Carbon dioxide enrichment in greenhouses

Carbon dioxide (CO₂) in our atmosphere is dramatically on the rise, and I have experienced it first hand.  An article really struck me that I found in the recently published edition of California Agriculture, "Possible Impacts of Rising CO₂ on Crop Water Use Efficiency and Food Security" . There in "figure 1" (here inserted on the left) is a graph depicting the carbon dioxide concentration measured from the year 1700 to the present.  It was the dramatic acceleration of atmospheric CO₂ concentration  that occurred during my life time that intrigued me (the darker green portion).  According to the "State of the Climate"  report in 2017 from the National Oceanic and Atmospheric Administration  and the American Meteorological Society, global atmospheric carbon dioxide was 405.0 ± 0.1 ppm in 2017, a new record high. 

 I measured CO₂ in greenhouses and in the atmosphere from about 1985 to 1990 when I evaluated the effectiveness of CO₂ enrichment on cut rose productivity and quality in central coast greenhouses. I still have slides (remember them) that indicate the atmospheric CO₂ concentration was 340 ppm!  This blog post is not about the impact of rising CO₂ on global warming (there's a lot of prudent information written about that). It's about the use of CO₂ in greenhouse production along the California coast. 

So what did our CO₂ enrichment evaluation indicate? What is the evidence for or against CO₂ enrichment?  

First some background. Simply stated, carbon dioxide, water and sunlight is used by plants in the process of photosynthesis to produce sugar and oxygen. The sugar is used by the plant for growth and eventually the flowers and foliage we harvest. Positive responses on flower crops were first reported in the 1960's in Colorado.  This first report on flowers and other similar reports from cold-winter climates, made it clear that when the sun was brightly shining in an unvented greenhouse, fast-growing crops were able to deplete the greenhouse CO₂  concentration rapidly  below the natural ambient CO₂ concentration and photosynthesis was significantly reduced. CO₂ enrich­ment to around 1000 ppm produced positive plant responses in many crops. 

The figure above is a generalized illustration of how the relative rate of photosynthesis is influenced by  greenhouse CO₂ concentration. This figure, produced in  the 1980s, shows the natural ambient CO₂ concentration as 340 ppm. This ambient concentration produces a relative photosynthesis rate of 100%.  

But in California, coastal greenhouses are never unvented very long when the sun is shining. Even in winter, CO₂ enrichment to 1000 ppm is only possible for just a few hours in early morning and evening when the vents are typically closed. Therefore in our coastal greenhouses it has always been suspected that there would be little benefit to enriching greenhouse air with CO₂  using a conventional enrichment method.  

In our first  experiment (1985-86) the conventional enrichment method was tested: the greenhouse atmosphere was enriched to 1000 ppm only in the morning and evening when the sun was shining and greenhouse vents were shut. The data followed:  CO₂ enrichment  did not improve production or quality of 'Bridal White' cut roses. But some  interesting observations directed us to try a different CO₂ enrichment method. 

CO₂ monitors within the rose plant canopy demonstrated that CO₂ levels were  being depleted to levels as low as 225 ppm during the day, even though vents were opened. Apparently, CO₂ was being absorbed quickly by the leaves and  greenhouse air was not moving sufficiently into the plant leaf canopy to replenish the CO₂ to normal levels (340 ppm).  Generally, a drop of CO₂ levels below 340 ppm, has a stronger effect on photosynthesis than enrichment above 340 ppm. In fact, the decrease in photo­synthesis when carbon dioxide levels drop from 340 to 200 ppm is similar to the increase of photosynthesis when carbon dioxide levels are raised from 340 to about 1300 ppm (again see figure above).

A second experiment was set up (1986 -87) to determine if "all day" carbon dioxide enrichment could improve flower production and quality. This time, carbon dioxide was distributed in drip irrigation tubing running along the ground of each production bed. Liquid CO₂ was released as a gas under pressure into the drip tubes when needed. The greenhouse ambient CO₂ level was maintained at about 1000 ppm at sunrise until vents were opened and then again if the vents were closed, up to 2 hours before sunset (as in the 1985-86 experiment). In addition, this time when the vents opened, the CO₂ level within the plant leaf canopy was maintained around 350 to 360 ppm. This technique eliminated any reduction of CO₂ under 350 ppm in the leaf canopy, but did not waste much CO₂ since the fresh air around the plant was around 340 ppm and therefore there was no strong diffusion gradient of CO₂ away from the plant. Essentially, the carbon dioxide was only being put where it was being used (near the leaves) and applied only when it was needed.

With this method, from summer to the following late spring, CO₂ enriched rose production increased by 12% over that of the untreated roses. The rose stem length increased about 1-inch above that of untreated roses. These stems had more girth and appeared robust. Flower bud dry weight was greater, so the flowers may have contained more petals or the petal size was larger.   At the time, the CO₂ cost was about 18 cents for each extra bud produced. In the heyday of the cut rose business the extra cost may have been worth it,  especially during the super profitable Valentine and Mother's Day markets.  This all day enrichment experiment was repeated beginning in the fall of 1988 in a different nursery on 'Royalty' cut roses, with similar positive trends forming in the data after six months. But what portended the demise of the cut rose industry in California, the local grower I was working with suddenly decided that production costs needed to go down immediately. Many dramatic changes were made at the nursery, which included the elimination of CO₂ enrichment, and consequently my experiment. I wish I had been able to collect more data to make stronger conclusions.

A current literature review indicates similar general findings around the world. Greenhouse vegetable, strawberry and flower crops grown in cold winter climates often improve dramatically with CO₂ enrichment but the benefits are limited in areas where climates require high greenhouse ventilation frequencies. See review article below. The most comprehensive "all day" enrichment experiment, similar to the 1986 rose experiments, is an evaluation of summer CO₂ enrichment of greenhouse tomatoes in Ontario Canada (2008). In this evaluation, CO₂ enrichment to a level above the ambient concentration in summer did not or only slightly increase fruit yield during the three years of the experiment. Marketable fruit yield in August 2005 was actually lower in the CO₂ enriched crop; most of these fruits had set and developed in the month of July, a month with the highest solar radiation and air temperatures of the three evaluated summers. (These temperatures probably adversely affected the tomatoes, CO₂ assimilation, and muddied the precision of the findings). Summer CO₂ enrichment did not affect fruit yield in summer 2006 (a typical weather pattern for the summer season) and slightly increased fruit yield in the summer 2007 (a cooler than usual summer season). They concluded that CO₂ enrichment was only valuable when environmental conditions resulted in favorable growing conditions and long enrichment periods. See article below. 

The benefit of CO₂ enrichment in climates such as along coastal California is limited because it is difficult to achieve higher than ambient  CO₂ concentrations for significant periods of time when the sun is shining. Under conditions of high solar radiation, the necessary prolonged periods of ventilation will limit the potential for CO₂ enrichment. CO₂ application methods and crop species will have an impact on the CO₂ enrichment benefits also. CO₂ delivered directly in and around the leaf canopy has some technical challenges but may have benefits in some crops. 

Next: How to maximize photosynthesis with only ambient CO₂ .