Investigation of the Radish Leaf Bioassay for Kinetins, and Demonstration of Kinetin-like Substances in Algae

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Abstract
This study is part of an investigation into the occurrence of plant growth substances in marine unicellular algae. Auxins and gibberellins have previously been detected. This paper reports the occurrence of phytokinins in algae, using the radish leaf test. The technique of the test is described and a few modifications examined. Radish leaves were shown to be more responsive than swede leaves. Some requirement for a minimum temperature below 7° C during the growing period of the plants appeared to be a factor for maximum response. The size and age of the leaf were shown to influence kinetin activity. Chemicals tested included benzyl adenine, a 9-substituted benzyl adenine, and kinetin. Kinetin showed lower activity than the benzyl adenines. Kinetin kept in the solid state at −20° C for 2 years was as active as freshly prepared kinetin. Gibberellic acid (GA2) at 10−5 g/ml sometimes showed activity equivalent to 10−5 g/ml kinetin. Indole-3-yl acetic acid at concentrations of 10−4 to 10−6 g/ml was inactive. The radish leaf test was successfully used to demonstrate phytokinins in extracts of two species of unicellular marine algae, and in marine phytoplankton samples. Amounts found were within the range 0.1 to 1.0 mg/kg with one exception of 10 mg/kg. Phytokinin activity in these extracts decreased over a period of a few weeks when stored at −20° C.

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