Photosynthetic Responses of Cup Plant (Silphium perfoliatum L.) to Salinity Stress in the Context of Sustainable Biomass Production

Soil salinity is recognized as a critical abiotic stress that limits plant growth on marginal lands. The cup plant (Silphium perfoliatum L.), a perennial bioenergy species with high biomass potential, has been proposed for cultivation on saline-degraded soils; however, its physiological responses to different types of salinity stress, particularly alkaline and neutral salt stress, remain insufficiently characterized. In the present study, the physiological responses of the cup plant to neutral (NaCl) and alkaline (NaHCO3) salt stress at concentrations of 100, 200, and 300 mM were evaluated in a pot experiment conducted under controlled conditions. The assessed indicators included relative chlorophyll content (CCI), chlorophyll fluorescence parameters (Fv/Fm, Fv/F0, PI), and gas exchange characteristics, namely net photosynthetic rate (PN), stomatal conductance (gs), transpiration rate (E), and intercellular CO2 concentration (Ci). Salinity reduced most physiological parameters, although some, such as maximum photochemical efficiency of PSII (Fv/Fm) and transpiration rate (E), did not show a clear dose-dependent response. Alkaline salt stress induced more pronounced reductions in the physiological parameters than neutral salt stress. At the first measurement, at the highest salt concentration, the chlorophyll content decreased by 49.0% and the PN parameter by 77.8% under NaHCO3 treatment, whereas under NaCl conditions the decreases were 29.0% and 51.3%, respectively, compared to the control. At 300 mM NaHCO3, the chlorophyll content and photosynthetic rate were substantially reduced compared with those recorded under the corresponding NaCl treatment. Even at the moderate salinity level of 100 mM NaHCO3, reductions in photosynthetic performance were detected relative to the control. Overall, photosynthetic efficiency and gas exchange in the cup plant were markedly impaired by salinity, particularly under conditions of high bicarbonate concentration. The results offer a deeper understanding of the physiological limitations of S. perfoliatum under acute salt stress and demonstrate that alkaline salinity, associated with elevated pH due to HCO3−, exacerbates stress effects beyond the osmotic and ionic impacts of neutral salinity. These results highlight the potential of S. perfoliatum for sustainable biomass production on salt-affected soils, supporting renewable energy generation and environmentally responsible land use.