Layering involves induction of adventitious roots on the shoot, while the shoot is still attached to the mother plant. In the case of litchi, how the branched form roots at an air layer site provide powerful information for not only understanding the process but also possibilities of further improvement over the existing situation and also solving the undesirable problems coming in the way of achieving maximum success in propagation through air layering.
The process of air layering involves removal of the bark, the cambium, and the phloem layer by cutting away carefully. The xylem, however, is left intact. This is known as girdling which is done with a sharp knife, make two parallel cuts about 1 1/2″ apart around the stem and through the bark and cambium layer. Connect the two parallel cuts with one long cut and remove the ring of bark leaving the inner woody tissue exposed. Scrape the newly bared ring to remove the cambial tissue to prevent a bridge forming of callus tissues.
A rooting hormone may be applied to the wound then apply a handful of soil ball or damp sphagnum moss as rooting media so that it surrounds the wounded portion of the stem. Wrapping with clear plastic type, the plastic around the placed soil medium ball or sphagnum moss and secure it (wrap it) by tiding with sutli/jute thread at the top and the bottom. This seal should be such that excess water can escape, but moisture will be retained. Tying the prepared soil ball and moss in place with string helps keep it in position while completing the process. In the case of litchi, stems of pencil size diameter or larger are best. Choose an area just below a node and remove leaves and twigs on the stem 3 to 4 inches above and below this point.
The physiology behind rooting is that the removal of the bark, cambium, and phloem, but not the xylem, prevents carbohydrates and photosynthates from flowing down the trunk past the girdling site but still allows water and mineral nutrients to flow upward to the leaves. This keeps the leafy portions of the shoot from drying out and maintains them with an adequate supply of nutrients. The removal of the actively growing cambium layer prevents the regeneration of phloem and healing over of the wound.
Because of this the carbohydrates and photosynthates flowing down the trunk collect at the girdling site. The presence of these excesses of carbohydrates and photosynthates (esp. auxin) at the girdling site, plus the presence of the moisture in the moist soil ball used as rooting media or the sphagnum moss, causes dormant adventitious buds in the area to grow into roots.
After the new roots have penetrated the soil medium ball or moss ball and are visible on all sides, remove the newly rooted plant from the parent plant with a sharp knife or pruning shears, cutting just below the ball of moss and roots. Ensure that when there are enough roots to sustain the shoot independently, the shoot is cut-off of the tree and then planted or potted. The litchi sticks are cut from the mother tree just at the base of the girdled area. Carefully remove the plastic without disturbing the roots or removing the soil ball or ball of moss and plant in a container using a good potting mixture or plant in a well-prepared soil bed/nursery bed or poly bags of suitable sizes.
The new plant will usually require some pampering until the root system becomes more developed. Provide shade and adequate moisture until the plant is well established. The better establishment of new transplants in the field may be due to the reason of salubrious climatic conditions, active growth period and root activity. The air layer is pruned to a straight, single stem with three-quarters of the original leaves removed, and then planted into either an 8-10″ size earthen pots, 8-10″ size poly bags or in nursery soil under the shade with proper care.
The Off-season air layering technique – an approach The off-season air layering approach mainly includes the air layering operation during spring (February – March) on selected shoots (1.0 – 1.5 cm diameter and 45 – 40 cm length), with proper root hormone (5000 ppm IBA) and proper rooting medium (enriched soil and moss grass) vis-à-vis maintenance of proper atmospheric conditions (91% relative humidity and 17.0 0C minimum and 39.0 0c maximum temperature) with the help of providing polythene sheet cover to entire plant and water application in the root zone as well as on the canopy. This method has been found prudent, and economically feasible as the cost of air layers comes to Rs. 6.40 per layer as against Rs. 6.00 per layer in the main season. This method provided plants ready for field planting within seven months in the same calendar year and thus saves time and money of farmer.
National Research Centre for Litchi