Overview of Research on Sepal Abscission and Persistence in Korla Fragrant Pear

Korla fragrant pear (Pyrus brestschneideri Rehd), abbreviated as fragrant pear, is ranked alongside Turpan grapes and Hami melon as one of the **three famous and premium melon and fruit specialties. It is a pear variety with excellent traits in color, aroma, flavor, storability and many other aspects, enjoys a high reputation in domestic and international markets, and has become one of the important export-oriented agricultural products generating foreign exchange. However, the fruit sepals of fragrant pear exhibit two conditions, abscission and persistence, and in addition, irregular changes in fruit shape and ribbed grooves on some fruits seriously affect the appearance quality of fragrant pear, reducing its commercial value and market competitiveness. There is almost no research abroad on sepal abscission and persistence in fragrant pear, and domestic research on this is also limited. This paper reviews the current research status of sepal abscission and persistence in fragrant pear fruit, and provides an outlook on future research, aiming to contribute to resolving the problems of persistent calyx and irregular fruit shape in fragrant pear.

1. Sepals, Fruit Shape and Rough-Skinned Fruit of Fragrant Pear

Sepals refer to the outermost whorl of a flower, the individual segments that make up the calyx, generally leaf-like and green, serving a protective function during the flowering period. The calyx and corolla together are called the perianth, and the calyx is composed of several sepals. In most plants the sepals fall off after flowering; for example, no sepal traces can be seen on the fruits of peach or citrus. In some plants the sepals remain on top of the fruit after flowering, called a persistent calyx. The sepals of pear fruit are an important characteristic describing the appearance quality of pear fruit. For example, as early as 1890, E. Koehne first divided the genus Pyrus into two groups based on fruit sepal abscission and persistence: the persistent-calyx group (achras) and the deciduous-calyx group (pashia). Based on the degree of sepal abscission and persistence, they are classified as: persistent, residual, deciduous; erect, reflexed. The calyx of fragrant pear includes two parts: the calyx tube and the sepals. The petals and stamens are borne on the calyx tube; at flowering the calyx tube is disc-shaped, and the lobes at the front end of the calyx tube are the sepals. Calyx abscission occurs when the lower part of the calyx tube falls off together with the sepals; in fragrant pear fruits that do not shed the calyx, the calyx tube develops as part of the fruit apex as the young fruit develops. Most fragrant pear fruits with sepal abscission have a depressed calyx end, with a fruit shape index of about 1.1, commonly known as female pear, de-calyxed fruit, or sunken-top fruit; this type of pear is nearly round and belongs to the regular-shaped fruit type. Most fragrant pear fruits with persistent sepals have a protruding calyx end, commonly known as male pear, persistent-calyx fruit, protruding-calyx fruit, protruding-top fruit, or pig-mouth pear. Research by Zhou Qishi showed that this protruding-calyx fruit exhibits hard calyx phenomenon, with flesh becoming hard, stone cells increasing, and the edible proportion decreasing. Zhang Dahai et al., through years of field investigation, experimental treatments and analysis, showed that fragrant pear has no representative fruit shape, generally varying within the range of nearly round, broadly ovate, elliptical and spindle-shaped, with a fruit shape index of 1.1-1.4. Rough-skinned fruit is a physiological disorder of pear fruit, also called green-headed fruit or hard-headed fruit. It occurs mostly in fragrant pears with the protruding-top fruit shape; such fruits have relatively high flesh firmness, relatively low soluble solids and vitamin C, and poor flavor and quality, belonging to inferior fruit.

2. External Conditions Affecting Sepal Abscission in Fragrant Pear Fruit

2.1 Rootstock Type According to a report by Ma Jiankang, under the same site conditions and tree age, fruit quality differs completely among different rootstock trees. For pear trees using Pyrus betulifolia, sour pear, fragrant pear and hybrid progeny of other varieties as rootstocks, the de-calyx rates of the produced fruits were 46.7%, 54.2% and 61.5%, respectively. According to a report by Wang Chunfang et al., the fruit shape indices of fruits obtained after top-grafting fragrant pear onto different pear varieties were: those using Gongli and Yali as rootstocks were both 1.3, those using Xuehuali and Laiyangli as rootstocks were 1.2 and 1.16 respectively, and the control fragrant pear was 1.18.

2.2 Plant Growth Regulators In recent years there has been considerable research on plant hormones and plant growth regulators. While studying sepal abscission and persistence, issues such as fruit shape, size and rough-skinned fruit are also often involved, but no in-depth research has been conducted, and it is mostly limited to studies on paclobutrazol and gibberellin.

2.2.1 Paclobutrazol Domestic horticultural scholars have done considerable research on the effect of paclobutrazol on sepal abscission and achieved certain results. According to a report by Li Baoguo et al., foliar spraying of 500-fold paclobutrazol on fragrant pear at the early full-bloom stage can effectively increase the de-calyxed fruit rate, generally by 25%-70%, and reduce the protruding-top fruit rate, with uniform fruit size, but fruit size decreases. Mei Longzhu et al. also pointed out that spraying paclobutrazol causes a decrease in single fruit weight, and single fruit weight decreases progressively with increasing concentration. Xu Qingxiu et al. treated fragrant pear with 300-2000 mg/kg paclobutrazol, among which spraying on trees at the flowering stage had the most obvious effect on fragrant pear fruit shape, and significantly reduced fruit surface protrusion and persistent calyx in fragrant pear. Treatment with two concentrations of paclobutrazol, 1000 mg/kg and 2000 mg/kg, respectively reduced protruding-top fruit from 21.3% and 27.3% on control branches of the same tree to 0, and respectively reduced the persistent-calyx fruit rate from 30.9% and 36.6% on the same tree controls to 0.4% and 0.5%, but the fruit-setting rate was significantly reduced, especially in the 2000 mg/kg treatment; at the same time, fruit shape became shorter, single fruit weight became smaller, and rough-skinned fruit also decreased to a certain extent. He also pointed out that the time at which paclobutrazol affects fragrant pear fruit shape is before flower drop, and spraying paclobutrazol on Chili pear and Canli pear also had similar effects. Gao Jiangsheng et al. proved that spraying 250-300 ppm paclobutrazol at the full-bloom stage of fragrant pear can significantly promote young fruit de-calyxing; the de-calyxed fruit rate of the control was 47.0%, and the four treatments were 35.5%, 64.9%, 75.0% and 83.3% respectively, with the highest de-calyx rate at the concentration of 300 mg/kg and the lowest at 150 mg/kg. At the same time, it reduced the proportion of protruding-calyx fruit and promoted fruit coloration. In summary, spraying a certain concentration of paclobutrazol on trees during the flowering stage of fragrant pear can effectively increase the de-calyx rate of fruit, make fruit size uniform, promote fruit coloration, reduce the protruding-top fruit rate, and lower the occurrence of rough-skinned fruit. However, there are also reports of disadvantages such as decreased single fruit weight and reduced fruit-setting rate. In addition, the mechanism of action of paclobutrazol within the tree is relatively complex, and the application effect is often influenced by many factors such as variety, tree vigor, cultivation management level and environmental conditions, and the effect varies markedly with different application methods and doses. Therefore, paclobutrazol as a de-calyxing agent for fragrant pear has not yet been fully popularized in production.

2.2.2 Gibberellin According to research by Xu Qingxiu et al., spraying 50 mg/kg gibberellin at the full-bloom stage of fragrant pear increased protruding-top fruit from 24.3% on control branches of the same tree to 96.9%, and increased the fruit shape index from 1.17 to 1.39, indicating that the effect of gibberellin is opposite to that of paclobutrazol.