While plant factories can be very efficient growing machines, the best results cannot be achieved without a sound knowledge base on which to make decisions. The first decision indoor growers must consider when starting up a plant factory is what type of artificial light they should use to grow their desired crops.
Fluorescent lamps are low-pressure mercury vapor lamps that use fluorescence to produce visible light. They require a ballast to regulate the current through the lamp, but the lower energy cost usually offsets the higher initial cost. The actual wavelengths of light produced depends on the type of phosphor used to produce the fluorescence. In the case of plants, the objective is to maximize the wavelengths in the blue and red zone of the spectrum. Because they contain mercury, many fluorescent lamps are classified as hazardous waste and must be disposed of appropriately.
Metal halide lamps, a type of high-intensity discharge lamps, differ from the fluorescent types. Mainly, the different wavelengths are produced by incorporating a suitable metal halide into the vacuum of the lamp, ensuring a specific wavelength of light will be produced. The only HID lamp to really catch on for horticultural purposes has been the high pressure sodium light (HPS). These have become the workhorses in most greenhouses where artificial lighting is used. The spectrum of an HPS lamp is distinctly different from the majority of fluorescent lamps used in horticulture. It is predominantly at the red end of the spectrum, so in addition to providing good photosynthetic potential, it also has the appropriate spectrum for flowering.
LEDs differ from fluorescent or metal halide lamps in that they are a solid state with a fixed wavelength. They have a long life and are robust and cheap to operate. However, there is a clear disadvantage to having a fixed wavelength if the plant variety requires a range of different wavelengths to perform at an acceptable level. This is less important when LEDs are being used to provide supplementary lighting in a greenhouse situation, as the solar radiation will provide the missing wavelengths, but in a plant factory (dependent solely on artificial light), providing a complete range of essential wavelengths is critical.
From an economics standpoint, blue LEDs are much more expensive to produce than red LEDs. It now appears that blue light plays an essential role not only in photosynthesis, but also in ensuring leafy green plants such as lettuce are more robust. There are two ways to approach this problem. Growers can either provide the majority of the light at the appropriate red-blue combination and then supplement this with a small quantity of white light, which covers all the missing wavelengths, or provide the plants with a full spectrum using different LEDs for each wavelength. The jury is still out on this important choice.
When I started my career in horticulture in the early 1950s, it was generally considered more than adequate to refer to light measurements in terms of the number of bright sunshine hours. This changed in the late ’50s into considerations of the measurement of light by photometric means (by foot candles or lumens). These measurements were based on the human eye’s response, and were totally inappropriate for evaluating the response of plants to different levels of light. Energy then became the appropriate measurement, with an emphasis on the amount of photosynthetically active radiation (PAR) over the wavelength range from 400-700 nm, expressed as units of energy flux or more recently as photon flux.
Even this method now has its critics, as all wavelengths are not created equal. For example, plants respond to different wavelengths in different ways. The two main photosynthetic drivers (red and blue) clearly have a major influence on photosynthesis, whereas other wavelengths can have a major influence on crop morphology and development. This was demonstrated by McCree in 1972 when he developed the concept of action spectrum for plants. This led to the development of the phytometric system, which provides a more flexible concept of light measurements for plants in which a calculated phytometric value would provide a much more accurate measurement of photosynthetic photon flux (PPF).
Light, Temperature and Carbon Dioxide Levels
The interaction of light, temperature and carbon dioxide levels is critical to ensure good crop productivity. The basic information on this was developed by P. Gaastra in the late 1950s, but it is possible this work is not particularly relevant for plant factories, where the environment can be closely controlled. Gaastra’s research involved measuring photosynthesis of individual plant leaves at three light intensities, two carbon-dioxide levels and two temperature levels. He found that at CO2 levels of 300 ppm, and temperatures of 68 and 86°F, the leaves are quickly light saturated. By increasing the CO2 concentration to 1,300 ppm and keeping the temperature at 68°F, photosynthesis increased.
Photosynthesis is limited at 300 ppm as the movement of the gas through the stomata in the leaves to the site of photosynthesis (chloroplasts) is determined by the difference in concentration between the CO2 in the outside atmosphere and that at the chloroplast. At higher light intensities, the CO2 concentration at the chloroplasts becomes zero, when the CO2 in the outside atmosphere is low (300 ppm), as the rate at which CO2 moves through the chloroplast depends on the difference in concentration in the outside air and the chloroplast.
Clearly, the higher the concentration in the outside air, the greater the photosynthesis rate. Temperature becomes a limiting factor because both the biochemical part of the photosynthesis process and the transport of the sugars away from the chloroplasts are temperature dependent. Increasing the temperature will further increase the rate of photosynthesis.
The above has been the basic philosophy for the supplementary use of carbon dioxide in greenhouse production for the past 50 years, and it has worked exceedingly well. However, there are new theories suggesting this model may not hold up when growing plants in an enclosed environment.
In a recent presentation at the 2014 International Conference on Healthcare in Brisbane, Australia, Duggan-Jones and myself showed that the Gaastra model might not apply. When measuring plant biomass as dry matter, there was always a good correlation between photosynthesis and dry matter production, although obviously more photosynthesis will result in a greater quantity of dry matter, but plant morphology may play a major role in determining what the actual end point will be.
We found the optimal temperature for dry matter accumulation was 77°F, when compared to 68 and 86°F, irrespective of light or CO2 levels. We also found there was little increase in dry matter accumulation above CO2 levels of 1,000 ppm. This was in spite of a clear-cut increase in dry matter accumulation with increased light levels.
Our results, which differ fundamentally from those of Gaastra, may be due to the fact that in our experiment, we were able to compare three light intensities with three temperature and three CO2 levels.
Gaastra only used two temperature and two CO2 levels. One possible explanation for our results is that when growing plants at a continuously high level of CO2, the stomata may not open so wide, or, alternatively, a smaller number of (or smaller-sized) stomata might develop under a high-CO2 environment.
The lack of response to higher temperatures—even at high CO2 levels—is even more unsettling, but it should be noted that increasing temperature tends to make leaves expand faster. If carbohydrates are limited, the leaves will be thinner. This certainly appeared to be the case with lettuce and cabbage seedlings.
It is starting to appear that in plant factories lit solely by artificial light, it will be necessary to establish specific lighting characteristics for specific crops. This is also likely to involve consideration of different temperature regimens and carbon dioxide levels. To date, virtually all of the commercialization in plant factories has involved leafy vegetables and yet the major greenhouse crops grown worldwide are fruit vegetables, tomatoes, sweet peppers and cucumbers. Clearly, we have a long way to go to fully replace current greenhouse technology.
This article was originally published in Practical Hydroponics & Greenhouses.