Enhancement of Papaya Axillary Shoot Proliferation in Vitro by Controlling the Available Ethylene

Abstract

The timing of ethylene concentration favorable for in vitro proliferation of papaya (Carica papaya L.) multiple shoot clusters through outgrowth of axillary buds was investigated. Exogenous ethylene in culture flasks was regulated during the first week by using a gas diffusion equilibration procedure, which used a transparent box as an indirect space for the ethylene injection and flasks sealed with 0.02 pm filters for diffusion and equilibration. There was a 36% increase in shoot number and a 50% increase of leaf number by adding 0.2 or 0.4ppm ethylene into the box the first week, followed by aerating the flasks the following two weeks. The level of endogenous ethylene was also modified by adding the ethylene biosynthesis precursor (ACC) and the inhibitors (AVG and CoCl2) to the culture medium in the flasks under sealed conditions. The greatest enhancement rate of shoot number (75%) was achieved with 2pM ACC. Shoot proliferation rates by applying 0.5μM AVG and 5μM CoC12 were enhanced by 23% and 49%, respectively. The relationships between shoot number and ethylene concentration during a three-week incubation period were analyzed to reveal the most favorable ethylene concentrations. Concentrations with the best proliferation were 0.34, 0.20 and 0.15ppm for Weeks 1, 2 and 3, respectively. The results indicated that a relatively higher ethylene level during the early incubation period, followed by low levels subsequently might be most favorable for improved rates of papaya axillary shoot proliferation.