CO₂-enrichment to 700 or 1000 ppm over a longer period increases growth and production of most crops by 20 to 30%. There are differences between crops in how they respond to CO₂-enrichment. This article discusses lettuce, cucumber and capsicum, and the next article will deal with special responses of tomato.

YOUNG PLANTS

In all crops CO₂ enrichment will increase the rate of CO₂-uptake (photosynthesis), which means that the plants have more sugars (assimilates) to grow on. In young crops with CO₂-enrichment, we will see an overall increase in growth: more and bigger leaves, taller and heavier plants. As bigger plants intercept more light, there is an extra increase of photosynthesis. The result of both the higher CO₂-concentration and the better light interception is a huge increase of initial growth of the young plants. CO₂-enrichment thus helps the newly planted crop to establish and to have a good start. It may also help to make the plants stronger and less susceptible to diseases.

After the initial young plant phase, we might see slightly different reactions in different crops. In all crops the leaves produce more assimilates in response to CO₂-enrichment, but crops differ in how they spend the extra assimilates.

LETTUCE

When lettuce is grown under high CO₂, all extra assimilates will go into the leaves, and maybe a bit into the roots. As the leaves form the harvestable product, all CO₂ is invested in harvested plant parts, and hardly any CO₂ is wasted. This explains why CO₂-enrichment is very effective in lettuce-type crops. With CO₂-enrichment, plants will be earlier on a sufficient weight and thus earlier ready for harvest, or they will end up at a higher harvest weight if grown for the normal length of time. A side effect may be that leaves are thicker and slightly hardy.

In an experiment in Littlehampton (UK) showed that threefold CO₂-enrichment (1000 ppm) increased the average weight of mid-winter lettuce by 25 to 40%, or reduced the growing period to reach a weight of 140 g by 10 to 12 days.

CUCUMBER

Cucumber plants have a very clear response to CO₂-enrichment. In a young stage, the leaves grow even bigger than normal and the plants look extra vigorous. First fruit harvest can be a few days earlier. The obvious first effect seems to diminish after a few weeks of CO₂ enrichment, but in a later stage CO₂-enriched cucumber plants usually have far more and taller laterals than unenriched plants. Each lateral will have its own photosynthesis and will produce many flowers and some cucumbers all the time. The extra assimilates may also speed up fruit growth, so that fruit reach their harvestable weight earlier, or end at a higher weight after the normal growing time. Generally, CO₂ plants has a strong positive effect on leaf growth, and on both fruit number and average fruit weight of cucumber.

In an experiment in Littlehampton CO₂-enrichment in the range 300 to 500 ppm increased fruit production by 540 g per m² for each 10 ppm increase in CO₂ (10.8 kg per m² extra in total), over a period of about 4 months in summer. An experiment in Naaldwijk (Holland) compared cucumber production at two CO₂ levels: 360 and 620 ppm over the summer. The high CO₂-level showed an increase in fruit production of 34%. In this case it was mainly due to increased number of fruits, and for a smaller part to increased average fruit weight.

CAPSICUM

Also in Capsicum CO₂-enrichment increases growth of young plants. When fruit set is limited by assimilate deficiency, e.g. in dark periods, CO₂ enrichment improves fruit set. In the later phase, however, Capsicum is slightly different from cucumber and tomato. Typical for Capsicum is that, while fruits are growing, the other plant parts like roots, stems, leaves etc. are strongly restricted in growth. The more fruit are growing, the more restricted leaf growth is. As CO₂-enrichment increases the number of fruit on the plant considerably, it clearly reduces the vegetative growth. Another way of looking at it, is that CO₂-enriched plants put a higher percentage of assimilates into their fruits. This means that CO₂-enrichment causes a shift in internal distribution of assimilates. As a result, production is very high production on the short term, but may be disappointing in the longer term due to insufficient growing capacity (too small plants). The remedy is fruit thinning and so maintaining a proper balance between producing fruit and building a plant for later production.

In an experiment in Holland with Capsicum in summer and autumn, CO₂-enrichment increased the first fruit production by almost 46% when 450 ppm was compared with 300 ppm. The increase in production was caused by a higher number of fruits, through a better fruit set. The first harvest started earlier. The average fruit weight was not much higher. However, the plants at high CO₂ were far smaller than those at low CO₂, and the later production would probably not have been so clearly raised by CO₂. Growers experience is that with a proper balanced in growth and fruit load, the effect of CO₂-enrichment is normally in the order of 12 to 15% when comparing moderate enrichment (450 - 500 ppm) to no enrichment at all.

TAKE CARE

CO₂-enrichment works, but it may work out wrong if the CO₂ level becomes too high and certainly if the CO₂ supplied is not pure. Therefore, do not enrich to a level higher than 1000ppm, and in younger or weak plants not above 700 ppm, to avoid other possible negative side effects. And be sure that CO₂ does not contain any dangerous contaminants like ethylene or sulphur dioxide.

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