ULTRASTRUCTURE AND HISTOCHEMISTRY OF LIPOIDAL DROPLETS IN VESSEL CONTACT CELLS AND ADJACENT PARENCHYMA CELLS IN COTTON PLANTS INFECTED BY FUSARIUM-OXYSPORUM F SP VASINFECTUM

Osmiophilic droplets developed in contact cells and adjacent vascular parenchyma cells of cotton accompanied by changes in cytoplasmic organization after vessel infection by Fusarium oxysporum f.sp. vasinfectum. In the early stages of the cellular response to vessel infection, the droplets were observed in dilations of ER cisternac, the membrane envelopes of mitochondria, and occasionally the membrane envelopes of plastids and nuclei. As the cellular response to vessel infection proceeded and cytoplasmic reorganization became apparent, the droplets were associated with vesicles or remained as individual droplets within the cytoplasm. The developmental sequence suggests that the osmiophilic substances in these droplets may be secreted through the plasmalemma, infused into apposition layers, and exuded into the infected vessels through pits. Sudan Black B staining and organic solvent extraction indicate that the osmiophilic droplets are lipoidal in nature. It is suggested that the lipoidal substances are terpenoids that could function as phytoalexins. © 1991.