What factors slow down the evaporation process
How air temperature affects plants
From CANNA research
It should come as no surprise that a large part of our research is concerned with the development of suitable temperature strategies for economical greenhouse production. However, the optimal temperature for a plant according to several factors. The reaction of a plant to the ambient temperature depends on the stage of development it is currently in. Plants have a kind of internal biological clock that determines how sensitive they are to changes in temperature.
Differences between air temperature and plant temperature
Most biological processes are accelerated at higher temperatures. This can have both positive and negative effects. Faster growth or formation of fruits is seen as an advantage in most cases. The excessive Plant breathing However, it is disadvantageous because it means that less energy is available for fruit development and the fruits become smaller. Some effects are short-term, while others are long-term. The plant's assimilation budget is influenced by temperature, for example, and has immediate effects. The initiation of flower formation however, is determined by the weather over a longer period of time.
In a figurative sense, this is comparable to traffic on a motorway. The plant's stomata are the exits through which traffic can exit the highway. If a lot of cars gather just before an exit, they have to slow down and the traffic jams. With fewer cars, the flow of traffic can be accelerated. The same thing happens with the air and water vapor molecules in the air. With a higher concentration of these molecules in the vicinity of the stomata (the "exit"), they can leave the stomata less quickly and a backlog occurs. This is exactly what happens when the satiety deficit is high. This means the plant is less able to cool itself, which causes stress. In addition, the water vapor condenses and forms a thin film on the leaf surface. This in turn creates optimal conditions for pathogens.
The Plant temperature and the air temperature are not the same because plants are able to get through evaporation to cool down and to warm up by exposure to sunlight. Plants naturally strive to find their optimal temperature. For this, air temperature, relative humidity and lighting conditions must be in balance. If these values are high, the plant warms up, which leads to a difference between the plant temperature and the air temperature. To cool down, the plant needs to increase its transpiration rate. Just like the temperature, the transpiration rate also depends on environmental conditions such as light, CO2 levels in the atmosphere and relative humidity and varies depending on the type of plant.
Plants consist of different parts, each of which reacts differently to temperature. The temperature of the fruit largely corresponds to the air temperature. If the air temperature rises, so does the fruit temperature and vice versa. However, the fruit temperature fluctuates less than the air temperature and a temperature increase or decrease in the fruit takes place (up to a few hours) more slowly than with the air temperature. The temperature of the flowers, however, is higher than the air temperature or the Leaf temperature, and the petals transpire to a much lesser extent than the rest of the leaves. The plant temperature on the upper side of the leaf crown is subject to higher fluctuations than on the underside of the leaf crown. The top side will also heat up more easily as a result of the sun's rays and, in bright light conditions, will reach temperatures that are above the air temperature.
The relative humidity of the environment depends on the temperature and the wind strength. Higher temperatures generally lead to increased perspiration. This is partly because the molecules move faster. But it is also a fact that warm air can absorb more water vapor. When there is no air movement, the air around the leaf becomes saturated with water vapor. This slows down evaporation. When the air becomes saturated with water, the water condenses and forms a film around the leaf. This in turn favors pathogens that can attack the plant.
The saturation deficit can be compared to the tachometer in a car. The more revolutions the engine makes, the higher the tachometer needle rises until it eventually gets into the red zone. An engine can take this for a short time. But if the car drives in this condition for a long time, the engine will inevitably be damaged. The same applies to plants: If the saturation deficit is too high over a longer period of time, the plant cannot recover the following night and permanent plant damage can occur (withered leaves or flowers).
The difference between the water vapor content in the air and the saturation point is called Satiety deficit. The higher the saturation deficit, the more water a plant loses through transpiration. However, if the satiety deficit is too high, it can lead to plant stress as the plant cannot replace the amount of water it loses through transpiration. For a short while this is not a big problem. The plant will absorb enough water the following night to regenerate.
By measuring the leaf thickness you can get an idea of the regeneration potential of the plant. The leaves can become thinner during the day because they lose water through perspiration. However, if a leaf is thinner one night than the night before, it suggests that the plant has not recovered. It is best to keep the satiety deficit low to avoid harm. However, under these conditions the plant is not stimulated to growth and activity, which in turn means that the plants are poorly armed against stress.
A comparison with a car's speedometer can illustrate a lot here. The more revolutions the engine makes, the higher the tachometer needle rises until it eventually gets into the red zone. This won't damage the motor immediately, but if the needle stays in the red for too long, damage is almost inevitable. For most plants, the saturation deficit should be between 0.45 and 1.25 kPa. The optimal value is 0.85 kPa. By the way, kPa (kilopascal) is the unit of measurement for pressure. The saturation deficit follows more or less the same pattern as the irradiance of the surroundings: it increases as soon as the sun shines, reaches its peak value around noon, and then gradually decreases again. To calculate the saturation deficit, air temperature, plant temperature and relative humidity must be known.
Most of the water in the atmosphere is in the form of water vapor. Water vapor is invisible, but it does have an impact on our wellbeing. When the humidity is high, we feel sticky, sweaty and not particularly comfortable. The visibility also depends on how much water vapor is in the air. Clouds are visible because the water vapor they contain has cooled down so much that the water molecules gradually condense and form small water droplets or even ice crystals in the air. We perceive that as clouds.
Plants are able to regulate the process of transpiration and cooling by using special plant organs: the Stomata. The stomata are specialized cells in the leaves that open and close. In this way they limit the amount of water vapor that can evaporate. The higher the temperature, the more evaporation the stomata allow because they open. It is difficult to measure the opening of the stomata. We can only estimate them based on the saturation deficit. The wider the stomata open, the more gases get in and out of the leaves.
Environmental factors influence the speed with which this process takes place (stomata conductance). Example: A higher humidity leads to a faster conductance, while higher CO2-Contents to inhibit stomal conductance. But conductance is also influenced by other non-environmental facts. For example through plant hormones and through the color (the wavelength) of the light that shines on the plant. The plant hormone abscisic acid regulates the concentration of ions in the stomata and causes the stomata to open very quickly (within a few minutes). Light with smaller wavelengths (approx. 400–500 nanometers (nm)), i. H. blue light, causes the stomata to open wider than with longer wavelengths (approx. 700 nm, red light).
This is a colored scanning electron micrograph (SEM) of the underside of a garden rose (Rosa sp.) Leaf showing an open stomata. A stomata is an extremely small pore that is bounded by two kidney-shaped protective cells. When the pore opens, gases can enter or escape. This is essential for photosynthesis. These pores close at night or during dry periods to prevent water loss.
Optimal day and night temperatures
Different processes take place in a plant during the day and at night. The optimal temperature for the plant is accordingly different. The transport of sugar usually takes place at night and is mainly towards the warmer parts of the plant. The leaves cool faster than fruits and flowers. Therefore, most of the available energy is directed to the parts of the plant that need that energy to grow and develop.
The optimal day / night temperature combinations were researched in 1949 in the world's first Phytotron, an air-conditioned research greenhouse at the California Institute of Technology. The trials confirmed that tomato plants grow taller with a combination of high temperatures during the light time of the day and a low temperature in the dark, compared to a situation where the temperature is constant all day. The phenomenon that plants can differentiate between day and night temperature fluctuations is known as thermoperiodism. It affects the formation of flowers and fruits, as well as growth.
The amount of sugar that is transported into the growth tissue, where this energy is needed to allow higher levels of plant breathing, can be restricted at higher night temperatures. In other words, growth can be restricted. It was also found that a combination of high daytime temperatures and low Night temperatures a Stem elongation can occur. A low night-time temperature improves the water balance within the plant and is therefore mainly decisive for the increased stem elongation. The temperature can therefore be used as an aid to regulate the height of the plants. But energy is also saved at low night-time temperatures. The term thermomorphogenesis describes the thermoperiodic effects on plant morphology.
The optimal air temperature also depends on the light intensity and the carbon dioxide content in the air. Plants work in a similar way to cold-blooded animals: their cycle and the extent of photosynthesis essentially correspond to the ambient temperature. If the temperatures are very low (different from species to species), even with so much light there is hardly any photosynthesis. The higher the air temperature, the more intense the photosynthesis is. When light and temperature are in equilibrium, the CO2-Content in the environment is the limiting factor. Is there enough CO2 available, the extent of photosynthesis increases with increasing temperatures, although other factors also play a role. Take the enzyme, for example RuBisCo.
RuBisCo is crucial for photosynthesis. In some cases, a process called photorespiration occurs. This is the case when the RuBisCo binds to the oxygen and not to the carbon dioxide, as happens in normal photosynthesis. The CO2-Content and the optimal temperature are lower at low light intensities than at high light intensities. The enzyme activity is also higher at higher temperatures.
Differences between day and night temperature (DIFF)
The DIFF-Concept deals with the relationship between day and night temperatures. The effects of the daily temperature fluctuations on the growth in length of the plant stems depends on the difference (diff) between day and night temperatures and less on the separate and independent reactions to day and night temperatures. In other words: It is precisely this temperature difference that is important, but also the question of which of the two is higher: the night or day temperature.
Foliage growth is not much affected by the difference, but the growth of the stalk sections with internodes is affected. Plants under a positive difference (day temperature higher than night temperature) grow taller than plants at zero diff. Plants that are at Zero diff grow, are in turn larger and have longer internodes than plants that grow under a negative difference. Other important morphogenetic responses to a negative difference (Day temperature lower than night temperature) are shorter petioles, flower stalks, flower stalks, and leaves.
Differences in internodal expansion and leaf expansion are the result of differences in the process of cell expansion or cell division. If the difference is negative, both processes are inhibited, which may be due to reduced gibberellin activity in the subapical meristem (a plant tissue that ensures growth). Gibberellin is a plant hormone that stimulates growth. DIFF has the greatest influence on stem elongation in a time of rapid growth. Therefore, compared to adult plants, seedlings are more sensitive to differences between day and night temperatures. A negative difference in the early stage of stem elongation is therefore important to limit the plant height.
Stem elongation can also be triggered by a brief drop in temperature (approximately two hours) during the daily 24-hour growth cycle. This temperature drop is generally carried out at the first twilight or just before it. The ability to react to temperature changes seems to be strongest in long-day, short-day and day-neutral plants in the first hours of light. A drop in temperature in the last two hours of the night therefore affects the height of the plants. This is usually easy to do in greenhouses in cooler climates, especially in autumn, as the nighttime temperature is already low at this time of the year.
The fluctuations in temperature sensitivity in stem elongation within the daytime and nighttime are possibly the result of an endogenous growth rhythm. A circadian growth rhythm (lasting approximately 24 hours) was found in chrysanthemums in 1994. The stem elongation is not constant in a 24 hour cycle of light and dark. Both short-day plants and long-day plants that are grown under flower-inductive conditions stretch faster at night than during the day. Orchids need a period of low nighttime temperature to bloom.
Differences between day and night temperatures is one of the techniques used by plant breeders. A minimum and maximum temperature for the crop is determined. The temperature can fluctuate calmly as long as the average temperature is maintained over a longer period of time. This strategy uses natural heat whenever possible.
Air temperature is an important environmental factor that affects plant development and growth. However, you can never look at the air temperature in isolation. Every factor in plant growth is related to every single one of the other factors, and the challenge is to find the weakest link in the chain. This article has addressed many of these factors. However, there are others that are just as important, such as: B. the water balance and thus - albeit indirectly - the transpiration. For everything that happens or will happen in a plant, the air temperature serves as the first control factor. The right air temperature is the first step on the long road to successful crop production.
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