A Preliminary Study on the Theory of Artificial Lighting in Facility Horticulture Production

About the author: Li Tao (1985-), male, from Shangluo, Shaanxi Province, PhD, research direction: photobiology of protected horticulture.

Predictive models and theoretical analyses show that the current significant increase in crop yield can only be achieved by improving photosynthetic capacity and efficiency, because increasing crop yield by increasing water and fertilizer supply has reached its limit [1], especially in the field of facility horticulture. For many years, practitioners of facility horticulture have focused their main efforts on crop water and fertilizer management, cultivation model innovation and greenhouse structure improvement, etc., to optimize the crop growth environment through these means, thereby indirectly improving crop photosynthetic capacity and thus increasing crop yield. Light, as the most critical environmental factor affecting plant growth, regulates plant growth and development by affecting plant photosynthesis and photomorphogenesis. For a long time, the growth and development of facility horticulture crops have been constrained by factors such as technology and cost. The cultivation of crops mainly relies on solar radiation. Due to factors such as different climate zones, geographical locations, or seasonal changes, crops inevitably face weak light environments. Prolonged weak light can lead to weak plant vegetative bodies, severe flower and fruit drop, slow plant growth and development, reduced yield, and low quality [2-4]. With the widespread application of modern facility horticulture technology worldwide, the impact of facility light environment on the production performance of horticultural crops has received increasing attention, and artificial lighting has become one of the key technologies in facility horticulture [5]. In order to enable the artificial lighting technology to be understood and mastered more quickly and correctly by relevant practitioners in the field of facility horticulture in China, this article provides a theoretical explanation of the photosynthetic characteristics of facility plants, the characteristics of facility light environment, the impact of light on crop production performance, China's daily solar radiation, and the ideas of artificial lighting, in order to provide a useful reference for the application of artificial lighting in facility horticulture.

Analysis of photosynthetic mechanisms in greenhouse plants

Photosynthesis is the process by which plants use light energy to assimilate CO2 and water to produce organic matter and release oxygen. Photosynthesis is not only the most important life activity in plants, but also the most important chemical reaction process on Earth. Almost all organic matter on Earth comes directly or indirectly from photosynthesis. The substances synthesized through photosynthesis are not only the fundamental source of energy for plants to synthesize their structural substances and maintain their life activities, but also the fundamental source of structural and energy substances for other living organisms [6-7].
Photosynthesis has long been a focus of attention and a hot topic in agronomy and biology research. In the field of protected horticulture, people use various artificial means to create optimal environments for crops in order to improve photosynthetic performance and thus increase yield. Many factors influence crop photosynthesis, with external factors mainly including light, CO2, temperature, humidity, mineral elements, and water. Currently, factors such as water, fertilizer, temperature, humidity, and CO2 in protected environments are largely adjustable and controllable. Under these conditions, crop photosynthesis depends entirely on the influence of the light environment. Therefore, improving the light environment to increase yield has become one of the key methods that cannot be ignored in protected horticulture.

Typically, when discussing photosynthesis, people first consider the photosynthetic rate of a single leaf, while the photosynthetic rate of the plant canopy is often overlooked. The photosynthetic rate of a single leaf only describes the adaptation and response characteristics of leaf photosynthesis to the environment under specific conditions, and does not indicate the photosynthetic capacity of the crop canopy. In other words, a high photosynthetic rate of a single leaf does not necessarily mean a strong canopy photosynthetic capacity, because canopy photosynthetic capacity is affected not only by the photosynthetic capacity of single leaves, but also by factors such as canopy light interception capacity and canopy light distribution. The photosynthetic rate of the plant canopy directly affects crop biomass and yield. Therefore, elucidating the mechanisms of single-leaf and canopy photosynthesis is crucial for theoretical research on artificial lighting.

The rate of photosynthesis is the amount of CO2 assimilated per unit photon per square meter of leaf per second. The light utilized by plants for photosynthesis is called photosynthetic active radiation (PAR, µmol/(m²·s)), which accounts for about 50% of total solar radiation. As shown in Figure 1, the light response curve of plant leaves for photosynthesis has several important nodes. When the PAR is 0 µmol/(m²·s) (i.e., under dark conditions, A), plants only perform respiration, that is, consuming organic matter and releasing CO2. When the light intensity increases to a certain point, the node where the amount of CO2 assimilated by photosynthesis equals the amount of CO2 released by respiration is the light compensation point (B), and the light intensity at this point is the light compensation light intensity. When the light intensity is higher than the light compensation light intensity, the amount of CO2 assimilated by photosynthesis is greater than the amount of CO2 released by respiration, and the rate of photosynthesis increases with increasing light intensity. At this stage, the photosynthetic rate is linearly related to light intensity (C), and its slope represents the light energy utilization efficiency of photosynthesis. The light energy utilization rate is highest at this stage in the entire photosynthetic light response curve. Therefore, in practical applications of artificial supplemental lighting, a suitable supplemental light intensity should be sought within this stage. When the light intensity increases to a certain level, the increase in leaf photosynthetic rate slows down until it stabilizes, meaning photosynthesis reaches its maximum value. This point is called the light saturation point (D), and the light intensity that causes the photosynthetic saturation point is called the saturation light intensity. For the canopy, the photosynthetic rate continuously increases with increasing light intensity, and the photosynthetic saturation point is not easily reached (solid line in Figure 1). This phenomenon is mainly due to the uneven light distribution within the crop canopy. Under high light intensity, although the top leaves of the canopy have reached the light saturation point, the leaves in the middle and lower parts of the canopy are still in a weak light environment. Therefore, in practical applications of artificial supplemental lighting, it is necessary to comprehensively consider the light intensity of the supplemental lighting fixtures and their installation location (such as combining top supplemental lighting with canopy supplemental lighting) based on the photosynthetic characteristics of individual crop leaves and the canopy to achieve the best supplemental lighting effect.

The light intensity under the facility conditions is significantly lower than the light intensity outside. Considering economic cost factors, more than 95% of greenhouses in China use plastic film as the covering material. The light transmittance of plastic greenhouses is as high as about 70% and as low as less than 50%[3]. Taking the Netherlands as an example, in the multi-span greenhouses covered with glass materials with a light transmittance of more than 90%, the light transmittance is usually around 70%. There are many factors that lead to low light transmittance in greenhouses, such as the absorption and reflection of light by the covering material itself, the shading of the greenhouse frame and equipment, external dust deposition, and internal condensation. Therefore, for most horticultural crops with strong light requirements, such as fruits, vegetables and cut flowers, the existing greenhouse light transmittance basically cannot meet the optimal growth requirements of crops in the greenhouse, especially in winter and spring and rainy weather, natural light is difficult to meet the normal growth and development of crops.
Furthermore, the light intensity distribution under greenhouse conditions is highly uneven. Affected by the dynamic changes in the greenhouse frame, crop canopy structure, and solar altitude angle, the crop canopy exhibits numerous and continuously varying light spots. Due to the influence of photosynthetic induction, these continuous dynamic changes in light limit the increase in plant photosynthetic rate [8-9]. Li et al. [10-11] found that dynamic changes in the light environment under greenhouse conditions led to a decrease of approximately 8% in the light energy utilization rate of potted anthuriums during their growing season. Within the crop canopy, the unevenness of light distribution is even more pronounced. [12] conducted a systematic measurement of the light environment of greenhouse tomato canopy. Six rows of crops were selected in the greenhouse, each row 5 m long. Within this range, the light intensity of 120 points was measured from 50 cm down from the top of the canopy (i.e., the middle and upper part of the canopy). The light intensity distribution on the horizontal plane of the canopy is shown in Figure 2. As shown in the figure, due to the mutual shading effect of the canopy structure, the light distribution in the horizontal direction is very uneven. In the vertical direction of the canopy, the light intensity was measured every 25 cm down from the top of the canopy, and the leaf area at the corresponding height of the canopy was measured at the same time, resulting in Figure 3. As shown in the figure, in the vertical direction, the light intensity received by the canopy leaves decreased exponentially with the increase of the leaf area index (i.e., the plant height decreased). Given that the photosynthetic rate of plant leaves has a nonlinear relationship with light intensity (Figure 1), the uneven distribution of light in the crop canopy reduces the light energy utilization efficiency. In summary, in the practical application of artificial supplemental lighting, in order to maximize the light energy utilization rate, it is necessary to comprehensively consider the supplemental light intensity and light distribution characteristics.

Overview of Daily Solar Radiation in China

Sun Youping et al. [14] used horizontal solar radiation data observed by 45 meteorological observation stations in China from 1973 to 2002 to establish a solar radiation map of China. This map records the monthly average solar radiation received by each region in each month. It can be summarized as follows: In December each year, the solar radiation in the northern region is the lowest, at 5~10 mol/(m2·day); however, from May to July each year, the solar radiation in the northwest region is the highest, at 45~50 mol/(m2·day). From October to March of the following year, the solar radiation is roughly distributed in a horizontal stripe shape in the east-west direction. From May to August each year, the distribution of solar radiation in the eastern and western regions of China is quite different. The solar radiation changes the most during the spring and autumn equinoxes each year. From March 21 to September 21 each year, the solar radiation time in the northern region of China is longer, but the maximum photon flux is lower; conversely, the solar radiation time in the southern region is shorter, but the maximum photon flux is higher. It is evident that the amount of solar radiation varies greatly in different regions and seasons. When applying artificial lighting, each region should take into full account the local solar radiation and local climate conditions.

Furthermore, in recent years, China's rapid industrialization has led to severe air pollution and frequent smog, especially in northern regions. Smog particles reduce the transmittance of solar radiation, thereby reducing daily solar radiation and severely impacting crop growth. Figure 4 compares the daily solar radiation in Wageningen, Netherlands, and Beijing, China, during the same period in 2015. Due to latitude and climate factors, Beijing's daily solar radiation was significantly higher than Wageningen's. However, due to the persistent extreme smog in North China during November, Beijing's daily solar radiation that month was as low as the Netherlands' (Figure 4, dashed box), specifically below 10 mol/(m²·day). Influenced by greenhouse covering materials and frameworks, the actual daily solar radiation reaching the plant canopy was below 6 mol/(m²·day), a level insufficient for fruit and vegetable growth. Therefore, smog significantly impacts China's daily solar radiation. Artificial lighting is a good way to address the challenges that extreme smog poses to the growth of greenhouse horticultural crops.

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