Graftage is a process that involves the choice of stock and scion species, creation of graft union by physical manipulation, healing of the union and acclimation of the compound plant. There are a number of methods applicable for conventional herbaceous grafting. Method of grafting depends on the kind of crops being grafted, preference and experience of the growers, and the kind or grafting machines or robots available.
Some of the most frequently used method is described as under:
Method # 1. Hole Insertion Grafting (HIG):
There are several modifications of HIG. It is a popular method of grafting among large commercial growers because it does not require additional labour needed for clipping, transplanting, cutting and clip removal. However, it requires a higher skill level as compared to tongue approach grafting (TAG).
In addition, healthy and stronger seedlings can be obtained because more of the vascular bundles can be connected in this method of grafting. Hole insertion hypocotyl grafting is used in watermelon because of its smaller seedling size than the size of rootstock i.e. squash or bottle gourd.
The seeds of watermelon are sown 7-8 days after the sowing of bottle gourd seeds and 3-4 days after sowing of squash seeds. Grafting is made at 7-8 days after the sowing of watermelon when both the scion and rootstock becomes uniform and strong enough to take the grafting operation easily.
The true leaf including the growing point should be carefully and thoroughly removed and a hole is made with a bamboo or plastic gimlet or drill at a slant angle to the longitudinal direction. The hypocotyl portion of the watermelon is prepared by slant cutting to have a tapered end for easy insertion.
The cave should be taken to avoid the insertion into the stem pith since this greatly interferes with formation of a rapid union and facilitates later protrusion of watermelon adventitious roots into the soil after downward elongation through the pith cavity of the rootstock.
For tomato and brinjal, the seeds of rootstock are sown 5-10 days before scion and grafting is made 20-25 days after the sowing of scion seeds. Rootstock seedlings having 2- 3 true leaves are decapitated 5-10 mm above the first node and a hole is made at a slant angle with a bamboo stick.
The scion having two true leaves is cut 10mm below the cotyledonary node and a slant cut is made on both sides of the scion hypocotyls to have a long and gradually tapering wedge. Insert the scion into the hole made in the rootstock and place the grafted plant in a high humidity conditioning room or chamber for subsequent healing. Special attention should be given to provide optimum conditions for rapid union i.e., high humidity, high temperature and adequate light.
Method # 2. Tongue Approach Grafting (TAG):
TAG is usually used by less experienced farmers and those who do not have a greenhouse with efficient micro-climate control system. It does not require special facilities and machines. However, it needs more space and labour as compared to other methods. TAG gives a higher rate of seedling survival even for beginners. It is the oldest and perhaps the most convenient method of grafting for herbaceous plants. The method can be used for basically any kind of plants such as cucurbits, solanaceous plants, etc. but it is extensively used in cucurbits.
Since the grafting operation would be much more efficient with both scion and rootstock seedlings having similar height, the seeds of scion (watermelon, muskmelon and cucumber) are sown 5-7 days earlier than the rootstock seeds. Carefully remove the growing point of rootstocks before grafting to reduce the unnecessary loss of nutrients for the bud growth and to promote the rapid union of graft interface.
Occasionally one cotyledon may also be removed when removing the growing point to ensure complete removal of the growth point and to avoid overcrowding in limited space on the greenhouse bench. Make the grafting cut on rootstock in downward direction and to that of scion in an upward direction at an angle of 30-40° to the perpendicular axis. The cut should be deep enough to allow the fusion of many vascular bundles as possible.
After the graft is completed, specially designed clips are placed to fix the graft position. Grafted plants are then planted together in a 9-12 cm diameter pots. The grafted plants are partially shaded for one or two days before placing them under normal greenhouse growing conditions.
After union remove the root and lower hypocotyl of the scion from the grafted plant by simple cutting off at the desired position, preferably at the closer position to the grafted position held by the clip. The clips are usually removed at later stages, shortly before transplanting.
In spite of the simple and easy grafting operation and higher rate-of survival, TAG is not extensively used by the commercial seedling growers due to the following reasons:
(i) More labour is required for grafting, cutting of rootstock and removal of clips after union.
(ii) Larger space is needed for growing of grafted plants as compared to other methods.
(iii) Frequent rooting from the scion after transplanting if the seedling is transplanted too deep.
Method # 3. Splice Grafting:
Splice grafting is very popular method of grafting among experienced growers and commercial plug seedling nurseries. It can be done by hand, machine and robot and can be applied to most vegetable crops. The major advantage of this method is the production of strong and healthy grafted seedlings since all the vascular bundles of the scion are fused with those of rootstock and the graft union is strong enough to take all the rough post-graft handling.
Remove one cotyledon and the growing point from the rootstock of cucurbits. After placing the scion on the root, ordinary grafting clips are used to fix the grafted position tightly together. In solanaceous crops, grafting is usually made at lower epicotyl and fixed with ordinary clips, elastic tube-shaped clip with side silt, or ceramic pins.
Method # 4. Cleft Grafting:
In herbaceous plants cleft grafting is somewhat different from those of woody plants. The seeds of the rootstock are sown 5-7 days earlier than those of the scion. The stem of the scion (at the fair-leaf stage) and the rootstock (at the 4-5 leaf stage) are cut at the right angles, each with 2-3 leaves remaining on the stem. The stem of the scion is cut in a wedge and the tapered end fitted into a cleft cut in the end of the rootstock. The graft is then held firm with a plastic clip.
Method # 5. Pin Grafting:
Pin grafting is basically the same as splice grafting. However, instead of placing grafting clips to hold the grafted position, specially designed pins are used to hold the graft in position. The ceramic pins made of natural ceramic are used so they can be left on the plant without any problem. These pins are very costly, therefore alternate methods are being used.
Method # 6. Tube Grafting:
The tube grafting makes it possible to graft small plants grown in plug trays two or three times faster than the conventional method. The smaller the plants, the more plants can be fitted into healing chambers or acclimation rooms. For this reason, tube grafting is popular among Japanese seedling growers.
The optimum growth stage for grafting varies according to the kind of plug tray used. Plants in small cells must be grafted at an earlier growth stage, and require tubes with a smaller inside diameter. First, the root stock is cut at a slant. The scion is cut in the same way. Elastic tubes with a side-slit tube, splicing the cut surfaces of the scions and rootstock together.
Method # 7. Micro-Grafting:
Micro-grafting refers to in vitro grafting using very small or micro explants (< 1/1000th mm3) from meristematic tissues. It is often carried out to eliminate the viruses from infected plants because virus particles do not exist in the apical meristem. Micro-grafting has been used in herbaceous plants to evaluate the physiology of grafting and determine the chemical basis of cell-to-cell contacts. This method provides rapid propagation of virus free plants although, it is expensive.