Chloroplasts in corn provide the structures for the corn plant to be the most productive of crops in converting light energy into usable forms of energy. Selection of features by humans over the past 8000 years assisted but natural selection of origin in hot dry Central America gave great assistance by evolving a C4 photosynthesis metabolism. Most plants have a photosynthesis system with an inefficiency that limits its productivity. This system, labeled as C3 photosynthesis, peaks in its ability to fully use total light intensity to about 3000 foot candles where-as unclouded sunlight has 10000 foot candles. In corn, with it C4 photosynthesis, it continues to produce carbs in direct relation intensity of the light with maximum photosynthesis in bright sunlight.
Carbon dioxide enters plants through holes in leaves called stomata. These structures also allow oxygen to escape from leaves to the benefit of all of us. Water vapors also go through the same stomata. Stomata open and close. At night they close with the benefit of avoiding unnecessary loss of water when photosynthesis cannot occur. But when plant tissue is stressed from lack of water, these stomata also close, limiting the water loss but also interfering with uptake of carbon dioxide for photosynthesis. C3 photosynthesis doesn’t make carbohydrates out of all the CO2 it absorbs, using some of it in other molecules. No problem when environment provides plenty of moisture, is generally cool and have long summer days, but some plant species that evolved under hot dry conditions evolved systems to overcome that limitation.
Teosinte, the species of origin for corn in Central America, has a C4 photosynthesis system. Plants with this character have additional structures in their leaves surrounding cells that perform photosynthesis. These cells function to reduce the loss of CO2 by causing these molecules to be recycled into more carbohydrates. The combination of extra enzymes and structures comes at some energy cost but the net gain is both more net carbohydrate and better utilization of CO2, even if stomata are closed.
Fortunately, corn that was moved out of the original dry hot environment, kept that C4 photosynthesis system. Along with that came the C4 photosynthesis advantages and its superior production of carbohydrates. Sorghum and sugar cane also are C4 plants but wheat, rice and soybeans are C3 and will not be able to match corn in carbohydrates per acre because of this trait. Although only about 3% of all plant species are C4, it does occur in a few plants in many plant families, suggesting that it can evolve independently. Researchers of other crops, such as rice, are trying to use genetic engineering methods to develop C4 photosynthesis, but it is not an easy task.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.