Accumulation of phenolic compounds in plants and nutrient solution of hydroponically grown peppers inoculated with Pythium aphanidermatum

Abstract

Root browning is a principal symptom of pythium root rot of pepper (Capsicum annuum) caused by Pythium aphanidermatum, but its biochemistry and physiological importance are poorly understood. Phenolic compounds were quantified over time in hydroponically grown pepper plants that were inoculated with zoospores of P. aphanidermatum or not inoculated (controls). The data were examined in relation to biotrophic colonization of the roots by the pathogen (roots remained white), necrotrophic colonization (roots turned brown), and plant growth. Colonization remained biotrophic in a temperature regime of 22:19 °C (temperature reduced at 2 days after inoculation) but became necrotrophic when the temperature was raised from 22 °C to 28 °C and when plants were kept continuously at 28 °C. Concentrations of free and cell-wall-bound phenolics, respectively, increased two- to three-fold and up to six-fold in inoculated roots compared with noninoculated roots maintained at 28 °C, and after the temperature was raised in the 22:28 °C regime; no differences were found in the 22:19 °C regime. Phenolic accumulation in inoculated roots coincided with, and paralleled, root browning. Pearson correlations of free and bound phenolics in relation to browning of the roots were moderate to high (r p = 0.40–0.90) and high (r p = 0.80–0.93), respectively. Observations of roots stained with p-nitroaniline indicated that phenolic compounds accumulated mainly in the brown as opposed to white roots. Increases in free and bound phenolics in the shoots coincided with phenolic accumulation and browning in the roots, but the leaves remained green. Phenolic compounds accumulated in nutrient solutions of all treatments, but levels were higher for inoculated plants with brown roots compared with those with only white roots and compared with the controls. Pythium aphanidermatum did not significantly affect dry mass of roots and shoots under conditions conducive to biotrophy (22:19 °C), but growth ceased and dry mass often declined when inoculated roots turned brown (28 °C). It is concluded that marked increases in concentrations of phenolics in roots, shoots, and nutrient solutions, and abrupt cessation of growth in hydroponically grown pepper plants inoculated with P. aphanidermatum were associated with the development of root browning.