Grafting

Recurrent mass selection requires very large numbers of seedlings, almost without limit. Grafting potato scions on to tomato rootstocks is a technique for producing very large numbers of true seeds of potato.

Selected potato scions can be grafted on to tomato rootstocks to induce flowering. Being unable to form tubers, the potato scion grows upwards as a vine, with a bunch of flowers every few inches. These flowers can be used for controlled cross-pollination. One good graft can produce up to fifty fruits, with up to 300 seeds per fruit, totalling some 15,000 true seeds.

There are several steps in this grafting.

Grow one tomato plant in each pot so that it is about six weeks old when the grafting is to take place. Each pot should be fairly large (about 12 inch diameter) to avoid any need for repotting later.

With a new razor blade, cut off the tomato stem with a horizontal cut, about 1/ inches above the first two leaves (photo in preparation). Then make a vertical cut down the centre of the stem to a depth of about one inch (photo in preparation).

Cut a vegetative shoot off the potato parent, several inches long (photo in preparation). Place it in water and take it immediately to the grafting house. Cut off a terminal shoot about one inch long (photo in preparation) for use as a potato scion. Slice the sides of the scion to make a wedge (photo in preparation).

Insert the wedge into the vertical cut in the tomato stem (photo in preparation).

Wrap the graft union in a ribbon of very thin, transparent, plastic film (photo in preparation). This ribbon serves two purposes. It binds the graft-union surfaces together, and it prevents the union from drying out. It should be fairly tight, but not too tight.

Place the entire pot inside a mist propagator for about five days, or until the potato scion shows signs of active growth (photo in preparation). Alternatively, put three sticks in the pot, and cover the entire pot with a plastic bag that is tied to the top of the pot, in order to maintain a high humidity (photo in preparation).

The two tomato axillary buds may start growing. If they do, cut them off. The tomato leaves should be removed later, when they show signs of senescence. The ribbon of plastic film may be removed soon after the scion starts growing and the graft union is well calloused over.

Place the pot in a greenhouse with a supporting string hanging from the roof. The potato vine may grow to a height of several feet. A few, small aerial tubers may form, and they should be removed.

There should be two grafts of each potato parent, mainly as an insurance in case one graft is lost for any reason. With 10-20

parents, there will be 20-40 grafts. With practice, cross-pollination among this many plants should not require more than 1-2 person-hours of work each day.

Tomato rootstocks are useful because we know that they graft easily and well. However, they have the disadvantage that they can get blight (Phytophthora infestans) and, if they do, the entire graft is in grave danger of being lost. There is also a risk of blight attacking the potato scion, particularly in the early breeding cycles. There are two ways of avoiding blight on both the stock and the scion. One is to spray them with a copper or dithiocarbamate fungicide. An alternative is to water the plants from the base of the pots, and to keep both the scion and the above-ground stock dry at all times. This is easily achieved in a greenhouse. Blight spores need free water to germinate and, if all the aerial parts of the grafted plant never get wet, they will not get blight. For complete safety, both methods may be used.

Some useful research, that can easily be undertaken by amateur breeders, would involve a search for alternative rootstocks that are immune to blight. Possible species include the common weed called 'thorn-apple' (Datura stramonium); wild species of Solanum, such as S. nigrum and S. dulcamara in Europe, or S. rostratum, known as prickly potato, or the buffalo burr, in North

America; eggplant (Solanum melongena); and tobacco (Nicotiana tabacum).

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Responses

  • casimiro
    What was potato grafted from?
    8 years ago
  • Kimi
    Is grafting a form of cross breeding?
    7 years ago

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