Structural and ultrastructural variations in in vitro and ex vitro rooting of microcuttings from two micropropagated Leucospermum (Proteaceae).

Abstract

In vitro rooting of two Leucospermum cultivars (L. cordifolium ‘Flame Spike’ and L. ‘Tango’) microcuttings was studied with different indole-3-butyric acid (IBA) treatments: A) including in a solid medium (0, 0.2, 1 and 2 mg L-1), B) by dipping the base of the shoots in 500-2500 mg L-1 for 5 s and C) including in a liquid medium (12.5 and 25 mg L-1) for 24-96h (induction step). After the treatments B and C, shoots were transferred to an auxin-free root elongation medium containing 20 g L-1 sucrose and solidified with 7g L-1 agar. The long-term presence of IBA in the culture-medium inhibited root development. The culture of shoots in a liquid medium with higher IBA concentrations and the application of IBA pulses at the base of the microcuttings proved to be suitable treatments to induce rhizogenesis. Culturing microcuttings in a growth regulator free solid medium after IBA treatments (B and C) triggered root initiation and differentiation. The results show that the microcuttings cultured in a liquid medium with IBA had the highest percentages of rooting. The sequence of anatomical changes during rooting was similar in vitro and ex vitro, the origin of the adventitious root in the vascular cambium. In vitro roots with an organized tissue system emerged from the microcutting stems 6 days after the root induction treatments. The acclimatized plantlets showed a suitable root system. Roots were formed ex vitro from the stem vascular cambium and connections were established between the stem vascular bundles and the roots, allowing root elongation. Some modifications were also observed in the basal rooting zone of stems in L. ‘Tango’