Heavy metal rich stone-processing wastewater inhibits the growth and development of plants

Large amounts of wastewater are generated from stone processing, which are toxic and cause serious environmental and health risks. To quantify the content of stone processing wastewater and estimate its effects on plant growth, we collected water samples from sewage outfall of four stone processing factories and nearby water bodies. The concentration of potential toxic metals were much higher in the wastewater than background controls. Wastewater inhibited plant primary root elongation, lateral root formation, and growth of aerial part. Seedlings treated with the effluents were unhealthy with deep purple leaves and usually died before flowering. Chlorophyll a/b contents and chloroplast number were reduced in those abnormal mesophyll cells. Transcriptional levels were decreased for chloroplast formation genes, but increased for those participated in chloroplast degradation and catabolism. Six out of nine tested senescence-associated genes were up-regulated. Furthermore, our results show that endogenous toxic metal levels indeed increased after wastewater treatment. Altogether, these results indicated that the potential toxic metals rich wastewater had significant inhibition on plant growth and led to senescence-associated program cell death, which could be helpful for the government and enterprises to understand the environmental risks and formulate reasonable wastewater emission standards for the stone processing industry.