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Patterns in the Growth, Oxygen Uptake, and Nitrogen Content of Single Colonies of Wheat Stem Rust on Wheat Leaves. W. R. Bushnell, Plant Physiologist, Crops Research Division, ARS, USDA, Cooperative Rust Laboratory, University of Minnesota, St. Paul 55101. Phytopathology 60:92-99. Accepted for publication 4 August 1969. DOI: 10.1094/Phyto-60-92.

Patterns of hyphal growth were determined for individual colonies of Puccinia graminis f. sp. tritici on wheat leaves 5 to 19 days after inoculation and related to the rates of O₂ uptake and total N content of 1.4-mm discs cut from positions within and adjacent to the colonies. Rates of colony elongation for partially incompatible host-parasite combinations (infection types 2 and 2-) were 18 to 55% of those for a compatible combination (infection type 4). The growth rate for each infection type was nearly constant from the time of first visible fleck until host leaves became senescent, even though a ring of chlorotic host tissue developed around colonies of incompatible combinations. Rates of O₂ uptake in the sporulating centers of the colonies of infection type 4 were 10 to 15 times the rates of healthy tissues, whereas the N content increased threefold. Tissues invaded by nonsporulating hyphae at the border of type 4 colonies had rates of O₂ uptake two to four times those of controls without detectable change in N content. Uncolonized tissues located just beyond hyphal tips at the apical or basal ends of type 4 colonies had no alteration in respiratory activity. In contrast, the rate of O₂ uptake in noninvaded barley tissues adjacent to colonies of powdery mildew was 120% of the rate in healthy tissue. With infection types 2 and 2-, the rates of O₂ uptake at colony centers were increased two- to four-fold, but only to rates one-fourth to one-half those of infection type 4 for equal amounts of fungus development. However, O₂ uptake increased slightly in the noninvaded tissues adjacent to colonies of infection type 2- when peripheral chlorotic rings were produced. Thus, an increase in respiration in hyphal-free tissues near advancing rust colonies could be detected only in the case of an incompatible host-parasite combination.