Phenols as a biochemical basis of resistance in wheat against Karnal bunt

The effects of the highly aggressive isolate KB-2 of the Karnal bunt pathogen (Neovossia indica) on phenol metabolism, peroxidase (POX) and its isoenzymes were studied in wheat. Two resistant genotypes, HD 29 of bread wheat (Triticum aestivum) and DWL 5023 of durum wheat (T. durum), and one susceptible bread wheat, WL 711, were used. In the bread wheats, total phenols reached a maximum 2 days after inoculation (d.a.i.). In the resistant durum line, total phenols did not change significantly for 6 d.a.i., but declined significantly at 10 d.a.i. Three phenolic compounds, caffeic acid, l-tyrosine and hydroquinone, were detected using thin-layer chromatography. The first two were detected at all times at and after inoculation, but hydroquinone was detected only in the resistant wheats at 6 d.a.i. The activity of POX was highest at 2 d.a.i. in the two resistant wheats, but increased more slowly to a peak at 6 d.a.i. in the susceptible wheat. The number of isoenzymes of POX detected by polyacrylamide gel electrophoresis (PAGE) changed after inoculation with KB-2. The maximum number of isoenzymes occurred at 2 d.a.i. in the two resistant wheats and at 6 d.a.i. in the susceptible wheat. Although the isoenzymes detected in seedlings were not identical to those detected in seeds, the PAGE banding patterns of seeds and seedlings were the same for the two resistant wheats. The potential use of the additional band at a relative mobility of 0·42 in seeds and 0·28 in seedlings as markers for Karnal bunt resistance is discussed.