Effects of drought stress on the leaf photosynthetic characteristics and chlorophyll fluorescence of sainfoin seedlings

To explore the effects of drought stress on the responses and adaptability of leaf photosynthesis and chlorophyll fluorescence in sainfoin (Onobrychis viciaefolia) seedlings, twenty different sainfoin seedling genotypes, derived from national and international strains, were used in this study. Water deficit stress was simulated with PEG-6000 at four osmotic potentials (−0.5, −1.0, −1.5 and −2.0 MPa, respectively), alongside a nil-PEG (CK) control treatment. Corresponding results demonstrated that drought stress inhibited the plant height of sainfoin seedlings. Additionally, with increasing drought stress, chlorophyll content, carotenoid content, transpiration rate, net photosynthetic rate, and stomatal conductance were reduced, while water use efficiency was improved. Initially, the stomatal limitation value increased with increasing drought stress, and then decreased, with a maximum value being observed at −1.0 MPa. Intercellular carbon dioxide concentration initially decreased and then increased, reaching a maximum value at −2.0 MPa; this indicated that non-stomatal factors limited photosynthesis. Although the non-photochemical quenching coefficient increased, the actual photochemical efficiency, maximum photochemical efficiency, electron transport rate, photochemical quenching coefficient, and photochemical quantum yield decreased with increasing drought stress. The comprehensive evaluation of the results based on the membership function value demonstrated that the novel P1, Onobrychis viciaefolia ‘Gansu’ sainfoin line and the 1626 genotype possessed the strongest drought tolerance, whereas genotypes 1668, 6, and 2049 possessed the weakest drought tolerance. Therefore, P1, Onobrychis viciaefolia ‘Gansu’ and 1626 sainfoin lines could be used for a variety of regional and production analyses in the future.