The effect of different concentrations of cadmium on some biochemical, physiologic and functional characteristics of European borage (Borago officinalis L.)

Introduction Soil contamination with heavy metals is one of the most important challenges related to the protection of water and soil resources. Heavy metals are metals that have a density of more than 5 grams per cubic centimeter. Among heavy metals, cadmium is of special importance due to its high solubility in water and quick and easy absorption by the plant root system. This element has been introduced as the fourth dangerous element for vascular plants (Kok et al., 2010). By accumulating in the root environment, cadmium can cause a decrease in growth, respiration, damage to the mechanisms involved in photosynthesis, and inhibit the activity of enzymes and the lack of nitrogen and phosphorus in the plant. Borage (Borago officinalis L.) is a valuable annual medicinal herb suitable for cultivation in many countries, including Iran. Borage (Borago officinalis L.) is considered as a native of both Europe and Asia. Several species around the globe fall under the denomination of “borage”. The presence of the highest γ-linolenic acid content in the seeds of borage makes borage distinctively important mainly for the nutraceutical and pharmaceutical research. γ-Linolenic acid is an omega-6 polyunsaturated fatty acid which cannot be synthesized in the body and hence falls into the category of essential fatty acids (Evesh et al., 2019). The present study was carried out with the aim of investigating the physiologic and biochemical responses of the medicinal plant Borage (Borago officinalis L.) to cadmium stress.   Materials and methods This experiment was conducted in the form of a completely random design in the greenhouse of Shahid Bahnar University, Bardsir College of Agriculture, Shahid Bahnar University, Kerman, at an average temperature of 25 degrees Celsius during the day and 20 degrees Celsius during the night, with a relative humidity of 60%. First, concentrations of cadmium (0, 1.25, 2.5, 5, 10, 20, 40, 80, 120, 160 mg kg-1) were prepared using cadmium chloride (Merck company) and the soil was uniformly contaminated with cadmium. The contaminated soil was incubated under constant humidity and temperature conditions for 30 days and then poured into pots with a capacity of 3 kg of soil. Plant seeds were disinfected with alcohol and 5% sodium hypochlorite solution and immediately washed several times with distilled water. Five seeds were planted in the pots containing the studied treatments and at a depth of 2 to 3 cm. Two months after planting, when the plant was in the 50% flowering stage, the growth and performance characteristics of the plant including the number of flowers, number of leaves, branch length, root length, shoot weight, root weight, flower weight were measured. Some biochemical characteristics (catalase, ascorbate peroxidase, guaiacyl peroxidase, protein, proline and photosynthetic pigments) were measured.   Results and discussion The studied treatments significantly (p≤0.01) affected the biochemical, physiologic and performance characteristics of European borage. Increasing the concentration of cadmium had an inhibitory effect on the growth parameters of the plant, and this effect was clearly evident in pollution above 80 mg kg-1. Number of flowers, number of leaves, flower weight, root weight, total weight, root length and branch length decreased significantly with increasing cadmium concentration. The flowering of the plant was 71% at the pollution level of 120 mg per kilogram. At this level of pollution, the root weight was reduced by 69.9%. The weight of aerial parts decreased by 59.7% at the pollution level of 160 mg kg-1. The activity of ascorbate peroxidase enzyme increased up to 20 mg kg-1 treatment and after that the activity of ascorbate peroxidase enzyme decreased with the increase of cadmium concentration. Proline concentration increased with increasing cadmium contamination. Photosynthetic pigments also decreased in high concentrations of cadmium. The cause of the adverse effects of cadmium on plants in contaminated soils can be attributed to more absorption of cadmium by the plant and growth disturbance caused by cadmium toxicity in the plant, reduction of growth rate, reduction of water absorption and absorption of other ions affecting the activities plant growth (Veselov et al., 2003), the reduction of cytokinin hormone activity, which has a significant effect on cell proliferation and growth, or the negative effect of cadmium on energy production in mitochondria was attributed (Fotohi et al., 2011). Cadmium causes disturbances in the overall metabolism of cells. (Xie et al., 2021) Cadmium toxicity in plants can lead to negative effects on the photosynthesis process. This action takes place through damage to some photosynthetic enzymes, especially those involved in the Kelvin cycle and chlorophyll biosynthesis (Mishra et al., 2006).   Conclusion The results showed that the yield of European borage was not affected by cadmium up to a concentration of 1.25 mg kg-1. The activity of antioxidant enzymes increased up to the treatment of 20 mg kg-1 and then decreased with increasing cadmium concentration, which indicates oxidative stress. Photosynthetic pigments and chlorophyll fluorescence index decreased with increasing cadmium concentration. Considering the tolerance and resistance of some medicinal plants to the conditions in soils contaminated with heavy metals, it is possible to use the cultivation of some medicinal plants as a solution for the management and exploitation of lands that have medium contamination with heavy metals. Considering the number of flowers, flower durability and high yield, the European borage plant can be a suitable candidate for cultivation in polluted areas, of course, additional studies with the approach of reducing the adverse effects of stress and reducing the concentration of cadmium element in the soil are needed.