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(I 0)) 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.