Topophysic effects differ between node and organogenic cultures of the eucalypt Corymbia torelliana × C. citriodora

Abstract

The eucalypt Corymbia torelliana 9 C. citriodora is planted widely in India, Brazil and Australia although plantation establishment has been limited by inadequate seed supply and low amenability to propagation via cuttings. This study optimised node culture and organogenic culture methods for in vitro propagation of Corymbia hybrids by identifying explant position (topophysic) effects on rooting, shoot elongation and shoot proliferation. Strong, negative morphogenic gradients in shoot elongation and proliferation capacity were evident from the cotyledonary node to the fourth or fifth node of seedlings when their nodes were transferred to node culture (without benzyladenine). These topophysic effects were related to differences in rooting capacity of individual nodes. Root formation in node culture was associated with formation of long multi-nodal axillary shoots, and so higher rooting of shoots from the cotyledonary node or first true-leaf node was associated with higher shoot proliferation. However, all nodes were equally capable of shoot proliferation in organogenic culture (with 2.2 lM benzyladenine), where rooting and rapid stem elongation did not occur. Most shoots (61-100%) from both node culture and organogenic culture were converted to plantlets, with plantlet conversion and primary root number not differing significantly among explant node positions. The strong topophysic effect in node culture, combined with the lack of a topophysic effect in organogenic culture, provides for an optimised clonal propagation system based on segregation of nodes from the same seedling into separate node and organogenic culture pathways.

Tissue Culture of Corymbia and Eucalyptus

“…The proliferation capacity of shoot tips, nodes, or axillary buds and the subsequent growth of their plantlets may also be influenced strongly by the position of the tree from which the explant was harvested. Maturation effects such as reduced rooting capacity, shorter internode length, and decreased stem growth [54,55,[59][60][61][62] can become evident from very early stages (i.e., from relatively low explant positions) during the development of eucalypt trees [44,[63][64][65][66][67].…”

“…Shoot proliferation is performed in the light, usually on semi-solid medium containing 58.4 mM or 87.6 mM sucrose (Table A1). Sucrose at 87.6 mM has been used in proliferation media for Corymbia species and hybrids even when 58.4 mM sucrose was used in the establishment medium [45,46,64,69,87]. A lowered sucrose level of 43.8 mM has been used during elongation [92] or both proliferation and elongation [93] of E. benthamii × E. dunnii shoots.…”

“…However, SP medium [108,[110][111][112], WPM minerals [113], WPM [114], B5 medium [115][116][117], N7 medium [86,118], JADS medium [98], EDM [119], and MS medium with White vitamins [89] have also been used. Callus is often induced in darkness, although basal organogenesis on shoots can occur in eucalypt shoot cultures that are maintained under light [45,46,64,98,120]. Callogenesis, shoot regeneration, and shoot elongation are performed on semi-solid media, typically containing 87.6 mM or, occasionally, 58.4 mM sucrose (Table A1).…”

“…Eucalypt shoots sometimes produce roots spontaneously in hormone-free medium or potting mix [45,46,64,88,92,96,104,113,119,143,144,150,161]. However, adventitious rooting on eucalypt shoots is usually induced with an auxin rooting hormone, typically IBA at a concentration between 0.49 and 49 µM (Table A1).…”

“…IAA at 57 µM induces roots on E. globulus and E. grandis shoots [14]. Cytokinins are almost always excluded from media during the root induction phase because they are potent inhibitors of adventitious rooting in eucalypt shoots [45,46,64]. However, low levels of cytokinin have occasionally been incorporated with auxins in root induction media (e.g., 0.04 µM BA with 5.4 µM NAA for E. urophylla [130] and 0.4 µM BA with 5 µM IBA for E. urophylla × E. grandis [164]).…”