The lychee (Litchi chinensis Sonn) an important sub-tropical evergreen fruit crop belonging to family Sapindaceae, is believed to have originated in China, where it has been grown in Southern Guangdong state for thousands of years. It is highly specific to climatic requirements and probably due to this reason its cultivation is restricted to few countries in the world.
The lychee tree is handsome, dense, round-topped and slow growing with evergreen leaves having 6-9 elliptic oblong and lanceolate abruptly pointed leaves. Colour of leaves varies from light green to dark green. Greenish white or yellowish flowers are borne in clusters. Fruits are round or heart shaped having thin, leathery skin. The colour of fruits varies with cultivar, and is red or rose or pinkish. The edible portion or fruit is the aril, which is immediately beneath the skin. Flavour of the aril varies with cultivar, which is distinctive. Seeds are bold but in some cultivars seeds are partially developed, due to failure of pollination, referred to as ‘chicken-tongue’ seed. The trees with small seeded fruits are prized because of the greater portion of pulp.
The origin of this cultivar is not known but the name indicates that it was selected for its superiority and named ‘China’. It is tolerant to hot waves and fluctuations in soil moisture, which cause fruit cracking. Rains at the time of fruit bud differentiation cause emergence of vegetative flush resulting in loss of crop. It bears fruits in cluster of 12-18. In some cases more than 30 fruits per cluster are also recorded. The plants bear less fruit in eastern and southern directions. Fruits are large in size, medium-heavy in weight, oblong in shape, and tyrant rose in colour with dark tubercles at maturity. The aril is creamy-white, soft, juicy, sweet having 18 to 17° brix TSS, 11 percent total sugar and 0.43 percent titratable acidity. Seeds are glaucous, dark chocolate in colour, oblong to concave or planoconvex in shape, medium in size (2.9 cm length and 1.5 cm diameter), and average in weight (3.49 g/seed).
Climate and Soil
Since, flower bud differentiation, flowering, fruit set, fruit quality and flavour development in Lychee is influenced significantly by temperature and humidity, it has adapted well in the sub-tropics where summer months are hot and wet and winter months are dry and cool. Hot summers free from hot wind and winters free from frost are essential.
Lychee cultivation is highly successful in areas having minimum temperature of 10°C from December to February and 38°C from April to June. However, temperature of 32° C during these months is considered to be optimum. It is highly specific to climatic requirement for its establishment, plant growth and fruiting, and consequently spread of area. A moist atmosphere, occasional rainfall, cool dry winter free from frost and hot winds are ideal for its cultivation. In lychee growing areas in Pakistan the temperature varies from 21° C to 37.8°C during flowering and fruiting. It has been observed that flower initiation in lychee requires comparatively low temperature. Seasonal variation in temperature is favourable for proper fruiting. A dry climate, free from rains for about 2 months before flowering induces flower bud differentiation, blossom and consequently give high production.The fog free dry winter, mild sub-tropical summer and intermittent pre-monsoon showers during April-May have been observed to be highly favourable for blossoming, better aril development and improvement in fruit quality. The sub-tropical to mild temperate climate in the foothills and valleys of the Himalayas are also suitable for lychee cultivation. Depending upon the temperature rise after winter the time of flowering and maturity is determined. No fruiting has been recorded when lychee has been grown in tropical conditions. However, on hills in southern states flowering is observed and harvesting commences in November-December.
In Pakistan, lychee is grown successfully on a wide range of soil types, which include sandy loams, laterite, alluvial sand, and calcareous soil, but the best lychee orchards are seen in alluvial sandy loam soils with good drainage and access to the water table. The performance of orchards is very poor on clay soil with poor drainage. Lychee grown in sandy soils have a root system network, while trees grown in clay soil have very poor root distribution. It grows well even in calcareous soil with 30 percent free lime content.
PRODUCTION OF PLANTING MATERIAL
Lychee is generally multiplied by vegetative methods of propagation as plants raised through sexual method (by seed) grow slowly, have a long juvenile period and do not produce fruit true to the type. However, earlier introduction in different parts of the country was perhaps through seeds, which enabled the selection of superior types and perpetuation the cultivar through vegetative means. The most commonly practiced method of vegetative propagation is air-layering, though cutting, grafting and budding have been found to be successful.
Air-layering, known as ‘marcottage’ in China is commercially practiced for large scale multiplication both in public sector and private sector nurseries. When and how this practice was adopted is not documented but the process of development and modification in the method of layering suggests that the method has gone through transformation. Earlier layering was done using clay soil having provision of watering, however, the air-layer practiced now uses growth hormone and nutrient mixed media of peat moss or coir pith, which is covered with polythene. For preparation of the air-layer a healthy terminal branch receiving good sunshine with a thickness of about 1.2-1.5 cm is selected and a 2.5 cm ring is made by removal of bark about 45-50 cm below the apical growth. The cambium layer is rubbed off and the woody portion is exposed. Rooting hormone (1000 ppm IBA) is used as paste or powder. A layer of moist sphagnum moss or coir pith is placed and wrapped with a piece (20 x 25 cm) of 400 gauge polythene sheet and tied properly at both ends to ensure supply of proper moisture which facilitates the development of roots. It is advised to enrich the rooting medium using organic nutrients. After about 50-60 days, the adequate root system develops from the upper end of the ring, which is visible through the polythene film. The layer is removed by making a sharp cut about 5 cm below the lower end of the ring, preferably in 2-3 stages. The detached layers are planted in partial shade. Success in rooting of the layer is determined by temperature and humidity. When night-time temperature falls to less than 20°C the root becomes brittle. Thus, June is considered to be best time for air-layering. In order to enhance the success of the detached layer, defoliation of leaves up to 50 percent is advocated. At the time of planting excess vegetative growth may be removed to maintain balance between the top and newly developed root system. Regular irrigation and weeding is done to facilitate better establishment and growth. Beds are kept weed free. Lychee layers become ready for field planting in 4-5 months. Growing of layers in the greenhouse has been found to enhance success.
Some nurseries practice, pot layering wherein a lower branch of mature wood is cinctured and the cut surface is buried in a pot or container filled with rooting medium. The pot is watered regularly. The roots develop in the cinctured portion of the branch in about 2 months. Then the branch is detached from the main plant by giving sharp cut, preferably in 2-3 stages. No repotting is required before transplanting in the field. Application of IBA (2000-5000 ppm) improves rooting and survival of the layers.
For large scale multiplication stooling is also recommended. In this method, planting is done closely at 1 x 2 m. Once the plant attains the required growth it is headed back to the stump during January-February which permits new shoot (stools) emergence from the stump within two months. A ring of 2 cm is made at the base of the newly emerged shoots and rooting hormone is applied. Then a mound of soil is raised around the shoots to encourage rooting and watering is done regularly. Profuse rooting occur in the stools within two months. These stools are detached and kept in the nursery for hardening and become ready for transplanting in July-August. In stooling, one must be careful not to allow the soil mound to dry, otherwise the rooting process is affected adversely. Therefore, the stool beds should be irrigated at weekly intervals from April-June.
Although this method is advocated it has not been practiced by nurserymen on a commercial scale. The propagation of lychee has also been tried through cutting under mist conditions. A high percentage of rooting was also obtained from the cutting treated with IBA and planted in April-May under mist. But this has not been adopted commercially.
Grafting and budding
Grafting in lychee is mainly practiced for changing scion cultivar or seedling tree or unproductive and old orchards by top working. The apical, side and approach grafting are mainly practiced. In apical grafting 10 cm long scion wood (non-terminal) with at least 2 slightly swollen buds gives better results. The technique of splice or tongue grafting is successful. Apical grafting has not been commercially used for large scale multiplication. Grafting appears to be promising provided seedling growth and percent germination improves. It is observed that the highest germination of lychee seeds could be obtained if fruits are harvested one week before maturity. A higher rate of growth in seedlings is possible under greenhouse conditions. Softwood grafting has been found to be successful in many nurseries. Budding of lychee has also been successful. However, much more work is required to be done before these methods become accepted practices.
Since, air-layering is a commercial practice, a large number of private nurseries have come forward for large scale multiplication of plants especially in lychee growing regions. It is estimated that about 300,000 lychee plants of different cultivars are produced annually. The regulatory framework to ensure the quality of plants is not in place, thus the creditability of public institutes or private nurseries determine the preference of growers. The cost of plants also becomes a factor in determining the preference of farmers.