Comparative Study on Resistance of C₃ and C₄ Xerohalophytes of the Genus Atriplex to Water Deficit and Salinity

The C₃ species Аtriplex verrucifera M. Bieb and the C₄ species A. tatarica L. were compared in terms of their photosynthetic activities, plant growth rates, and the content of water, Na⁺, K⁺, and proline under water deficit and salinity conditions caused by polyethylene glycol (PEG) and NaCl solutions having equal osmotic potentials (–0.6 MPa). Nonstressed A. tatarica plants accumulated twofold larger biomass compared with A.verrucifera; they also featured a tenfold higher amount of proline and were superior in photosynthetic water use efficiency. Compared to A. tatarica, А. verrucifera plants were more resistant to osmotic stress: they exhibited normal growth rates at low osmotic potential of the medium, retained their water and K⁺ content in tissues, and preserved the rates of dark respiration and water use efficiency. Despite the attribution to C₄ plants, A. tatarica showed low resistance to PEG-induced osmotic stress and accumulated significant amounts of proline, whereas the C₃ species accumulated proline only under excess of Na⁺, i.e., under salinity. The principal component analysis of various parameters (content of proline, water, K⁺, Na⁺, and the K⁺/Na⁺ ratio) revealed the factors significant for adaptation of the examined plant species to water stress. In A. tatarica, the main role belongs to proline, Na⁺, and water, whereas this role is played by Na⁺ and proline in А. verrucifera. Potassium ions proved to be an indispensable component of adaptive mechanisms in both plant species. Under salt stress, the C₃ species accumulated proline, which served as an antioxidant and osmoprotector and also balanced the cell osmotic potential. The examined C₄ species accumulated the osmolyte proline under PEG-induced water stress. The results provide evidence that representatives of the genus Atriplex employ different strategies of adapting to water deficit and salinity.