Abstract
Callus tissues derived from hypocotyl of lettuce seedlings (Lactuca sativa L. cv. Waya head) were cultured on a synthetic medium with the objective of obtaining the most favorable condition for their organ differentiation. The obtained results are summarized as follows:
1. The formation of adventitious buds on the callus tissue was markedly enhanced by culturing on the medium containing relatively lower concentration of 2, 4-D (2, 4-dichlorophenoxyacetic) acid: 0.01mg/l plus relatively higher concentration of KIN(kinetin: 0.5mg/l), whereas the formation of adventitious roots was rarely observed.
2. The cell suspensions which had been prepared by means of passing through 50- and 100-mesh stainless steel sieves consisted of aggregated cells and large vacuolated cells. In the liquid culture of this suspension, it produced the numerous cell clumps on the medium containing 2, 4-D 0.01mg/l plus KIN 0.05mg/l at the 3 rd week. Shoots and leaves were produced from the cell clumps during additional 2nd week with KIN 0.1mg/l alone under continuous illumination.
3. When the hypocotyl segments of lettuce seedlings, which had been previously subcultured for a week on the medium being supplemented with 2, 4-D (0.1mg/l) plus KIN (0.1mg/l), were transferred to the medium with KIN (0.1mg/l) alone, it resulted in inciting shoot formation on the cultured tissues. On the contrary, it was benefitial for maintenance of callus proliferation for additional five weeks without shifting their growth pattern, when the hypocotyl inocula were transferred to the medium devoid of KIN after one week's preculture.
4. Within six days after explanting onto the suitable medium for callus growth, the cultured hypocotyl tissues started to differentiate into proembryo-like bodies.
Microscopic observation showed that preferential cell division was restricted to distribute the zone near epidermis and vascular tissues of the hypocotyls.
5. From an inspection of the present data, therefore, it will be assumed that a differentiation of proembryo-like body is considered to be a consequence of predisposing factors occurred in the initial culture of lettuce hypocotyl growing on the medium at the appropriate concentrations of 2, 4-D and KIN, which factors may be associated with difference of totipotency in the inocula composed of various kind of tissues. Whereby KIN seems to contribute the achievement of bud formation.
