Differential Physiological Responses Of Copper-Sensitive and Copper-Tolerant Elsholtzia Species to Copper Toxicity: the Character of Cell Walls and their Subfractions

Elsholtzia splendens is one of a few species for copper (Cu) accumulators. The present study compares the physiological responses and differences in Cu accumulating ( E. splendens ) and non-accumulating ( E. cypriani ) species to Cu stress. In this study, 30-day-old seedlings of E. splendens and E. cypriani were treated with 0.16 (control), 25, 50, or 100 μM CuSO 4 for 7 days, respectively. The growth and Cu concentrations of two species, thiobarbituric acid reactive substance and reactive oxygen species deposition in roots, were measured. The root cell wall and its fractions of two species with treated 50 μM CuSO 4 were isolated to determine the Cu adsorption capacity of the root cell wall. Our results revealed that E. splendens showed higher tolerance to excess Cu, and more adverse effects on plant growth, root morphology, and severe oxidative damage were observed in E. cypriani compared to E. splendens . Cu was more likely to be accumulated in the roots of two species relative to their shoots. The Cu adsorption of the root cell wall in E. splendens was higher than that of E. cypriani . Pectin from E. splendens adsorbed Cu persistently up to 360 h after exposure while equilibrium was reached at 168 h in E. cypriani . With Cu treatment, methyl esterification in the cell wall was upregulated and uronic acid content decreased in E. cypriani . However, only downregulation in methyl esterification in the cell wall was observed with Cu treatment in E. splendens . These findings indicate that the comparatively better growth and higher tolerance to Cu displayed by E. splendens can be mainly attributed to higher Cu adsorption capacity of pectin in the cell wall.