Cytokinin activities of phenylurea derivatives ? Bud growth

Abstract

The induction of cell division in isolated stem-pith of tobacco, a characteristic of the class of plant growth regulators called cytokinins (Skoog, Strong and Milles, 1965), is caused by about 300 structural variants of phenylurea (Bbuce, Zwar and Kefford, 1965; Bruce and Zwar, 1965). Another biological activity of cytokinins is their accelera tion of the growth of buds (Wickson and Thimann, 1958; Miller, 1961). This paper reports the activities of some phenylurea derivatives in the growth of lateral pea buds retarded in their growth by the activity of the apical bud. The synthesis and purification of the phenylurea derivatives was described by Bruce and Zwar (1965). The methods used were similar to those of Sachs and Thimann (1964). Pea (Pisum sativum cv. Alaska1) plants were grown in soil in a glasshouse with 25° C day/200 C night temperature. Uniform plants about 14 days old were used and each treatment was applied to approximately 25 plants. The youngest buds that were exposed and therefore were accessible to treatment were at the fourth node above the cotyledons. These buds were treated directly with a solution of 7 per cent Car bo wax 1500 in 50 per cent (wjv) aqueous ethanol without or with the compound at 0.05 per cent (w/v) concentration. In some treatments the solution applied to the buds contained 10-5 M gibberellic acid. Each bud received 5 [xl of this solution and a similar dose 3 days later. The lengths of buds were measured 7 days after the first treatment, when control buds were 1 to 2 mm long. As an index of the effect of compounds upon bud growth, the mean length of treated buds is expressed as a percentage of the mean length of control buds which received only Carbowax in aqueous ethanol. The activities of the ureas in cell division, as determined by Bruce and Zwar (1965), are included for comparison. These cell division acti vities were obtained by culturing expiants of tobacco stem pith on a sucrose-salts medium containing a series of concentrations of the com pound under test. After 14 days of culture, nests of new cells could be