Foliar Abscission in Cotton. III. Macroscopic and Microscopic Changes Associated with Natural and Chemically Induced Leaf-Fall

1. The stage of physiological maturity is very important in regulating the abscission of cotton leaves. Different anatomical and physiological responses occur when defoliants of different chemical classes are used to induce abscission. 2. Cotton pulvinoid structures can be divided into four age classes to facilitate a more exact means of studying anatomical and physiological changes occurring during maturation. 3. Morphologically, differentiation of the abscission zone in cotton occurs very late in leaf maturation. Microscopically, the first evidences of differentiation in this zone are thinning of the cell walls and a poor staining reaction of cells within this specific zone. 4. Tracheary elements are plugged rapidly on the distal side of the abscission zone by tyloses which form concomitantly with the general hydrolysis of cell walls in the abscission zone. 5. Healing of the abscission wound is accomplished by the formation of a periderm layer four or five cells in from the abscission wound. Formation of the cork tissue occurs after the cell division preceding abscission. 6. The abscission of cotton leaves induced by chemicals appears to follow two general patterns. Defoliants such as Endothal (disodium-3,6-endoxy-hexahydrophthalate) induce and accelerate the dropping of leaves in much the manner of naturally abscising leaves, which involves secondary cell division. On the other hand, defoliants such as Shed-A-Leaf (sodium chlorate-metaborate) and amino triazole (3-amino-1,2,4-triazole) stimulate a rapid hydrolysis of walls across the abscission zone with little or no cell division preceding separation.