Feedstock Development and Production
The SBAR Center of Excellence uses an integrated approach to ensure sustainable regional production of biofuels and other high-value products. The foundation of this process is improved feedstocks (raw materials to supply an industrial process) and sustainable production (cultural practices) of feedstocks. We are specifically optimizing the production of guayule and guar, both of which have been successfully cultivated in the arid regions of the Southwestern United States. Guayule (Parthenium argentatum) is a woody perennial shrub native to the Chihuahuan Desert of central Mexico and the Big Bend area of Texas. There is presently no commercial production of guayule for natural rubber; however, advances in feedstocks, co-products, and agronomic practices will support and expand the bio-economy of this industry. Guar (Cyamopsis tetragonoloba) is native to the Indian sub-continent where it grows under drought, heat, and high salinity conditions, and does well in marginal soils. These conditions and soils are also found in the production areas encompassed by this project. Guar gum is an industrial product used in food additives and drilling muds, and comes from the seed endosperm. The U.S. imports approximately 80% of its guar beans from India, but demand has increased over the past decade and domestic production will significantly help meet this demand.
- Genetics and Plant Breeding
- Phenotypic Characterization
- Reduce or Eliminate Flowering in Guayule
- Sustainable Feedstock Production
- Soil Health
Genetics and Plant Breeding
Guayule and guar feedstock development means improving biomass quantity and quality, and yield of desired products through genetics and traditional breeding. The goal is to deploy superior genotypes to regional growers. Traditional breeding is dependent upon having genetic resources (germplasm) from which to identify useful traits. Germplasm may include lines with only one or two desirable traits, to lines that after minimal breeding have the desirable traits and uniformity to be grown commercially.
In both guayule and guar, we are evaluating available lines from the USDA National Plant Germplasm System. Evaluation is for the major components of yield in both crops, primary products (rubber and gum), disease and insect resistance, and water use efficiency. Breeding programs will incorporate noteworthy lines/plants, and use them in the development of improved lines. In addition, we are generating new genetic diversity in guayule by taking advantage of its facultative mode of reproduction (sometimes sexual and sometimes asexual reproduction), and in guar by crossing elite lines with male-sterile plants. An additional aspect of the research is bioengineering of guayule to improve rubber yield.
A plant’s phenotype, its observed physical and biochemical characteristics, is a result of both its genetic makeup and environmental influences. Phenotyping germplasm is essential in plant breeding programs, but information available for guayule and guar in the USDA National Plant Germplasm System is minimal and sometimes conflicting. Thus, we are phenotyping each guayule and guar germplasm line at multiple locations, allowing for the estimation of environmental effects on different traits.
A tractor equipped with high throughput sensors to measure biomass, canopy temperature, plant height, and plant architecture is used to phenotype guayule. Other measurements include rubber/resin quantity and quality, leaf classification and leaf area index, flowering time and amount, germination ploidy, and leaf waxes.
In guar, directly measured phenotype parameters include plant height, plant architecture, stem morphology, leaf size and shape, flowering time, raceme length, pod/seed size and number, and biomass yield under different water management and planting schemes. These data will not only be invaluable in breeding programs, but will help in deciding traits/lines best suited for different environments and cultural practices in the southwestern United States.
Reduce or Eliminate Flowering in Guayule
A novel approach to improve biomass quantity and quality, and potentially rubber and resin quantity and quality, is to reduce or eliminate flowering in guayule. Guayule flowers indeterminately (when water is not limiting), and flowers can account for up to 10% of the total biomass at harvest. However, flowering structures contain less than 1% of the total rubber in the plant. Thus, we hypothesize that by reducing flowering, the photosynthate used to produce flowers would be available to add biomass and possibly rubber at harvest. The objective of this aspect of the project is to reduce or eliminate flowering in guayule by controlling the expression of flowering-related genes. Candidate genes to be down-regulated have been identified, constructs necessary for transformation created, and transformations are underway.
Sustainable Feedstock Production
Common questions from growers considering a new crop are: (1) how much and when should we irrigate, and (2) how do we control weeds, and other insect/pest problems? Thus, much of our work is focused on answering these questions.
Irrigation Efficiency Management:
We are conducting irrigation and remote sensing experiments in both drip and furrow irrigated guayule and guar on different soil types. With potential future decline in available irrigation water, it is now imperative to conduct multiple trials to assess different delivery systems, irrigation amounts/regimes, and timing of irrigation to maximize yield and water use efficiency. From this information, we will develop an app to assist growers in making decisions about irrigation, salinity, and nutrient management.
Guar is a minor crop that has been grown commercially in the southwestern U.S. there are nine herbicides registered for use in guar. Guayule is a newer crop in which tolerance to herbicides was originally assessed by typically spraying young, 8-inch guayule transplants. There are currently three herbicides and one defoliant registered for use in guayule based on this research. Controlling early season weeds is critical due to the slow growth of guayule seedlings. In absence of herbicides, weed control requires expensive hand weeding. Since guayule is now direct seeded, we are focused on determining the herbicide tolerance of germinating and small seedlings during crop establishment. Additional preemergence and postemergence herbicides will be tested and registered herbicides will be retested for guayule tolerance during germination and stand establishment including the tolerance of very young guayule seedlings (2 to 6 leaf plants) to postemergence herbicides.
Expanding the Growing Range:
Expanding the cultivation range of guayule and guar is important, and addressed by understanding and testing for cold tolerance among the available germplasm lines. In guayule, we will use metabolomics to understand cold stress and acclimation, using time course data collected and plants screened at -8°C and -9°C, with hundreds of metabolites analyzed from acclimated and non-acclimated plants, and from plants that are freeze tolerant. Soil temperature for germination is the major limiting factor in expanding the growing range of guar. We will germinate (in an incubator) six elite guar cultivars at germination temperatures from 10°C-24°C (50°F-75°F). The results will then be used in test plantings at latitudes north of the present growing range.
Optimum Plant Densities:
New varieties and establishment systems require a reassessment of cultural practices such as planting density in both guayule and guar. In guayule, two lines are direct seeded at two locations and thinned to 3, 6, 12, 18, and 30in. spacing between plants. Guar lines with different plant architecture (branching, basal branching, non-branching) demand new tests looking at different plant densities and distance between rows.
The soil microbiome community affects plant growth and health, and plants have a corresponding effect on the soil microbiome community. Dryland ecosystems, such as the desert southwestern United States, occupy nearly half of the Earth’s surface. A recent survey confirmed that increasing aridity reduces the diversity of soil bacteria and fungi in dryland soils. Optimization of production will depend on understanding the desert soil microbial communities essential to supporting plant health and nutrient cycling, and how they change under different cultural practices. We will analyze microbial diversity and temporal diversity, and identify key microbial community populations in soil samples collected from guayule and guar plots. This will allow us to monitor the impacts of our agricultural management practices on the integrity of the soil microbiome, and correlate the soil microbiome with plant growth and productivity.
Progress Made Thus Far
The Feedstock Development & Production Team established guayule yield tests with the available USDA germplasm at three locations in Arizona. At the 1-year-of-age harvest, measurements included total biomass; rubber and resin content; and rubber and resin yield. There was significant variation observed for all traits. The final harvest will take place in spring 2020.
The Team has completed phenotypic evaluations of the available USDA guayule germplasm including germination at 11 different temperatures (to identify the optimum planting temperature), and ploidy levels. Remote sensing-related traits at 1-year-of-age exhibited significant variation among germplasm lines.
Guar yield tests (30-guar germplasm lines), established in June at three locations (Tucson, AZ; Las Cruces, NM; and Clovis, NM), were evaluated for simple phenotypic (height, branching, flowering) characteristics and harvested in November/December 2019. Seed characteristics and yield will be evaluated next. Initial results show a significant variation among lines.
Guayule flowers can account for up to 10% of the total biomass at harvest. However, flowering structures contain less than 1% of the total rubber in the plant. To reduce flowering by controlling the expression of flowering-related genes, candidate genes have been identified, constructs necessary for transformation created, and transformations are underway.
Guayule irrigation experiments are underway at the University of Arizona’s Maricopa Agricultural Center and Bridgestone’s Eloy farm. Maricopa has sandy loam soil and Eloy has clay soil. Each site includes five subsurface drip irrigation treatments [50%, 75%, 100%, 125%, and 150% of ET (evapotranspiration)], and one to three flood treatments (50%, 100%, and 150% ET). Weekly drone images, soil moisture profiles, and bimonthly biomass and rubber measurements help in developing a crop coefficient curve for guayule. The crop coefficient, along with remotely sensed multispectral and visual indices, can then be compared to biomass production and rubber yield over the range of water application rates. The establishment of a new flood-irrigated experiment at Eloy will evaluate the effect of limiting water during specific growth stages, the effect of irrigation schedule, and harvesting date on rubber/resin and biomass yield.
Guar irrigation experiments at Clovis, New Mexico (High Plains), during 2018 and 2019, compared the effects of full irrigation, and then limited water at pre-plant, vegetative (early season), and reproductive (late season) stages. Measurements included soil moisture, yield, biomass, and remotely sensed indices. Experiments during the first year indicated that only one-foot total depth of applied water during the entire irrigation season was sufficient and that turning off water during the reproductive stage resulted in only a minor decrease in yield. Although there was higher than normal precipitation during 2018 (one foot), this experiment demonstrated the extremely low water requirement of guar in Clovis, New Mexico. Data from the 2018 season has been incorporated into the WINDS and Aquacrop models in order to evaluate the effects of specific soil moisture levels on crop yield. The team is currently evaluating 2019 data.
In addition to soil moisture and canopy measurements, we are evaluating soils for physical, chemical, and biological profiles. By taking 54 soil samples in each field, we have established a baseline that will allow us to understand the effect of irrigation on soil parameters, and the effect of soil parameters on guayule growth rate, biomass, rubber, and resin production. This will ultimately result in the development of a web-based app to help in irrigation scheduling.
The WINDS (Water-use, Irrigation, Nitrogen, Drainage, and Salinity) model runs in VBA/Excel for analysis of the guayule and guar irrigation experiments. The WINDS model was reprogrammed in python in order to run on a server, connect to a database and produce web pages that provide irrigation information to users. The database was also connected by the cloud to field sensors detecting soil moisture and crop canopy temperature. These sensors will be installed in the new guayule irrigation experiment in March 2020. This system is still under development.
Herbicide studies on two different soil types (a fine-textured clay soil, and a course-textured sandy soil) have yielded data on herbicide damage to guayule seedlings. The six herbicides (acetochlor, bensulide, ethalfluralin, pendimethalin, s-metolachlor, and sulfentrazone) investigated were applied either pre-emergent or incorporated pre-plant. Another test evaluated tolerance of guayule to carfentrazone applied at 2, 3.6, 5.6 and 10.4 leaves per plant. Plant density trials for guayule were harvested at 1-year-of-age. Planting density trials continue in guar, and results are being evaluated.
Research and Results