Heating of susceptible sugarcane leaves (4 h at 35 degrees C) renders them resistant, for 24 h, to the effects of helminthosporoside. Membrane ATPase activity is reduced by 50% as a result of the heat treatment. When the leaves again become susceptible (after 24 h), membrane. ATPase activity is fully restored. 2. Inhibitors of membrane ATPase activity protect susceptible leaves from the effects of helminthosporoside (KF, EDTA, and octylguanidine). 3. Helminthosporoside activates (30%) membrane ATPase in microsomes from susceptible, but not heat-treated (resistant) leaves. Once heat-treated leaves again become susceptible, helminthosporoside activation of membrane ATPase activity resumes. 4. A plot of the production of helminthosporoside-induced symptoms, and membrane ATPase activity as a function of the reciprocal of the absolute temperature reveals that both have sharp breaks at 32 degrees C. 5. Protoplasts of susceptible cane are rendered insensitivity to the effects of the toxin in a medium deficient in K+ and Mg2+. When these ions are added, cell sensitivity to the toxin is restored. Since K+ uptake in plants is mediated by membrane ATPase, a connection with this enzyme activity can be made to cell sensitivity to the toxin.
