Iron Nitrates Stimulate Phenolic Compound Production by Melissa officinalis L.

Iron plays a key role in plant chemical processes and photosynthesis. The aim of this study was to investigate the effect of various iron concentrations on total phenolic compound levels and photosynthetic efficiency in Melissa officinalis. It is a very popular herb, widely cultivated worldwide for its medicinal properties. Herbs were grown under controlled laboratory conditions. Gas exchange parameters, chlorophyll index, and total content of phenolic compounds were determined. Additionally, iron, manganese, copper, and zinc concentrations in either soil or plant tissues were analyzed. The hypothesis that iron supplementation significantly affects photosynthesis and total phenolic compounds in lemon balm was confirmed by one-way analysis of variance. In particular, the beneficial biofortification at 100 and 200 µg/g iron doses generates a moderate increase in phenolic compounds and simultaneously does not hamper plant growth and photosynthetic efficiency. However, iron doses above 500 µg/g induce plant stress and trigger defense mechanisms that significantly increase phenolic compound production. This is accompanied by a decrease in biomass and photosynthetic efficiency.