TIME-COURSE OF BIOSYNTHESIS OF PHENOLICS AND LIGNIN IN ROOTS OF WHEAT GENOTYPES DIFFERING IN MANGANESE EFFICIENCY AND RESISTANCE TO TAKE-ALL FUNGUS

Inhibition of lignin biosynthesis in Triticum aestivum L. roots by Mn deficiency has been suggested as the mechanism of reduced resistance of Mn-deficient wheat roots to infection by the take-all fungus (Gaeumannomyces graminis var. tritici). This study evaluated phenolics and lignin accumulation in roots of wheal genotypes differing in Mn efficiency (measured as growth and yield in Mn-deficient soils) and take-all resistance. Seedlings of the Mn-inefficient, take-all sensitive genotype Bayonet and the Mn-efficient, more take-all resistant genotype C8MM were grown in nutrient solution without added Mn for 18 d and then transferred to a Mn-deficient sandy soil fertilized with Mn at 0 or 30 mg kg(-1) Both genotypes had Mn-deficient roots and shoots at the time of transfer to the soil. Roots of both genotypes were inoculated with the take-all fungus 0, 1, 3 and 7 d after transfer. Twenty-four hours after inoculation, take-all fungus penetrated the root stele of take-all sensitive Bayonet but not of more resistant C8MM wheat. Rates of phenolics and lignin accumulation in roots declined steadily during growth in soil for up to 8 d, were higher in mature, fully differentiated parts of the root system compared to distal, younger root tissue, and were higher in Bayonet than in C8MM. Manganese fertilization did not significantly influence rates of phenolics and lignin accumulation but reduced depth of radial penetration by hyphae in both genotypes. The rate of phenolics accumulation was positively (r = 0.91 to 0.96) correlated with the rate of lignin accumulation. Mn-efficient C8MM had a higher rate of lignin accumulation per unit of phenolics than Mn-inefficient Bayonet over a wide range of phenolics synthesis rates. From this we suggest that C8MM has a more efficient mechanism for conversion of phenolics to lignin, the trait which appears related to higher take-all resistance of this genotype.