Other benefits of plant growth retardants

Plant growth retardants, or PGRs, can do much more than control plant stretch. They can make for darker green plants and improve profits, too.

Many growers understand that plant growth retardants (PGRs) are an effective tool to help control plant stretch (Photo 1). But did you know there are other benefits to using certain PGRs as well? The PGRs that block the gibberellic acid (GA) pathway control growth because GA is a plant hormone that stimulates cell elongation in plants. By inhibiting GA, there is less cell elongation (i.e., stretch). PGRs that work in this manner include those that contain ancymidol, chlormequat chloride, daminozide, fluprimidol, paclobutrazol and uniconazole, and are the focus of this Michigan State University Extension article.

Photo 1. Cosmos bipinnatus ‘Sonata Carmine’ four weeks after PGR treatment. Left: Control (no PGR spray). Right: 3,000 ppm daminozide spray at 2 quarts per 100 ft2.

Greener leaves are one additional benefit to using these types of PGRs. This happens for two reasons. First, the cells are smaller, so chlorophyll is more concentrated in the cell. Second, there is an increase in chlorophyll production because some metabolic energy is diverted from GA synthesis into chlorophyll production.

Plants treated with PGRs can also exhibit improved water use and less water stress. This is likely due to the reduction in leaf size that PGRs provide, which require less water than their larger-leafed, non-PGR-treated counterparts. Some researchers also believe that the blocked GA pathway causes an increase in abscisic acid production, which promotes stomatal closure that reduces water loss and improves water use as well.

In some cases, use of PGRs may also help suppress disease. Fluprimidol and paclobutrazol both block sterol production in fungi, which fungi need to grow. Although neither product will provide season-long suppression of fungi development, it certainly is an added PGR benefit.

Of course, PGR applications require purchasing the products and labor to apply. Brian Whipker of North Carolina State University addresses this issue by performing an economic analysis of PGR use. His analysis shows that PGR-treated crops can be spaced tighter, which potentially translates into increased profit per pot. For details of his analysis, see the Greenhouse Grower article “How PGRs Make You Money.”

For more detailed information on how the PGRs that block GA pathway work, see the short video “Three Additional Benefits of Plant Growth Regulators,” authored by Whipker. For much more detailed information about PGRs in general, see the publication “Selecting and Using Plant Growth Regulators on Floricultural Crops” published by Virginia Cooperative Extension and written by Joyce Latimer and Whipker, or visit the MSU Floriculture website on PGRs.  

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