Carbon dioxide (CO2) is an essential requirement for photosynthesis and can be somewhat overlooked by newer growers. Being odorless, invisible, and only a small fraction of our atmosphere, CO2 often doesn’t get the same attention as nutrients, lights, and other plant-growth factors.
The use of CO2 enrichment to boost yields, quality, and growth rates under hydroponic production is, however, widely used in commercial greenhouse horticulture and has an even greater potential in enclosed growing spaces. While simply pumping in some additional CO2 may seem like a straightforward option, the use of this technology is a little more complex if its potential is to be maximized and problems minimized.
Ambient CO2 levels in air are a little more than 400 ppm (or 0.04 per cent by volume), however, plant tissue contains an average of 45 per cent carbon that comes entirely from CO2. By boosting CO2 levels surrounding the leaf surface, above ambient levels, the rate of photosynthesis increases up until the point where some other factor, such as the speed at which plant enzymes will work, is reached.
Essentially, the transfer of CO2 from the surrounding air to the reaction centers in the leaf chloroplasts depends both on the concentration difference between the air and these sites, and the intervening biochemical resistance in various leaf tissues. This means that while CO2 enrichment will boost photosynthesis, there comes a point were further increases will not occur and plant damage becomes a possibility. Determining this optimal level of CO2 enrichment for a particular plant or stage of growth is where the application of CO2 needs some careful thought.
Carbon dioxide enrichment has become more popular in recent times with hydroponic growers using a range of low- and high-tech options to boost CO2 levels. The most common methods of generating CO2 include burning hydrocarbon fuels and the use of compressed, bottled CO2. Smaller growers with a very limited growing space may use dry ice (solid, very cold CO2) which releases CO2 as it “melts” under warm conditions.
Fermentation or the decomposition of organic matter (composting and fungi) are still effective but less accurate ways of boosting CO2 levels through natural processes. Whichever method is used to generate CO2, levels should be regularly monitored, either with a hand-held CO2 meter or as part of the environmental control system in the growing area.
If CO2 enrichment is to be applied, then determining the correct level is as important with this gaseous element as it is with nutrient levels. The benefits and levels of CO2 enrichment is crop dependent, but most plants respond well to levels in the range of 500–1,500ppm. Below 200ppm, CO2 begins to severely limit plant growth, but more than 2,000ppm of CO2 becomes toxic to many plants.
More than 4,000ppm is a risk to humans. An excess of CO2 will cause crop damage in the form of CO2 toxicity, which is often misdiagnosed as mineral deficiencies or disease symptoms. Mild CO2 toxicity can cause stunting of growth, or leaf-aging type symptoms, while excessive levels may cause leaf damage such as chlorosis (yellowing), necrosis (death of leaf tissue), curling and/or thickening of the leaves.
There is much debate over which level of enrichment is ideal for each crop, under various different growing conditions, however, the most economic use of CO2 is in enriching crops to above ambient levels, but not more than 1,200ppm. Most commercial growers enrich to within the range of 600-800ppm where an increase in growth and yields of between 20-30 per cent are common.
While CO2 enrichment is largely used on fruiting crops such as tomatoes, capsicum, and cucumber, it can benefit a wide range of plant species. Indoor gardens with ornamental, potted, and flowering plants also respond to CO2 enrichment with increased rates of growth and leaf area, increased rates of flowering, more lateral breaks, earlier flowering, greater flower number, reduced flower drop, and increased flower diameter as well as improved leaf color and reduced time to maturity. Carbon dioxide also assists with root development on cuttings and clones in many species and may be applied via enrichment of the air or through the use of carbonated mist.
To make the most of CO2 enrichment, other growth factors need to be considered and manipulated. Carbon dioxide enrichment will produce the best results in terms of plant growth, yield increases, and shortening the time to maturity where there is high light to power rapid levels of photosynthesis.
If light is insufficient or below the light saturation point for the crop, then boosted CO2 levels cannot be fully utilized by the plants. Temperature also plays a role in the efficient use of CO2. Under conditions of high light and CO2 enrichment, temperatures can be run higher than they would normally, and this maximizes the effect of additional CO2.
Studies have shown that for tomato plants, a threefold level of CO2 enrichment will increase net photosynthesis by about 50 per cent in both dull and bright light, but if leaf temperature is also raised (to 86°F), the increase in net CO2 fixation can be as high as 100 per cent in bright light. This means that while boosting CO2 in an indoor hydroponic system will boost growth rates, consideration should be given at the same time to manipulation of the other environmental factors of light and temperature if the valuable CO2 is to be used with the highest degree of efficiency.
Another often overlooked factor is CO2 distribution around the plants. Simply releasing or generating CO2 for enrichment into the growing area is often not sufficient to get the maximum rate of photosynthesis unless this is directed and circulated over leaf surfaces. A stale boundary layer of moist air, depleted in CO2 due to photosynthesis, can form directly around the leaf surface and this needs frequent removal and replenishment.
Whatever source of CO2 generation is being used, it is vital that the enriched atmosphere is thoroughly mixed so the valuable CO2 is delivered to plant surfaces for uptake and assimilation. Small mixer fans can be used to gently circulate the air away from the source of CO2 generation and toward the crop.
To monitor this process, hand-held CO2 meters are useful to check levels in and around the canopy rather than just at the point of CO2 release. Keeping a check on CO2 levels inside a small growing area is vitally important, no matter what the source of CO2 used. It can be difficult to judge how much CO2 the plants are taking up and in tightly sealed growing environments, CO2 accumulation can occur and cause plant damage.
CO2 enrichment is undoubtedly a great growth-promoting tool for hydroponic growers, however, it has its limitations and risks.
Plants have the ability to adjust and adapt to increasing CO2 levels, so that over time, acclimation occurs. When CO2 enrichment is first introduced to a crop, there is a rapid increase in photosynthesis and growth, but as plant growth continues, the effect of the increased CO2 levels becomes less and less so that by the time the crop is completed, overall yields were not as high as the increase in early yield.
Numerous studies have reported this effect with plants grown continuously at high CO2 levels having a photosynthetic rate that tends to decrease with time. If a crop grown at elevated CO2 levels is suddenly given only ambient CO2, it will recover back to normal rates of photosynthesis within five days.
Some growers have attempted to prevent this acclimation of crops to high CO2 levels by only supplying CO2 intermittently, or avoiding the use of CO2 enrichment until a vital stage of development, such as flowering or fruit set, has been reached when the boost in photoassimilate is most valuable to yields. Studies have shown the problem of CO2 acclimation can be reduced or eliminated if the plant has strong “sinks” for the assimilate produced in the leaves.
These sinks for assimilate include rapidly developing tissues such as buds, flowers, and fruits. Plants with a low sink strength often end up with carbohydrate accumulating in the leaves under CO2 enrichment, which in turn triggers acclimation and a reduction in photosynthesis. Despite the issue of plant acclimation to high CO2 levels limiting the overall potential boost to growth, CO2-enriched plants still produce photosynthetic rates higher than those grown at ambient CO2 levels.
Carbon dioxide enrichment is a worthwhile tool for indoor and greenhouse growers which is well proven in a wide range of crop species to increase growth rates and yields. However, as with most high-tech techniques, it requires monitoring, attention to detail, and careful consideration of the effect on biochemical processes. If CO2 is to be used at maximum efficiency, correct rates of application, adjustments to light and temperature, timing of enrichment, and consequences of CO2 acclimation all need consideration.