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 ‘popularis35-44' (P. popularis) and the hybrid P. talassica Kom × (P. euphratica + Salix alba L.) to examine genotypic differences in nutrientselectivity under NaCl stress. One-year-old seedlings ofP. euphratica and one-year-old hardwood cuttings ofP. popularis were usedin a short-term study (24 hours), while in a long-term study, up to 4 weeks, two-year-old seedlings ofP. euphratica and the hybrid P.talassica Kom × (P. euphratica + Salix alba L.) were compared. In the short-term study, K+ concentration in the xylem sap ([K+]xylem)of P. euphratica significantly increased after salt stress was initiated, and maintained 1-2 fold higher than control levels during theperiod of salt stress (24 hours). Xylem Ca2+ and Mg2+ concentrations ([Ca2+]xylem, [Mg2+]xylem) in P. euphratica resembled the patternof K+ despite a lesser magnitude in elevation. However, [K+]xylem, [Ca2+]xylem and [Mg2+]xylem in P. popularis exhibited a transient in-crease at the beginning of salt treatment, thereafter, they all returned to control levels at 4 hours and no further rise was observed inthe following hours. Xylem Na+/K+, Na+/Ca2+ and Na+/Mg2+ in P. popularis increased sharply upon NaCl stress and steadily reachedthe maximum at 24 hours. In contrast, xylem Na+/K+, Na+/Ca2+ and Na+/Mg2+ in NaCl-treated plants of P. euphratica did not signifi-cantly increase during the period of salt stress (24 hours). Noteworthy, Na+/K+ markedly declined after the onset of stress. These re-sults 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+/K+, Na+/Ca2+ and Na+/Mg2+ in salinised plants of the hybrid abruptly increased after 4 days of stress, and then continu-ously increased to reach the highest level at day 8 or day 15. In comparison, the magnitude of Na+/K+, Na+/Ca2+ and Na+/Mg2+ eleva-tion in the xylem of P. euphratica was much lower during the observation perio