AgriLife study sheds light on nodulation in legume crop guar

Texas A&M AgriLife scientists are conducting several research projects to improve producers’ understanding of guar and the legume’s value to their operations in the Rolling Plains and South Plains.

Guar has been grown in Texas for more than a century, but acreage of the crop in the state is relatively low, said Curtis Adams, Texas A&M AgriLife Research crop physiologist in Vernon.

Lack of nodulation on guar roots is one of the producer concerns addressed in a recent AgriLife Research study by Adams and Calvin Trostle, Texas A&M AgriLife Extension Service agronomist in Lubbock, along with Santanu Thapa, AgriLife Research postdoctoral research associate in Vernon.

Nodulation is the process of forming nodules on the roots of legume plants. Nodules are root structures that legumes make to house bacteria capable of using nitrogen gas from the air to form fertilizer that the plant can use to grow.

The team conducted a controlled environment study to compare the impact of environmental and management factors on guar nodulation and crop nitrogen uptake, Adams said.

Guar is grown in semi-arid regions and produces a seed containing galactomannan gum, which is a product used in a variety of food and industrial applications as a lubricant, binder, thickener or hardener, he said.

“As a legume, Rhizobium bacteria in the soil will associate with guar roots and potentially develop nodules where the bacteria converts atmospheric nitrogen into fertilizer for the plant and soil,” he said, adding that “the plant is also drought tolerant and uses relatively little water.”

Thapa said guar is unfamiliar to most people, but it is a part of their lives nonetheless.

“Guar gum is a common ingredient in the food we eat every day,” he said. “It is used extensively in oil and gas exploration, and in so many other ways.”

The majority of the world’s guar is grown in India and Pakistan, and the U.S. has had variable and relatively low acreage over time, Trostle said. In the U.S., guar is mostly grown across the Southern Great Plains region where the climate is suitable.

“Guar being a legume and adapted to a semi-arid region’s dryland agriculture is important,” Trostle said. “There are few legumes that would be adapted in this type of environment. That is why this work is especially important, to get potential nitrogen fixation in a legume rotational crop where it doesn’t rain a lot.”

Adams said despite the potential nitrogen benefits of the crop, there is a worldwide perception that guar does not nodulate effectively.

“So, we tested the effects of contrasting soils, a sandy loam and a clay loam, and Rhizobium inoculants on nodulation and plant growth in two guar varieties,” he said.

Although Rhizobia bacteria often occur naturally in soils, Rhizobium inoculants are crop-specific bacterial cultures prepared in the lab and applied to the seed or in-furrow at planting to increase the likelihood of root nodulation, Adams said.

He said because guar acreage is not large in the U.S., there is a lack of inoculant products specific to guar.

“In our study, we tested one commercially available inoculant and a custom inoculant prepared by a microbiologist colleague, both containing bacterial strains thought to create nodules on guar roots that fix nitrogen,” he said.

Thapa said two iterations of the 50-day study were run in 2017. Plant growth, plant nitrogen concentration, measures of yield potential, root nodule number, nodule weight and other parameters were determined.

“The results of this study clearly showed in different soils that guar is capable of producing plenty of nodules,” Adams said. “The soils we tested are representative of the semi-arid soils on which guar is produced around the world.

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“We saw very different nodule characteristics in each soil, with a high number of nodules of low weight in the clay loam soil and low number of nodules with high weight in sandy loam. In the end, the amount of nitrogen supplied to the plants was similar between soils.”

The difference in nodule characteristics between the soils may have resulted from differences in Rhizobia population already in the soil, Adams said.

The study showed no effect of the inoculants on the number or size of nodules or plant nitrogen uptake, Thapa said.

“We are not sure why the inoculants had no effects, but it likely has to do with survival or competitiveness of the inoculant bacteria, or naturally occurring levels of Rhizobia in the soil,” he said.

“Based on the results of this study, we expect guar will nodulate and supply nitrogen in the field, as long as the conditions are right,” Adams said. “Factors like drought or low soil levels of Rhizobia bacteria could prevent nodulation.”

But little is known about the effects of external factors on guar nodulation, he said, so there are still many questions to answer.

Trostle said recent discussion with an inoculant manufacturer may provide AgriLife the opportunity to work with experimental products to expand biological nitrogen fixation in semi-arid dryland agriculture.

Trostle said four additional federally funded projects, three led by AgriLife Research, are aimed at providing more information for producers on guar in relation to guar agronomics, wheat rotation, plant breeding/adaptation and bioenergy.

And, he said, if the production builds up, Texas growers of guar seed have a market in Brownfield. The guar is processed into several different products, either for supplying specialty manufacturers who do their own refining or direct use in commercial products.