What Limits Yield – The Source Or The Sink? Does It Matter?

DR. DENNIS B. EGLI

LEXINGTON, KENTUCKY

 Crop Physiologists often analyze the yield production process in grain crops by dividing the process into two components – the source and the sink. The source is the photosynthetic machinery that supplies the raw materials and energy for plant growth. The sink is the seed that utilizes simple sugars from the source to grow. This simple division helps us understand a very complex system and makes it easier to determine what is limiting yield. If yield is limited by the source (photosynthesis), efforts to increase yield should focus on increasing photosynthesis. If the size of the sink (number of seeds per acre) is limiting, increasing photosynthesis will do no good – the number of seeds must be increased.

Source vs sink seems like a simple system – its either one or the other.  Unfortunately, it is not nearly as simple as it seems. Analysis of plant growth and yield production is rarely simple.

Generally speaking, yield is source limited. The size of the sink (seeds per acre) is determined during flowering and seed set by the supply of simple sugars from photosynthesis (the source). Matching seed number to source activity adjusts the reproductive output of the crop to the productivity of the environment and usually prevents a sink limitation. This adjustment occurs between growth stages R1 (initial bloom) and R5 (beginning seed fill) in soybean and from roughly 10 to 15 days before to 20 days after silking in corn.

High photosynthesis during this period usually results in a large number of seeds and high yield, while low photosynthesis results in fewer seeds and lower yield. The source is in control during this period. The crop can usually tolerate some stress during vegetative growth, but stress that reduces photosynthesis during the critical period will reduce sink size (seed number) and yield.

As promised, there are exceptions to this simple source limitation. If your corn population is too low, there will not be enough flowers on the ear(s) to handle all of the simple sugars from photosynthesis and the crop will be sink limited. The source could support more seeds, but there are not enough flowers. The number of seeds limits yield. 

Soybean is not sink limited during flowering and seed set. The soybean plant is flexible, it responds to the supply of simple sugars from photosynthesis by producing branches with more nodes and more flowers increasing sink size. Fifty percent flower and small pod abortion in high-yielding soybean crops shows that the potential sink size is much larger than the actual sink size. There is no sink limitation. 

 Corn is sink limited at low populations because corn lacks the flexibility to increase the number of flowers to match the supply of simple sugars. Over the years breeders favored single-ear hybrids which reduced corn’s flexibility and made it susceptible to sink limitations. Corn producers increase the number of flowers per acre to avoid sink limitations by increasing population. The plant does the adjusting for soybean producers.

Corn populations increased steadily since the beginning of the high input era (~1940) to avoid sink limitations as productivity increased. Soybean populations, in comparison, stayed constant and, in recent years, declined, as the plant increased flower number to avoid a sink limitation. This difference is due to the flexibility of the plant or the lack thereof.

Most corn producers prize ears that are filled to the tip at maturity. Completely filled ears (there was no flower or small seed abortion) can indicate high yield or they can indicate a sink limitation (population was too low) with yield left in the field because there were not enough flowers. Unfortunately, there is no uncomplicated way to determine if well-filled ears are good news or bad news. 

Crops are normally source limited (assuming adequate corn populations) during the critical period for seed number determination, but what about seed filling? Determining seed number is only the first part of the yield production process – the seeds still have to grow to their mature size. Source-sink relationships during seed filling often depend upon changes in the environment.

Seed number will be in balance with the capacity of the crop to fill the seeds if the environment doesn’t change from the critical period for seed number determination through seed filling. A productive environment that is maintained until maturity will produce large numbers of seeds and fill them to their normal size.

What if the environment changes after seed number is fixed?  If the environment deteriorates (i.e., (the rains stop, for example, and source activity is reduced), there will not be enough simple sugars to fill the seeds and the seeds will be smaller and yield will be reduced. Sink size was set too large for the deteriorating source during seed filling. In other words, plants don’t always get it right because they can’t predict the weather. 

What if the environment improves after seed number is fixed(i.e., rains come after a dry critical period)? Now the source is larger than the sink and the capacity of the individual seed to respond to the larger supply of simple sugars will determine what happens to yield. The crop will be sink limited if the seed cannot respond to the increase in the supply from the source. If the crop cannot convert the increase in source activity during seed filling into higher yield, yield will be sink limited. Corn seeds often fall into this category.

If the seed can respond to the increase in source activity during seed filling, seeds will be larger, and yield will be increased. Soybean seeds fit into this category; improved conditions during seed filling often result in larger seeds and higher yields (i.e., the crop is source limited). 

The response to improved environmental conditions during seed filling is always limited by the physical characteristics of the seed and pod. All seeds have a maximum potential size – after all you can’t expect to find a golf ball in a soybean pod, so there is a limit to how much yield can be recovered when the environment improves after stress reduces seed number. 

But as often happens in life, there is no downside limit – there is no limit to how much  stress during seed filling can reduce seed size and yield in both corn and soybean.

Thinking about sources and sinks helps us better understand the yield production process. It provides us with insights into the response of crop productivity to the environment, the effect of population on crop yield and many other aspects of crop yield. These insights lead to more informed management decisions that ultimately improve the bottom line.

“Flix qui potuit rerun cognoscere causes” (Fortunate is he who understands the cause of things (Virgil, Italian poet, 70 – 19 BC).  ∆

DR. DENNIS B. EGLI: University of Kentucky