When plants transpire, they move water from the roots to the shoots and release it into the atmosphere. Transpiration influences several other important processes, including nutrient uptake and photosynthesis. Understanding how plant transpiration works can help indoor cannabis cultivators foster an ideal environment for optimum transpiration rates.
Figure 1. Plant transpiration. Image by brgfx on Freepik
The transpiration process
Transpiration occurs primarily through small openings in the leaves called stomata. The stomata regulate water loss by pulling water up through the tiny tubes of the xylem[1] and releasing it as water vapor. This transpiration stream supplies water for photosynthesis and carbohydrate production. It also helps maintain turgidity: plant cell and tissue rigidity. In addition, transpiration transports nutrients throughout the plant.
The transpiration process is passive, meaning that the plant doesn’t expend energy on it. Instead, the driving force for water movement up the plant is the difference in water potential[2] between the root zone and the atmosphere, explained by the cohesion-tension theory. Because water evaporates through the leaves, the water potential in the leaves is lower than in the stem, which in turn is lower than in the roots, creating negative water potential. Think of it as suction. Since water naturally flows from areas of higher to areas of lower water potential, plants draw water upward from the roots to the leaves. Water molecules also adhere to the walls of the xylem and to each other, forming a continuous column of upward-moving water. Because transpiration is based on the difference in water potential, evaporation from the leaves obviously plays the most significant role in determining the transpiration rate.
Why transpiration is important
Several plant functions depend on an optimum transpiration rate, including:
- Water uptake: If transpiration is too slow, water uptake is reduced. However, if transpiration is too fast and water loss through the leaves begins to exceed what the roots can supply, plants close their stomata to limit further water loss, which also reduces water uptake.
- Nutrient uptake: Because nutrients dissolve in water, their absorption depends on water uptake. If water uptake is reduced because the transpiration rate is too slow or fast, it can result in nutrient deficiencies.
- Carbon dioxide (CO2) absorption: A critical component of photosynthesis, CO2 is absorbed through the stomata. If they close due to water stress, then CO2 absorption—and the rate of photosynthesis—decreases.
- Plant temperature: Transpiration dissipates heat and lowers the leaf surface temperature, so the plant’s ability to survive heat and drought stress heavily depends on the transpiration rate.
- Turgor pressure: By pushing the plasma membrane against the cell wall, turgor pressure, the cause of turgidity, keeps plants upright and maintains their form and structure. It also supports cell expansion for apical growth and enables stomata to open for CO2 intake during photosynthesis. When plants lose turgidity, they become flaccid and wilt.
- Removal of excess water: Transpiration allows the plant to release any water it does not need. Most of the water plants take up is released through evaporation.
How environment impacts transpiration
Several environmental factors influence the rate of transpiration, including relative humidity (RH), vapor pressure deficit (VPD) and air temperature at or above the canopy.
As RH increases, transpiration decreases (Figure 2).[3] Because transpiration releases moisture into the air, it increases humidity. When humidity gets too high, transpiration slows because the air already holds abundant moisture, making it harder for water molecules to evaporate. High humidity can also promote mold and fungal growth. On the contrary, if humidity is too low, excessive transpiration can occur, leading to water stress and nutrient imbalances.
Figure 2. Effect of humidity on plant transpiration. [4]
Temperature also influences transpiration. Increasing the temperature increases the transpiration rate until it plateaus (Figure 3). Overheating a grow room can cause plants to lose more water than they can take up, leading to wilting and stress, while low temperatures can slow the transpiration rate too much.
Figure 3. Effect of temperature on plant transpiration. [5]
Research also shows that an increase in photosynthetically active radiation and air velocity promote transpiration.[6]
Managing transpiration in the grow room
Because the environment strongly affects plant transpiration, growers can manipulate conditions in their greenhouse or grow room to control the transpiration rate.[7]
Irrigation management is critical. Underwatering can cause too little transpiration, resulting in wilting and nutrient deficiencies. However, providing more water than the plants need can suffocate roots, hindering the transpiration process. Make sure to provide the right amount of water at the right time and monitoring moisture levels in the growing medium.
Because light intensity influences photosynthesis, more light leads to higher transpiration rates, requiring plants to take up more water to cool themselves and support metabolic processes. When using intense lighting, make sure that adequate water is in the growing medium to support increased transpiration without causing dehydration.
Use fans, air conditioning and/or exhaust systems to keep the temperature within the desired range and ensure proper air circulation, which also helps regulate humidity and temperature to prevent hot spots and even distribution of water vapor. Good air circulation also helps plants exchange gases more efficiently and reduces the risk of pests and disease. Consider all these factors in selecting the appropriate HVAC system for your operation.
Finally, consider using different tools and practices to monitor transpiration. A simple, cost-effective option is to weigh plants before and after watering. If planted in pots, the whole pot should be weighed, while an entire section is weighed for NFT[8] systems, using either suspended or digital scales. The difference in weight indicates how much water has been lost to transpiration. Infrared thermometers or thermal imaging cameras can be used to measure leaf temperature: Cooler leaves mean higher transpiration; warmer leaves mean the transpiration rate is too low. But the easiest way to monitor transpiration is to use transpiration sensors, which measure water loss and provide real-time data on transpiration rates.
Our next blog post will discuss VPD rates, which provide growers with even more information on transpiration rates, not to mention humidity and temperature. All three parameters are interconnected, and VPD measurements are accurate indicators of them.
Emerald Harvest Team
[1] The plant vascular system is composed of the xylem, which transports water and nutrients upward from the roots to the shoots, and the phloem, which translocates sucrose and other photosynthates throughout the plant.
[2] Water potential quantifies the tendency of water molecules to flow from one area to another.
[3] Zhu, Yong, Zefeng Cheng, Kun Feng, et al. 2022. “Influencing factors for transpiration rate: A numerical simulation of an individual leaf system.” Thermal Science and Engineering Progress 27: 101110. https://doi.org/10.1016/j.tsep.2021.101110.
[4] DGmann. “Transpiration Humidity Graph.” Wikipedia, May 24, 2013. https://en.wikipedia.org/wiki/Transpiration#/media/File:Transpiration_Humidity_Graph.svg.
[5] DGmann. “Transpiration Temperature Graph.” Wikipedia, May 24,2013. https://en.wikipedia.org/wiki/Transpiration#/media/File:Transpiration_Temperature_Graph.svg
[6] Zhu, Yong, Zefeng Cheng, Kun Feng, et al. 2022. “Influencing factors for transpiration rate: A numerical simulation of an individual leaf system.” Thermal Science and Engineering Progress 27: 101110. https://doi.org/10.1016/j.tsep.2021.101110.
[7] Stanghellini, C. and W.Th.m. van Meurs. 1992. “Environmental control of greenhouse crop transpiration.” Journal of Agricultural Engineering Research 51: 297-311. https://doi.org/10.1016/0021-8634(92)80044-S.
[8] Nutrient film technique
Frank
Great article please provide more