Threshold effect of the groundwater depth on the photosynthetic efficiency of Tamarix chinensis in the Yellow River Delta

Aims Groundwater is a major factor that affects the growth and distribution of Tamarix chinensis in the Yellow River Delta (YRD). The response relationship between the photosynthetic physiological processes of T. chinensis and the depth to groundwater (DGW) remains unclear. This paper aims to elucidate the response pattern of photosynthetic efficiency to the DGW and to determine the suitable DGW for the growth of T. chinensis. Methods Three-year-old T. chinensis, a dominant species in the YRD, was used as the research object, and seven DGW treatments of 0-1.8m under low salinity conditions were simulated to determine and analyse the biomass of T. chinensis, the light responses of leaf gas exchange parameters, and the photosynthetic productivity classification under different DGW treatments. Results There were significant threshold effects of the DGW on the net photosynthetic rate (P-n), transpiration rate (T-r), and water use efficiency (WUE) of T. chinensis leaves, indicating that photosynthetic parameters had significant DGW threshold effects. Different DGW treatments significantly changed the water and salt conditions in the T. chinensis-planted soil column, thereby significantly affecting T. chinensis growth, photosynthetic capacity, and WUE. With increasing the DGW, the soil relative water content and soil salt content gradually decreased, while the soil solution absolute concentration and T. chinensis biomass first decreased and then increased, followed by another decrease. The growth and photosynthetic capacity of T. chinensis at a high DGW (>= 1.2m) were significantly higher than those at a low DGW (<= 0.6m). At the medium DGW of 0.9m, the biomass, photosynthetic capacity, water consumption associated with transpiration, and water and light use efficiencies of T. chinensis all reached their highest values. When the DGW was increased from 0.9m to 1.8m or decreased from 0.9m to 0.3m, the reduction of P-n in T. chinensis changed from a stomatal to a non-stomatal limitation. At the shallow DGW (0.3m) and the deep DGW (1.8m), the reduction in P-n was largely affected by photosynthetically active radiation. The 1.03-1.44m DGW was referred to as the "high-yield and high-efficiency DGW", at which T. chinensis exhibited good growth, high photosynthetic capacity, and high-efficiency physiological water use characteristics. Conclusions Differences in the DGW changed the soil water content and salt content, and the soil solution absolute concentration was a key factor that affected the growth and photosynthetic efficiency of T. chinensis. However, soil drought stress at the deep DGW and soil salt stress at the shallow DGW were the main factors that affected the photosynthetic physiological processes of T. chinensis. T. chinensis exhibited adaptability characteristics of adequate tolerance to drought stress but intolerance to salt and flooding stresses.