ACCUMULATION OF TOTAL PHENOLICS DUE TO SILICON APPLICATION IN ROOTS OF AVOCADO TREES INFECTED WITH Phytophthora cinnamomi

Abstract

The accumulation of soluble and wall-bound phenolics and phenolic polymers in Persea americana Mill. roots from thirteen-year-old Hass on Edranol trees exposed to the pathogen Phytophthora cinnamomi and treated with water soluble potassium silicate, was investigated. Following elicitation, the conjugated and non-conjugated phenolic metabolites present in the induced root tissue were extracted and quantifi ed. From March 2005 to January 2006, three applications (Si x 3) of soluble potassium silicate per season resulted in signifi cantly higher concentrations of crude phenolic compounds in the roots compared to the untreated control. From March to May 2006, the control treatment (133.66 μg.l-1; 109.08 μg.l-1) resulted in higher crude phenolic levels compared to Si x 3 (94.61 μg.l-1; 67.98 μg.l-1). Signifi cantly higher crude phenolic concentrations in avocado roots were obtained in Si x 3 during March and May 2006 (94.61 μg.l-1; 67.98 μg.l-1) when compared to potassium phosphonate (Avoguard®) (49.07 μg.l-1; 59.46 μg.l-1). Glucoside bound phenolic acid concentrations in trees treated with Si x 3 differed signifi cantly from the untreated control for the period from January to May 2006. Concentrations of glucoside bound phenolic acids obtained with Si x 3 treatment are comparable to that of potassium phosphonate (Avoguard®) with exceptions during March 2005 and May 2006. Three silicon applications per season resulted in signifi cantly lower cell wall bound phenolic acid concentrations on avocado roots compared to the control treatment during May and Sept 2005, and March and May 2006. This trend was negated during January 2006 when a signifi cantly higher cell wall bound phenolic concentration was obtained in Si x 3 (0.71 μg.l-1) than the control (0.36 μg.l-1). Three silicon treatments per season resulted in signifi cantly lower cell wall bound phenols during May, July and Sept 2005 and May 2006 compared to Si x 1, with higher concentrations obtained in Si x 3 only during Jan 2006 (0.71 μg.l-1 vs. 0.38 μg.l-1). Results indicate that potassium silicate application leads to lower cell wall bound phenolics. Silicon treatment of avocado trees resulted in fewer identifi able phenols in avocado roots compared to the untreated control and potassium phosphonate (Avoguard®) treatments. HPLC separation of hydrolysed phenolic acids extracted from roots revealed all non-conjugated phenolic acid hydrolysed samples to contain 3,4-hydroxibenzoic acid. The glucoside bound samples of both the potassium phosphonate and the untreated control treatments contained 3,4-hydroxibenzoic acid and vanillic acid, while the control also contained syringic acid in the hydrolysed glucoside bound extract. These results indicate that potassium silicate application to avocado trees under P. cinnamomi infectious conditions increase total phenolic content of avocado root tissue. centration in different avocado cultivars, although marked differences were found between plant parts, with avocado leaves containing the highest levels, followed by fruit mesocarp, root, seed and skin extracts. In avocado some phenolics may act as antioxidants and induce resistance. These phenolic antioxidants are present in plant lipophylic regions. The soluble phenol fl avan-3-ol epicatechin is an antioxidant and acts as a trap for free radicles (Vidhyasekaran, 1997). Diene (1-acetoxy-2-hydroxy-4-oxo-hen-eicosa-12-,15diene) inhibits mycelial growth (Prusky et al., 1982; Prusky et al., 1983) and spore germination (Prusky et al., 1982), and is degraded by lipoxygenase extracted from avocado peel. An 80% increase in the specifi c activity of lipoxygenase in peel extracts occurs coincident with a rapid decrease of diene in fruit peel (Prusky et al., 1983). Epicatechin inhibits lipoxygenase in vitro, and may act as a regulator of membrane-bound lipoxygenase. Epicatechin concentration in avocado fruit peel is inversely correlated with lipoxygenase activity and decreases signifi cantly when lipoxygenase increases (Marcus et al., 1998). It is suggested that epicatechin plays a role in induced resistance by inhibiting lipoxygenase. Diene decrease is regulated by lipoxygenase activity, which in turn is regulated by a decrease in the antioxidant, epicatechin, con-