Meteorological-driven modeling of comprehensive quality evaluation for ruby green pomelo (Citrus grandis)

【Objective】Ruby Green pomelo [Citrus grandis (L.) Osbeck], a recently introduced cultivar from Thailand, is characterized by green peel and red flesh at maturity. Its trees exhibit vigorous growth and a short juvenile phase before fruiting. Its fruit is characterized by desirable flavor, strong resistance to storage and transport stress, and advantageous off-season productivity, a high import price per single fruit (200- 300 yuan per fruit), and broad market prospects, which has set off a“boom”in domestic planting. However, significant regional differences in fruit quality and economic viability have been observed in Guangxi, where subtropical and South Asian monsoon climates (hot- humid summers, milddry winters, and hot and humid all year round) pose challenges to standardized cultivation practices. The purpose of this study is to clarify the regulation mechanism of climate factors on the quality formation of Ruby Green pomelo, determine the key climate-sensitive indicators, and simultaneously build a comprehensive quality evaluation model for pomelo by screening the core quality traits.【Methods】The field investigation and sampling after the introduction of Ruby Green pomelo were carried out in seven representative orchards in Guangxi (Nanning, Yulin, Baise, Hechi, etc.), with a total of 7 sampling points, and 35 resources of Ruby Green pomelo were collected. The meteorological data of temperature, rainfall, illumination time, and relative humidity were collected throughout the whole development stage of fruit. Sensory evaluation of 35 fruits was carried out by a tasting panel composed of 20 members, and 24 quality indexes of all fruits were evaluated, including morphology (peel thickness, fruit weight), soluble solid content (SSC), titratable acidity (TA), solid-acid ratio, nutrients (vitamin C, Ca, Mg) and secondary metabolites (limonin and nomilin). Correlation analysis was used to quantify the relationship between climate variables and quality indicators. Principal component analysis (PCA) transformed the data into seven core components (eigenvalues ＞ 1) by dimensionality reduction, which explained the cumulative variance of 82.70%. A weighted evaluation model (F) is established to grade and rank the suitability of each region.【Results】Although the seven sampling points were located in a subtropical monsoon climate, there were differences in microclimate in each orchard. We obtained accurate orchard meteorological data for analysis through the intelligent agriculture cloud platform of Jiejiarun Company (http://www.jjr.vip). The comprehensive score ranking of sensory evaluation of Ruby Green pomelo is: YouJiang (YOQ) ＞ HuanJiang (HNJ) ＞ BaMa (BMA) ＞ RongXian (ROG) ＞ YongNing (YNG) ＞ BoBai(BBA) ＞ LongAn (LGA). The results of sensory evaluation were influenced by water content, bitterness, and the sugar-to-acid ratio of fruit, but also easily influenced by individual taste preference. Correlation analysis shows that temperature played a dominant role in quality regulation. The average temperature (＞ 25 ℃ ) during fruit expansion period strongly increased the number of fruit capsules (r=0.970, P ＜ 0.01), but it induced the accumulation of limonin in the case of more extreme temperatures, which affected the sweetness and sourness of the fruit. Soluble solid content was positively correlated with the duration of light during maturity (r=0.509). Rainfall negatively regulated TA (r=-0.724, P ＜ 0.01) and promoted the synthesis of nomilin (r=0.776, P ＜ 0.01), indicating the biphasic effect of water stress. Among the mineral elements, calcium was negatively correlated with average temperature (r=-0.884, P ＜ 0.01), and magnesium positively correlated with rainfall (r=-0.821, P ＜ 0.01). Potassium and zinc showed no climate sensitivity (P ＞ 0.05). Among the seven sampling points, the dominant areas of YOQ, HNJ and LGA had the highest comprehensive score (F=0.05-0.32), and were characterized by the balance of heat and water resources (effective accumulated temperature ＞750 Ac (℃· d)/ month and rainfall of 1200-1450 mm). The single fruit weight of YOQ was the largest, and its juice yield (79.12%) was relatively high. The fruit of HNJ performed well in SSC (11.61° Brix), solid-acid ratio (25.47) and vitamin C content (66.58 mg · 100 g- 1 ). Although the fruit of LGA is seedless, its titratable acidity was very high (0.86 mg · L- 1 ). Principal component analysis (PCA) gave priority to seven quality components: nutrient elements (0.92), peel thickness and titratable acidity (0.87), fruit size (0.84), soluble solid content and fruit shape (0.81), bitterness (0.78), capsule number (0.75) and seed character (0.70). The model confirmed the relationship between climate and fruit quality, and the comprehensive score showed that HNJ, YOQ and LGA ranked the highest.【Conclusion】 The synergistic effect of temperature, light and relative humidity is the key to fruit quality formation of Ruby Green pomelo. Based on principal component analysis, the key quality parameters include fruit nutrient elements, peel thickness and titratable acidity, fruit size, fruit shape and soluble solid content, nomilin and limonin abundance, capsule number and seed characteristics. The comprehensive evaluation results also show that the ranking of comprehensive performance in each place is: HNJ ＞ LGA ＞ YOQ ＞ ROG ＞ YNG ＞ BMA ＞ BBA. To sum up, it is suggested that standardized planting schemesshould be adopted in areas with high comprehensive scores to promote the improvement and stability of fruit quality, so as to obtain geographical indication certification. The future research must integrate soilmicroorganism interaction and field cultivation management into the“climate-soil-management”coupling model, and adopt artificial intelligence algorithm to improve the prediction accuracy of the comprehensive evaluation model. This study provides scientific basis for the adaptability of introduction of green pomelo in subtropical areas.