Development of a guayule production system for low-allergenic rubber in Australia

Abstract

Guayule (Parthenium argentatum Gray) produces high quality, low-allergenic rubber that provides, potential for the crop to be commercialised. Previous intermittent attempts to commercialise guayule in the USA and Australia highlighted the need for further research to increase rubber yields and decrease production costs. Recently, considerable progress has been made in the USA with the release of high yielding lines.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 guayule lines under different environmental conditions; germplasm trials were conducted at two sites in Southeast Queensland over nearly three years and involved testing USDA lines (AZ-1 to AZ-6) against two older lines (N 565 and 11591). Secondly, to gain greater understanding of mechanisms affecting dormancy, studies on the effects of promoters, inhibitors, light quality and seed coat on germination were undertaken. The third objective was to investigate the effects of seed and planting treatments on crop establishment at Gatton.Germplasm evaluation trials confirmed that both Chinchilla and Gatton were suitable for guayule production. Environmental conditions at Gatton, with clay soils rich in nutrients and a comparatively narrow temperature range were better than at Chinchilla. However, at Gatton, the high clay content which results in slow drainage after heavy summer rains, leads to water logging and may result in crop death due to soil borne diseases such as Rhizoctonia solani.Overall performance for plant growth and yields of dry matter, rubber and resin of the new lines, AZ-1 to AZ-6, was generally better than the old lines. AZ-1 and KL-2 maintained the best combinations of desirable traits, including plant uniformity, early vigorous growth, increased dry matter, and increased rubber and resin yields. Of these two, AZ-2 was better than AZ-1 due to its comparatively more uniform plant growth.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; rubber yields were 48% to 61% higher than those of the old lines. They produced rubber yields of 789 kg/ha and 771 kg/ha, respectively, at 32 months at Gatton; rubber yields were 14% to 25% higher than those of the old lines. AtChinchilla, when plants were 33 months old, AZ-1 and AZ-2 produced rubber yields of 717 kg/ha and 787 kg/ha, respectively; the yield increase over the old lines was between 86% and 107%. Thus AZ-1 and AZ-2 were consistent performers at both sites, largely due to higher biomass; rubber content was intermediate in value.AZ-1 and AZ-2 also produced resin yields of 752 kg/ha and 663 kg/ha, respectively, at 17 months. Resin yields of these two lines at 32 months were 1158 kg/ha and 1115 kg/ha respectively. At Chinchilla, AZ-1 (1318 kg/ha) and AZ-2 (1476 kg/ha) produced high resin yields. Thus resin yields of AZ-1 and AZ-2 were consistently high across sites. At both sites, resin yields were higher than rubber yields.Harvesting season affected rubber content. At Gatton, in spring, 17 month old plants produced a mean rubber content of 7.7%, while, in summer, when the plants were 32 month old, rubber content dropped to 6.4%. At Chinchilla, 33-month-old plants harvested in spring produced a rubber content of 7.4%, similar to the spring value at Gatton. By contrast, resin content appeared to be little affected by the season.A series of experiments was performed on both whole seed and naked embryo, under light and complete darkness, to gain a greater understanding of dormancy mechanisms. Both seed coat and light had significant effects on germination of guayule seed which varied in age from 19 day old to 8 month old. The seed coat had a much smaller effect on germination under light (up to 16%) than under dark conditions (25-26%). More rapid germination for embryo may indicate that the seed coat acts as a mechanical barrier to radicle emergence or to gas exchange, although this could be equally well explained by lower concentration of inhibitor in the embryo being more quickly overcome by light; the latter would support the promoter/inhibitor hypothesis of dormancy.This study provided evidence to suggest that seed dormancy/germination in guayule isregulated by the balance of germination promoters and inhibitors.