Relative growth rate (RGR) showed no significant difference with increasing external Cu concentrations up to 9 mmol L−1 (ANOVA, Tukey test, p > 0.05), but decreased under exposure to 23 mmol L−1 Cu, at which point it was found to be significantly lower than the control (ANOVA, Tukey test, p < 0.05). Relative to the control, the reduction in RGR in the 23 mmol L−1 Cu treatment was 35% ( Fig. 1A). Total leaf area and plant height were also significantly lower than the control in the 23 mmol L−1 Cu treatment (ANOVA, Tukey test, p < 0.05, in both cases; Fig. 1B and C). Plants treated with 23 mmol L−1 Cu exhibited chlorosis from around the third week of treatment.