Performance of improved guayule lines in Australia

Guayule (Parthenium argentatum Gray) is a potential source of commercial natural rubber. Its commercialisation depends mainly on economical plant production. The objective of this study was to evaluate the performance of improved lines in Australia. Seeds from five improved lines (AZ-1, AZ-2, AZ-3, AZ-5 and AZ-6) and two previously developed guayule lines (N 565 and 11591) were obtained from the Agricultural Research Service (ARS) of the United States Department of Agriculture (USDA). Seedlings from these lines were grown in a glasshouse for 3 months and later transplanted in a field experiment in early September 2001. Plant height and width were monitored from transplanting to 62 weeks at regular intervals. After 62 weeks, plant dry matter production, rubber and resin content, and yields were analysed. Plant height and width of the improved lines were higher than N 565 and 11591. Plant dry matter, rubber and resin yields were significantly different among lines. Of the five lines, AZ-1 and AZ-2 produced rubber yields of 620 and 550 kg/ha, respectively and these yields were significantly greater than for N 565 (371 kg/ha) and 11591 (391 kg/ha). AZ-1 and AZ-2 also produced significantly higher resin yields, 727 and 668 kg/ha, respectively, than those for N 565 (436 kg/ha) and 11591 (325 kg/ha). Rubber and resin yield increase of lines, AZ-1 and AZ-2, were in the range of 41–68% and 53–123%, respectively over N 565 and 11591. AZ-1 tended to produce higher rubber and resin yields than AZ-2 but exhibited highly variable plant height (CV=25%) and width (CV=41%) indicating potential for further genetic improvement. AZ-2 offers the best combination of desirable characters including early vigour, uniformity and comparatively higher rubber and resin yields.     

国内外巴西橡胶树替代作物及技术研发现状

天然橡胶是重要的工业原料和战略物资.近年来其供需日趋紧张,而目前大面积商业化种植生产天然橡胶的巴西橡胶树,只能种植在亚洲和非洲等地的热带地区.为此,美国、欧盟的某些机构相继开展了巴西橡胶树替代作物的研究,并在近期取得了重要进展.这对中国及世界其它国家以巴西橡胶树种植为主的天然橡胶产业构成了潜在威胁.为了对目前国内外非巴西橡胶树生产天然橡胶的研究和开发现状进行较为深入系统地了解,本文对国内外巴西橡胶树替代产胶作物及技术开发利用现状进行概括分析,并建议迅速确立专项开展银胶菊及转基因橡胶草等替代巴西橡胶树产胶作物及技术调研研究.     

The formation of rubber particles in developing cortical parenchyma of Parthenium argentatum plants exposed to the low temperatures of fall and winter of the Chihuahuan Desert

In this paper we have examined rubber particle formation in cortical parenchyma cells of guayule (Parthenium argentatum Gray) plants exposed to the low temperature of the fall and winter of the Chihuahuan Desert that stimulates rubber formation.Plants were transplanted to field plots in May in Ft. Stockton, TX. In September most of the cortical parenchyma in cross-sections 3 cm from the stem tips and from the base of the stems were mature, containing a central vacuole, tonoplast and a parietal cytosol with chloroplasts, mitochondria, peroxisomes and a few rubber particles. In 8-month-old plants examined in January many of the cortical parenchyma cells 3 cm from the stem tip were immature and completely filled with cytosol containing a high population of rubber particles. At this stage of development there was a thickened secondary cell wall but no central vacuole, tonoplast or parietal cytosol and all the rubber particles were formed in the central cytosol. In the cortical parenchyma in early stages of central vacuole development, by the coalescing of microvacuoles, the rubber particles were located in the developing central vacuole and in the cytosol being appressed toward the cell wall. In 8-month-old plants in January, the cortical parenchyma at the base of the stems was more mature than cells near the stem tips. These cells showed a later stage of development of a central vacuole from microvacuole fusing and contained a high population of mixed sizes of rubber particles and a thin layer of parietal cytosol containing chloroplasts, mitochondria and rubber particles with a partly developed tonoplast. There was clear evidence of the fusion of small rubber particles with other rubber particles that may represent a mechanism for the growth of rubber particles.     

Development and evaluation of a guayule debarker

Guayule (Parthenium argentatum Gray) stores its rubber primarily within the cells of its bark, which constitutes only 30% of the dry weight of the whole shrub. Debarking guayule soon after harvesting at the crop site would substantially improve the efficiency of latex extraction technology by reducing transport costs to the processing plant and increasing the processing capacity of the latex extraction plant. The objective of this study was to design, fabricate and evaluate an efficient debarking machine for guayule. The bark removal unit consists of a pair of grooved rubber rollers rotating in opposite directions at different speeds, drawing and crushing the cut stems without chipping the core. The clearance between the rollers is adjustable and can handle different stem sizes ranging from 6 to 45 mm. The speed difference between the rollers creates a shearing action, required for peeling the bark. The separation unit consists of a fan attached to metal tubing, a discrimination chamber for grading processed material and a water trough where lighter material is removed by flotation. The debarking system was tested to determine bark removal performance as well as separation of bark from other processed plant material. A debarking efficiency of up to 95% and maximum separation efficiency of 75% were found from the evaluation. The prototype debarking machine produced a throughput capacity of up to 450 kg/h which can be increased by converting it to a full-size field machine.     

Guayule production on the southern high plains

Guayule has long been known as a potential source of natural rubber. Native guayule populations are scattered throughout 300,000 km² of rangeland in the Chihuahuan Desert of Texas and Mexico. The only native indigenous U.S. stands occur in the Trans Pecos region of southwest Texas, and represent the most northern extension of the plant's habitat. Maximum air temperatures of over 38 °C are frequent and minimum temperatures of −23 °C have been recorded. Guayule has been successfully cultivated across the arid and semi-arid Southwestern U.S., but new production areas need to be identified in order to meet the expected world-wide shortage of natural rubber by 2020. The objective of our study was to determine if guayule production could be successful farther north on the Southern High Plains near Halfway, TX. Guayule seedlings were transplanted May 18, 2006 at the Texas AgriLife Research Station at Halfway. Seed used included four released lines, AZ-1, AZ-2, AZ-3, and AZ-4; a released USDA cultivar (11591); and three unreleased breeding lines, N9-3, N6-5, and N13-1. Guayule cold damage was estimated in June of 2007 and 2008 using the following index: (1) no damage, (2) slight - injury of terminals to 6 cm, (3) moderate - 2/3 of plant volume injured, (4) severe - all aerial portions killed but resprouting, and (5) complete - beyond recovery with no regrowth. Plant harvests were conducted in April 2008 and March 2009. Following the 2006/2007 winter the cold damage index ranged from 1.2 in 11591 to 3.8 in AZ-1. The minimum air temperature was −15 °C. There was minimal cold damage during the 2007/2008 and 2008/2009 winters. Biomass of the 24 month-old shrubs harvested in 2008 varied from 9639 kg/ha in 11591 to 13,393 kg/ha in AZ-4. Shrub biomass in 2009 ranged from 26,721 kg/ha in 11591 to 32,951 kg/ha in N6-5. Rubber yield in 2008 was 222 and 639 kg/ha in AZ-3 and N6-5, respectively. Line AZ-3 yielded 717 kg/ha of rubber in 2009 while line AZ-4 yielded 2006 kg/ha. The critical factor in winter survival appeared to be the temperature regime that existed in the late fall and early winter: a gradual and progressive onset of low temperatures that allowed the shrubs to be dormant when freezing temperatures occurred. Lines 11591 and N6-5 had the least cold damage and hold promise for establishment and rubber production on the Southern High Plains of Texas. Certain production criteria make this area an ideal guayule production site: the long-term annual rainfall averages 460 mm, irrigation water salinity is less than 1 E.C. and is pumped from only 90 m, and center pivot sprinklers are available for establishing guayule by direct-seeding.     

Development and evaluation of a guayule seed harvester

Guayule flowers continuously and seeds mature at different times; thus, a harvesting method that is gentle and non-destructive is required. The main objective of this study was to develop a simple but effective harvesting system capable of dislodging and collecting seed without damaging the plant. A single-row guayule seed harvester was developed after investigating different methods of seed dislodgement and collection. The harvester consisted of a seed dislodgement mechanism that removes seeds by vibration of spring steel rods. An axial flow fan placed directly over the vibrating rods catches the seed before it falls to the ground. The seed is then conveyed by the air stream through an inflatable canvas tube to a collection bin. The design and development of the seed catching system was based on terminal velocity of the unthreshed seed. Vibration frequencies ranging from 9.9 to 14.5 Hz and amplitude from 4 to 5.5 cm were used to test the harvester in the field. Harvested seed was analysed for percentages of clean seed in the harvested material, seed loss and immature seeds harvested. The performance of the machine was evaluated based on harvesting capacity and efficiency. Guayule seed harvested by the machine ranged from 1.73 to 7.18 kg/ha and harvest efficiency varied from 77 to 91%. The percentage of immature seeds removed from the crop during mechanical harvesting was as low as 0.1%. Although there was a trend for better efficiency from higher vibration frequencies, the variation was not significant. Nearly 21% of the harvested sample was clean which was much higher than that reported by other researchers. The harvester can be converted into a multiple row machine to increase its capacity.     

Direct seeding as an alternative to transplanting for guayule in southeast Queensland

Guayule (Parthenium argentatum Gray) is a source of high quality rubber and low-allergenic latex. Commercial potential of guayule to produce high value latex products has increased due to the increased incidence of deadly diseases in humans. The objective of this study was to investigate the potential of direct seeding in southeast Queensland as an alternative to establishment by the current high cost transplanting method. Experiments were conducted for 2 years at Gatton, Queensland using physical and chemical seed treatments. NaOCl plus GA₃, osmo-priming with polyethylene glycol, seed pelleting and seed tape planting were tested. Planting depth had a significant influence on seedling emergence. Emergence at 18 mm depth (1.5 seedlings/m) was significantly reduced compared with 10 mm depth (2.9 seedlings/m). Osmo-priming was effective in increasing germination from 36 to 47%. It also improved seedling emergence and vigour. Osmo-priming significantly increased establishment (7.5 seedlings/m) compared with untreated seed (3.3 seedlings/m). NaOCl plus GA₃ did not have a significant influence on germination (38%) or seedling emergence (5.0 seedlings/m) but increased survival at 42 days after planting. Establishment as a percentage of emerged seedlings was high for both osmo-primed and NaOCl plus GA₃ treatments with 89 and 88% respectively, whereas untreated seed had only 70% survival. Osmo-priming also increased vigour as indicated by increased root length (101 mm), shoot height (123 mm), and seedling dry matter (379 mg/seedling) compared with the control (83, 107 mm and 206 mg/seedling, respectively).     

Chemical constituents and physical properties of guayule wood and bark

About 15 Parthenium species grow in the North American continent with Parthenium argentatum (guayule) as the only species containing harvestable amounts of the rubber latex. The predicted commercialization of the guayule plant for its hypoallergenic latex will result in a significant amount of waste fiber or bagasse biomass that can also be put to use for making wood, paper, and other chemical products, as well as in energy production. Thus, the guayule wood and bark fibers can be considered a new source of plant biomass that may be used as a direct substitute for forest-based wood fiber. However, little information is available on the chemical composition of the wood and bark tissues of guayule (P. argentatum). The objectives of this study were to determine the chemical and fiber composition of guayule and to compare it with other wood fiber sources.Three germplasm lines of mature guayule (Cal-6, AZ-101, and G7-15) and another species of Parthenium (P. tomentosum, PT), juvenile soft maple (Acer rubrum), a deciduous tree, and milkweed (Asclepias syriaca L.) that has long fibers were the plant sources. Separate wood and bark tissues were analyzed for hot water, 1% sodium hydroxide, and alcohol–toluene extracts. In addition, the lignin, holocellulose, alpha-cellulose, and pentosan contents were determined.All the chemical components in the wood fibers for the Partheniums were equal to or greater than the juvenile maple tree. Milkweed had higher alpha-cellulose and lower alcohol–toluene extract contents than both the guayule and soft maple. The guayule bark fibers had more chemical extracts than the wood fibers. The specific gravity of guayule wood was greater than the deciduous wood species. However, the fiber lengths of soft maple wood, guayule wood, and milkweed are similar. Based on the chemical composition, P. argentatum and P. tomentosum could serve as raw materials for the paper and chemical industries as well as for energy production.     

The physiological and biochemical responses of guayule to the low temperature of the Chihuahuan Desert in the biosynthesis of rubber

Guayule (Parthenium argentatum Gray) is a rubber plant indigenous to the Chihuahuan Desert of Northern Mexico and Southwestern Texas. In this review we report the nature of the physiological, cellular and biochemical responses of these plants to the low temperature of the desert in the biosynthesis of rubber.

Studies on rubber formation in guayule in several field plantings support the conclusion that the low temperature of the fall and winter months of the Chihuahuan Desert promotes a rapid increase in rubber biosynthesis. There are definite changes in the cortical parenchyma during the biosynthesis and deposition of rubber. Discrete rubber particles are formed in the parietal and interior cytosol. Following the digestion of the cytosol in the interior of the cell and rubber particle fusion leaves the parenchyma with rubber deposits throughout the cells. The rubber transferase (RT) bound to the washed rubber particles (WRP-RT) catalyzes the formation of different length cis-1, 4-polyisoprene chains that collectively form the rubber polymer. The activities of the WRP-RT and the endoplasmic reticulum 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) increase with exposure of the plant to low temperature and it is conceivable that these two enzymes play a pivotal role in the biosynthesis of rubber during this period. Thus guayule plants indigenous to the Chihuahuan Desert have developed a genetic system capable of responding to the low temperatures of the fall and winter of the desert culminating in the formation of rubber.     

Guayule as a feedstock for lignocellulosic biorefineries using ammonia fiber expansion (AFEX) pretreatment

Natural rubber latex extraction from guayule leaves behind greater than 90% (by weight) of agricultural residue as a feedstock suitable for conversion to biofuels via a thermochemical or biochemical route. Untreated guayule shrub and bagasse (after latex extraction) has shown to be very recalcitrant to enzymatic hydrolysis, necessitating application of a chemical pretreatment to enhance cellulase accessibility. The objective of this work was to carry out detailed compositional analysis, ammonia fiber expansion (AFEX¹) pretreatment, enzymatic hydrolysis and ethanol fermentation for various guayule-derived biomass fractions. Plant feedstocks tested were derived from two sources; (a) a mature 2007 AZ-2 whole guayule shrub plant obtained from USDA/ARS² research fields, and (b) the guayule latex-extracted commercial grade bagasse (62505) from Yulex Corporation. Compositional analysis and enzymatic hydrolysis were carried out using standard NREL³ protocols (www.nrel.gov/biomass/analytical_procedures.html). AFEX pretreatment was carried out using concentrated ammonium hydroxide at elevated temperatures for desired residence times in a pressurized reactor. Yeast fermentations on biomass hydrolyzates were carried out micro-aerobically using Saccharomyces cerevisiae (424A strain) in shake flasks.AFEX pretreatment was found to substantially improve overall enzymatic digestibility by 4-20 fold for both untreated guayule shrub and latex-extracted bagasse. Maximum glucan and xylan conversion achieved for the latex-extracted bagasse was 40% and 50%, respectively. The yeast was readily able to ferment both glucose and xylose to ethanol from the guayule bagasse hydrolyzate with or without external nutrient supplementation (i.e., yeast extract and tryptone). Our results highlight the possible utilization of guayule as a feedstock for lignocellulosic refineries co-producing natural rubber latex and biofuels. However, further process improvements (e.g., lignin/resin extraction and cellulose decrystallization using a modified AFEX process) are necessary to increase the effectiveness of ammonia-based pretreatments for further enhancing enzymatic digestibility of guayule-derived hardwood biomass.