Rhizosphere process of hyperaccumulation of chromium and nickel by <italic>Leersia hexandra</italic> Swartz

Phytoremediation technology is considered to be one of the most promising remediation methods for complex contaminated soil. Leersia hexandra Swartz is a chromium hyperaccumulator first discovered and reported in China， and also a nickel and copper hyperaccumulator plant. Previous studies have been carried out on the physiological and molecular characteristics of chromium， nickel， and copper accumulation in L. hexandra. However， the rhizosphere process and the mechanism of symbiotic regulation of co-hyperaccumulation of chromium and nickel by L. hexandra remain unclear. Therefore， in this paper， L. hexandra was selected as the research object， and the spatiotemporal variation of pH value in rhizosphere microdomain under the heavy metal stress of chromium and nickel， and the influence on enzyme activity in the rhizosphere soil of L. hexandra were studied by using the planar optode technique and in-situ enzyme spectrometry. The results showed that： 1） under the stress of heavy metals chromium and nickel， the rhizosphere pH value of L. hexandra will be significantly reduced. The pH value of the soil around the rhizosphere in the CK group fluctuated between 6.57 and 7.13， but decreased sharply under the compound stress， and the pH value fluctuated between 6.00 and 6.57. Rhizosphere acidification is an important mechanism for the activation and absorption of heavy metals by hyperaccumulator plants， which may also be an important mechanism for the hyperaccumulation of chromium and nickel in L. hexandra. 2） The rhizosphere zone of L. hexandra was the hotspot of enzyme activity. Under the stress of chromium and nickel， the activity of β-glucosidase increased， and the activity of acid phosphatase decreased. The enzyme activity in the CK group fluctuated between 400 pmol/（cm2·h） and 600 pmol/（cm2·h）， while the enzyme activity in rhizosphere soil decreased to 200 to 400 pmol/（cm2·h） under heavy metal stress. These results indicated that the rhizosphere region of L. hexandra had a significant effect on enzyme activity under the stress of chromium and nickel， which may be related to the physiological mechanism of the rhizosphere of L. hexandra under stress.