Photosynthesis in guar: Recovery from water stress, basic parameter estimates, and intrinsic variation among germplasm

Authors: SHRESTHA, RAJAN - Texas A&M University; Adams, Curtis

Submitted to: Journal of Crop Improvements
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2022
Publication Date: 9/12/2022
Citation: Shrestha, R., Adams, C.B. 2022. Photosynthesis in guar: Recovery from water stress, basic parameter estimates, and intrinsic variation among germplasm. Journal of Crop Improvements. 37(5):626-646. https://doi.org/10.1080/15427528.2022.2121348.
DOI: https://doi.org/10.1080/15427528.2022.2121348

Interpretive Summary: Guar is a legume produced in semi-arid climates worldwide, primarily for the galactomannan gum or ‘guar gum’ in its seed endosperm, which is used in many food and industrial applications. Little is known about the photosynthetic physiology of guar. Having more knowledge is important for guar improvement and modeling. To address this, two greenhouse studies were conducted. The photosynthetic rate of guar peaked at 33.29 µmol (CO2) m-2 s-1 a light level of 1500 µmol (photons) m -2 s -1. The respiration rate of guar was 2.62 µmol (CO2) m-2 s-1. Other critical photosynthetic parameters were likewise measured or modeled. Photosynthesis in guar was resilient to water stress. Persistent water stress reduced growth, specific leaf area, and other growth parameters of guar, but the plants had rapid and full recovery of photosynthetic functions when re-watered. Guar varieties differed in their capacity to recover to some degree. There was relatively little variation among guar varieties in photosynthetic rate and photosynthesis was not associated with most plant growth parameters, at least in point measurements. The results of this study expand our understanding of photosynthesis is guar, with applications in variety improvement and plant growth modeling.

Technical Abstract: Little is known about the photosynthetic physiology of guar (Cyamopsis tetragonoloba L. Taub), a legume crop, including how photosynthetic parameters intrinsically vary among germplasm and their recovery from water stress. To address this, two greenhouse studies were conducted: Study-1 to compare photosynthetic light response (AN–I) curves and related parameters in three contrasting guar genotypes under optimal and post-water deficit conditions; and Study-2 to quantify photosynthetic parameters in 44 guar genotypes and explore inter-relationships with plant growth parameters. In Study-1, the mean net photosynthetic rate (AN) statistically peaked at 1500 µmol (photons) m -2 s -1, though the maximum AN [33.29 µmol (CO2) m-2 s-1] was modeled to occur at 1950 µmol (photons) m -2 s -1. The light compensation point (Icomp), dark respiration rate (RD), and maximum quantum yield ('(I0)) were modeled to be 49.9 µmol (photons) m-2 s-1, 2.62 µmol (CO2) m-2 s-1, and 0.0526 µmol (CO2) µmol-1 (photons), respectively. Photosynthesis in guar was resilient to water stress. Despite reductions in growth, specific leaf area (SLA), and other growth parameters, persistently drought-stressed guar plants, on average, exhibited rapid and full recovery of photosynthetic functions when watered. Genotypes differed in their capacity to recover to some degree. In Study-2, AN differed only between two of the 44 genotypes tested, corresponding to the minimum and maximum AN values. There were no relationships between AN and most plant growth parameters. This finding suggested there is low potential to use point measurements of AN as a selection parameter for increased guar productivity.