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Rural Industries Research & Development Corporation
by Jonathan Lidbetter, Tony Slater, Pauline Cain, Michelle Bankier and Slobodan Vujovic May 2003
RIRDC Publication No 02/140 RIRDC Project No DAN-181A
Eriostemon australasius (austral eriostemon, pink waxflower) provides an excellent floral display of numerous five petalled star-like pink flowers two to four centimetres across. Eriostemon belongs to the family Rutaceae and is closely related to other commonly cultivated Australian genera such as Boronia, Crowea, Correa, Geleznowia (Yellow Bells), Philotheca, Phebalium and Zieria.
This species has been available in the cut flower trade for over 60 years, but mainly as a bush-picked product. Early development saw one relatively easy to propagate variety enter the nursery trade but little further development occurred, mainly due to difficulty and/ or slowness in propagation. Preliminary examination of natural populations suggested that huge variation in flower colour and form existed. It remains a desirable cut-flower and relatively popular nursery plant but is often considered difficult to maintain in cultivation in all but the best drained soils or cooler climates.
The combination of a need to propagate desirable forms and the possibility of developing a disease resistant scion/ rootstock combination led to this project determining the opportunities for grafting this species.
Field Selection
Seven natural populations of Eriostemon australasius were surveyed from Oakdale (55 km WSW of Sydney) to Nabiac (30 km S of Taree) to identify elite clonal material. Significant variation was found in petal colour, petal width, flower diameter, leaf size and plant vigour. Sixty superior or unusual forms were identified. Grafting of scion material from natural populations (or tissue culture) onto seedlings has enabled over forty varieties to be rapidly brought into cultivation. From these, collaborators selected five elite varieties in 2001 and an additional twenty-two in 2002. Colour photographs showing natural variation and the form and detail of the top 10 selections are available in the associated RIRDC Short Report 02/117.
Seed germination
Techniques were developed for the germination of Eriostemon australasius seed using concentrated sulfuric acid and gibberellic acid to break the hard seed coat and stimulate germination respectively. Seedling emergence reached levels approaching 50% with 800 seedlings germinated for use as rootstocks or varietal evaluation.
Intergeneric grafting
Grafts of the commercial form onto thirteen hardy rootstock species from eight related genera all formed initial unions and started to grow. The use of three grafting methods and three protection methods to prevent the scions from drying out all successfully assisted in establishing graft unions. However, in all cases scions failed to establish a long-term compatible combination with deaths occurring within the first six months. Although not totally precluding compatibility of other varieties as scions or rootstocks it strongly suggests that Eriostemon australasius is incompatible with each of the species tested.
Grafting onto seedling E. australasius
Seedling rootstocks allowed the rapid formation of successful graft unions with .semi-hardened off. grafted plants back on drip irrigation in five weeks. Grafts were successful using scion material from either cultivated stock, collections from natural populations or direct from tissue culture. One hundred and eighty successful grafts were achieved out of 480 grafts with a success rate averaging 38% (ranging from 0-100% per variety). Success rate depended on scion and rootstock quality and stage of maturity. Eighty percent of successfully grafted scions flowered from late July to September 2001, eight to fifteen months after grafting. All grafted plants flowered in 2002 with most two-year-old plants over one metre tall.
Grafting onto cuttings of E. australasius
The use of cultivated grafted motherstock material to provide scions for grafting onto cutting grown rootstocks increased the success rate to 85%. So successful were these grafts that a number of rootstocks grafted with more than one scion variety were flowering with up to four scion varieties p plant at eight months of age. Although missing some of the vegetative vigour of seedlings, cutting grown rootstocks compensate through uniformity and consistent results.
Cutting propagation
Initial cutting trials with material collected from natural populations were unsuccessful. However when cultivated grafted motherstock were used for scion material strike rates improved dramatically Success was extremely variable depending on the variety. Some varieties produced roots readily wi up to 80% of cuttings struck within five weeks whilst others failed to achieve more than 2-3% after seven months. Applications of high concentration (proprietary) auxin preparations were more successful stimulated rooting than lower rates. Regular re-dipping of unstruck cuttings appeared to improve results of slow to root varieties.
Release to collaborators
Over 450 cutting grown or grafted plants of selected varieties have been propagated for release to collaborators as part of this project. Further cutting material from motherstock and/or tissue culture also available.
Conclusion
In conclusion this project has successfully established a protocol for the rapid introduction of new field selections of E. australasius into cultivation through grafting, significantly expanding the form available to industry. Over 40 new varieties have been introduced to cultivation with twenty-five of these generating early interest. However, in the absence of an identifiable compatible hardy rootstoc the cost of grafting is difficult to justify except for introduction of material into cultivation and early bulking up. Preliminary cutting trials have identified techniques to propagate some of the new varieties already and at this stage appears the most appropriate method of vegetative propagation. The development successful seed germination techniques allows any future breeding and selection program to procee quickly. Alternatively the generation of a variable seedling population could be used for disease resistance screening to identify tolerant individuals within the species. The lack of a compatible rootstock limits the development of this species to areas where it can alrea be grown successfully. However, the new flower forms and longer flowering season created by the identification of early and late varieties will greatly assist market penetration through extended presence and greater range of product.
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