3-Chlorophenoxypropionic Acid as a Plant Growth Regulator in Fruit Crops: A Critical Review of Crop-Specific Responses and Unresolved Mechanisms

3-Chlorophenoxypropionic acid (3-CPA; cloprop; CAS 101-10-0), formulated commercially as the propionamide-dominant preparation Fruitone CPA, is a synthetic auxin-class plant growth regulator with an experimental record across four horticultural crops spanning approximately six decades. This review synthesizes the available 3-CPA-specific literature on peach (Prunus persica), pineapple (Ananas comosus), grape (Vitis vinifera), and pecan (Carya illinoensis), keeping 3-CPA carefully distinguished from the structurally related chlorophenoxyacetic and pyridine compounds with which it is readily confused in the broader fruit-growth-regulation literature. The evidence is strongest in peach and pineapple. In peach, fruitlet thinning emerges as the product of four interacting conditions, developmental timing, cultivar identity, applied dose, and tree physiological status, with developmental timing the most consistently corroborated across independent research programs and no mechanism of action demonstrated. In pineapple, three biologically distinct responses are documented over four decades and multiple cultivars: inhibition of natural-differentiation flowering, delayed fruit maturation, and reduced crown size. Pineapple is also the only crop system in which 3-CPA-treated tissue was directly assayed for hormonal markers: ethylene-pathway indicators increased under 3-CPA treatment while flowering was simultaneously inhibited, the opposite of ethephon’s flowering-inducing effect, and the underlying mechanism remains, in the original investigator’s words, not known. Grape and pecan each contribute a single preliminary observation that extends the documented activity range without providing corroborative depth. Across all four systems, directional polar auxin transport flux has not been measured under 3-CPA treatment, so pool-level hormone data cannot resolve the mechanistic question that most interpretive frameworks require. This review covers the available 3-CPA-specific literature rather than a global systematic search; it makes no application or dosing recommendations, and modern validation under contemporary germplasm and applicable regulatory frameworks would be required before any historical finding could inform current practice. Our contribution is to organize a dispersed record, enforce compound-specific evidence boundaries, and define the questions that the six-decade literature leaves open.