Effects of Salt Stress on Chlorophyll Fluorescence and the Antioxidant System in Ginkgo biloba L. Seedlings

Ginkgo biloba L. (ginkgo) is generally regarded as a tolerant species to environmental stresses. However, its tolerance mechanisms are not well understood, particularly for salt stress. To evaluate the species' physiological responses to salt stress, 3-year-old ginkgo seedlings were exposed to a range of salinity levels (0% to 1.0% NaCl). A significant reduction in maximum (F-v/F-m) and actual (Phi(PSII)) quantum yields of photosystem II (PSII) photochemistry and the nonphotochemical quenching (q(N)) coefficient only occurred in late treatment stages at the salinity levels of 0.6% to 1.0%. As salt concentration increased, the response time and chlorophyll (Chl) fluorescence indices decreased. Overall, the activities of superoxide dismutase (SOD) and peroxidase (POD); contents of catalase (CAT), reduced glutathione (GSH), and flavonoids; and scavenging rate of free radicals enhanced under salinity stress. These data indicate that ginkgo seedlings are tolerant to low salt stress, and enzymatic and nonenzymatic antioxidant systems seem to work synergistically to reduce lipid oxidation under NaCl stress because malondialdehyde (MDA) content did not increase. Correlation and principal component analyses determined that water potential, Chl fluorescence parameters, activities of POD and SOD, contents of CAT and flavonoids, and hydroxyl ((OH)-O-center dot) and diphenyl picrylhydrazyl (DPPH) free radical scavenging capability were sensitive to salt stress. These parameters can be used for in vitro or rapid and nondestructive monitoring of the responses of ginkgo seedlings to salinity stress. It is of significance to understand the tolerance mechanisms of ginkgo to salt stress, reduce the harm of NaCl and other snow-melting agents to ginkgo as shade trees, and develop new salt-tolerant varieties.