ILSOYADVISOR POST

Understanding Plant Hormones in Soybeans

As agronomists or producers, it’s our responsibility to understand how to properly manage a soybean crop for a given field’s conditions each season. Our goal should be to reduce as much stress to the soybean plant as we can, ultimately creating the right hormone balance. However, there are always situations that cause issues and for this we must continue to learn and develop on-farm trials to better understand how we should react when faced with a soybean agronomic issue.
 
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 results in aborted 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 three beans per pod instead of one or two. 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.
 
Agronomic issues that causes stress/hormone response:
  1. Planting late: Fewer flower locations, reduced root development, less time to store energy
  2. Untreated soybean seeds: Slow early vigor, diseases attack cells
  3. Saturated soils: Slow root development, lack of oxygen
  4. Dry soils: Slow movement of water and nutrients, and slow root growth
  5. Soil compaction: Slow cell division or elongation, low transpiration
  6. High air temperatures and/or drought: Closed stomates, reduce transpiration and photosynthesis, reduce cell division or elongation
Key Plant Hormones:
Cytokinin: 
  • A plant hormone that is responsible for above ground branching and leaf size (canopy development)
  • Produced near the root tips and is transported through the plant by transpiration
    • Dry and hot conditions slow the movement of water and nutrients into and around the plant, internally lowering the supply and disrupting the balance
  • Increases flower, seed development, seed size
  • 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
  • Needs to be properly balanced with auxins. An imbalance will create a smaller rooting system or a smaller vegetable /flowering system.
Auxins: 
  • A plant hormone that tends to influence the root systems (size and depth).
  • Produced in the above-ground plant parts.
Ethylene:
  • A plant hormone and 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 abortion
    • Increased cytokinin in the plant can help minimize this issue or its impact on yield components.
  • During prolonged stress, the stomata close and can impact photosynthesis (reduced CO2 intake).
  • Prolonged closure can lead to cell death and the production of ethylene.
ABA:
  • A plant hormone that can reduce vegetative growth (node development, flower production and pod set) when levels become too high.

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. Since 2015, he has been a regional agronomist with AgriGold in West Central Illinois. His previous experience includes 10 years as a sales agronomist in the GROWMARK-FS system. 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. If you have any questions for him about this article, he can be reached at steinacher@ilsoy.org.


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