Important reasons why autumn shoots are the main fruiting mother branches of longan

Longan, like other subtropical evergreen fruit trees, grows new shoots throughout the year, and in the Fuzhou area it flushes 4-5 times a year: 1 spring flush, 1-2 summer flushes, 1 autumn flush, and winter shoots are less common, with large numbers of winter shoots emerging only under warm winter conditions. According to Huang Jinsong (1962), who observed adult Wulongling and Youtanben longan trees, in terms of quantity, autumn shoots were relatively numerous and common, accounting for 33.02% of the total shoots. However, if the year's crop was small or absent, the number of spring and summer shoots was greater; over 4 years the average spring shoots accounted for 23.99% of total shoots, summer shoots for 43.03% (including summer-extended autumn shoots), and autumn shoots for 22.98%. In shoot length, spring shoots were the longest (average 9.87 cm), autumn shoots next (7.61 cm), and summer shoots the shortest (6.67 cm). According to observations in Sichuan, however, spring shoots were numerous and autumn shoots the fewest. 

Longan trees have a relatively high sprouting rate but low branching ability. New longan shoots usually extend from the terminal bud of already matured shoots; therefore, 2-4 years after planting, attention should be paid to shaping and pruning, cutting back the extending branches to help the canopy expand rapidly. At the early stage of new shoot sprouting, the scales covering the bud begin to loosen, and the bud color changes from yellow-brown to yellow-green; 5-10 days after bud break, the leaflets on the first leaf of the new shoot begin to flatten. In the early stage of new shoot sprouting (about 15 days), the increase in length and thickness is most obvious, after which it shows mainly in the thickening and maturing of the shoot. From the start of sprouting to the leaves turning light green, a new shoot takes as little as 15 days or as long as 30 days; from light green to dark green until the next sprouting takes as little as 20 days or as long as 45 days. Therefore, completing one shoot-flush cycle takes as little as 50 days or as long as 80 days. The average shoot-flush cycle for each flush is: spring shoots 60 days, summer shoots 50 days, autumn shoots 45-50 days, and winter shoots do not turn green for a long time. Establishing the number of days for each shoot type to complete a growth cycle provides a scientific basis for formulating cultivation measures such as fertilization, pest and disease control, and spraying agents or girdling to control winter shoot emergence. 

Longan shoots are of four types: spring shoots, summer shoots, autumn shoots and winter shoots. The criteria for classifying each shoot flush are: whenever 70%-80% of the shoots in the whole tree canopy within a month have tender shoots 1-2 cm long, it is defined as the shoot type of that month; for example, February-April is spring shoots (February is early spring shoots, March is spring shoots, April is late spring shoots); May-July is summer shoots (May is early summer shoots, June is summer shoots, July is late summer shoots); August-October is autumn shoots (August is early autumn shoots, September is autumn shoots, October is late autumn shoots); November to January of the following year is winter shoots (November is early winter shoots, December is winter shoots, January is late winter shoots). The above standardized classification criteria help eliminate confusion in distinguishing shoot types and provide a basis for phenological observation of shoot emergence periods. 

1) Spring shoots Usually emerge from autumn shoots that did not develop into flower panicles the previous year, or from post-harvest autumn shoots and old branches. The timing of spring shoot sprouting is related to temperature and variety. In the Fuzhou area it is roughly around late January; for warm-winter or early-maturing varieties such as Bayizao and Chike, spring shoot bud break can be advanced to around late December. Spring shoots grow vigorously from mid-February to mid-March; most spring shoots stop elongating in mid-April and basically mature in late April. Fruiting trees have few spring shoots; however, trees with few flower panicles that year or with large numbers of flower panicles undergoing "flushing" have many and vigorous spring shoots. Because spring shoot emergence occurs at low temperatures, the growth period is long and the growth vigor is relatively weak, making it difficult to form good fruiting mother branches for the following year. However, the emergence of spring shoots plays an important role in improving longan photosynthesis, increasing nutrient accumulation, and promoting summer shoot emergence and thereby extending autumn shoots. Therefore, spring shoots are important basic shoots for longan. 

2) Summer shoots Generally emerge from the current year's spring shoots or from fruit-bearing branches and old branches that did not sprout the previous year. Summer shoots have 1-2 emergence periods from early May to mid-to-late July. Early-to-mid May is the peak emergence period of the first summer shoots, with relatively few shoots; the second peak emergence period is from mid-to-late June to mid-July, when high temperatures and sufficient rainfall make this the peak of summer shoot emergence and growth. For young, vigorous trees or those under good management, in mid-to-late July new shoots will again emerge from the tips of the current year's early-emerged summer shoots, forming a second summer flush. Summer shoot emergence is closely related to the current year's crop load and management level. If the current year's crop is heavy, nutrients are largely consumed for fruit development, so summer shoots emerge very few; conversely, they emerge many. For moderately fruiting plants, strengthened fertilizer and water management can produce more summer shoots on non-fruiting shoots. Summer shoots grow thick and strong and have many leaves, and are usually very good important basic shoots for extending autumn shoots. Therefore, the number and vigor of summer shoots have obvious effects on increasing the number of autumn shoots, enlarging fruit, and reducing late fruit drop. Strengthening the application of pre-flowering fertilizer and appropriate cultivation management such as thinning flower panicles and fruit clusters to promote large numbers of summer shoots is an important basic shoot for mitigating and overcoming alternate bearing in longan and achieving stable yields. However, trees with heavy crops produce few summer shoots. 

3) Autumn shoots There are mainly two kinds of autumn shoots: one is the autumn shoot extended from a summer shoot as the base branch (commonly called summer-extended autumn shoot); this kind of autumn shoot sprouts early and is strong, and is the most important fruiting mother branch for the following year. The other is the post-harvest autumn shoot sprouting from the axillary buds at the top of the fruiting mother branch after harvest. For early-maturing longan varieties and in the longan production areas of southern China, if irrigation facilities are available and sufficient pre-harvest fertilizer can be applied in time, most can produce two post-harvest autumn shoots, which can also become good fruiting mother branches. However, if there is fertilizer but no water supply, the post-harvest autumn shoots produced grow thin and weak and are difficult to become good fruiting mother branches. Strengthening agricultural measures and cultivating a sufficient number of strong autumn shoots is an important key to annual high yield in longan. 

4) Winter shoots To correctly judge what constitutes a winter shoot, it should be determined from the period of longan flower bud differentiation. Before entering physiological flower bud differentiation, new shoots that cannot mature and cannot undergo morphological flower bud differentiation are called winter shoots. In Fuzhou, longan enters physiological flower bud differentiation in December, so new shoots that sprout and emerge in November-December are all winter shoots. The emergence of winter shoots is related to winter temperature and moisture levels; under warm and rainy winter conditions, winter shoots easily sprout. Winter shoots grow slowly, are thin and weak, are not plump, and are prone to freeze damage. At the same time, winter shoot growth consumes large amounts of nutrients accumulated in the tree, so winter shoot emergence is unfavorable for flower bud differentiation the following year. Therefore, in cultivation, fertilizer and water control, root cutting, girdling and growth regulators are often used to control them, avoiding winter shoot emergence and regulating the contradiction between vegetative growth and reproductive growth, thereby achieving the goal of high yield and good quality. 

Determining the number of autumn shoots and the shoot-release period. Shoots that emerge in longan from August to October are autumn shoots. Adult fruiting trees of mid-to-late-maturing varieties generally produce only 1 post-harvest autumn shoot. They sprout and grow from late August to early September after harvest, but low-age fruiting trees with suitable climate, sufficient water and fertilizer, and strong tree vigor will produce a second autumn shoot in mid-to-late October; low-age fruiting trees of early-maturing varieties generally produce two autumn shoots, the first sprouting from early August to early September, and the second from late September to early November. Therefore, for mid-to-late-maturing varieties, the aim is to release 1 autumn shoot from late August to early September and control the sprouting of the second autumn shoot; for those low-age fruiting trees with strong vigor, management should be strengthened to release 2 autumn shoots to prevent winter shoot emergence. For low-age fruiting trees of early-maturing varieties, it is preferable to release 2 autumn shoots, that is, one in early August and one from late September to early October.

Timely pruning and orchard cleaning, and releasing autumn shoots on schedule. If longan fruiting trees are not pruned and are allowed to grow naturally, their branches may show the following conditions: branches clustered, weak and overlapping; some branches growing excessively with prominent dominant branches, while some branches have difficulty sprouting and leave empty positions; branch groups are uneven in strength, and the canopy is difficult to make full and complete. For this reason, through pruning, dead branches, weak branches, overlapping branches, diseased and pest-infested branches, and excessively vigorous branches within the canopy are removed to control the number of shoots, concentrate nutrients, promote healthy new shoot growth, and cultivate a good hemispherical canopy. For adult fruiting trees, if the crop is relatively heavy, the tree vigor is relatively weak, and only 1 uniform autumn shoot is planned, pruning can be slightly later to retain more leaves and help restore tree vigor, and can be carried out after harvest when the branches begin to sprout new shoots (late August to early September); if the current year's crop is relatively small, the tree vigor is relatively strong, fertilizer and water conditions are good, and 2 autumn shoots are planned, pruning should be carried out immediately after harvest to promote early autumn shoot sprouting. For low-age fruiting trees, if fertilizer and water are sufficient and 2 autumn shoots can be produced after harvest, pruning should be early rather than late, and should also be carried out immediately after harvest; for trees that bear too much fruit in the current year and are relatively weak in growth, leaves should be retained as much as possible after harvest to prevent the trunk from being exposed to the sun and declining, and pruning should be carried out when the leaves of the emerging autumn shoots turn green. At the same time, orchard cleaning should be carried out together with pruning, removing harmful weeds, dead branches and fallen leaves in the orchard and concentrating them for deep burial or burning, then spraying once with a mixture of carbendazim at 800 times plus dichlorvos at 1000 times to keep the orchard clean, prevent weeds from competing with fruit trees for fertilizer, water and light, reduce pest and disease damage, and promote early new shoot sprouting.

The annual shoots of longan contain two sections with different functions: the fruiting mother branch and the base branch. The fruiting mother branch refers to the last segment of new shoot on the annual shoot, which grows well and can flower and bear fruit the following year, and is called the fruiting mother branch. The vegetative branches below the fruiting mother branch (spring and summer shoots) are collectively called the base branches of the fruiting mother branch. In our longan production areas, taking the common saying "summer-extended autumn shoot" as the entire shoot being a good fruiting mother branch is a conceptual error, because strictly speaking the summer shoot is the base branch, and the autumn shoot extending from it is the true fruiting mother branch. In recent years, Guangdong and Guangxi have emphasized promoting two post-harvest autumn shoots as fruiting mother branches; in this way, below the first post-harvest autumn shoot, including spring and summer shoots, all are called base branches of the fruiting mother branch, and only the second post-harvest autumn shoot is the fruiting mother branch. This is conceptually the same as the division of fruiting mother branch units by Lu Meiying (1997). Autumn shoots mainly emerge from the terminal buds of vegetative base branches or from the axillary buds of fruiting mother branches after harvest. Autumn shoots emerging from vegetative branches as base branches (that is, spring-extended summer shoots then extended autumn shoots, and summer-extended autumn shoots) are of the best quality, with the best flower formation and fruit set. According to observations by Chen Wenxun (1964), the panicle formation rate of autumn shoots emerging from vegetative branches as base branches reached 40.0%-72.3%, while the panicle formation rate of post-harvest autumn shoots was 23.0%-40.1%. At the same time, a good fruiting mother branch must be a two-level shoot, that is, on the basis of a vegetative branch as the base branch, an autumn shoot fruiting mother branch is further extended, while a single post-harvest autumn shoot or a late-emerging post-harvest autumn shoot can hardly become a good fruiting mother branch. Summer shoots or, in high-yield years, latent buds on perennial branches can also form fruiting mother branches, but this situation is very rare. This is because mature summer shoots must extend and produce autumn shoots; from the time to complete one shoot-flush period, summer shoots take 50 days, and if summer shoots emerge in July, under normal management conditions they will generally extend autumn shoots in September. Therefore, establishing the updated concept that autumn shoots are the main fruiting mother branches of longan is crucial for formulating high-yield and high-quality cultivation measures for longan.