Dr. Fadi Al-Daoud (Greenhouse Vegetable Specialist, Ontario Ministry of Agriculture, Food and Rural Affairs, Harrow, ON)
Dr. Xiuming Hao (Research Scientist, Agriculture and Agri-Food Canada, Harrow, ON)
Table of contents:
- What is CO2?
- CO2 Enrichment and Plant Physiology
- CO2 Enrichment and High Light and Heat
- So, is CO2 enrichment always a good idea?
When exposed to light, plants grow by converting carbon dioxide (CO2) and water into sugar and oxygen; this is a simple way to view photosynthesis. About half of the amount of CO2 that is fixed by plants during photosynthesis is released back into the atmosphere during respiration. This input and output of CO2 is a passive process that happens by diffusion through the leaf stomata where open stomata will facilitate this exchange and closed stomata will hinder it.
Pre-industrial levels of atmospheric CO2 gas were around 280 parts per million (ppm), but today’s levels are around 400 ppm. Atmospheric levels of CO2 gas also fluctuate with the seasons. They increase with warmer spring weather that promotes aerobic activity to breakdown dead organic material, and they peak in early summer when vegetation is in full bloom undergoing photosynthesis and consuming CO2. The consumption of CO2 continues as the growing season progresses until the atmospheric levels reach a low at the end of the season. The cycle begins again with the breakdown of dead organic matter in autumn and winter which releases CO2 back into the atmosphere.
Theoretically, if CO2 is required for photosynthesis and CO2 levels are increased so too should the rate of photosynthesis, plant growth and food production. But is it possible to have too much of a good thing when it comes to CO2 and its effect on plants? And how do plants respond to CO2 enrichment under different environmental conditions?
This article aims to answer these questions by providing the latest scientific data on the effect of CO2 enrichment on plant growth and physiology, and its interaction with temperature, heat, and nutrition.
What is CO2?
CO2 is composed of one carbon (C) and two oxygen (O) atoms. It is a colourless and odourless gas at room temperature, but under higher pressure CO2 gas is converted into liquid. Solid CO2, better known as dry ice, is produced under high pressure and low temperatures. CO2 gas is soluble in water where it forms a weak acid known as carbonic acid (H2CO3). It is more dense than dry air, and it is transparent to visible light. However, CO2 gas absorbs and reemits infrared radiation. This allows it to trap heat and act as a greenhouse gas.
CO2 Enrichment and Plant Physiology
Crops grown in controlled environment agriculture (CEA) production systems such as greenhouses and vertical farms in warehouses consume CO2 as they undergo photosynthesis when exposed to light. A typical greenhouse crop will use 4.8-9.6 kg per hour per acre. This process can drop the concentration of CO2 from ambient levels (around 400 ppm) to as low as 100 ppm if the production site is not ventilated. Such low levels of CO2 can cause significant plant health issues and losses in productivity. It is recommended to ventilate a vegetable greenhouse at a rate of 1 air exchange per hour to replenish the CO2 in the growing space. Enriching the air of vegetable greenhouses with CO2, commonly referred to as CO2 fertilization, has also become common practice. It is typically done by using flue gas from boilers that burn natural gas to generate heat and CO2, or by purchasing liquid CO2 that is vaporized and pumped into the greenhouse. The quality of CO2 derived from flue gas must be monitored to ensure there are no harmful side effects from byproducts such as ethylene (C2H2), nitric oxide (NO), nitrogen dioxide (NO2), and carbon monoxide (CO).
Current recommendations for greenhouses suggest that when vents are closed CO2 levels should be maintained at around 1000 ppm on a sunny day and 600 to 700 ppm on a cloudy day, and when vents are open more than 10% CO2 levels should be maintained at 400 ppm or ambient levels. But what about other weather conditions like hot cloudy days or sunny cold days? To help answer these questions let’s first look at how CO2 affects crop physiology:
1. Photosynthesis – Plants uptake CO2 passively through their stomata by diffusion. Therefore, the higher the level of CO2 is outside the leaf, the higher the level is inside the leaf. Higher levels of CO2 lead to more photosynthesis and are associated with a 15-30% increase in vegetable production, on average. Boosts in production as high as 50% have been observed in tomatoes and peppers and 73% yield increases have been observed in cucumbers. One study showed an increase of 72% in lettuce head mass when plants were grown under 1000 ppm as compared to 200 ppm CO2. However, a growing environment with more than 1200 ppm of CO2 resulted in lower rates of photosynthesis and reduced yield for greenhouse cucumbers.
The level of CO2 in the growing environment is not the only factor to consider because the duration of the exposure of plants to CO2 enrichment also affects photosynthesis (dose = quantity + duration). If plants are exposed to elevated levels of CO2 for an extended period of time, they undergo a process called photosynthetic acclimation where the rate of photosynthesis drops, and the benefits of elevated levels of CO2 are not as pronounced as they were when the plants were first exposed to them. This drop in photosynthetic activity may be associated with a drop in chlorophyll content and a build-up of starch and other sugars in the leaves that may interfere with the function of chloroplasts. Furthermore, plant leaves may become chlorotic, deformed, rolled and brittle under prolonged CO2 enrichment. This is thought to be due to an imbalance in the ratio of carbon and nitrogen in the plant. One way to reduce photosynthetic acclimation in the crop is by feeding it high levels of nitrogen (N). Cucumbers that were grown with high N exhibited less photosynthetic acclimation than those that were grown with lower N levels.
2. Transpiration – When greenhouse CO2 concentration is elevated above ambient levels it results in lower stomatal conductance and transpiration rates in plants, almost 10% less in some cases. This means that less water is evaporating from the leaves through the stomata, and less water is being taken up by the plant. This reduction in transpiration can increase the water use efficiency by up to 60% in plants and allows producers to reduce their water bill. However, lower rates of transpiration can be detrimental to plants on a hot day because transpiration is one of the primary ways plants cool themselves when greenhouses reach high temperatures. The reduction in stomatal conductance and transpiration by CO2 enrichment is more severe under cloudy (low light) than under sunny (high light) conditions. This effect also varies with plant species and is more pronounced in eggplants compared to tomatoes, cucumbers, and peppers. Therefore, CO2 enrichment should be minimized during hot days, especially on cloudy days and on eggplants, to allow plants to maximize their transpiration rate and their ability to cool down.
3. Food Quality – In general, elevated levels of CO2 increase sugar and antioxidant levels and decrease protein and mineral levels in vegetables. However, variability does exist between different crops and different varieties of the same crop depending on the concentration of CO2. Some tomato plants, for example, may produce higher quality and better tasting fruit under elevated CO2 conditions, whereas other tomato plants may have reduced levels of lycopene – the pigment that gives tomatoes their red colour. Also, some cucumbers grown under high CO2 levels have elevated levels of sugar but lower levels of dietary fiber. The colour of lettuce and strawberry crops can change depending on the level of CO2 they are grown in, and higher levels of healthy phenolic compounds have been observed in lettuce grown under CO2 enriched conditions. Overall, plants produce larger fruit that have lower nutrient concentrations when grown under elevated CO2 levels. This may be due to a dilution effect when plants increase their fruit size, but they don’t produce more nutrients to fill the larger fruit.
The effect of high levels of CO2 on sugar and protein levels in the fruit is also dependent on the availability of nutrients for the crop. This is similar to the effect of N on photosynthetic acclimation. Studies have shown that under low N and high CO2 concentrations cucumber quality improved, but no increase in yield was observed. In contrast, when cucumbers were grown under high N and high CO2 levels the concentration of sugar and dietary fiber in the fruit did not change while yield increased. Similarly in tomatoes, the effect of CO2 enrichment on the levels of sugars and lycopene can be positive or negative depending on the nutrient levels available to the crop.
4. Improved Stress Tolerance – CO2 enrichment enhances the ability of plants to tolerate stresses like drought and heat because of elevated levels of soluble sugars and antioxidants. These compounds help protect plant cells from damage in stressful growing conditions. But this is counteracted by the associated reduction in transpiration rates that may be detrimental under hot conditions, as mentioned above.
CO2 Enrichment and High Light and Heat
The increase in photosynthesis when crops are grown under high CO2 levels is greater at higher light levels than at lower light levels, in general. Does that mean that increasing both CO2 and light levels is a good idea? Not necessarily. For example, it has been shown that CO2 enrichment increases antioxidant levels in lettuce when grown under high light conditions and when grown under a higher red to blue light ratio, but high light levels have the opposite effect on sugar levels because high light does not increase sugar levels in lettuce as lower light levels do under CO2 enriched conditions.
Another study showed that CO2 enrichment reduced yield of marketable tomato fruit during bright and hot days of August in southwestern Ontario. Furthermore, tomato plants grown under high CO2 and high light conditions may suffer from short leaf syndrome (SLS) where plants develop short, thick, curled, and crisp, dark grey-green leaves. SLS has also been shown to be aggravated by not only high light but also high temperatures. One of the reasons for SLS is high temperatures, especially high nighttime temperatures, that reduce tomato fruit setting and sink strength and cause an imbalance between sink and source. Less severe SLS was observed when plants were grown in high plant density because it may have resulted in more shade and lower light levels (lower source strength) in the crop canopy. Therefore, one practical solution to prevent SLS is to use high stem density by introducing additional stems in the spring.
So, is CO2 enrichment always a good idea?
The current recommendations for greenhouses to maintain CO2 levels around 1000 ppm on sunny days and 600 to 700 ppm on a cloudy day when vents are closed, and 400 ppm when vents are open more than 10% still apply. However, greenhouse growers might also want to consider reducing CO2 levels during hot days, especially on cloudy days. Vertical farmers should also consider enriching with CO2, if they don’t already, and optimizing the levels depending on their light and nutrient recipes. Just like other environment parameters, proper control of greenhouse CO2 levels relies on properly functioning sensors. CO2 sensors should be calibrated on a regular basis according to manufacturers’ recommendations to ensure accurate measurements are taken. As greenhouse environment control systems becomes more dynamic with more sophisticated decision-making algorithms that optimize set points in real time, so too will CO2 control. In the near future, CO2 set points will be changing in real time to adjust to changing weather conditions (light and temperature), which will improve greenhouse efficiency overall.
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