Studies on the Response of Orchid Roots to 2.4-dichlorophenoxyacetic Acid (2.4-d)

Abstract

Studies were conducted to elucidate the nature of the response of orchid roots to 2, 4-dichlorophenoxyacatic acid [2, 4-D] by approaching it through morphological, anatomical, and fine-structural investigations. The roots of orchids [Dendrobium Lady Hay and Oendrobium J. Thomas] either attached to plantlets or excised from plants responded to 2,4-0 at 0.5-1.0 ppm level by forming tumorous growths at the tips. The sequence of tumor formation was observed under light and electron microscopes. During the first 3 days, root tips expanded radially and mitotis in meristematic area decreased drastically. Then, the enlarged cortical cells degenerated and the cortex collapsed. Although cells in the epidermis, root cap, and initial zones persisted longer than cortical cells, they too eventually degenerated. Mitotic cells at the periphery of the vascular cylinder replaced the cortical cells which degenerated and produced a population of new cells. After a prolonged lag period these cells started to proliferate at about 25 days after sub-culture in medium with 2, 4-D. The rapid division of these cells resulted in tumor growth. At the ultrastructural level, multivesicular bodies CMVB] and nucleoli were studied in relation to cell expansion growth and to RNA metabolism during the Formation of tumors. MVB appeared only after sub-culturing on 2, 4-D containing medium, MVB frequently became associated with the cell wall and deposited internal vesicles in the wall space. The possible relation of MVB to cell expansion is discussed. In the normal root tips, the area occupied by the granular region within the nucleolus and the degree of dispersion of granular components progressively increased within a vascular initial, dividing peripheral vascular cells, and elongating peripheral vascular cells. The surface intruded L-zone [lightly staining zone] of the nucleolus in the initial cells progressively moved into the nucleolus of the dividing and elongating cells. During the first few days of culture in 2, 4-D medium, the granular region in the nucleolus increased and the L-zone moved into the nucleolus. These changes may be due to increased RNA synthesis necessary for cell expansion. Dedifferentiating cells occasionally contained multiple nucleoli. This was related to the acceleration of nucleolar reorganization of cells in the lag phase with a very low demand for ribosomal RNA. Before the rapid cell proliferation phase, the granular region of the nucleolus increased rapidly. This was considered to be related to the increased production of ribosomal RNA which is a prerequisite for the rapid cell division in tumor formation.