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.