Culturing geranium at different doses of Zn from 0-1.0 g m -3 (Zn 0 to Zn 1.000 ) revealed that Zn is an antioxidant promoter, apart from its micronutrient essentiality. Zn 0.250 was the critical concentration for maximum content (0.21 %) of total essential monoterpene oil(s). At Zn 0.005 -Zn 0.250 , net photosynthetic rate, and contents of chlorophyll and essential monoterpene oil(s) were affected. The maximum peroxidase activity was obtained at Zn 0.250 , with the production of biomolecule geraniol. We found an oxido-reducible reaction of Zn in the formation of monoterpene essential oil(s) and possibly major constituents of geraniol.