Differences in Ionic, Enzymatic, and Photosynthetic Features Characterize Distinct Salt Tolerance in Eucalyptus Species

In the face of rising salinity along coastal regions and in irrigated areas, molecular breeding of tolerant crops and reforestation of exposed areas using tolerant woody species is a two-way strategy. Thus, identification of tolerant plants and of existing tolerance mechanisms are of immense value. In the present study, three Eucalyptus ecotypes with potentially differential salt sensitivity were compared. Soil-grown Eucalyptus plants were exposed to 80 and 170 mM NaCl for 30 days. Besides analysing salt effects on ionic/osmotic balance, and hydrolytic enzymes, plants were compared for dynamics of light-induced redox changes in photosynthetic electron transport chain (pETC) components, namely plastocyanin (PC), photosystem I (PSI) and ferredoxin (Fd), parallel to traditional chlorophyll a fluorescence-based PSII-related parameters. Deconvoluted signals for PC and Fd from PSI allowed identification of PC and PSI as the prime salinity-sensitive components of pETC in tested Eucalyptus species. Eucalyptus loxophleba portrayed efficient K⁺-Na⁺ balance (60-90% increased K⁺) along with a more dynamic range of redox changes for pETC components in old leaves. Young leaves in Eucalyptus loxophleba showed robust endomembrane homeostasis, as underlined by an increased response of hydrolytic enzymes at lower salt concentration (~1.7-2.6-fold increase). Findings are discussed in context of salinity dose dependence among different Eucalyptus species.