While products containing plant growth hormones have been around for years, there has been a renewed interest recently. If you're considering these options, be sure to approach them as you would any new product; learn exactly what they are and whether they'll benefit your particular production system.
Plant hormones that are often contained in these products are indole-3-butyric acid (IBA) and kinetin. These chemical compounds are plant growth hormones and fall within broader plant hormone categories: IBA is a type of hormone called auxin, while kinetin is a type of cytokinin. These growth hormones are classified differently based on their chemical structure, but they can cause similar physiological responses in the plant.
Auxin and cytokinin are critical growth hormones in plant development and are naturally present within the plant at variable concentrations throughout the season. Their presence and activity are different from other hormones which act more in an on-off manner and are present only at specific times. Cytokinins regulate many cellular processes and stimulate cell division. Cytokinin is synthesized largely in root tissue and then travels upward to the shoots; some production also occurs in developing leaves. Nutrients will move and accumulate in plant tissue where higher levels of cytokinin are expressed. Auxins are primarily produced in areas that are experiencing rapid growth such as shoot tissue, young leaves and developing seeds. Auxins inhibit primary root elongation but do promote lateral root development. Senescence (death) and shedding of leaves is regulated by both auxin and cytokinin. During the reproductive stages, the abortion of flowers as well as flower and seed development are regulated and promoted by auxin and cytokinin.
Typically, these products are to be applied in the early vegetative stages. Research documenting the effect of growth hormone application onto foliage is largely focused on applications that occur near flowering because of auxin and cytokinins critical roles in seed development. Therefore there is little available research to document the impact earlier applications would have in a field setting. What can we learn though by looking at the impact the application of these hormones have when applied later in the growing season?
Plant responses to cytokinin and auxin have been variable. Here we will focus primarily on field research (soybean and corn) although laboratory research reports also are available. In one study, cytokinin was applied to soybean at R1 (initial flowering) yet no difference in the pod number, seed number, seed weight, or seed yield resulted in comparison to an untreated control (Nagel, 2001). Other researchers looked at the effect of cytokinin when applied to two soybean varieties (small and large seeded) at R1 or R3. The varieties did not perform the same which shows that varietal differences exist. The small-seeded variety had increased seed weights and seed yield following treatment at R3, whereas, the large-seeded variety had increased seed weight and pod number but not increased seed yield with the R1 treatment. The application of growth hormones may increase pod numbers, seed weight or seed yield but this will vary based on varietal sensitivity and correct application timing (Cho, 2002). Laboratory research conducted on immature corn ears (prolific and non-prolific types) showed cytokinin to have little or no effect on ear growth or development. Ear growth and development was increased though when auxin was included in the treatment mixture (Leal-León, 2002).
Although there is a possible benefit to applying these growth hormone products, it is possible to also cause detrimental effects to the plant. With the foliar application of these products the normal level of these hormones within the plant are increased, causing a redirection of the plant's energy. In some instances, plants with cytokinin applied have appeared smaller and have a less developed root system. Although reduced plant height and root system differences may not correlate to yield differences, it is important to realize that the plant is being impacted by these treatments.
Also, the concentrations of cytokinin and IBA in marketed products are often unclear and therefore the results obtained may not be similar to the results mentioned from the noted research. It is also difficult to know how the plant will respond to growth hormones applied at early vegetative stages since previous research has been focused largely on later applications.
Nagel et al. 2001. Annals of Botany. 88:27-31.
Cho et al. 2002. Plant Growth Regulation. 36:215-221.
Leal-León et al. 2002. Plant Cell, Tissue and Organ Culture. 71:133-139.
Portions of this text, written by Lori Abendroth and Roger Elmore, are taken from a Crop Watch article (University of Nebraska extension newsletter) published May 20, 2005.