With most of the state experiencing at least moderate drought conditions and high temperatures, soybeans are exhibiting symptoms of water stress. A visual indication of soybean water stress includes flipped leaves (see photo). The flipped leaves expose a silver-green underside which reflects light. In more severe cases, the outer leaves of the trifoliate will close together to reduce the leaf area exposed to sunlight and reduce water loss. Water-stressed soybeans will grow slower and have smaller leaves compared to soybeans growing with adequate soil moisture.
Soybean yield potential is influenced by total number of pods per plant, number of beans per pod, and seed size. Stress conditions during soybean reproduction can reduce yield by affecting one or more of these yield components. Vegetative growth, flowering, pod development, and seed filling stages overlap allowing the plant to compensate for short periods of stress. Currently in Ohio, many soybeans are beginning to flower. In a normal year, 60 to 75% of soybean flowers will abort, but this number can increase in a stressful year. However, flowering can occur through the beginning of the R5 growth stage (beginning seed). If water stress is alleviated prior to the R5 growth stage, some flowers and pods can still be produced compensating for the flowers that were aborted earlier. If water stress persists, soybean yield will be reduced, especially as plants enter the R4 growth stage (full pod). Yield reductions at this time result mainly from reductions in total pod number per plant. At the R6 growth stage, water stress will cause a reduction in seed size.
References:
Casteel, Shaun. 2012. Signs of Drought Stress in Soybean. Soybean Station, Purdue University. http://www.agry.purdue.edu/ext/soybean/news/2012/2012_0627SOYDroughtSigns.pdf [URL verified July 2012]
Pedersen, Palle. 2004. Soybean Growth and Development. Iowa State University Extension. http://extension.agron.iastate.edu/soybean/production_growthstages.html [URL verified July 2012]
Source: OSU