Genotypic Variation in Nutrient Selectivity in Populus under NaCl Stress

We used a salt-resistant poplar genotype Populus euphratica and two salt-sensitive genotypes, Populus ‘popularis 35-44’ (P. popularis) and the hybrid P. taiassica Kom×(P. euphratica + Salix alba L.) to examine genotypic differences in nutrient selectivity under NaCl stress. One-year-old seedlings of P. euphratica and one-year-old hardwood cuttings of P. popularis were used in a short-term study (24 hours), while in a long-term study, up to 4 weeks, two-year-old seedlings of P. euphratica and the hybrid P. taiassica Kom×(P. euphratica + Salix alba L.) were compared. In the short-term study, K⁺ concentration in the xylem sap (K⁺_xyjem) of P. euphratica significantly increased after salt stress was initiated, and maintained 1-2 fold higher than control levels during the period of salt stress (24 hours). Xylem Ca²⁺ and Mg²⁺ concentrations (Ca²⁺_xyiem,Mg²⁺_xyiem) in P. euphratica resembled the pattern of K⁺ despite a lesser magnitude in elevation. However,Ca²⁺_xyIcni andMg²⁺_xyIem in P. popularis exhibited a transient increase at the beginning of salt treatment, thereafter, they all returned to control levels at 4 hours and no further rise was observed in the following hours. Xylem Na⁺/K⁺, Na⁺/Ca²⁺ and Na⁺/Mg²⁺ in P. popularis increased sharply upon NaCl stress and steadily reached the maximum at 24 hours. In contrast, xylem Na⁺/K⁺, Na⁺/Ca²⁺ and Na⁺/Mg²⁺ in NaCl-treated plants of P. euphratica did not significantly increase during the period of salt stress (24 hours). Noteworthy, Na⁺/K⁺ markedly declined after the onset of stress. These results suggest that P. euphratica had a higher nutrient selectivity in face of salinity. A same trend was observed in a 4-week study. Xylem Na⁺/K⁴, Na⁺/Ca²⁺ and Na⁺/Mg²⁺ in salinised plants of the hybrid abruptly increased after 4 days of stress, and then continuously increased to reach the highest level at day 8 or day 15. In comparison, the magnitude of Na⁺/K⁺, Na⁺/Ca²⁺ and Na⁺/Mg²⁺ elevation in the xylem of P. euphratica was much lower during the observation period. In conclusion, salt-tolerant genotype P. euphratica maintained a higher nutrient selectivity under saline stress, as compared to the two salt-sensitive genotypes. The high capacity for nutrient uptake and transport presumably contributes to the salt tolerance of P. euphratica in a longer term.