Root nodulation in guar: Effects of soils, Rhizobium inoculants, and guar varieties in a controlled environment

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.