Editor’s note: This is the second in a series of three articles on the role of hormones in soybean plants.
Soybean reproduction is all about supporting and increasing cell division. Cell division drives flower fertility and survival, pod set, beans per pod and, ultimately, seed size and weight.
Soybean plants have a unique trigger which occurs at R1, but often goes unnoticed. The first flowers appearance triggers a signal in the plant to focus its energy on increasing root growth. If you recall from our first article, the roots are the major source of the plant hormone, cytokinin. What the first flowers tell the plant is to ramp up cytokinin production because it’s going to need it to set pods and fill beans.
The average soybean raceme, or node, has the capability of setting well over 10 pods, but most set 2 to 5 pods on average. We understand that many soybean flowers, approximately 60 to 70%, typically end up aborting. But why does this occur? Plant stress is a major cause, more specifically two plant hormones, ABA (abscisic acid) and ethylene, are the culprits. These two stress hormones are the main reason we see flower, pod and seed abortion, and end up with a mere 2 to 5 pods per node.
When a plant experiences stress it must burn energy to deal with the stress. Because of this spent energy, the soybean plant is unable to utilize these same resources to develop new pods and seeds. New developing plant parts need energy to survive and any energy deficiency in the plant causes new pods and seeds to simply abort, resulting in yield loss. After all, a plant’s one main purpose in life is to reproduce and ensure the survival of the species. It really makes no difference to the plant if it produces 1 seed or 200+ seeds per plant—survival is the goal.
In crop production, Mother Nature can be as mean or as kind as she wants. Consider the weather during a soybean’s reproduction crop stage, typically the two hottest and driest months of the year. It’s no surprise our plants are under a lot of repeated stress. Stress drives a plant to naturally respond by producing these same two hormones at increased levels.
When ABA levels rise in the plant, plant growth slows or, in extreme conditions, even stops nearly all above-ground plant activity. Since developing pods are above ground this can have very negative implications for yield.
Ethylene can cause cell death. It is a gas given off by the stressed cell and affects all the cells around it. When ABA and ethylene levels are high, they tend to suppress the important growth promotion hormones; cytokinin, auxin and gibberellic acid. Foliar applications of these hormones during the reproduction cycle or during high-stress periods can lead to significant yield advantages.
Soybean reproduction is all about supporting and increasing cell division. Cell division is strongly driven by cytokinin. Cytokinins are produced in the roots and are passively transported upward with water, via transpiration. When field conditions limit transpiration in any manner, be it lack of water (drought), high humidity or high temperatures, cytokinin becomes limiting. When cytokinin is limited, cell division is slowed and when cell division slows during reproduction we end up with few seeds at harvest.
When these types of stress conditions occur, foliar applications of cytokinin can have significant yield implications. In past research conducted by Stoller USA, foliar applications have proven to develop upwards of 20 percent more pods and beans per plant. Keep in mind yield comes from three components: plants per acre, seeds per plant and seed size. The more pods we can protect and set, and seeds we can develop, the greater the yield potential for your crop.
About the Author: Rob Jarek
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