Lemon balm (Melissa officinalis L.) is an important medicinal and aromatic plant belonging to the Lamiaceae family. Besides essential oil, lemon balm also contains several phenolic substances, with rosmarinic acid (RA) being the most important one. For a pharmaceutical use, a minimal RA content of 1% is required by the European Pharmacopoeia. The use of various kinds of plant protection nets is increasingly popular. They have been proposed for the prevention of leafhopper infestations in the cultivation of medicinal and aromatic plants, like lemon balm, as well. As they also lead to a light reduction, this work wanted to determine how growth, yield, and quality of lemon balm plants are influenced by the use of nets with shading properties at two different locations. A moderate light reduction of about 10-15% was tested at the experimental site in Gross-Gerau (GG), and a stronger light reduction of about 50% in Rauischholzhausen (RH). The plant stands were harvested for three years, in two cuts per year. For the determination of possible interaction effects, different genotypes and harvest stages were investigated as additional factors. Regarding the plant parameters, no significant influence of the moderate light reduction was observed. The stronger light reduction, however, led in many cases to changes that are typical signs of the shade avoidance syndrome of the plants. The yield level in RH was on average higher than in GG. No effect of the moderate shading in GG on biomass or leaf yield, dry matter content of the leaves, or leaf:stem-ration was observed. The stronger light reduction in RH, however, decreased the dry matter content of the leaves, and, partly, biomass and leaf yield, especially for quite recently shaded plants. The measured quality parameters total phenolic content (TPC), antioxidant capacity (ORAC assay), as well as essential oil and RA content were not reproducibly affected by the moderate light reduction in GG. Also under the stronger light reduction in RH, essential oil content was not reproducibly influenced. TPC, antioxidant capacity, as well as RA content, however, were decreased under the stronger light reduction in RH. No reproducible pattern could be observed regarding a genotype effect on the tested growth, yield, and quality parameters, except for the essential oil content, which was highest in the genotype Lemona . Differences were greater between the two experimental sites, indicating a greater importance of the environmental conditions than the genotypes for the investigated parameters. It can be concluded that the use of plant protection nets with a stronger light reduction can be regarded as negative under the tested cultivation conditions. However, the use of plant protection nets with only a moderate light reduction seems to be possible for the cultivation of lemon balm under comparable cultivation conditions.
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