The role of non-glandular emergences in avoiding ozone (O₃) damages by preventing its entrance into leaf tissues was previously suggested to the O₃-tolerant species Croton floribundus (Euphorbiaceae). However, this function against O₃ damage has been underestimated due to the covering wax layer mostly composed of saturated hydrocarbon with low reactivity to this gas. To evaluate the role of these structures in conferring tolerance to O₃, we mechanically removed the non-glandular emergences from leaf blades of C. floribundus, submitted the plants to acute O₃ fumigation, and assessed the morphological and microscopic alterations. Plants with intact leaves showed the same phenotype of control samples but showed microscopic markers of accelerated senescence. These alterations indicated a whole-plant response to O₃. Plants bearing leaves with emergences removed exhibited specific morphological symptoms and microscopic markers of O₃. The leaf emergences encompass a barrier for volatiles contention, preventing O₃ damage to leaf tissues of C. floribundus. The absence of these structures possibly disperses defense volatiles quickly and allows this species to become vulnerable to O₃ effects. This study highlights the relevance of surface structures in confer resistance of plants against O₃ damages over biochemical defenses alone.

Intact leaves of Croton floribundus were compared to leaves with non-glandular emergences mechanically removed to assess the role of these structures in preventing O₃ stress. Samples from the ozone fumigation experiment and filtered air plants were taken 24 h and 48 h after the exposure. Part of the samples was immediately observed under bright and UV light (fresh material), and part fixed in Karnovsky’s solution, dehydrated, embedded in historesin, and sectioned at 1.5 – 2 µm thickness using a microtome. Slides were stained with toluidine blue O and mounted in water. Observations of both fresh and fixed samples were performed in an epifluorescence microscope equipped with a Hg lamp, blue and green filter sets.