**1. Introduction**

272 Soybean – Genetics and Novel Techniques for Yield Enhancement

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As we know, sexual hybridization is the primary means of creating genetic variation in the conventional breeding methods, all kinds of new varieties are mainly developed through this means. However, sexual hybridization can only be carried out between species. The distant-graft mutagenesis technology developed rapidly in recent years just made it possible to bypass the natural obstacles of incompatibility or hybrid embryos sterile of sexual hybridization between distantly-related species and integrate the different sources of rootstock and scion organically, and allow the scion to grow, develop and fruit normally. As the sequence of it, a wide range of mutations can be induced under the adverse conditions of distant grafting and all kinds of the unique new plant types can also be created. Qian (1993), the well-known Chinese scientist, had ever pointed out: "We should develop a technical science---The graft-transformed science of plants, whose role must be no less than genetic engineering."

Compared to other mutagenesis technologies, the distant-graft mutagenesis technology can integrate the advantages of rootstock and scion and further expand the genetic base of target crop. Besides, it characters as: (1) the mutated traits stabilize quickly, which may shorten the breeding period; (2) the operation technique is simple and easy to grasp; (3) the required test conditions and input are comparatively lower than other mutagenesis technologies. Now, the distant-graft mutagenesis technology has been applied in resources innovation and genetic improvement of crops such as wheat, millet, cotton, peanut, mungbean, corn, and soybean etc, and a lot of new plant materials have been created, from which many new varieties with the strong resistance to stress and diseases or the improved yield and quality have been bred out. For example, sweet potato/mungbean, castor/cotton, yam/soybean, sweet potato/soybean and castor/soybean etc are all bred out from the progeny of distant graft. Here we introduce the history, definition, types of distant graft, the procedure and key influencing factors for soybean distant-graft mutagenesis, the types and the possible mechanism of soybean distant-graft mutagenesis and prospect of soybean distant-graft mutagenesis, hoping that this technology can be popularized to widely use in resources

Distant-Graft Mutagenesis Technology in Soybean 275

wedge shape, so that the rootstock and scion can develop a new plant; cleft grafting is that the scion with wedge-shaped end is inserted the spliced mouth of the stem of rootstock; As for splice graft, all the stems of rootstock and scion are cut into the inclined planes with the same angle and length, and then they are bounded together. Other graft methods such as whip and tongue graft, saddle graft, bridge graft, inarch graft etc. are sometimes used in practices. In addition, according to the combination pattern of rootstock and scion, the graft methods are including single Shoot to root grafts, Y shaped grafts with one scion, interstock grafts, Y shaped grafts with two scions, Y shaped grafts with Y shaped shoots and A shaped

The plants with tuber roots are suitable for distant grafting as rootstocks, such as ginger, lily, sweet potato, potato, yam, etc. In addition, other plants with thick stem are also appropriate for grafting as rootstocks, for example, castor, sunflower, sesame, maize,

The stem node near the surface of soil is often chose for graft site by the way of side-plug when grafting, so it is first assured that stem of rootstock is thick enough to prevent splitting of stem while the scion is inserting the hole of rootstock. In order to increase the stem diameter of rootstock, some necessary methods such as ahead of sowing or cutting off the

Under normal circumstances, the rootstocks are sowed earlier 40-45 days than the scion of soybean. But the climate conditions and the rootstocks types may influence the growth and development of rootstocks, the best age of rootstocks are about 50 days after emergency

7-9 DAM soybean seedlings is best for grafting according to our practices, and that seedling

When grafting, the surface soil near to the stem of rootstock is first scraped away a little with the bamboo blade. Second the nearest internode from the topsoil is decided to use as graft site. Then the surgical blade is sterilized with alcohol cotton and cut off the section of the internode vertically. After that, the sterilized bamboo stick with the wedged end is rotatedly inserted into the cut section at an angle of 30-45 degree along the stem. The size and depth of drilled hole is depended on the stem thickness of scion seedling and the length of wedged end of the scion seedling. Generally, the depth of hole is 1.0-1.5 cm and the

age is too small or too big all have important effect on the graft survival rate.

grafts with two rootstocks (Jia and Han, 2010).

**4.1 Selection and cultivation of rootstock** 

**4.1.1 Choice of rootstock** 

**4.1.2 Cultivation of rootstock** 

main stem are generally adopted.

**4.2.1 The grafting age of rootstocks** 

(DAM) under good growth season.

**4.3 Grafting operation** 

**4.2.2 The grafting age of soybean scions** 

**4.3.1 Drilling a hole at the stem of rootstock** 

diameter of the hole is about 0.2-0.3 cm.

sorghum and so on.

**4. Procedure for soybean distant-graft mutagenesis** 

**4.2 Appropriate grafting age of rootstocks and soybean scions** 

innovation and genetic improvement of soybean and to promote the process of soybean new varieties.
