Effect of Bacillus simplex as a growth promoting bacterium on some growth characteristic of Pepper (Capsicum annuum L.) Seedling at Different Water Conditions

Background an Objectives
Water shortage and the occurrence of drought is a major challenge in agriculture. By 2050, Iran will experience a 2.6 0C increase in temperature and a 35% decrease in rainfal. Plant growth promoting bacteria could alleviate the water stress effect on crops. Auxin and ACC-deaminase production are best known for their mechanisms of action, but recently extracellular polysachrid (EPS) production was considered. EPS could support bacterial life by absorbing water and keeping the hydrative environment around bacteria. In such situations, plants are indirectly affected by increasing available water as well as getting enough time to genetic responses to water deficit. Bacteria of the genus Bacillus are considered as one of the most important microbes in the rhizosphere of plants. Their ability to form dormant spores resistant to heat and dehydration makes them a desirable option in biofertilizers production. Moreover, using extracellular polymer producing strains for plants inoculation maybe more successful compared to other bacteria. So far, no study has been reported on the effect of EPS producing Bacillus simplex inoculation on plants growth. Therefore, this research study was carried out to investigate the effect of this bacterium on pepper seedling under different watering conditions.
Methodology
Bacillus simplex selected as a plant growth promoting bacterium (PGPB) due to the ability of auxin and ACC-deaminase production. This bacterium was obtained from the soil biology lab collection at the University of Maragheh. Its tolerance to water shortage and EPS production was evaluated by culturing it on nutrient agar medium treated with polyethylene glycol (PEG) to simulate water stress conditions. To assess the effects of EPS-producing B. simplex on pepper growth, a greenhouse experiment was conducted. The experiment was conducted as factorial in a completly randomised design with three replications. The experimental treatments consisted of B. simplex inoculation and three different soil water states, namely 80%, 60%, and 40% of the soil's field capacity water content.
The plants were harvested after 50 days of planting date and various parameters were recorded. These included total dry weight, leaf surface area, leaf dry weight, root dry weight, root fresh weight, root volume, specific leaf area (SLA), specific leaf weight (SLW), leaf weight ratio (LWR), root length, root surface area, root diameter, and root surface area density.


Findings
Based on visual evaluations, it was observed that the bacterial colonies of B. simplex exhibited significant changes under water stress conditions compared to non-stressed conditions. The colony color changed from milky to semi-transparent, and its fluidity and stickiness increased in PEG nutrient agar containing PRG compared to nutrient agar medium.
Its inoculation could significantly increase the total dry weight of aerial organs by 23%, 27% and 46% and the dry weight of the leaves by 15%, 19% and 37%, respectively, under normal watering, moderate and severe water stress conditions. The leaf area increased respectively by 12% and 29% just in the conditions of moderate and severe water stress. Bacterial inoculation in normal watering and medium water stress conditions increased the root dry weight by 37% and 42%, respectively, but the wet weight of root in severe water stress decreased by 28%. The inoculation of studied bacterium caused to increase root volume by 27%, 31% and, 51% and root surface by 16%, 20% and 38% respectively, under normal watering and modarate and severe water stress conditions. Root diameter, root tissue density and root surface density increased, by 38%, 29% and 17%, respectively, at severe water stress conditions by bacterial inoculation. The regression relationship between root traits and the total dry weight of the aerial part of pepper showed that there is a direct and significant correlation between them. Root dry weight, root fresh weight, root volume, explanation factor, root length, root surface density, root tissue density, and root surface area showed positive correlations with the total dry weight of the pepper seedlings. The coefficients of determination (R2) for these relationships were 0.86, 0.86, 0.86, 0.90, 0.91, 0.91, 0.60, and 0.91, respectively. However, there was no correlation between the total dry weight and leaf indicators.

Conclusion
Based on the correlation coefficients between the total dry weight of pepper seedlings and their root and leaf traits, it appears that the increase in yield by B. simplex is associated with improvement in the characteristics of both leaves and roots, particularly under severe water stress conditions. Bacterial metabolites, such as auxin, are known to stimulate root growth, root size, weight, and branching, which ultimately leads to a larger soil volume accessible to the plant and improved nutrient availability, thus enhancing overall plant yield. Therefore, based on the results the use of B. simplex might improve the establishment of pepper plants in all soil water conditions, particularly under water deficit conditions.