Radiation, Transpiration, Leaf Area Index, and Everything in Between
Radiation, or light, is a major factor in designing complex growing environments, as it affects all other climate parameters. This post will cover:
- The light spectrum
- Components of PPF – photosynthetically active radiation
- Greenhouse lighting types
- Radiation for cultivation in growing facilities
- Radiation intensity and evapotranspiration
- Cannabis and lighting types
The visible light spectrum (PAR – photosynthetically active radiation) is between 400 – 700 10-9 meters (nanometers).
- Light quantity or intensity refers to the total number of photons received per unit area at a given time.
Light quality refers to the relative proportion of photons received at each wavelength per unit area. Wavelengths shorter than 400 nanometers are called UV and are commonly separated into 3 areas, all of which are powerful and can cause skin diseases for humans:
- UVA – 320-400 nanometer
- UVB – 290-320 nanometer
- UVC – 200-290 nanometer
On the other side of the spectrum, wavelengths greater than 700 (nm) create heat. It is believed that UVA radiation strongly influences the creation of secondary metabolites in the plants. For example, THC and other molecules that have a role in protecting the flowers. It is common to say that UV radiation activated the plant’s defense mechanism.
As a rule of thumb, when converting watts/m2 to µmol/ m2/s, the ratio is 1:2. On an average sunny day, the energy flux could be around 1,000 watts/m2 , which roughly translates to 2,000 µmol/ m2/s.
Components of PPF – Photosynthetically Active Radiation
There are 2 types of chlorophyll in plants. Prior to extensive research in recent years, wavelengths in between the red and blue were considered less useful for the plants. Today we know that these wavelengths are crucial for various processes involved in the creation of phycoerythrin, phycocyanin, and carotenoids and more. Blue wavelength is not the highest in quantity, but its energy is very high. Blue light is important for stomatal opening and chlorophyll synthesis. Red, or far-red light, determines elongation in plants. In fact, plants exposed to blue light grow smaller in size, compared to plants that receive more red light.
Greenhouse Lighting Types
Until recently, high-pressure sodium lights (HPS) were widely used in agriculture. Yet, the spectrum it creates is rather narrow and focuses mostly around the orange and red wavelengths. Metal halide lighting is more useful and has a wider spectrum but proved to be unreliable and in need of frequent replacement. LED lighting is currently rather expensive, in some cases up to 5 times the price of an HPS light. Additionally, LED lighting requires a hefty investment in infrastructure, due to different light distribution.
Radiation for Cultivation in Growing Facilities
Daily light requirements –10-15 megajoule (MJ/m2 day). The daily amount of radiation can differ tremendously between seasons. In a semi-arid climate, during the summer, the amount of radiation can reach 30 MJ/m2/day over 14 hours of daylight. The same location during the winter will get only about 6-7 MJ/m2/day over 10 hours of daylight.
Radiation Intensity and Transpiration
Inverse Square Law
The inverse square law states that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. If a light source’s location doubles in distance, the coverage area multiplies by 4.
This simple physical law can help us understand the amount of light we would like to have for a given plant’s setting.
Leaf Area Index
Leaf area index (LAI) can be calculated by how many layers of foliage can cover 1 square meter. The values usually range from 1 to 4.5. The absorption coefficients for radiation don’t change in a ratio of 1:1 compared to the LAI. The reason for that is that in modern agriculture, plants are grown in rows, and do not get the light uniformly and equally from all directions. Also, leaves on the lower part of the plant tend to be less active. When compared to younger leaves, we see calcium accumulation on these lower, older leaves.
LAI and Transpiration
In “Transpiration of greenhouse crops: an aid to climate management” (1987), Dr. Cecilia Stanghellini calculates the transpiration of plants with LAI value of 3, in different humidity and temperature levels. Her research provides a baseline of how to calculate the amount of water released to the air, depending on the amount of radiation.
In 70% relative humidity and 20°C in the dark, plants transpire 36 grams of water per m2/ hour. What’s interesting is that as soon as there’s a very small amount of radiation (30 Wm-2), the transpiration rate more than doubles, and in the same climate conditions is about 80 grams of water per m2/ hour. In conditions of 250 Wm-2, the transpiration rate is 158 grams of water per m2/ hour. For 1,000 Wm-2, it’s 288 grams of water per m2/ hour.
Typically, above a certain temperature plants will transpire less, due to several reasons. Mostly due to the ability of the plant to close its stomata to avoid water loss. A state in which a plant transpires heavily doesn’t necessarily mean that it’s the most active. It’s usually only true for leafy greens. Some fruit bearing plants can lose control over their stomata and lose significant amounts of water uncontrollably. Some evidence shows that irrigating with saline water can reduce nighttime transpiration by up to 30%.
Cannabis and Radiation Types
HPS light (high-pressure sodium) made the plant grow taller because of the amount of red light. The other 2 lighting types were different versions of LED with higher levels of blue light, but with a combination of red light as well. The NS1 is the LED type with a better combination of blue and red light, and therefore had the highest amount of cannabinoids and overall yields. Additionally, we know that a combination of ~30°C and radiation of 1,000 – 1,500 µmol/m2/s will result in optimal photosynthesis in cannabis. (Chandra et al., 2008)