Matthews, Stella
(2024)
Effect of nano-encapsulated bio-stimulant on the growth, yield, and quality of chilli (Capsicum annuum L.).
PhD thesis, University of Nottingham.
Abstract
Bio-stimulants have demonstrated significant positive outcomes in enhancing plant health, boosting soil fertility, mitigating the adverse effects of environmental stress on plant growth, improving tolerance to heavy metal contamination in soil, and augmenting crop quality and yield. Limited studies have reported the usage of bio-stimulants in soilless medium to enhance plant growth, yield and quality. There are lack of innovative microbial and plant-based bio-stimulants for soilless cultivation systems. Only scarce information is available on the combined effect of bio-stimulants of various origins. Furthermore, microbial-based bio-stimulants have short shelf-life, which makes them not suitable for soilless systems. The hypothesis of the study is co-application of nano-encapsulated bio-stimulants, specifically endophytic plant growth-promoting bacteria and moringa leaf extract, may positively impact the growth, yield, and quality of chilli plants cultivated in soilless media. The main aim of this study is to evaluate the synergistic effect of nano-encapsulated bio-stimulants consisting of endophytic bacteria and moringa leaf extract in soilless medium.
To achieve this objective, this research was conducted using encapsulated nano-bio-stimulant to overcome the rapid degradation and increase the absorption efficiency by plants. Seventy endophytic bacteria were screened for various biochemical assays and plant growth-promoting traits in chilli seedlings. Fresh and dry moringa leaf was extracted using aqueous and polar solvents via maceration, decoction, and infusion methods to determine the phytochemical contents and plant growth-promoting efficiency of the extracts. Later, the selected endophytic bacteria was incorporated with nano-alginate whereas moringa leaf extract was encapsulated into nano-emulsion. The population of bacteria in nano-gel was determined after 6 months of shelf-life, and nano-emulsion was characterized using zetasizer. A completely randomized block design (RCBD) was adopted for this experiment. There were two treatments, T1 (Liquid fertilizer) and T2 (Liquid fertilizer and nano-encapsulated bio-stimulant) with five replications of each treatment. Each replication consists of eight plants. The efficiency of encapsulated nano-biostimulant consisting of endophytic bacteria and moringa leaf extract applied via drenching method were evaluated on the seed germination, vegetative growth, yield, and post-harvest quality of chilli. Post-harvest analyses include chroma value, phenolic, flavonoid, ascorbic acid, and capsaicin content of chilli fruits. Finally, chilli root microbiome analysis was done using next-generation sequencing (NGS) to determine the impact of nano-encapsulated bio-stimulant on bacterial population in chilli roots.
Bacillus subtilis and Streptomyces panaciradicis were selected from seventy endophytic bacteria isolated from the leaf and root of neem, sesbania, moringa, and chilli plants. The selection was based on their potential to exhibit beneficial traits in biochemical assays, reduce mean seed germination time, increase germination energy, enhance Chilli's root colonization, and improve chilli seedlings' vegetative growth. Aqueous extraction of dry moringa leaf produced more flavonoids (74.89 quercetin equivalent mg/ml) compared to polar solvent extraction methods. A combined application of moringa leaf extract and endophytic bacteria demonstrated a positive response on the root and aboveground biomass and yield of chilli. Aqueous extraction of dry moringa leaf could be formulated into a stable oil-in-water nano-emulsion with particle size below 230 nm, polydispersity below 0.4, and a zeta potential value of more than -30 mV. Endophytic bacteria were formulated into a nano-alginate gel and could maintain a bacterial population of 1 x 108 cfu/ml throughout the storage period of six months. Nano-encapsulated bio-stimulants, when applied together on a soilless medium, increased the yield, flavonoid, phenolic, and ascorbic acid content of chilli. The total yield of chilli plants treated with nano-encapsulated bio-stimulant was 575.41g, whereas chilli plants not treated with bio-stimulants recorded a 522.16g yield. Chilli plants treated with bio-stimulant showed a higher L value of -15.38±11.9 and chroma valuef of 8.77±5.56 in chilli fruits compared to -8.29±7.97 and 5.30±6 in chillies not treated with bio-stimulants. This finding is in agreement with the enhanced photosynthesis activity of chilli plants treated with bio-stimulants and the metabolites produced by endophytic bacteria such as rhodopsin, rhodoxanthan, lycoxanthan, rubixanthin, echinenone, carotene, zeaxanthin, and astaxanthin. Post-harvest quality analysis revealed the potential of nano-encapsulated bio-stimulants in increasing flavonoid content in red chillies to 15.7 mg/L, phenolic contents in green chillies to 0.908 mg/L and ascorbic acids in red chillies to 144.29 mg/L compared to chillies that were not treated with bio-stimulants. Chillies without bio-stimulant treatment recorded 13.0 mg/L of flavonoids, 0.82 mg/L of phenolic contents and 124 mg/L of ascorbic acid respectively. Root microbiome analysis revealed that nano-encapsulated bio-stimulant treatment has the majority of unique groups of bacteria that did not exist in other treatments. The principal variation of bacteria between treatments was assessed based on Weighted UniFrac and Bray-Curtis distances, which explained 48.11% and 44.96 % of the total observed variation.
This study has proved the potential of applying nano-encapsulated bio-stimulants to positively promote the growth, yield, and quality of chilli cultivated in a soilless medium. It is a pioneer research on the effect of nano-formulated bio-stimulants in a soilless medium. However, further research on the bio-stimulant dosage, application time, and method could maximize the benefit of bio-stimulant on crops.
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