title: Structural and biochemical characteristics of citrus flowers associated with defence against a fungal pathogen.
abstract: The constitutive characters of plants can be structural or biochemical and play an important role in their defence against pathogens. Citrus postbloom fruit drop (PFD) caused by Colletotrichum spp. is one of the most important fungal diseases of citrus. The pathogen infects the flowers, leading to premature fruit drop and reducing citrus production. However, flower buds smaller than 8 mm long are usually not infected by Colletotrichum spp. Thus, this study investigated whether there are constitutive mechanisms in flower buds related to Colletotrichum spp. infection. We studied flower buds that were 2, 3, 4, 8, 12 and 15 mm long and petals, after anthesis, of sweet orange 'Valência' using light and scanning electron microscopy and histochemistry. We evaluated the effect of volatile organic compounds (VOCs) in flowers (R-limonene and linalool) on the in vitro growth of Colletotrichum acutatum. We found that the arrangement of the epidermal papillae in the petal primordia, the occurrence of prismatic crystals and the distribution of oil glands are the main differences between buds smaller than 8 mm and buds 8-15 mm long. Osmophores at the tips of petals produced and accumulated phenols, terpenes and lipophilic compounds. Flower buds smaller than 8 mm long have constitutive structural and biochemical barriers to Colletotrichum spp. infection. In addition, this is the first time that osmophores have been reported in citrus. Our study shows that natural terpenes of Citrus flowers inhibit the fungal growth in vitro, highlighting the potential use of terpenes for the chemical control of PFD in citrus.
authors.affiliation: Universidade de São Paulo, Escola Superior de Agricultura "Luiz de Queiroz", Cx. Postal 9, CEP 13418-900, Piracicaba, SP, Brazil joaopaulormarques@yahoo.com.br.
authors.forename: João Paulo Rodrigues
authors.initials: JP
authors.lastname: Marques
authors.affiliation: Universidade de São Paulo, Escola Superior de Agricultura "Luiz de Queiroz", Cx. Postal 9, CEP 13418-900, Piracicaba, SP, Brazil.
authors.forename: Lilian
authors.initials: L
authors.lastname: Amorim
authors.affiliation: Fundo de Defesa da Citricultura - FUNDECITRUS, Av. Adhemar Pereira de Barros, 201 | CEP: 14807-040, Vila Melhado, Araraquara, SP, Brazil.
authors.forename: Geraldo José
authors.initials: GJ
authors.lastname: Silva-Junior
authors.affiliation: Universidade de São Paulo, Escola Superior de Agricultura "Luiz de Queiroz", Cx. Postal 9, CEP 13418-900, Piracicaba, SP, Brazil.
authors.forename: Marcel Bellato
authors.initials: MB
authors.lastname: Spósito
authors.affiliation: Universidade de São Paulo, Escola Superior de Agricultura "Luiz de Queiroz", Cx. Postal 9, CEP 13418-900, Piracicaba, SP, Brazil.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Appezzato-da Gloria
keywords: Antimicrobial activity; Citrus sinensis; Colletotrichum acutatum; calcium oxalate crystals; citrus essential oils; osmophores; plant anatomy; volatile organic compounds