From Micronutrients to Potentially Toxic Elements: Physiological Responses of Canavalia ensiformis to Copper and Iron

Background: The increase in potentially toxic elements (PTEs) in the soil is worrying, especially in agricultural soils due to the bioaccumulation factor. Copper (Cu) and iron (Fe) are micronutrients, responsible for important functions in the plant body, but the high availability of these elements in the soil can cause soil contamination and toxicity in plants; consequently, they can be considered PTEs. Objectives: The focus of this study is to understand the physiological responses (pigments, gas exchange, growth, biomass, accumulation) of Canavalia ensiformis to high levels of Cu and Fe in the soil, in isolation, and to identify which PTE is most harmful to its development. Methods: Two experiments (Cu and Fe) were conducted simultaneously in a greenhouse. Treatments of 50, 150, 250, and 350 mg dm^-3 of soil for each element (CuSO₄*5H₂O and FeSO₄*7H₂O) were incorporated into the soil (Oxisol) of each experimental unit (4 dm³ pot), in addition to the control. C. ensiformis seeds were sown directly in soil enriched with Cu and Fe, respectively, and after emergence they were cultivated for 90 days. Results: Changes in chlorophyll levels caused direct effects on gas exchange, shoot biomass, root development, nodulation, and total plant biomass. The tolerance of the species is dependent on chlorophyll levels and gas exchange. There was accumulation of both PTEs in the roots and low translocation to the shoot. Conclusions: The plants were tolerant to Fe treatments; however, they were not tolerant to Cu treatments (T150-T350). Excess Cu was more detrimental to plant development.