Mitotic Inhibition and Histological Changes Induced by Helminthosporium victoriae Toxin in Susceptible Oats ( Avena byzantina C. Koch) 1

Paraffin sections and root squashes of susceptible parent and resistant‐mutant oats were examined for the effects of various Helminthosporium victoriae toxin treatments on their histology and mitosis. Susceptible roots showed slight protodermal cell plasmolysis after 8 hours exposure to toxin and severe plasmolysis after 24 hours, while resistant tissue was unaffected. In susceptible tissue, root cap, meristematic, and cortex cells were less affected than protodermal cells. The frequency of mitotic figures in culture filtrate solution (containing 0.1 units of toxin/ml) treated susceptible root tips decreased to almost zero during 8 hours of treatment, at which time the resistant tissue mitotic frequency was equal to its control. Anaphases and telophases disappeared first in treated susceptible tissue, then metaphases, and last prophases, a few of which persisted until the last measurement at 8 hours. A colchicine method was used to estimate the normal minimum cell division cycle time at 13 to 14 hours. The relationship between the disappearance of the different mitotic phases within a short duration and the cell cycle time suggests that any of the cell cycle stages G 1 , S or G 2 may be sensitive to toxin. The mitotic frequency of resistant tissue is unaffected by ninhydrin negative (partially purified) toxin but is temporarily inhibited by ninhydrin positive toxin (culture filtrate).