Final answer:
The chemical pinching agent that effectively overcomes apical dominance is ethephon, which releases ethylene and promotes the growth of lateral shoots by reducing auxins.
Step-by-step explanation:
The chemical pinching agent that overcomes apical dominance is ethephon. Auxin is a plant hormone that is responsible for maintaining apical dominance through inhibiting the growth of lateral buds. While auxins are crucial in promoting flower development, fruit setting and ripening, and a variety of other plant growth functions, it is the application of ethephon, a plant growth regulator that releases ethylene, that can help overcome apical dominance. Ethylene can promote the abscission of leaves and fruits and can trigger the growth of side branches by reducing the concentration of auxins in the plant's apical meristem after being applied exogenously.