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Moreover, Hevea causes life-threatening Type I latex allergy which is triggered by the presence of protein in the latex. Latex allergy caused by Hevea latex has become a major health hazard since the 1980s with the increased use of latex products (medical gloves and condoms) to control the outbreak of deadly diseases like Acquired Immune Deficiency Syndrome (AIDS). Of the three types of allergy, Type I immediate hypersensitivity or IgE mediated anaphylactic reaction is life threatening and is caused by proteins in the Hevea latex. It was reported that almost 10% of the population in the USA are allergic to Hevea latex.

Guayule (Parthenium argentatum Gray) is a semi-arid plant that produces high quality natural rubber and low-allergenic latex with the potential to become commercial crop. Natural rubber possesses high performance properties that cannot be achieved by synthetic forms. These include resilience, elasticity, abrasion resistance, efficient heat dispersion, impact resistance, and malleability at cold temperatures. Due to its superior quality, natural rubber is an essential raw material for many products. It is often blended with synthetic rubber for various products but many are 100% natural rubber (e.g. medical gloves and condoms). Although synthetic rubber is non-allergenic, it is becoming more expensive due to increased petroleum prices. Australia imported $ 1,045 million worth of rubber tyres, tubes and related products during the financial year 2001-2002 that accounted for 0.9% of total imports.

Previous intermittent attempts to commercialise guayule in the USA and Australia highlighted the need for further research to increase rubber yields and/or decrease production cost. Latest research has resulted in considerable progress in the release of high yielding lines. New releases produced significantly higher rubber and resin yields compared to existing lines in environmental conditions in the USA, however, these new lines have not been tested in the Australian environment. Therefore, the broad objective of this research was to investigate the commercial potential of guayule in Australia. Experiments were conducted with three specific objectives, firstly, to evaluate the performance of new USDA guayule germplasm under different environmental conditions within two potentially suitable areas in southeast Queensland.

Secondly, to gain further understanding of the mechanisms of seed dormancy to facilitate direct seeding. The third objective was to investigate the potential of direct field seeding as an alternative to expensive transplanting.

Evaluation of USDA germplasm
Results of germplasm evaluation trials verified that both environments Chinchilla (300 km northwest from Brisbane) and Gatton (80 km west of Brisbane) were suitable for guayule production. Overall performance of plant growth, dry matter, rubber and resin yields (stem and branches) of six new guayule lines at two sites in Queensland was generally better than the old lines (N565 and 11591). Of all the lines, AZ-1 and AZ-2 were the best for both environmental conditions. They produced early vigorous growth, increased dry matter, increased rubber and resin yields. They also can be harvested in shorter duration (two years) in good soils and environmental conditions. Of these two lines, AZ-2 was preferred over AZ-1 due to its comparatively high uniformity.

AZ-1 and AZ-2 produced rubber yields of 567 kg/ha and 611 kg/ha respectively in the second year (17 months) at Gatton from stem and branch that was 48% to 61% higher than the old lines.

These two lines (AZ-1 and AZ-2) produced rubber yield of 717 kg/ha and 787 kg/ha respectively from stem and branch in the third year (33 months) at Chinchilla. Thus the yield increase over the old lines was between 86% and 107%. No ratoon studies were performed.

AZ-5 produced the highest rubber yields from stem and branch at both the second and third year at Gatton, 565 kg/ha and 966 kg/ha respectively. This is over 43% increase in yield over the old lines. However, the uniformity of AZ-5 was lower than that of AZ-1 and AZ-2 indicating that further selection is possible. Also it did not perform better at Chinchilla.

The yield improvement in both rubber and resin was due mainly to improved biomass and was evident for most of the new lines. Rubber yields approaching 1000 kg/ha were achieved under well irrigated, fertile conditions indicating the improved returns for these new lines. Higher yields may be achieved under higher plant populations (this study used a plant population of 19,000 plants/ha but 27,500 plants/ha has been used in the U.S.). No studies were performed on ratoon crops although measurements were made on root yields at Chinchilla; the recommendation in the U.S. has been to harvest at two years and ratoon annually each spring with a final harvest in year five when the roots as well as tops are harvested.

At Gatton, plant death was noted in some plots after the soil became waterlogged after summer rain. This was ascribed to Rhizoctonia sp. and confirms the finding of previous researchers who found that slow draining cracking clay soils were prone to disease under these conditions.

Seed dormancy
Seed germination experiments revealed that dormancy or germination in guayule was regulated by the balance of growth promoters and inhibitors. Dormancy and germination of guayule seed depend upon the corresponding shift in the threshold levels of growth promoters and inhibitors.

Endogenous or exogenous growth regulators can shift the balance to inhibit or induce germination. Further, the level of dormancy or rate and extent of germination depends upon the degree of shift from the threshold levels.

High levels of dormancy with freshly harvested seed are due to high levels of inhibitor (abscissic acid or ABA and/or phenolic compounds) and/or low levels of promoter (gibberellic acid or GA).

Parent environmental conditions during seed development determine the levels of growth regulators (GA and ABA) and hence the dormancy.

The seed coat also contains inhibitors that affect the balance between promoter and inhibitor.

Therefore, gibberellins are required to overcome germination constraints imposed by the seed coat and ABA-related embryo dormancy. The seed coat also acts as mechanical barrier to the emerging radicle.

Light requirement or response to gibberellin in germination of guayule is mediated through phytochrome, a plant hormone. Therefore, light and its quality are also important to activate phytochrome that induces the synthesis of gibberellins to promote germination. Further, the light requirement in germination can be totally replaced with exogenous gibberellin acid. However, the required amount or concentration varies according to the level of dormancy.

Direct seeding
Direct seeding trials indicated that establishment by direct seeding is possible. However, high quality seed, fine textured uniform seedbeds, shallow precision planting, adequate soil moisture throughout the first three weeks and efficient weed control are required to achieve good establishment by direct seeding. Direct seeding is a challenge due to small seed size (1 mg) and the need to have a shallow depth. Soil moisture must be sustained long enough to allow germination and establishment.

Osmopriming with polyethylene glycol was very effective in increasing germination, emergence and establishment. It also improved seedling growth at the initial stage.

Implications and recommendations
This research has identified suitable lines for guayule production in south-east Queensland. These lines have greatly improved rubber and resin yields due to improved biomass compared with old lines. Improvement in both these attributes will increase the economic viability of the crop.

Australia has large areas of land with soils and climate suitable to guayule production. Yulex Corporation (U.S. based) has recently expressed interest in setting up a pilot plant in Australia to process the crop. There is thus considerable opportunity for import replacement from guayule production; however, the major use for guayule rubber may come from high end medical uses which need a low-allergenic, high quality source.

Further increases in yield are possible through higher plant population, harvest after two years and ratoon crops. Further research is necessary to characterise performance under these conditions. It was evident from the variability of growth of some varieties (such as AZ-5) that further selection could be undertaken with potential for increased yields.

A note of caution is needed regarding soil type. Guayule appears to perform well on slow draining cracking clay soils until waterlogging occurs. This can result in rapid plant death and may necessitate planting of the crop on well drained soils such as sandy loams. Further research is being conducted at the University of Queensland to investigate this aspect.

Greater understanding of guayule seed dormancy has been achieved especially in relation to the effect of the embryo (germination inhibitor) and seed coat which is both a mechanical barrier to radicle emergence and a location for germination inhibitor. This understanding is very useful in treating seed to improve germination and thus establishment, both by direct seeding and transplanting.

Direct seeding trials demonstrated that the crop can be established by this method albeit under well controlled conditions. This can greatly reduce establishment costs and increase the attractiveness of the crop commercially. Where the crop is grown under dryland conditions, further research is necessary to identify sowing times and cultural practices such as mulching which would increase establishment rates.

A further issue that was recognised in undertaking the present project was the need to develop more efficient methods of harvesting and processing seed. Seed is produced over an extended period during the summer and shatters readily once it is physiologically mature. Seed size becomes an issue for cleaning and processing. Research on these areas is being undertaken at the University of Queensland and will facilitate the production of clean seed of high quality for direct seeding.

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