Penstemon, with more than 250 species native to North America, holds signi fi cant aesthetic and ecological value in Utah, supporting diverse pollinators. Despite their signi fi cance, the survival of penstemon is threatened by challenges such as habitat loss, climate change, and Utah ' s naturally high soil salinity. To address these challenges and understand their adaptability, this study evaluated the salt tolerance of two penstemon species [ Penstemon davidsonii (Davidson ' s penstemon) and Penstemon heterophyllus (foothill penstemon)] under controlled greenhouse conditions. The aim was to develop baseline information for nursery production and landscape use that utilize reclaimed water for irrigation. Plants were irrigated weekly with a nutrient solution at an electrical conductivity (EC) of 1.0 dSm - 1 as control or a saline solution at an EC of 2.5, 5.0, 7.5, or 10.0 dSm - 1 for 8 weeks. Half of the plants were harvested after four irrigation events, and the remaining plants were harvested after eight irrigation events. At harvest, visual rating (0 = dead and 5 = excellent without foliage salt damage), plant width, number of shoots, leaf area, shoot dry weight, leaf greenness [Soil Plant Analysis Development (SPAD)], stomatal conductance, and canopy temperature were collected to assess the impact of salinity stress. In both species, salt damage was dependent on the salinity levels and length of exposure. After four irrigation events, both species exhibited foliage damage that increased in severity with rising EC. The most severe damage was observed in plants receiving saline solution at an EC of 10.0 dSm - 1 . After eight irrigation events, P. davidsonii exposed to a saline solution with an EC of 10.0 dSm - 1 received a visual rating of 0, whereas P. heterophyllus had a visual rating of 0.4. Both species exhibited salinity -induced effects, with variations observed in the speci fi c parameters and the degree of response. Penstemon davidsonii exhibited signi fi - cant salinity stress, as indicated by reduced leaf area, shoot dry weight, SPAD reading, and stomatal conductance with increasing EC of the saline solution. In addition, in both species, at both harvests, canopy temperatures increased either linearly or quadratically by 8% to 36% as the EC levels of the saline solution increased. These results indicate that P. davidsonii was more sensitive to salinity stress than P. heterophyllus .

Penstemons are a diverse group of flowering plants valued for their ability to enhance the visual appearance of urban landscapes. Penstemon barbatus (Cav.) 'Rocky Mountain' (rocky mountain beardtongue) are widely used in landscapes, but their tolerance to soil salinity remains poorly understood. This study aimed to investigate the effects of salinity levels at electrical conductivities (ECs) of 1.0 (nutrient solution), 2.5, 5.0, 7.5, and 10.0 dS center dot m-1 on two penstemons (P. barbatus and P. strictus). Penstemons were irrigated with nutrient or saline solution for 8 weeks and various growth and physiological data were recorded before harvest. Salinity stress degraded the visual quality of penstemon species and led to a reduction in the growth rate and biomass production. Leaf burn and necrosis were observed in penstemons because of salinity stress. The visual score of P. barbatus and P. strictus decreased with increasing EC levels in the saline solution. When irrigated with saline solution at an EC of 7.5 dS center dot m-1, Penstemon barbatus and P. strictus had severe-to-moderate foliar salt damage with average visual scores of 1.7 and 2.5, respectively (0 = dead plant; 5 = excellent plant without any foliar damage). The two penstemon species had severe foliar salt damage or were dead when irrigated with saline solution at an EC of 10.0 dS center dot m-1. There were 87% and 92% decreases in the leaf area of P. barbatus and P. strictus, respectively, when irrigated with saline solution at an EC of 10.0 dS center dot m-1 compared with those in the control. Although not statistically significant, there were 7% to 18% decreases in shoot dry weight of P. barbatus when irrigated with saline solutions at ECs of 2.5 to 10.0 dS center dot m-1 compared with control. However, P. strictus displayed declines of 13% to 31% in shoot dry weight as the salinity levels of the irrigation solution increased. As the salinity levels increased, the net photosynthetic rate (Pn), stomatal conductance (gs), and transpiration (E) rates decreased. Furthermore, sodium (Na+) and chloride (Cl-) contents of P. barbatus and P. strictus increased with the increase in salinity levels of the treatment solution. Consequently, P. barbatus and P. strictus demonstrated sensitivity to salinity stress at ECs of 7.5 and 10.0 dS center dot m-1. This study provides important insights for their effective utilization in landscaping practices within saline-prone areas.