Why do plants die when they are covered with oil?

Plants, like many other species, depend on the surface transport of a number of compounds. Key among such compounds are oxygen and carbon dioxide. Plants produce oxygen during the process of photosynthesis and use carbon dioxide as a reactant for the same. They also consume oxygen and release carbon dioxide during cellular respiration. The exchange of gases with atmosphere takes place through the leaves, more specifically through pores known as stomata. When the surface of a plant is covered with oil, stomata will be covered as well. This would mean that the plant will no longer be able to exchange gases with the atmosphere and will not be able to survive.

In a somewhat similar fashion, an oil layer over ocean water kills the marine species. This is because the oil layer prevents any exchange of gases between water and atmosphere, which causes a depletion in dissolved oxygen levels and a rise in carbon dioxide levels, thereby killing the organisms.

Hope this helps.