Sweat It Out for Good Results!
By Steve Berlow
Transpiration
So what the heck is transpiration and how does it affect the way your plants grow?
Transpiration is the evaporation of water from the leaf surface to the air.
It mainly takes place through stomata, minute pores present on either the upper, lower or commonly both surfaces of a leaf. In terrestrial plants, especially herbs, transpiration takes place on both surfaces of the leaf.
The lower surface, however, contributes to the most water loss.
The uptake of water by plants is mainly in response to the demands of transpiration upon the plant.
Since transpiration is the movement of water through the plant to replace the water lost from the leaves, it is essentially responsible for generating sufficient force for absorption and translocation of water (think of it as suction).
If absorption lags behind transpiration, the stomata will close as the first line of defense against dehydration, thereby slowing the rate of water loss from the leaves.
Now this might seem like a good thing (and it is normally) but the paradox or irony of this response is that when the stomata close (even partially), not only is the rate of water loss from the leaves slowed ( the good thing), but so is the uptake of water (a potentially bad thing).
So why is this important? For several very important reasons.
First, if you slow the uptake of water, you will also slow the uptake and translocation of nutrients into the plant.
This uptake and translocation is directly proportional to the rate of transpiration, which, in effect, pulls water up through the plant.
Secondly, if transpiration is limited, then the plant’s ability to disperse metabolically generated heat is also limited.
Now if this internally generated heat dispersal is now limited, one of two things can happen: the plant can build up internal heat to the point where death occurs, or the plant can slow down its metabolic rate to reduce the amount of heat generated.
Luckily for us (and the plants!) there are feedback mechanisms that stop the plant from overheating. ...
the main mechanism is the reduction of Co2 through the closure of the stomata, which then limits gas exchange.
(There are other feedback mechanisms as well.)
The point to all of this is, if you limit the maximum rate of beneficial transpiration, then you will, by default, limit the rate of growth.
The ideal scenario is where the stomata are open for the maximum period of time during the lights on period without causing dehydration or stress - we call this scenario beneficial or positive transpiration.
Beneficial or positive transpiration will in turn maximize not only your water/nutrient uptake, but the metabolic heat dissipation and gas exchange, as well.
This in turn maximizes growth and development rates.
So now that we know this very important information, how do we put it into practice and affect maximum beneficial transpiration rates? EASY! Transpiration depends primarily upon the prevailing environmental conditions. As a rule, a drier environment accelerates transpiration. Now before you madly go off and try to obtain a relative humidity of 0% to get a temperature of 100 degrees, remember that for optimal uptake or transpiration we cannot lose more water than we uptake. If this happens then the stomata will close and try to limit water loss. Environmental conditions taken to extremes can cause the death of the plant, especially if excess temperatures are involved. Then we would be back to square one. What we are trying to obtain is the most favorable environment for maximum beneficial transpiration, not out-right maximum transpiration. We need to consider and control other important environmental factors besides temperature and humidity.
What Good Is Transpiration?
The advantage of transpiration might be a sort of “win by default”. It is essential to the life of a land plant to absorb carbon dioxide from the atmosphere. It seems that the stomatal mechanisms have evolved because of this requirement, and the disadvantageous consequence is excessive or negative transpiration. Transpiration is necessary since it allows nutrient transport, helps maintain optimum turgidity for growth, and accounts for removing large amounts of heat from the leaves produced through metabolic or environmental means.
Environmental Impact On Stomata
Stomata of higher plants are differentially affected by prevailing environmental conditions. Most of the time two or more environmental variables influence stomatal movement initiated by the first variable. In other words, the primary variable initiates a response, and a second or secondary variable amplifies or reverses the response. For instance, adequate water in the growing medium initiates opening, while water depletion with higher temperature closes the stomata. Similarly, higher carbon dioxide initiates closing, but if given with adequate light favors opening. It is therefore wise to understand the effect of one variable on stomata and interactions of different variables on these minute pores.
The Effect Of Light
Light has both quantitative and qualitative effects on stomata. Stomata of most plants open at sunrise and close in darkness, allowing CO2 entry to be used in food production during the lights on or day period. Opening generally requires about an hour, and closing is often gradual throughout the afternoon. Stomata close faster if plants are suddenly exposed to darkness. The minimum light level for the opening of stomata in most plants is about 1/1000 to 1/30 of full sunlight (950W/m sq.), just enough to cause photosynthesis. High irradiance levels cause wider apertures. Blue light (wavelengths between 430 and 460nm) is nearly 10 times as effective as red light (wavelengths between 630 and 680nm) in producing a given stomatal opening. This is one reason why all good light sources should have an adequate blue spectrum. There is only a slight response to green light. (Note: Most good quality high output HPS lamps have an adequate blue spectrum to facilitate maximum stomatal response.)
Effects Of CO2
In most plants, low CO2 concentration in the leaves favors opening because the plant is trying to absorb more Co2. Higher external CO2 concentration partially opens stomata enough to increase photosynthesis. Indoor crops sometimes lack enough CO2 for maximum growth and this is especially serious if inadequate ventilation is provided. Carbon dioxide levels should not exceed 1000 to 1200 ppm over the ambient because it causes stomatal closure.
Effect Of Relative Humidity (RH)
Stomata of most plants are highly susceptible to atmospheric humidity. They close when the difference between the water vapor content of the air and that of the plant exceeds a critical level. Low RH coupled with low light intensity causes a rapid closing response. Saturated atmospheres tend to close stomata.
Effect Of Temperature
High temperatures (over 30 to 35°C) usually cause stomatal closing. This can be a response to water depletion or increased respiration. Low to freezing temperatures tend to close stomata.
Effect Of Wind
Sometimes stomata partially close when leaves are exposed to gentle breezes, possibly because more CO2 is brought close to the stomata, increasing its diffusion into the leaves. Wind can also increase the transpiration, leading to excessive water depletion and stomatal closing.
Effect Of Water Status
Water content close to field capacity favors wider opening of stomata. With increased transpiration, water depletion leads to gradual stomata closing. This is an adaptive response to defend the plant against the development of wilting symptoms, which leads to severe dehydration through excessive loss of water. Similarly, water logging coupled with high temperature favors closing of stomata.
By Steve Berlow
Transpiration
So what the heck is transpiration and how does it affect the way your plants grow?
Transpiration is the evaporation of water from the leaf surface to the air.
It mainly takes place through stomata, minute pores present on either the upper, lower or commonly both surfaces of a leaf. In terrestrial plants, especially herbs, transpiration takes place on both surfaces of the leaf.
The lower surface, however, contributes to the most water loss.
The uptake of water by plants is mainly in response to the demands of transpiration upon the plant.
Since transpiration is the movement of water through the plant to replace the water lost from the leaves, it is essentially responsible for generating sufficient force for absorption and translocation of water (think of it as suction).
If absorption lags behind transpiration, the stomata will close as the first line of defense against dehydration, thereby slowing the rate of water loss from the leaves.
Now this might seem like a good thing (and it is normally) but the paradox or irony of this response is that when the stomata close (even partially), not only is the rate of water loss from the leaves slowed ( the good thing), but so is the uptake of water (a potentially bad thing).
So why is this important? For several very important reasons.
First, if you slow the uptake of water, you will also slow the uptake and translocation of nutrients into the plant.
This uptake and translocation is directly proportional to the rate of transpiration, which, in effect, pulls water up through the plant.
Secondly, if transpiration is limited, then the plant’s ability to disperse metabolically generated heat is also limited.
Now if this internally generated heat dispersal is now limited, one of two things can happen: the plant can build up internal heat to the point where death occurs, or the plant can slow down its metabolic rate to reduce the amount of heat generated.
Luckily for us (and the plants!) there are feedback mechanisms that stop the plant from overheating. ...
the main mechanism is the reduction of Co2 through the closure of the stomata, which then limits gas exchange.
(There are other feedback mechanisms as well.)
The point to all of this is, if you limit the maximum rate of beneficial transpiration, then you will, by default, limit the rate of growth.
The ideal scenario is where the stomata are open for the maximum period of time during the lights on period without causing dehydration or stress - we call this scenario beneficial or positive transpiration.
Beneficial or positive transpiration will in turn maximize not only your water/nutrient uptake, but the metabolic heat dissipation and gas exchange, as well.
This in turn maximizes growth and development rates.
So now that we know this very important information, how do we put it into practice and affect maximum beneficial transpiration rates? EASY! Transpiration depends primarily upon the prevailing environmental conditions. As a rule, a drier environment accelerates transpiration. Now before you madly go off and try to obtain a relative humidity of 0% to get a temperature of 100 degrees, remember that for optimal uptake or transpiration we cannot lose more water than we uptake. If this happens then the stomata will close and try to limit water loss. Environmental conditions taken to extremes can cause the death of the plant, especially if excess temperatures are involved. Then we would be back to square one. What we are trying to obtain is the most favorable environment for maximum beneficial transpiration, not out-right maximum transpiration. We need to consider and control other important environmental factors besides temperature and humidity.
What Good Is Transpiration?
The advantage of transpiration might be a sort of “win by default”. It is essential to the life of a land plant to absorb carbon dioxide from the atmosphere. It seems that the stomatal mechanisms have evolved because of this requirement, and the disadvantageous consequence is excessive or negative transpiration. Transpiration is necessary since it allows nutrient transport, helps maintain optimum turgidity for growth, and accounts for removing large amounts of heat from the leaves produced through metabolic or environmental means.
Environmental Impact On Stomata
Stomata of higher plants are differentially affected by prevailing environmental conditions. Most of the time two or more environmental variables influence stomatal movement initiated by the first variable. In other words, the primary variable initiates a response, and a second or secondary variable amplifies or reverses the response. For instance, adequate water in the growing medium initiates opening, while water depletion with higher temperature closes the stomata. Similarly, higher carbon dioxide initiates closing, but if given with adequate light favors opening. It is therefore wise to understand the effect of one variable on stomata and interactions of different variables on these minute pores.
The Effect Of Light
Light has both quantitative and qualitative effects on stomata. Stomata of most plants open at sunrise and close in darkness, allowing CO2 entry to be used in food production during the lights on or day period. Opening generally requires about an hour, and closing is often gradual throughout the afternoon. Stomata close faster if plants are suddenly exposed to darkness. The minimum light level for the opening of stomata in most plants is about 1/1000 to 1/30 of full sunlight (950W/m sq.), just enough to cause photosynthesis. High irradiance levels cause wider apertures. Blue light (wavelengths between 430 and 460nm) is nearly 10 times as effective as red light (wavelengths between 630 and 680nm) in producing a given stomatal opening. This is one reason why all good light sources should have an adequate blue spectrum. There is only a slight response to green light. (Note: Most good quality high output HPS lamps have an adequate blue spectrum to facilitate maximum stomatal response.)
Effects Of CO2
In most plants, low CO2 concentration in the leaves favors opening because the plant is trying to absorb more Co2. Higher external CO2 concentration partially opens stomata enough to increase photosynthesis. Indoor crops sometimes lack enough CO2 for maximum growth and this is especially serious if inadequate ventilation is provided. Carbon dioxide levels should not exceed 1000 to 1200 ppm over the ambient because it causes stomatal closure.
Effect Of Relative Humidity (RH)
Stomata of most plants are highly susceptible to atmospheric humidity. They close when the difference between the water vapor content of the air and that of the plant exceeds a critical level. Low RH coupled with low light intensity causes a rapid closing response. Saturated atmospheres tend to close stomata.
Effect Of Temperature
High temperatures (over 30 to 35°C) usually cause stomatal closing. This can be a response to water depletion or increased respiration. Low to freezing temperatures tend to close stomata.
Effect Of Wind
Sometimes stomata partially close when leaves are exposed to gentle breezes, possibly because more CO2 is brought close to the stomata, increasing its diffusion into the leaves. Wind can also increase the transpiration, leading to excessive water depletion and stomatal closing.
Effect Of Water Status
Water content close to field capacity favors wider opening of stomata. With increased transpiration, water depletion leads to gradual stomata closing. This is an adaptive response to defend the plant against the development of wilting symptoms, which leads to severe dehydration through excessive loss of water. Similarly, water logging coupled with high temperature favors closing of stomata.
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