Potassium fertilization arrests malate accumulation and alters soluble sugar metabolism in apple fruit.

Wen Zhang,Yanping Guo,Nishang Zhang,Xian Zhang,Xiaolin Ren,Yufei Wang,Zhengyang Zhao

doi:10.1242/bio.024745

Wen Zhang, Yanping Guo + Show 5 more

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https://doi.org/10.1242/bio.024745

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Journal: Biology open	Publication Date: Jan 1, 2018
Citations: 19	License type: CC BY 4.0

Affiliation: North West Agriculture and Forestry University

Abstract

ABSTRACTEffects of different potassium (K) levels, which were K0 (no fertilizer), K1 (71.5 g KCl plant−1 year−1), K2 (286.7 g KCl plant−1 year−1), and K3 (434 g KCl plant−1 year−1), were evaluated based on sugar and organic acid metabolism levels from 70–126 days after bloom (DAB) in the developing fruit of potted five-year-old apple (Malus domestica, Borkh.) trees. The results indicate that K fertilization promoted greater fruit mass, higher Ca2+ and soluble solid levels, and lower titratable acid levels, as well as increased pH values at harvest. With the application of different levels of K fertilizer, fructose, sorbitol, glucose and sucrose accumulation rates significantly changed during fruit development. Fruit in the K2 group had higher fructose, sucrose and glucose levels than those in other treatment groups at 126 DAB. These changes in soluble sugar are related to the activity of metabolic enzymes. Sucrose synthase (SS) and sorbitol dehydrogenase (SDH) activity in the K2 treated fruit was significantly higher than those in other treatment groups from 70–126 DAB. Malate levels in K-supplemented fruit were notably lower than those in non K-supplemented fruit, and K3 treated fruit had the lowest malate levels during fruit development. Cytosolic malic enzyme (ME) and phosphoenolpyruvate carboxykinase (PEPCK) activity significantly increased in fruit under the K2 treatment during 112–126 DAB and 98–126 DAB, respectively. In addition, Ca2+ concentration increased with increasing K fertilization levels, which promoted a maximum of 11.72 mg g−1 dry weight in apple fruit. These results show that K levels can alter soluble sugar and malate levels due to the interaction between sugars and acid-metabolic enzymes in fruit.

Highlights

Apples (Malus domestica, Borkh.), belonging to the Rosaceae family, are one of the most important cultivated fruit crops grown worldwide
Malic acid accumulation in apple fruit is well known to be under the tight control of several key enzymes, such as phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) and malate dehydrogenase (MDH)
Effect of different levels of K fertilization on fruit characteristics during fruit development A profile of apple fruit growth was established by measuring the changes in fruit mass, pH, soluble solid levels and titratable acid under different K-level conditions (Fig. 1)

Summary

Introduction

Apples (Malus domestica, Borkh.), belonging to the Rosaceae family, are one of the most important cultivated fruit crops grown worldwide. The metabolic mechanisms of sugars and organic acids in fruit play important roles in fruit yield and quality, and determine fruit organoleptic characteristics In apples, both sorbitol and sucrose are first synthesized in leaves, translocated to and used in fruit, in which the carbohydrate level consists of about 70% sorbitol and 30% sucrose (Klages et al, 2001). In the metabolic pathways of the dicarboxylate malate, the initial formation of malate is carboxylation of phosphoenolpyruvate (PEP) in the cytosol, the degradation of decarboxylation of malate and oxaloacetate (OAA), and those that allow conversion between tri- and dicarboxylates These require fixation of CO2 on a carbon skeleton derived from hexose catabolism, which is achieved by the carboxylation of PEP, catalyzed by the phosphoenolpyruvate carboxylase (PEPC). There are close relationships between sugar accumulation and acid metabolism during fruit quality formation (Yao et al, 2011)

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