Li 6400 01

We measured A_max, g and E parameters using a LI-COR 6400XT infrared gas analyser (IRGA; LI-COR Biosciences, Lincoln, NE, USA). All measurements were conducted using six seedlings of each species under each treatment from 11:00 to 14:00 h on August 17, 2014, which was a sunny day. Light was supplied by an LED lamp (LI-6400-02B, LI-COR Biosciences), and the light levels were kept at 1500 μmol·m⁻² s⁻¹, which is above the light saturation point for all four species according to the light response curves before the measurements. The external CO₂ was provided by portable tanks containing a CO₂/air mixture with a concentration of 400 μmol·mol⁻¹. A CO₂ injector (LI-6400-01, LI-COR Biosciences) was used to control the output of the tanks. The temperature was maintained at 25–28 °C, and the relative humidity was kept at 25–30%. Then, we divided A_max by E to calculate the instantaneous water use efficiency (WUE_i).
We examined the chlorophyll fluorescence parameters of leaves that had been kept under dark conditions for 30 min in the morning between 05:00 and 06:00 h on August 17, 2014. We also measured the maximum quantum yield of photosystem II (PSII, F_v/F_m) with the formula (F_m − F_o)/F_m, using an LI-6400-40 leaf chamber fluorimeter (LI-COR Biosciences).

Net photosynthesis, Ci, gs, and E of leaves were measured simultaneously by an open gas exchange system (LI-6400-XT, LI-COR, Lincoln, NE, USA) fitted with a leaf chamber fluorometer (6400-40, LI-COR, Lincoln, NE, USA), using equations developed by von Caemmerer and Farquhar (1981) (link). CO₂ concentration was maintained at a constant level of 380 μmol mol^-1 by using a CO₂ injector (LI-6400-01, LI-COR, Lincoln, NE, USA) with a high-pressure liquid CO₂ cartridge source. Air temperature and relative humidity were maintained at 20°C and 40%, respectively. Photosynthetic active radiation was fixed at 200 μmol m^-2 s^-1.