A soybean doesn’t care about obtaining 100 bu per acre or if it gets 10-18 podding sites or if it is planted on April 15th or June 15th. A soybean plant is genetically programmed to develop a root system to bring in nutrients and water, and to produce nodules and nitrogen with rhizobia. Soybeans care about developing a canopy that captures sunlight to produce energy to produce a single offspring, and more would be great.
Like humans, plants have hormones that signal action within and throughout the plant. Plants use these different hormones to initiate cell division and expansion (growing root, growing leaf), program cells for their specific function (root cells, flower cells) and to become aware of environmental stress (hot, dry, saturated soils) and how to react (slow down growth or abort flowers, pods and seeds).
Stress on a soybean plant requires the extra use of stored energy to minimize the impact on the plant.  When a plant reallocates energy and resources, it is essentially robbing from what was going to be spent to produce leaves, flowers, pods and seeds. Once these resources are spent, the plant must restructure demand to meet current supply—this means to abort unfertilized flowers, pods or seeds.
Once a balance has been restored and the plant is under less stress (good growing conditions resume), the soybean plant can increase the demand to utilize the current/future supply of carbon-based energy.  This means creating new podding sites, or flowers, at the top of the plant, retaining more pods, and producing 3 beans per pod instead of 1 or 2. This process can change many times throughout the growing season and is influenced through the balance or imbalance of plant hormones. Stress leading to yield reduction environments increases production of ABA (abscisic acid) and ethylene, which creates an imbalance with cytokinin and auxins.
Soybean production is often a recipe including variety selection, seed treatments, planting dates, fertility, and/or fungicide and insect applications. But in reality, it is about putting together a system using all of these to reduce the environmental stress on the plant that ultimately creates a hormone imbalance and restricts growth and yield.
As agronomists or producers, it’s our responsibility to understand how to properly manage a soybean crop for a given field and a given season to reduce as many stresses as we can, ultimately creating the right hormone balance in the plant. However, there are always situations that cause issues and for this we must continue to educate and develop on-farm trials to better understand how one should react when faced with a soybean agronomic issue.
Agronomic issues that causes stress/hormone response:
  1. Planting late: Less flower location, reduced root development, less time to store energy
  2. Untreated soybean seeds: Slow early vigor, disease attacks cells
  3. Saturated soils: Slow root development, lack of oxygen
  4. Dry soils: Slow movement of transpiration of water and nutrients, and root growth
  5. Soil compaction: Slow cell division or elongation, low transpiration
  6. High air temperatures and/or drought: Close stomates, reduce transpiration and photosynthesis, reduce cell division or elongation
Key Plant Hormones:

1. Cytokinin: 

a. A plant hormone that is responsible for above ground branching and leaf size (canopy development)

b. Produced near the root tips and is transported through the plant by transpiration

i. Dry and hot conditions slow the movement of water and nutrients into and around the plant, internally lowering the supply and disrupting the balance

c. Increases flower, seed development, seed size

d. During R1 a signal is sent to the root tip to increase the production of cytokinin so that It can be remobilized to the developing flowers and pods

e. Needs to have a proper balance with auxins.  An imbalance will create a smaller rooting system or a smaller vegetable /flowering system

2. Auxins: 

a. A plant hormone that tends to influence the root systems (size and depth).

b. Produced in the above-ground plant parts

3. Ethylene:

a. A plant hormone that is a gas given off by cells during times of increased stress, leading to cell death. Once this takes place the neighboring cell will have the same reaction, followed by cell destruction. This will shorten reproductive stages, and increase pod and seed abortion1. Increased cytokinin in the plant can help minimize this issue or its impact on yield components

b. During prolonged stress, the stomata close and can impact photosynthesis (reduced CO2 intake)

c. Prolonged closure can lead to cell death and the production of ethylene

4. ABA:

a. A plant hormone that can reduce vegetative growth (node development, flower production, and pod set) when levels become too high

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About the Author: Todd Steinacher

Steinacher is an ISA CCA Soy Envoy alum and currently supports ISA on agronomic content as well as serving as an Illinois CCA board member. He was recently awarded the 2020 IL CCA of the Year & the 2021 International CCA of the Year. He has over 15 years agronomic experience, currently working with AgriGold and GROWMARK previously. Steinacher has an associate degree from Lincoln Land Community College, a B.S. in agronomy and business from Western Illinois University and a master’s degree in crop science from the University of Illinois at Urbana-Champaign.