Agricultural Lighting Parameters

In a previous article, the role of LED illumination in green-house environments for growth of crops were discussed. The ability to colour blend various LED sources to match the Photosynthetic absorption curve of different crops and also change intensity levels for each wavelength for maximum productivity, are big advantages that LED lighting can provide for horticultural lighting applications.

In this article, several important parameters that need to be measured to encourage the growth and productivity of crops are discussed. Measuring these parameters and monitoring them on a day to day basis, is the key to promoting highly efficient plant growth. The role of monitoring equipment and particularly the Spectrometer developed by Allied Scientific Pro (the Lighting Passport) in performing this task is examined.

Before examining different monitoring parameters for agricultural lighting, the “Action Spectrum” needs to be described. It is important not to confuse the Action Spectrum with Absorption Spectrum of a plant as there is a significant difference between the two.

Plants absorb radiation mostly in the 400-700 nm visible range and convert CO2 uptake and water into oxygen and glucose. The amount of absorption in each wavelength depends on the cellular structure of the plant and may differ from species to species somewhat, however it stays mostly in the visible range. Action Spectrum describes the wavelengths that are most important to photosynthesis and drive its processes. Figure 1, shows a typical plot of absorption and action spectra and the comparison between them.

Figure 1: Comparison of action spectrum and absorption spectrum

As observed in the figure, the plants have high absorption in the red and blue wavelengths and smaller absorption in green wavelengths. This explains the green colour of most plants since a high percentage of the green wavelegth in sunlight is reflected off the plants. There is a similarity in shape between the action spectrum and the absorption spectrum. This Action Spectrum concept was been pioneered by KJ Mcree in the 70’s who studied the Action Spectrum or Photosynthetic Active Radiation (PAR) for a variety of plants.

Another parameter of importance is the ratio Oxygen molecules emitted by plants, P, to number of incident Photons Ia. This defines the quantum yield of photosynthesis &Phi.

In 1922, Otto Warberg and E.Negelein (Reference) examined this process and came up with the ratio of 4 to 1, implying that a minimum of 4 photons are needed to release one molecule of oxygen. This was the accepted theory for almost 20 years. Later experimentation with improved and different techniques by Emerson and Daniels showed this ratio to be 8:1. The primary reason this ratio differed from Warberg's hypothesis was the incorrect assumption that ΔO2=-ΔCO2 or in other words absorption of carbon dioxide is equal to emission of oxygen molecules. Other researchers have separately measured the two parameters which produced more accurate results. Subsequent experiments focusing on radioactive CO2 intake confirmed the 8:1 ratio.

Figure 2: Photosynthetic Quantum yield vs. Wavelength

The other important parameters for horticultural lighting are: Spectrum, PPFD, YPFD, DLI, Intensity in Lux, Red/Blue, Red/Far Red. A measuring instrument which can quantify each of these parameters is very useful for horticultural lighting applications and green-house crop growers. However, one has to understand the usefulness of measuring these parameters first.

Spectrum: For artificial lighting it is important to know the spectrum. While using a mixture of LED’s, one can choose a combination of wavelengths which are most important to the photosynthetic process and drive it more efficiently. For a broad-band light such as High Pressure Sodium (HPS), one needs to know the amount of Photosynthetic Active Radiation (PAR) in the spectrum of the lamp. Choosing the correct wavelengths combinations with LED’s, it is possible to produce healthier plants with thicker leaves and increased branching and flowering.

PPFD: Photosynthetic Photon Flux Density takes into account the fact that red light is twice as effective as blue light per incident watt. This is due to the fact that the number of photons per unit of energy is proportional to wavelength. Therefore larger wavelengths such as red, carry more photons as compared to blue wavelengths which carry less. For this reason, in horticulture it is common practice to use PPFD in units of mol/m2/s (1 mol=6.023x1023 photons) instead of energy units. Since the number of photons for each wavelength is more important that the actual energy of the photon, PPFD is a more suitable unit to measure the light intensity in quantum units when promoting photosynthesis. This unit conversion assumes that each photon of light equally contributes to the photosynthesis process and does not take into account the quantum yield.

Figure 3: PPFD spectrum of a Fluorescent lamp superimposed on McCree Reference Action Spectrum.
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Figure 4: YPFD plot superimposed on McCree reference action spectrum.

DLI: Another important parameter is Daily Light Integral which is defined as the total number of photons impinging per square meter in one day. DLI is measured in units of mol/m2/d and each plant has a specific requirement of DLI for its growth. Values ranging between 6-18 mol/m2/d are common depending on the particular plant. There is a relationship between PPFD and DLI which is given by:

DLI=PPFD x light hours per day x (3600/1000,000).

One can see from this formula that there is a trade-off between PPFD and number of light hours required to achieve a certain DLI value. If there is a certain amount of natural lighting available for a green-house, it has to be subtracted from the original DLI value for proper artificial lighting fixture calculations. Taking into account the DLI, PPFD and number of light hours per day, one calculate the total number of fixtures required in a green-house to illuminate the crops

Light intensity in Lux or Foot-candles: In low light levels the process of respiration is dominant and plants consume oxygen. The process of respiration is independent of light intensity and hence the plot of Reaction Rate of Respiration vs. Light intensity is flat. As light intensity increases from darkness and photosynthesis begins, there is more production of oxygen and at a particular light intensity level, the amount of oxygen consumed by respiration becomes equal to the amount of oxygen produced by photo-synthesis. This is called the compensation point and past this point where the amount of oxygen produced, exceeds the amount of oxygen consumed, the plant can add to its reserves, grow and reproduce. Not all plants have the same compensation point and some reach this point at higher intensities than others. Typically crop plants such as soybean and corn, reach the compensation point at higher intensities. Figure 5 below shows the compensation point concept.

Figure 5: Compensation point, respiration and photosynthesis.

R/FR and R/B: Several studies by horticulture scientists have examined the ratio of R/FR and R/B and its effect on the growth and productivity of plants. For example studies have shown that low blue light from warm white LEDs cause increased stem elongation and leaf expansion. On the other hand, high blue light from cool white LEDs has resulted in more compact plants. In general the presence of blue light has significant effects on morphology of plants aside from Photo-synthesis. The R/FR stimulates physiological processes such as flowering, setting winter buds and vegetative growth.

Unique meter that can measure many parameters: In the past Botanists and greenhouse growers had to use 4 different meters to measure a) the spectrum of light shining on their crops, b) the amount of photosynthetic Active Spectrum c) the intensity in Lux or Foot-candles and d) R/FR ratios. This process has been very tedious and the data has to be transferred separately to desktop computers to perform the analysis.

Allied Scientific Pro has developed the first smart phone Spectrometer that communicates via bluetooth to a smart device and along with the new APP, “SGAL” can measure all these parameters at once.

This remarkable device has diaries where one can store the monitored measured values along with a picture of the plant being monitored on any given day, eventually generating a record illlustrating the plants growth.

There are several available PAR Action Reference spectra including the McCree Action Spectrum which is most widely used. It is also possible to compare several measured spectra in PPFD or YPFD units.

This meter is an ideal tool for green-house growers and horticulturalists who want to monitor their plant growth, come up with excellent light blending recipes and keep records of the plant growth. Figure 6, shows the SGAL software used with lighting passport and several measurements of different parameters that it is capable of doing. This unique meter is the first smart-phone Spectrometer in the world and the SGAL software is very user-friendly.

Figure 6:  SGAL APP can measure many parameters all at once, reducing the processing time and the need for several different pieces of equipment.
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