Roles of citrus secondary metabolites in tree and fruit defence against pests and pathogens

Plants have evolved several mechanisms to protect themselves from different biotic and abiotic challenges, including pest and pathogen attacks. One of the most important mechanisms is the secondary metabolites (SM) (terpenes, phenolics and nitrogen/sulphur-containing compounds). The plants have synthesised a diverse group of compounds in different concentrations (mostly in very small quantities) and have important roles in plant defence, survival, adaptation, and reproduction. Citrus crops are among the most essential cultivated fruit groups and are rich in terpenoids and phenolics. Besides the well-known benefits of these compounds on human health, they have a significant role in plant/fruit defence against biotic and abiotic challenges. This review aims to highlight the importance of these compounds (such as limonene, citral, saponin, hesperidin, quercetin, tangeritine, caffeic acid, p-coumaric acid, scoparone, etc.) and discuss their roles in tree and fruit defence against pests and pathogens. In today’s world, where there is an essential human impact on nature, a need is raised to reduce pesticides against pests and pathogens. Therefore, understanding the roles of these SM and their induction is believed to have a significant positive impact on the production and/or storage of horticultural crops and may help reduce the use of synthetic agrochemicals. In line with this information, present review was aimed to provide a background information for researchers, farmers, horticulturalists and technology developers about the potential benefits of SM on plant/fruit defence and a guideline about their induction.