Effect of microbial biostimulants on growth and physiology of lettuce (Lactuca sativa var. prizehead) under Dicyandiamide stress

Effect of microbial biostimulants on growth and physiology of lettuce (Lactuca sativa var. prizehead) under Dicyandiamide stress

Background and objectives

Dicyandiamide is a nitrification inhibitor that slows down the nitrification process (the conversion of ammonium to nitrate) by inhibiting the activity of nitrifying bacteria, and its accumulation in the soil may lead to plant toxicity. The efficiency of nitrogen use in soils is generally low, and a large portion the nitrogen applied to soil becomes unavailable to plant roots through leaching, volatilization, and denitrification. One of the most important forms of nitrogen in the soil is nitrate (NO₃⁻). Due to its high mobility in the soil, nitrate is easily leached by irrigation or rainfall and enters groundwater resources. To address this problem, the use of nitrification inhibitors has been considered an effective method to reduce the rate of ammonium conversion to nitrate. One approach to mitigate the harmful effects of nitrification inhibitor and prevent their entry into the nutrient cycle is through their immobilization in the soil and plant roots using plant-microorganism bioremediation technologies. The presence of useful microorganisms around the roots led to substantial pollutant degradation, suggesting that inoculation of roots holds promise as a method to remediate pollution

Material and Methods

In the present study, to investigate the effect of different levels of dicyandiamide and microbial biostimulant on growth and physiological traits of lettuce, a factorial experiment was conducted in a completely randomized design. The experiment included different levels of dicyandiamide (0, 5, 20, and 40 mg kg-1 of soil) and the application of a combination of symbiotic fungus and growth-promoting bacteria (inoculated and non-inoculated), with four replications under soil-based cultivation in the research greenhouse of University of Mohaghegh Ardabili. The cultivation was carried out in the autumn of 1401.In this experiment, traits such as root and leaf fresh weight, number of leaves, total chlorophyll content, carotenoid, anthocyanin, soluble protein content, soluble sugars, flavonoids, malondialdehyde, leaf relative water content, ascorbate peroxidase, catalase, and guaiacol peroxidase were studied. To identify bacterial strains resistant to the nitrification inhibitor dicyandiamide, a total of 15 bacterial strains were obtained from the Faculty of Soil Science at University of Shahid Beheshti. One of these strains, Bacillus anthracis, was selected and used as the chosen strain.

Result

The results indicated that as the dicyandiamide (DCD) amount in the soil increased from 0 to 40 mg/kg, root symbiosis percentage, fresh root weight, and fresh leaf weight decreased by 29%, 27%, and 30%, respectively. Additionally, inoculation with the bio-stimulant increased fresh root weight and fresh leaf weight by 36% and 27%, respectively. Total chlorophyll content decreased from 8.39 mg/g FW in the 5 mg/kg DCD treatment to 7.11 mg/g FW in the 40 mg/kg DCD treatment. The 40 mg/kg DCD treatment showed an approximately 24% increase in carotenoid content compared to the DCD-free control treatment. Inoculated plants exhibited 33% higher total chlorophyll and 50% higher carotenoid content than non-inoculated plants. The highest activity of guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) enzymes was obtained in the 20 mg dicyandiamide treatment without bio-stimulant application, while the lowest activity was observed in the dicyandiamide control treatment without bio-stimulant application.

Conclusion:

"In general, it is concluded that the application of the nitrification inhibitor dicyandiamide (DCD) at rates of 20 and 40 mg/kg soil resulted in a significant reduction in leaf and root fresh weight, root colonization, and total chlorophyll content. However, it increased carotenoid and flavonoid content, and antioxidant enzyme activities. Inoculation of plant roots with the fungus S. indica and the bacterium Bacillus anthracis improved growth and physiological traits."

Keywords: Nitrification inhibitor, Plant growth-promoting bacteria (PGPR),Serendipita indica, Lettuce