Effects of rain-shelter cultivation on leaf photosynthetic characteristics and fruit quality in pear

【Objective】Rain- shelter cultivation has been widely implemented in fruit production to cope with adverse environmental stresses and enhance the fruit quality. However, its physiological effects on photosynthetic parameters, sugar-acid metabolism and related metabolic enzymatic activity in pear remain poorly understood. This study investigated the impact of rain- shelter cultivation on leaf photosynthetic performance, fruit appearance, and internal quality of the green-skin pear cultivar Zheli No. 6 at various developmental stages. This study aims to provide theoretical reference for optimizing cultivation practices in the humid and rainy pear-growing regions of southern China.【Methods】Field experiments were conducted at the National Pear Industry Technology System Production Demonstration Orchard in Jiantou Village, Xikou Town, Jianning County, Fujian Province, to compare the aforementioned indicators of pear trees grown in rain-shelter (plastic greenhouse) and open-field cultivation patterns. Leaf and fruit samples from Zheli No. 6 pear trees were collected at 30, 50, 70, 100, and 120 days after full bloom (DAFB). Leaf photosynthetic parameters, including chlorophyll content, net photosynthetic rate, stomatal conductance, and intercellular CO2 concentration, were measured using a chlorophyll meter (SPAD) and a portable photosynthesis system (LI-6400XT), respectively. Fruit internal qual-ity traits, such as soluble sugars, pericarp lignin, and phenolic compounds, were analyzed using spectrophotometric methods, and the levels of organic acids were quantified via the high- performance liquid chromatography (HPLC). In addition, the activities of sugar metabolism- and organic acid metabolismrelated- enzymes were quantified using commercially available reagent kits, comprising sucrose synthase (SS), hexokinase (HXK), fructokinase (FRK), sucrose phosphate synthase (SPS), soluble acid invertase (S- AI), and phosphoenolpyruvate carboxylase (PEPC), NADP- malic enzyme (NADP- ME), NAD-malate dehydrogenase (NAD-MDH), as well as NAD-isocitrate dehydrogenase (NAD-IDH), and citrate synthase (CS). Furthermore, the Pearson correlation analysis of soluble sugars, organic acids, and their key metabolic activities were performed to identify key factors contributing to metabolic differences between cultivation systems.【Results】For fruits of Zheli No. 6 grown under open-field cultivation, the brown russet unevenly appeared on the skin beginning at 70 DAFB, and the russet area expanded progressively during fruit development. At fruit maturity (120 DAFB), the russet area covered most of the fruit skin and resulted in rough brown-skin fruits. However, for fruits of Zheli No. 6 grown under rain-shelter cultivation, same russet spots appeared near lenticels starting at 100 DAFB, and the fruit skins kept a smooth green surface until fruit maturity. The significantly lower lignin content in the skin of rain-shelter cultivation fruits (with no or sparse russet) compared to open-field cultivation fruits (with more russet) at the same developmental stage, might be associated with the changes in lignin content. At the early fruit developmental stage (30 DAFB), the single fruit mass, transverse diameter, and longitudinal diameter of Zheli No. 6 under rain-shelter cultivation were significantly higher than those under open-field cultivation; however, no significant differences were observed in these traits between cultivation methods during 50-100 DAFB. By contrast, at fruit maturity (120 DAFB), these traits under rain-shelter cultivation were significantly lower than those under open-field cultivation. Compared with open- field cultivation, Zheli No. 6 under rain- shelter conditions had significantly higher leaf chlorophyll content and stomatal conductance, while the net photosynthetic rate remained comparable between the two cultivation patterns. This suggested that rain-shelter cultivation did not impair leaf photosynthetic efficiency. Rather, leaves under rain-shelter conditions likely underwent adaptive regulation to enhance photosynthetic capacity and increase assimilate production, thereby potentially supporting sugar accumulation in developing fruits. Analysis of soluble sugar content revealed that sorbitol was the most abundant sugar at early fruit development stage. Fructose, glucose, and sucrose levels increased steadily throughout development, with fructose becoming the dominant sugar at maturity. Significant differences in total and individual sugar contents were observed between the two cultivation systems in mature fruits. Under rain-shelter cultivation, mature Zheli No. 6 fruits had significantly higher levels of total sugars, sucrose, and glucose than those grown in open fields. Correlation analysis showed that sucrose content was significantly and positively associated with SS activity, while glucose content was positively correlated with HXK activity. These findings suggested that elevated SS and HXK activities may contribute to the increased sucrose and glucose levels under rain-shelter cultivation. Organic acid profiling showed that malic, citric, and quinic acids followed similar accumulation patterns in both cultivation systems, with malic acid being the predominant component. However, the malic acid content in mature fruits was significantly lower under rain-shelter cultivation, while the citric and quinic acid contents were significantly higher. The differences in total organic acid content were mainly driven by variations in malic acid levels. Correlation analysis revealed significant associations between malic acid content and the activities of PEPC and NADP-ME, indicating that these enzymes played key roles in the reduced malic acid content observed under rain-shelter cultivation.【Conclusion】Rain-shelter culti-vation can significantly enhance both the appearance and internal quality of pear fruits and is suitable for application in the humid, and rain-rich pear-growing regions of southern China.