| Interspecific interactions between plants (competition or facilitation) are the theoretical basis of establishment of intercropping or mixed cropping system. It is well documented that complementary use of soil nutrients between intercropped plants has greatly increased efficienct of nutrient utilization, and thus enhanced total yields. Under the soil with heavy metal contamination, however, whether and how intercropped plants affect bioavailable of metal in soil, and thus affect metal uptake by plants have not been well studied. Here I hypothesize that plant interactions may altered the rhizospheric environment, and further change the bioavailability of metal and affect metal uptake by plants. I conducted both field and greenhouse experiments to test these hypotheses. The results obtained in these experiments are as follows:1The responses of crop to soil contaminated with heavy metal under interspecific interactionsMetal accumulating in edible parts of crops varied with types of metals and crops. Pb concentration was higher in edible parts of leafy vegetables (cabbage and pakchoi), Cd concentration was higher in edible parts of tomato and pakchoi, and Cu was higher in edible parts of maize and pakchoi.Crop co-planting tended to decrease in Pb accumulation and increase in Cd accumulation, especially, when co-planted with Japanese clover, Pb in edible parts of maize, cabbage and pakchoi were much lower, and Cd in edible parts of tomato and pakchoi were obviously higher relative to they grown alone, respectively. Cu in edible parts of maize was higher when co-planted with tomato than when maize monoculture, but Cu concentrations were not significantly affected by planting patterns in tomato, cabbage and pakchoi.2The responses of maize and tobacco different genotypes to soil contaminated with cadmium under interspecific interactionsCompared to monoculture, for each genotype of maize or tobacco, Cd concentration, Cd uptake and Cd bio-concentration factor were significantly higher when co-planted with Japanese clover, but Cd transfer factor had no significant difference.3Rhizosphere environment and Cd bioavailability of soil under plants interspecific interactionsRhizosphere environment and Cd bioavailability in soil were significantly affected by plant patterns. Compared to monoculture, for each genotype of maize or tobacco, the colonization rates of arbuscular mycorrhizal fungi were higher when co-planted with Japanense clover, especially, for two tobacco genotype, the colonization rates of arbuscular mycorrhizal fungi of tobacco (K326) and tobacco (Yunyan-87) in Cd3treatment were increased by21%and15%, respectively. Compared to monoculture, soil pH was significantly lower and soil exchangeable Cd concentration was significantly higher when crop co-planted with Japanese cover.4The different rhizosphere effects in root barrier experimentCd concentrations and Cd uptake of maize and tobacco were significantly affected by different root barrier treatments. When maize co-planted with Japanese clover, the increment of Cd uptake in co-planted system was mainly determined by root effect, followed by solution effect, and finally by mycorrhizal effect. With soil Cd elevated, mycorrhizal effect decreased, and root effect and solution effect increased. When tobacco co-planted with Japanese clover, the increment of Cd uptake in co-planted system was also mainly determined by root effect, followed by mycorrhizal effect, and finally solution effect. With soil Cd elevated, root effect decreased, and mycorrhizal effect and solution effect increased.5Interspecific relationships of crops under heavy metal stressUnder soil contaminated with several heavy metals or single cadmium, compared to monoculture, the biomass or yield of maize, tomato and tobacco significantly increased when co-planted with Japanese cover. In maize and tomato intercropping system, maize was the dominant crop, biomass or yield increased when compared to maize alone. The biomass or yield of cabbage and pakchoi were not significantly affected by plant patterns in our experiment. With soil Cd elevated, the biomass of different maize genotypes first increased and then decreased, and biomass of different tobacco genotypes was gradually increased, and the co-planted plant, Japanense clover, was not significantly affected in biomass. This indicated that the mechanisms of competition and facilitation of plant interspecies enhanced the preferences of different functional group plants to Cd in mildly Cd-contaminated soil.6The safety production assessment of crops and the application of plant interspecific interations under soil contaminated with heavy metalAccording to the maximum permissible concentration (MPC) standard of the National Standard Agency in China, the edible parts of tomato, cabbage and pakchoi had potential risk of Pb or Cd pollution, only maize (under all planting patterns) can be safely consumed. Considering the Cd concentration of maize edible part can be enhance by intercropping, especially, co-planting with Japanese clover of legume, the results suggest that maize monoculture is the optimum crop planting pattern grown in soil contaminated by electronic wastes.Cd concentrations in edible parts had significant difference among maize different genotypes. According to the MPC standard, the edible parts of Zhengdan-958and Shentian-1were safe in mildly Cd-contaminated soil (<1mg/kg), and all maize genotypes had Cd pollution risks when soil Cd concentration is more than lmg/kg. Considering Shentian-1had higher Cd uptake than Zhengdan-958, the results suggest that Shentian-1has the potential for satety production and phytoremediation when soil Cd concentration is less than lmg/kg.Tobacco (K326) had high biomass under soil Cd stress, and both bio-concentration factor and transfer factor of Cd are more than1, especially, when co-planted with Japanese clover, K326had substantial quantities of Cd uptake. The results suggest that tobacco (K326) has the potential for phytoextraction when soil contaminated with cadmium.
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