Should I use a PAR meter for greenhouse climate control?

My greenhouse climate and irrigation controllers are years old and use a lux sensor for light measurement. I was recently told that I should be using a PAR (Photosynthetically Active Radiation) meter. This is apparently much better than a lux meter. Is this correct?

Answer by RICK DONNAN

Answer
The simple answer is yes, a PAR meter is more appropriate than a lux meter for climate/irrigation control. However, they are also more expensive, so it becomes a matter of choice as to whether it is beneficial to change. It is difficult to put a value on changing to a PAR meter, because there are many other factors involved.
For example, if your climate and irrigation management is very good, then the change would be worthwhile. However, if your control systems and their management are sub-optimal, then the money to change light meters might be better spent elsewhere for better benefit.

Lux and PAR spectra
Figure 1 shows the spectrum each type of meter is measuring. The unit on the horizontal axis is the nanometre (= 10-8 m = 1/100,000,000 m). The unit given by a Lux meter is lux or klux, and that by a PAR meter is Watts/m2 (W/m2).

 

Lux and PAR spectra

Figure 1. Lux and PAR spectra. (Image illumitex.com)

The human eye detects a narrow band of wavelengths based around a peak of 555 nm in the yellow band. This is measured by a Lux meter, the most common version of which is a light meter for cameras. The spectrum that plants respond to is quite different, other than both are within the visible range, which is between 400 and 700 nm. Plant response is much wider and has shallower peaks in the blue and red bands.

There is no simple relationship that enables lux to be converted to PAR as it is influenced by other factors, especially the light intensity.

Solar radiation spectrum
This is a good opportunity to discuss the influence of solar radiation upon plants, especially in greenhouses.

Figure 2 shows the spectra of solar radiation, both as it arrives at our outer atmosphere and also as it arrives at Earth’s surface. The difference is that gasses in the atmosphere absorb some of the radiation in specific bands. For now I’ll be referring to the radiation that we need to cope with on Earth.

Solar radiation spectra

Figure 2. Solar radiation spectra in space and on earth.
(Image Geosciencebigpicture.com)

There are three major zones of solar radiation—ultraviolet (UV 100–400 nm), visible light (especially PAR 400–700 nm), and infra-red (IR – heat energy 700–3000 nm). The bulk of radiation is visible and IR, of which visible contributes about 45% and IR about 55%, but these percentages can change slightly, influenced by factors such as cloud cover.

Photosynthesis
Plant leaves use PAR light for photosynthesis—in simplest terms: to produce sugars for plant growth. Photosynthesis increases with increasing radiation intensity. It also increases with increasing plant temperature, but plateaus at about 20 degrees C beyond which there is no further increase.

Increased CO2 levels can give some increase in photosynthesis, but whether this is worthwhile is influenced by other factors.

Respiration
Whereas photosynthesis takes in energy from the sun to produce sugars, respiration uses some of that energy to power the processes within the plant. The amount of growth of the plant depends upon its net energy input, that is the overall net photosynthesis, which is the total photosynthesis minus the total respiration.

A major influence on the amount of respiration is plant temperature. However, unlike photosynthesis, respiration continues to increase with rising temperature up to a limit of about 40 degrees C. The result of this is that as plant temperature rises, photosynthesis plateaus at 20 degrees C, but respiration continues to rise, hence net photosynthesis falls.

Managing radiation
High solar radiation levels plus high air temperatures in summer are common in many countries, especially Australia. The challenge is that while high PAR input could increase growth, because it is coupled with high levels of IR, net photosynthesis is reduced. Also, plants suffer from direct damage due to induced high plant temperatures.

Research is ongoing into splitting off the IR band from the PAR band and using this heat outside the greenhouse, rather than directly heating the greenhouse and the crop. However, this is not yet commercial.

Therefore, the current practice is to screen out enough of the IR radiation for plant comfort and to maximise net photosynthesis. This usually aims for a 24-hour plant temperature of around 20 degrees C. In hot summer conditions this requires the use of screening to reduce the level of incoming radiation. The major practical options are moveable internal screens or external whitewash. These were discussed in PH&G Issue 161, November 2015.

Ozone layer
If you refer to Figure 2, you can see where gases in the atmosphere absorb some of the incoming solar radiation. This is most important for the UV band, a large proportion of which is absorbed by the ozone (O3) layer, which is a layer in the stratosphere about 15 km above the Earth. It absorbs virtually all the dangerous UVC (100–280 nm), most of the UVB (280–315 nm) and some of the least harmful UVA (315–400 nm). If the short wave length radiation was not blocked by the ozone layer, life on Earth would be in great danger.

On an annual cycle, a hole appears in the ozone layer above the Antarctic. In the 1970s, scientists discovered that the hole was getting larger due to the destruction of some of the ozone. The culprit was gases containing Fluorine, Bromine or Chlorine, especially ChloroFluoroCarbons (CFCs—used in refrigerators, air conditioners, etc.), and also Methyl Bromide (MeBr—used as a soil sterilant).

Most countries agreed to relatively quickly phase out the use of these products, and the UN two years ago announced that the hole appeared to have stabilised and was expected to be back to the previous size by mid-century. This has two major implications: that international cooperative action can work; and that the time scale of 60 to 80 years turnaround is very long.

Unfortunately, climate change science is much more complex, and compared to the 1970s the debate has become highly politicised. Both climate change ‘sceptics’ and most ‘believers’ are working from ideology and largely ignoring good science, other than to sometimes selectively pick and often misinterpret isolated aspects that suit their belief. Ω

PH&G May 2016 / Issue 167


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