Abstract
Integrating solar modules into agricultural production constitutes a novel type of agricultural industry. We evaluated the effect of setting opaque plastic solar modules on greenhouse roofs on the crop growth inside greenhouse. The opaque plastic agricultural films simulating the material of solar modules and the greenhouse roofs covered with these films were used, and the yield and nitrate content of pak choi (Brassica chinensis ‘Bekamaru’) and rape (Brassica napus ‘Dragon’) under these films were measured. The results indicated that the yield of pak choi did not change considerably by a simulated photovoltaic (SPV) roof with a shading rate of 38% compared with an uncovered plastic (PL) roof. However, during the first and second planting periods, the yield of rape under the PL roof substantially exceeded that under the SPV roof by 31% and 34%, respectively, indicating that the effect of shading on the yield of rape was greater than that on the yield of pak choi. In addition, the appearance of pak choi and rape also changed under the SPV roof, such as fewer leaves, lower chlorophyll content, and larger specific leaf areas. Nevertheless, the nitrate content of crops grown under the SPV roof exceeded that of crops grown under the PL roof. In conclusion, based on the expression of yield and growth of crops, pak choi is suitable for cultivation in greenhouses that are equipped with photovoltaic systems. However, to prevent plants from accumulating excessive nitrate, attention must be focused on the amount and frequency of nitrogen fertilizers application.
Solar radiation is an essential factor in the cultivation of a variety of agricultural crops. It provides direct kinetic energy for photosynthesis, biochemical reactions, and cell elongation of crops, and it is a crucial factor influencing the appearance of plants (Zheng et al. 2021). Specifically, the combination of radiation intensity and light wavelength required for crop growth may affect the potential yield and quality of crops. When crop plants are shaded, their growth and development will be decreased greatly by the reduced light intensity (Khalid et al. 2019; Shafiq et al. 2021).
Shade response increases the leaf area and chlorophyll content because of the acceleration in the rate of photosynthesis. (Liu et al. 2021). For example, when the level of shading increases, the chlorophyll content and rate of photosynthesis of Bangladhonia (Eryngium foetidum L.), wheat (Triticum aestivum L.), and lettuce (Lactuca sativa L.) increase (Arenas-Corraliza et al. 2021; Liu et al. 2021; Moniruzzaman et al. 2009).
When crops are shaded, they tend to exhibit plant development, internode elongation, leaf area expansion, and increased specific leaf area (SLA). During crop development, the majority of crops are sensitive to changes in the light environment, and their yield and quality are affected accordingly (Arenas-Corraliza et al. 2021; Cossu et al. 2021; Gommers et al. 2013; Liu and Yang 2012). The yield of tomatoes cultivated in greenhouses with 47% shading of photosynthetically active radiation (PAR) is greater than that of tomatoes cultivated in greenhouses with 55% PAR reduction. In addition, decreased light intensity alters the crop quality (Kanski et al. 2021). Under the low-light environments, both dry weight and sweetness of tomatoes decrease, and the total phenolic content decreases considerably (Kanski et al. 2021). When the level of shading increases, the β-carotene, vitamin C, and fiber contents of Bangladhonia decrease (Moniruzzaman et al. 2009). The nitrogen, phosphorus, and potassium contents of wheat are decreased when grown in a shading environment. Shading reduces the level of light intensity and affects the photosynthetic process, thereby causing lettuce to accumulate nitrate (Cometti et al. 2011; Liu and Yang 2012). When plants are grown under shading, their quality decreases during subsequent storage. For example, when baby spinach (Spinacia oleracea L.) is cultivated under shade netting, its ascorbic acid content decreases by 12% to 33% during storage after harvest (Bergquist et al. 2007). Because light intensity affects the concentration of ascorbic acid in crops, shading not only affects growth and yield but also causes significant changes in quality after harvest (Bergquist et al. 2007; Mozafar 1994).
When the upper part of a plant canopy is placed under different levels of shading, the internal light intensity of the microclimate changes inside the facility, resulting in fluctuations in micrometeorological factors such as temperature, humidity, and wind speed. Specifically, when the light intensity decreases as a result of shading, both the temperature and vapor pressure deficit decrease (Kittas et al. 2012; Liu et al. 2021). Kittas et al. (2012) reported that the temperature decreases by 1.7 °C, on average, whereas tomatoes are grown under 30% to 49% shading, which decreases the vapor pressure deficit at the upper part of the tomato canopy by ∼2 kPa, thereby affecting their yield and quality.
As a result of the gradual depletion of global fossil fuel energy resources, the majority of countries have started to develop clean, renewable energy to replace fossil fuel energy. An agrivoltaic system, which is an emerging technique, combines both agriculture and solar energy. Solar modules directly affect crop production by influencing the photosynthetic process, i.e., the effect of shading on crop growth, development, and yield. Unlike photoselective nets (or shade nets), solar modules are opaque and can evenly reduce optical radiation. When the angle of sunlight changes, the shade generated by solar modules moves as a block, resulting in an irregular distribution of crop yield and quality. During this study, we used a simulated photovoltaic (SPV) greenhouse to explore the potential effect of shading created by solar modules on crop production. We also investigated whether pak choi and rape can be cultivated in such facilities without any negative effects.
Materials and Methods
Plastic roof and SPV roof greenhouse
This experiment was conducted at the Taiwan Agricultural Research Institute in Taichung City, Taiwan (24.03125°N, 120.68873°E). Opaque plastic agricultural films (Plastika Kritis, Heraklion, Greece) were installed on the eastern roof of a 78-m2 greenhouse with a length of 13 m and a width of 6 m to simulate solar modules. The opaque PL (plastic) agricultural films were north-south orientated. The shaded area accounts for approximately 38% of the roof area (shading rate of 38%), which means that there is a transmittance rate of 62%. This section was regarded as the SPV roof. As the control treatment, transparent, uncovered PL films were installed on the western roof of the greenhouse. This section was regarded as the PL roof (Fig. 1A).
Effects of the configuration of the plastic (PL) roof, simulated photovoltaic (SPV) roof (gray lines), and raised-bed planting system (green lines) on the greenhouse (A) and crop (pak choi or rape) growth inside the greenhouse (B). The circle (○) indicates the location of the pyranometers. The square represents (□) the temperature and humidity sensor.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
Experimental crops.
Pak choi (Chinese cabbage, Brassica chinensis ‘Bekamaru’) and rape (Chinese rape for leafy greens, Brassica napus ‘Dragon’) were purchased from Tokita Seed (Saitama, Japan) and Pan-Asian Seeds (Tainan City, Taiwan), respectively, and used as testing materials. Seeds of both species were cultivated from Jul 2018 to Jan 2019, and cultivated twice in succession. The first and second planting periods of pak choi were from 7 to 30 Jul 2018 and from 31 Aug to 26 Sep 2018, respectively. The first and second planting period of rape were from 1 to 8 Nov 2018 and from 21 Dec 2018 to 22 Jan 2019, respectively. Peat soilless mixture BVB 6D and BVB 9D (BVB Substrates, Maasland, Netherlands) was used as the planting medium, and the seeds were cultivated in three raised-bed planting systems in the greenhouse. Each raised-bed planting systems had a length of 12 m and width of 1 m (Fig. 1A). The experimental design adopted a completely randomized design, and each raised-bed planting system had one repetition; three repetitions were performed. The 4 kg/bed organic fertilizer (4N–3P–3K, 70% organic matter; New Paradise No. 1; Sinon, Taichung City, Taiwan) was evenly mixed with the soil medium before seeding, and dibble planting was used to sow the seeds. During the planting stage, a water-soluble 20N–20P–20K fertilizer (JR Peters, Allentown, PA, USA) diluted at 1500× was added to the plants daily through drip irrigation by a pump for 15 min during the morning and afternoon. Plants were divided into two treatment group. One groups included plants cultivated under an SPV roof with a shading rate of 38%, and the other group included plants cultivated under a transparent uncovered PL roof.
Micrometeorological monitoring.
To record the photosynthetic photon flux density (PPFD) (μmol⋅m−2⋅s−1) and radiometric density (W⋅m−2), the greenhouse was equipped with two types of pyranometers, which were installed 10 cm above the plants. A thermometer and a hydrometer were also installed 40 cm above the raised-bed planting systems. Two pyranometers were used for each raised-bed, and the configuration positions are shown in Fig. 1. A CR1000 data logger system (Campbell Scientific, Logan, UT, USA) was used to record the temperature, humidity, PPFD, and radiometric every 1 min, as well as the average hourly and daily micrometeorological measurements. The temperature and humidity sensor was set in the middle of raised-bed planting system 1 and raised-bed planting system 2 in the SPV area (Fig. 1A).
Measurements
Growth and biomass.
Plants aboveground were cleaned before determining their growth characteristics, such as plant height, number of leaves, fresh weight, and dry weight (oven-dried at 80 °C for 72 h). A portable area meter (LI-3000C; LI-COR Biosciences, Lincoln, NE, USA) was used to measure the area of each leaf. A chlorophyll meter (SPAD 502 Plus; Konica Minolta, Tokyo, Japan) was used to measure the chlorophyll content of both sides of the largest leaf as the soil plant analysis development (SPAD) value (Ling et al. 2011). The SLA was measured as the ratio of the leaf area to the leaf dry weight. The specific leaf weight (SLW) was the ratio of the leaf dry weight to the leaf area.
Photosynthetic rates.
A portable photosynthetic instrument (LI-6400XT; LI-COR Biosciences, Lincoln, NE, USA) was used to evaluate the photosynthetic process from 9:30 AM to 12:00 PM. The largest fully open leaf was clamped from its center, and the leaf chamber was placed at the upper one-third of the leaf. Each leaf was clamped for 3 min at a gas flow rate of 150 μmol⋅s−1 (with an atmospheric carbon dioxide content of ∼400 ppm). The average value of the measurements obtained at 80, 90, and 100 s was used. Each measurement included the net carbon dioxide exchange rate (μmol⋅m−2⋅s−1 CO2) of the leaves from four plants, and the average value of three plants was used.
Nitrate content.
The plant leaves were diluted 100 times with water and evenly crushed. Nitrate test strips (Merck, Darmstadt, Germany) were soaked in the solution and left to stand for 1 min; then, a reflectometer (RQflex 20, Merck) was used to measure the nitrate content (Itoh et al. 2015).
Statistical analysis
The measured data were entered into a spreadsheet (Microsoft Excel; Microsoft, Redmond, WA, USA). An analysis of variance was used for statistical analysis and to assess differences in every measured parameter via SAS-EG 7.0 (SAS Institute Inc., Cary, NC, USA). Means separation was analyzed using least significant difference tests (P < 0.05).
Results and Discussion
Effect of solar module shading on the optical radiation of the greenhouse
The two planting periods of pak choi spanned from July to September (summer in Taiwan). Consequently, the optical radiation intensity was higher when compared with that of other seasons. During the first and second planting periods, the average light intensity values under the PL roof were 114 and 113 W⋅m−2, respectively, whereas the average light intensity values under the SPV roof were 71 and 70 W⋅m−2 (approximately 62.3% and 61.9% transmission rates), respectively. Therefore, during the two planting periods, the average light intensity under the PL roof and under the SPV roof differed by ∼43 W⋅m−2 (Fig. 2).
Changes in the average daily irradiance under the plastic (PL, ○) and simulated photovoltaic (SPV, ▲) roofs of the greenhouse-cultivated pak choi (A) and rape (B). The first and second planting periods of pak choi were from 7 to 30 Jul and from 31 Aug to 26 Sep 2018, respectively. The first and second planting periods of rape were from 1 to 28 Nov 2018 and from 21 Dec 2018 to 22 Jan 2019, respectively.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
The two planting periods of rape spanned from November to January (winter in Taiwan). Consequently, the optical radiation intensity was lower than that during the other seasons. During the first and second planting periods, the average light intensity values under the PL roof were 71 and 75 W⋅m−2, respectively, whereas the average light intensity values under the SPV roof were 53 and 39 W⋅m−2 (approximately 74.6% and 52% transmission rate), respectively. Therefore, during the two planting periods, the average light intensity values under the PL roof and under the SPV roof differed by 18 and 36 W⋅m−2, respectively.
The pak choi and rape plants cultivated under the SPV roof, which simulated the shade of solar modules, were evidently shorter than their counterparts cultivated under the uncovered PL roof. During the growth of pak choi, the light intensity under the SPV roof was 62% transmission under the PL roof. During the growth of rape, the optical radiation intensity under the SPV roof was 63% transmission under the PL roof. Although the atmospheric optical radiation intensity decreased as the seasons changed, the light intensity under both roofs did not considerably differ, and the optical radiation intensity of the greenhouse evenly decreased as the seasons changed.
In general, the internal light intensity of solar modules changes depending on the arrangement. In addition, the internal temperature and humidity change when the light intensity changes (Kadowaki et al. 2012; Marrou et al. 2013b).
Effect of solar module shade on yield
Yield.
During the first planting period of pak choi, the yields under the PL and SPV roofs were 2.21 and 2.19 kg⋅m−2, respectively. During the second planting period of pak choi, the yields under the PL and SPV roofs were 2.28 and 2.11 kg⋅m−2, respectively. During the first and second planting periods, the yields of pak choi under the SPV roof were 0.02 and 0.17 kg⋅m−2, respectively, which were less than those under the PL roof, but the difference was nonsignificant under PL and SPV. During the first and second planting periods, when the plants were grown under the shade of solar modules with a shading rate of 38%, the yields under the SPV roof were 1% and 8%, respectively, which were lower than those under the PL roof (Fig. 3A). Ma et al. (2016) examined the effects of three light intensities on the fresh weight of pak choi. They discovered that a light intensity of 360 μmol⋅m−2⋅s−1 resulted in the optimal fresh weight, whereas light intensity exceeding 540 μmol⋅m−2⋅s−1 decreased the fresh weight by 58%. They also reported that the average light intensities under the PL and SPV roofs during the growth of pak choi were 114 and 70 W⋅m−2, respectively, which were equivalent to 507 and 312 μmol⋅m−2⋅s−1, respectively. Therefore, they concluded that the light intensity under the SPV roof was suitable for the growth of pak choi. Furthermore, Kumpanalaisatit et al. (2022) cultivated pak choi in ground-mounted agrivoltaic facilities with higher shading. They reported that the yield of pak choi in such facilities was 92% lower than that of the control group because the solar modules were closer to the ground and the shading rate was higher. However, they believed that the benefits of the electricity generated outweighed the losses.
Effects of the plastic (PL) and simulated photovoltaic (SPV) roofs on the yields of pak choi (A) and rape (B) during the two planting periods. Asterisks indicate significant differences between PL and SPV according to the least significant difference (P < 0.05). The first and second planting periods of pak choi were from 7 to 30 Jul and from 31 Aug to 26 Sep 2018, respectively. The first and second planting periods of rape were from 1 to 28 Nov 2018 and from 21 Dec 2018 to 22 Jan 2019, respectively.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
During the first planting period of rape, the yields under the PL and SPV roofs were 1.58 and 1.09 kg⋅m−2, respectively. During the second planting period of rape, the yields under the PL and SPV roofs were 2.35 and 1.55 kg⋅m−2, respectively. During the first and second planting periods of rape, the yields under the PL roof were significantly higher than those under the SPV roof by 31% and 34%, respectively, indicating that the effect of shade on the growth of rape was stronger than that on the growth of pak choi (Fig. 3B). Evaluation of ground-mounted agrivoltaic facilities also revealed that 11% to 50% of the fresh weight of Japanese rape and European rape (B. napus L.) was lost under solar modules shade (Kirimura et al. 2022; Zheng et al. 2021). Generally, shade decreases the yield of crops, with a higher shading rate indicating a greater loss of yield (Cossu et al. 2014; Touil et al. 2021; Trypanagnostopoulos et al. 2017). If solar modules are installed at a higher level, then the effect of shade on the growth of plants may decrease. For example, when lettuce is grown at shading rates of 30% and 50%, if solar modules are installed at 4 m or higher above the ground to increase light transmission, then the yield of lettuce does not considerably decrease (Valle et al. 2017). Previous research has demonstrated that increasing the amount of light received by crops by 1% may increase their yield by 0.5% to 1% (Marcelis et al. 2006). Increasing the distance between the solar modules and the canopy of crops may also increase light exposure.
Difference of light saturation points.
When pak choi and rape were cultivated at a shading rate of 38%, their yields under the PL and SPV roofs differed considerably despite the temperature and humidity being kept relatively constant. For instance, the yield of pak choi cultivated under the SPV roof decreased by 1% to 7%, whereas the yield of rape cultivated under the SPV roof decreased by 31% to 34%. Both pak choi and rape belong to genus Brassica (family Brassicaceae) and are regarded as headless cabbages. However, they react differently to shade, thereby confirming that the shade provided by solar modules and the different light intensities caused by shade affect the yield of different species (Kirimura et al. 2022; Ma et al. 2016). In this study, the light saturation point of pak choi was ∼1300 μmol⋅m−2⋅s−1 (294 W⋅m−2), and the photosynthesis of rape increased as the light intensity increased. In contrast, the light saturation point of rape was ∼1500 μmol⋅m−2⋅s−1 (330 W⋅m−2), indicating that rape required more light compared with pak choi (Fig. 4). Consequently, rape cultivated under low-light conditions exhibited low photosynthetic efficiency and low yield. Ma et al. (2016) reported that the fresh weight of pak choi increased as the light intensity increased, and that the maximum fresh weight was achieved when the light intensity reached ∼414 μmol⋅m−2⋅s−1. However, they reported that higher light intensities decreased the fresh weight of pak choi as a result of stomatal closure, which decreased the carbon dioxide concentration in the stomata. Compared with other Brassica species, such as Brassica oleracea and B. napus, rape has a higher light saturation point (1500–1600 μmol⋅m−2⋅s−1), presumably because of its leaf net carbon dioxide assimilation and Rubisco total activity (Dellero et al. 2021; Taylor et al. 2020). According to Taylor et al. (2020), although the photosynthesis of rape rapidly decreases under shade, rape requires only 10 min of light exposure to achieve stable net carbon dioxide assimilation. When sunlight is blocked by regular opaque materials above plants, the fluctuating shade may negatively affect the photosynthetic process and reduce the yield (Varella et al. 2011).
Light saturation points of pak choi (○) and rape (●) under different levels of light radiation. Each measurement included the net carbon dioxide exchange rate of the leaves from four plants, and the average value of three plants was used.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
Photosynthetic response of pak choi and rape cultivated under shade.
In this study, we examined the photosynthetic process of pak choi and rape cultivated under PL and SPV roofs (Fig. 5). The results indicated that regardless of the planting period, the photosynthetic capacity of crops cultivated under the PL roof was higher than that of crops cultivated under the SPV roof. During the first and second planting periods, the photosynthesis rates of pak choi were 10.9 and 8.5 μmol⋅m−2⋅s−1, respectively, under the PL roof and 8.1 and 6.1 μmol⋅m−2⋅s−1, respectively, under the SPV roof. During the first and second planting periods, the photosynthesis rates of pak choi under the SPV roof were lower than those under the PL roof by 26% and 29%, respectively. During the first planting period, the photosynthesis rates of rape under the PL and SPV roofs were 8.8 and 5.9 μmol⋅m−2⋅s−1, respectively. During the second planting period, the photosynthesis rates of rape under the PL and SPV roofs were 24.4 and 11.6 μmol⋅m−2⋅s−1, respectively. During the first and second planting periods, the photosynthesis rates of rape decreased by 33% and 52%, respectively. Shade resulted in a greater decrease in the photosynthesis rate of rape than in pak choi, particularly during the second planting period (Fig. 5). Generally, rape is adapted to cool seasons (Decoteau 2000; Yaniv et al. 1995). In this study, the average temperature during the second planting period was 20 °C, which was 3.2 °C lower than that during the first planting period. However, the light intensities during the two planting periods under the PL roof were similar. Therefore, because of the decreased temperatures, the photosynthesis rate of rape during the second planting period was higher than that during the first planting period. Because the reduced light intensity decreased the rate of photosynthesis, when shade blocked the sunlight, the photosynthesis rate of rape during the second planting period decreased more rapidly than during the first planting period (Franklin 2008). During the second planting period, the average light intensity under the SPV roof was only 38.7 W⋅m−2, which was 14 W⋅m−2 lower than that during the first planting period (52.7 W⋅m−2). During the second planting period, the photosynthesis rate under the SPV roof was 52% lower than that during the first planting period. Three B. napus cultivars, Dragon, vincent, and Fluke sweet, grown under 0%, 50%, and 70% shade nets had net photosynthetic rates that were significantly decreased with an increasing degree of shade (Huang and Sung 2016), indicating that rape required higher light intensity to maintain the photosynthetic process (Yao et al. 2017).
Effects of the plastic (PL) and simulated photovoltaic (SPV) roofs on the photosynthesis rates of pak choi and rape during the two planting periods. Asterisks indicate significant differences between PL and SPV according to the least significant difference (P < 0.05). Each measurement included the net carbon dioxide exchange rate of the leaves from four plants, and the average value of three plants was used. The first and second planting periods of pak choi were from 7 to 30 Jul and from 31 Aug to 26 Sep 2018, respectively. The first and second planting periods of rape were from 1 to 28 Nov 2018 and from 21 Dec 2018 to 22 Jan 2019, respectively.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
Effect of shade on the growth of pak choi and rape.
Shade affects the yield and growth of pak choi and rape. In this study, we investigated the effect of shade on the growth of pak choi (Fig. 6). The results indicated that regardless of the planting period, the number of leaves, leaf area, SPAD value, and SLW of plants cultivated under the PL roof were larger than those of plants cultivated under the SPV roof. Only the SLA of plants cultivated under the SPV roof was higher than that of plants cultivated under the PL roof. Shade affected rape more than it affected pak choi. Consequently, the plant height, number of leaves, and leaf area of rape cultivated under shade were smaller than those of pak choi (Fig. 7).
Effects of the plastic (PL) and simulated photovoltaic (SPV) roofs on the plant height (A), leaf number (B), leaf area (C), soil plant analysis development (SPAD) value (D), specific leaf area (SLA) (E), and specific leaf weight (SLW) (F) of pak choi during the two planting periods. Asterisks indicate significant differences between PL and SPV according to the least significant difference (P < 0.05). The first and second planting periods of pak choi were from 7 to 30 Jul and from 31 Aug to 26 Sep 2018, respectively. The first and second planting periods of rape were from 1 to 28 Nov 2018 and from 21 Dec 2018 to 22 Jan 2019, respectively.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
Effects of the plastic (PL) and simulated photovoltaic (SPV) roofs on the plant height (A), leaf number (B), leaf area (C), soil plant analysis development (SPAD) value (D), specific leaf area (SLA) (E), and specific leaf weight (SLW) (F) of rape during the two planting periods. Asterisks indicate significant differences between PL and SPV according to the least significant difference (P < 0.05). The first and second planting periods of pak choi were from 7 to 30 Jul and from 31 Aug to 26 Sep 2018, respectively. The first and second planting periods of rape were from 1 to 28 Nov 2018 and from 21 Dec to 22 Jan 2019, respectively.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
The rape cultivated under the SPV roof were 10% to 13% shorter than those cultivated under the PL roof (Fig. 7A). In contrast, the heights of the pak choi plants cultivated under the SPV and PL roofs did not differ considerably (Fig. 6A). Japanese rape (komatsuna, Brassica rapa L. var Perviridis Grop cv. Natsurakuten) is similar to rape and has a similar growth pattern in agrivoltaic systems (Kirimura et al. 2022). Kirimura et al. (2022) reported that the height of Japanese rape cultivated under solar modules shade was 64% to 79% that of the control rape, in other words, 21% to 37% shorter.
The number of rape leaves decreased considerably in the presence of shade (Fig. 7B). The number of pak choi leaves also decreased; however, the decrease was not statistically significant (Fig. 6B). Generally, the number of leaves plays a role in the plant yield (Maseko et al. 2017). Therefore, a decrease in the number of pak choi and rape leaves is associated with a reduced yield (Fig. 3).
The loss of leaf area of rape was greater than that of pak choi. During the first and second planting periods, the rape plants cultivated under the SPV roof lost 42% and 35%, respectively, of their leaf area, whereas the pak choi plants cultivated under the same roof lost 24% and 7%, respectively, of their leaf area (Figs. 6C and 7C). Therefore, shade directly reduced the number of leaves and leaf area of rape. In general, the effect of shade on the size and number of leaves varies depending on the species involved (Kittas et al. 2012; Marrou et al. 2013a; Moniruzzaman et al. 2009; Tani et al. 2014; Zheng et al. 2021). For instance, Zheng et al. (2021) reported that when lettuce was grown in a greenhouse beneath east/west-aligned solar modules, the shade provided by full-density and half-density solar modules decreased the number of leaves but increased the leaf area. These results indicate that when lettuce was grown under shade, assimilated substances were prioritized for leaf expansion but did not increase the number of leaves. Therefore, the leaves and plants appeared larger. Shade also caused the number and size of tomato leaves to increase. However, this effect differed from that observed in pak choi and rape, because shade reduced the number and size of pak choi and rape leaves. The authors also discovered that the plants cultivated under the SPV roof were shorter than their counterparts. Kumpanalaisatit et al. (2022) made the same observation and discovered that the pak choi plants cultivated in an agrivoltaic system were shorter and had fewer leaves compared with their counterparts.
The SPAD values of pak choi and rape cultivated under the SPV roof were considerably lower than those of pak choi and rape cultivated under the PL roof (Figs. 6D and 7D). In addition, the SPAD value during the second planting period was lower than that during the first planting period. In general, the SPAD value represents the chlorophyll content, nutrition, and photosynthetic capability of a crop (Arenas-Corraliza et al. 2021; Kirimura et al. 2022; Liu and Yang 2012; Tani et al. 2014). Shade reduces the photosynthetic capacity of crops. This phenomenon was reported by Tani et al. (2014) in chessboard-like photovoltaic greenhouses, which reduced the SPAD value of lettuce. In another study, Kirimura et al. (2022) reported that the Japanese rape plants cultivated in agrivoltaic systems had a low SPAD value, but the difference between the treatment group and the control group was not statistically significant. Arenas-Corraliza et al. (2021) reported that some crops, such as cereal/seed crops, increased their chlorophyll content when they were cultivated under shade to increase their SPAD value and maintain their photosynthetic capability. These findings confirmed that the effect of shade on the SPAD value of crops varies depending on the species involved. However, in this study, we discovered that pak choi and rape cultivated in greenhouses under solar module shade had low SPAD values.
The SLA represents the response of plants to shade. A larger SLA indicates a thinner leaf, which can be used to understand the coping strategies of crops as they adapt to shade (Gommers et al. 2013; Marrou et al. 2013a). In this study, we investigated the SLA of pak choi plants cultivated under an SPV roof (Fig. 6E). The results indicated that during the first and second planting periods, the SLAs of pak choi plants cultivated under the SPV roof were 16% and 33%, respectively, which were larger than those of pak choi plants cultivated under the PL roof. These results revealed the status of the pak choi leaves under the SPV roof, which had decreased light intensity. The SLA of rape plants cultivated under the SPV roof was also larger than that of rape plants cultivated under the PL roof (Fig. 7E). During the first and second planting periods, the SLAs of rape cultivated under the SPV roof were 17% and 23%, respectively, which were larger than that of rape cultivated under the PL roof. Tani et al. (2014) grew lettuce in a greenhouse with thin transparent solar modules installed on its roof. They discovered that the SLA of the treatment group was higher than that of the control group. They also discovered that the leaves of the treatment group were thinner and larger. According to Wolff and Coltman (1990), because shade reduces the weight of plants, the SLA increases if the leaf area remains constant.
The SLW is the leaf weight per unit of leaf area. It represents the leaf thickness. The SLW is the inverse of the SLA. It allows researchers to understand the thickness of leaves grown under shade. In this study, regardless of the planting period, the SLW of pak choi and rape cultivated under the SPV roof was lower than that of pak choi and rape cultivated under the PL roof (Figs. 6F and 7F). During the first and second planting periods, the SLWs of pak choi cultivated under the SPV roof were 14% and 22%, respectively, which were lower than those of pak choi cultivated under the PL roof. During the first and second planting periods, the SLWs of rape cultivated under the SPV roof were 16% and 20%, respectively, which were lower than those of rape cultivated under the PL roof. According to Marrou et al. (2013a) and Tani et al. (2014), a lower SLW (or higher SLA) indicates thinner leaves and fewer mesophyll cells as plants adjust their photosynthetic process to adapt to reduced light under shade.
Shade-induced changes in the nitrate content.
Shade alters the yield, growth, and nitrate content of pak choi and rape. In this study, the nitrate contents of plants cultivated under PL and SPV roofs were compared (Fig. 8). The results indicated that, regardless of whether the plants were pak choi or rape, during the two planting periods, the nitrate contents of plants grown under the covered SPV roof were clearly higher than those of plants grown under the uncovered PL roof. During the first planting period, the nitrate contents of pak choi cultivated under the PL and SPV roofs were 6046 and 8269 mg⋅kg−1, respectively (Fig. 8A). The nitrate content of pak choi cultivated under the SPV roof was 37% higher than that of pak choi cultivated under the PL roof. During the second planting period, the nitrate contents of pak choi cultivated under the PL and SPV roofs were 4106 and 5600 mg⋅kg−1, respectively. The nitrate content of pak choi cultivated under the SPV roof was 36% higher than that of pak choi cultivated under the PL roof. However, during the two planting periods, the nitrate contents of pak choi did not differ considerably. During the first planting period, the nitrate contents of rape cultivated under the PL and SPV roofs were 6452 and 8213 mg⋅kg−1, respectively (Fig. 8B). The nitrate content of rape cultivated under the SPV roof was 27% higher than that of rape cultivated under the PL roof. During the second planting period, the nitrate contents of rape cultivated under the PL and SPV roofs were 4663 and 7288 mg⋅kg−1, respectively. The nitrate content of rape cultivated under the SPV roof was 56% higher than that of rape cultivated under the PL roof. During the two planting periods, the nitrate contents of rape differed considerably. Except for the nitrate content of crops cultivated during the second planting period under the PL roof, the nitrate contents of crops cultivated during all the other planting periods exceeded the nitrate standard of 5000 mg⋅kg−1 (Official Journal of the European Union 2011) set by the European Union. In this study, drip irrigation was used to cultivate pak choi and rape, which may have caused the plants to accumulate additional nitrate. The shade provided by the SPV roof also increased the accumulation of nitrate. According to Liu and Yang (2012), leafy vegetables such as lettuce, pak choi, and spinach easily accumulate nitrate, with those grown in greenhouses accumulating more nitrate compared with those grown on land. This phenomenon is linked to the light intensities because the nitrate content of plants increases as the level of shade increases (Cometti et al. 2011; Liu and Yang 2012). In this study, the PL cover of the greenhouse roofs and the shade provided by the solar modules caused the pak choi and rape plants to accumulate additional nitrate.
Effects of the plastic (PL) and simulated photovoltaic (SPV) roofs on the nitrate contents of pak choi (A) and rape (B) during the two planting periods. Asterisks indicate significant differences between PL and SPV according to the least significant difference (P < 0.05). The first and second planting periods of pak choi were from 7 to 30 Jul and from 31 Aug to 26 Sep 2018, respectively. The first and second planting periods of rape were from 1 to 28 Nov 2018 and from 21 Dec 2018 to 22 Jan 2019, respectively.
Citation: HortScience 58, 11; 10.21273/HORTSCI17240-23
Conclusion
In this study, we first used opaque PL agricultural films to create a shady environment that simulated the shade of solar modules installed on greenhouse roofs. The results indicated that the shade caused the crops to change their outer appearance to adapt to the low-light conditions. The plants also decreased their leaf count and chlorophyll content, increased their SLA, and adopted certain strategies to maintain their growth under these conditions. The experimental results confirmed that pak choi is more suitable than rape in agrivoltaic systems because its growth in the presence of shade (solar modules) and its growth in the absence of shade (control group) do not differ considerably. Regardless of whether the plant is pak choi or rape, shading results in the accumulation of nitrate. Therefore, greenhouses equipped with photovoltaic systems must determine how to prevent yield reduction and nitrate accumulation while using fertilizers, particularly nitrogenous fertilizers. Agrivoltaic systems may be a potential agricultural production method in the future. To further understand the effect of solar module shade on crop production, researchers must continue to examine the adaptability of different species to solar module shade and the effect of solar module shade on the yield and quality of different species to provide a reference for selecting suitable crop species.
References Cited
Arenas-Corraliza MG, López-Díaz ML, Rolo V, Moreno G. 2021. Wheat and barley cultivars show plant traits acclimation and increase grain yield under simulated shade in Mediterranean conditions. J Agron Crop Sci. 207:100–119. https://doi.org/10.1111/jac.12465.
Bergquist SAM, Gertsson UE, Nordmark LTG, Olsson ME. 2007. Ascorbic acid, carotenoids, and visual quality of baby spinach as affected by shade netting and postharvest storage. J Agr Food Chem. 55:8444–8451. https://doi.org/10.1021/jf070396z.
Cometti NN, Martins MQ, Bremenkamp CA, Nunes JA. 2011. Nitrate concentration in lettuce leaves depending on photosynthetic photon flux and nitrate concentration in the nutrient solution. Hortic Bras. 29:548–553. https://doi.org/10.1590/S0102-05362011000400018.
Cossu M, Murgia L, Ledda L, Deligios PA, Sirigu A, Chessa F, Pazzona A. 2014. Solar radiation distribution inside a greenhouse with south-oriented photovoltaic roofs and effects on crop productivity. Appl Energy. 133:89–100. https://doi.org/10.1016/j.apenergy.2014.07.070.
Cossu M, Sirigu A, Deligios PA, Farci R, Carboni G, Urraccia G, Ledda L. 2021. Yield response and physiological adaptation of green bean to photovoltaic greenhouse. Front Plant Sci. 12:655851. https://doi.org/10.3389/fpls.2021.655851. [accessed 7 Oct 2022].
Decoteau DR. 2000. Vegetable crops. Prentice-Hall, Inc., Hoboken, NJ, USA.
Dellero Y, Jossier M, Bouchereau A, Hodges M, Leport L. 2021. Leaf phenological stages of winter oilseed rape (Brassica napus L.) have conserved photosynthetic efficiencies but contrasted intrinsic water use efficiencies at high light intensities. Front Plant Sci. 12:659439. https://doi.org/10.3389/fpls.2021.659439. [accessed 7 Oct 2022].
Franklin KA. 2008. Shade avoidance. New Phytol. 179:930–944. https://doi.org/10.1111/j.1469-8137. 2008.02507.x.
Gommers CMM, Visser EJW, St Onge KR, Voesenek LACJ, Pierik R. 2013. Shade tolerance: When growing tall is not an option. Trends Plant Sci. 18:65–71. https://doi.org/10.1016/j.tplants.2012.09.008.
Huang YC, Sung Y. 2016. Effect of shading on the growth and nitrate content in rape (Brassica napus L.). Horticulture NCHU. 41:55–70. (in Chinese).
Itoh H, Nomura K, Shiraishi N, Uno Y, Kuroki S, Ayata K. 2015. Continuous measurement of nitrate concentration in whole lettuce plant by visible-near-infrared spectroscopy. Environ Control Biol. 53:205–215. https://doi.org/10.2525/ecb.53.205.
Kadowaki M, Yano A, Ishizu F, Tanaka T, Noda S. 2012. Effect of greenhouse photovoltaic array shading on Welsh onion growth. Biosyst Eng. 111:290–297. https://doi.org/10.1016/j.biosystemseng.2011.12.006.
Kanski L, Kahle H, Naumann M, Hagenguth J, Ulbrich A, Pawelzik E. 2021. Cultivation systems, light intensity, and their influence on yield and fruit quality parameters of tomatoes. Agronomy (Basel). 11:1203. https://doi.org/10.3390/agronomy11061203. [accessed 13 Oct 2022].
Khalid MHB, Raza MA, Yu HQ, Sun FA, Zhang YY, Lu FZ, Si L, Iqbal N, Khan I, Fu FL, Li WC. 2019. Effect of shade treatments on morphology, photosynthetic and chlorophyll fluorescence characteristics of soybean (Glycine max L. Merr.). Appl Ecol Environ Res. 17:2551–2569. https://doi.org/10.15666/aeer/1702_25512569.
Kirimura M, Takeshita S, Matsuo M, Zushi K. 2022. Effects of agrivoltaics (photovoltaic power generation facilities on farmland) on growing condition and yield of komatsuna, mizuna Kabu, and spinach. Environ Control Biol. 60:117–127. https://doi.org/10.2525/ecb.60.117.
Kittas C, Katsoulas N, Rigakis N, Bartzanas T, Kitta E. 2012. Effects on microclimate, crop production and quality of a tomato crop grown under shade nets. J Hortic Sci Biotechnol. 87:7–12. https://doi.org/10.1080/14620316.2012.11512822.
Kumpanalaisatit M, Setthapun W, Sintuya H, Jansri SN. 2022. Efficiency improvement of ground-mounted solar power generation in agrivoltaic system by cultivation of bok choy (Brassica rapa subsp. Chinensis L.) under the panels. International J Renewable Energy Development. 11:103–110. https://doi.org/10.14710/ijred.2022.41116.
Ling Q, Huang W, Jarvis P. 2011. Use of SPAD-502 meter to measure leaf chlorophyll concentration in Arabidopsis thaliana. Photosynth Res. 107:209–214. https://doi.org/10.1007/s11120-010-9606-0.
Liu WK, Yang QC. 2012. Effects of short-term treatment with various light intensities and hydroponic solutions on nitrate concentration of lettuce. Acta Agric Scand B Soil Plant Sci. 62:109–113. https://doi.org/10.1080/09064710.2011.580366.
Liu X, Gao S, Liu Y, Cao B, Chen Z, Xu H. 2021. Alterations in leaf photosynthetic electron transport in Welsh onion (Allium fiftulosum L.) under different light intensity and soil water conditions. Plant Biol. 23:83–90. https://doi.org/10.1111/plb.13165.
Ma Q, Cao X, Wu L, Mi W, Feng Y. 2016. Light intensity affects the uptake and metabolism of glycine by pakchoi (Brassica chinensis L.). Scientific Reports. 6:21200. https://doi.org/10.1038/srep21200.
Marcelis LFM, Broekhuijsen AGM, Meinen E, Nijs EMFM, Raaphorst MGM. 2006. Quantification of the growth response to light quantity of greenhouse growth crops. Acta Hortic. 711:97–103. https://doi.org/10.17660/ActaHortic.2006.711.9.
Marrou H, Wery J, Dufour L, Dupraz C. 2013a. Productivity and radiation use efficiency of lettuce grown in the partial shade of photovoltaic panels. Eur J Agron. 44:54–66. https://doi.org/10.1016/j.eja.2012.08.003.
Marrou H, Guilioni L, Dufour L, Dupraz C, Wery J. 2013b. Microclimate under agrivoltaic systems: Is crop growth rate affected in the partial shade of solar panels? Agr For Meteorol. 177:117–132. https://doi.org/10.1016/j.agrformet.2013.04.012.
Maseko I, Beletse YG, Nogemane N, du Plooy CP, Misimwa TR, Mabhaudhi T. 2017. Productivity of non-heading Chinese cabbage (Brassica rapa subsp. chinensis) under different agronomic management factors. S Afr J Plant Soil. 34:275–282. https://doi.org/10.1080/02571862.2017.1295324.
Moniruzzaman M, Islam MS, Hossain MM, Hossain T, Miah MG. 2009. Effects of shade and nitrogen levels on quality Bangladhonia production. Bangladesh J Agric Res. 34:205–213. https://doi.org/10.3329/bjar.v34i2.5791.
Mozafar A. 1994. Plant vitamins: Agronomic, physiological and nutritional aspects. CRC Press, Boca Raton, FL, USA.
Official Journal of the European Union. 2011. Commission regulation (EU) No 1258/2011 of 2 December 2011 amending regulation (EU) No 1886/2006 as regards maximum levels for nitrates in foodstuffs. 54:L320/15-17. https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2011:320:0015:0017:EN:PDF. [accessed 7 Sep 2022].
Shafiq I, Hussain S, Raza MA, Iqbal N, Asghar MA, Raza A, Fan YF, Mumtaz M, Shoaib M, Ansar M, Manaf A, Yang WY, Yang F. 2021. Crop photosynthetic response to light quality and light intensity. J Integr Agric. 20:4–23. https://doi.org/10.1016/S2095-3119(20)63227-0.
Tani A, Shiina S, Nakashima K, Hayashi M. 2014. Improvement in lettuce growth by light diffusion under solar panels. Nogyo Kisho. 70:139–149. https://doi.org/10.2480/agrmet.D-14-00005.
Taylor SH, Orr DJ, Carmo-Silva E, Long SP. 2020. During photosynthetic induction, biochemical and stomatal limitations differ between Brassica crops. Plant Cell Environ. 43:2623–2636. https://doi.org/10.1111/pce.13862.
Touil S, Richa A, Fizir M, BIngwa B. 2021. Shsding effect of photovoltaic panels on horticulture crops production: a mini review. Rev Environ Sci Biotechnol. 20:281–296. https://doi.org/10.1007/s11157-021-09572-2.
Trypanagnostopoulos G, Kavga A, Souliotis M, Tripanagnostopoulos Y. 2017. Greenhouse performance results for roof installed photovoltaics. Renew Energy. 111:724–731. https://doi.org/10.1016/j.renene.2017.04.066.
Valle B, Simonneau T, Sourd F, Pechier P, Hamard P, Frisson T, Ryckewaert M, Christophe A. 2017. Increasing the total productivity of a land by combining mobile photovoltaic panels and food crops. Appl Energy. 206:1495–1507. https://doi.org/10.1016/j.apenergy.2017.09. 113.
Varella AC, Moot DJ, Pollock KM, Peri PL, Lucas RJ. 2011. Do light and alfalfa responses to cloth and slatted shade represent those measured under an agroforestry system? Agrofor Syst. 81:157–173. https://doi.org/10.1007/s10457-010-9319-6.
Wolff XY, Coltman RR. 1990. Productivity of eight leafy vegetable crops grown under shade in Hawaii. J Am Soc Hortic Sci. 115:182–188. https://doi.org/10.21273/JASHS.115.1.182.
Yaniv Z, Schafferman D, Zur M. 1995. The effect of temperature on oil quality and yield parameters of high- and low-erucic acid Cruciferae seeds (rape and mustard). Ind Crops Prod. 3:247–251. https://doi.org/10.1016/0926-6690(94)00041-V.
Yao XY, Liu XY, Xu ZG, Jiao XI. 2017. Effects of light intensity on leaf microstructure and growth of rape seedlings cultivated under a combination of red and blue LEDs. J Integr Agric. 16:97–105. https://doi.org/10.1016/S2095-3119(16)61393-X.
Zheng J, Meng S, Zhang X, Zhao H, Ning X, Chen F, Omer AAA, Ingenhoff J, Liu W. 2021. Increasing the comprehensive economic benefits of farmland with even-lighting agrivoltaic systems. PLoS One. 16:e0254482. https://doi.org/10.1371/journal.pone.0254482. [accessed 13 Oct 2022].