Plant breeders are often confronted with some diffi¬culties which prevent successful hybridization. Some plants do not set seed when self-pollinated, that is, they are self-sterile. There are others which show cross incompatibilities. If a pistil carrying functional female gamete fails to set seed, following pollination with viable and fertile pollen, capable of bringing about fertilization in another pistil, the two are said to be incompatible and the phenomenon is termed sexual incompatibility. Sexual incompatibility may be interspecific (between individuals of different species) or intraspecific (between individuals of the same species). The latter is also called self-incompatibility. Incompatibility is a gene-physiological process. Owing to these natural barriers, horticulturists and plant breeders often fail to secure the desired hybrids. Some of the more common difficulties encountered by them are:
1. Disharmony in the time of flowering of the two Parents.
2. The relatively short life of the pollen.
3. Failure of pollen to germinate on the stigma
4. The slow growth of the pollen tubes so that tin i reach the embryo sac in time.
5. Bursting or dying of pollen tube in the style.
6. The failure of the sperm to fuse with the egg.
The disharmony of flowering of parent plan!:, cnn i solved by altering the environmental condition audi I temperature and photoperiod or light.
It is common experience that the pollen losen 11 viability soon after it is shed from the anther. The pol In > a| grasses is short-lived. In Sorghum, no seed could be oht..-1111 < i when pollen was used five hours or more after collection from the dehiscing anthers. Pollen grains of Gossypiun were found to retain their viability upto the 24th hour ah. I collection. The longevity of pollen is governed by a complex of factors such as flowering time, temperature, relativi humidity and light. Whatever may be the viability, it can IN prolonged by storing the pollen under proper conditions.
Air-dry pollen of Coffea, stored in the ordinary way, loses its germination power within a week; but when kept i 11 a desiccator, it retains that power for more than a month The pollen of Saccharum spontaneum spread out on a watch glass in a diffused light remained viable for only about six hours, the same pollen stored in vials plugged with cotton and kept at room temperature for 12 to 24 hours are found viable for more than a week. Generally, the sub-freezing temperature and 25-50 percent relative humidity (RH) extend the longevity of pollen. The pollen of apple stored at -190°C was as effective as fresh pollen even after two years. To increase the longevity of pollen, some anhydrous materials, the diluents, are mixed with pollen. Lycopodium powder, caesin, egg albumen are some of the diluents.
