Abstract
Legume-rhizobium symbiosis has been heavily investigated for their potential to enhance plant metal resistance in contaminated soil. However, the extent to which plant resistance is associated with the nitrogen (N) supply in symbiont is still uncertain. This study investigates the effect of urea or/and rhizobium (Sinorhizobium meliloti) application on the growth of Medicago sativa and resistance in metals contaminated soil (mainly with Cu). The results show that Cu uptake in plant shoots increased by 41.7%, 69%, and 89.3% with urea treatment, rhizobium inoculation, and their combined treatment, respectively, compared to the control group level. In plant roots, the corresponding values were 1.9-, 1.7-, and 1.5-fold higher than the control group values, respectively. Statistical analysis identified that N content was the dominant variable contributing to Cu uptake in plants. Additionally, a negative correlation was observed between plant oxidative stress and N content, indicating that N plays a key role in plant resistance. Oxidative damage decreased after rhizobium inoculation as the activities of antioxidant enzymes (catalase and superoxide dismutase in roots and peroxidase in plant shoots) were stimulated, enhancing plant resistance and promoting plant growth. Our results suggest that individual rhizobium inoculation, without urea treatment, is the most recommended approach for effective phytoremediation of contaminated land.
Highlights
Heavy metal contamination in soil is considered a widespread problem due to growing anthropogenic disturbance and industrial development [1,2]
Previous experimental results showed a decrease in the biomass of soybean nodules when grown with N, inferring that N addition reduces rhizobia performance [50]. These findings suggest that plants in symbiosis with rhizobium are more resistant to Cu, and the enzyme ACC deaminase may provide an explanation for these results [28]
The relationship between the content of N and Oxygen Free Radical (OFR) was negatively correlated. These results indicate that N supply in legume-rhizobium symbiosis reduces oxidative stress under Cu pollution
Summary
Heavy metal contamination in soil is considered a widespread problem due to growing anthropogenic disturbance and industrial development [1,2]. Copper (Cu) contamination is one of the most widespread and damaging types of heavy metal pollution in terrestrial ecosystems [3,4]. Cu is an essential microelement involved in plant biological processes, it is highly toxic if absorbed in excess by plants. Excessive Cu in the soil can cause growth inhibition by interfering with a number of plant processes, including photosynthesis, respiration, oxygen superoxide scavenging, and cell wall metabolism and lignification [5]. Due to the impacts of Cu contamination on agriculture, it is crucial to remediate and restore Cu-contaminated soil. Chemical, and/or biological treatments are used to minimize Cu contamination [6,7]
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