Water stress is one of the major limiting factors of crop growth and productivity worldwide. Plectranthus amboinicus (Jamaican Thyme) is an aromatic, spicy and medicinal plant which is also valued in the pharmacological industry. Although the plant is generally known to tolerate some levels of drought, its response to sequences of different water stresses is unknown. In this study, we investigated the effect of flooding and drought on the growth and development of Plectranthus amboinicus. The treatments were regular watering (RW), flooding (FL), drought (DR), and rehydration (RH) after drought. The result showed that plant height was significantly (p < 0.001) reduced in DR and FL plants while fresh aboveground weight was increased by ca. 17.4% in FL compared to RW. Stomatal conductance and transpiration rates were higher in FL plants but reduced drastically in DR. Moreover, chlorophyll fluorometric indices including maximum quantum yield efficiency of Photosystem II (Fv/Fm) and potential photosynthesis capacity (Fv/Fo) were significantly (p < 0.001) increased in the FL but decreased in DR plants. Leaf relative water content was significantly (p < 0.01) highest in the RH followed by the RW, and the least in DR and DH. Leaf cell electrolyte leakage was significantly (p < 0.05) higher in the RH followed by the RW while FL recorded the least cell electrolyte leakage. These results indicate that Jamaican thyme regulates physiological characteristics to tolerate flooding and demonstrate that DR poses a severe threat to plant development.
