Effects Of The Spraying With Exogenous GABA And Calcium On Fruit Quality And Calcium Absorption And Transport Of ’Honeycrisp’ Apple

As the largest apple producing country in the world,the output and sales of apple of our country all rank first in the world.The apple industry has become a pillar industry in the fruit area,making important contributions to poverty alleviation,rural revitalization and ecological civilization.Apple fruit is rich in vitamins,amino acids,protein and other nutrients,and is an indispensable part of people’s daily diet.Calcium is one of the essential elements of apple,which can regulate the metabolism of apple and has important significance to maintain the stability of cell membrane.Calcium deficiency in apple is easy to cause physiological diseases such as bitter pox and reduce fruit quality.γ-aminobutyric acid（GABA）is a natural amino acid in plants,which plays an important role in maintaining fruit quality and improving nutritional value.Therefore,in this study,’Honeycrisp’apple was used as materials,different concentrations of GABA were sprayed at the young fruit stage,expansion stage and maturity stage to screen the appropriate GABA concentration in apple production.Furthermore,the effects of GABA application alone and mixed application of GABA and calcium fertilizer on calcium（Ca）absorption and transport and fruit quality at different stages of fruits were analyzed.It aimed to provid the guidance for reducing the incidence of bitter acne and improving fruit quality of’Honeycrisp’apples in production.Through two years of continuous experiment,the main results were obtained:1.The application of different concentrations of GABA at young fruit stage,expansion stage and maturity stage increased the contents of glutamate（Glu）and GABA in fruits,and the Ca content in leaves and fruits changed to different degrees compared with the control.Exogenous GABA also improved the quality of ripe fruits to different degrees:Compared with CK,different concentrations of GABA increased single fruit weight,and the increase range of single fruit weight was smaller when GABA concentration was over 20 mM.The content of soluble solid in’Honeycrisp’fruit was increased by 1 mM-100 mM GABA,the titrable acid was decreased,and the ratio of solid to acid was increased.Spraying GABA alone had little effect on fruit coloring.Spraying GABA could also improve the firmness of fruits and the crispness of fruit flesh.The 20 mM of exogenous GABA was the appropriate concentration.2.Exogenous GABA and Ca affected the absorption and metabolism of GABA.For Glu,mixed spraying with 20 mM GABA+0.3%CaCl₂ significantly increased the content of Glu in fruits at different stages,and the effect of GABA content was the most significant.The content of GABA in bagged fruits at young fruit stage increased by 52.6%,67.1%and75.2%,respectively.The effect of 20 mM GABA alone was better than 0.3%CaCl₂ alone.At the same time,at different stages,20 mM GABA,0.3%CaCl₂ and 20 mM GABA+0.3%CaCl₂ mostly up-regulated the genes related to GABA absorption and transport,such as MdGAD1,MdGAD2,MdGAD3,MdSSADH1,MdSSADH2 and MdGABA-T.It promoted the GABA shunt.3.Exogenous GABA and Ca changed the content of mineral elements in fruits.Compared with CK,20 mM GABA,0.3%CaCl₂ and 20 mM GABA+0.3%CaCl₂ had a certain effect on the content of Ca and Mg in fruits.The effects of different exogenous treatments on Ca and Mg contents at different stages were roughly the same.Mixed spraying with 20 mM GABA+0.3%CaCl₂ significantly increased the Ca content in leaves and fruits,and decreased the Mg content.The contents of Ca and Mg were the highest at young fruit stage,and decreased gradually with the development of fruits.At the same time,at the expansion and maturation stages of fruits,the content of Ca was higher and the content of Mg was lower in fruits without bagging under the same exogenous treatment compared with bagging treatment.4.Exogenous GABA and Ca affected the absorption and transport of Ca.The 20 mM GABA,0.3%CaCl₂ and 20 mM GABA+0.3%CaCl₂ could improve the activity of Ca²⁺-ATPase in different degrees and promote the increase of CaM content in fruits at different stages.At the same time,the relative expression levels of MdMCU2,MdACA3,MdTPC1 and MdACA3 related to Ca absorption and transport were higher at young fruit stage and expansion stage,and the relative expression levels of CaM were higher at maturity stage.These results indicated that exogenous GABA and Ca could up-regulate the related genes of Ca absorption and transport,and promote the absorption and transport of Ca in apple fruits.5.Exogenous GABA and Ca improved fruit quality.The single fruit weight was significantly increased by spraying 20 mM GABA+0.3%CaCl₂,1200 times sugar alcohol calcium and 1200 times sugar alcohol calcium+20 mM GABA.1200 times sugar alcohol calcium+20 mM GABA had the most obvious effect on fruit hardness,20 mM GABA+0.3%CaCl₂ treatment significantly improved the ductility of the peel compared with CK.Exogenous GABA and Ca treatments could increase the soluble solid content,reduce the titratable acid content and improve the fruit flavor.On the whole,under the same exogenous treatment,the intrinsic quality of non-bagged fruits was better than that of bagged fruits.