Urea application enhances cadmium uptake and accumulation in Italian ryegrass

Italian ryegrass (Lolium multifolorum Lam.) has a potential phytoextraction capacity for cadmium (Cd), which is considered as the most toxic heavy metal (HM) pollutant in the farmland. The promotion effect of urea application on Italian ryegrass growth has been clarified, while it is not clear whether and how urea application affects Cd accumulation in Italian ryegrass under Cd stress. A 2-year pot experiment was conducted to investigate the effect of urea application on Cd accumulation and related mechanisms by uptake inhibition and kinetics experiments. The results showed that both shoot biomass and Cd concentration under Cd stress were increased by up to 213.37% and 84.74% in 2016 and 38.15% and 47.11% in 2017 after urea application, respectively. The shoot Cd accumulation reached maximum value (910.23 and 630.09 μg pot^-1 in 2016 and 2017, respectively) at the level of 300 kg ha^-1 urea. Superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities were significantly increased by urea application. Compared with control group, urea application significantly improved inhibition ratio of 2, 4-dinitrophenol (DNP) rather than LaCl₃ and Ca²⁺. Cadmium uptake kinetics experiment showed that urea application significantly decreased the K_m value and improved the α value (P < 0.01), but no significant effect on the V_max value (P > 0.05). In conclusion, we proposed that the higher affinity to Cd²⁺ of the membrane transporter after urea application promoted the active uptake of Cd, which contributed to the effective Cd accumulation capacity in Italian ryegrass.