As climate change threatens the concept of traditional growing seasons and as communities push for more local agriculture, the idea of being able to de-seasonalize production offers increased productivity and opportunities for greenhouse growers. To fully grasp how Sollum Technologies’ solution can effectively support this transition, we must better understand how natural light changes seasonally and why this variation affects plant growth. At the end of this discussion, it will become clear that giving producers access to a fully programmable and responsive lighting solution is key to multi-season and year-round agriculture.
Greenhouse products account for approximately half of total Canadian vegetable exports and their growing seasons are somewhat more flexible than field agriculture due to the ability to control the growing environment. Tomatoes, peppers, and cucumbers represent the majority of crops grown in Canadian greenhouses – 98% of total annual greenhouse production by weight[ii]. Greenhouse peppers are typically planted between the end of November and early February and harvested until December 3. For tomatoes, several scenarios exist and these are illustrated in the following graphic:
Compared to peppers and tomatoes, cucumber growing schedules are much more variable: producers may choose to plant in winter, spring, or fall, and two to four times per year with each crop-producing fruit for 60 to 150 days.
Conventionally, greenhouse crop scheduling has strongly depended on the availability of natural light to drive photosynthesis and optimize plant metabolism. Without supplemental lighting, greenhouse growers align crop schedules with periods of adequate natural light levels, which vary greatly through the day and year according to the time of year, solar elevation angle, weather conditions, etc. These variations can affect crops in multiple ways, such as by causing heat stress and wilting under excessive light, flower and fruit abortion with insufficient light and morphological responses to changes in spectrum. Understanding the variability of light quantity (i.e., photoperiod and intensity) and quality (i.e., spectrum) is the first step to creating the optimum light conditions for a greenhouse crop; allows growers to best manage the light they receive while implementing dynamic lighting allows growers to create the light conditions they want.
Before diving into the next two sections, we would like to point out that some of this content is covered in a previous Sollum white paper, Natural Lightv, which provides a detailed background on the science of sunlight, light spectra, intensity, and differences between types of light fixtures. Here, we expand on topics specifically related to diurnal and seasonal natural light variation, but we do encourage you to check out all our white papers to gain more knowledge on these and other topics.
Natural light conditions change over the course of the year both with respect to spectrum, intensity and photoperiod. Light intensity is measured as the amount of light received over a given surface and time frame and changes throughout the year due to the changing angle of incoming solar radiation, or the solar incidence angle. Assuming a near constant solar output during the year, changes in the solar incidence angle impact the area upon which solar radiation lands, causing an increase or decrease in light intensity experienced at a given point. The maximum solar incidence angle is 90°, at which point the Sun is directly over a target area which results in a lower incident area, thus a higher intensity (Fig. 2). At a lower angle of incidence, solar radiation lands on a larger area, resulting in more diffuse light conditions and a lower intensity on the surface.
During the summer months, the Northern Hemisphere experiences a higher solar incidence angle due to the Earth’s tilted axis. This decreases the surface area over which the light lands, effectively increasing the intensity and resulting in warm weather conditions. The opposite occurs during the winter months as the Southern hemisphere leans towards the Sun and the Northern Hemisphere leans away from it, leading to lower intensities and cooler conditions in the Northern Hemisphere (Fig. 3).
Seasonal trends in incident light intensity can be observed using solargraphy, which uses a pinhole camera and long exposures to capture the journey of the Sun over extended periods of time. Figure 4, for example, is a solar graph developed between August 16, 2014, and December 21, 2014vii in Georgian Bay, Ontario. The figure clearly demonstrates changes in the Sun’s movement over time, with the higher, longer arcs corresponding to August solar conditions while the lower, shorter arcs correspond to December conditions. As such, we see how the Sun is higher in the sky and for longer periods during summer months, then decreases until December 21 (i.e., winter solstice). The solar graph thus demonstrates how both the solar incidence angle and daylength (i.e., photoperiod) change throughout the year, the latter being critical for attaining the optimum light sum (i.e., daily light integral) for plant growth and managing the flowering response in long- day and short-day plants.
Fig. 4: 127-day solargraph (2014.08.16 – 2014.12.21), Georgian Bay, Ontario, CA. Image credit: Bret Culp.
Similarly, the solar incidence angle changes throughout the day and is lowest at sunrise, increases until midday then decreases until sunset. As a result, light intensity is typically highest around midday and lowest at sunrise and sunset. Combined with the previously discussed seasonal variations, the result is that the highest light intensities in North America are achieved midday during the summer months, while the lowest overall light intensities occur at sunrise/sunset in winter months.
To summarize, light intensity is impacted by seasonal changes in the solar incidence angle and diurnal changes in the solar elevation angle, both of which impact light’s trajectory to the surface and are further impacted by atmospheric conditions
The solar elevation angle has further implications on light quality as it influences the preferential diffusion of different wavelengths. Our earlier white paper, Natural Light, delved into the phenomenon of Rayleigh scattering through which light is scattered by atmospheric molecules. Strongly dependent on wavelength, Rayleigh scattering more effectively deflects short wavelengths towards the Earth at high solar elevations, giving the sky its blue appearance during the day. This phenomenon also explains the dominance of red during sunset and sunrise as the lower solar elevation angle results in a longer path which, combined with Rayleigh scattering, results in more deflection of blue wavelengths away from the surface.
At the University of Helsinki, Kotilainen et al. (2020) studied the diurnal and seasonal variations in light spectrum, and particularly the photosynthetically active radiation (PAR)ix. The study focused on changes in the blue:green (B:G), blue:red (B:R) and red:far red (R:FR) photon ratios due to variation in solar elevation angle, atmospheric water vapor content, and ozone column thickness.
According to the study results, the R:Fr ratio showed the greatest seasonal variation. While the lowest solar elevation angle consistently produced the lowest ratio, mid-range angles (0-20°) produced the highest R:Fr ratios between June and August, and high angles (>20°) produced the greatest ratios between September and June (Fig. 7). Further, B:G ratio was mostly consistent throughout the year, peaking between March and May. Throughout the day, the B:G ratio was highest at low solar elevation angles. The B:R ratio was also highest at low solar elevation angles but there was no significant difference between seasons.
While these results suggest that blue light levels are highest at low angles of solar elevation, this initially appears to contrast the discussion of Rayleigh scattering which states that lower solar elevation angles result in red dominance. However, herein lies another nuance of spectrum and atmospheric absorption. In their study, Kotilainen et al. (2020) reported the highest B:R and B:G at solar elevation angles of >0°, therefore below the horizon. At this point, the Sun is low enough that it is not visible but high enough to produce low light levels. Referred to as “blue hour”, this occurs before sunrise and after sunset, during which blue wavelengths dominate due to the ozone-mediated absorption of red and orange light (i.e., Chappuis absorption).
It is important to note that photon ratios are also affected by atmospheric moisture content, which also varies according to the season. The variation in photon ratios thus cannot be solely attributed to changes in the solar elevation angle. This is especially true for the R:Fr. This being the case, specific trends will change based on geography and climate.
Changes in light quality, such as changes in photon ratios, matter due to the wavelength-specific sensitivity of plant photoreceptors including chlorophyll, phytochromes and cryptochromes. Photoreceptor activity has significant impacts on plant growth, gene expression, plant-plant interactions, metabolism, and morphology. For example, the ratio of R:Fr has a direct impact on shoot elongation, leaf characteristics and flowering response of certain species. The ratio of red to blue has been extensively studied with respect to pigmentation, shoot growth and leaf characteristics, while the ratio of blue to green is being increasingly studied.
i. Pest Management Program Agriculture and Agrifood Canada. (2017). Crop Profile for Greenhouse Pepper in Canada, 2017 (4th ed.). Ottawa, Ontario: Agriculture and Agri-Food Canada. Retrieved from https://publications.gc.ca/site/eng/9.883980/publication.html
ii. LaPlante, G., Andrekovic, S., Young, R. G., Kelly, J. M., Bennett, N., Currie, E. J., Hanner, R. H. (2021). Canadian Greenhouse Operations and Their Potential to Enhance Domestic Food Security. Agronomy, 11(1229). https://doi.org/10.3390
iii. Pest Management Program Agriculture and Agrifood Canada. (2021). Crop Profile for Greenhouse Tomato in Canada, 2020 (5th ed.). Ottawa, Ontario: Agriculture and Agri-Food Canada. Retrieved from https://publications.gc.ca/site/eng/9.883980/publication.html
iv. Pest Management Program Agriculture and Agrifood Canada. (2021). Crop Profile for Greenhouse Cucumber in Canada, 2020 (4th ed.). Ottawa, Ontario: Agriculture and Agri-Food Canada. Retrieved from https://publications.gc.ca/site/eng/9.896773/publication.html
v. Dupras, G. (2020). White Paper: Natural Light. Montreal, Québec: Sollum Technologies. Retrieved from https://www.sollumtechnologies.com/fr/notre-solution
vi. Journey North: Reasons for Seasons–Exploring the Astronomy of Spring. (2019). Retrieved October 31, 2021, from https://journeynorth.org/tm/mclass/ReasonsBack.html
vii. Culp, B. (2014, December 29). Solargraphy (Exposure of Time, Space and Weather). Retrieved October 31, 2021, from https://www.bretculp.com/bret-culp-photography-blog/2014/12/solargraphy-128-day-exposure-of-time-space-and-weather/
viii. Sustainable By Design: Altitude Angle. (n.d.). Retrieved October 31, 2021, from https://susdesign.com/popups/sunangle/altitude.php
ix. Kotilainen, T., Aphalo, P.J., Brelsford, C.C., Book, H., Devraj, S., Heikkila, A., Hernandez, R., Kylling, A., Lindfors, A.V., Robson, T.M. 2020. Patterns in the spectral composition of sunlight and biologically meaningful photon ratios as affected by atmospheric conditions. Agricultural and Forest Meteorology. 291. https://doi.org/10.1016/j.agrformet.2020.108041
x. Fosbury, R., Koch, G., Koch, J. 2011. Ozone: twilit skies and (exo-)planet transits. The Messenger. 143. 27-31.