Effectiveness of Biochar Biofertilizer on Some Morphological, Physiological and Biochemical Traits of Marigold (Calendula officinalis L.) UnderCadmium Stress

Soil pollution by heavy metals, especially in industrial regions is one of the main environmental problems. Cadmium (Cd) is a heavy metal that causes oxidative stress in plants and has many destructive effects on product quality. Nowadays, various methods are used to reduce the negative effects of high concentrations of heavy metals in the soil. In this regard, using biochar is a cost-effective and environmentally-friendly method and its influence on the reduction of heavy metals bioavailability of soil is an important advantage. Biochar is a carbon-rich material obtained by pyrolysis of biomass, such as agricultural residues and manures in conditions without oxygen or with limited oxygen content.

In this study, a factorial experiment was conducted in a completely randomized blocks design with three replications on Marigold (Calendula officinalis L.) medicinal plant with six levels of Cd (0, 1, 3, 5, 7, and 10 mg/l) and three levels of biochar (0, 1.5, and 3 w/w). The effect of experimental treatments was investigated separately and combined on the morphological (wet and dry weights of aerial parts and roots), physiological (the amount of chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid), and biochemical (soluble sugar, catalase, peroxidase, and proline) characteristics of this medicinal plant. Means comparisons were done by Duncan's multiple range test at a probability level of 5%.

The results showed that different concentrations of Cd decreased the wet and dry weights of roots and aerial parts of the plant. The most reduction effect was related to the concentration of 10 mg/l of Cd. The interaction effect of biochar and Cd was significant only on the dry weight of aerial parts. The effect of stress caused by increasing the concentration of Cd on the plant's physiological processes was different. Considering that the first effect of Cd on the plant is the reduction of photosynthesis and chlorosis of the leaves, at the highest level of Cd, the amount of total chlorophyll decreased by 40% compared to the control sample, but the amount of carotenoid increased by 50% (p<0.05). The reduction of chlorophyll content under Cd stress can be due to oxidative damage and inhibition of different stages of chlorophyll synthesis. However, the increase in carotenoids in response to heavy metal stress occurs because these molecules, as part of the non-enzymatic antioxidant defense system, play a protective role against oxidative stress. In contrast, the use of biochar treatment caused a significant increase in the wet weight of aerial parts, the amount of chlorophyll a and carotenoid. Also, the interaction effect of the treatments indicated that at different levels of Cd, the characteristics of the dry weight of aerial parts, the amount of chlorophyll b, and the total chlorophyll increased with the increase in the biochar level. Among the investigated biochemical traits, the interaction effect of the treatments was significant only on the amount of soluble sugar (p<0.05) and catalase (p<0.01). This means that the absorption of Cd by biochar and the reduction of its toxicity effect on seedlings provided the conditions for more production of soluble sugar and catalase. In fact, biochar had high adsorption of Cd due to its high cation exchange capacity, high specific surface, and presence of functional groups.

In total, the results showed the biochar capacity to stabilize and inactivity Cd absorption. Therefore, the incorporation of biochar to soil can improve Cd bioavailability by plants in the phytoremediation, although the effect of type and plant variety on the amount of decreasing Cd stress should not be ignored.