Could carbon dioxide be the seventeenth essential plant nutrient element? It’s an interesting question that doesn’t have a definitive answer. One might argue that since both carbon (C) and oxygen (O) are already accepted essential plant nutrient elements, so why should their combination also be designated?
If the CO2 molecule is to be designated as an essential plant nutrient element, why not the water molecule (H2O) also? Such reasoning could be never ending. Carbon dioxide does meet all the criteria for being designated as an essential plant nutrient element. By criteria, I mean the following criteria that have been accepted as proposed by D.I. Arnon and R.R. Stout in 1939:
- Omission of the element in question must result in abnormal growth, failure to complete its life cycle or premature death of the plant
- The element must be specific and not replaceable by another
- The element must exert its effects directly on growth or metabolism and not by some indirect effect, such as by antagonizing another element present at the same level.
For some, the designation as being essential as an element may be the factor that would eliminate CO2, which is a molecule, from being considered an essential plant nutrient element.
CO2 properties and characteristics and how it affects plant growth
Without CO2, there would be no plant life because it is CO2 combined with hydrogen (from water) in the process called “photosynthesis” that forms a carbohydrate, which are the basic building block for all plant life. As with all biological systems, photosynthesis is not particularly simple in terms of how it works as well as the factors that affect its function. But, in simple terms, a molecule of H2O is split and combined with a molecule of CO2 in the presence of chlorophyll and light to form a carbohydrate as is illustrated in the following chemical equation:
carbon dioxide (6CO2) + water (6H2O)
in the presence of light and chlorophyll yields
carbohydrate (C6H12O6) + oxygen (6O2)
Merriam Webster’s Collegiate Dictionary defines photosynthesis as formation of carbohydrates from carbon dioxide and a source of hydrogen (as water) in the chlorophyll-containing tissues of plants exposed to light.
When the first product of photosynthesis was determined, it was found that there are two pathways for carbohydrate formation, one being the formation of a 3-carbon carbohydrate and the other a 4-carbon carbohydrate. From this came the designation of plants as being either C3 or C4 based on which was the first product of photosynthesis, a 3- or 4-carbon carbohydrate.
Is this finding a big deal? Yes indeed. Most plant species are C3, while most grasses, which includes all the major food grain crops, such as corn, wheat, rice, sorghum, etc., are C4 plants. C3 plants are quite responsive to the concentration of CO 2 in air surrounding them, while C4 are less so.
C3 plants are sensitive to high light intensity, are not as drought tolerant and are more sensitive to changing growing conditions both in the rooting medium and surrounding atmosphere as compared to C4 plants.
Experiments have shown that in many situations it is the maintenance of a constant level of CO2 in the air surrounding the plant as being equally important as its concentration. Therefore, air movement over plant leaves, as well as air movement into and within the plant canopy, can significantly affect plant growth and yield.
Good examples are the orientation of corn rows so that the predominate directional wind currents will move down between the rows and not be impeded when having to move across the rows, and then the making of provisions for air movement up through a greenhouse tomato plant canopy rather than trying to push or pull air through the canopy.
Photosynthesis primarily takes place within leaf stomata—unique leaf structures where the exchange of water and air takes place. Guard cells surrounding the stoma control its opening and closing.
When the CO2 concentration in the air surrounding the plant is high, there is danger that such high concentrations can result in the closure of stomata. At what CO2 concentration this occurs varies with other factors, but experience has shown that stoma closure is more likely to occur when the CO2 air concentration is greater than 800 ppm.
It has been shown that the correlation between plant growth rate and CO2 concentration is not linear, with the rate of growth declining with each increasing increment of CO 2 concentration as is shown in Table 1.
There is an issue regarding the value for CO2 enrichment of air surrounding the plant in terms of the cost-benefit ratio, a return in increased plant growth and product yield versus the cost for the CO2 and its distribution in the air surrounding the plant and within the plant canopy. Light intensity and duration, combined with air temperature and the moisture and nutritional status of the plant, are correlated factors that will determine the extent of the CO2 effect. Therefore, just increasing the CO2 concentration of the air surrounding the plant does not automatically result in a significant increase in plant growth.
Under optimal conditions the rate of photosynthesis versus the CO2 concentration of the air surrounding the plant can be significant. This was illustrated from results obtained when the CO2 concentration of the air within a greenhouse tomato canopy was being continuously monitored. Within just a few minutes at dawn, when the morning light reached the plants in the greenhouse, the CO2 concentration of the air within the plant canopy dropped by over 50 ppm, and did not return to the atmospheric level of about 325 ppm until the ventilation fans came on.
The ability of a plant to suck CO2 from the atmosphere probably has an equilibrium point, a point that will vary with growing conditions, i.e., light intensity, moisture conditions, plant characteristics, stoma status, etc. For example, how far could the CO2 concentration within that tomato plant canopy be drawn down?
That level would be correlated with the rate of photosynthesis, decreasing with each increment decrease in the CO2 air concentration. It also should be noted that the air in contact with plant leaves is held in place by surface leaf characteristics as well as the surface tension properties of the leaf itself; therefore that air is not easily displaced even when there is gentle air movement over the plant leaf surface.
The beneficial effects of CO2 have been well established under various circumstances, but with varying results. Simply adding CO2 to the air surrounding the plant will not automatically result in a significant increase in plant growth or product yield. Therefore, the grower needs to weigh the potential benefits of CO2 enrichment against the costs, and the probable potential for no significant effect as well as possible adverse effects.