Dark-induced Anomalous Hypocotyl Growth in Colchicine-treated Lettuce Seedlings

Abstract

Central nervous system (CNS) depressant drugs such as barbiturates and tranquillizers have been observed: ( a ) to depress cell multiplication in different types of animal cells growing in culture (Dawson, 1972; Jackson, 1975); (b) to interfere with the normal germination and growth behaviour of angiosperm seedlings and pollen grains (Kordan, 1977a, ft, 1978, 1979a, ft; Kordan and Mumford, 1978, 1979); and (c) to inhibit transpiration in excised barley leaves (Ramiandrasoa-Luong et al., 1979). Although gaseous oxygen is not listed in the medical literature as being a CNS depressant agent, hyperbaric oxygenation in clinical anaesthetic practice has been observed to exert toxic effects under certain conditions, the basic mechanism of oxygen toxicity not being understood (Ledingham, 1971). Hyperbaric oxygen has been observed to exert greater adversity of action on radicle and hypocotyl growth than on cotyledonary growth in germinating lettuce seedlings (Borthwick and Robbins, 1928). Colchicine, a weak anaesthetic and CNS depressant drug (Jackson, 1975) that also causes metaphase arrest in most animal and plant cells (Sharma and Sharma, 1972; Jackson, 1975), also exerts greater adversity of action on radicle and hypocotyl growth than on cotyledonary growth in germinating lettuce seedlings, this phenomenon being the subject of this brief communication. Sixteen 'Pyrex' unspouted crystallizing dishes (94x50 mm) were each lined on the bottom with Whatman No. 1 (9 cm diam.) filter-paper discs. The paper in eight dishes was wetted thoroughly with glass-distilled water and in the other eight dishes with 1 per cent (w/v) colchicine (BDH Chemicals Ltd., Poole). Fifty lettuce achenes {Lactuca sativa L. cv. Grand Rapids) were placed in each crystallizing dish, each dish was covered with a 'Pyrex' Petri dish lid and sealed with 'Parafilm'. Eight dishes were placed in an 18 h light (25-26 °C) and 6 h dark (21-22 °C) daily cycle and the other eight dishes were placed in continuous darkness (26-27 °C). On day 3 all germinated and ungerminated achenes in two dishes from each of the glass-distilled water and colchicine treatments in the light and dark environments were washed thoroughly with glass-distilled water and transferred to glass-distilled water on filter paper in^sealed crystallizing dishes in their respective light and dark environments for seven more days. All treatments were harvested for examination on day 10. High percentages of achene germination were evident in all treatments in both the light and dark environments (Table 1). Washing one set of the lightand dark-germinated seedlings with glass-distilled water and transferring them to glass-distilled water for 7 additional days did not bring about any apparent differences in their growth behaviour from that manifested by the 10-day-old unwashed seedlings in the duplicate treatments (Table 1) (Plate 1 a-h). Thus, the washing procedure neither impaired nor enhanced the growth behaviour of the seedlings compared with that of the unwashed seedlings. Radicle but not cotyledonary growth was completely inhibited in all lightand darkgerminated colchicine-treated seedlings (Table 1) (Plate lc, d, g, h). The cotyledons of