IN VITRO CULTURE OF PLANT MATERIAL AS AN AID TO HYBRIDIZATION

Modern plant breeding comprises essentially the induction of variation in plant material followed by programmed selection. The most important way of inducing variation is by hybridization but many attempts to produce hybrids fail, even when pollinations are made between parents known to be fertile in legitimate crosses. The plant breeder needs to identify the causes of these failures and to take steps to overcome them.

Failures to produce a hybrid may be categorised broadly into three groups:

1. Pre-fertilization - pollen stigma or pollen-style incompatibilties which prevent the gametes from coming together. The pollen may not germinate; it may fail to penetrate the stigma; the pollen tubes may not grow down the style or they may grow so slowly that the female gametes are no longer receptive by the time the male gametes reach the egg apparatus.
2. Fertilization - the gametes are incompatible or perhaps in rare cases apomixis may have been triggered off by pollination before gametic fusion can occur.
3. Post zygotic - lack of coordination between mother parent, endo-sperm and embryo preventing the development of seeds capable of germinating to produce normal plants.

Some of the failures can be overcome by simple manipulative treatments. Pre-fertilization failures may be avoided sometimes by making the cross in the opposite direction, by cutting off the stigma, by injecting pollen into the hollow style or directly into the ovary or by adding a growth substance, ostensibly to delay senescence of the egg apparatus. Technically more demanding in vitro techniques for the cultivation of plant organs are important aids to the production of wide crosses. Embryo culture in particular is a useful technique for overcoming post zygotic failures; in vitro fertilization may have a role to overcome pre-fertilization failures and there are exciting prospects for avoiding all the causes of failure already mentioned, and in particular fertilization failures, by protoplast fusion.