Application of chitosan lactate, selenite, and salicylic acid as an approach to induce biological responses and enhance secondary metabolism in Melissa officinalis L.

Given the beneficial aspects of elicitation, plant elicitors are being extensively used in recent years to increase the production of desired secondary metabolites in vitro. However, the potential of elicitors to be used in vivo is still limited due to the insufficient knowledge of their biological activity and the dependence on a number of factors, especially the elicitor type and the duration of the elicitation. For this reason, further in vivo studies are still needed to unravel the biological basis of the elicitation process. Therefore, the aim of the study was to compare the effect of three chemically different substances (150 mg/L chitosan lactate, ChL; 10 mg/L selenium as selenite, Se; 100 mg/L salicylic acid, SA; a mixture of ChL+Se+SA in the proportion of 1:1:1) on some aspects of primary and secondary metabolism in Melissa officinalis L. The main objective of the research was to determine the potential of these foliar-applied compounds in the elicitation of phenolic metabolites in vivo. The strongest elicitation effect was found after the use of ChL and SA, and a slightly weaker effect was noted after the application of Se; in turn, the mixture of the elicitors generally did not cause changes in the level of phenolic metabolites. Moreover, regardless of the elicitor type, their highest concentrations were recorded on the 8th day of elicitation, while no significant increase was generally observed on the 4th day. Among the analyzed metabolites, rosmarinic acid hexoside was most strongly induced by the elicitation, and its level increased even several times compared with the control. A very effective increase in the accumulation of rosmarinic and lithosperimic acids was detected as well. The elicitors also increased the antioxidant activity of the plant extracts. No visual signs of toxicity of these compounds or their effect on plant biomass were found, although there were some elicitor-induced changes in parameters related to oxidative stress (increase in H2O2 and O2-˙ accumulation and lipid peroxidation) and photosynthesis (reduction of chlorophyll fluorescence indicators and photosynthetic pigment concentrations). We recommend that ChL or SA alone should be applied for effective and successful elicitation of intact lemon balm in controlled pot cultivation. This method can be an easy and cost-effective alternative to in vitro, suspension, or hydroponic cultures of M. officinalis and can be widely used for acquisition of high-quality plant raw material with improved nutraceutical and pro-health values.