Effects Of Potassium On Fruit Quality And Its Association With Metabolic Pathway Of Trehalose 6-phosphate In Apple

Potassium is an important mineral element for plant growth and development,which is closely related to fruit quality.Fruit yield and fruit quality both mainly depend on carbon metabolism,including sorbitol,glucose,fructose,sucrose,starch,malic acid,citric acid,and other carbohydrates.However,effect of potassium metabolic pathways on soluble sugar,organic acid,starch or other metabolic substance in fruit are still not very clear.In this experiment,the different growth periods of apple fruit were as experimental materials.The experimental treatments included four potassium-level treatments,potassium coupled with nitrogen or phosphorus treatments,exogenous soluble sugar treatments,and leaf-thinned treatment.The results suggested that potassium promoted soluble sugar content and inhibited organic acid content on both gene transcription level and protein level.This experiment showed the regulating mechanism between potassium and carbon metabolism in apple fruit,and also provided the theoretical support on potassium regulation with fruit growth and development.The main results are described as follows:1.Apple trees were treated with four potassium concentrations(K0?K1?K2 and K3;K2O:0,100,200 and 300 g plant-1)which were treated as experimental materials.The data were analyzed included fruit growth index,quality index,soluble sugar content,organic acid content,and enzyme activity.The results suggest that different concentrations of potassium treatments promoted fruit yield and fruit quality.K2 treatment significantly improved fruit pH,soluble solid content,fruit yield,and sugar acid ratio,while K3 treatment had the most obviously influence on organic acid level.On the one hand,potassium indirectly influenced the activity of acid invertase,sucrose synthase,and sucrose phosphate synthase,which resulted in higher sucrose,fructose and glucose accumulation during growth and development periods.On the other hand,potassium via fruit pH value effected on malic acid content and malate-related enzyme activity.In addition,potassium coupled with nitrogen and phosphorus treatments had higher fruit mass and soluble solid content than those in control treatment.2.There is a close relationship between potassium and calcium content with fruit growth and development,via analysis of potassium concentration and calcium concentration in different potassium-treated fruit.When potassium content is lower than 11.85 ± 0.29 mg g-1Dry Mass(DM),the calcium concentration in fruit increased with rapidly increasing potassium level.However,when potassium content is higher than 11.85 ± 0.29 mg g-1 DM,the calcium level in fruit decreased with the rapidly increasing potassium content.From 11.85 to 12 mg g-1DM is the most optimized potassium concentration for calcium accumulation.3.The mRNA library was established with potassium-treated and control apple fruit.Samples were sequenced by Illumina HiSeqTM 4000.The results show that potassium influenced the gene expression of apple fruit at 70 and 90 days after full bloom(DAFB).Moreover,metabolic pathway had the largest amount of differential genes in potassium-treated fruit.By combining metabonomics and transcriptome data in potassium-treated fruit and control fruit,it was found that potassium notably influenced metabolite contents and metabolic gene expressions which participate in glucose metabolism pathway,starch synthesis pathway or trehalose 6-phosphate metabolic pathway at 70 DAFB.Potassium also notably influenced metabolite contents and metabolic gene expressions of malate metabolic pathway at 90 DAFB.4.Starch,soluble sugar,and phosphorylation product contents were analyzed in fruit sample without potassium treatment.In addition to Arabidopsis,potato,grape,and other plants,there also is trehalose 6-phosphate in apple fruit.With fruit growth and development,trehalose 6-phosphate content firstly decreased and then presented a flat trend.Under nature,leaf-thinned treatment,or exogenous soluble sugar treatment conditions,trehalose6-phosphate level of apple fruit always kept up with changes of sorbitol content,but not with sucrose content.Trehalose 6-phosphate was more sensitive to sucrose or glucose content than sorbitol level in fruit.Moreover,trehalose 6-phosphate and sorbitol were collaboratively related to starch content in fruit.5.In order to understand the relationship between trehalose 6-phosphate and soluble sugar in potassium treated fruit,soluble sugar and the relative phosphorylated product content,protease,and SnRK1 activity were all analyzed.The results suggest that reducing trehalose6-phosphate level in developing fruit was promoted by lower trehalose 6-phosphate synthase activity and higher sorbitol oxidase activity under increasing potassium level condition.Potassium treatment resulted in changes of the balance between trehalose 6-phosphate and sorbitol content in fruit.Moreover,higher potassium level promoted higher correlations between Tre6P/sorbitol ratio and fructose,sucrose,or glucose level in fruit.6.Comparing potassium-treated fruit to control fruit,not only soluble solid content and fruit size were notably increased,but also fruit firmness was obviously improved.The result shows that oxygen species and ultra oxygen anion content in treatment group were much lower than those in control group.Ascorbic acid content and the related enzyme activity intreatment group were much higher than that in control group.In exo-trehalose treated fruit,MDHAR,DHAR,and APX activity were significantly increased.But only two of them,DHAR and APX activity,were changed similarly with those enzyme activity in potassium treated fruit.Moreover,potassium inhibited trehalase activity in developing fruit.DHAR,APX and TREH together regulated and influenced oxygen metabolism in potassium treated fruit.In generally,potassium not only influenced organic acid content via pH value,but also via trehalose 6-phosphate level effected on fruit firmness,starch content,and soluble sugar concentration.