Table of Contents:

A to Z guide for using carbon dioxide in the greenhouse:

1. CO₂ enrichment in greenhouses
2. CO₂ for growth
3. How crops respond to CO₂ enrichment
4. CO₂ enrichment in tomatoes
5. CO₂ enrichment from burning fuel
6. CO₂ from the central burner
7. Enrichment with pure CO₂

CO₂ OR CARBON DIOXIDE

CO₂ or carbon dioxide is a gas that normally occurs in air. Fresh air consists of 78% nitrogen, 21% oxygen and only 0.034% CO₂, which is usually indicated as 340 ppm (parts per million, or millilitre per litre). CO₂ in low concentrations is not dangerous for humans; it is also the gas that gives the prickles in fizzy drinks. The advised maximum concentration in working places is 5000 ppm.

CO₂ arises from burning carbon-containing material like wood and fossil fuel, and from decomposition of organic material. The atmospheric CO₂ concentration has been below 300 ppm for many centuries, but is rising rapidly in the last decades due to CO₂ emission by industry and traffic. The increased atmospheric CO₂ level contributes to the so-called greenhouse effect and global warming. It is therefore a concern to minimise CO₂ emission.

CO₂ THE BASIS OF PLANT GROWTH

Plants absorb CO₂ from the surrounding air through the pores (stomata) in the leaves. Inside the leaves, carbon dioxide is transformed into sugars and other carbon-containing substances. CO₂ uptake and the forming of sugars is known as CO₂ - assimilation or photosynthesis. Because this process requires light as the source of energy, CO₂ is taken up only at day time. The carbon-containing products formed in the assimilation process are called assimilates. They are used together with mineral nutrients and abundant water to build the plant tissues and organs. Therefore CO₂ uptake is the basis of plant growth.

When the main growing conditions such as light, temperature and water conditions are good, the plants will consume CO₂ at a high rate. The better the light conditions, the more CO₂ will be absorbed by the plants (see Figure 1). And also, the more CO₂ is available, the more will be absorbed. This explains that photosynthesis and growth rate can be increased by supply of additional CO₂ in the greenhouse air, i.e. by CO₂ enrichment.

CO₂ DEPLETION

When there is no CO₂ supply and insufficient air refreshment in a greenhouse, the plants may consume more CO₂ than comes into the greenhouse. This makes the CO₂ drop to a level below the normal outside level of 340 ppm. Such a condition with a lower-than-outside CO₂ concentration in the greenhouse is called CO₂ depletion. If the CO₂ level is as low as 200 ppm, photosynthesis is reduced to only half the photosynthesis rate at the normal CO₂ level of 340 ppm. CO₂ depletion causes the plants to be unable to utilise the good light conditions, and when this situation lasts longer, growth and production will drop dramatically.

CO₂ ENRICHMENT

As shown in the figure, the effect of CO₂ enrichment is greatest when the CO₂ concentration is low, and the effect decreases at higher CO₂ concentration. The line in the figure levels off at about 800 ppm and is even horizontal beyond about 1000 or 1200 ppm. So increasing the CO₂ concentration from very low up to 350 ppm or 400 ppm is very effective; between 600 and 800 ppm supply is less effective and increasing it further than 1000 is not very useful at all.

In general, a two- or threefold increase of the normal CO₂ concentration in a greenhouse, i.e. enrichment to 700 or 1000 ppm compared to 340 ppm, increases photosynthesis and therefore growth and production by 20 to 30%. In some crops, mainly flower crops, it may also improve the product quality. But .....

VENTILATION

When the greenhouse air contains more CO₂ than the outside air, some CO₂ will be lost by air exchange. With ventilators closed, the air exchange is only small, just caused by leakage. Of course at more ventilation, more CO₂ must be supplied to maintain a certain level above the outside level. The need to ventilate makes it very expensive or even impossible to maintain high CO₂ levels on a sunny day. This is very unfortunate, because plants could favour very much from extra CO₂ under sunny conditions and there is no easy a solution available.

On the other hand, when no CO₂ is supplied, ventilation is beneficial as it brings in fresh air with CO₂. But even during ventilation, depletion may occur, because sometimes the plants consume more CO₂ than the ventilation brings in. As long as the CO₂ level is low, CO₂ enrichment is very effective. Moreover, as long as the greenhouse CO₂ level is below-outside, all CO₂ supplied will be absorbed by the plants, especially when it is released under or in the plants.

In general, limited CO₂ supply will be feasible whenever there is CO₂-depletion and also at around-normal levels when the ventilation rate is only small. Really high CO₂ levels are feasible only when the greenhouse is closed.

POSSIBILITIES

CO₂ enrichment is applied on a very large scale in greenhouses in many countries, for instance in The Netherlands, where CO₂ is obtained from burning natural gas. As CO₂ enrichment is often combined with heating or heat storage, it can be done relatively cheaply. The effects of CO₂ enrichment are well understood and appreciated, and most growers in The Netherlands have made great investments especially for CO₂ enrichment.

In New Zealand, however, CO₂ enrichment is more expensive, because of different prices and conditions. Nevertheless there are possibilities to increase the production moderately with CO₂ enrichment, mainly in periods with little or no ventilation. This will be the topic of following articles.

CO2 enrichment in greenhouses boosts growth and production of all crops

The rates of photosynthesis and therefore of growth increase with increasing CO2 concentrations and ncreasing light levels.