Grafted plants in vegetables can have a direct or indirect benefits viz. resistance to diseases, nematodes; tolerance to low temperature, high temperature, salinity, flooding; winter hardiness; enhanced mineral and water uptake; high fertilizer use efficiency; improvement of fruit yield and quality; translocation studies; shoot growth promotion; earliness; propagation and transformation; size control; ornamental value, etc.
ROOTSTOCKS FOR GRAFTING
Major rootstock species used for grafting in vegetables is presented in Table 1.
Table 1. Major rootstock species used for grafting in vegetables
|Brinjal||Solanum torvum, S. integrifolium, S. melongena|
|Watermelon||Lagenaria siceraria, Cucurbita spp., Benincasa hispida|
|Cucumber||Cucurbita spp., Cucurbita ficifolia|
|Melons (greenhouse)||Cucurbita spp., C. moschata x C. maxima, Benincasa hispida, Cucumis melo|
|Melons (open field)||Cucurbita spp., C. moschata x C. maxima, Cucumis melo|
I. Solanaceous vegetables
Tomato and brinjal are commonly grafted by cleft grafting or tube grafting or side grafting. The growth rate differs according to the species of rootstock used. The number of days from sowing to grafting varies accordingly.
1. Cleft Grafting/ Wedge grafting
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 four to five-leaf stage) are cut at right angles, each with 2-3 leaves remaining on the stem. The stem of the scion is cut in wedge shape and the tapered end is fitted into a cleft cut in the end of the rootstock. The graft is then held firm with a plastic clip.
2. Splice Grafting/ Tube grafting / Slant-cut grafting
The optimum growth stage for grafting of tomato varies according to the kind of plug tray used. In brinjal, the seeds of S. torvum must be sown a few days earlier than those of the other rootstock species. Plants in small cells must be grafted at an earlier growth stage and require tubes with a smaller inside diameter. The rootstock is given a slant cut. The scion is also cut in the same way. The cut ends of the scions are then inserted into the tube, splicing the cut surfaces of the scions and rootstock together.
3. Tongue approach grafting/ Side-by-side grafting
Matching 45° incisions are cut in scion and rootstock stems, approximately 3/4 through the stem, to create "tongues". The stem tongues are then joined together so that the cut surfaces are in contact. Parafilm is wrapped tightly around the graft union to prevent moisture loss. After 5 days, begin to cut the RS top & the scion roots from grafted plant such that the two are completely separated within 3 days.
1. Tongue Approach Grafting
In this method, seeds of cucumber are sown 10-13 days before grafting, and pumpkin seeds 7-10 days before grafting, to ensure uniformity in the diameter of the hypocotyls of the scion and rootstock. The shoot apex of the rootstock is removed so that the shoot cannot grow. The hypocotyls of the scion and rootstock are cut in such a way that they tongue into each other. Then the graft is secured with a plastic clip. The hypocotyl of the scion is left to heal for 3-4 days and then crushed between the fingers. The hypocotyl is cut off with a razor blade 3 or 4 days after being crushed. Melon plants are also mainly grafted by this method.
2. Hole-insertion grafting
Rootstock seedlings should have one small true leaf, and scion seedlings should have one or two true leaves. With a pointed probe, remove the true leaf, the apical meristem (undifferentiated cells), and the axillary buds from the topmost growing point of the rootstock plant. Use the probe to create a hole in the top of the rootstock where the tissue was removed. Cut the scion below the cotyledons at a 45° angle on two sides to form a wedge & insert it into the rootstock. Mist with water and place in healing chamber.
3. One Cotyledon Grafting/ Splice grafting
Rootstock seedlings should have at least one true leaf, and scion seedlings should have 1 or 2 true leaves. Rootstock is cut at a 45° angle so one cotyledon remains and one is removed. The scion is cut at a 45° angle below the cotyledons, where its diameter matches that of the rootstock. Then the two cut stem surfaces are brought together, and hold them in place with a grafting clip. Mist with water and place in healing chamber.
4. Side Grafting
A slit is cut all the way through the stem of the rootstock below the cotyledons using a sharp knife or razor blade. The slit should be just long enough to insert the scion. A probe or toothpick is inserted into the slit to hold it open. The scion is cut below the cotyledons at a 45° angle on two sides to form a wedge, and the prepared scion is inserted into the slit of the rootstock. The probe or toothpick if used is then removed. The scion is hold in place with a grafting clip, mist with water, and shifted to healing chamber.
HEALING AND ACCLIMATIZATION
Grafted plants are usually healed and acclimated in a plastic tunnel for better survival. The tunnel is covered with materials which provide shade and maintain humidity inside: silver/white cheese-cloth (outside) and transparent film (inside). During acclimatization, it is recommended to keep light levels at about 3 to 5 Klx. Before grafting the scion and rootstock should be exposed to sunshine for 2-3 days and water is withhold to avoid spindly growth. The stems of scions and rootstock should have a similar diameter. After grafting, the grafted plants are kept at about 30°C and with more than 95% relative humidity for 3 days of healing to promote the survival. Gradually, the relative humidity is lowered and the light intensity is increased. During healing and acclimatization, keep a constant air temperature in the tunnel to maintain high humidity. If wilting is observed, foliar spraying of grafted plants is done with water.
About Author / Additional Info:
Working as a Senior Scientist at ICAR-Indian Agricultural Research Institute, Pusa, New Delhi