone dehydrogenase（Gal LDH）in fruits at color-changing and mature stages,thereby increasing the content of Vc in fruits.These results indicated that PGA treatment promoted Vc metabolism in strawberry fruit,thereby increasing Vc content.For two application methods,100mg/L PGA treatment had the best effect in enhancing Vc metabolism,and the effect of foliar spraying was better than that of root irrigation.5.PGA treatment enhanced the activities of sucrose phosphate synthase（SPS）,sucrose synthase（SS）,neutral invertase（NI）and acid invertase（AI）in fruits at color-changing and mature stages,thereby increasing the contents of sucrose,fructose and glucose in fruits.These results suggested that PGA treatment promoted sugar metabolism in strawberry fruit,which further increased the contents of sugars and improved fruit quality.For two methods applied,100mg/L PGA treatment showed the best effect in enhancing sugar metabolism,and the effect of foliar spray was better than root irrigation.PGA treatment could inhibit the activities of citric acid synthase（CS）,NAD-malic enzyme（NAD-ME）,NADP-malic enzyme（NADP-ME）,NAD-malic dehydrogenase（NAD-MDH）and NADP-malic dehydrogenase（NADP-MDH）in the fruit at color-changing and mature stages,thereby reducing the contents of citric acid（CA）and malic acid（MA）in the fruit.This result suggested that PGA reduced the contents of acids by inhibiting the acid metabolism in strawberry fruit.For two applied methods,100mg/L PGA treatment had the best inhibitory effect on the acid metabolism,and the effect of foliar spray was better than root irrigation.Meanwhile,PGA treatment increased the contents of soluble solids and soluble sugars in fruit during color-changing and mature stages and decreased the total acid content,which further increased the sugar-acid ratio.For two methods applied,100mg/L PGA treatment had the best improvement in sugar-to-acid ratio,and the effect of foliar spray was more effective than root irrigation.