How soybeans respond to drought stress
By Roger Borges, Professor of Agronomy, University of Wisconsin-Madison, Extension Soybean and Small Grain Specialist , 608-262-7975 rborges@wisc.edu
The physiologic response
Drought stress will lead to a lower rate of water uptake by the soybean roots. An imbalance between water uptake by roots and water loss by transpiration causes the plant to wilt. The plant will defend itself from water loss by closing the stomata (the pore openings at its leaf surface). Closing the pores certainly helps the plant to avoid fast dehydration. However, closed pores also keep the plant from exchanging carbon dioxide and oxygen between its internal tissue and outside atmospheric air. Also, stopping (or drastically reducing) the flow of water through the plant reduces the uptake of nutrients. All factors described above cause the soybean plant to reduce its metabolism to idle speed in order to survive the drought period and resume growth once moisture is no longer limited. Reduction of photosynthetic rate signals the plant to reduce its own yield goal. In response to lower turgor pressure (wilting) the soybean plant might grow smaller leaves, slow or stop growth and flowering, abort flowers, abort pods, abort grain and drop leaves.
Philosophical assumption
Soybean plants don't produce grain because we want the grain. They produce grain because the grain is the seed for the next generation of plants.
Practical implication of drought stress on soybean production
Once you understand this philosophical assumption, it becomes easier to understand the strategies that the soybean plant might use in its attempt to dodge the stress and guarantee the survival of the soybean species? Common strategies are dropping flowers, dropping pods, reducing seeds per pod, aborting seeds within pods, and reducing the size of seed. The strategies used by the soybean plant will depend on its stage of physiological development and the severity of the drought stress.
Drought effects on flowering soybeans
A flower does not represent a large investment towards a seed, so a soybean plant won't hesitate to abort a flower in order to adjust its yield potential to growing conditions. Even in a normal year, 50 to 70 percent of the flowers are aborted naturally. Soybeans usually flower during 30 to 40 days. Those characteristics give soybeans a great ability to minimize (or recover from) potential yield losses caused by a stressful condition during flowering. However, if drought stress is severe enough, the plants might abort an excessive number of flowers and lower its yield potential. The soybean plants might slow or stop growth. Soybean leaves might be smaller, wilting, curling, standing upright or weeping. Eventually, the plant will drop leaves in an attempt to preserve moisture. Depending on the duration and severity of the drought stress, soybean plants may or may not resume growth and flowering once water becomes available.
Drought effects on podding soybeans
Individual pods represent a bigger investment towards seed production and are regarded by the plant as more valuable than individual flowers. Consequently, a soybean plant will try to retain as many pods as possible. However, even in normal years, a few soybean pods are dropped naturally. Most pod dropping happens in response to stressful growing conditions or physical damage. Younger pods tend to drop first. The average pod size may also be reduced (fewer seeds per pod). The soybean plant is likely to slow or stop growth. Leaves might be smaller, wilting, curling, standing upright, or weeping. The plant might drop some leaves. Obviously, the soybean plant does not know how long the drought stress will last. Consequently, the plant reduces its own yield goal in order to improve its capacity to fill the pods that it decides to keep. Typically, podding overlaps with flowering. If the drought ends soon enough, the soybean plant might resume flowering and compensate for some or all flower and pods that were aborted.
Drought effects on seed filling soybeans
The soybean plant might use two strategies to cope with drought during seed filling -- reduce the number of seeds or reduce the size of the seed. Producing as many viable seeds as possible is the primary goal of a soybean plant. Compared with seeds that are more developed, the young seeds will require more resources before becoming viable. Consequently, the soybean plant might abort some of the younger seeds or reduce the average size of the seeds that are already further developed. Plant growth will likely stop. Leaves might senesce and fall off earlier. The occurrence of atypical insect pest and diseases might appear unusually high. Typically, there is little or no overlap between seed filling and flowering. Consequently, both seed abortion and reduction in seed size represent direct and irreversible reductions in yield potential.
The spotty nature of drought stress on soybean fields
As in most other crops, drought-related symptoms on soybean fields are spotty. Several factors contribute to the irregular nature of the drought stress symptoms. The plants are not responding directly to the lack of rain. They are responding to limited soil moisture. Rains and soil moisture are related, but they are not the same thing. Soil works like a sponge. It quickly soaks in water. Then, the soil slowly loses water through evaporation or plant transpiration. Differences in topography, soil types, soil depth, surface cover, the presence of tiles and soil compaction zones are just a few variables that will determine how much water will be stored at each field spot. Also, uneven distribution of rain, crop size, nutrient availability, soil pH, weeds, insects, diseases, and other factors contribute to the spotty nature of soybean response to drought stress.
We tend to blame the drought for all stressed areas of the field. However, it is to our advantage to take a closer look at each stressed area of the field. The stressed areas may have less water available for the crop compared with the rest of the field, but not necessarily! Even though lack of rain is likely responsible for the appearance of stressed areas in the field, only a thorough investigation of each spot will identify the direct cause of the stress symptoms. Often, drought stress symptoms are directly related to soil and crop management issues such as fertility, insects, diseases, weeds, soil compactions, and plant density. For instance, small nutrient deficiencies might go unnoticed during a normal year. However, when soil moisture is deficient the soybean plant will have more difficulty taking up the deficient nutrient. So, the question is… Is there anything we can do to manage drought stress during the current season or minimize drought stress for future seasons?
References
Soybean and wheat development and management
http://soybean.agronomy.wisc.edu/ppt/00_cca_training_files/frame.htm
Effects of drought on soybeans http://www.okstate.edu/OSU_Ag/oces/timely/soybean.htm
Managing Soybean Under Drought Stress
http://www.vaes.vt.edu/tidewater/soybean/soybeanup/0207/0207.html#L2
