To examine H2O2 accumulation at the cellular level, the tissues of the stained leaves were subjected to microscopic analysis. Some mesophyll cells stained brownish as early as 3 hpi (Figure 2G). Beginning at 6 hpi, the number of stained cells increased significantly. At 12 hpi, stained cells showed a deep brown color, indicating strong accumulation of H2O2, and contained granules that also stained brown. The walls of some stained cells were broken, resulting in an outflow of granules (Figure 2H). At 24 hpi, most of the stained mesophyll cells collapsed and the epidermal cells were also deeply stained (Figure 2I); at 36 hpi, the guard cells were stained and both epidermal and mesophyll cells collapsed (Figure 2J). These data revealed that H2O2 accumulation occurred before hypersensitive necrosis was visible.