Phenolic Composition of Basil Plants Is Differentially Altered by Plant Nutrient Status and Inoculation with Mycorrhizal Fungi

Basil (Ocimum basilicum) has a high market demand and has economically important uses in culinary applications, food flavorings, fragrance additives, and pharmaceutical and industrial products. Basil plants are known to produce phenolic compounds that are potentially beneficial to human health. Quality of basil used in certain fresh and dry products is a function of its production of secondary metabolites, including phenolics. Nutrient availability, particularly phosphorus (P) can alter plant production of secondary metabolites. Root infection by arbuscular mycorrhizal fungi (AMF) can also alter plant nutrient status and therefore may also alter production of phenolic compounds. The objectives of this study were to assess (1) whether P-availability and inoculation with AMF influences the phenolic composition of basil plants; and (2) how phenolic composition is related to plant nutrient status. Four cultivars of basil (‘Cinnamon’, ‘Siam Queen’, ‘Sweet Dani’, and ‘Red Rubin’) were inoculated (Glomus intraradices) or not with AMF and grown in a soilless substrate with a complete liquid fertilizer containing either 16 mg l^-1 P (low-P) or 32 mg l^-1 P (high-P). Fourteen-week-old plants were assessed for growth (biomass), AMF colonization, concentrations of anthocyanins, total phenolics, specific polyphenolics, and mineral nutrients. In general, increased P-rate and inoculation with AMF increased fresh and dry biomass production. Non-inoculated plants were not colonized by AMF and AMF colonization of inoculated plants was not influenced by P-rate. Across all cultivars, increased P-rate enhanced uptake (concentration and content) of P and Ca. Inoculation with AMF enhanced uptake of N, K, S, Mg, and B in low-P plants and enhanced uptake of S, B, and Zn in high-P plants. Differences in uptake of other nutrients between low-P and high-P plants and between non-inoculated and AMF-inoculated plants were a function of plant size. Rosmarinic acid was the predominant polyphenolic produced by all cultivars and differences in rosmarinic acid content between treatments were a function of plant size. Increased P-rate and inoculation with AMF enhanced production (concentration and content) of several polyphenolics and resulted in treatments altering the phenolic composition in the whole plant and aboveground structures. Across all cultivars, P-rate and AMF inoculation only had a similar production enhancing effect on one phenolic compound – chicoric acid. Results indicate that fertilizer management can be used to alter phenolic composition of basil and that AMF inoculation may provide an additional strategy for optimizing basil quality beyond benefits obtained from just altering plant nutrient status.