Becoming C4

​Cells between the upper and lower epidermis of corn leaves make up the mesophyll and the vascular bundles (veins). These cells carry out many normal cell functions of producing proteins and anti-pathogen substances but most notable is the photosynthesis performed in the chloroplasts.  Most (95%) plant species have mesophyll cells located immediately adjacent to the epidermis.  These species have a C3 system of photosynthesis.  This system obviously works ok even with some inefficiency in the final conversion to sugar. However, this inefficiency results in excess water consumption for that final step and even higher use of oxygen instead of releasing it to the environment.  The problem is accelerated at higher temperatures in which more oxygen is consumed and less sugar produced.
 
Some plant species of tropical origin, such as the Teosinte species from which corn was developed, separated the photosynthesis two steps making the process more efficient even when under tropical conditions.  With a change in the enzymes involved, the first step finishes with a 4 carbon ring instead of a 3 carbon ring.  This compound is then moved to another cell’s chloroplasts for the final combination with carbon dioxide molecules and production of sugar.  In C4 plants like corn, the mesophyll cells are not lined up close to the epidermis but dispersed closer to the vascular bundles where that second stage of sugar production takes place.  Bundle sheath cells, in C4 plants, have the specialized chloroplasts that make the final product.  Being adjacent to the mesophyll cells is essential to the efficiency of the process.
 
The effect of this cell arrangement and the slight change in enzymes in the chloroplasts of C4 plants and participation of the bundle sheath cells allows corn to become a greater user of light intensity than most crops.  Whereas photosynthesis in soybeans and wheat peaks out at about 3000 ft candles (32000 lux) of light intensity, photosynthesis rate in corn increases to the brightest of sunlight (10000 ft candles or 107000 lux).  It also explains why corn photosynthetic rates decreases with slight changes in light intensity such as shading within the canopy as well with clouds.  On the other hand, this cell arrangement and unique photosynthesis process makes corn one of our best crops at removing CO2 from the atmosphere and for storage of chemical energy captured from the sun.