Leaf Abscission in Soybean: Cytochemical and Ultrastructural Changes following Benzylaminopurine Treatment

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Abstract
6-benzylaminopurine (BAP) delays leaf abscission of soybean Glycine max (L.) Merr. Abscission of the distal pulvinus of primary leaves was induced in 12-d-old seedlings or explants by removal of the leaf blade. BAP applied to the cut end of the pulvinus following leaf blade removal delayed abscission. Discoloration of the pulvinus occurred before abscission commenced and the number of grana in chloroplasts within cortical parenchyma cells of the pulvinus decreased over time following leaf blade removal. BAP prevented discoloration of pulvinus tissues and a decrease in grana number. Starch grains within amyloplasts of cells of the starch sheath in the pulvinus disappeared following leaf blade removal, whereas starch accumulated within the abscission zone prior to abscission. BAP prevented this apparent redistribution of starch and instead promoted an increase in starch within plastids of cortical parenchyma cells of the pulvinus. During the abscission process, cells within the separation layer enlarged and their nuclei and nucleoli became more evident prior to their separation from one another. Cell separation resulted from breakdown of middle lamellae and partial degradation of primary cell walls. Cycloheximide applied directly to the external surface of the abscission zone inhibited abscission in a similar way to the BAP treatment. These results suggest that BAP prevents abscission by altering patterns of starch distribution in the pulvinus and abscission zone and by inhibiting the synthesis of proteins that typically appear de novo in induced abscission zone tissues.

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