No abstract

Eucalypts are among the world's most widely planted trees, but the productivity of eucalypt plantations is limited by their often-low amenability to true-to-type propagation from cuttings. An alternative approach to cutting propagation is tissue culture, which can be used to micropropagate valuable genotypes rapidly while simultaneously preserving germplasm in vitro. This review describes the use of tissue culture methods such as shoot culture, organogenesis, and somatic embryogenesis for micropropagating eucalypts. This review also discusses the use of cool storage, encapsulation, and cryopreservation methods for preserving eucalypt germplasm and delaying tissue maturation under minimal-growth conditions.

Hybrids between Corymbia torelliana (F.Muell.) K.D.Hill & L.A.S.Johnson and C. citriodora subsp. variegata (F.Muell.) A.R.Bean & M.W.McDonald are used extensively for plantation forestry in subtropical eastern Australia; however, plantation establishment has been hampered by inadequate seed supply and variable amenability to propagation as rooted cuttings. The present study investigated node-culture and organogenic-culture methods for in vitro propagation of two families of juvenile Corymbia hybrids. The effects of nutrient and auxin concentrations on root formation, shoot elongation and shoot proliferation, and subsequent conversion to plantlets in an in vitro soil-less system, were assessed. The response to the nutrient concentration differed between organogenic and node cultures. Half-strength, compared with full-strength, medium with auxin increased shoot proliferation by 58% for one family in the node culture, in which many shoots form roots and elongate rapidly in the absence of cytokinin. However, full-strength medium increased shoot proliferation by 213% over that in the half-strength medium for the other family in the organogenic culture, in which shoots do not form roots in the presence of cytokinin, and nutrient uptake occurs via callus and stem tissue rather than roots. The auxin, naphthalene acetic acid, sometimes stimulated root formation and shoot elongation in the node culture, although it had no effect on the final number of shoots. Most shoots (57–100%) subsequently formed roots in the in vitro soil-less medium. The high rates of proliferation and plantlet conversion in the present study provide the means, not only for simultaneous laboratory storage and field-testing of Corymbia clones before selection of desired genotypes, but also for en masse plantlet production of selected clones or families for plantation establishment.

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.

A protocol was developed for short-term preservation and distribution of the plantation eucalypt, Corymbia torelliana 9 C. citriodora, using alginateencapsulated shoot tips and nodes as synthetic seeds. Effects of sowing medium, auxin concentration, storage temperature and planting substrate on shoot regrowth or conversion into plantlets were assessed for four different clones. High frequencies of shoot regrowth (76-100%) from encapsulated explants were consistently obtained in hormone-free half-and full-strength Murashige and Skoog (MS) sowing media. Conversion into plantlets from synthetic seeds was achieved on half-strength MS medium by treating shoot tips or nodes with 4.9-78.4 lM IBA prior to encapsulation. Pre-treatment with 19.6 lM IBA provided 62-100% conversion, and 95-100% of plantlets survived after acclimatisation under nursery conditions. Synthetic seeds containing explants pre-treated with IBA were stored for 8 weeks much more effectively at 25°C than at 4°C, with regrowth frequencies of 50-84% at 25°C compared with 0-4% at 4°C. To eliminate the in vitro culture step after encapsulation, synthetic seeds were allowed to preconvert before sowing directly onto a range of ex vitro nonsterile planting substrates. Highest frequencies (46-90%) of plantlet formation from pre-converted synthetic seeds were obtained by transferring shoot tip-derived synthetic seeds onto an organic compost substrate. These plantlets exhibited almost 100% survival in the nursery without mist irrigation. Pre-conversion of non-embryonic synthetic seeds is a novel technique that provides a convenient alternative to somatic embryo-derived artificial seeds.

No abstract

No abstract

No abstract