RESUMO
Ionic silver (Ag+) is being investigated as a residual biocide for use in NASA spacecraft potable water systems on future crewed missions. This water will be used to irrigate future spaceflight crop production systems. We have evaluated the impact of three concentrations (31 ppb, 125 ppb, and 500 ppb) of ionic silver biocide solutions on lettuce in an arcillite (calcinated clay particle substrate) and hydroponic (substrate-less) growth setup after 28 days. Lettuce plant growth was reduced in the hydroponic samples treated with 31 ppb silver and severely stunted for samples treated at 125 ppb and 500 ppb silver. No growth defects were observed in arcillite-grown lettuce. Silver was detectable in the hydroponic-grown lettuce leaves at each concentration but was not detected in the arcillite-grown lettuce leaves. Specifically, when 125 ppb silver water was applied to a hydroponics tray, Ag+ was detected at an average amount of 7 µg/g (dry weight) in lettuce leaves. The increase in Ag+ corresponded with a decrease in several essential elements in the lettuce tissue (Ca, K, P, S). In the arcillite growth setup, silver did not impact the plant root zone microbiome in terms of alpha diversity and relative abundance between treatments and control. However, with increasing silver concentration, the alpha diversity increased in lettuce root samples and in the water from the hydroponics tray samples. The genera in the hydroponic root and water samples were similar across the silver concentrations but displayed different relative abundances. This suggests that ionic silver was acting as a selective pressure for the microbes that colonize the hydroponic water. The surviving microbes likely utilized exudates from the stunted plant roots as a carbon source. Analysis of the root-associated microbiomes in response to silver showed enrichment of metagenomic pathways associated with alternate carbon source utilization, fatty-acid synthesis, and the ppGpp (guanosine 3'-diphosphate 5'-diphosphate) stringent response global regulatory system that operates under conditions of environmental stress. Nutrient solutions containing Ag+ in concentrations greater than 31 ppb in hydroponic systems lacking cation-exchange capacity can severely impact crop production due to stunting of plant growth.
RESUMO
Scutellaria baicalensis is a well-studied medicinal plant belonging to the Lamiaceae family, prized for the unique 4'-deoxyflavones produced in its roots. In this study, three native species to the Americas, S. lateriflora, S. arenicola, and S. integrifolia were identified by DNA barcoding, and phylogenetic relationships were established with other economically important Lamiaceae members. Furthermore, flavone profiles of native species were explored. 4'-deoxyflavones including baicalein, baicalin, wogonin, wogonoside, chrysin and 4'-hydroxyflavones, scutellarein, scutellarin, and apigenin, were quantified from leaves, stems, and roots. Qualitative, and quantitative differences were identified in their flavone profiles along with characteristic tissue-specific accumulation. 4'-deoxyflavones accumulated in relatively high concentrations in root tissues compared to aerial tissues in all species except S. lateriflora. Baicalin, the most abundant 4'-deoxyflavone detected, was localized in the roots of S. baicalensis and leaves of S. lateriflora, indicating differential accumulation patterns between the species. S. arenicola and S. integrifolia are phylogenetically closely related with similar flavone profiles and distribution patterns. Additionally, the S. arenicola leaf flavone profile was dominated by two major unknown peaks, identified using LC-MS/MS to most likely be luteolin-7-O-glucuronide and 5,7,2'-trihydroxy-6-methoxyflavone 7-O-glucuronide. Collectively, results presented in this study suggest an evolutionary divergence of flavonoid metabolic pathway in the Scutellaria genus of Lamiaceae.
