Effects of soil microbe inoculation on the growth and photosynthetic physiology of Elsholtzia splendens under copper stress

Abstract

Phytoremediation, an emerging cost effective and ecologically benign technology for the decontamination of soils, is the use of plants and their associated microbes for environmental cleanup. The efficiency of phytoremediation depends mostly on the establishment of robust plant-microbe interactions. Elsholtzia splendens is a Labiatae plant well known for its high copper tolerance, which is widely distributed on copper deposits as well as non-contaminated areas. It has been considered for the phytoremediation of copper polluted soils; however, little is known about the role of the soil microbe community in the roots of E. splendens in adapting the non-contaminated plants to copper stress. In order to evaluate the effects of inoculation with soil microbes on the copper tolerance of E. splendens, this experiment studied the effect of inoculation with soil microbes on the seedling dry weight, plant morphology, survival rate, copper content and leaf gas exchange parameters of E. splendens under copper stress using a mesocosm system. The experiment included four soil treatments: the addition of copper (copper stress); soil microbe inoculation; addition of copper and soil microbe inoculation; and no additional copper or soil microbe inoculation (control). The results were as follows: (1) The plant number, plant height, basal diameter, biomass, and stem weight ratio of E. splendens under copper stress were lower than in the control. Inoculation with soil microbes significantly increased the plant number, plant height, biomass, and stem weight ratio of E. splendens under copper stress, indicating an alleviation effect on the inhibition of copper on the growth of E. splendens. Compared with the control, the copper content of roots, stems, leaves, inflorescences and seeds were increased under copper stress. Under copper stress, soil microbe inoculation significantly decreased copper content in the stem and leaf organs of E. splendens, but did not markedly affect copper content in the other organs. (2) In autumn, the diurnal variation in net photosynthetic rate (Pn) in the leaves of E. splendens under all treatments took on a one-peak curve. Inoculation with soil microbes significantly increased the daily mean Pn and daily mean transpiration rate (Tr), and significantly decreased the daily mean stomatal conductance and daily mean intercellular CO2 concentration (Ci) of E. splendens under copper stress. (3) Under copper stress, the maximum net photosynthetic rate (Pnmax), light saturation point (LSP), apparent quantum yield (AQY), maximum rate of carboxylation (Vcmax), maximum rate of electron transport (Jmax) and the triose phosphate use rate (TPU) of E. splendens significantly decreased, while the light compensation point (LCP) significantly increased. Inoculation with soil microbes significantly increased the Pnmax, LSP, AQY, Vcmax, Jmax and TPU, but decreased the LCP. In conclusion, inoculation with soil microbes can effectively alleviate the damage of copper stress to E. splendens by increasing the photosynthetic ability, enhancing the light energy utilization and carbon assimilation and promoting the accumulation of organic matter. Inoculation with soil microbes can increase the growth of plants under copper stress, which could have potential applications in the phytoremediation of soils contaminated with heavy metals.