The receptivity of the stigma is very short and if the pollen is not transferred to it at the right time, it fails to germinate. Attempts to prolong the receptivity of the stigma have been unsuccessful. Even if the pollen is viable, it may
ail to germinate on the stigma due to other reasons, ccasionally, this is due to boron deficiency and this may be corrected by spray with boric acid.
The germination of the pollen may not be as difficult as the subsequent growth of the pollen tubes. This barrier may operate either at the level of the stigma itself thereby preventing further elongation of the pollen tube or at the various levels in the style so that their tips burst before reaching the ovary. In Brassica nigra, to overcome this difficulty, the stigma is treated with cutinase enzyme.
In some cases, the maximum length attainable by the pollen tubes of the male parent is inadequate for enabling them to reach the ovules. To be successful in such cases, one should try reciprocal crosses; in case this also remains unsuccessful, then one will have to amputate the style and reduce it to a suitable length. It is obvious that the cut-end of the style is not always suitable for pollen germination as the stigma.
Doak and others (1932) proposed a different method in which the middle portion of the style is removed, and the upper and lower portions are then joined together and held in place.
Yasuda (Japanese worker) grafted the style upon the ovary in Petunia violacea. He cut off the style of one flower and glued it with gelatin to the ovary of another flower whose style had been previously cut away. In order to prevent the falling apart of the grafted style, he supported it against an iron-wire, tying the support and the style with a thread. When the operation was successful the grafted style grew normally and attained its usual size. This method requires a great deal to manipulative skill and so it has never been tried by other workers.
Daves amputated the style in Lathyrus odoratus and pollinated the cut end. Amputation of the style to a length, appropriate to that the pollen tube of the male parent, was also employed in a cross between Zea and Tripsacum.
In some cases, the cut-end of the style may nftt be suitable for the germination of the pollen as the stigma. For this, Swaminathan recommended the use of an artifisial stigma by applying an agar-sucrose-gelatin-mediun. on the stub and covering it with moist cotton wool after pollinatiotn This method was also used to effect a cross between two species of Nicotiana and also in the compatible specie,, of Solarium.
Yet another method of overcoming the difficulty cauwd by an extremely slow growth of the pollen tube, would bo n direct introduction of the pollen grain into the ovary. Thil method is termed intra-ovarian pollination. Bosio (1940) tried intra-ovarian pollination in Helleborus and Paeonm Kanta and Maheswari (1960) have shown intra-ovariail pollination in Papaver somniferum, Argemone mexicann, A. ochroleuca. A pollen suspension is prepared in distilled water. The flowers were emasculated and the surface of the ovary was sterilised in ethanol and two punctures wen made on the ovary - one for introducing the pollen suspen sion and the other to permit the escape of air present in thl ovarian cavity. Then the pollen suspension was injected into the ovary with a help of a syringe and the punctures then sealed with petroleum jelly. In the injected ovary the pollen grains germinate, ovary grew normally and viable seed: were produced. This method is used in plants where zone of incompatibility lies in the stigma and style. If the pollen grains require some special substances for germination, these may be added to the pollen suspension before injection.
There is also another method to overcome the barriers to incompatibility. The ovules and pollen grains are grown side by side in a nutrient medium which would favour the growth of both. Kanta (1962) in Papaver somniferum cultured pollen and ovules simultaneously on Nitsch's nutrient medium in the same vial. All the stages from germination of pollen to double fertilization were observed and mature seeds containing viable embryos obtained in culture.
This technique was also employed in Petunia axillaris by Rangaswamy and Shivanna in 1967. Such test-tube fertilization suggests the new possibilities of overcoming incompatibilities.
