Adventitious Rooting in ‘Hopi’ Sunflower: Function and Anatomy1

Abstract

AbstractThe ‘Hopi’ sunflower (Helianthus annuusL.) is a primitive openpollinated variety that produces numerous adventitious buds (nodules) on exposed lower stalks under greenhouse conditions. Similar conditions do not induce such budding in more advanced hybrids. The root buds remain dormant when exposed to air, but begin rapid growth into adventitious roots when covered with damp soil. Objectives of this study were to determine the effects of adding basal soil around stalks of Hopi and hybrid ‘894’ sunflower on adventitious root development, contribution of the roots to plant standability, and anatomy of the roots. The addition of one or two 100‐mm layers of soil around lower stalks induced significantly more (1% level) adventitious root growth above the original soil level on stalks of Hopi variety than on hybrid 894 stalks. When the addition of soil was delayed until plants were flowering, extensive adventitious rooting still occurred in Hopi sunflower, but little occurred in hybrid 894. Applying horizontal forces to stalks 0.3 and 0.6 m above the original soil level caused significantly less stalk deflection in Hopi than in hybrid 894 plants. The greater resistance to stalk deflection in Hopi was attributed to greater plant stability furnished by more extensive adventitious rooting. Anatomical studies revealed that the internal anatomy of adventitious roots was nearly identical to the internal anatomy of primary roots for both Hopi and hybrid 894. Roots of hybrid 894 had larger intercellular spaces in the cortex and a higher xylem to cortex ratio than comparable roots of the Hopi variety.