Collecting pollen for genetic resources conservation 25

Pollen grains are the mature male (haploid) gametophyte of higher plants. These tiny organs (10-100 IJ.m in diameter) develop in the anthers (Stanley and Linskens, 1974). When the flower opens the anthers become exposed to the environment and dehydration triggers a mechanism for release of the pollen. In the process, the pollen also dehydrates. Though the pollen of most species can withstand drying (Stanley and Linskens, 1974; Towill, 1985; Hoekstra, 1986; Hoekstra etal., 1989b), it may lose some vigour and viability before it lands on a compatible female receptive surface, depending on the mode of dispersal, availability of vectors and environmental factors such as humidity and temperature. There, it produces a tube after rehydration, and this grows into the female tissues to ultimately deliver its two sperm cells to the embryo sac. Their small size and desiccation tolerance render pollen grains particularly suitable for storage. In fact, pollen storage is a common practice in breeding programmes to bridge the gap between male and female flowering time and to improve fruit setting in orchards (Franklin, 1981; Towill, 1985). However, relatively limited use has been made thus far of pollen for long-term germplasm conservation. One reason for this is the short longevity of pollen grains relative to seeds. Longevity varies widely among species. Also, methods for testing viability are generally cumbersome and time-consuming. Furthermore, there is limited experience in the survival and fertilizing capacity of cryopreserved pollen that is more than five years old (Towill, 1985). Another serious drawback of using pollen in germplasm conservation for the time being is that mature pollen grains cannot develop independently into whole plants. The advantages and disadvantages of the use of pollen for germplasm storage are discussed further below. The different factors that