Promoting the Growth of Butterhead Lettuce by Bio-Fermented Extract from Purple Non-Sulfur Bacteria in Genus 𝙍𝙝𝙤𝙙𝙤𝙥𝙨𝙚𝙪𝙙𝙤𝙢𝙤𝙣𝙖𝙨
Keywords:
good agricultural practices, growth, butterhead, bio-fermented extract, plant hormoneAbstract
Background and Objectives: Purple non-sulfur photosynthetic bacteria (PNSB) can produce several compounds that benefit plant growth, such as 5-aminolaevulinic acid, auxin, and cytokinin. As a result, they are used to produce bio-fermented liquid fertilizers for various applications. However, the use of PNSB-based bio-fermented liquid fertilizers is affected by variations in suitable strains, bio-fermentation methods, and application to different cash crops, as well as by limited basic knowledge of photosynthetic microbial strains in Thailand compared with countries with highly advanced agricultural research, such as Japan. Consequently, this research aims to investigate photosynthetic bacteria for cash crop cultivation, starting with isolating the bacteria, followed by selecting suitable strains for the production of bio-fermented extract fertilizers, and developing practical application models for cash crop cultivation. Growth tests were conducted on butterhead lettuce, a high-value leafy vegetable that is popular among consumers who prefer fresh vegetables and is promoted for organic cultivation.
Methodology: Soil and water samples were collected from agricultural and natural areas in nine provinces in northern and central Thailand. Photosynthetic bacteria were isolated using GM broth. Pure cultures of the isolates were then screened to select efficient isolates for producing bio-fermented extract and promoting plant growth. Growth rate, 5-aminolevulinic acid (5-ALA) production, indole-3-acetic acid (IAA) production, protein digestion, cellulose digestion, and phosphate solubilization were tested. The selected isolates were classified by species of photosynthetic bacteria, and the effects of their bio-fermented extracts was tested on butterhead lettuce (cv. MJ-7) grown in pots under eight treatment conditions. In addition, plant growth and yield data were recorded.
Main Results: From the fieldwork conducted in nine provinces in northern and central Thailand, a total of 108 samples were collected, consisting of 44 soil samples and 64 water samples. Fifty-nine pure photosynthetic bacterial isolates were obtained. Growth rate testing showed that 51 isolates exhibited good growth after 7 days of incubation. The ability of these isolates to promote plant growth was further tested in two respects: biodegradation capacity and plant hormone production. The biodegradation tests showed that 15 isolates were able to solubilize inorganic phosphate. The cellulose and protein degradation tests revealed that two isolates were able to degrade cellulose, and three isolates were able to degrade protein (gelatin). Regarding the production of the two plant growth-promoting substances, all isolates produced 5-ALA, while 50 isolates produced IAA. Based on the evaluation of these six characteristics, 12 isolates were selected for species identification using PCR and DNA sequencing of the 16S rRNA gene. The resulting nucleotide sequences were compared with those available in the NCBI database, and a phylogenetic tree was constructed using MEGA 12 software. It was found that all 12 selected isolates belonged to the genus Rhodopseudomonas, with four isolates classified as Rhodopseudomonas sp. and eight isolates classified as R. palustris. Three efficient isolates, UT-7, UT-8, and UT-67, were selected to produce bio-fermented extract, which was then tested on the growth of butterhead lettuce under eight different treatments. The treatments revealed that the canopy width of butterhead lettuce was greatest in treatment 8, at 38.33 ± 2.69 centimeters, which was significantly different from other treatments (p ≤ 0.01).
Conclusions: From 108 soil and water samples collected from agricultural and natural areas in nine provinces in northern and central Thailand, 59 photosynthetic bacterial isolates were obtained. Three isolates, UT-7, UT-8, and UT-67, were selected based on their efficiency and ability to produce plant-beneficial substances. In addition, their effects on the growth of butterhead lettuce were tested. The bio-fermented extract from isolate UT-7 significantly promoted plant height, while the bio-fermented extract from UT-67 significantly increased the number of leaves. This extract also showed a tendency to increase fresh weight; however, this increase was not statistically significant compared with the control group.
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