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Guar (Cyamopsis tetragonolobus L. Taub.) is a leguminous crop plant that produces a seed containing galactomannan gum, which is used in many applications as a lubricant, binder, thickener, and emulsifier, among other functions. A recent spike in worldwide guar consumption has been driven largely by the industrial uses of the gum. As a legume, the primary ecosystem service agricultural producers expect from guar is nitrogen fixation, but the common perception is that guar does not nodulate effectively. The objective of this replicated and repeated greenhouse study was to test the effects of contrasting soils, representing typical alkaline soils in which guar is cultivated worldwide, and Rhizobia inoculants on nodulation and plant growth characteristics in two guar varieties. In the study, abundant nodulation was observed. The soils had contrasting effects on nodulation, with a high nodule number of low weight (497 mg/24.3 nodules) in the clay loam and low nodule number of high weight (583 mg/8.64 nodules) in the sandy loam, but no effect of the inoculants was observed. The difference in nodule characteristics between the soils may have resulted from differences in indigenous Rhizobium population sizes and/or species available to associate with guar roots. The lack of inoculant effects may have resulted from high indigenous levels of Rhizobium in the soil and/or low concentrations of Rhizobium or uncompetitive Rhizobium delivered through the seed-applied inoculants. Based on the results of this study, we expect that guar will nodulate and fix nitrogen effectively in field conditions, given that these processes are not inhibited by exogenous factors, such as water stress. Achieving increases in nodulation and nitrogen fixation above baseline levels may require a concerted effort to develop effective inoculants for guar, which may offer a unique opportunity to expand biological nitrogen fixation in agriculture in semi-arid regions.

Guar [Cyamopsis tetragonoloba (L.) Taub] is a legume grown worldwide, tolerant of hot, dry, and saline conditions. Phenotyping is a key bottleneck in guar improvement, with belowground root nodule phenes being largely overlooked. To address this bottleneck, a greenhouse study was conducted to evaluate phenotypic variability and associations in 14 nodule and plant growth parameters in 50 guar genotypes over 48 days. Nodule mass per plant differed among genotypes and had moderate heritability (P = 0.011, H2 = 0.40), while nodule number did not differ and had low heritability (P = 0.247, H2 = 0.14), and the parameters were not correlated. Nodule mass was correlated with total plant N (r = 0.56), plus most biomass parameters, indicating phenotypic links between plant capacity to support nodulation, N2 accumulation, and plant biomass production. Stem diameter may be useful as an indirect, high-throughput selection trait for increased nodulation in guar, as there was a positive association between stem diameter and nodule mass. Branch number and stem diameter, which have great agronomic significance, had wide phenotypic variability and were highly heritable. Reproductive development was delayed in branched genotypes, which may have implications in breeding guar for environments that differ in growing season length. Despite low heritability, there was a wide range in specific leaf area (SLA) that may be exploited by breeders to optimize drought tolerance and N use efficiency traits. These analyses give novel and confirmatory insights on the nature and associations of phenotypic character traits in guar, particularly nodule traits, providing added tools for breeders in developing desired and beneficial plant ideotypes.

Legume inoculants are products that introduce legume-specific rhizobia bacteria to the soil and promote the process of biological nitrogen fixation. Guar (Cyamopsis tetragonoloba L. Taub.), also known as clusterbean, is a legume produced in semi-arid regions around the world, primarily for industrial uses of guar gum, but there is only one known commercial inoculant products for the crop, which is not available worldwide. The USDA-ARS National Rhizobium Germplasm Collection has four Rhizobium strains listed as compatible with guar: USDA 3089, USDA 3385, USDA 3386, and USDA 3595. The objective of this research was to test the efficacy of four peat-based, seed-applied inoculants derived from these strains, as well as a mixture of all four strains, compared with un-inoculated plants in controlled greenhouse conditions. Among the tested strains, USDA 3089 and USDA 3386 had the greatest positive impacts, as they increased nodule weight, total assimilated nitrogen, biomass production, and protein production per plant relative to the un-inoculated control. USDA 3385 and the Mix likewise increased nodule weight. No inoculant affected other measured aboveground plant morphological and productivity parameters or nodule number. These Rhizobium strains, particularly USDA 3089 and 3386, were identified as beneficial and recommended for further testing in the field in development of an effective inoculant for guar.

Demand for galactomannan gum from guar, commonly known as guar gum, has exponentially increased in the last few decades due to increased use in oil and natural gas, food, cosmetics, paper and other industries. The United States of America is the major importer of the guar gum in the world, which is mainly grown in deserts of India and Pakistan. There is need to increase guar production in non-traditional areas to ensure steady supply and reduce market volatility. Expanding guar acreage into cooler regions of the southern High Plains or in similar agroclimatic regions of the world is an attractive option. Therefore, a study was conducted in incubators to assess genetic variations among currently available guar cultivars for germination and early growth under cooler temperature ranges. Six temperatures (13, 16, 19, 22, 25, and 28 °C) and six guar cultivars (Kinman, Lewis, Matador, Monument, Santa Cruz and Judd 69) were used. Guar cultivars exhibited significant variations for germination percentage, mean germination time and seed vigor index under lower temperatures, which narrowed as the temperature increased closer to optimum. Mean seed germination percentage of cultivars reached maximum at 22 °C. However, speed of germination and seed vigor indices, which are indicators of quick and uniform establishment of guar in the field, continued to increase till 28 °C. Kinman, which was the most promising guar cultivar under cooler temperature, reached maximum germination percent at 16 °C and had the lowest germination time and highest seed vigor index among cultivars below 16 °C. In contrast, Matador reached maximum germination at 26 °C. Germination and early seedling traits indicate that it is better to plant guar in warm soils and if needed to plant in cooler conditions, cultivars like Kinman should be used.

U.S. castor production is limited in part due to only one labeled herbicide (trifluralin) for effective weed control. Field studies were conducted in south Texas and in the Texas High Plains in 2008, 2009, 2012, and 2013 to evaluate castor tolerance and weed efficacy following the application of preemergence (PRE) herbicides. During years of little or poor rainfall, no castor injury was noted following the application of PRE herbicides. However, when rainfall occurred soon after application of PRE herbicides, linuron doses of 1.12 kg/ha or greater resulted in stunting of castor plants and reduced stands when compared with the non-treated control. Acetochlor, clomazone, pendimethalin, and trifluralin consistently caused the least injury to castor plants. However, castor bean yield in many instances was not affected by herbicide injury. Diuron at doses of 1.12 kg/ha or greater and the combination of diuron plus linuron at 0.56 kg/ha or greater effectively controlled (> 80%) smellmelon ( Cucumis melo L. var. Dudaim Naud.) and Palmer amaranth ( Amaranthus palmeri S. Wats), but provided inconsistent control (0–99%) of Texas millet [ Urochloa texana (Buckl.) R.Webster]. Results of these studies indicated that castor tolerance to PRE applications of acetochlor, clomazone, and pendimethalin was excellent, thus providing the greatest opportunity for future use in castor production.