Cell Division and Expansion in Petals during Flower Development and Opening in Eustoma grandiflorum

We investigated morphological changes in petal cells during flower development and opening in Eustoma grandiflorum. The morphology of petal epidermal cells was observed by scanning electron microscopy, and their number was determined. The numbers of adaxial and abaxial epidermal cells increased during flower development. Increase in these numbers terminated before flower opening earlier in abaxial than in adaxial epidermal cells. Measurements of cell number and area showed that the petal growing stage during flower development and opening can be divided into four phases: cell division and expansion, cell division, cell division and expansion, and cell expansion. Adaxial epidermal cells in the petal blade showed a conical papillate shape whereas adaxial epidermal cells in the petal claw were longitudinally elongated in shape. Abaxial epidermal cells were longitudinally elongated in both petal blade and claw. The ultrastructure of petal cells at the bud stage and the open stage was observed by transmission electron microscopy. In the petal cells at the bud stage, nuclei and several plastids were observed, although the cells were mainly occupied with vacuoles. Relatively large spherical electron-dense bodies were observed only in the vacuoles of adaxial epidermal cells at the bud stage. The petal cells were largely occupied with enlarged vacuoles at the open stage. We conclude that petal growth in Eustoma is divided into four phases, based on the activities of cell division and expansion, and that petal growth in the final phase is mainly due to cell expansion with marked enlargement of vacuoles.