2.3. Trait measurements and calculations
After washing the impurities on the surface of the collected root systems with distilled water, the relevant parameters were measured. The roots of each plant were scanned using the Epson Perfection V850 Pro Scanner (Epson, Los Alamitos, CA USA), then the scanned root images were numbered and stored in a computer. Each image was analyzed using the root analysis system software Win-RHIZO Pro 2013 (Regent Instruments Inc., Canada) to measure total root length (TRL), root surface area (RSA), root diameter (RD), and root volume (RV). After scanning, the root system and the aboveground part were put into different empty envelopes respectively, and dried them to constant weight in an 80 ℃ oven. The dry weight of each part was weighed with the electronic balance (0.1mg). The specific root length (SRL) was calculated as the ratio of total root length to biomass, the root mass fraction (RMF) was root biomass to individual plant biomass, the specific root area (SRA) was surface area to biomass, and the root tissue density (RTD) was biomass to root volume.