This is a coloured scanning electron micrograph (SEM) of the lower leaf surface of a Garden Rose Rosa sp., showing an open stoma. A stoma is a tiny pore bordered by two kidney-shaped guard cells. Opening the pore allows gases to be enter and exit leaf tissues, which is essential for photosynthesis. They pore closes at night or during dry periods to prevent water loss.
Plants are able to regulate the process of transpiration and cooling by using specialised plant organs called stomata. The stomata are specialised cells in the leaves which can open or close, limiting the amount of water vapour that can evaporate. The higher the temperature rises, the more the stomata will evaporate when they are open. It is difficult to measure the aperture of the stomata, so we can use the VPD to estimate this. As the stomata open wider, more gases can move into and out of the leaves.
Most of the water in the atmosphere is present in the form of water vapour. Water vapour is invisible, but we can notice its presence through how comfortable we feel (higher humidity makes us feel sticky and less comfortable). Visibility is also affected by how much water vapour there is in the air. Clouds are visible because the water vapour they contain has cooled off to the point where the water molecules begin to condense and form tiny droplets of water or even ice crystals in the air. We can see these as clouds.
Overall, a comparison with the rev-counter of a car can be made. As the engine speed increases, the needle of the rev-counter goes higher and enters the red zone. This will not damage the engine immediately, but it will if the needle stays in the red zone for too long. For most plants, the VPD should be between 0.45 and 1.25 expressed in kilo Pascal (kPa the unit for pressure) with an optimum of around 0.85kPa. The VPD follows more or less the same pattern as the ambient irradiance levels; in the morning it rises, as the sun start shining, reaching a peak around noon and then gradually decreases again. To calculate the VPD, the air temperature, plant temperature and relative humidity must first be known.
Not surprisingly, a great deal of research work has been done into proper temperature strategies for efficient greenhouse production. However, the optimum temperature for a plant depends on a range of factors. A plant’s reaction to the atmospheric temperature around it depends on which stage of development that plant is in. Plants have a kind of biological clock which determines their sensitivity to temperature.
Vapour Pressure Deficit (VPD) can be compared with a rev-counter in a car. As the engine speed increases, the needle on the rev-counter turns and enters the red zone. This will not damage the motor immediately, but it will if the car continues to drive like this for a prolonged period. The same applies for plants: when the VPD is too high for a longer period of time, the plant is not able to recover the subsequent night and irreversible plant damage can occur (burned leaves or petals).
The optimum day and night temperature combinations were investigated in the world’s first air-conditioned greenhouse, a phytotron, at the California Institute of Technology in 1949. The experiments demonstrated that tomato plants grew taller under a combination of a high temperature during the light period and a lower temperature during the dark period than when the temperature was kept constant. This ability of the plants to ‘distinguish’ between temperature variations during the day and night is called thermoperiodism, and it has an effect on flowering, fruiting and growth.
Stem elongation can also be caused by a short temperature drop (of about two hours) during the 24-hour daily growth cycle, generally at or just before the first daylight, but during the dark period. Responsiveness to temperature changes seems strongest during the first hours of the light period in long-day plants, short-day plants and day-neutral plants. Thus a temperature drop during the last two hours of the night will affect plant height. This is usually easy to accomplish in greenhouses during the autumn of cool climate zones because of the naturally low night temperature.
The amount of sugar that is transported to growing tissue, where the energy is needed to fuel higher levels of respiration, can be restricted when night temperatures are higher, and thus growth can also be restricted. It was also found that stem elongation can occur with a combination of high day-time temperatures and low nocturnal temperatures. A low nocturnal temperature improves the water balance in the plant which is the main reason for increased stem elongation. So temperature can be used as a tool for regulating plant height, but low nocturnal temperatures can also save energy. The term thermomorphogenesis is used to describe the thermoperiodic effects on plant morphology.
Temperature is a key factor in plant growth and development.The optimum temperature for a plant depends on a range of factors.
Humidity measures the amount of water vapour in the air. A reduced humidity percentage in the air causes water loss from plants’ tissues. But a high humidity level can cause the development of microbial and fungal pathogens.
You’ll find a variety of models available, which allow for increased customization.
The ideal room and humidity temperatures depend on the plants’ growth phase. These factors determine the level of your plants’ photosynthesis and other biochemical processes.
The ideal humidity levels for your grow space depend on the optimum temperatures needed for your plants to thrive and the growth stage they’re in.
To achieve top-quality crops, you must control and maintain optimum temperature and humidity levels in your grow room. If the environment is too cold or too hot, your crops will deteriorate.
- Vegetative phase, the ideal temperature for growth is 24 degrees Celsius and the best humidity level is 60%.
- Flowering stage, the ideal temperature/ humidity ratio is 28 degrees C/ 50%.
- When temperature levels exceed 30 degrees Celsius, plants’ enzymes don’t perform at optimum rates and the photosynthesis process stops.
To measure the grow room temperature and humidity levels, you should use a hygrometer and thermometer. The digital options are the best as they provide current readings.
To prevent your grow space from getting too hot, you can use extraction fans. Also, intake fans will help you circulate air through the leaves of your plants and remove any stale air.
If the humidity levels are not right, your plants’ growth will slow down. To identify and control the temperature in your grow room, use top-quality thermometers. You’ll find a variety of tools available on the market such as wireless temperature and humidity sensors, digital display thermometers, thermal hygrometers, and thermometer-hygrometers with external probes.
The ideal room humidity and temperatures depend on the plants' growth phase. Learn how to control temperature and humidity in your grow room!