Effect of wastewater discharge on root anatomy and radial oxygen loss (ROL) patterns of three mangrove species in Southern China

The effects of wastewater discharge on radial oxygen loss (ROL) and root anatomy varied among mangrove species. ROL of Bruguiera gymnorrhiza (L.) increased from 22.44 ng cm^-2 min^-1 in the control (just fresh water) to 31.09 ng cm^-2 min^-1 when received normal wastewater (NW) and to 44.22 ng cm^-2 min^-1 when treated with strong wastewater (10NW). However, discharge of both NW and 10NW caused 28% decreases of ROL in the root tip of Excoecaria agallocha L., and the decreases in Acanthus ilicifolius L. were even more significant, with 45% when treated by 10NW. The changes of ROL were related to the root anatomy. Among three species, A. ilicifolius had the highest proportional cross-sectional area of aerenchyma air spaces, suggesting that the internal oxygen transfer to root tip was the fastest. However, the area of aerenchyma air spaces in the root tip of 10NW treated A. ilicifolius was significantly reduced while area of epidermis and hypodermis (E+H) increased leading to less oxygen supply to root tip. Compared to B. gymnorrhiza and E. agallocha, the (E+H) layer of A. ilicifolius was the thinnest, and the cells without suberized walls were loosely packed in all three treatments. These results suggested that the root anatomy and ROL of B. gymnorrhiza was least affected by wastewater discharge, followed by E. agallocha, and A. ilicifolius was the most susceptible species thus was not suitable for treating strong wastewater.