Seedling progenies are extremely variable and small fruit size appears to be a dominant characteristic. Therefore, through the centuries improved cultivars have resulted merely from selection amongst existing cultivars or selections amongst open-pollinated seedlings from such cultivars. Selection is based particularly on large fruit size, high edible portion, crisp flesh, good flavour and high sugar content. In so doing, heavy and regular yields appear to have been sacrificed. Future selection has to include marketing characteristics, such as early or late harvest, a long shelf-life and a pure white aril for the canned product.
In Fujian province, China, a selection programme was initiated over 30 years ago to select abort-seeded longan cultivars. Five promising strains with abort-seeded fruits, namely, 'Minjiao No.1', 'Minjiao No.2', 'Minjiao No.3', 'Minjiao No,4' and 'Minjiao No.5' have been successfully selected (Huang et al., 2000) (Figure 11). Among them 'Minjiao No,4' is the most promising due to its stable and higher rate of seed abortion, large fruit size, thick and good quality aril, and high yield.
For germplasm collections, various programmes have been established in longan growing centres worldwide. The national longan germplasm repository of China was established in the Fujian Academy of Agricultural Sciences in 1981 and so far 202 cultivars have been collected and preserved. There are also collections of longan in Fujian, Guangdong and Taiwan Province of China. Collections of accessions are also reported in Hawaii, Florida, Australia, Indonesia, Israel and Thailand.
Figure 11. 'Minjiao No.1' - an abort-seeded longan cultivar (from: Huang et al., 2000).
In China, biotechnology research has achieved great progress in terms of its contribution to longan production, especially genetic improvement through modem high technology (Lai et al., 2000). Some of the biotechnology research includes tissue and organ culture, i.e., studies on plant regeneration obtained from explants such as cotyledons, immature embryos, anthers, leaves of bearing trees, stem apices and stem axes. Plant regeneration has been successfully carried out via somatic embryogenesis from calli derived from the cotyledon of an immature embryo. A breakthrough was achieved in the induction and long-term maintenance of friable embryogenic calli, and somatic embryogenesis at high frequency. Single cell cloning technology and a highly effective protoplast regeneration system have been established from embryogenic suspension cell lines by isolating single cells and protoplasts. Agrobacterium-mediated genetic transformation has been established and transgenic plantlets of longan have been obtained (Zeng et al., 2000). Culture of shoot tips and shoot segments has been successfully carried out to obtain virus-free plantlets for overcoming witches' broom disease.