The micronutrient boron causes the development of adventitious roots in sunflower cuttings

Abstract

Three-day-old light-grown sunflower seedlings were de-rooted and incubated in nutrient solutions that either contained or lacked boric acid (B). In the absence of B, in the majority of the seedlings, no adventitious roots were formed. The micronutrient B caused the development of numerous roots in the lower part of the hypocotyl. The effect of B occurred without the supply of any phytohormones. A dose-response curve of B-induced rooting yielded an optimum concentration of 0.1 mM boric acid. Histological studies revealed that cell divisions occurred in the control but no root primordia developed. In cuttings that were incubated in B (0.1 mM) root primordia were observed that rapidly developed into well-differentiated adventitious roots. Sunflower cuttings that were planted with their cut end in vermiculite that was moistened with nutrient solutions without B degenerated after several weeks. In the presence of B the cuttings formed numerous adventitious roots that entirely replaced the tap root system of intact seedlings. The rooted cuttings developed into sturdy adult sunflower plants. Our results are discussed with respect to the possible role of B in the evolution of vascular from prevascular plants.