Scanning electron microscopic study of modified chloroplasts in senescing broccoli florets

Structural changes in chloroplasts of broccoli (Brassica oleracea L., Itali ca group) florets during senescence were examined using light microscopy, S canning electron microscopy (SEM) with freeze-fracture technique, and trans mission electron microscopy (TEM) to better understand the process of chlor oplast degradation, particularly at the advanced stage of senescence. Light microscopy revealed that chloroplasts, which initially were intact and gre en, became obscure in shape, and their color faded during senescence, Small , colored particles appeared in cells as the florets approached the final s tage of senescence and became full- to dark-yellow in color, Scanning elect ron microscopy showed that stroma thylakoids in the chloroplast initially w ere parallel to each other and grana thylakoids were tightly stacked. As se nescence advanced, the grana thylakoids degenerated and formed globules, Th e globules became larger by aggregation as senescence progressed, and the l arge globules, called "thylakoid plexus," formed numerous vesicles, The ves icles ultimately were expelled into the cytosol, and the light microscope r evealed many colored particles in the senescent cells. These results indica te that the degradation of chloroplasts in broccoli florets progresses syst ematically, with the final product being colored particles, which are visib le in yellow broccoli sepal cells.