Fusaric acid treatments induced a greater proportion of the total ion content to leak out from mature tomato leaf tissues than from young leaf tissues. Delaying senescence by treating the plants with kinetin reduced the effects of the toxin on leaf tissue as measured by chlorophyll degradation. T-toxin treatments of maize leaves also induced greater percentage leakage from older leaves than from young leaves. The higher chlorophyll contents of tomato plants grown with high nitrate nitrogen levels compared to plants grown with low nitrate levels indicated greater juvenility in the former tissues than in the latter. The ratio of phospholipids to free sterols increased with increasing levels of nitrogen fertilization, indicating possible changes in membrane composition. These results indicate that young tissues or tissues in which the juvenile state is prolonged by growing the plants with high levels of nitrate nitrogen or by kinetin treatments have greater non-specific resistance to toxins than mature of senescing tissues.