Protected cultivation of plants means a grower can manipulate just about all aspects of the crop's environment to optimize growth, development and yields. However it is important to understand what environmental influences affect certain plant processes, so that mistakes which could actually damage or slow plant growth don't ever happen.

CO₂
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. A by-product of this process is oxygen being released into the environment. When artificial lights are used to grow plants, the aim is to provide just the right intensity and wavelengths foroptimal 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.

What is important is 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 - 1800ppm 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 26(79F) - 30 C(86F) 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.

Day-Night Temperatures
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, stomatal 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 15 C (59F) - 20 C(68F) 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.

Maintaining night temperatures just a few degrees C(F) below optimal day time temperatures will have a positive effect on plant growth and development for most flowering plant species. This is largely because the majority of plants on this planet have adapted over millions of years for these types of naturally occurring conditions and we need to be aware that biorhythms and dinural cycles still exist even in artificially created growing environments.