Synthetic cytokinins are commonly applied to ornamental plants to promote lateral branching. This gives them a full, bushy appearance, and makes them more marketable. The same sort of growth is deemed desirable in herbs, but these chemicals are not approved by the Food and Drug Administration for use on edible plants, so the only alternative available to growers is to pinch off the apical meristems by hand, a labor-intensive, and consequently expensive, procedure.
It occurred to Mr. Kellett that another alternative might be to spray the plants with suspensions of harmless bacteria that are known to produce cytokinins; this was the focus of his SARE project. Mr. Kellett grew suspensions of three bacterial species, Rhizobium leguminosarum, Corynebacterium fascians, and Agrobacterium gluteans. These he sprayed on hydroponically-grown Genovese basil three times per week, every other week, for six weeks. Some plants were left untreated as controls, and others were treated with 10, 15, or 20 mg/liter concentrations of synthetic cytokinins. When the plants were six weeks old they were harvested, and height, lateral branch length, average distance between internodes, and dry leaf mass were measured.
Mr. Kellett reports considerable variability of all dependent parameters within treatments, and no significant differences between treatments. Notably, even the sprays of artificial cytokinins did not produce any effect, by comparison with the untreated control. He suggests that cytokinin concentrations in the bacterial suspensions may have been too low, but that would still not account for the lack of observed effect in the case of the artificial cytokinins. He suggests also that the cytokinins may have been unable to penetrate the cuticle of the basil. If this is the case, Mr. Kellett’s idea may still be valid for herbs with less impenetrable cuticles.