Gentian ‘Little Pinkie’ and the production of explants as influenced by ethephon and growth duration in vitro

A successful in vitro propagation system for gentian (Gentiana sp.) 'Little Pinkie' as a potted plant depends on a rapid multiplication rate to produce shoots that, when excised (explants), result in highly branched plants. As a part of an ongoing research programme to improve our understanding of this gentian grown in vitro, the effect of growth duration, nodal position from which explants were taken, and inclusion of ethephon in the culture medium were investigated. In the absence of ethephon, increasing the duration of growth from 5 to 8 weeks did not improve branching. Furthermore, the multiplication rate did not change, as after 8 weeks most explants from the tip position became floral, resulting in the total number of usable explants generated being reduced by 30%. Not every explant taken from different nodal positions grew equally, however: fresh weight and number of new shoots per explant taken from the second node below the tip were respectively, 1.5 and 3 times more than the explants taken from the tip. In contrast, when ethephon was applied, after 8 weeks growth in vitro, explants produced twice as many shoots per explant as the control, and only 10% of the tip-derived explants flowered. The number of shoots produced per explant taken from the second node below the tip was 4.5 times more than tip-derived explants. In spite of the improvement in branching however, ethephon in the growing medium decreased the multiplication rate as a result of a decrease in the length of the longest shoots rendering fewer shoots as useable explants. We conclude that culture for 5 weeks in the absence of ethephon is preferable for in vitro propagation of 'Little Pinkie'. Higher multiplication rates may be achievable if ethephon can be used to stimulate the number of branches formed, but also subsequently release those branches into rapid growth.