Differential Changes in Some Antioxidant Enzymes and Biomolecules in Leaves of Tomato Infected by Phytophthora infestans (Mont.) de Bary

In Cameroon, tomato yields remain low due to attack by pathogens and insects. Phytophtora infestans (PI) is a phytopathogenic fungus responsible for the downy mildew of tomatoes, a fungal disease responsible for enormous economic losses. To contribute to the search for means of control, the stimulatory effect of the tomato defence system by extracts of some plants was evaluated. Tomato plants were treated with the aqueous macerates of Callistemon citrinus (C. citrinus) and Cymbopogon citratus (C. citratus) at 10% (W/V). After 4 days of spraying with the macerates, plants were inoculated with a virulent strain of PI. Tomato leaves collected at 2-day intervals for 10 days were used to determine the contents of chlorophyll a, proteins, carbohydrates, lipids, amino acids and proline. The activities of the main antioxidant enzymes were also assessed: Ascorbate Peroxidase (APX), Phenylalanine Ammonia Lyase (PAL), Peroxidase (POX), Superoxide Dismutase (SOD). The results showed that treatment of tomato plants with aqueous macerates of C. citrinus and C. citratus and their inoculation with PI induced an increase in chlorophyll a, protein, carbohydrate, lipid, amino acid and proline contents in tomato leaves; an increase in APX, PAL, POX, and SOD activities. Our results suggested that increased metabolite synthesis on the one hand, and increased enzymatic activities, on the other hand, could mitigate the oxidative damage that occured during pathogen expansion. Aqueous macerates of C. citrinus and C. citratus could be used as natural products to stimulate the defence system in tomato against Phytophtora infestans.