Effects Of Different Potassium Level On Sugar And Acid Metabolism In ’Gala’ Apple Fruit

Potassium(K) is an essential mineral element for fruit development and quality formation.It is importantto research the effects of different potassium levels on carbonmetabolism in apple fruit. In this study, fruits were harvested from six-year-old ‘Gala’ apple trees(M. domestica Borkh.) grafted on M. 26self-rooted rootstock which are growing in an experimental orchard of Northwest A&F University(Yangling, Shaanxi, China). The 4 treatments were as follows:0, 71.5, 286.7, 434 g KCl Plant-1?yr-1, which wasequivalent to 0, 0.5, 1.5, 2.5 kg K2O100 kg-1fruit. In order to evaluate the effects of different K levels on apple fruit development, the changes of fruit growth, sugar and acid contents, sugar-metabolic and malate-metabolic enzymes were analyzed. The main results were as follows:1.Supplying K fertilizer could significantly increase Kcontents in fruit tissues, and then promote the rise of the fruit weight, firmness and SSC, and alsoreduce the content S of titratable acid.Thus,K element was able to improvefruit quality at harvest, especially 1.5 kg K2O100 kg-1 fruittreatment. However, excess K levels would be harmful forplant growth and display negtive effects. The results indicated that quality traitsexcept for titratable acidcontentsin fruit were improved by suitable K supplies, and inhibited by the excess K fertilizer.2.With the levels of K supplyincreased, Kand Ca contents significantly increased during fruit development. And 1.5 kg K2O100 kg-1fruittreatment performed best effects on fruit developing.However,magnesium contentsshowed no obvious correlation with K levels.3. It was found that different K supply had obviously effects on fructose(all stage, especially during86-99 DAB) and sucrose(99-112 DAB) accumulation,but slightly changes in sorbitol and glucose levels,especially with 1.5 kg K2O100 kg-1 fruittreatment. During fruit development, the increasing activities of AI(73-99 DAB), SS(73-99 DAB) and SDH(throughout all stages) provoked a higherfructoseaccumulations. Meanwhile, the activities of SPS(86-112 DAB) and SDH considerably influenced on sucrose contents. In addition, there was a highly correlations between K elementand activities of SDH, AI and SPS. Itwas also noteworthy that2.5 kg K2O100 kg-1fruitwas unbenifit for accumulating sugars.4.Malic acid is a dominant acid in apple fruit. There were significantlynegative correlations between K content and malic acid contents.During early degradation stage ofmalic acid, K treatments can significantlyimprove the activity of cy MDH and inhibit PEPC activity which could affect malic acid accumulation.However, the changes of cy MDH and PEPC activities were not theonly two factors, and the K levels in fruit could influence malic acid as well. During the laterperiod of fruit enlargement stage,the malate accumulation could be reductedby the increasingcy ME(112-125 DAB) and PEPCK(99-125 DAB)activitieswith different K levels.5. It wassuggested that sugar-acid ratio in fruit was improved with suitable K supply. The ratiowas positively correlated with potassium levels in a certain range.The suitableratio was under1.5 kg K2O100kg-1 fruittreatment, andexcess K supply would cause the ratio falling.What is more, the effect of suitable contents of K would promote more sugar accumulationsand the malate reduction, which improved the formulation ofquality inapple fruit.