Alleviation of drought stress damages by melatonin and Bacillus thuringiensis associated with adjusting photosynthetic efficiency, antioxidative system, and anatomical structure of Glycine max (L.)

Abstract

These experiments were performed to study the effect of exogenous treatment with melatonin at 100 μM and seed treatment with Bacillus thuringiensis (10^6-8 CFU/cm³) on growth, physio-biochemical characters, antioxidant enzymes, and anatomical features of soybean plants cv. Giza 111 under drought conditions. The findings showed that leaves number, nodules number, branches number, relative water content (RWC), chlorophyll content, and maximum quantum efficiency of PSII (Fv/Fm) were significantly reduced in soybean under drought stress. In addition, anatomical structure of stems and leaves were negatively affected in stressed plants. Moreover, proline, electrolyte leakage (EL%) lipid peroxidation (MDA), superoxide (O₂ ^·-), hydrogen peroxide (H₂O₂), and antioxidant enzymes, such as catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), were significantly increased under drought stress. However, application of melatonin or Bacillus caused an improvement in growth characters, such as branches number, and increased chlorophyll a and b content, RWC as well as Fv/Fm in drought stressed soybean plants. Furthermore, melatonin and Bacillus treatments showed a significant decrease in EL%, MDA, O₂ ^·- and H₂O₂, besides regulating the activity of antioxidant enzymes under drought stress. The stems and leaves anatomical structure, such as lamina thickness, lower and upper epidermis thickness, number of xylem vessels/bundle, stem diameter, xylem vessels diameter, and phloem thickness, were improved under drought conditions with melatonin and Bacillus treatments. Therefore, the outcomes of this investigation recommended the use of melatonin as foliar spray and Bacillus thuringiensis as seed treatment, which could regulate a number of stress-responsive mechanisms to protect the stressed soybean plants, improve their growth under drought stress.

Keywords: Drought; Enzymes activity; Melatonin; Plant growth-promoting bacteria; ROS; Soybean.

Fig. 1

Effect of melatonin and Bacillus thuringiensis on number of leaves (A), branches number (B), and nodules number (C) in soybean plants under drought stress during 2022 and 2023 seasons. The value within the same column with the different letter is significantly different at 5 % probability level by DMRT.

Fig. 2

Effect of melatonin and Bacillus thuringiensis on chlorophyll a (A), chlorophyll b (B), minimum fluorescence (Fo) (C), maximam fluorescence (Fm) (D), and maximum quantum efficiency of PSII (Fv/Fm) (E) in soybean plants under drought stress during 2022 and 2023 seasons. The value within the same column with the different letter is significantly different at 5 % probability level by DMRT.

Fig. 3

Effect of melatonin and Bacillus thuringiensis on total soluble sugars (A), total phenolics compound (B), relative water content (C), and proline content (D) in soybean plants under drought stress during 2022 and 2023 seasons. The value within the same column with the different letter is significantly different at 5 % probability level by DMRT.

Fig. 4

Effect of melatonin and Bacillus thuringiensis on electrolyte leakage (A), lipid peroxidation (MDA) (B), hydrogen peroxide (C), and superoxide level (D) in soybean plants under drought stress during 2022 and 2023 seasons. The value within the same column with the different letter is significantly different at 5 % probability level by DMRT.

Fig. 5

Effect of melatonin and Bacillus thuringiensis on CAT (A), SOD (B), and POD activities (C) in soybean plants under drought stress during 2022 and 2023 seasons. The value within the same column with the different letter is significantly different at 5 % probability level by DMRT.

Fig. 6

Effect of melatonin and Bacillus thuringiensis on anatomical characters of soybean stems under drought stress conditions. A. Control B. Drought (D) C. D + Bacillus D. D + Melatonin.

Fig. 7

Effect of melatonin and Bacillus thuringiensis on anatomical characters of soybean leaves under drought stress conditions. A. Control B. Drought (D) C. D + Bacillus D. D + Melatonin.