Monoterpenes are a group of biogenic volatile organic compounds (BVOCs) known for their role in plant response to environmental stresses. Their composition and emission variation under stress conditions are closely related cellular and intercellular functioning. But several aspects of their function unclear. This thesis firstly investigated the physiological (leaf gas exchange, water status, and redox balance) responses of tomato plants exposed to well-watered conditions and deficit irrigation in combination with exogenous monoterpenes (Chapter 2) at 1.25, 2.5 and 5 mM concentrations. Exogenous monoterpenes decreased foliar ROS accumulation by enhancing enzymatic antioxidant capacity, thereby reducing oxidative damage. However, this does not appear to protect photosynthetic performance. Whether endogenous monoterpenes (Chapter 3 & 4) had similar effects was explored by exposing three transgenic tobaccos (Nicotiana tabacum) with upregulated monoterpene precursor genes and individually inserted genes for (-)-α/β-pinene (PG11), myrcene (MG1), and (-)-limonene (LG12) and their wild-type (WT) to drought stress (withholding water) before and during stem elongation. Significant monoterpene emissions were detected in PG11 and LG12 but not in MG1 and WT. The results provided basic information of the impact of overproduction of monoterpene in a non-native emitter and suggested trade-offs in growth and development. The LG12 line was selected to determine whether enhanced endogenous limonene emission provides additional antioxidative protection to plants under water deficit. LG12 plants maintained leaf water status under drought, showed increased emission rate as soil moisture declined initially, then decreased dramatically as drought persisted. LG12 plants also had decreased gas exchange and increased lipid peroxidation compared to WT plants. After re-watering, these adverse effects were reversed, with the highest ascorbate peroxidase (APX) activity and lowest lipid damage in LG12, which may benefit plants' long-term recovery from drought. These findings highlight the complexity of monoterpene interactions with plant stress response and oxidative regulation. They suggest a better understanding of the physiological and biochemical significance of monoterpenes in plant drought response might assist efforts in enhancing drought tolerance.