Selective Toxin ofHelminthosporium victoriae: Thermal Relationships in Effects on Oat Tissues and Protoplasts

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Abstract
Electrolyte leakage induced in oat leaves susceptible to toxin from H. victoriae was minimal at temperatures below 12.degree. C; above 12.degree. C, the rate of leakage increased with increases in temperature. Protoplasts from leaves were held at temperatures of 0-40.degree. C, and the fluidity of membranes was measured by ESR with 5-doxylstearic acid as a spin label. A membrane phase transformation was observed at 12.degree. .+-. 1.degree. C. The temperature correlation between toxin-induced leakage and membrane fluidity suggests that loss of electrolytes occurs through the membrane matrix, possibly via a shuttle-type carrier. Toxin rapidly killed oat mesophyll protoplasts as shown by the use of fluorescein diacetate a vital stain. Collapse of protoplasts followed death at 35.degree. C but not at 23.degree. C. The vital stain showed that protoplasts are as sensitive to toxin as are leaf tissues and intact roots. Tissues treated with toxin at 0.degree. C and held at that temperature had no toxin-induced leakage. Tissues exposed to toxin at 0.degree. C, washed, and then warmed to 23.degree. C lost as much as did tissues held at 23.degree. C throughout. Presumably, the initial step in toxic activity can occur at 0.degree. C, but processes leading to electrolyte leakage occur only at higher temperatures.

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