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

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 (A_N–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 (A_N) statistically peaked with 1500 μmol (photons) m ⁻² s ⁻¹, though the maximum A_N [33.29 μmol (CO₂) m⁻² s⁻¹] was modeled to occur with 1950 μmol (photons) m ⁻² s ⁻¹. The light compensation point (I_comp), dark respiration rate (R_D), and maximum quantum yield (Ф(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, A_N differed only between two of the 44 genotypes tested, corresponding to the minimum and maximum A_N values. There were no relationships between A_N and most plant growth parameters. This finding suggested there is low potential to use point measurements of A_N as a selection parameter for increased guar productivity.

KEYWORDS:

• Clusterbean
• crop improvement
• crop physiology
• drought stress
• legume

Acknowledgments

We acknowledge Tom Wacek, formerly of Urbana Laboratories, for supplying the custom Rhizobium inoculant that was used in these studies. We thank John Ray Cunningham for maintenance of the greenhouse facility in which the studies were conducted. We acknowledge the Germplasm Resources Information Network (GRIN) of the USDA-ARS for supplying the seeds for this project.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was funded by the USDA National Institute of Food and Agriculture (NIFA), Agricultural and Food Research Initiative (AFRI) Competitive Program grant #2018-67019-27873, and NIFA Hatch Project #1011694.