In vitro propagation of wavy-leaved Indian paintbrush (Castilleja applegatei Fern.)

Castilleja spp. (Indian paintbrush, Orobanchaceae) are desirable ornamental plants with showy floral bracts that are native throughout much of the western U.S. Propagation of these hemiparasites by seed is usually successful only when a host species is present, and asexual propagation through traditional methods has proven to be extremely difficult. In this study we present an effective shoot culture micropropagation system for Castilleja applegatei Fern., the wavy-leaved Indian paintbrush. In vitro shoot tips of three C. applegatei clones were cultured for 28 days on woody plant medium (WPM) supplemented with four concentrations of each of three cytokinins: 6-benzylaminopurine (BA), 6-(y,y-dimethylallylamino)-purine (2iP) and zeatin. Responding explants, shoot number per responding explant and shoot length were determined. In vitro rooting of microcuttings cultured for 42 days on media containing three concentrations of indole-3-butyric acid (IBA) were also evaluated. A high percent response to shoot induction was observed across all treatments, ranging from 94.5 to 100%. C. applegatei responded best to zeatin, which resulted in both the highest mean shoot number and mean shoot length among the cytokinins tested. The best overall shoot multiplication occurred on media with 4.0 μM zeatin, yielding a mean shoot number of 4.11 and mean shoot length of 3.95 cm. The mean highest rooting response (66.7%), root number (13.21) and root length (2.73 cm) were obtained on WPM supplemented with 10 μM IBA. Significant clone × treatment interactions existed for all variables except mean root number, thus the optimum treatments for both shoot multiplication and root induction were clone-dependent. Rooted microcuttings were acclimated ex vitro with an average success rate of 81.2%. This study demonstrates the considerable potential of an in vitro shoot culture system for the asexual propagation of Castilleja spp.

Research highlights▶ An effective shoot culture micropropagation system for Castilleja applegatei Fern., the wavy-leaved Indian paintbrush was developed. ▶ Twenty-eight days after subculturing 1 cm shoot tips, the best overall multiplication took place on WPM supplemented with 4.0 μM zeatin, which produced a mean shoot number of 4.11 and mean shoot length of 3.95 cm. ▶ The highest in vitro rooting response (66.7%), mean root number (13.21) and mean root length (2.73 cm) were obtained after 42 days on WPM supplemented with 10 μM IBA. ▶ Significant clone × treatment interactions existed for all variables except mean root number, thus the optimum treatments for both shoot multiplication and root induction were clone-dependent.