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Five distinctive developmental stages were chronologically suggested. Cells at Stage I and II were essentially free of cytoplasmic or vacuolar abnormalities and the cytoplasm contained numerous electron-dense mitochondria with well-developed cristae. At Stage III, there were a localized dilation of mitochondria matrix and a partial-diluted cytoplasm in mesophyll cells. At Stage IV, characterized by high levels of fresh weight and osmolality, most mesophyll cells were seen to be ruptured, resulting in a general mixing of cell contents and diluting cytoplasm. It can be explained as an irreversible senescence phenomena that tonoplast in mesophyll cell was ruptured partly, corresponding to rapid increase in petal cell size and turgidity. Petal turgidity was due to an increase of content in soluble sugar. At Stage V, there was a loss of petal fresh weight. With a loss of turgidity, most mesophyll cells have collapsed completely. There were a notable plasmolysis in vasculature. The activity of protease in petals was found to increase between Stage II and III, and then decreased rapidly at Stage IV, resulting in the decrease of total protein content before senescence. Unexpectedly, there were stomata in hibiscus petals. Ultrastructural disorganization, like as a broken tonoplast, was observed in mesophyll cells at Stage IV. ABA and the stomata on petal might promote the disorganization. The final stages of senescence involved breakdown of cellular organization leading to hydrolysis of previously separated compartments. The cellular disorganization triggered during the flowers are still in the process of opening may be one of the earliest physiological signal that senescence is under way.

140 Ultrastructural Changes during the Senescence of Petals in Hibiscus syriacus L.