The effect of copper stress on inter-trophic relationships in a model tri-trophic food chain.

Abstract

Soil fertility and management are paramount in ensuring food security for the growing populations. The use of agri-chemical and products containing heavy metals inadvertently threaten both food security and the surrounding ecosystems from contamination, loss of productivity or ecosystem service. In the present study a series of experiments on the toxic and adaptive responses of wheat plants to copper-induced stress were conducted to establish the effects of different levels of Cu (0 – 200 mg kg -1) on growth, nutrient levels and the total plant proteins of wheat seedlings (Triticum aestivum) using pot experiments. A tri-trophic food chain soil → plant → herbivore → predator was established as plants were infested with grain aphids (Sitobion avenae) which were subsequently fed to predatory ladybirds (Adalia bipunctata). Multiple measurements were conducted which deduced that Cu was taken up from soil into the plant tissues accumulating in the shoot and ear. The rate of growth and flag leaf length were affected by levels of Cu in the soil but total plant mass and ear weights were not. Wheat shoots and ears were analysed for N (crude protein) P, K and it was found that the levels of Cu in soils affected the levels of protein in both the shoot and the ear while the levels of P and K remained unaffected. Total populations of aphid and aphid fecundity appeared to be unaffected by the Cu stress-induced plants and no significant relationships between levels of N in plant tissues or flag leaf length were found. Ladybirds also appeared to be unaffected by the levels of Cu in soils as consumption rate or change in mass between the treatments was not significant. While the present study does not support a critical threshold for Cu levels in agricultural soils it can conclude that biological control methods are unaffected by levels of Cu in the soil.