Abscission or shedding occurs when the hydrolytic enzymes cellulase and pectinase weaken cell walls and middle lamellae of cells in the abscission zone at the base of the pedicel. Any condition that increases shedding must increase the activity of these enzymes. Hormones affect the synthesis of cellulase and pectinase in the abscission zone. IAA inhibits their synthesis whereas ethylene stimulates the synthesis of these hydrolytic enzymes. Ethylene also slows movement of IAA and causes its destruction by oxidative decarboxylation. ABA may also increase cellulase and pectinase activity by stimulating ethylene production (and possibly by a more direct effect).