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Evidence for Cytokinin in Bacterial Leaf Nodules of Psychotria punctata (Rubiaceae)

^a Department of Botany and Plant Pathology, Iowa State University, Ames, Iowa 50010

Cytokinin activity based on two bioassays was at least 100-fold higher in Psychotria punctata leaf discs with bacterial nodules than in discs without them. Nodulated discs from young leaves yielded 0.4 to 6 µg of cytokinin (zeatin equivalents) per g fresh weight of leaf tissue, whereas non-nodulated discs from the same leaves yielded 0 to 0.003 µg per g fresh weight. These estimates probably include free-base cytokinins and, if present, any nucleoside cytokinins precipitable by acidic silver nitrate. Cytokinin concentrations in Psychotria leaf nodules appear to be higher than normally found in green leaves of other plants. In l-butanol-acetic acid-water (12:3:5, v/v), the one peak of activity chromatographed with an R_F similar to zeatin's, but both number and identity of the active substance(s) remain unknown. These findings suggest that a cytokinin is produced by bacteria in leaf nodules of P. punctata and that it is involved in the symbiosis.