Plant Growth Regulation

Christopher S. Walsh

The regulation of plant growth and form has been a topic of intense biological research. In horticulture, considerable scientific effort has been spent studying hormonal development, with the goal of improving fruit productivity and quality. Much of this work has focused on the exogenous applications of plant growth regulators (PGRs). PGRs are chemicals that mimic hormonal effects on development. In some cases, the hormone itself can be reapplied as a PGR. For example, gibberellin can be applied in the field to reduce russet on apples, and ethylene can be applied in storage to preripen peaches and nectarines. But, in most cases, PGRs are exogenously applied chemicals that are structurally similar to the endogenous plant hormone but are resistant to inactivation in the field. Consequently, PGRs can mimic endogenous hormones but can be synthesized and applied far less expensively than the endogenous plant hormone.

While this discussion is intended to give relevant information to answer production questions, it presents an overview of processes and materials, rather than exact chemical recommendations. Chemical registrations, product names, and formulations are constantly changing. For this discussion to remain useful, it is necessary to present a broader picture of strategies needed, rather than focusing on particular products and rates. Timely information on rates and products is available by consulting commercial labels and local recommendations.

PGR usage in temperate fruit trees can be grouped into four broad categories: (1) regulation of tree vigor and the enhancement of flowering, (2) chemical thinning, (3) control of preharvest drop of fruit, and (4) specialty applications in targeted situations. In these four categories, fruit growers are most likely to use chemical thinning and preharvest drop control annually, if the appropriate materials are registered for their crop. Regulation of vigor, enhancements of flowering, and specialty applications are available but are typically used on a small proportion of fruit acreage, such as young trees, or to address problems inherent in a particular cultivar.

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