Effect of the application of benzyladenine pulse on organogenesis, acclimatisation and endogenous phytohormone content in kiwi explants cultured under autotrophic conditions

In traditional in vitro culture, explants grow enclosed in a non-ventilated vessel at high relative humidity with phytohormones continuously present and sucrose as the main energy source. Under such conditions explant growth is far from normal. In this paper, explants of Actinidia deliciosa were cultured in MS medium supplemented with sucrose, benzyladenine and gibberellic acid under autotrophic conditions in glass boxes flushed with air enriched with 600 μl l-1 CO2 for the first 20 days and then transferred to MS medium until the end of the culture period. The effect of benzyladenine was assayed in two regimes of application: in cultures for 20 days in the medium or only 24 h in the presence of benzyladenine with the aim of improving shoot proliferation and acclimatisation. The longest explants were those grown under ventilation and pulsed for 24 h with benzyladenine. These explants also rooted spontaneously, whereas those grown with continuous benzyladenine under ventilation or without ventilation grew and rooted poorly. The highest amount of endogenous isoprenoid cytokinins were found in the longest explants grown under ventilation and pulsed for 24 h with benzyladenine; under these conditions zeatin riboside represented two thirds of the entire cytokinin pool. These explants presented the highest amount of indole-3-acetic acid, while abscisic acid content was high in explants cultured under non-ventilated conditions. No differences were observed between explants cultured under ventilation regardless of their exposure to benzyladenine. The longest explants, which also performed best in acclimatisation, also presented a high indole-3-acetic to abscisic acid ratio.