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Abstract
The effects of four levels of soil water content (100%, 80%, 60%, 40% SWC) on M. spicata and O. dictamnus were studied. Drought stress (40% SWC) resulted in a great reduction of the plant height and biomass in both plants. Leaves decreased in size and exhibited signs of chlorosis with local reddish patches. Leaf anatomical observations principally comprised an accumulation of anthocyanins in the vacuoles of the mesophyll cells and a diminishing of the intercellular space volume. Leaf blade thickness did not undergo any alteration in M. spicata, whereas in O. dictamnus it became greatly reduced. The decrease of SWC from 100% to 40% reflected an increase in the number of leaf peltate hairs producing the essential oil, an increase in the leaf essential oil yield, an increase in the density of mesophyll cells, and a decrease in the number of stomata in both plants. Furthermore, drought stress had a negative effect on chlorophyll a + b, chlorophyll fluorescence, photosynthetic rate, transpiration rate, and stomatal conductance. GC-MS analysis of the essential oils derived from control M. spicata and O. dictamnus plants (100% SWC) and from drought-stressed plants (40% SWC) disclosed differences in their qualitative and quantitative composition.