The uptake and transport of salt ions (Na + , Cl -), macronutrients (K + , Ca 2+ , Mg 2+ ) and abscisic acid (ABA) response to increasing soil salinity were examined in 2-year-old seedlings of Populus euphratica and a hybrid, P. talassica Kom × (P. euphratica + Salix alba L.). Leaf burn symptoms appeared in the hybrid after 8 days of exposure to salinity when soil NaCl concentration increased to 206 mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354 mM. Leaf necrosis was the result of excess salt accumulation since the injury followed an abrupt increase of endogenous salt levels. Compared with the hybrid, P. euphratica exhibited a greater capacity to exclude salt ions from leaves under increasing salinity, especially Cl -. Salt treatment altered nutrient balance of the hybrid, leaf K + , Ca 2+ and Mg 2+ concentrations significantly declined and the same trends were observed in roots with the exception of K + . Although K + levels decreased in salinised P. euphratica, increasing salinity did not affect the levels of Ca 2+ and Mg 2+ in leaves, but did increase the uptake of these nutrients when salt stress was initiated. NaCl-induced increase of ABA concentration in xylem sap [ABA] was observed in the two tested genotypes, however xylem [ABA] increased more rapidly in P. euphratica and a fivefold increase of xylem [ABA] was recorded after the first day of exposure to salt stress. Therefore, we conclude that the increase of Ca 2+ uptake may be associated with the rise of ABA, and thus contributes to membrane integrity maintenance, which enables P. euphratica to regulate uptake and transport of salt ions under high levels of external salinity in the longer term.

The effect of a 20-day NaCl treatment on shoot growth, transpiration, ion uptake and transport, and intracellular ion compartmentation was investigated in regenerated plants of Populus euphratica Oliv. and Populus tomentosa Carr. Plants watered with 100 mM NaCl for 8 days and then 200 mM NaCl for 12 days exhibited soil NaCl concentrations of 60 and 95 mM, respectively. Unit transpiration rates and relative growth rates of P. tomentosa were restricted more by salinity as compared with P. euphratica. Salinized P. tomentosa exhibited leaf necrosis whereas no damage was seen in stressed P. euphratica. Compared with P. tomentosa, P. euphratica had considerably lower rates of net root uptake and transport of salt ions (Na+ and Cl–) to the shoots under salinity. The relatively lower unit transpiration rates of P. euphratica and the lower salt concentrations in the xylem of salinized P. euphratica contribute to its greater capacity for salt exclusion. X-ray microanalysis showed that P. euphratica had a greater ability to restrict radial salt transport in roots by blocking apoplasmic salt transport and sequestering more Cl– in cortical vacuoles. In addition, P. euphratica maintained higher K+ uptake and transport than P. tomentosa in the presence of high external Na+ concentrations.

Stomatal regulation is crucial to reduce water consumption under drought conditions. Extracellular ATP (eATP) serves as a signaling agent in stomatal regulation; however, it is less known whether the eATP mediation of stomatal aperture is linked to apyrases (APYs), the principal enzymes that control the concentration of eATP. To clarify the role of APYs in stomatal control, PeAPY1 and PeAPY2 were isolated from Populus euphratica and transferred into Arabidopsis. Compared with the wild-type Arabidopsis and loss-of-function mutants (Atapy1 and Atapy2), PeAPY1- and PeAPY2-transgenic plants decreased stomatal aperture under mannitol treatment (200 mM, 2 h) and reduced water loss during air exposure (90 min). The role of apyrase in stomatal regulation resulted from its control in eATP-regulated stomatal movements and increased stomatal sensitivity to ABA. The bi-phasic dose-responses to applied nucleotides, i.e., the low ATP (0.3–1.0 mM)-promoted opening and high ATP (>2.0 mM)-promoted closure, were both restricted by P. euphratica apyrases. It is noteworthy that eATP at a low concentration (0.3 mM) counteracted ABA action in the regulation of stomatal aperture, while overexpression of PeAPY1 or PeAPY2 effectively diminished eATP promotion in opening, and consequently enhanced ABA action in closure. We postulate a speculative model of apyrase signaling in eATP- and ABA-regulated stomatal movements under drought.