Photosynthesis – The Ultimate Yield Producer

DR. DENNIS B. EGLI

LEXINGTON, KENTUCKY

The keys to high yield include variety selection, planting date, population, fertilizer and control of weeds and pests. Photosynthesis doesn’t make the list, but there is no yield without photosynthesis. High yield requires high photosynthesis. 

Think about it – a corn producer plants about 20 pounds of seed per acre and, 100 to 120 days later, harvests 250 bushels (14,000 pounds)of corn and leaves about 14,000 pounds of leaves, stems and husks, plus an unknown amount of roots, in the field. That 20 pounds of seed produced more than 28,000 pounds of plant material per acre. That is the miracle of photosynthesis.

Years ago, scientists pondering how a small seed could produce a large plant, concluded that the sustenance for growth must come from the soil. But when they grew a willow tree in a large soil-filled pot they found no decrease in the weight of the soil,  so they mistakenly concluded that growth came from water. Finally, in the early 1800’s, they demonstrated that the increase in plant weight came from carbon dioxide in the air (with a small contribution from water) in the presence of sunlight – in other  words, they discovered photosynthesis.

Photosynthesis in green plant tissues uses the energy in sunshine to convert carbon dioxide into simple sugars that are the building blocks for all plant tissues. Energy from respiration of these simple sugars is used to acquire nitrogen and to make starch, protein, oil, cellulose  and all the many compounds that make up a plant.

Photosynthesis requires energy (from the sun), carbon dioxide, warm temperatures (but not too warm), water (mostly just evaporates from the leaves), and mineral nutrients to function. 

Photosynthesis not only feeds us, but, for most of recorded history, it provided the energy to cook our food, heat our homes and, more recently, to move us from place to place. The plant tissues that ultimately, over geologic time, became coal and petroleum came from photosynthesis.

Photosynthesis of a field of corn or soybean reaches a maximum when the leaves completely cover the ground because only sunlight that is intercepted by the leaves is used in photosynthesis. Only weeds benefit from sunlight that reaches the soil. Maximum yield requires complete ground cover near the beginning of  reproductive growth.

We don’t often think about it, but crop management is all about providing the ideal environment for photosynthesis. We irrigate, fertilize, adjust row spacing, and control weeds and pests in large part to maximize photosynthesis. Managing for maximum yield is maximizing photosynthesis.

Biochemists tell us that there are two types of photosynthesis (there is a third, but it doesn’t appear in any common crops). Most crops have C3-type photosynthesis (first stable product is a 3-carbon sugar) while only a few crops use the C4 system (first stable product is a 4-carbon sugar). 

C4 crops have higher photosynthesis rates and a greater tolerance to high temperatures, while the photosynthesis rate of C3 crops increases when the carbon dioxide concentration in the air goes up. Carbon dioxide levels in the air increased from 280 ppm at the beginning of the industrial revolution to roughly 426 ppm today. This increase   contributed to higher yields of C3 crops and is also causing climate change. C4 crops do not respond to higher carbon dioxide levels. 

Interestingly, most of the crops that feed the world (rice, wheat, barley, soybean, peanuts, potatoes, all the grain legumes)  have C3 photosynthesis, while the more productive C4 photosynthesis is found only in corn, sorghum, and millet. 

The rate of photosynthesis – the amount of carbon fixed per acre per day – is directly related to the crop growth rate (pounds of dry matter per acre per day) and to yield – the higher the growth rate, the higher the yield. 

Yield was reduced when we shaded soybean communities during reproductive growth to reduce the sunlight they received and photosynthesis (Egli, 1993). A 30% shade treatment reduced yield by 28%, while a 63% treatment reduced yield by 58% averaged over 2 years and 2 varieties. One could show the same response to water deficits or poor fertility. Reducing photosynthesis reduces yield.   

The duration of photosynthesis is also important, especially during the seed-filling period – the longer the seed-filling period, the more time there is for photosynthesis and the higher the yield. There is evidence in several crops that selection for higher yield by plant breeders increased the length of the seed-filling period and yield. The length of the vegetative growth period is not always related to yield.

Photosynthesis – the ability of a green leaf to use energy in sunlight to fix carbon is the fundamental process that makes agriculture possible. Growing crops is basically a matter of managing photosynthesis. We will depend on photosynthesis as long as our food comes directly or indirectly from green plants. Strange as it may seem, this basic process that feeds us also produced the fossil fuels that may ultimately kill us if we continue to burn them, increasing the carbon dioxide concentration in the air and causing climate change. 

Remember – “As long as you have food in your mouth, you have solved all questions for the time being.” (Franz Kafka, Novelist, 1883-1924).

Adapted from Egli, D.B. 2021. Applied Crop Physiology. Understanding the Fundamentals of Grain Crop Management. CABI.

References

Egli, D.B. 1993. Cultivar maturity and potential yield of soybean. Field Crops Research 32: 147 -158.   ∆

DR. DENNIS B. EGLI: University of Kentucky