It is not uncommon for the soybean plant to abort flowers and small pods as the soybean plant adjusts to the surrounding environment. The soybean plant does this by forming abscission layer at the base of the flower of pod resulting in the loss of nutrient flow. Researchers have tried to figure out why these layers form as well as trying to minimize their occurrence as a way to increase the yield of soybeans. This research has identified several factors that contribute to these losses.
A soybean plant will abort up to 80 percent of its flowers or small pods depending on the environment in a normal year. Good growing conditions early in the spring can result in the soybean plant producing more flowers or pods than it needs, resulting in flower or pod abortion later in the summer as the plant adjusts to the growing season and the available photosynthates that have been produced through photosynthesis.
Heat or drought stress during the reproductive stages of beginning pod to full seed will increase flower or pod abortion at higher than normal rates. It has been shown that temperatures above 104 degrees severely limited pod formation. Water stress alone has been shown to decrease pod formation by 20 percent, and this affects seeds per pod and seed weight, but not to the extent of the number of pods. Stressed plants will mature earlier than normal which results in a shorter fill period lowering the seed weights.
More than 76 percent of the soybean root is located in the upper 16 inches of soil and more than 50 percent of these are located in the top 8 inches. The total rooting depth has been measured from 4.9 to 6.5 feet in normal growing conditions. The roots grow at a rate of .5 inches per day under normal conditions from planting to the R1 stage and this growth increases to 2.3 inches per day from R1 to the R3 growth stage. Root growth continues up until the R6 stage and root growth after the R5 stage is variety specific.
The soybean plant will take photosynthates and use them to promote root growth if drought stress is experienced for 3 to 5 days reducing the amount available for seed production. The soybean growth rate will return to normal if available water returns to normal.
Nitrogen fixation is affected by water stress and this reduces the amount of nitrogen that is needed for seed fill because reduced nitrogen is available for protein formation. The decrease in the N fixation is affected by limited water availability and the reduced availability of oxygen to the nodules lowering rhizobia bacteria creating a nodule carbon shortage.
Plant hormones have an effect on flower and pod abortion as well. Auxin and cytokinin and how they influence yield have been researched for many years. It has been found that basal flowers have a higher concentration of cytokinin when compared to the distal flowers, or those flowers at the top of the plant, resulting in reduced flower and pod abortion at the plant. Research shows that removal of the base flowers resulted in a retention of the distal flowers. Post applications of cytokinin have been applied to soybeans and it has been found that it increases flowers but this increase is dependent on the environment. Synthetic Auxins have been applied as well and its application results in a decrease in flower retention.
The good news is that soybeans are one of the most resilient plants that we produce as a commercial crop. If conditions improve, the soybean plant will return to normal growth and new growth is common after water deficits improve. Yield potential can improve with new pod formation and an increase seed size and weights.
References and additional information:
- Mark A. Licht, David Wright, Andrew W. Lenssen (2013) Soybean Response to Drought. Iowa Sate University http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1190&context=extension_ag_pubs
- Kaori Nonokawa, Makie Kokubun, Takayuki Nakajima, Teiji Nakamura & Ryuji Yoshida (2007) Roles of Auxin and Cytokinin in Soybean Pod Setting, Plant Production Science. http://www.tandfonline.com/doi/pdf/10.1626/pps.10.199
Note: The information in this issue is based upon field observations and third party information. Since variations in local conditions may affect the information and suggestions contained in this issue, LG Seeds disclaims legal responsibility therefore. Always read and follow label instructions.
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Download a copy of this technical bulletin: Tech_344 - Flower and Pod Abortion in Soybeans