Scanning electron microscopic study of modified chloroplasts in senescing broccoli florets.

摘要

Structural changes in chloroplasts of broccoli florets during senescence were examined using light microscopy, scanning electron microscopy (SEM) with freeze-fracture technique, and transmission electron microscopy (TEM) to better understand the process of chloroplast degradation, particularly at the advanced stage of senescence. Light microscopy revealed that chloroplasts, which initially were intact and green, became obscure in shape, and their colour faded during senescence. Small, coloured particles appeared in cells as the florets approached the final stage of senescence and became full-to dark-yellow in colour. SEM showed that stroma thylakoids in the chloroplast initially were parallel to each other and grana thylakoids were tightly stacked. As senescence advanced, the grana thylakoids degenerated and formed globules. The globules became larger by aggregation as senescence progressed, and the large globules, called 'thylakoid plexus', formed numerous vesicles. The vesicles ultimately wereexpelled into the cytosol, and the light microscope revealed many coloured particles in the senescent cells. The degradation of chloroplasts in broccoli florets progressed systematically, with the final product being coloured particles, which are visiblein yellow broccoli sepal cells.