Growing medium amendments effect on growth, secondary metabolites and anti-streptococcal activity of two species of Plectranthus

Plants belonging to the genus Plectranthus are versatile aromatic perennial shrubs that are commonly used in cuisine, medicine and landscaping. A study was carried out to determine the effects of different growing medium amendments on growth, secondary metabolites and anti-microbial properties of Plectranthus amboinicus (Lour.) Spreng and Plectranthus ‘Nicoletta’. The greenhouse pot-experiment tested three different amendments to Promix-BX™ growing medium potting mix as follows: 120g of K-humate; 120g vermicasts; and 1.16gN20P20K20. Rooted cuttings were grown in 670g of Promix-BX™ potting mix with or without an amendment, and finally harvested after 100days. Plants grown in the vermicasts had significantly (P<0.01) high fresh weight as compared with plants grown in the K-humate or the N20P20K20 but there was no significant (P>0.05) difference between the two species. Vermicasts application significantly (P<0.05) reduced plant mass density but increased specific leaf area index as compared to the N20P20K20, K-humate or the Promix-BX™ alone treatment. Overall, cavacrol production was higher in P. amboinicus whereas total carotenoid and phenolic production were higher in P. ‘Nicoletta’. The N20P20K20 treatment gave the most yield of secondary metabolites while the Promix-BX™ alone gave the least. Both species had the same Minimum Bactericidal Concentration (MBC) of ca. 500μg/mL and Minimum Inhibitory Concentration (MIC) of ca. 250μg/mL, which were all higher than that of the pure carvacrol (ca. 198μg/mL). In conclusion, the different growing medium amendments had different degree of effect on the growth and phytochemistry of these two P. amboinicus and P. ‘Nicoletta’. Whereas vermicasts was most effective on plant growth and development, N20P20K20 enhanced secondary metabolite content. Both species demonstrated similar effect on growth reduction in Streptococcus pyogenes. The results of this pot-experiment are only preliminary. Future research should focus on field plot experiments for general conclusion on cultivation practices.