Changes in the Phyllosphere-associated Bacteria of Leafy Greens Caused by Environmental Factors Such as Solar Radiation

Introduction: The phyllosphere associated bacteria represents a dynamic population in constant change, probably affected by extrinsic factors such as agricultural practices and climatic conditions. Among climatic factors, solar radiation has been highlighted as a major factor affecting bacterial colonization and survival. However, the impact of solar radiation on specific bacteria phyla of leafy greens has not been addressed.

Purpose: This study was conducted to determine the effect of solar radiation on the bacterial community of red-pigmented baby leaf lettuce using cultivation dependent and independent techniques.

Methods: Baby lettuce, grown in open field, were subjected to two different solar radiation intensities using a light-excluding plastic mulch to reduce the sunlight exposure. Four weeks before harvest, the field was divided in two plots; one of them was covered, while the other was kept uncovered. Photosynthetically active radiation (PAR) was measured weekly and the cumulative PAR was calculated to distinguish the treatments as 4265±356 µmol/m²/s (uncovered) and 3115±313 µmol/m²/s (covered). The impact of the solar radiation on the bacterial communities was quantified using qPCR primers for the 16S ribosomal sequences of the most predominant phyla, including Gammaproteobacteria, Betaproteobacteria, Alphaproteobacteria, Bacteroidetes, Actinobacterias and Firmicutes, as as well as total bacteria.

Results: The total bacterial population size was not affected by solar radiation treatments. However, significant differences were observed in the relative abundance of bacterial community between treatments. In general, solar radiation reduced the relative abundance of Gammaproteobacteria, while it increased the relative abundance of Betaproteobacteria. Obtained results, also, confirmed that Proteobacteria, Bacteroidetes, Actinobacterias and Firmicutes were the most abundant phyla in baby pigmented lettuce.

Significance: This study described, for the first time, the impact of the solar radiation on specific groups mong the phyllosphere-associated bacteria in pigmented baby lettuce. The obtained results are relevant, but should be completed to determine if the changes on the relative abundance of specific phyla might have an impact on the safety of leafy greens.