How do growth-regulating sprays delay fruit drop before harvest?

In fruit farming, many fruits—such as apples, pears, oranges, lemons, and grapefruit—are prone to dropping from the tree during the final stages of maturation. To mitigate this, growers apply dilute sprays of growth-regulating substances, such as naphthaleneacetic acid (NAA), to delay preharvest drop.1

These substances function by mimicking or supplementing auxins, a class of plant hormones that regulate various developmental processes, including abscission (the natural separation of plant organs like leaves or fruit from the stem). By applying these synthetic compounds, farmers can effectively prolong the period the fruit remains attached to the tree, allowing for more flexible harvesting schedules and reducing potential crop loss. Other substances, such as the chemical spray Alar, are also used to reduce fruit drop while simultaneously improving fruit quality, including firmness and color.231