Eco-friendly bio-encapsulation from sodium alginate-trehalose-kaolin and its performance evaluation in improving plant growth under salt or/and drought conditions

Abstract

Plant growth-promoting bacterium plays a significant role in improving plant tolerance to abiotic stresses. However, there are low survival and poor effect in field application, especially in unfavorable environments. Our previous study suggested that encapsulation of Bacillus pumilus G5 from polyvinyl alcohol‑sodium alginate could improve plant growth and soil fertility under drought and salt soil conditions. However, in the G5 microbeads, the polyvinyl alcohol could not be degraded after entering the soil, and the loss of viable bacteria was severe during the drying process. Achieving a more eco-friendly and efficient formulation based on biodegradable polymers can have significant effects on increasing the quantity and quality of agricultural products. Herein, G5 has immobilized in the composite wall of sodium alginate-trehalose-kaolin microbeads and then evaluated the performance, and applied on the Pharbitis nil under salt or/and drought stress by pot experiment. A 2 % sodium alginate, 1 % trehalose, and 1 % kaolin formulation for the coating films resulted in optimal G5 microbeads embedding efficiency, viable bacteria, degradation rate, and sustained release. Also, the G5 microbeads exhibited longer storage life than that of the G5 suspension. Scanning electron microscopy revealed that the G5 microcapsules had a near-spherical structure with a particle size of around 1000 μm forming a continuous dense composite wall membrane with obvious protrusions and folds on the surface, which facilitated the release of the G5 strain. The interior of the G5 capsule was rough and suitable for bacterial attachment. Infrared spectroscopy showed that the G5 microcapsules are a simple physical mixture with no chemical reaction between the excipients, making the G5 microcapsules chemically stable. The inclusion of the G5 microcapsules considerably induced Pharbitis nil seedlings growth and biomass under drought and/or salt stress. In the rhizosphere soil of Pharbitis nil, the G5 microcapsules increased the total cultivable bacteria population, the activities of invertase, urease, phosphatase, and catalase, and the contents of available nitrogen and available phosphorus. We concluded that a suitable formulation by bio-encapsulation with eco-friendly excipients for alleviating drought and/or salt stress in plants will be advantageous in sustainable agriculture.