LAUSR

Abstract 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: 120 g of K-humate; 120 g vermicasts; and 1.16 g N 20 P 20 K 20 . Rooted cuttings were grown in 670 g of Promix-BX™ potting mix with or without an amendment, and finally harvested after 100 days. Plants grown in the vermicasts had significantly (P  20 P 20 K 20 but there was no significant (P > 0.05) difference between the two species. Vermicasts application significantly (P  20 P 20 K 20 , 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 N 20 P 20 K 20 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 . 500 μg/mL and Minimum Inhibitory Concentration (MIC) of ca . 250 μg/mL, which were all higher than that of the pure carvacrol ( ca . 198 μg/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, N 20 P 20 K 20 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.