Enhancement of papaya axillary shoot proliferation in vitro by controllingthe available ethylene

The timing of ethylene concentration favorable for in vitro proliferation o f papaya (Carica papaya L.) multiple shoot clusters through outgrowth of ax illary buds was investigated. Exogenous ethylene in culture flasks was regu lated during the first week by using a gas diffusion equilibration procedur e, which used a transparent box as an indirect space for the ethylene injec tion and flasks sealed with 0.02 mu m filters for diffusion and equilibrati on. There was a 36% increase in shoot number and a 50% increase of leaf num ber by adding 0.2 or 0.4 ppm ethylene into the box the first week, followed by aerating the flasks the following two weeks. The level of endogenous et hylene was also modified by adding the ethylene biosynthesis precursor (ACC ) and the inhibitors (AVG and CoCl2) to the culture medium in the flasks un der sealed conditions. The greatest enhancement rate of shoot number (75%) was achieved with 2 mu M ACC. Shoot proliferation rates by applying 0.5 mu M AVG and 5 mu M CoCl2 were enhanced by 23% and 49%, respectively. The rela tionships between shoot number and ethylene concentration during a three-we ek incubation period were analyzed to reveal the most favorable ethylene co ncentrations. Concentrations with the best proliferation were 0.34, 0.20 an d 0.15 ppm for Weeks 1, 2 and 3, respectively. The results indicated that a relatively higher ethylene level during the early incubation period, follo wed by low levels subsequently might be most favorable for improved rates o f papaya axillary shoot proliferation.