Effects of Synthetic Phenylurea and Nitroguanidine Cytokinins on Dormancy Break and Sprout Growth in Russet Burbank Minitubers

Abstract

Minitubers often exhibit a more protracted dormant period relative to their field-grown counterparts and this can have a detrimental impact on plant establishment. Treatment of dormant Russet Burbank minitubers with the synthetic cytokinins N-(2-chloro-4-pyridyl)-N′-phenylurea (CP) or 1-(α-ethylbenzyl)-3-nitroguanidine (NG) resulted in the premature termination of tuber dormancy. In contrast, treatment with the thidiazoyl-urea cytokinin TZ did not stimulate sprouting. Both CP and NG were more effective than the naturally occurring cytokinin zeatin and, unlike zeatin, both stimulated sprouting in minitubers during early storage. Unlike treatment with gibberellic acid (GA) which often induced excessive sprout growth, sprouts developing from minitubers treated with either cytokinin were short, thick, and robust. Upon transfer to room temperature, the endogenous content of ABA in apical bud periderm discs isolated from control minitubers declined more than 60% over a 15 day period. Treatment with either CP or NG had no appreciable effect on this decline while treatment with GA diminished the decrease in ABA content. Treatment with either synthetic cytokinin resulted in a large and sustained increase in endogenous ethylene production that was detectable immediately prior to visible sprout growth. In contrast, GA treatment resulted in a much smaller increase that was observed after sprout growth commenced. These results indicate that either CP or NG treatment can be effectively used to prematurely terminate tuber dormancy and induce sprout growth.