Phenolic content as an indicator of tolerance of cowpea seedlings to Sclerotium rolfsii

The role of phenolics in plant tolerance to pathogen infection is well documented. The objective of the present preliminary investigation was to study phenolic metabolites involved in the tolerance or susceptibility of cowpea (Vigna unguiculata Walp.) cultivars to Sclerotium rolfsii Sacc. and to use their presence as a possible screening tool. Total, free acid, ester-bound and cell wall-bound phenolics of 10 cowpea cultivars were quantified. In healthy seedlings, the tolerant cultivars displayed the higher phenol content than the susceptible cultivars. In S. rolfsii infected seedlings, the highest increase was found from 48 h after inoculation. The net effect of inoculation was a 630% increase in total phenolics (soluble and insoluble) in the stem of tolerant cultivars while the total phenolic content increased only by 212% in the stems of susceptible cultivars. Although, no significant difference (P = 0.05) was detected among cultivars, in terms of free acid phenolics, the amount of ester-bound and cell wall-bound phenolics significantly increased, therefore demonstrating a similar trend to the one observed for the total phenolic content. These preliminary results showed that the presence of phenolics before and after S. rolfsii infection may be used as a rapid screening method for detection of tolerance to S. rolfsii damping-off and stem rot of cowpea.