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, 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.