Growth and Leaf Gas Exchange in Populus euphratica across Soil Water and Salinity Gradients

Desert riparian forest in the Tarim River Basin, northwestern China, is an important ecosystem that provides valuable services. Unfortunately, this valuable ecosystem has undergone a great decline in the past decades, primarily as a result of flow regime alterations (such as a reduction of flooding and groundwater decline in riparian zones) due to human activities and/or climate change. Therefore, the evaluation of riparian plant response to soil water and salinity gradients is important for understanding the ecosystem decline and further for any restoration effort.

Populus euphratica responses to stress have attracted much attention possibly due to the ecosystem degradation over the past decades. However, the adaptive strategies and tolerance of P. euphrarica seedlings to drought and salinity stresses are not well understood.

To assess the morphological and physiological responses of the P. euphratica seedlings to soil water and salinity gradients, and determine the critical water and salinity levels that allow the seedlings to grow, researchers measured the growth and leaf gas exchange of 2-year-old P. euphratica seedlings across soil water and salinity gradients under controlled conditions.

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 (F_v/F_m), the effective quantum-use efficiency of PSII (F_v'/F_m'), and photochemical quenching (qP) under mild (18% soil water content, SWC; 18.3 g/kg soil salt content, SSC) and moderate (13% SWC, 22.5 g/kg 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 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).

The 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 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, the seedlings suffered from the severe stress. The result was published in Photosynthetica in September 2013.