Comparative analyses of alterations in carbohydrates, amino acids, phenols and lignin in roots of three cultivars of Xanthosoma sagittifolium infected by Pythium myriotylum

The putative role of carbohydrates, amino acids, phenols and lignin in the susceptibility of Xanthosoma sagittifolium to Pythium myriotylum root rot disease was investigated by comparing the alterations of these groups of molecules in the roots of the white (susceptible), red (tolerant) and yellow (resistant) cultivars. At Day 8 after inoculation, total soluble sugars decreased. There was a positive correlation between this decrease and the root rot disease index for the white, the red and the yellow cultivars. At Day 2 proline was no longer detected in the yellow cultivar, while its content was 83% and 84% of the initial values in the white and red cultivars, respectively. At the same time, total amino acid content increased in the three cultivars. However, this increase was negatively correlated to the disease only in the yellow cultivar (r(p) = 0.940, P < 0.05). The amounts of phenols and lignin increased in the three cultivars. The increase in phenols was correlated to the disease (r(p) = 0.951 and 0.890; P < 0.05) for the red and yellow cultivars, respectively. Thin layer chromatography revealed five amino acids (arginine, proline, glycine, alanine and tyrosine) in the healthy roots of the white and the red cultivars, and three amino acids (arginine, glycine and alanine) in the yellow cultivar. Following inoculation, glycine and flavanol-like phenols (lambda(max) = 220nm) were detected in the three cultivars. The content of a non-categorised group of phenols (lambda(max) = 270nm) detected in all cultivars increased after inoculation in the red and yellow cultivars. Phenols of the hydroxycinnamic group (lambda(max) = 350nm) were specific to the inoculated roots of the red and the yellow cultivars. Glycine and flavanol-like phenols thus seem to have accumulated in X. sagittifolium roots after inoculation by P. myriotylum, and phenols of the hydroxycinnamic group seem to be specific to the red (tolerant) and the yellow (resistant) cultivars.