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.
Highlights
Plant species are attacked by a wide range of pathogens and herbivores, which can alter host survival, growth and reproduction (Wittstock and Gershenzon 2002; Agrios 2005)
This study aims to describe the anatomical structure of flower buds at different developmental stages to identify the presence of constitutive defences to infection caused by C. acutatum
C. sinensis plants receiving high doses of Ca possibly show a large number of calcium oxalate (CaOx) crystals in flower buds and, are more resistant to infection caused by C. acutatum
Summary
Plant species are attacked by a wide range of pathogens and herbivores, which can alter host survival, growth and reproduction (Wittstock and Gershenzon 2002; Agrios 2005). Plants defend themselves against these enemies using a combination of structural characters and biochemical reactions, which can be either constitutive or induced by attack (Wittstock and Gershenzon 2002; Agrios 2005; Hanley et al 2007). Constitutive structural traits important to pathogen resistance include cell walls with different compositions, waxy epidermal cuticles, cuticles thickness, trichomes, idioblasts, sclereids and crystalliferous cells (Hudgins et al 2003; Smith et al 2006; Freeman and Beattie 2008; Koch and Ensikat 2008)
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