Abstract
Untreated wastewater used for irrigating crops is the major source of toxic heavy metals and other pollutants in soils. These heavy metals affect plant growth and deteriorate the quality of edible parts of growing plants. Phytohormone (IAA) and exopolysaccharides (EPS) producing plant growth-promoting rhizobacteria can reduce the toxicity of metals by stabilizing them in soil. The present experiment was conducted to evaluate the IAA and EPS-producing rhizobacterial strains for improving growth, physiology, and antioxidant activity of Brassica juncea (L.) under Cd-stress. Results showed that Cd-stress significantly decreased the growth and physiological parameters of mustard plants. Inoculation with Cd-tolerant, IAA and EPS-producing rhizobacterial strains, however, significantly retrieved the inhibitory effects of Cd-stress on mustard growth, and physiology by up regulating antioxidant enzyme activities. Higher Cd accumulation and proline content was observed in the roots and shoot tissues upon Cd-stress in mustard plants while reduced proline and Cd accumulation was recorded upon rhizobacterial strains inoculation. Maximum decrease in proline contents (12.4%) and Cd concentration in root (26.9%) and shoot (29%) in comparison to control plants was observed due to inoculation with Bacillus safensis strain FN13. The activity of antioxidant enzymes was increased due to Cd-stress; however, the inoculation with Cd-tolerant, IAA-producing rhizobacterial strains showed a non-significant impact in the case of the activity of superoxide dismutase (SOD), peroxidase (POX) and catalase (CAT) in Brassica juncea (L.) plants under Cd-stress. Overall, Bacillus safensis strain FN13 was the most effective strain in improving the Brassica juncea (L.) growth and physiology under Cd-stress. It can be concluded, as the strain FN13 is a potential phytostabilizing biofertilizer for heavy metal contaminated soils, that it can be recommended to induce Cd-stress tolerance in crop plants.
Highlights
The cultivated area in the world is decreasing due to urbanization and industrialization on fertile agricultural lands [1,2]
The minimum relative water contents (RWC) was observed in the leaves of uninoculated plants while the maximum RWC was given by the plants inoculated with FN13 that was 5.6% higher than uninoculated unstressed control plants
All the strains were effective in enhancing the RWC of Brassica juncea (L.) ranging from 3.0% to 5.6% but the improvement was not significantly different from control plants
Summary
The cultivated area in the world is decreasing due to urbanization and industrialization on fertile agricultural lands [1,2]. The use of agrochemicals and intensive cultivation may result in deterioration of land resources and environmental quality [3,4]. To meet the irrigation water requirements, farmers use wastewater to grow crops around metropolitans and industrial cities that cause soil and environmental pollution [5,6]. Untreated wastewater is used by farmers to grow crops that is the major source of toxic heavy metals and other pollutants in soils [7] which limits crop production and pose a serious threat to food security in peri-urban areas.
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