Vegetative propagation of Corymbia henryi and its hybrids in South Africa through cuttings and mini-grafting techniques.

Abstract

Eucalyptus species are the most commonly planted hardwood species in South Africa, composing 42% of the total plantation area in the country but as the forestry planation areas in South Africa are not expanding, the only way to increase timber supply is to use the available land productively. This also increases the demand to efficiently propagate eucalyptus species, in an easy and economically viable way. This can be achieved through optimising vegetative propagation techniques which ensures mass multiplication of superior genotypes/phenotypes and the maintenance of individual characteristics in order to ensure that a plant is genetically identical to the original/donor plant. Spotted gum (Corymbia species) and their inter-specific hybrids have been identified as promising taxa for commercial forestry in South Africa due to their superior survival and growth across a broad range of edaphic and climatic conditions. The major benefits of these hybrids include superior growth characteristics, disease and insect resistance and frost tolerance, making them desirable for propagation. The hybrids of Corymbia torelliana and C. citriodora are some of the extensively used Corymbia hybrids since they combine frost tolerance (C. citriodora) and disease resistance (C. torelliana). The widely used vegetative propagation techniques include propagation through rooted cuttings and grafting. The aim of the study was to investigate the vegetative propagation of Corymbia henryi (C. henryi) hybrids through the rooting cuttings, and the grafting of C. henryi. The propagation of the hybrids through cuttings was carried out in three experiments, comparing cutting material collected from coppice stumps and ramets. The cuttings were collected from 14 C. henryi hybrid genotypes grown in Zululand. The cuttings were maintained in a rooting tunnel for four weeks before being placed in greenhouse conditions to acclimatize to the natural environment. The use of ascorbic acid (AA) (40 mg L-1) with Seradix® 2 (IBA 3g kg-1) yielded the highest rooting incidence, ranging from 37.5-55.5%, with the lowest rooting resulting from the application of propiconazole with Seradix®2 (IBA 3g kg-1) at >2%. The combination of AA with IBA has shown to be synergistic, since AA acts by protecting the rooting hormone from oxidation therefore allowing it to enhance tolerance of the plants to greenhouse conditions and alleviate stress. The cuttings selected from coppice had a higher rooting survival than those collected from hedge material. The genotype had an effect on the rooting success of cuttings, with the highest rooting percentage occurring with C. torelliana x C. citriodora subsp. variegata hybrids, with rooting ranging from 25-70%, while a C. torelliana x C. henryi hybrid had the least rooting success (%). The Corymbia species are considered difficult-to-root therefore the results have shown that these species can be propagated through rooting if the better rooting hybrid genotypes are selected. There is still a need to perform more trials to test the genotypes that have been found to root better in order to reach the commercial requirements of rooting rate of 70% and above. The propagation through grafting allows for the union of more than one genotype, whether belonging from the same species or different species and offers propagation of species which may be hard-to-root therefore cannot be produced through cuttings. The grafting experiment was carried out to optimize grafting techniques by comparing mini-grafting and conventional grafting techniques for Corymbia henryi. The rootstocks were grown and maintained at the ICFR nursery until time of grating. The scion material was collected from the Zululand region from C. henryi provenance mix. The grafting and mini-grafting was carried out in the grafting tunnel at the Institute for Commercial Forestry Research (ICFR) nursery, which had continuous mist for the duration of the experiments. The methods applied on the grafts were cleft and splice grafting. The evaluations made were based on the grafting method applied, comparing age of the grafts (mini- grafting and conventional grafting) and the effect of different treatments applied onto the grafts which included the control treatment, use of Parafilm® to tie the graft union, covering the grafts with polyethylene plastic (for one week) and use of an antitranspirant, Vapor Guard®. The control treatment, where grafts were tied with Parafilm® had the highest graft survival (33.3%), with the lowest graft survival on grafts covered with polyethylene plastic (%).Grafting onto four- month old seedlings(mini-grafting) had the highest survival at 55.6%, compared to grafting onto 10-month old seedlings (conventional grafting), with a survival of 22.2%. Mini-grafting has been tested over some species due to its ease of handling and is being evaluated for its potential in the propagation of Corymbia henryi. This type of grafting offers advantages of efficient management of plants that are grown in the nursery, allowing ease in irrigation, nutrition and pest disease control. The use of younger seedlings allows for grafting to be a commercially viable technique due the reduced time to grow the plant, flexible and pliable cambium layers of younger rootstocks and rapidness of grafting these younger seedlings. Mini-grafting was evaluated with the use of commercially available anti-transpirants and antioxidants, to assess effect of these treatments on graft success. The different ancillary treatments applied were the control, use of Parafilm® to tie the graft union, the use of anti-transpirants such as Nu-Film 17®, Vapor Gard® and Greenstim®, and the use of an antioxidant, ascorbic acid. Foliar application of ascorbic acid had the highest graft survival (60%) compared to the other treatments applied. The use of anti-transpirants in grafting of C. henryi was not successful therefore more research needs to be done on the commercially available anti-tranpirants in alleviating water stress in Eucalyptus species, as each crop may have different requirements for anti-transpirant application and doses may differ from one crop to another. Vegetative propagation of Corymbia henryi and its hybrids shows some potential for future use, but more research needs to be done to optimise these techniques to be able to have an impact on the commercial scale and in research outputs.