Effects of copper on growth, radial oxygen loss and root permeability of seedlings of the mangroves Bruguiera gymnorrhiza and Rhizophora stylosa

Mangrove wetlands have experienced significant contaminant input such as copper (Cu), aggravated by rapid urban development. This study aimed to investigate the possible function of root permeability in metal detoxification. Pot trials were conducted to evaluate the responses of root permeability in relation to metal (Cu) exposure in seedlings of two mangroves: Bruguiera gymnorrhiza and Rhizophora stylosa. Copper inhibited plant growth and root permeability of the two species significantly (due to decreases in root porosity, thickening of exodermis and increases in lignification), leading to a significant reduction in radial oxygen loss (ROL). A negative correlation between soil Cu and ROL from root tip was also observed. The observed metal uptake by excised roots further indicated that increased lignification would directly prevent excessive Cu from further entering into the roots. In summary, the two mangroves reacted to Cu by producing an impermeable barrier in roots. Such an inducible barrier to ROL is likely to be an adaptive strategy against Cu toxicity. This study reveals new evidence of a structural adaptive strategy for metal tolerance by mangrove plants.