In apple, the glycoside phloridzin (1.27) and its aglycone phloretin are thought to inhibit the apple scab fungus Venturia inaequalis. Phloridzin is also an ortho-dihydroxyphenolic compound, and, like chlorogenic acid, can also be easily converted to a reactive quinone upon attack by a pathogen. Raa (1968) demonstrated that oxidation products of phloridzin inhibit fungal pectinases. Fungal pectinases hydrolyze pectin, a cell wall compound that is abundant in the middle lamella and plays a role in cell adhesion. Thus, by inhbiting pectinases, the ability of the fungus to hydrolyze and invade the plant cell wall would be compromised. Although phloridzin and phloretin are toxic at high concentrations, based on the poor correlation between resistance to scab and the concentration of phenolic compounds such as phloridzin, they are probably not the factors that account for the actual resistance of apple cultivars to this fungus (Nicholson and Rahe, 2004). This was further substantiated by Leser and Treutter (2005), who investigated the effect of nitrogen supply on the scab susceptibility of the susceptible apple cultivar 'Golden Delicious' and the resistant cultivar 'Rewena'. Increased nitrogen supply was hypothesized to stimulate growth and decrease the levels of phenolic compounds. This was shown to indeed be the case. Consistent with this hypothesis, the susceptible cultivar became more susceptible under high nitrogen supplements. The resistant cultivar, however, did not become susceptible, even though the levels of phenolic compounds, including phloridzin, decreased.
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