Seed Size and Yield of Grain Crops
Dennis B. Egli
LEXINGTON, KENTUCKY
Advertising constantly reminds us that large is better, but does this philosophy carry over to crop seeds? Do varieties with large seeds produce higher yields?
The tremendous variation in seed size (measured as weight per seed or seeds per pound) within and among grain crops makes this an interesting and relevant question. Seeds of cereals (wheat or rice) are small (30 to 40 mg/seed or 15,120 to 11,340 seeds/pound); corn is much larger (~ 320 mg/seed or 1418 seeds/pound). Soybean fall in between (~175 mg/seed or 2592 seeds/pound). Some grain crops have really small seeds - canola, for example, produces seeds that weigh only 10 mg apiece (45,360 seeds/pound) – and some are really large - a broad bean variety produces a seed that weighs in at 2000 mg (227 seeds/pound).
Seed size also varies among varieties within a species. Seeds of commercially available soybean varieties range in size from 150 to 200 mg/ seed (3024 to 2269 seeds/pound). While seeds of maize hybrids vary from 250 to 340 mg/seed (1814 to 1334 seeds/pound).
Does this variation have anything to do with yield? A scientist’s favorite answer comes into play here – it depends! It depends on the source of the variation in size – is it genetic or environmental? Genetics controls differences among species and varieties within a species. But the environment where the seed is produced also affects size, so a varieties’ seed size will vary depending upon the environment during seed filling.
Let’s discuss genetic differences first. If we know why seeds are large or small, we will know if size affects yield. Seed size has no effect on yield when size is determined by seed growth rate (large seed grow rapidly, small seeds grow slowly). A seed growing rapidly - a large seed - requires more sugars per day than a seed growing slowly – a small seed. So, it takes fewer large seeds to utilize the daily supply of sugars from the leaves than small seeds. To put it another way, seed number compensates for differences in seed growth rate (seed growth rate up, seed number down) to match the supply of sugars. The total growth rate of all the seeds on the plant is the same for large and small seeds, so yield stays the same because yield is determined by the supply of sugars from photosynthesis. All the genetic variation in seed growth rate and seed size does is determine how yield is packaged.
The first breeders working with soybean in the 1930’s and 40’s thought they could increase yield by selecting for large seeds. They successfully increased seed size, but much to their surprise, yield did not change! The number of seeds decreased as size increased and yield stayed the same.
This compensation of size and number is clearer when we compare species. Corn produces a large seed and high yield but only a modest number of seeds per acre. Rice yields are often almost as high as corn and rice seeds are 10 times smaller than corn, but rice produces 10 times as many seeds as corn. No one would argue that wheat is a low yielding crop, but its seeds are small and seed number is high. Yield is determined by the productivity of the plant community; genetic differences in seed growth rate and seed size are not important.
Size will be related to yield when a seed is larger because it grows for a longer time (longer seed-filling period). A longer seed-filling period gives seeds more time to use sugars from the plant to accumulate dry weight resulting in higher yield. The length of the seed-filling period is under genetic control and longer seed-filling periods exist in some varieties of soybean and corn, but they are not nearly as common as high seed growth rates. Most of the genetic differences in size are due to seed growth rate, so the best bet is to assume that genetic differences in size are not related to yield. You will be right most of the time.
Seed size is almost always related to yield when size is determined by environmental conditions when seeds are developing on the plant (during seed filling). Unfortunately, in this case, size is usually related to lower yields. Water stress or high temperatures during seed filling will shorten the period, reducing seed size and yield. We all remember years when late season drought caused early maturity (shortened the seed-filling period), resulting in smaller seeds and lower yields. If the environment improves during seed filling (it starts to rain again), seed size and yield may increase. There are physical limitations on seed size in most crop species, so the potential to increase seed size is limited. Unfortunately, the potential reductions in seed size and yield are larger than the potential increases in seed size and yield. Environmental effects have a much larger downside than an upside.
Seed size – yield relationships are complicated, but we can understand the complications if we consider the source of the variation in size. Genetic differences (variation among varieties and species) are almost always not related to yield (except in the rare case when seed-fill duration determines size), while environmental effects usually result in differences in yield. ∆
Dennis B. Egli: Plant and Soil Sciences Department, University of Kentucky