Effects of fruit load on the flavor quality and texture formation of medium and late maturing citrus cultivars in primary fruit period

【Objective】This study aims to explore the effects of different levels of tree fruit load treatments on the flavor quality and fruit texture of five late maturing citrus cultivars in their early bearing stage，including Red Tangerine（Citrus reticulata cv. Hongju）（RT），（C. clementine × C. sinensis）Osbeck var. Meiguotangju（MGTJ），（C. reticulata × C. sinensis）var. Okitsu No.44（Okitsu），（C. reticulata × C. sinensis）var. Ehime No. 28（Ehime） andC. grandis（L.）var. Huyou（Huyou），in order to provide a theoretical basis for identifying an optimal tree fruit load interval for each cultivar and the improvement of flavor quality during their primary fruit period.【Method】In 2019，four-year-old fruit-bearing trees were taken as materials. A single-level of tree fruit load treatment was set under the conditions of protected cultivation and the total soluble solid content/ titratable acidity content（TSS/TA）ratio change in fruits was used as the main indicator to judge the ripening process of each cultivar. In 2020，two different levels of tree fruit load treatments（low fruit load：I；high fruit load：II）were set to determine the relevant indicators of flavor quality during the near-maturity period of the fruit，including the TSS，TA，soluble sugar and organic acid contents in the flesh and peel，as well as the fruit texture indexes of the fully-ripened fruits. The correlation between fruit load and the fruit quality index were also analyzed. 【Result】The contents of sucrose，fructose，glucose and citric acid in the flesh of RT were increased to different degrees under the relatively lower fruit load treatment. In Okitsu，the contents of sucrose，fructose and glucose increased，while contents of citric acid decreased and malic acid increased. However，under low fruit loading，the content of glucose in Ehime decreased，glucose content was increased，malic acid content decreased. The ratio of sugar/acid in different cultivars changed at harvest time. In RT，the final TSS/TA ratio under high load treatment was 12.7% lower than that from the low load treatment. In contrast，the final TSS/TA ratio of Okitsu and MGTJ under the high load treatment were 7.3% and 7.1% higher than that of the low load treatment，respectively. In RT，tree fruit load level showed extremely significant positive correlation（P<0.01，the same below）with fruit skin hardness，as well as a significant positive correlation（P<0.05，the same below）with its fruit skin firmness and flesh organic acid content. In Huyou，tree fruit load level showed extremely significant and positive correlation with fruit skin toughness，as well as significant positive correlation with its peel soluble sugar content. In Ehime，the tree fruit load level showed a significant positive correlation with fruit skin hardness as well as extremely significant positive correlation with peel soluble sugar content. In Okitsu，the tree fruit load level showed a significant positive correlation with peel organic acid content as well as extremely significant positive correlation with flesh soluble sugar content. In MGTJ，the tree fruit load level showed extremely significant positive correlation with flesh hardness，as well as a significant positive correlation with its flesh toughness. However，among all the pulp indexes oftested varieties，there was no significant correlation between fruit firmness and tree fruit load levels.【Conclusion】 This study suggests that RT，Ehime and Huyou achieve a higher ratio of TSS/TA and better flavor under the relatively lower tree fruit load treatment whereas the ratio of TSS/TA and flavor quality of Okitsu and MGTJ are better under a relatively higher tree fruit load treatment in their early bearing stages. This study provides reference data to support the further improvement of production and quality control of citrus fruits.