In Part 1 of this series we learned that, as autotrophs, plants crave carbon dioxide to feed themselves through photosynthesis. In general, the more CO2, the more photosynthesis, the faster and bigger plants will grow. Knowing this relationship, greenhouse growers and indoor farmers often enrich the plant environment with CO2 to maximize yields and increase product output. Crop productivity is even greater when elevated levels of CO2 are co-optimized with light intensity, air temperature, VPD, and fertilizer.
The question I ask in Part 2 of this series: Do plants crave high CO2 levels?
CO2 and Crop Nutrition
Recent studies have shown a correlation between rising CO2 levels in the atmosphere and diminishing nutrient levels in plants. Over the past 50 years, as CO2 levels increased from 320 ppm to 400 ppm, food crops have been fixing more carbon dioxide into carbohydrates. At the same time, however, their vitamin content has decreased an average of 8%. And it’s not just minerals that have declined, it’s protein too. Cereal grains, such as wheat, rice, and barley – the bread of life – all have lower levels of protein than they did 50 or 100 years ago. It seems plants aren’t able to absorb nutrients at the same rate they can fix carbon.
There are at least three possibilities for this phenomenon:
1) Soils are depleted of nutrients and there’s not enough for plants to absorb;
2) Plants are growing so quickly that they do not have enough time to absorb the same quantity of nutrients before being harvested; and/or
3) The edible parts of plants are so plumped with carbon that the relative proportion of other nutrients is reduced.
There are probably a dozen other possibilities.
What do you think? Why are plants becoming less nutrient dense over time?
Figures: Direct effect of rising atmospheric carbon dioxide (CO2) on the concentrations of protein and minerals in crops. The top figure shows that the rise in CO2 concentration progressively lowers protein concentrations in wheat. The lower figure shows that a doubling of CO2 concentration from preindustrial levels (280 ppm) has diminished the concentration of essential minerals in 125 wild and crop plants, and also lowers protein concentrations in barley, rice, wheat and potato. Figure source: Experimental data from Ziska et al. 2004 (top figure), Taub et al. 2008, and Loladze 2014 (bottom figure).1,2,3
CO2 and CEA
So where does this leave Controlled Environment Agriculture (CEA)? Is it possible that crops grown inside CO2-enriched environments are less nutritious than those grown at ambient CO2 levels?
Not necessarily. In CEA, indoor farmers are constantly striving to find the right balance between yield and quality. They want to produce large yields to increase their per pound sales. But they also want to produce a safe and enjoyable product that consumers will come back for. Quality is always front-and-center for the greenhouse grower and indoor gardener. It has to be, if growers are going to charge $4/head of lettuce or $1000/lb for cannabis. The value-add is the customer experience: the taste, touch, look, smell, and feel of the product. To achieve this value proposition, CEA growers must optimize all their inputs, including, but definitely not limited to, CO2 levels and nutrient delivery.
CEA growers who target 1000 or 1500 ppm of CO2 in their indoor plant environment (much higher than the “elevated” 400 ppm in the earth’s atmosphere) must also increase the availability of other inputs to keep their crop in balance. These inputs include fertilizer recipes to prevent nutrient deficiencies; VPD levels to prevent over-evapotranspiration and maladies such as leaf tip burn; air temperatures to maintain the metabolic activity and energy needed to grow; and light to electrify the photosynthetic process. When all these inputs are in balance, the crop will grow quickly and strongly, producing large quantities of high-quality product. But if these inputs are out of balance, plants grown indoors will be susceptible to quality and nutrient deficiencies like their open-field counterparts, and growers will have expended resources without achieving the full potential of their crop, limiting profit margins and reducing the economic viability of indoor farming.
CO2: Creating A Balance
Carbon dioxide is what plants crave to photosynthesize, feed themselves and eventually (potentially) feed us. There can be too much of a good thing, however, if CO2 fixation is greater than nutrient uptake. Plants may grow bigger and faster under elevated CO2, but that doesn’t necessarily make them more nutritious or delicious. To achieve both, all the inputs must be in balance. By having control over so many environmental parameters, CEA farmers are uniquely positioned to overcome the imbalance observed in soil-grown crops, not only improving the nourishment of plants but also of the people who eat them.
CEA Farmers: We want to hear from you!
- Have you experimented with CO2 levels and observed differences in plant health or product quality?
- Have you tested the vitamin, mineral, and protein content of your crops? How about flavonoids, terpenes, and other chemical compounds?
If you’re interested in learning more about how rising atmospheric CO2 levels are affecting the health of our food system, check out these resources:
1Taub, D. R., B. Miller, and H. Allen, 2008: Effects of elevated CO 2 on the protein concentration of food crops: A meta‐analysis. Global Change Biology, 14, 565-575, doi:10.1111/j.1365-2486.2007.01511.x.
2Ziska, L. H. M., C. F.; Goins, E. W., 2004: Quantitative and qualitative evaluation of selected wheat varieties released since 1903 to increasing atmospheric carbon dioxide: Can yield sensitivity to carbon dioxide be a factor in wheat performance? Global Change Biology, 10, 1810-1819, doi:10.1111/j.1365-2486.2004.00840.x.
3Loladze, I., 2014: Hidden shift of the ionome of plants exposed to elevated CO2 depletes minerals at the base of human nutrition. eLife, 3, e02245, doi:10.7554/eLife.02245.