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Background and aims Populus bolleana Lauche. (P. bolleana) and Populus euphratica Oliv. (P. euphratica) separately survive in mild and moderate alkaline soil conditions. The aim of this study was to explore the underlying mechanism for the different alkaline tolerance in the two poplar species.

Methods Young saplings of two poplar species were grown in moderate alkaline soil, and the young and old leaves of the two poplars were separately analyzed by ion concentration, allocation and distribution, transcript variation of different genes involved in ion transport and nitrogen assimilation, nitrogen metabolism, organic acid, leaf pigments, and redox responses.

Results Excess Na+ under alkali stress was mainly allocated to old leaves in P. bolleana. However, excess Na+ was allocated to both young and old leaves in P. euphratica, and was balanced by enhanced levels of Mg2+, Ca2+, and SO4 (2-), with no change in oxidative parameter. The reduction of nitrate nitrogen occurred under alkali stress in both species; P. euphratica acclimated to alkali stress by more flexible regulation of N metabolism and nitrate absorption than P. bolleana.

Conclusions Our results strongly indicated different alkali tolerance mechanisms in P. bolleana and P. euphratica. P. bolleana protects young tissues via profound accumulation of Na+ and confining damage effects into the old leaves under alkali stress, while P. euphratica can effectively compartmentalize excess Na+, keep its ion balance, and adjust nitrogen transport and metabolism in both young and old leaves to avoid alkali damage.