Morphological aspect of blossom-end rot fruits of tomato

A high incidence of blossom-end rot (BER) fruits is a serious problem in hydroponics of tomato, especially in hot summer season. We examined the morphological aspects based on light and electron microscopic observations. The fluid-soaked area appeared on the surface of fruits at first, finally the color changed to black and corky dark brown. The cells that exhibited abnormal structure appeared at the part of epidermal and subepidermal layers at first. Electron microscopic observation showed that (1) disruption of plasma membrane and tonoplast occurred, (2) the cell wall was wavy-shaped, (3) the endoplasmic reticulum was broken down, (4) the plastids became swollen, etc. Plasma membrane and tonoplast are the sensitive parts to cell death affected by BER. The peripheral cells around necrotic tissues of BER had normal internal structure, but the plasma membrane was detached from the cell wall, suggesting occurrence of plasmolysis. The phenomenon of plasmolysis was rarely observed at other parts of fruits distant from the part of BER and in the cells of normal fruits. Those findings suggested that the distal part of fruits was under osmotic stress when BER appeared. In developed BER fruits, the sub-epidermal cells were broken down. Some cells around broken cells showed deposition of lignin, and peripheral cells showed meristematic structure like callus. It is thought that they are the response of wound healing of tomato fruits.