Abstract： 【Objective】The economic characteristics of pears, including fruit size, skin color, sugar and acid composition, sugar-to-acid ratio, and fruit firmness, significantly impact the commercial value of pear fruit and are of interest to various stakeholders such as producers, traders, consumers, and researchers. Glucose, fructose, sucrose, and sorbitol are the primary soluble sugars in pears, while malic acid, citric acid, tartaric acid, and quinic acid are the major organic acids, with fructose and sucrose playing crucial roles in determining fruit sweetness. Therefore, enhancing these traits is fundamental for pear production. Current strategies for improving fruit quality focus on enhancing environmental conditions like water availability, soil quality, nutrient levels, and light exposure. While studies have explored the use of external agents for foliar application to enhance fruit quality, there remains a research gap concerning the pivotal enzymes that regulate sugar and acid metabolism, impacting sugar and acid levels in the fruit. This research deficiency impedes the development of effective quality enhancement products and the establishment of a robust theoretical framework. Gamma-aminobutyric acid (GABA) and melatonin (MT) are naturally occurring compounds with reported potential in regulating fruit quality, yet limited research has been conducted on their exogenous application to enhance red-skinned pear quality. This study investigates the effects of exogenous GABA and MT treatments on Hongsubao red-skinned pears, analyzing changes in the activity of sugar metabolism-associated enzymes (soluble acid invertase, neutral invertase, sucrose synthase, and sucrose phosphate synthase) and sugar content at various fruit development stages. The objective is to elucidate the key enzymes influencing sugar accumulation in Hongsubao pears and provide a theoretical foundation for developing quality control strategies for this pear variety. 【Methods】Experimental materials in this study consisted of Hongsubao pear fruits sourced from the Zhengzhou Fruit Tree Research Institute of the Chinese Academy of Agricultural Sciences, specifically from the National Horticultural Germplasm Resource Library Zhengzhou Pear Sub-library. During the fruit enlargement phase, pear fruits underwent treatment with γ-aminobutyric acid (GABA) solutions at concentrations of 5 mmol∙L-1 (G1), 10 mmol∙L-1 (G2), and 20 mmol∙L-1 (G3), as well as melatonin (MT) solutions at concentrations of 50 μmol∙L-1 (M1), 100 μmol∙L-1 (M2), and 200 μmol∙L-1 (M3) via foliar application. Distilled water served as the control (CK) for foliar spraying. The impacts on individual fruit weight, longitudinal and transverse diameters, coloration of the sun-exposed fruit surface, peel firmness, soluble solids content, sugar and acid levels, sugar-to-acid ratio, and activities of sugar metabolism-related enzymes were assessed throughout the developmental stages of pear fruits. 【Results】The optimal concentrations of exogenous γ-aminobutyric acid (GABA) and melatonin (MT) treatments notably enhanced various characteristics of Hongsubao pears, encompassing individual fruit weight, fruit coloration on the sunlit side, fruit firmness, soluble solids content, sugar-acid content, and sugar-acid ratio. Particularly, treatment M1 significantly boosted individual fruit weight, sunlit side coloration, firmness, sucrose, sorbitol, fructose, total sugar content, sweetness value, and sugar-acid ratio by 18.48%, 351%, 12.21%, 176.62%, 52.24%, 57.21%, 50.81%, 54.99%, and 25.54%, respectively. Treatment M2 notably increased soluble solids content by 11.79%, while treatment G1 enhanced glucose content by 32.74%. Exogenous treatments also substantially elevated the activities of sugar metabolism-related enzymes during the progression of Hongsubao pears. For instance, G2 treatment raised soluble acid invertase (S-AI) activity by 69.21% at 66 days after full bloom (DAF), G1 treatment increased neutral invertase (NI) activity by 81.44% at 66 DAF, M3 treatment enhanced sucrose synthase (degradation direction) (SS-Ⅰ) activity by 72.53% at 108 DAF, G3 treatment increased sucrose synthase (synthesis direction) (SS-Ⅱ) activity by 17.77% at 94 DAF, and M1 treatment augmented sucrose phosphate synthase (SPS) activity by 79.42% at 154 DAF. In conclusion, foliar application of a 50μmol∙L-1 melatonin solution significantly improved individual fruit weight, color differentiation, and soluble solid content of Hongsubao pears, key economic traits of the fruit. Moreover, it notably increased sucrose phosphate synthase activity, leading to elevated levels of fructose, sorbitol, sucrose, and organic acids in pear fruit, thereby enhancing sugar-acid ratio, sweetness, and overall fruit quality.
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