There are three types of photosynthesis: C3, C4 and CAM.  C3 photosynthesis occurs in woody, round-leafed plants, which are 95% of all plants.  C4 and CAM photosynthesis is found in drier, hotter land plants, grasses, sedges, grains, with CAM in cacti and bromeliads.  It is a more efficient, process with the first intermediary compound including four carbon atoms where the "round leaves" start with three, hence the "C3" and "C4" labels. 

        Where C4 and CAM appear virtually identical they are rarely discussed separately. C4 and CAM are more efficient in water and energy use.  The functional difference is that CAM plants employ an "idle" function that saves energy and water.  It is employed during times of stress; high temperature or lack of water.

        During dry periods CAM stomata are closed during the day.  CO2 captured overnight is stored in solution to be processed when the light returns.  C4 plants photosynthesize faster under high heat and light conditions than C3 plants, but it is now clear the increased rate is due to higher operating temperatures and the fact that chemical reactions double in rate for every 10 Celsius degrees they are raised.  The only remarkable fact is that this was only recently confirmed and accepted.

           The C3 class plants respond very well to additional carbon dioxide in the atmosphere when grown in greenhouses and we have demonstrated they respond well to soil-sourced carbon dioxide. (See CO2 Root Absorption)

 The C4 class includes all grasses and grains.  These are warmer area plants that were reported to show about half the response to increased CO2 as did the C3 plants in early studies.  New work has resulted in a dispute over this fact and there will appear to be confusion in the literature.  We believe the rates will be found to be the same or greater for C4 plants when the science is settled..

In the on line science journal “CO2  Science,” Vol 6, Number 40, 1 October 2003, authors Keith Sherwood and Craig Idso presented a study showing a 53% increase in the dry weight biomass for tall Fescue (Festuca arundacea) grown in a CO2 rich atmosphere.  They observed a 14% reduction in lignin, to us the indigestible part of the plant. 

Fescue is a C4 plant and the increase occurred when the grass was grown in an atmosphere with 700 ppm carbon dioxide; roughly double that of today’s air.  In summation they wrote, “…therefore it can be concluded that atmospheric CO2 enrichment was a hugely positive factor, made better use of the applied nitrogen such that additional nitrogen had no effect.”  

        Their conclusion is important as C4 grass and grain crops have previously been treated in the literature as having a physiology less amenable to increased CO2 than C3 plants and perhaps less accepting of earth delivered CO2 as their root structures are spare compared to C3 plants.  In the Sherwood-Craig study the differences were non-existent.  The chemistries of the two reaction  pathways show no reason why there should be a difference.  This is very encouraging as C4 plants are responsible for all our grain and are of primary economic importance in our economy.