We grow grain crops for their seeds, so it makes sense that we should give some thought to how seeds grow. After all, the production of yield is not just about leaves and photosynthesis, seeds are also important. 

Seeds of grain crops vary widely in color, size, the proportions of oil, protein, and starch, and structure. Canola and millet, for example, produce a tiny seed (45,400 seeds per pound, 10 mg seed) while broadbean seeds are huge (227 seeds per pound, 2000 mg seed). Corn (1513 kernels per pound, 300 mg kernel), soybean (2270 seeds per pound, 200 mg seed) and wheat (11,350 seeds per pound, 40 mg seed) fall in between. Interestingly, these genetic differences in size, as well as genetic differences within a species, are usually not related to yield. Corn kernels are, strictly speaking, a fruit (pericarp is fused to the seed coat surrounding the endosperm and the embryo) that is high in starch, while soybean seed has two cotyledons that are high in oil and protein.

In spite of all this variety, seed growth characteristics are very uniform across species. If you understand growth of one species, you understand them all - only the details differ. 

Growth of an individual seed starts with pollination, followed by a period of rapid cell division (lag phase) when all basic seed structures are produced. Rapid accumulation of dry weight then continues until the seed stops growing at physiological maturity (PM) (maximum seed dry weight) (Fig. 1). The increase in seed size as the seed grows requires water movement into the seed to drive the increase in cell volume which reaches a maximum before the seed reaches PM. The increase in volume and maximum seed weight can be limited by seed structures (seed coat or pod wall). The concentration of water in the seed declines steadily during seed growth reaching a characteristic level at PM (approximately 55% for soybean and  34% for corn) (Fig. 1). Seeds of all grain crops would follow a pattern similar to that shown in Fig. 1 for soybean.

Obviously, all the seeds on the plant don’t start growing at the same time. We measured as much as 35 days between the start of growth of the first and last pods on a soybean plant, but roughly 80% of the pods started growth within a 16-day period. Others reported 4 to 8 days between the start of growth of seeds at the base of a corn ear and those at the tip. All the seeds on a plant (or ear) do not reach physiological maturity at the same time, but the variation is much less than when they start growing.                 

Seeds are metabolic powerhouses, producing the oil, protein, and starch that give them their value from sucrose and amino acids imported from the mother plant. The rate of growth of an individual seed is regulated, in part, by the supply of sucrose and amino acids from the mother plant, but the characteristics of the seed are also important. For example, the growth rate of a canola seed will always be much less than the rate of a corn kernel.  The seed is no longer connected to the plant’s vascular system at PM, so yield production is finished – no more sugars or amino acids move into the seeds. Growth sage R7 (one normal pod on the main stem has reached its mature pod color) is a useful whole plant indicator of physiological maturity in soybean, although all the pods on the plant have not reached PM.  Black layer (a black layer at the bottom of the kernel, growth stage R6) is the accepted indicator of PM in corn.

Since the seed is cut off from the plant, the loss of water by the seed after PM is controlled by environmental conditions. Seeds will dry rapidly if it’s hot and dry, but the water content may increase in rainy weather. Soybean seeds usually dry faster than corn, probably because the corn husks slow the movement of water out of the seed.

There are reports of yield losses after PM that are not caused by harvest losses. Seed moisture concentrations at PM are high enough to support respiration (loss of carbon) which would continue until the seed dries to a level that stops metabolic activity. This continued metabolic activity (respiration) could cause yield losses. The amount of yield loss would probably depend upon how fast the seeds dry (i.e., environmental conditions). 

Flowers or immature seeds (in the lag phase) will abort if plants are stressed. Seeds rarely abort once they start rapid growth; they may grow fast or slow depending upon the supply of raw materials from the mother plant, but they won’t just give up the ghost and stop growing. 

Yield is often related to the duration of seed filling. The longer seeds grow, the larger they are, and the higher the yield. Genetic differences in seed size resulting from longer seed-filling periods are associated with higher yields. Environmental conditions during seed filling also influence duration of seed fill, seed size, and yield. Moisture stress will accelerate leaf senescence, shorten the seed-filling period, and reduce yield. We found that just 3-days of stress in greenhouse experiments with soybean accelerated senescence and reduced yield even though the plants were well watered after the stress. No stress during seed filling may be a requisite for super-high yields. 

The length of the seed filling period, seed size, and yield also depend upon temperature – increasing temperature shortens the seed-filling period and lowers yield. This relationship provides another mechanism by which the higher temperatures associated with climate change can reduce yield. 

The production of yield is not just a matter of photosynthesis using solar energy to produce sucrose and amino acids and dumping them into storage containers (seeds). The seeds play a vital role in the process, and we cannot hope to completely understand the production of yield without considering the seed. Understanding how seeds grow makes us better managers. Or as the Italian poet Virgil (70-19 BC) put it “Flex que potuit cognoseve causes (Fortunate is he who understands the cause of things)”.

Adapted from Egli, D.B. (2017). Seed Biology and Yield of Grain Crops. CABI. 219 pp.