Ultrastructural Changes of Hibiscus syriacus L. during the Petal Senescence

This study was undertaken to explain the phenomena of senescence in Korean hibiscus (Hibiscus syriacus) flower, as an ephemeral flower. The death of flower makes it obvious that a sensitive, tightly controlled program for cell death must exist. Korean hibiscus flower provides a good model system of senescence, because events happen so quickly. 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 partially diluted cytoplasm in mesophyll cells. At Stage IV, most mesophyll cells seem to be ruptured, resulting in a general mixing of cell contents and diluting cytoplasm. It can be explained as an irreversible senescence phenomena where tonoplast in mesophyll cell ruptured partly, corresponding to rapid increase in petal cell size and turgidity. There were losses of petal fresh weight and turgidity at Stage V, followed by complete collapse of most mesophyll cells. Ultrastructural disorganization, like a broken tonoplast, was observed in mesophyll cells at Stage IV. Also, the electrolyte leakage increased from Stage III to Stage IV. The final stages of senescence involved the breakdown of cellular organization leading to hydrolysis of previously separated compartments. It is suggested that the loss of membrane integrity triggered while flowers are still in the process of opening may be one of the earliest physiological signals that senescence is under way.