Effects of Monoterpenes on Physiological Processes During Seed Germination and Seedling Growth

The mechanisms by which monoterpenes released by certain species affect the germination and growth of other plants remain to be elucidated. The information that is currently available suggests that they interfere with some physiological processes, including respiration, mitosis, the action of phytohormones, antioxidant defence systems and membrane properties. The data obtained by our group and other data from the literature have enabled us to propose a mechanism of action for α-pinene, the most active monoterpene that has been identified in our works. α-Pinene acts on mitochondria as an inhibitor of electron flow through cytochrome-oxidase pathway. This action likely leads to an increase in mitochondrial reactive oxygen species (ROS) production and consequently, to membrane lipoperoxidation. These effects are manifested in intact tissues, as indicated by the inhibition of KCN-sensitive respiration (cytochrome-oxidase pathway), stimulation of KCN-insensitive respiration (mitochondrial alternative oxidase and extramitochondrial oxidases), activation of lipoxygenase and antioxidant enzymes, and increased concentrations of lipoperoxide products. A comparison of the effects of α- pinene on different plant species has revealed that the potential of α-pinene to induce cellular oxidative stress depends on the balance between the generation of ROS and the capacity of the antioxidant defence system. Seeds with high concentrations of lipoxygenase seem to be more susceptible to α-pinene-induced oxidative damage. Many of the cellular activities that have been reported to be altered by monoterpenes require the metabolic energy of ATP and/or membrane integrity. A perturbation on mitochondrial functions may thus be a primary mechanism of the phytotoxic action of some monoterpenes, particularly during seed germination and initial seedling growth when ATP production depends mainly on mitochondrial metabolism.