Carbon Dioxide is an odorless gas which makes up about 300 ppm of our atmosphere, yet dried plant material contains an average of 40% carbon which comes entirely from CO2. Therefore we need to consider CO2 to be a major plant nutrient, one that affects growth rate, yield and one that needs to be supplied in adequate quantities if crop growth is going to be maximized.
CO2 – Carbon Dioxide
The main plant process a grower needs to consider is ‘photosynthesis’ as this is what drives growth, development and production. Photosynthesis is a reaction with occurs within the leaf tissue and requires light of the correct wavelength, water and carbon dioxide to produce assimilates (sugars) which are used for growth and development. As a by-product oxygen is released into the environment. When artificial lights are used to grow plants, the aim is to provide just the right intensity and wavelengths for optimal photosynthesis. Hydroponic plants also usually have more than sufficient water and nutrients, so the limiting factor in the process of photosynthesis in an enclosed environment, then becomes the availability of carbon dioxide (CO2). In a well sealed growing environment situation, CO2, under good lighting, begins to limit photosynthesis very rapidly. Since ambient CO2 levels in the air are around 360 ppm, which is relatively low, this can be used up by even a small population of actively photosynthesizing plants within a couple of hours. In fact CO2 can drop away to only a few ppm in well sealed growing environment and when this happens, if the CO2 is not replaced, photosynthesis and plant growth stops.
Not only is it important to prevent CO2 depletion, but enrichment to levels much greater than atmospheric levels is known to boost plant growth by over 40%. The level of enrichment and the timing of enrichment, since all methods of CO2 enrichment have a cost involved. Obviously since plants only require, take up and use CO2 when photosynthesizing in light, enrichment only needs to occur when the lights are on or during day light hours. Enrichment at night is pointless since the extra CO2 won’t be taken up by the plants and will just accumulate. Secondly, enrichment levels need to be high enough to replace the CO2 used by the plants and to increase the levels of CO2 in the environment to a level where it will accelerate photosynthesis and therefore plant growth. Levels of 800 – 1800 ppm have proven to be optimal for the majority of crops grown under protected cultivation and having CO2 monitoring equipment then becomes important to make sure this level is reached and maintained. CO2 enrichment will have its greatest effect on accelerating photosynthesis and growth where other factors are also optimal – that is there is sufficient light for photosynthetic reactions and temperatures are not limited. Temperatures can be run a little higher where CO2 is enriched and light levels are at optimum levels – generally in the range 27(80F) too 32 C(92F) day temperatures for most flowering and fruiting plants.
CO2 enrichment to levels of at least 800 ppm has been shown to increase the growth rate, yields and early harvests of many crops and is certainly economically viable for most high value crops. Supplying CO2 The two most commonly methods used for CO2 enrichment of a growing area are burning of hydrocarbon fuels such as natural gas or propane, and compressed, bottled CO2. There are actually a few other, less practical ways – these are dry ice, fermentation, burning of candles and oil lamps and decomposition of organic matter.
CO2 generators are widely available for use in growing areas and this is less expensive than using bottled CO2. The major problem with burning fuel to create CO2 is that heat is produced as a by product – this may be useful under cooler conditions, but not if the environment is already sufficiently warm. As the CO2 is introduced to the greenhouse, it needs to be thoroughly mixed with use of a circulation fan.
Compressed, bottled CO2: Safer option for plant enrichment – in that no toxic by-products or additional heat can be produced.. Compressed CO2 comes in cylinders stored under high pressure (1600-2200 psi). Equipment such as a pressure regulator, flow meter and solenoid valve and timer are required to set up this type of enrichment system. CO2 is injected into the growing area via the pressure regulator and flow meter which is controlled via a solenoid and timer. One pound of compressed CO2 gas contains about 8.5 cubic feet of CO2 gas at normal atmosphere pressure.
Very small tightly sealed growing areas can use dry ice to provide CO2 enrichment – this also gives some cooling effect. Dry ice is solid, very cold CO2 and needs to be stored and handled with care. Dry ice can also ‘melt’ very rapidly in warm conditions, so may need to be well managed to ensure a continual supply of CO2 at the correct level.
No matter which method of enrichment is used it is important to firstly bring the environment up to the predetermined level and then constantly replenish to this level as the plants absorb the CO2. The rate of CO2 absorption will change with plant size, temperatures and light level and this is why constant monitoring of levels in important.
For the majority of flowering and fruiting plants produced hydroponically, plant growth and flowering will be optimal under conditions where the night temperature is lower than the day temperature. Most plant species exhibit these ‘Diurnal rhythms’ where certain plant process such as the rate of growth of the flower buds, stomata opening, discharge of perfume from flowers, cell division and metabolic activity, occur more rapidly at a certain time within a 24 hour period. For example, photosynthesis in most plants is known to reach a maximum just before noon, and cell division also seems to always reach a maximum just before dawn. Many species flower or grow well only when temperatures during the part of the diurnal cycle that normally comes at night are lower than temperatures during the day. Also light given during the normal night period may actually inhibit some plant processes.
Plants such as tomatoes seem to be particularly sensitive to the alternation in temperature between day and night: they produce more flowers when night temperatures are lower than day temperatures – this effect in plants is called ‘Thermoperiodism’, and is common amongst many plant species. Pepper plants also require lower night than day temperatures for good production, it has been found that many more buds on pepper plants will actually develop into open flowers when night temperatures are at least 6 C(11F) lower than day temperatures. Where day and night temperatures remain at similar levels on a long term basis, flowering and fruiting can be adversely affected, particularly where temperatures are warm. Bud, flower and fruitlet abscission is much more common on crops which do not receive lower night temperatures and this often limits production of crops such as tomatoes and peppers under tropical conditions.
Night temperatures for most plants are optimal at around 18 C (65F) too 24 C(75F) lower than day temperatures, provided day temperatures are held at optimal levels for photosynthesis. At night, where the ‘sinks’ which receive the assimilates (sugars) produced via photosynthesis, become cooler, transport of sugars into these is promoted. ‘Sinks’ on most plants are the developing flower buds, flowers and fruit which have the greatest affinity for the sugars produced by the plant. The ‘Source’ is the producer of the assimilates – usually the leaves, but sometimes also the stem in some plant species. So cooler ‘sinks’ get more assimilate pumped into them at night than if they remained as warm as they were during the day light hours.
Apart from the physiological effects on plant growth and flower development, having a lower night temperature setting has other beneficial effects on plant processes. Firstly root pressure is greater at night under cooler conditions – this increases the pressure in the xylem vessels, so that calcium and other plant growth compounds which are carried in the xylem stream are forced out to the leaf tips and into developing buds, flowers and fruits. This turgor pressure is often essential in the prevention of tip burn as it ensures calcium is carried to the very edges of the leaves. Often, this root or xylem pressure can be seen in the form of ‘guttation’ which are visible droplets of water which can be seen at the tips of leaves on plants in the early morning. It is this root or xylem pressure which also acts to ‘pump up’ the plant during the cooler night temperatures particularly after a day when transpiration rates and warm temperatures have resulted in some wilting and loss of turgour.
Maintaining cooler night temperatures also ensures that plant respiration does not occur at too greater rate. Respiration uses up valuable assimilates and the rate of respiration increases rapidly with temperature. Under very warm night temperature conditions, night respiration can burn nearly as much assimilate as has been produced via photosynthesis and can severely limit plant growth.
There are a few devices which can be used to measure and monitor CO2 levels in your growing environment. There are a range of CO2 sensors available – from simple ‘syringe’ test kits which allow a grower to take a sample of the air in the growing environment and determine the CO2 level, to using timed devises and the use of electronic controls and meters which accurately monitor CO2 levels and display this on an LCD readout.
CO2 enrichment to increase plant growth and yields is a well proven method of crop production which can benefit even the smallest grower and is widely used on a commercial scale.