Use of growth retardants for banana (Musa AAA cv. Grand Naine) shoot multiplication in temporary immersion systems

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Temporary immersion culture (TIS) offers several advantages over solid medium for banana shoot multiplication, e.g. TIS results in an increase in the multiplication rate and improves the quality of the plantlets. For a commercial application of this technique large vessels are required. When using 10-litre culture vessels an excessive growth of the shoots (leaves and pseudostem) was obtained, which limited the final number of shoots to be produced per flask and reduced the production capacity in the growth room. Labor costs also increased, since handling is more difficult when dividing and subculturing large shoots during the multiplication stage. The effect of growth retardants ancymidol (ANC), paclobutrazol (PBZ) and daminozide (DAM) in liquid shake cultures and TIS was investigated in order to reduce the size of the shoots and allow a better use of the space inside the culture vessel. In liquid shake cultures ANC and PBZ, independently of the tested concentrations, promoted bud cluster formation with reduced size and compact shape. Shoots multiplied with ANC or PBZ (2.5 mg l−1) after five subcultures, recovered their normal morphology after transfer to a hormone-free medium without growth retardants. However, during the acclimatization stage, plants multiplied in ANC (2.5 mg l−1) containing media showed reduced height in comparison with control plants and plants multiplied in PBZ and DAM containing medium. The application of PBZ and ANC in TIS (1-litre flasks) stimulated bud proliferation. Both compounds were also effective in controlling the excessive growth of the shoots and in inducing the formation of compact bud clusters. Shoots multiplied in TIS in presence of PBZ (2.5 mg l−1) were successfully transferred to semisolid or liquid rooting media in traditional culture vessels or TIS. The developed protocol was further scaled up in 10-litre TIS vessels.