The Timing Of Water Stress And Soybean Yield

DR. DENNIS B. EGLI

LEXINGTON, KENTUCKY

When will it rain? How much will it rain? These questions keep soybean growers constantly checking the Weather Channel during the summer. In Kentucky, the average rainfall during the summer months is less than the average maximum potential water use by the crop. The higher temperatures associated with climate change will increase the potential use, but the mismatch will depend upon what happens to rainfall. Throw in soils that have limited water holding capacity and the amount and distribution of rainfall becomes a critical component of maximum yield production.

Water stress reduces yield, but all stresses are not equal. The effect of stress depends upon when it occurs. Some growth stages are more susceptible than others.

The life cycle of a soybean plant can be divided into three phases. First comes vegetative growth (Phase I) then flowering and seed set (Phase II) and finally seed filling (Phase III). All grain crops fit into this scheme, although there is some variation in details among crops. Soybean, for example, continues Phase I until the end of Phase II. Other crops, corn for example, have a clearer separation between Phases. Phase I in soybean starts at seedling emergence and continues until growth stage R5. Growth stage R1 marks the beginning of Phase II and growth stage R5 its end. Phase III begins at R5 and ends at growth stage R7.

This scheme is especially useful because it relates directly to the determination of the two yield components (seeds per unit area and weight per seed) that combine to make up yield [i.e., Yield = (seeds/unit area) x (weight/seed)]. Seed number is determined during Phase II while environmental variation in weight per seed is determined during Phase III. 

Most of the environmental effects on yield are expressed by variation in seed number simply because it is determined first. Phase II is the first chance the crop has to adjust its reproductive output to environmental conditions. Weight per seed responds to the environment during phase III. 

Thinking about when water stress occurs during these three phases helps us understand its effect on yield.

Vegetative growth (phase I) is probably least sensitive to water stress. Water stress during this phase will reduce growth. This reduction, however, will probably not carry over to affect yield if it starts raining during Phase II, assuming that there is enough leaf area to provide complete ground cover by the beginning of phase II. If the stress is severe enough to prevent complete ground cover, yield will be reduced even if it rains during phase II and III. The size of the vegetative plant is usually not closely associated with yield, 

Stress during phase II will probably reduce yield because it will reduce the number of seeds the crop produces. Seed number is related to canopy photosynthesis and the availability of sugars during Phase II, so any reduction in photosynthesis as a result of stress will reduce seed number and probably yield. Increases in weight per seed can compensate for small reductions in seed number (assuming it starts raining again during Phase III). There is a limit though to how much weight per seed can increase and how much compensation can occur.

Stress during Phase III will reduce weight per seed and yield. Water stress during Phase III accelerates leaf senescence (the destruction of the photosynthetic apparatus in the leaf that occurs normally during seed filling), and shortens the seed-filling period, reducing weight per seed and yield. 

Stress during seed filling may not be as obvious as earlier stresses, so it can be a ‘hidden’ stress. The stressed leaves go through their normal senescence pattern (turn yellow and fall off the plant), they just do it faster. If well-watered plants are not available for comparison, the yield reduction may not be noticed until harvest. The early maturation of soybean plants on hilltops provides a classic example of stress during seed filling. The plants on the hilltops were growing in shallower soils, so they experienced stress and matured before the plants in lower areas that had access to more soil water. 

We found in greenhouse experiments that the acceleration of senescence by water stress could not be reversed by adding water to relieve the stress. We stressed soybean plants for three days early in the seed-filling period (Phase III) and then watered them to relieve the stress, but the accelerated senescence continued, and yield was reduced by nearly 20% due entirely to smaller seeds.  

The ‘hidden’ stress can be an important source of lower yields, since short periods of water stress can occur in the absence of wide-spread significant drought. The sensitivity of senescence to short-term water stress suggests that high yields may require a complete absence of water stress during seed filling.

When water stress occurs, the big question is – will it reduce yield? It all depends upon how much stress and when it occurs. Soybean can take some stress during vegetative growth without affecting yield. Phases II and III are the critical periods - stress during these periods will probably reduce seed number and/or weight per seed which will result in lower yields. The old Spanish proverb that says, “civilization and anarchy are separated by seven meals” (quoted by J. Cribb in ‘The Coming Famine’) illustrates perfectly the dangers of a lack of water.    

 Adapted from Egli, D.B. 2021. Applied Crop Physiology. Understanding the Fundamentals of Grain Crop Production. CABI. 178 pp. ∆

DR. DENNIS B. EGLI: University of Kentucky