Role of the Hypodermis and Secondary Cell Wall Thickening in Basal Stem Internodes in Resistance to Strawbreaker Foot Rot in Winter Wheat

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Abstract
Anatomical characteristics of the 1st elongated internode of 11 winter wheats were studied to determine their association with resistance to Pseudocercosporella herpotrichoides. Based on disease indices, ''VPM-1'' was highly resistant: ''Cappelle-Desprez'', ''Cerco'', ''Golils'' and ''Rubigus'' were resistant; ''Viking'' was intermediate: and ''Daws'', ''Nugaines'', ''Sprague'', ''Stephens'' and Selection 101 were susceptible to strawbreaker foot rot in the field. Resistance was correlated with hypodermis width and number of hypodermal cell layers, but not with epidermal cell wall thickness, lumen diameter, or stem diameter. Cell wall thickening and lignification occurred earlier in resistant than in susceptible wheats. The fungus invaded all wheats extensively, damaging parenchyma and vascular tissues, particularly phloem. Parenchyma cell walls were more susceptible to damage than lignified cell walls. Epidermal cell walls became thickened and lignified prior to contact with the fungus. Pith parenchyma cell walls became thickened and lignified in diseased tissue. Lignified cell wall appositions (lignitubers) were larger and formed in greater profusion at penetration sites in epidermal cells of stems of resistant than of susceptible wheats. Resistance to P. herpotrichoides is correlated with structural attributes of the host, these differences become more apparent as host maturity approaches.