May 2, 2013
With the warmer conditions and warming soil temperatures planters started rolling across Nebraska over the weekend and early this week. By mid week, however, field work stopped due to rain, snow, and a return to colder temperatures. Now growers are wondering how this cold stress and the potential for inbibitional chilling may affect soybean seeds that have been planted.
Soybean Germination Process
See updated cold stress injury threshold in April 21, 2015 CropWatch article.
The germination stage of soybean consists first of a very fast uptake of water (imbibitional phase) followed by a much slower uptake of water (osmotic phase). Chilling during the first phase can cause severe problems because the imbibed water is needed to rehydrate the cotyledons and embryo to the point that cell membranes become functional. Cold temperatures interfere with proper hydration of those membranes. The imbibitional phase is typically not very long (less than 24 hours) and can occur with relatively little soil moisture since the seed is dry. Thus, getting a cold rain 0-24 hours after you plant can lead to chilling injury in soybean and lower stands. A study by UNL researcher W.J. Bramlage et al. showed that when the soybean seed coat was removed, imbibition injury occurred within 30 minutes. With a seed coat, imbibition is slower and a longer exposure would be needed before chilling injury occurs.
A key point to consider is that chilling injury is likely to be greater if soil temperatures were cold (less than 40° F) at planting rather than becoming cold 24 or more hours after sowing. The longer the seed is in the ground at warm soil temperatures before cold temperatures occur, the less likelihood there is of chilling injury. The bottom line is: Plant your soybeans if you think the soil temperatures won't get cold (less than 40°F) for at least 24 hours. If you planted two or more days before the cold rain, there should be no imbibitional injury due to cold temperature.
During the second phase of germination, the fully functional membranes (after imbibitional hydration) create an osmotic situation in which water diffuses into the living cells. Osmotic water uptake is slow with cold temperatures. Chilling during this phase causes little direct injury to the germinating seedling. Cold temperatures will, however, slow emergence.
Be Aware of Soil Moisture Content
More important than cold soil temperatures after imbibitional water uptake is the soil moisture content. Cold soil delays the germination to emergence process, but cold soil plus soggy soil conditions of some duration can substantively reduce soybean emergence. Why? Soil-borne pathogens love soggy soil. Since the period of seedling germination to emergence takes longer due to the cold, those pathogens will have more time to infect the seedling cotyledons and access their carbohydrate, protein, and lipid reserves. The seedling needs those cotyledon reserves to live on until the unifoliolate leaves start photosynthesis and form more carbohydrates. This is why fungicide seed treatments are crucial if planting soybeans in April or early May and in cold, wet conditions. Ultimately, it is soggy soil with cold temperatures that kill your germination.
For additional research see Dynamics of Imbibition of Soybean Embryos by C.W. Vertucci et al. in Plant Physiology, the journal of the American Society of Plant Biologists.
Jim Specht, UNL Professor of Agronomy and Horticulture
Jenny Rees, Extension Educator