The effects of competition and excessive copper concentration on seedling dry matter and copper, phosphorus, potassium, and magnesium contents in common bean (Phaseolus vulgaris) and redroot pigweed (Amaranthus retroflexus)

Soil contamination with heavy metals is one of the most serious problems in the agricultural environments. Due to excessive injudicious and unregulated use of agrochemicals such as pesticides, bactericides and fungicides, which contain Cu as an active component, contamination of agricultural soils with Cu is often occurred. In many plant species, excess copper (Cu) toxicity is an important disorder that limits uptake and accumulation of mineral nutrients. The root system of crops in contact with the roots of neighbor plants (crop or weed) has different characteristics in terms of heavy metals adsorption compared to the pure stand of crops. The position of the roots of heavy metal accumulator weeds may affect the absorption of such metals by adjacent crops in infected conditions. The heavy metal accumulator weeds by reducing or evacuating metals from the rhizosphere of the adjacent plants, cause the plants to be immune to the toxicity of metals. Biomass reduction is a dominant effect of copper toxicity in plants. Application of high copper doses (50 and 75 µM) in nutrient solution, decreased the dry weight of common bean plants in comparison with control. Contamination of soil can affect ecological interactions between plants such as the weed-crop competition. Positive plant-plant interactions are expected to be especially beneficial under high abiotic stresses and therefore, the facilitation effects could be enhanced by increasing stress intensity. Weeds possess the ability to accumulate heavy metal and nutrients, in comparison with crops. The role of nurse plants in facilitating plant community has been applied in severe conditions. Metal hyper-accumulator plants have a positive effect on co-occurring species in met al-rich soils. Redroot pigweed (Amaranthus retroflexus L.) is a serious weed, which affects common bean production. Amaranthus retroflexus and Amaranthus spinosus are known as metal accumulators. We hypothesized that in contaminated fields, the effects of heavy metals on crop growth may be modified by the presence of a metal accumulator weed such as redroot pigweed.

To investigate copper uptake and accumulation of mineral nutrients by common bean in association with redroot pigweed under excessive copper conditions, a factorial experiment based on a completely randomized design with four replications was carried out in a research greenhouse of Bu-Ali Sina University in 2016. Experimental factors were three Cu doses of 1 (control), 25 and 50µM CuSO4 and five replacement intercropping ratios of bean (B) and redroot pigweed (P) (1B: 0P, 0.75B: 0.25P, 0.5B: 0.5P, 0.25B: 0.75P, 0B:1P). The experiment was carried out in a hydroponic condition. 15 days after seedling establishment, the seedlings of both plants were transferred to the hydroponic conditions with a Hoagland solution, and Cu doses were applied in the Hoagland solution. 30 days after growing the plants in above condition, biomass dry weight and concentration of Cu, K, P and Mg in the leaves and roots of both plants as well as copper translocation factor were determined.

The results showed that under three doses of copper, the highest concentration of copper in bean was obtained in sole crops, and with increasing replacement intercropping ratios of pigweed, the copper concentration was decreased in bean. In pigweed, the highest concentration of copper was observed in 0.75B: 0.25P proportion. At 1µM CuSO4, potassium concentration in the roots and the leaves of bean in 0.25B: 0.75P proportion, was decreased by 7 percentage in comparison with that of the sole crops, and at 25 µM CuSO4, by 10 percentage and at 50 µM CuSO4, by 15 percentage were increased compared to the sole crops, respectively. At 25 µM CuSO4, phosphorus and magnesium concentration in the roots of bean in 0.25B: 0.75P proportion was increased by 17 and 18 percentage, and at 50 µM CuSO4, by 21 and 26 percentage, respectively in comparison with the sole crops, respectively. Association of pigweed to the bean, decreased Cu pollution in bean plants, which could be related to the balance of nutrient elements uptake by bean under excessive copper conditions. Under excessive copper conditions, the higher copper uptake by pigweed, improved the growth of bean.

The results of this study showed the facilitative effects of pigweed plant in decreasing the Cu toxicity stress effects to the bean and indicated that pigweed could be as a potential Cu accumulator to improve bean plant growth under the Cu stress. Here we showed that the presence of some heavy met al accumulator plants such as pigweed in common bean farms, might be a useful practice in contaminated soils. In addition, these facilitation effects of such weeds, emphasize ecological management of weeds rather than weed eradication.