*2.2. Hydroponic Experimental Design*

Rice seeds (*Oryza sativa* L.) were sterilized with 70% (*v*/*v*) ethanol for 10 min and rinsed three times with deionized water. The sterilized seeds were germinated and grown in vermiculite for 2 weeks prior to the hydroponic experiment. Vermiculite on the root surfaces was gently removed in tap water, and then plants were transferred into a 100 mL glass jar containing half-strength Hoagland's solution (mg/L: 57.52 ammonium phosphate, monobasic; 1.43 boric acid; 328.2 calcium nitrate; 0.04 cupric sulfate·5H2O; 1.68 Na2EDTA·2H2O; 1.3 ferrous sulfate·7H2O; 120.38 magnesium sulfate, anhydrous; 0.91 manganese chloride·4H2O; 0.008 molybdenum trioxide; 303.3 potassium nitrate; 0.11 zinc sulfate·7H2O, PhytoTechnology Laboratories Inc., Lenexa, KS, USA). After a 5-day acclimation period, rice seedlings were exposed to 50 and 250 mg/L C3N4 with or without 10 mg/L As (sodium arsenate, Na3AsO4) or Cd (cadmium chloride, CdCl2); additionally, As and Cd single analyte exposures were established as metal controls. Five biological replicates were established for

each treatment. The plants were grown for 14 days. At harvest, all seedlings were rinsed with deionized water three times to remove the surface-attached C3N4 and As and Cd. The fresh biomasses of shoots and roots across all treatments were recorded, and all tissues were stored at −80 ◦C until further analysis.
