Influence of root restriction and ethylene exposure on apical dominance of petunia (Petunia x hybrida Hort. Vilm.-Andr.)

Reducing rooting volume restricted root growth during the production of Petunia x hybrida 'Orchid' and resulted in an unfavorable increase in apical dominance. Exposing young petunia seedlings to ethylene counteracted the effects of root restriction. Rooting volumes of 9, 28, 58, or 160 mL restricted the development of lateral shoots, thereby increasing apical dominance compared to plants grown in 162 mL rooting volumes. Ethephon, an ethylene-producing compound, increased the development of lateral shoots of seedlings grown in rooting volumes ranging from 28 mL to 160 mL. At a rooting volume of 9 mL, ethylene exposure was not capable of reducing the growth of the main shoot; apical dominance remained strong in both the control and ethephon-treated plants. Because lateral shoot development was not restricted by rooting volumes greater than 160 mL, exposing these plants to ethylene did not result in supplementary lateral shoot development. Levels of indole-3-acetic acid (IAA), isopentenyladenosine (iPA), and zeatin riboside (ZR) decreased on a whole shoot basis as rooting volume decreased from 162 to 58 mL. Indoleacetic acid levels in ethephon-exposed plants decreased 20% compared to the control. The cytokinins iPA and ZR showed no response to ethylene exposure; however, the ratio of auxin/cytokinin decreased 24% compared to the control. The decrease in the auxin/cytokinin ratio was associated with an increase in the number and length of lateral shoots.