Importance of the antioxidant capacity of phenolic metabolites in plant resistance against oxidative stress

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Our previous results demonstrated that phenolic metabolites (PHENs) present in leaves, by their effects on UV-induced fluorescence, represent internal probes that can reveal imbalances between plant growth and photosynthesis. Besides their effects on fluorescence, PHENs have antioxidant properties that may play a protecting role against oxidative stress in plants. The long term objective of this research program is to induce the accumulation of PHENs in order to protect plants against oxidative stress that may enhance their productivity and food quality. The global approach of this research program will be first to improve our estimations of PHEN antioxidant activities directly in leaves (in planta), then test if growth regulators and cultural practices could enhance PHEN concentrations and antioxidant activities with minimal metabolic costs, and finally demonstrate if increased PHEN concentrations in leaves result in higher resistance against oxidative stress caused by excess light and heavy metals. More specifically, we will first define an index of PHEN antioxidant activities in planta based on a set of parameters measured in leaves from several (crop, aromatic and medicinal) species, using notably cyclic voltammetry to assess the concentration of oxidisable compounds and their redox potential. In the 2nd phase, experiments with dwarf sunflower ("Sunspot") plants will demonstrate how the induction of PHEN accumulations by growth regulators (salicylic acid, Actigard®, jasmonic acid) and cultural practices (short term nitrogen deficiency or root hypoxia) can enhance PHEN antioxidant capacities and this, at minimal metabolic costs. Finally, we will test if "activated" plants that have increased PHEN concentrations, enhanced antioxidant capacities, and ideally, no sign of oxidative damages (normal growth and photosynthesis) are more resistant than normal plants against oxidative stress caused by photoinhibition (excess of light intensity) or copper.