Sodium and chloride distribution in roots and transport in three poplar genotypes under increasing NaCl stress

Abstract

We investigated the effects of increasing soil NaCl on growth, water flow, ion transport and intracellular compartmentation of Na + and Cl − in 1-year-old seedlings of Populus euphratica Oliv. and 1-year-old rooted cuttings of P.× euramericana cv. I-214 (cv. Italica) and P. ‘ popularis 35-44’ ( P. popularis). Relative growth rates of leaf area (RGR A) and shoot height (RGR H) of cv. Italica and P. popularis were severely restricted by increasing salinity, whereas in P. euphratica both were not significantly inhibited . Salinised trees of P. euphratica experienced 10% leaf area loss during a 30-day study, however, cv. Italica and P. popularis shed over 50% of their initial surface area. Leaf necrosis of the two salt-sensitive genotypes (cv. Italica and P. popularis) was attributed to excessive salt accumulation and reduced water loss. The rapid built-up of leaf salt in these two genotypes was mainly the result of high ion concentrations in the transpiration stream. An artificially generated lower shoot-to-root ratio, which was reached by removing approximately 50% of total leaf area from shoots of cv. Italica and P. popularis prior to the salt treatment did not enhance their salinity tolerance since root-to-shoot salt fluxes were largely independent of water flow. Compared with cv. Italica and P. popularis, P. euphratica maintained considerably higher unit leaf transpiration rates with lower salt concentrations in the transpiration stream during the period of salt stress. Therefore, salt tolerance of P. euphratica likely depends on its ability to restrict salt transport to leaves. X-ray microanalysis of root compartments showed that there was genotypic difference in the pattern of ion compartmentation. Cv. Italica exhibited a greater capacity to accumulate salt in cortical vacuoles compared with P. popularis, even though both were considered as salt-sensitive genotypes. P. euphratica was more effective than the other two genotypes to block apoplasmic ion transport and sequester Cl − in cortical vacuoles at high salinity. These limited ion loading into the xylem during radial transport, and thus contributed to the restriction of subsequent axial transport.