Plant growth regulators (PGRs) are a tool used to manipulate vegetative and reproductive growth, flowering, and fruit growth and development. PGRs have been successfully used in agriculture for decades to amend plant growth characteristics and maximize yield and thus grower profit. Foliar-applied PGRs are routinely used in various fruit crops for flower and fruit thinning, improving fruit set, growth and development, controlling vegetative growth, and reducing fruit drop. Citrus is no exception to the use of PGRs, which can provide significant economic advantages to citrus growers when used appropriately.

According to the Florida state legislature, PGRs are defined “as any substance or mixture of substances intended, through physiological action, for accelerating or retarding the rate of growth or maturation or for otherwise altering the behavior of ornamental or crop plants or the produce thereof, but not including substances intended as plant nutrients, trace elements, nutritional chemicals, plant inoculants, or soil amendments.”

Most of the PGRs are plant hormones, naturally occurring plant compounds. A plant hormone is a chemical signal produced in one part of the plant and then transported through vascular bundles to another part, where it triggers a response. Hormones regulate plant responses to various biotic and abiotic stimuli. PGRs are synthetic analogues of naturally occurring plant hormones (PGRs and hormones are interchangeable throughout this document). There are five classic groups of PGRs: auxins, gibberellins, cytokinins, abscisic acid, and ethylene (Table 1).

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Tripti Vashisth, W. Chris Oswalt, Mongi Zekri, Fernando M. Alferez, and Jamie D. Burrow

University of Florida, IFAS Extension.
2019–2020 Florida Citrus Production Guide: Plant Growth Regulators