Introduction

The push for clean, sustainable and environmental friendly sources of renewable energy as a consequence of climate change, and the bid to meet the global energy demand have led to increased use of solar energy or PV panels in homes and offices. This trend has led to land use crisis between food production and energy (Weselek et al. 2019) as PV installations on open areas remains the most cost effective option (Fraunhofer ISE 2015). There is a growing concern about the loss of agricultural land to the more lucrative PV energy production. Unfortunately, with the global population projected to rise from its current 7.7 billion to 8.5 billion in 2030 (UN Report 2020), the competition for limited agricultural land is expected to rise with greater impacts in very densely populated regions with small land area.

Agrophotovoltaics (APV) is an emerging technology that integrates food production under PV panels (Trommsdorff et al. 2021). Organic photovoltaic (OPV) cells are small, flexible and semi-transparent type of PVs that uses organic semi-conductors to produce electricity from sunlight (Cheng et al. 2018; Hou et al. 2018). It has absorption properties that can be modified to complement the light spectral range (400–700 nm) of photosynthesizing plants (Ravishankar et al. 2011).

Measurement of RF and OPV transmittance

To establish the spectra range of light transmitted by the RF and OPV materials, the materials were passed through a high resolution ASD FieldSpec 4 (Malvern Panalytical) following the procedure: (i) optimize integration time; (ii) collect white reference (WR) with fiber facing sphere opening and lamp on; (iii) hold front of sample in front of sphere opening and ensure no light is esca** to the sides, take measurement; (iv) hold back of sample in front of sphere opening and ensure no light is esca** to the sides, take measurement; (v) repeat iii and iv for all samples.

At the end of the measurement, RF had the least transmittance (0.01) of BL while P1 and P2 had relatively higher transmittance (0.11 and 0.09, respectively). For RL transmittance however, RF was highest (0.89) followed by P1 (0.64) and P2 (0.09) (Fig. 1).

Statistical analysis

To test for significance of treatment means, analysis of variance (ANOVA) was carried out for all data using GenStat 10.3 discovery edition, and post-ANOVA mean separation was achieved using least significant difference (LSD) at p < 0.05. GraphPad Prism 9 was used to produce the graphs.

Results

Effect of OPV-RF transmittance, genotype and interaction on yield and yield attributes of spinach in greenhouse and field conditions

The effect of OPV-transmittance on leaf yield and yield attributes of spinach in greenhouse and field conditions are shown in Fig. 2. OPV-transmittance significantly influenced FSW, DSW and TED (p ≤ 0.05) but did not affect LMA in the greenhouse (Fig. 2A–D). P0 and P1 were similar in FSW and DSW and were higher than P2. TED was higher in P1 (15.27 ± 2.29) and P2 (10.99 ± 3.62) than P0 (5.79 ± 4.25) (Fig. 2E). The effect of RF on yield and yield attributes of spinach was significant (p < 0.05) for FSW, DSW, TBW, LMA and TED in the field (Fig. 2F–J). RF0 was significantly higher than RF1 in all five yield traits.

Fig. 2
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Effect of OPV-RF Transmittance, Genotype and Interaction on Leaf Yield and Yield Traits of Spinach in Greenhouse and Field Conditions. Bars show mean and standard deviations. Bars with different letters are significantly different at p < 0.05. P0: Plants grown under no OPV in greenhouse; P1: plants grown under OPV with transmittance peak of 0.64; P2: plants grown under OPV with transmittance peak of 0.11; RF0: plants grown under no red-foil in the field; RF1: plants grown under red-foil with transmittance peak of 0.89

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There was a clear case of genotypic effect in almost all the yield traits measured. With the exception of LMA (Fig. 2D and I) where eland was superior to bufflehead, bufflehead genotype was consistently higher in FSW, DSW, TBW and TED than eland in greenhouse and field (Fig. 2).

Significant OPV-genotype interaction was recorded in FSW, DSW and TBW in the greenhouse. Bufflehead grown in open (BP0) and bufflehead grown under P1 (BP1) had superior FSW, DSW and TBW than the others. Interaction effect on LMA and TED were not significant (Fig. 2A–E). Likewise, the effect of RF-genotype interaction did not significantly affect yield attributes of spinach in the field (Fig. 2F–J).

Effect of OPV-RF transmittance, genotype and interaction on growth and SPAD value of spinach in greenhouse and field conditions

The effects of OPV-RF transmittance on PH, LN, LA and SPAD value of spinach are shown in Fig. 3. OPV transmittance did not significantly affect (p > 0.05) PH, LN, and SPAD value of spinach under greenhouse conditions (Fig. 3A–D). Significant effect was recorded for LA with P0 (36.63 ± 16.04) showing comparable (p > 0.05) LA with P1 (29.29 ± 12.49) but was larger than P2 (p < 0.05) (Fig. 3C). In a similar pattern, there was no significant effect (p > 0.05) of RF transmittance on PH, LN, LA, and SPAD value of spinach in the field (Fig. 3E, F).

Fig. 3
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Effect of OPV-RF Transmittance, Genotype and Interaction on Growth and SPAD Value of Spinach in Greenhouse and Field Conditions. Bars show mean and standard deviations. Bars with different letters are significantly different at p < 0.05. P0: Plants grown under no OPV in greenhouse; P1: plants grown under OPV with transmittance peak of 0.64; P2: plants grown under OPV with transmittance peak of 0.11; RF0: plants grown under no red-foil in the field; RF1: plants grown under red-foil with transmittance peak of 0.89

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Bufflehead genotype performed significantly better than eland in PH, LN, and LA across both environments (Fig. 3). Plants grown in the greenhouse outperformed the plants raised under field conditions in all growth traits measured. The highest LN in the field was 8.25 compared to 19.50 in the greenhouse. The tallest plant on the field was about 10.05 cm high in contrast to 16.70 cm in the greenhouse. SPAD value did not differ between genotypes of spinach either in field or in the greenhouse.

Under controlled greenhouse conditions, the response of bufflehead and eland genotypes to OPV-transmittance were similar (p > 0.05) in PH and SPAD value but varied (p < 0.05) in LN and LA. Interactions of bufflehead and no-OPV (BP0), bufflehead and OPV1 (BP1) and bufflehead and OPV2 (BP2) were statistically similar but performed higher than interactions of eland with no-OPV (EP0), eland and OPV1 (EP1) and eland with OPV2 (EP2) which were also similar in LN. The interaction BP0 (48.25 ± 13.79) and BP1 (40.57 ± 6.65) produced the largest LA in contrast to the interaction EP2 (18.68 ± 3.31) which was the least. In like manner, the interaction effect of RF and genotype did not show significant difference for PH, LN, LA, and SPAD value of spinach under field conditions.

Photo-irradiance responses of ETR, Y(II), Y(NPQ) and Y(NO) in OPV and control conditions

The photo-irradiance responses of ETR, Y(II), Y(NPQ) and Y(NO) are shown in Figs. 4 and 5 for greenhouse and field, respectively. ETR increased progressively with increasing light intensity across genotypes and environments until a saturation point is reached beyond which ETR started to decline. The response was similar in OPV, RF and control plants at low light intensities (< 400 PPFD). However, at higher light intensities (> 500 PPFD) variation became evident. OPV and RF plants generally showed reduced ETR at > 500 PPFD compared to control. Eland genotype was more sensitive to light at > 500 PPFD compared to bufflehead. OPV and RF plants generally showed higher photochemical efficiency than control plants across environments and genotypes. Y(II) declined progressively with increasing light intensity (> 200 PPFD). P2 plants showed higher values of Y(NO) with increasing light intensity across genotypes compared to P0 and P1 plants in the greenhouse (Fig. 4). However, there was no variation in Y(NO) across RF and genotypes in the field (Fig. 5).

Fig. 4
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Photo-Irradiance response curves of ETR, Y(II), Y(NO) and Y(NPQ). ETR: electron transport rate; Y(II): effective photochemical yield; Y(NO): yield of non-photochemical energy losses via heat dissipation and fluorescence at the reaction centers; Y(NPQ): yield of non-photochemical energy losses via heat dissipation at the antenna. P0: control; P1: plants grown under OPV with transmittance peak of 0.64; P2: plants grown under OPV with transmittance peak of 0.11; Data are Mean ± SD (n = 3)

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Fig. 5
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Photo-Irradiance response curves of ETR, Y(II), Y(NO) and Y(NPQ) of Spinach Plants Grown in Field. ETR: electron transport rate; Y(II): effective photochemical yield; Y(NO): yield of non-photochemical energy losses via heat dissipation and fluorescence at the reaction centers; Y(NPQ): yield of non-photochemical energy losses via heat dissipation at the antenna. RF0: control; RF1: plants grown under red-foil with transmittance peak of 0.89. Data are Mean ± SD (n = 3)

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Effect of OPV-RF transmittance, genotype and interaction on photosynthesis of spinach

The effects of OPV-RF transmittance on photosynthetic parameters of spinach are shown in Fig. 6. P1 and P2 significantly reduced ETR and Y(NPQ) compared to P0 which recorded higher values. In addition, P2 (0.76 ± 0.01) and P1 (0.74 ± 0.01) had higher Y(II) than P0 (0.72 ± 0.02) (Fig. 6B). Y(NO) was not affected by OPV transmittance in the greenhouse. Likewise, RF significantly reduced ETR, Y(NO) and Y(NPQ) in the field while Y(II) was significantly improved (Fig. 6E–H). There was no significant variation in the two genotypes for all traits measured both in greenhouse and field conditions (Fig. 6). The response of the two genotypes to different light transmittance in the greenhouse and in field were similar (p > 0.05) for all photosynthetic traits measured across the duration of the study.

Fig. 6
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Effect of OPV-RF Transmittance, Genotype and Interaction on Photosynthesis of Spinach in Greenhouse and Field Conditions. Bars show mean and standard deviations. Bars with different letters are significantly different at p < 0.05. P0: Plants grown under no OPV in greenhouse; P1: plants grown under OPV with transmittance peak of 0.64; P2: plants grown under OPV with transmittance peak of 0.11; RF0: plants grown under no red-foil in the field; RF1: plants grown under red-foil with transmittance peak of 0.89; Y(II): effective photochemical yield; Y(NO): yield of non-photochemical energy losses via heat dissipation and fluorescence at the reaction centers; Y(NPQ): yield of non-photochemical energy losses via heat dissipation at the antenna

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Discussion

Low OPV transmittance reduced yield attributes of spinach in greenhouse

The transmittance properties of the materials used over a plant can influence physiological processes, and consequently growth and yield attributes of the plant. It is a very important parameter because; it regulates the amount and quality of light absorbed by leaves. It also provides a basis to explain variation in photosynthetic efficiencies as a function of different light spectra it transmits. According to Liu and Van lersel (2021) light intensity and quality can affect physiological and yield processes. In this study, P1 with a higher transmittance of red light (RL) (64%) and blue light (BL) (11%) was statistically similar (p > 0.05) to P0 in FSW, DSW, TBW and was superior to P0 in TED. However, P2 with lower transmittance of RL (11%) and BL (9%) influenced lower values in all the yield traits (Fig. 2). The higher root weight of P0 plants (0.15) in contrast to the higher TED of P1 (15.27) and P2 (10.99) is an indication that OPV plants utilized greater proportion of the absorbed light energy for shoot production as against root development. LMA was not significantly different among treatments although P1 and P2 had insignificantly higher value (0.02) compared to P0 (0.01) implying that the plants did not sufficiently adjust their leaves to reduced light. The reduced yield and yield traits as a consequence of reduced light observed in this study corroborates previous reports on other crops (Islam et al. 1993; Mu et al. 2010; Wang et al. 2015; Schmierer et al. 2021).

There was no significant difference in plant height, and leaf number which is consistent with the report of Chidburee et al. (2017) in Curcuma alismatifolia grown under RL condition. P2 plants recorded higher LA than P1 and P0 by maintaining higher photochemical efficiency and the least non-photochemical energy losses to heat and fluorescence at the reaction centers (Fig. 6). Reduced LA as a function of reduced light was reported by Schmierer et al. (2021) on the contrary. The leaf is an indispensable part of a plant where virtually all photosynthetic activities take place, and for spinach, it is also the economic part of the plant. Plants are able to harvest light energy from the sun by virtue of its antenna contained in the chloroplast of leaves (Lambers et al. 2008), and could adjust their antenna system to suit the prevailing environmental condition through leaf and chlorophyll movements (Muller et al. 2001). For instance, at low light intensity, plants tend to increase the size of their antenna and reduce it at very high light intensity (Fryer et al. 1998) as a protective mechanism to regulate the quantum of light energy that is absorbed and transmitted to the reaction centers, which explains why P2 with the least transmittance recorded the largest LA.

RF transmittance reduced yield attributes of spinach under extreme cold conditions in the field

RF transmitted majorly monochromatic RL (89%) with very infinitesimal proportion of BL (< 1%), which impacted on shoot weight, total biomass weight and photosynthesis of spinach in extreme field condition. RF in contrast to full irradiance significantly reduced FSW, DSW, TBW, LMA and TED under field conditions. The reduction in shoot weight, biomass, leaf mass per area and total energy distribution of spinach across genotypes is suggestive of the RF’s inability to supply complementary blue light spectra which is a major contributor to photosynthesis as it has been reported to influence leaf expansion, stomata increase and opening as well as maximum and effective quantum yields of photosystem II photochemistry (Miao et al. 2015) which directly impacts crop yield. This report is in tandem with earlier reports of Matsuda et al. (2008) and Hogewoning et al. (2010) who asserted that a combination of BL and RL could promote plant biomass, photosynthesis and chlorophyll concentration of plants under similar light intensity. PH, LN, and LA were unaffected by RF transmittance. Plants under RF1 were unable to adjust their leaves due to the quality and intensity of light transmitted (Liu and Van lersel 2021) in addition to extreme cold weather. Similarity in growth pattern of plant under RL to control was previously corroborated by other scientists (Chidburee et al. 2017; Ezzaeri et al. 2018; Anusiya and Sivachandiran 2019; Liu et al. 2019; Hassanien et al. 2022).

SPAD value was unaffected by OPV-RF transmittance in greenhouse and field conditions

OPV and RF transmittance did not significantly affect SPAD value of spinach across environments which is consistent with the findings of Yang et al. (2019) who reported non-significant effect of shading on chlorophyll a and b content of three forage species; Schmierer et al. (2021) who reported comparable chlorophyll a and b contents of super dwarf rice for three shading levels in the greenhouse; Hassanien et al. (2022) who found non-significant increase in SPAD value of shaded chili pepper compared to control plants. Chlorophyll content of a leaf could be a very good determinant of photosynthetic efficiency and chloroplast development as they influence the ability of plants to harvest solar radiation for photosynthesis. The similarity in SPAD values of shaded and control plants could be an advantage to an APV farmer in that, they can produce crops and electricity from a limited space without affecting the chlorophyll content of the crop negatively. The non-significant increase in chlorophyll content of plants grown under OPV and RF is an acclimation to insufficient light which could be beneficial since the nutritional value of leafy vegetables tend to increase with larger chlorophyll content.

Genotype, genotype-OPV interaction and genotype-RF interaction affected growth and biomass of spinach

The genotypes expressed clear genotypic variation in growth and yield attributes at both environments. Bufflehead influenced significantly higher FSW, DSW, TBW, PH, LN and LA than eland genotype across both environments. Genotypes responded differently to OPV-transmittance in the greenhouse. Interaction of BP0 and BP1 were comparable in FSW, DSW, TBW, LMA and TED but were superior to BP2, EP0, EP1 and EP2 interactions, respectively. With an exception of LN where interaction of BRF1 and BRF0 recorded higher values than ERF0 and ERF1, all other measured traits did not significantly differ across all interacting factors in the field. The variation in growth and yield attributes of spinach as a consequence of genotype across both environments could be implicated on their preference for shade environments (Jiwuba et al. 2020). Bufflehead genotype which is shade loving showed statistical similarity for most growth and yield indices when grown under full irradiance and when grown under P1 which could suggest that P1 transmitted sufficient irradiance necessary for normal photosynthesis and other physiological processes of the plant in contrast to P2 which significantly reduced growth and yield attributes across genotypes. Conversely, Eland genotype which is shade shy showed reduced growth and yield attributes under shade. This report is consistent with Malaviya et al. (2020) who reported differential responses of Megathyrsus maximus genotypes to shading intensity.

Effect of OPV-RF transmittance on photosynthesis of spinach

The lower CO2 assimilation rates of P1, P2 and RF1 plants were due to reduced light effect of the materials. OPV and RF transmittance generally influenced higher Y(II) values in both environments. However ETR, Y(NPQ) and Y(NO) were lower than control. The higher photochemical efficiencies of OPV and RF treated plants and the lower non photochemical energy losses at the antenna Y(NPQ) and reaction centers Y(NO) could be implicated on the effect of the materials in reducing the incident radiation reaching the plants thereby minimizing the potential for photo-oxidative damage. P0 and RF0 plants were unable to effectively convert greater proportion of their excitation energy to photochemistry due to higher non-photochemical energy losses via the regulated Y(NPQ) and unregulated (Y(NO) pathways. Increased photosynthetic efficiencies and decreased non photochemical quenching in reduced light have been reported in other crop species (Gong et al. 2015; Wang et al. 2016; Song and Li 2016; Ravishankar et al. 2021; Schmierer et al. 2021). In addition, by reducing the incident radiation, the amount of absorbed light reaching the reaction centers was greatly reduced. The low Y(NO) values recorded relative to Y(NPQ) across all levels suggest that both photochemical energy conversion and protective regulatory mechanisms are efficient and that the plants efficiently managed the incident radiation (Busch et al. 2009).

Effect of OPV-RF transmittance on photo-irradiance response curves

The photo-irradiance response curves followed a similar trend in both OPV and RF plants. At increasing light intensities (> 400 PPFD) after saturation, CO2 assimilation rate was higher in P0 than P1 and P2. At similar light intensities, P1 and P2 converted higher proportion of their excitation energy to photochemistry as against P0 where substantial proportion was dissipated as heat at the antenna via the Y(NPQ) pathway (Fig. 2). P2 plants expressed the highest non photochemical energy losses at the reaction centers via the Y(NO) pathway and the least regulated energy losses to heat via the Y(NPQ) pathway, an expression of its inability to protect itself from excess light as a consequence of been grown under reduced light intensity. In contrast, P1 had low non photochemical energy losses at the reaction centers and comparable photo protective ability to P0 across both genotypes due to their prior acclimation to higher incident radiation in the growth environment which is in agreement with Ravishankar et al. (2021) and Schmierer et al. (2021) who reported higher photochemical efficiency, higher non-regulated energy losses to heat and fluorescence, and lower regulated non-photochemical energy losses to heat in plants acclimated to reduced light prior to commencement of rapid light curves.

For field plants, RF1 did not vary much from RF0 (Fig. 3). With the exception of CO2 assimilation rate which was slightly higher in RF0, effective photochemical yield, yield of non-photochemical energy losses to heat and fluorescence at the reaction centers, and yield of regulated non-photochemical energy losses at the antenna were comparable in bufflehead genotype. Variation however, was more pronounced in eland genotype. In general, plants grown under high OPV or RF transmittance handled excess light more efficiently than those grown under low OPV transmittance. This finding corroborates Schmierer et al. (2021) who reported that on exposure to strong light, plants grown under the highest light intensities handled excessive light energy more efficiently.

Conclusions

The transmittance properties of PV materials installed over plants in an APV system is fundamental as it influences both light quality and intensity. Light quality and intensity can affect physiological and yield processes of plants. In this study, we confirmed significant reduction in shoot weight and total biomass of spinach grown under very low light intensities as a function of the transmittance properties of the OPV cell used (P2). Interestingly, P1 with high transmittance properties and good balance of blue and red light was comparable to control in most of the growth and yield traits measured which confirms our hypothesis. In addition, it influenced better shoot to root distribution than control. In general, greenhouse plants outperformed field plants due to the improved micro-climate environment of the greenhouse relative to the field.

Light quality in terms of spectra or colors of light that a plant is exposed to can also influence performance. This was confirmed in this study as RF reduced shoot and total biomass production of spinach in the field and could be implicated on its inability to transmit other complementary light spectra aside RL. OPV-RF transmittance did not affect plant height (PH), leaf number (LN), and SPAD value but leaf area (LA) was highest in P2. Photochemical energy conversion was higher in P1, P2 and RF1 in contrast to control due to lower levels of non-photochemical energy losses through the Y(NO) and Y(NPQ) pathways.

Photo-irradiance curves showed that plants grown under reduced light (P2) did not efficiently manage excess light when exposed to high light intensities. Bufflehead genotype showed superior growth and yield traits than eland across OPV-RF levels.

Conclusively, agrivoltaics is an emerging technology that could potentially provide the key to the food and energy challenges in the tropics without compromising crop yield. This study is also a confirmation that photovoltaics can successfully be integrated with crop production without significant reduction in yield traits. P1 installed over spinach plants in the greenhouse competed favorably with the control plants in fresh and dry shoot weight, fresh and dry biomass weight and leaf mass per area. However, decrease in shoot and biomass yield was observed with RF1 plants in the field, and also with P2 plants in the greenhouse principally as a result of insufficient light. It is therefore important to strike the right balance in terms of using the right PV materials, transmittance, distance from the plant, installation angle etc., to resolve some of the challenges at least in the short term. Perhaps with further experimentations, the ideal mix with respect to the panel type, transmittance, installation angle and height for enhanced efficiency of the system will be obtained. On the basis of these findings, it is recommended that OPV cells with transmittance properties greater than or equal to 11% in BL and 64% in RL be used in APV systems for improved efficiency.