Critical effects of gas exchange parameters in Tamarix chinensis Lour on soil water and its relevant environmental factors on a shell ridge island in China's Yellow River Delta

Photosynthetic parameters of the leaves of a three-year-old Tamarix chinensis Lour tree growing on a shell ridge island were assessed under different soil water conditions with a LI-6400 portable photosynthesis system. The objective of the study was to relate the critical effects of the net photosynthetic rate (Pn), transpiration rate (Tr), water use efficiency (WUE) and other physiological parameters of T. chinensis Lour to soil water and to classify its productivity. The direct and indirect effects of environmental factors under different relative water content (RWC) regimes on Pn and Tr were calculated using path analysis. The results indicated that when the RWC was between 36% and 85%, both the net photosynthetic rate (Pn) and transpiration rate (Tr) exhibited a double-peak curve. The decline in Pn was mainly caused by stomatal limitation in morning and non-stomatal limitation in afternoon. The maximum value of Pn (8.3 ± 1.7 mmol m−2 s−1) appeared at an RWC value of 75%. There was significant difference in the response of water use efficiency (WUE) among levels of RWC: diurnal changes in WUE exhibited a modest single-peak curve when the RWC was greater than 49% and were highest (daily average, 2.56 ± 0.68 μmol mol−1) when the RWC was 49%. According to the quantitative relations between the Pn, Tr, WUE and RWC, the suitable RWC for T. chinensis Lour growth is between 36% and 75%. Based on photosynthetic and physiological parameters, soil water availability and productivity was classified and evaluated. An RWC of more than 93% or less than 36% resulted in medium yield and efficiency, and an RWC between 19% and 27% was classified as low yield and efficiency. RWCs ranging from 75% to 85% and 49% to 75% were classified as high yield-medium efficiency and high yield-high efficiency, respectively. Our statistical analysis indicated that photosynthetic active radiation (PAR) was the most important ecological factor affecting Pn and Tr, followed by air CO2 concentration (Ca). With the intensification of water stress, the atmospheric temperature (Tc) evidently restrained Pn and Tr.