Let There Be Light: How Wavelength Composition, Intensity, and Duration Affect Plants

For several years, a colleague and I conducted a series of greenhouse tomato experiments in order to evaluate cultivar performance, and the effects of rooting media, plant population and topping on plant growth and fruit yield.

These experiments were conducted in two greenhouses located just a few miles from each other, yet if you went from one greenhouse to the other, you would think that the experiments were entirely different due to the plants’ appearances.

The tomato plants in the glass-covered greenhouse had a dull-green leaf color and long internode stems, while the tomato plants in the fiberglass-covered greenhouse had a lustrous dark-green leaf color and short internode stems.

The differences between the plants were a direct result of the greenhouse coverings. When light passes through any substance, even the atmosphere, its intensity (energy level) decreases and there is a shift in wavelength distribution. The extent of these changes depends on the characteristics of the atmosphere and substance through which the light is transmitted.

The wavelength shift that occurs when light passes through a greenhouse glazing material is evident in two ways. One, light reflecting from the internal surfaces is shifted into longer wavelengths and trapped as heat. Secondly, the glazing material will either diffuse the light or allow it to pass through directly.

In the above experiment, I found that fiberglass filters out the long (red) wavelength radiation, producing diffused light, whereas glass filters out the shorter (blue) wavelengths and maintains the direct-light characteristic. Today, there are tables in many greenhouse books that give the transmission properties of various glazing materials.

One time, when I was investigating a newly developed hydroponic rooting system—again using tomato as the test crop—I conducted my experiments in a double-walled polyethylene-covered greenhouse.

The obtained tomato fruit yield and quality were beyond my expectations. No doubt the new hydroponic rooting system was a contributing factor, but it wasn’t the primary one. No, this high yield was mainly due to the filtering of the sunlight that passed through a thin water film that had been trapped between the two polyethylene sheets.

Read also:

• Red Light, Blue Light: Balancing LED Efficiency with Performance
• Greenhouse Glazing and its Effect on Photosynthesis
• Light Transmittance Through Greenhouse Glazing

To get a “feel” for the changes in light characteristics, one can record photosynthetically active radiation (PAR) with a PAR light meter both inside and outside a greenhouse on various days, correlating each reading with that day’s atmospheric conditions (clear and cloudless, high humidity, etc.). The PAR meter is partially affected by the spectral composition of the light striking it; however, this instrument cannot determine wavelength shift. (Nonetheless, there is little data on the spectral wavelength shifts that occur when light passes through glazing materials—something that needs to be done.)

Several years ago, I consulted with a South Carolinian greenhouse tomato grower who had kept daily records of noontime PAR readings. He was one of four growers—three located in South Carolina, one in Georgia—whose fruit yield and quality were lower than what they had obtained in previous years. My repeated visits didn’t uncover what might be the possible cause(s) until that first grower remarked that measured incoming radiation was considerably higher than what he had recorded in previous years.

That was the clue that led me to obtain weather station measurements of solar radiation; however, the only solar information being recorded was daily minutes of sunshine. After correlating minutes of sunshine versus fruit yield obtained by these four growers in previous years and the current year, a colleague and I came to the conclusion that if the monthly minutes of sunshine exceeded 10,000, then fruit yield and quality would be lower than if minutes of sunshine were less.

From this experience, I recommend installing moveable shade in the greenhouse to be pulled over the plant canopy during periods of high radiation (mainly during the noon hour). Such a result may not apply at all latitudes, however, since these observations were made during the spring months in South Carolina, whose latitude is approximately 35^oN and solar radiation intensity can be high.

These four greenhouse tomato growers’ experience of low yield and poor-quality fruit occurred during drought conditions—cloudless days and low amounts of suspended atmospheric moisture, which results in high direct radiation intensity impacting the plants. In environments where the atmosphere is moist, the reverse phenomenon exists.

For example, I have walked fields of wheat and alfalfa in Saudi Arabia and have seen the vigorous growth and high yields being obtained. This is due to high amount of suspended water vapor and particulate matter in the desert environment, which produces diffuse radiation (and is one reason the desert seems so bright to the eyes).

So, whenever something unexpected happens, I have learned from these and other experiments to not initially look down, but up to determine what the light conditions are—or were—before I make any further judgment.