Carry-over effect of Thidiazuron on banana in vitro proliferation at different culture cycles and light incubation conditions.

Thidiazuron (TDZ) is an active cytokinin that was shown to induce increased shoot proliferation and habituation in black walnut, Phaseolus lunatus and evergreen azalea, which are tree species but has not been widely investigated in bananas. Unlike other cytokines commonly in use that are adeninebased, TDZ is a urea based cytokinin and therefore is  non-degradable by cytokinin-oxidase enzymes in plant tissues. This quality causes TDZ to be persistent in tissues hence transforming them from cytokinin dependence to cytokinin autonomy. This therefore makes use of TDZ cost effective but there is lack of information on this quality in banana micropropagation. A study was therefore conducted to investigate the carry over effect of varying concentrations of TDZ and 22.2 mM benzylaminopurine (BAP) as control on proliferation of five banana cultivars on a hormone free medium under various incubation conditions. The results showed that TDZ had a carry-over effect that enabled shoots to continue proliferating on a hormone free medium as the culture cycles increased and that this effect was significantly (P<0.05) higher than that of BAP. Accumulation of TDZ to high levels resulted in suppression of shoot proliferation but on exposing such tissues to a cytokinin-free medium in subsequent subcultures would result in increased shoot proliferation and elongation. The results further showed dark conditions enhanced higher proliferation rates than light conditions in some cultivars suggesting that banana in vitro proliferation is a photomorphogenically responsive process that is enhanced under dark conditions.