ESTABLISHMENT OF A NOVEL TISSUE-CULTURE METHOD, STEM DISC CULTURE, AND ITS PRACTICAL APPLICATION TO MICROPROPAGATION OF GARLIC (ALLIUM-SATIVUM L)

A restricted part of the undeveloped stem of the garlic clove, called the ''stem disc'', which is just under the basement of the immature fo liage leaves, proved to be a very potent explant for the micropropagat ion of garlic. Twenty to thirty tissue-cultured shoots consistently we re differentiated from a single clove during 1 month of culture on phy tohormone-free Linsmaier and Skoog medium. In addition, more than 90% of the shoots formed bulblets in vitro during an additional I month of culture. Pretreatment of the garlic bulbs at 4 degrees C for approxim ately 8 weeks before preparing the stem discs enhanced both shoot deve lopment and bulblet formation. This novel method for culturing garlic was designated the stem-disc culture method. Shoot development in this type of in vitro culture apparently is divided into four stages: expa nsion of tissue zones surrounded by the basal parts of the immature fo liage leaves, formation of dome-shaped structures, bud differentiation directly from each dome, and development into shoots and bulblets. Th e dome-shaped structures appeared within 5 days of the onset of cultur e and had developed independently into shoots approximately 1 cm high 3 weeks later. Histological observations showed that both the internal cell organization and formation process of the dome-shaped structures were similar to those in the meristem. In addition, events leading to the formation of these dome-shaped structures appeared to be initiate d by vigorous cell division in the epidermis of concentric tissue zone s surrounded by the basements of immature foliage leaves. The results of several field trials showed that the stem-disc culture method is us eful for the production of garlic seed plants, including virus-free pl antlets. Furthermore, it is a novel field cultivation system for garli c in that the seedlings produced by in vitro-induced bulblets are used as seed instead of the usual cloves.