No synergy between P and AMF inoculation to improve Sun Protection Factor production in Anadenanthera colubrina (Vell.) Brenan leaves

Secondary metabolite production in medicinal plants and other plants is improved by arbuscular mycorrhizae, plant stress, as well as nutritional factors. The compounds responsible for Sun Protection Factor from Anadenanthera colubrina (Vell.) such as flavonoids and proanthocyanidins may also increase by manipulation of plant stress or nutrient status, and inoculation with arbuscular mycorrhizal fungi (AMF). However, specific gaps regarding the viability of this agricultural tool need clarification, especially regarding the role of Phosphorus (P), one of the mycorrhizal symbiosis regulators and widely employed in plant cultivation. Therefore, this study aimed to understand the role of P in regulating SPF in Anadenanthera colubrina when cultivated in association with arbuscular mycorrhizal fungi (AMF). To this end, two concentrations of P₂O₅ (8 mg dm⁻³ and 50 mg dm⁻³) were evaluated, and two inoculation treatments were tested: control and seedlings inoculated with AMF inoculant. After 150 days in a greenhouse, the leaves were collected to obtain the ethanolic extracts and used to assess the SPF, production of flavonols, proanthocyanidins, saponins, pigments, and in vitro antioxidant activity. The presence of higher P levels in the soil (50 mg dm⁻³) renders mycorrhizal inoculation dispensable for the enhancement of SPF, production of photoprotective compounds, and antioxidant activity as plants grown under this condition showed a six-fold increase compared to non-inoculated seedlings kept in soil with a lower P level. On the other hand, leaves of A. colubrina seedlings inoculated with AMF and cultivated in soil supplemented with 8 mg P dm⁻³ displayed a SPF with a 4.9-fold increase and the biosynthesis of photoprotective and antioxidant compounds enhanced by more than five-fold compared to those not inoculated. Despite these benefits, these cultivation practices did not increase the accumulation of carotenoids and chlorophylls. Mycorrhizal inoculation and phosphate fertilization are agricultural practices that, independently, contribute to the increased SPF, biosynthesis of secondary metabolites, and antioxidant activity in A. colubrina leaves.