David A. Dierig

Root traits contributing to plant productivity under drought.

Geneticists and breeders are positioned to breed plants with root traits that improve productivity under drought. However, a better understanding of root functional traits and how traits are related to whole plant strategies to increase crop productivity under different drought conditions is needed. Root traits associated with maintaining plant productivity under drought include small fine root diameters, long specific root length, and considerable root length density, especially at depths in soil with available water. In environments with late season water deficits, small xylem diameters in targeted seminal roots save soil water deep in the soil profile for use during crop maturation and result in improved yields. Capacity for deep root growth and large xylem diameters in deep roots may also improve root acquisition of water when ample water at depth is available. Xylem pit anatomy that makes xylem less “leaky” and prone to cavitation warrants further exploration holding promise that such traits may improve plant productivity in water-limited environments without negatively impacting yield under adequate water conditions. Rapid resumption of root growth following soil rewetting may improve plant productivity under episodic drought. Genetic control of many of these traits through breeding appears feasible. Several recent reviews have covered methods for screening root traits but an appreciation for the complexity of root systems (e.g., functional differences between fine and coarse roots) needs to be paired with these methods to successfully identify relevant traits for crop improvement. Screening of root traits at early stages in plant development can proxy traits at mature stages but verification is needed on a case by case basis that traits are linked to increased crop productivity under drought. Examples in lesquerella (Physaria) and rice (Oryza) show approaches to phenotyping of root traits and current understanding of root trait genetics for breeding.

Development of DArT Marker Platforms and Genetic Diversity Assessment of the U.S. Collection of the New Oilseed Crop Lesquerella and Related Species

The advantages of using molecular markers in modern genebanks are well documented. They are commonly used to understand the distribution of genetic diversity in populations and among species which is crucial for efficient management and effective utilization of germplasm collections. We describe the development of two types of DArT molecular marker platforms for the new oilseed crop lesquerella (Physaria spp.), a member of the Brassicaceae family, to characterize a collection in the National Plant Germplasm System (NPGS) with relatively little known in regards to the genetic diversity and traits. The two types of platforms were developed using a subset of the germplasm conserved ex situ consisting of 87 Physaria and 2 Paysonia accessions. The microarray DArT revealed a total of 2,833 polymorphic markers with an average genotype call rate of 98.4% and a scoring reproducibility of 99.7%. On the other hand, the DArTseq platform developed for SNP and DArT markers from short sequence reads showed a total of 27,748 high quality markers. Cluster analysis and principal coordinate analysis indicated that the different accessions were successfully classified by both systems based on species, by geographical source, and breeding status. In the germplasm set analyzed, which represented more than 80% of the P. fendleri collection, we observed that a substantial amount of variation exists in the species collection. These markers will be valuable in germplasm management studies and lesquerella breeding, and augment the microsatellite markers previously developed on the taxa.

Lesquerella commercialization efforts in the United States

Abstract Lesquerella fendleri (Gray) Wats. is a promising new crop for cultivation in the western United States. Its oil-bearing seed contains high amounts of the hydroxy fatty acid (HFA), lesquerolic acid (C20:1-OH), suitable as a raw material for many types of industrial applications. Currently the U.S. imports castor seed oil as its chief source of HFA, used primarily in the production of lubricants and plastics. Lesquerella could complement castor oil imports as well as provide new applications and products. Lesquerella meal is being tested as a feed source. Other lesquerella species contain varying amounts and types of HFA. L. fendleri appears to be highly cross-pollinated, and as a consequence, considerable genetic variation exists for traits such as seed oil content, HFA, yield, flowering time, and growth habit. Rapid progress is being made toward full commercialization of lesquerella with private companies and government agencies working together in parallel studies on yield improvement and on finding new uses for the seed oil. Cooperative plantings began in 1990 and included 12 ha in central Arizona and Texas. Planting for 1991 has been increased to 30 ha and covered a wider range of climatic and soil conditions. The seed oil from these plantings is being used to formulate and test lubricant and cosmetics products. Agronomic management practices being investigated include planting methods, time of harvest, and irrigation scheduling. Fortunately, existing farm equipment with minor modification can be used for planting and harvesting the crop. Genetic and germplasm improvement studies are also being conducted and covers germplasm evaluation, selection, and hybridization.

Growth and yield characteristics of Lesquerella fendleri as a function of plant density

Abstract There is little information available regarding the response of Lesquerella fendleri to planting density. The effects of plant density on growth, yield, and yield components of L. fendleri were studied in central Arizona at plant populations of 250u2008000, 500u2008000, 750u2008000, 1u2008000u2008000, and un-thinned controls of 1u2008658u2008000 plants/ha in 1992–1993 and 1u2008500u2008000 plants/ha in 1993–1994. In the 1992–1993 tests, increasing density resulted in a significant decrease in plant width and number of branches, while causing an increase in taproot length in populations up to 750u2008000 plants/ha. There were no effects on seed size; however, density significantly reduced the number of seeds/silique for the 750u2008000 and 1u2008658u2008000 plants/ha treatments. Seed yield/plant and harvest index/plant were significantly reduced with increasing density, but yield/ha increased with density up to 1u2008000u2008000 plants/ha. At 1u2008658u2008000 plants/ha, shattering was excessive and resulted in loss of seeds. The greatest seed yields/ha were obtained at 750u2008000 plants/ha and 1u2008000u2008000 plants/ha densities. The oil content did not vary significantly from 500u2008000 to 1u2008658u2008000 plants/ha, but treatments in this range produced seeds with significantly higher oil content than the 250u2008000 plants/ha treatment. Differences in oil yield/ha reflected differences in seed yield/ha. The 1993–1994 harvest did not show significant differences for seed and oil yields among density treatments. Higher temperatures in 1993–1994 were believed to increase the oil content and seed yield in the higher plant densities. Plant densities of 750u2008000 and 1u2008000u2008000 plants/ha are recommended for lesquerella growing areas comparable to central Arizona. At these densities, maximum seed yields should be obtained even with annual temperature fluctuations. Also, maximum plant heights were obtained at these densities, minimizing seed loses during combine harvest.

Lesquerella growth and selenium uptake affected by saline irrigation water composition

Abstract A greenhouse study was conducted to determine the effects of waters differing in salt composition on growth and selenium (Se) accumulation by lesquerella (Lesquerella fendleri Gray S. Wats.). Plants were established by direct seeding into sand cultures and irrigated with solutions containing either (a) Cl− as the dominant anion or (b) a mixture of salts of SO42− and Cl−. Four treatments of each salinity type were imposed. Electrical conductivities of the irrigation waters were 1.7, 4, 8, and 13 dS m−1. Two months after salinization, Se (l mg l−1, 12.7 μM) was added to all solutions as Na2SeO4. Shoot growth was significantly reduced by increasing Cl-salinity. Regardless of salinity type, concentrations of Ca2+, Mg2+, Cl−, total-S, and Se were higher in the leaves than the stems, whereas K+ and Na+ were higher in the stem. Leaf-Se concentrations were not significantly affected by Cl-based irrigation waters, averaging 503 mg Se kg−1 dry wt across salinity levels, whereas leaf-Se decreased consistently and significantly from 218 to 13 mg kg−1 as mixed salt salinity increased. The dramatic reduction in Se was attributed to SO42−:SeO42− competition during plant uptake. The strong Se-accumulating ability of lesquerella suggests that the crop should be further evaluated as a potentially valuable phytoremediator of Se-contaminated soils and waters of low to moderate salinity in areas where the dominant anion in the substrate is Cl−.

Evaluation of new Lesquerella and Physaria (Brassicaceae) oilseed germplasm

The seed oil of Lesquerella and the closely related genus Physaria (Brassicaceae) is rich in hydroxy fatty acids (HFAs). HFAs and their derivatives are used to produce a variety of industrial products including lubricants, nylon-11, plastics, drying agents, protective coatings, surfactants, cosmetics, and pharmaceuticals. Lesquerella fendleri is being developed as a new crop for arid regions of the southwestern United States as an alternative source of HFAs. Between 1995 and 2001, 66 accessions from 28 species of Lesquerella were collected in the United States, 33 accessions from four species were collected in Mexico, and 41 accessions from 15 species of Physaria were collected from the southwestern United States. Mean seed mass ranged from 0.54 to 2.30 mg for Lesquerella compared to 1.70 to 5.80 mg for Physaria. Seed oil content ranged from a high of 32.2% in Lesquerella to a high of 35.4% in Physaria. The fatty acid profile of all species of Physaria and most of the lesquerolic-acid-rich species of Lesquerella contained from 30 to 55% lesquerolic acid, although several species contained >60%. These collections of wild germplasm provide a diverse gene pool that should enhance our breeding program in developing a domestic source of HFAs.

Collection and evaluation of new Lesquerella and Physaria germplasm

Lesquerella and the closely related genus Physaria are prime candidates as a new, alternative source of hydroxy fatty acids (HFA) for industrial uses. At present, castor oil is the only natural source of these HFA for commerce. To expand the germplasm base of these taxa for our breeding program and the National Germplasm System (NPGS), we collected species from a wide geographic region in the United States. In 1993, 44 accessions of Lesquerella fendleri, 38 accessions from nine other Lesquerella species, and three accessions of two different Physaria species were obtained. In 1994, 41 additional accessions of L. fendleri and 51 accessions of ten other Lesquerella species were collected. A total of 20 different species of Lesquerella and two Physaria species were collected over the two years. Oil characteristics and seed size of eight of these Lesquerella and one Physaria species have not previously been reported. Twelve of these taxa have not been available in the NPGS. Populations of L. fendleri collected in Arizona had higher seed-weights than those collected from Texas and New Mexico. Some of the new accessions had seed-weights higher than germplasm presently in the USDA-ARS breeding program. Diverse growth habits were found in populations from all three states. We believe that other species of Lesquerella and Physaria could later be developed as a source of HFA in regions other than the arid southwestern United States. Accessions of Lesquerella douglasii had the same or higher seed-oil content than L. fendleri and also higher seed yields. Two accessions of Physaria newberryi had seed-weights that ranged between 4.5 and 6.2 g/1000 seeds compared to that of 0.25 to 1.1 g/1000 seeds for L. fendleri. Seed-oil contents of P. newberryi averaged 30.8% compared to 23.8% for L. fendleri. Preliminary seed increase and evaluation of oil content and composition were completed on 14 L. fendleri populations from the 1993 collection under field conditions in Phoenix, Arizona. Accessions were compared to the original populations collected in 1993 and 1994 for growth habit, seed size, and seed-oil content and quality.

Characterization of Vernonia galamensis germplasm for seed oil content, fatty acid composition, seed weight, and chromosome number

Abstract Vernonia galamensis (Cass.) Less. has potential for domestication as a new industrial oilseed source of natural epoxy fatty acids. Thirty-four accessions from the six subspecies of the Vernonia galamensis complex were characterized for seed weight, seed oil content, and fatty acid composition. The mean seed weight of the six subspecies was 3.42 g/1000 seeds, with a range of 2.46 g to 5.43 g for ssp. mutomensis and ssp. afromontana, respectively. The mean seed oil content for the six subspecies was 30.8%, ranging from 25.1% to 39.2% for ssp. mutomoensis and ssp. lushotoensis, respectively. The 18:1 epoxy fatty acid (vernolic acid) was predominant. The mean vernolic acid content for the six subspecies was 72.5%, and ranged from 66.9% to 76.6% for ssp. mutomoensis and ssp. lushotoensis, respectively. Within the four varieties of ssp. galamensis, in which most of the current domestication effort is centered, oil contents ranged from 31.8% to 38.4%, and 18:1 epoxy fatty acid ranged from 68.0% to 77.0%. Mean levels of other fatty acids within the species were about 14% for linoleic acid (18:2), 7% for oleic acid (18:1), and from 2 to 3% for both palmitic acid (16:0) and stearic acid (18:0). The basic chromosome number for Vernonia galamensis was found to be n = 9. It was concluded that selection for improved levels of both oil and vernolic acid contents should be possible within a germplasm enhancement and plant breeding program.

Lesquerella fendleri seed oil content and composition: Harvest date and plant population effects

Abstract Yield losses of Lesquerella fendleri L. seeds, caused by shattering and seed oil content reduction by excessive desiccation could be minimized by the appropriate choice of harvest dates. However, no information is available on harvest dates to maximize economic returns at any planting density. This research was undertaken to study the effects of harvest dates and plant populations on oil yield and composition in L. fendleri. Mechanically seeded field plots of L. fendleri were thinned at 103 days after sowing (DAS) to density treatments of 250,000, 500,000, 750,000, 1,000,000, and an nonthinned control of 1,500,000 plants/ha. These plants were grown in 1993–1994 at The University of Arizona, Maricopa Agricultural Center in central Arizona, and were harvested at 203, 229, 243, and 264 DAS. Eight fatty acids, which make up 87–96% of mature seed oil, were investigated. These included palmitic (C16: 0), palmitoleic (C16: 1), stearic (C18: 0), oleic (C18: 1), linoleic (C18: 2), linolenic (C18: 3), lesquerolic (C20: 1-OH), and auricolic (C20: 2-OH) fatty acids. Generally, seed at the earliest harvest of 203 DAS had lower concentrations of palmitoleic, stearic, oleic, lesquerolic, and auricolic acids, and lower total oil content than those harvested at 229, 243, and 264 DAS. Also at 203 DAS, the 750,000 plants/ha treatment produced the highest fatty acid content yield, although both this oil and that from the 1,000,000 plants/ha treatment lacked auricolic acid. During this period, the major contributing fatty acids to oil content were oleic, linoleic, linolenic, and lesquerolic. Harvest at 229 DAS resulted in an increase in oil content for all treatments except the 750,000 plants/ha. Lesquerolic acid was the dominant oil for the nonthinned control. At 243 DAS, higher oil contents were observed in the 250,000, 750,000, and 1,000,000 plants/ha densities, with lesquerolic acid again the dominant fatty acid. At final harvest (264 DAS), lesquerolic acid content was virtually the same among the density treatments. No significant differences for seed yield, fatty acid composition or content were found among plant populations. The seed yield averaged over all population densities was 1344 kg/ha with a fatty acid content of 21.9%, giving an oil yield 267 kg/ha (calculated as total fatty acid content).

Initial selection and breeding of Lesquerella fendleri, a new industrial oilseed

Abstract Considerable research and developmental efforts are being devoted by both the public and private sectors to commercialize Lesquerella fendleri as a new industrial oilseed crop for the production of hydroxy fatty acids. Selection and breeding research initiated in 1984 at the USDA-ARS, US Water Conservation Laboratory, Phoenix, Arizona has made progress in increasing seed, oil and lesquerolic acid yields. Yields of 1700 kg/ha of seed, 30% oil and 59% lesquerolic acid have been obtained in replicated trials. Single plant selections developed during the interium are traceable to two populations: one to a single collection made in Arizona, PI 331165, and the other to nine different accessions collected from various sites in Texas. Variation among selections within each population exhibited relatively the same amount of genetic variability. Neither population appeared to be superior as a source of variation. The need for acquisition and utilization of new sources of germplasm is indicated. Topcross yield performance of six selected lines appeared to be superior to that of comparable halfsib family progeny, validating the existence of heterosis, which is expected due to the high degree of natural cross pollination observed. These data should serve as a benchmark upon which future progress in varietal and population development can be measured.