2.3 Determination index and method
1) Photosynthetic traits: After the experiment, at 10:00 pm on July 12, 2019, we selected 2–3 mature leaves from the same node of each plant to measure their physiological traits with a Li-Cor 6400 Portable Photosynthesis system (LI-6800, USA). The main observed photosynthetic characteristic indexes were net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO2 concentration (Ci). The following conditions were maintained during the measurements: CO2 concentration: 360–400 μmol·mol-1; flow rate: 500 μmol·s-1; leaf temperature: 23 ± 0.4 °C; relative humidity: 60 ± 5%; light illumination: 750 μmol‧m-2·s-1.
2) Water use efficiency (WUE): The WUE was recorded using the following formula (Pn and Tr were defined in the previous section):
\begin{equation} WUE=P_{n}/T_{r}\nonumber \\ \end{equation}
3) Leaf water content (LWC): This was obtained by a conventional weighing method.
At the end of the experiment, the leaves of the four species were removed and they were kept in an oven at 80 °C for 24 h to record their dry weight. The LWC was recorded using the following formula:
\begin{equation} LWC=\left(\frac{W_{\text{fresh}}-W_{\text{dry}}}{W_{\text{fresh}}}\right)\times 100\%\nonumber \\ \end{equation}
4) Aboveground and belowground biomass (AGB and BGB): After the determination of photosynthetic traits, the aboveground branches and leaves of the plants were separated from the underground roots, and the roots and soil were separated and washed clean with a 60 target quasi-soil screen (0.28 mm aperture). Finally, the aboveground and underground parts were placed in an oven at 65 °C and dried to constant weight. The ABG and BGB of the plants was obtained by weighing them with an electronic balance with an accuracy of 0.001 g.