Growth and leaf gas exchange in Populus euphratica across soil water and salinity gradients

Soil water and salinity conditions of the riparian zones along the Tarim River, northwest China, have been undergoing alterations due to water use by human or climate change, which is expected to influence the riparian forest dominated by an old poplar, Populus euphratica. To evaluate the effects of such habitat alterations, we examined photosynthetic and growth performances of P. euphratica seedlings across experimental soil water and salinity gradients. Results indicated that seedlings were limited in their physiological performance, as evidenced by decreases in their height and biomass, and the maximal quantum yield of photosystem II (PSII) photochemistry (Fᵥ/Fₘ), the effective quantum-use efficiency of PSII (Fᵥ′/Fₘ′), and photochemical quenching (qP) under mild (18% soil water content, SWC; 18.3 g kg⁻¹ soil salt content, SSC) and moderate (13% SWC, 22.5 g kg⁻¹ SSC) water or salinity stress. However, seedlings had higher root/shoot ratio (R/S), increased nonphotochemical quenching (NPQ), and water-use efficiency (WUE) relative to control under such conditions. Under severe (8% SWC, 27.9 g kg⁻¹ SSC) water or salinity stress, P. euphratica seedlings had only a fifth of biomass of those under control conditions. It was also associated with damaged PSII and decreases in WUE, the maximal net photosynthetic rate (P Nₘₐₓ), light-saturation point (LSP), and apparent quantum yield (α). Our results suggested that the soil conditions, where P.euphratica seedlings could grow normally, were higher than ∼ 13% for SWC, and lower than ∼22.5 g kg⁻¹ for SSC, the values, within the seedlings could acclimate to water or salinity stress by adjusting their R/S ratio, improving WUE to limit water loss, and rising NPQ to dissipate excessive excitation energy. Once SWC was lower than 8% or SCC higher than ∼28 g kg⁻¹, the seedlings suffered from the severe stress.