Enhancement of crop photosynthesis by diffuse light: quantifying the contributing factors

Radiative cooling and indoor light management enabled by a transparent and self-cleaning polymer-based metamaterial

Transparent roofs and walls offer a compelling solution for harnessing natural light. However, traditional glass roofs and walls face challenges such as glare, privacy concerns, and overheating issues. In this study, we present a polymer-based micro-photonic multi-functional metamaterial. The metamaterial diffuses 73% of incident sunlight, creating a more comfortable and private indoor environment. The visible spectral transmittance of the metamaterial (95%) surpasses that of traditional glass (91%). Furthermore, the metamaterial is estimated to enhance photosynthesis efficiency by ~9% compared to glass roofs. With a high emissivity (~0.98) close to that of a mid-infrared black body, the metamaterial is estimated to have a cooling capacity of ~97 W/m2 at ambient temperature. The metamaterial was about 6 °C cooler than the ambient temperature in humid Karlsruhe. The metamaterial exhibits superhydrophobic performance with a contact angle of 152°, significantly higher than that of glass (26°), thus potentially having excellent self-cleaning properties.

Air quality improvements can strengthen China's food security

Air pollution exerts crucial influence on crop yields and impacts regional and global food supplies. Here we employ a statistical model using satellite-based observations and flexible functional forms to analyse the synergistic effects of reductions in ozone and aerosols on China's food security. The model consistently shows that ozone is detrimental to crops, whereas aerosol has variable effects. China's maize, rice and wheat yields are projected to increase by 7.84%, 4.10% and 3.43%, respectively, upon reaching two air quality targets (60 μg m-3 for peak-season ozone and 35 μg m-3 for annual fine particulate matter). Average calories produced from these crops would surge by 4.51%, potentially allowing China to attain grain self-sufficiency 2 years earlier than previously estimated. These results show that ozone pollution control should be a high priority to increase staple crop edible calories, and future stringent air pollution regulations would enhance China's food security.

Campomanesia xanthocarpa (Myrtaceae: Myrtoideae) seedlings reveal morpho-physiological plasticity under shade conditions

Plants exposed to different light intensities generate physiological, morphological, and anatomical changes conducting to plasticity. Thus, this characteristic establishes the ability of plants to present phenotypic adjustments by the same genotype under different environmental conditions. The objective of this study was to verify the morphophysiological alterations in Campomanesia xanthocarpa (Mart.) O. Berg (guabiroba) seedlings cultivated in different shading levels. The seedlings were grown for 21 months under full sun or 30%, 50%, and 80% under shading. Growth, photosynthetic pigments, gas exchange rate, chlorophyll fluorescence, and leaf anatomy were evaluated. In all the treatments subjected to shading, plasticity mechanisms involved structural and physiological changes such as an increase in leaf area and chlorophyll content (total and Chl a), reduction in leaf thickness, and increased gas exchange and quantum yield of photosystem II. The guabiroba seedlings can be cultivated in full sun or different shading environments; even under high shading intensity (80%), the plants showed vigor similar to those produced in a sunny environment. These results confirmed our hypothesis about guabiroba acclimation capacity to shading, noteworthy information for nurseries, orchards, agroforestry systems, or forest restoration in a wide range of light environments.

Leaf shape, planting density, and nitrogen application affect soybean yield by changing direct and diffuse light distribution in the canopy

When attempting to maximize the crop yield from field-grown soybean (Glycine max (L.) Merr.) by means of improving the light conditions for photosynthesis in the canopy, it is crucial to find the optimal planting density and nitrogen application rate. The soybean plants that were the subject of our experiment were cultivated in N-dense mutual pairs, and included two cultivars with different leaf shapes; one cultivar sported ovate leaves (O-type) and the other lanceolate leaves (L-type). We analyzed the results quantitatively to determine the amount of spatial variation in light distribution and photosynthetic efficiency across the canopy, and to gauge the effect of the experimental parameters on the yield as well as the photosynthetic light and nitrogen use efficiency of the crop. Results indicate that the different leaf shapes were responsible for significant disparities between the photosynthetic utilization of direct and diffuse light. As the nitrogen fertilizer rate and the planting density increased, the soybean plants responded by adjusting leaf morphology in order to maximize the canopy apparent photosynthetic light use efficiency, which in turn affected the leaf nitrogen distribution in the canopy. Despite the fact that the light interception rate of the canopy of the L-type cultivar was lower than that of the canopy of the O-type cultivar, we found its canopy apparent photosynthetic nitrogen and light use efficiency were higher. It was interesting to note, however, that the nitrogen and light use efficiency contributions associated with exposure to diffuse light were greater for the latter than for the former.

Evaluation of the enhancement of photosynthetic rate in a komatsuna (Brassica rapa L. var. perviridis) canopy with upward lighting using an optical simulation in a plant factory with artificial light

In a plant factory with artificial light (PFAL), upward lighting is expected to prevent senescence and decrease in the photosynthetic capacity of the lower leaves in the canopy. Upward lighting may also increase the photosynthetic rate of a canopy by improving its photosynthetic photon flux density (PPFD) distribution. However, the net photosynthetic rate (Pn) of leaves is lower when the abaxial surface is irradiated than that when the adaxial surface is irradiated. The aim of this study was to estimate the PPFD in a PFAL and the Pn of plants using three-dimensional plant models and optical simulation. First, we measured the Pn of komatsuna (Brassica rapa L. var. perviridis) leaves under different conditions of the proportion (p_ad ) of PPFD on the adaxial surface to total PPFD on both surfaces and developed an equation for the light response curve of photosynthesis considering p_ad . When PPFD was low, except when it was 30 and 70 µmol m^-2 s^-1, Pn increased as p_ad increased, because the absorptance also increased with p_ad . Under high PPFD conditions, Pn was maximized at 67-83% of p_ad because the light would be distributed more efficiently for photosynthesis. Next, using optical simulation and the developed equation, we estimated the photosynthetic rate of a komatsuna canopy (CPn) under downward and upward lighting. The CPn increased by 1.08-1.13 times by combining downward and upward lighting due to the increase in the photosynthetic photon flux (PPF) of light incident on the canopy and the decrease in the spatial variation of PPFD on the leaves in the canopy. As the depreciation of lamps for upward lighting accounts for 7.5-9.0% of the production cost in a PFAL, even if the depreciation of lamps for upward lighting increased, enhancement of CPn by upward lighting would be cost-effective. We performed optical simulations under 220 conditions and evaluated them using CPn as an index. Moreover, we provided the proportion of PPF of upward lighting that improved CPn and discussed the reason for this improvement. The result shows that optical simulation is useful for evaluating the lighting design in a PFAL and analyzing the effects of the lighting design on the light environment and photosynthesis.

A Novel LED Light Radiation Approach Enhances Growth in Green and Albino Tea Varieties

Light, as an energy source, has been proven to strongly affect photosynthesis and, thus, can regulate the yield and quality of tea leaves (Camellia sinensis L.). However, few comprehensive studies have investigated the synergistic effects of light wavelengths on tea growth and development in green and albino varieties. Thus, the objective of this study was to investigate different ratios of red, blue and yellow light and their effects on tea plants' growth and quality. In this study, Zhongcha108 (green variety) and Zhongbai4 (albino variety) were exposed to lights of different wavelengths for a photoperiod of 5 months under the following seven treatments: white light simulated from the solar spectrum, which served as the control, and L1 (red 75%, blue 15% and yellow 10%), L2 (red 60%, blue 30% and yellow 10%), L3 (red 45%, far-red light 15%, blue 30% and yellow 10%), L4 (red 55%, blue 25% and yellow 20%), L5 (red 45%, blue 45% and yellow 10%) and L6 (red 30%, blue 60% and yellow 10%), respectively. We examined how different ratios of red light, blue light and yellow light affected tea growth by investigating the photosynthesis response curve, chlorophyll content, leaf structure, growth parameters and quality. Our results showed that far-red light interacted with red, blue and yellow light (L3 treatments) and significantly promoted leaf photosynthesis by 48.51% in the green variety, Zhongcha108, compared with the control treatments, and the length of the new shoots, number of new leaves, internode length, new leaf area, new shoots biomass and leaf thickness increased by 70.43%, 32.64%, 25.97%, 15.61%, 76.39% and 13.30%, respectively. Additionally, the polyphenol in the green variety, Zhongcha108, was significantly increased by 15.6% compared to that of the plants subjected to the control treatment. In addition, for the albino variety Zhongbai4, the highest ratio of red light (L1 treatment) remarkably enhanced leaf photosynthesis by 50.48% compared with the plants under the control treatment, resulting in the greatest new shoot length, number of new leaves, internode length, new leaf area, new shoot biomass, leaf thickness and polyphenol in the albino variety, Zhongbai4, compared to those of the control treatments, which increased by 50.48%, 26.11%, 69.29%, 31.61%, 42.86% and 10.09%, respectively. Our study provided these new light modes to serve as a new agricultural method for the production of green and albino varieties.

Consequences of intra-canopy and top LED lighting for uniformity of light distribution in a tomato crop

In the past decade, the potential of positioning LED lamps in between the canopy (intra-canopy) to enhance crop growth and yield has been explored in greenhouse cultivation. Changes in spatial heterogeneity of light absorption that come with the introduction of intra-canopy lighting have not been thoroughly explored. We calibrated and validated an existing functional structural plant model (FSPM), which combines plant morphology with a ray tracing model to estimate light absorption at leaflet level. This FSPM was used to visualize the light environment in a tomato crop illuminated with intra-canopy lighting, top lighting or a combination of both. Model validation of light absorption of individual leaves showed a good fit (R² = 0.93) between measured and modelled light absorption of the canopy. Canopy light distribution was then quantified and visualized in three voxel directions by means of average absorbed photosynthetic photon flux density (PPFD) and coefficient of variation (CV) within that voxel. Simulations showed that the variation coefficient within horizontal direction was higher for intra-canopy lighting than top lighting (CV=48% versus CV= 43%), while the combination of intra-canopy lighting and top lighting yielded the lowest CV (37%). Combined intra-canopy and top lighting (50/50%) had in all directions a more uniform light absorption than intra-canopy or top lighting alone. The variation was minimal when the ratio of PPFD from intra-canopy to top lighting was about 1, and increased when this ratio increased or decreased. Intra-canopy lighting resulted in 8% higher total light absorption than top lighting, while combining 50% intra-canopy lighting with 50% top lighting, increased light absorption by 4%. Variation in light distribution was further reduced when the intra-canopy LEDs were distributed over strings at four instead of two heights. When positioning LED lamps to illuminate a canopy both total light absorption and light distribution have to be considered.

Novel Approaches for Sustainable Horticultural Crop Production: Advances and Prospects

Reduction of plant growth, yield and quality due to diverse environmental constrains along with climate change significantly limit the sustainable production of horticultural crops. In this review, we highlight the prospective impacts that are positive challenges for the application of beneficial microbial endophytes, nanomaterials (NMs), exogenous phytohormones strigolactones (SLs) and new breeding techniques (CRISPR), as well as controlled environment horticulture (CEH) using artificial light in sustainable production of horticultural crops. The benefits of such applications are often evaluated by measuring their impact on the metabolic, morphological and biochemical parameters of a variety of cultures, which typically results in higher yields with efficient use of resources when applied in greenhouse or field conditions. Endophytic microbes that promote plant growth play a key role in the adapting of plants to habitat, thereby improving their yield and prolonging their protection from biotic and abiotic stresses. Focusing on quality control, we considered the effects of the applications of microbial endophytes, a novel class of phytohormones SLs, as well as NMs and CEH using artificial light on horticultural commodities. In addition, the genomic editing of plants using CRISPR, including its role in modulating gene expression/transcription factors in improving crop production and tolerance, was also reviewed.

Fillable and unfillable gaps in plant transcriptome under field and controlled environments

The differences between plants grown in field and in controlled environments have long been recognized. However, few studies have addressed the underlying molecular mechanisms. To evaluate plant responses to fluctuating environments using laboratory equipment, we developed SmartGC, a high-performance growth chamber that reproduces the fluctuating irradiance, temperature and humidity of field environments. We analysed massive transcriptome data of rice plants grown under field and SmartGC conditions to clarify the differences in plant responses to field and controlled environments. Rice transcriptome dynamics in SmartGC mimicked those in the field, particularly during the morning and evening but those in conventional growth chamber conditions did not. Further analysis revealed that fluctuation of irradiance affects transcriptome dynamics in the morning and evening, while fluctuation of temperature affects transcriptome dynamics only in the morning. We found upregulation of genes related to biotic and abiotic stress, and their expression was affected by environmental factors that cannot be mimicked by SmartGC. Our results reveal fillable and unfillable gaps in the transcriptomes of rice grown in field and controlled environments and can accelerate the understanding of plant responses to field environments for both basic biology and agricultural applications.

Optimization of Photosynthetic Photon Flux Density and Light Quality for Increasing Radiation-Use Efficiency in Dwarf Tomato under LED Light at the Vegetative Growth Stage

Dwarf tomatoes are advantageous when cultivated in a plant factory with artificial light because they can grow well in a small volume. However, few studies have been reported on cultivation in a controlled environment for improving productivity. We performed two experiments to investigate the effects of photosynthetic photon flux density (PPFD; 300, 500, and 700 μmol m⁻² s⁻¹) with white light and light quality (white, R3B1 (red:blue = 3:1), and R9B1) with a PPFD of 300 μmol m⁻² s⁻¹ on plant growth and radiation-use efficiency (RUE) of a dwarf tomato cultivar (‘Micro-Tom’) at the vegetative growth stage. The results clearly demonstrated that higher PPFD leads to higher dry mass and lower specific leaf area, but it does not affect the stem length. Furthermore, high PPFD increased the photosynthetic rate (Pn) of individual leaves but decreased RUE. A higher blue light proportion inhibited dry mass production with the same intercepted light because the leaves under high blue light proportion had low Pn and photosynthetic light-use efficiency. In conclusion, 300 μmol m⁻² s⁻¹ PPFD and R9B1 are the recommended proper PPFD and light quality, respectively, for ‘Micro-Tom’ cultivation at the vegetative growth stage to increase the RUE.

Acclimating Cucumber Plants to Blue Supplemental Light Promotes Growth in Full Sunlight

Raising young plants is important for modern greenhouse production. Upon transfer from the raising to the production environment, young plants should maximize light use efficiency while minimizing deleterious effects associated with exposure to high light (HL) intensity. The light spectrum may be used to establish desired traits, but how plants acclimated to a given spectrum respond to HL intensity exposure is less well explored. Cucumber (Cucumis sativus) seedlings were grown in a greenhouse in low-intensity sunlight (control; ∼2.7 mol photons m^-2 day^-1) and were treated with white, red, blue, or green supplemental light (4.3 mol photons m^-2 day^-1) for 10 days. Photosynthetic capacity was highest in leaves treated with blue light, followed by white, red, and green, and was positively correlated with leaf thickness, nitrogen, and chlorophyll concentration. Acclimation to different spectra did not affect the rate of photosynthetic induction, but leaves grown under blue light showed faster induction and relaxation of non-photochemical quenching (NPQ) under alternating HL and LL intensity. Blue-light-acclimated leaves showed reduced photoinhibition after HL intensity exposure, as indicated by a high maximum quantum yield of photosystem II photochemistry (F _v /F _m ). Although plants grown under different supplemental light spectra for 10 days had similar shoot biomass, blue-light-grown plants (B-grown plants) showed a more compact morphology with smaller leaf areas and shorter stems. However, after subsequent, week-long exposure to full sunlight (10.7 mol photons m^-2 day^-1), B-grown plants showed similar leaf area and 15% higher shoot biomass, compared to plants that had been acclimated to other spectra. The faster growth rate in blue-light-acclimated plants compared to other plants was mainly due to a higher photosynthetic capacity and highly regulated NPQ performance under intermittent high solar light. Acclimation to blue supplemental light can improve light use efficiency and diminish photoinhibition under high solar light exposure, which can benefit plant growth.

Environmental factors modulate the diffuse fertilization effect on gross primary productivity across Chinese ecosystems

Diffuse radiation allocated by cloud cover and aerosols can promote vegetation photosynthesis, which is known as the diffuse fertilization effect (DFE). As an important uncertain factor regulating the DFE, understanding the role of environmental conditions in the response of terrestrial ecosystems to diffuse radiation is vital for quantitative and intensive studies. By using a light use efficiency model and statistical methods with satellite data and ChinaFLUX observation data, the optimal environmental range of DFE was estimated, the indirect role of vapor pressure deficit (VPD) and air temperature (T_a) on DFE was explored, and the relative contribution of diffuse photosynthetically active radiation (PAR_dif) on gross primary productivity (GPP) was analyzed across Chinese ecosystems under different sky conditions. The results showed that the DFE increased with leaf area index (LAI), but distributed a unimodal curve along with VPD and T_a, both of which had an optimum range that was lower in the forest (or cropland) and higher in the grass (or desert) ecosystem. When considering the co-effect of VPD and T_a, the strongest positive effect of DFE was found at 0-5 h Pa and 20-25 °C. Based on path analysis, PAR_dif promoted GPP and served as the main controlling factor in forest ecosystems predominantly through a direct pathway from half-hourly to the daily scale, while T_a and VPD occupied the dominant position at single-canopy ecosystem sites. When the aerosol optical depth (AOD) increased, the relative contribution of PAR_dif increased in multiple-canopy ecosystems and decreased in single-canopy ecosystems; when the sky conditions changed from sunny to cloudy, the relative contribution of PAR_dif was higher in the forest ecosystem and increased significantly in the grass ecosystem. These findings offer a more comprehensive understanding of the environmental effects of regulating DFE on GPP across ecosystems.

Different responses of two Chinese cabbage (Brassica rapa L. ssp. pekinensis) cultivars in photosynthetic characteristics and chloroplast ultrastructure to salt and alkali stress

Salt and alkali stress affected the photosynthetic characteristics of Chinese cabbages. A salt-tolerant cultivar maintained its tolerance by ensuring the high ability of photosynthesis. The synthesis of organic acids and carbohydrates in leaves played important roles in improving the photosynthetic capacity of alkali-tolerant plants. Soil salinization has become an increasingly serious ecological problem, which limits the quality and yield of crops. As an important economic vegetable in winter, however, little is known about the response of Chinese cabbage to salt, alkali and salt-alkali stress in photosynthetic characteristics and chloroplast ultrastructure. Thus, two Chinese cabbage cultivars, 'Qinghua' (salt-tolerant-alkali-sensitive) and 'Biyu' (salt-sensitive-alkali-tolerant) were investigated under stresses to clarify the similarities and differences between salt tolerance and alkali tolerance pathways in Chinese cabbage. We found that the root of Qinghua, the leaf ultrastructure and net photosynthetic rate (Pn), stomatal conductance (Gs), water use efficiency (WUE), maximum photochemical quantum yield of PSII (Fv/Fm) and nonphotochemical quenching (NPQ) were not affected by salt stress. However, Biyu was seriously affected under salt stress. Its growth indexes decreased by between 60 and 30% compared with the control and the photosynthetic indexes were also seriously affected under salt stress. This indicated that the salt-tolerant cultivar Qinghua improved the photosynthetic fluorescence ability to promote the synthesis of organic matter resulting in salt tolerance. In contrast, under alkali treatment, the root of Biyu was affected by alkali stress, but could still maintain good growth, and root and leaf structure were not seriously affected and could maintain the normal operations. Biyu improved its tolerance by improving the water use efficiency, regulating the synthesis of organic acids and carbohydrates, ensuring the synthesis of organic matter and ensured the normal growth of the plant.

Atmospheric opacity has a nonlinear effect on global crop yields

Agricultural impacts of air pollution, climate change and geoengineering remain uncertain due to potentially offsetting changes in the quantity and quality of sunlight. By leveraging year-to-year variation in growing-season cloud optical thickness, I provide nonlinear empirical estimates of how increased atmospheric opacity alters sunlight across the Earth's surface and how this affects maize and soy yields in the United States, Europe, Brazil and China. I find that the response of yields to changes in sunlight from cloud scattering and absorption is consistently concave across crops and regions. An additional day of optimal cloud cover, relative to a clear-sky day, increases maize and soy yields by 0.4%. Changes in sunlight due to changes in clouds have decreased the global average maize and soy yields by 1% and 0.1% due to air pollution and may further decrease yields by 1.8% and 0.4% due to climate change.

The impact of global dimming on crop yields is determined by the source-sink imbalance of carbon during grain filling

Global dimming reduces incident global radiation but increases the fraction of diffuse radiation, and thus affects crop yields; however, the underlying mechanisms of such an effect have not been revealed. We hypothesized that crop source-sink imbalance of either carbon (C) or nitrogen (N) during grain filling is a key factor underlying the effect of global dimming on yields. We presented a practical framework to assess both C and N source-sink relationships, using data of biomass and N accumulation from periodical sampling conducted in field experiments for wheat and rice from 2013 to 2016. We found a fertilization effect of the increased diffuse radiation fraction under global dimming, which alleviated the negative impact of decreased global radiation on source supply and sink growth, but the source supply and sink growth were still decreased by dimming, for both C and N. In wheat, the C source supply decreased more than the C sink demand, and as a result, crops remobilized more pre-heading C reserves, in response to dimming. However, these responses were converse in rice, which presumably stemmed from the more increment in radiation use efficiency and the more limited sink size in rice than wheat. The global dimming affected source supply and sink growth of C more significantly than that of N. Therefore, yields in both crops were dependent more on the source-sink imbalance of C than that of N during grain filling. Our revealed source-sink relationships, and their differences and similarities between wheat and rice, provide a basis for designing strategies to alleviate the impact of global dimming on crop productivity.

Assessing diversity in canopy architecture, photosynthesis, and water-use efficiency in a cowpea magic population

Optimizing crops to improve light absorption and CO2 assimilation throughout the canopy is a proposed strategy to increase yield and meet the needs of a growing population by 2050. Globally, the greatest population increase is expected to occur in Sub-Saharan Africa where large yield gaps currently persist; therefore, it is crucial to develop high-yielding crops adapted to this region. In this study, we screened 50 cowpea (Vigna unguiculata (L.) Walp) genotypes from the multi-parent advanced generation inter-cross (MAGIC) population for canopy architectural traits, canopy photosynthesis, and water-use efficiency using a canopy gas exchange chamber in order to improve our understanding of the relationships among those traits. Canopy architecture contributed to 38.6% of the variance observed in canopy photosynthesis. The results suggest that the light environment within the canopy was a limiting factor for canopy CO2 assimilation. Traits favoring greater exposure of leaf area to light such as the width of the canopy relative to the total leaf area were associated with greater canopy photosynthesis, especially in canopies with high biomass. Canopy water-use efficiency was highly determined by canopy photosynthetic activity and therefore canopy architecture, which indicates that optimizing the canopy will also contribute to improving canopy water-use efficiency. We discuss different breeding strategies for future programs aimed at the improvement of cowpea yield for the Sub-Saharan African region. We show that breeding for high biomass will not optimize canopy CO2 assimilation and suggest that selection should include multiple canopy traits to improve light penetration.

Canopy photosynthesis of six major arable crops is enhanced under diffuse light due to canopy architecture

Diffuse radiation generally increases photosynthetic rates if total radiation is kept constant. Different hypotheses have been proposed to explain this enhancement of photosynthesis, but conclusive results over a wide range of diffuse conditions or about the effect of canopy architecture are lacking. Here, we show the response of canopy photosynthesis to different fractions of diffuse light conditions for five major arable crops (pea, potato, wheat, barley, rapeseed) and cover crops characterized by different canopy architecture. We used 13 years of flux and microclimate measurements over a field with a typical 4 year crop rotation scheme in Switzerland. We investigated the effect of diffuse light on photosynthesis over a gradient of diffuse light fractions ranging from 100% diffuse (overcast sky) to 11% diffuse light (clear-sky conditions). Gross primary productivity (GPP) increased with diffuse fraction and thus was greater under diffuse than direct light conditions if the absolute photon flux density per unit surface area was kept constant. Mean leaf tilt angle (MTA) and canopy height were found to be the best predictors of the diffuse versus direct radiation effect on photosynthesis. Climatic factors, such as the drought index and growing degree days (GDD), had a significant influence on initial quantum yield under direct but not diffuse light conditions, which depended primarily on MTA. The maximum photosynthetic rate at 2,000 µmol m^-2  s^-1 photosynthetically active radiation under direct conditions strongly depended on GDD, MTA, leaf area index (LAI) and the interaction between MTA and LAI, while under diffuse conditions, this parameter depended mostly on MTA and only to a minor extent on canopy height and their interaction. The strongest photosynthesis enhancement under diffuse light was found for wheat, barley and rapeseed, whereas the lowest was for pea. Thus, we suggest that measuring canopy architecture and diffuse radiation will greatly improve GPP estimates of global cropping systems.

The Effect of Low-Haze Diffuse Glass on Greenhouse Tomato and Bell Pepper Production and Light Distribution Properties

Diffuse greenhouse glass can increase the production and growth of several crops, by scattering the incoming direct sunlight, which results in a better and more homogeneous light distribution in the crop canopy. Tomato and bell pepper growers in Belgium tend to install low-haze diffuse glass with a double anti-reflection (AR) coating. These glass types have a limited diffuse effect but have a higher light transmission compared to standard float glass. Therefore, tomato growers often increase stem density to maximize light interception. However, a denser crop could counteract the positive effects of diffuse glass on the vertical light distribution. In this study, the effect of low-haze diffuse glass with an AR coating was evaluated for different cropping densities for tomato and bell pepper taking into account the vertical light distribution throughout the crop canopy. Tomato plants with two stem densities (3.33 and 3.75 stems.m⁻²) and bell pepper plants (with only one stem density of 7.1 stems.m⁻²) were evaluated in a greenhouse compartment with diffuse and reference float glass during a full growing season. For tomato, a significant production increase of 7.5% was observed under diffuse glass during the second half of the growing season but only for the low stem density. The benefit of diffuse glass appears most relevant during sunny clear skies and on the sun-side-facing rows of the crop. For bell pepper, no significant production increases were noted between regular float or diffuse glass, because a bell pepper crop is typically covered with thermal screens to prevent sunburn on the fruits during sunny days. The vertical light distribution and the usefulness of AR-coated diffuse glass depends on the crop type and should be optimized accordingly by altering the stem density, leaf pruning strategy, row orientation, or crop variety.

Substantial differences occur between canopy and ambient climate: Quantification of interactions in a greenhouse-canopy system

Organ temperature and variation therein plays a key role in plant functioning and its responses to e.g. climate change. There is a strong feedback between organ, especially leaf, temperature and the climate within the canopy (canopy climate), which in turn interacts with the climate outside the canopy (ambient climate). For greenhouses, the determinants of this interplay and how they drive differences between canopy and ambient climate are poorly understood. Yet, as many experiments on both regular greenhouse crops and field crops are done in greenhouses, this is crucial to know. Therefore, we designed an experiment to quantify the differences between ambient and canopy climate and leaf temperature. A path analysis was performed to quantify the interactions between components of the greenhouse canopy-climate system. We found that with high radiation the canopy climate can be up to 5°C cooler than the ambient climate, while for cloudy days this was only 2°C. Canopy relative humidity (RH) was up to 25% higher compared to ambient RH. We showed that radiation is very important for these climate differences, but that this effect could be partly counteracted by turning off supplementary light (i.e. due to its indirect effects e.g. changing light distribution). Leaf temperature was substantially different, both higher and lower, from the canopy air temperature. This difference was determined by leaf area index (LAI), temperature of the heating pipe and the use of supplementary light, which all strongly influence radiation, either shortwave or thermal radiation. The difference between leaf and ambient air temperature could be decreased by decreasing the LAI or increasing the temperature of the heating pipe.

Photosynthetic characteristics and chloroplast ultrastructure of welsh onion (Allium fistulosum L.) grown under different LED wavelengths

BACKGROUND

The optimized illumination of plants using light-emitting diodes (LEDs) is beneficial to their photosynthetic performance, and in recent years, LEDs have been widely used in horticultural facilities. However, there are significant differences in the responses of different crops to different wavelengths of light. Thus, the influence of artificial light on photosynthesis requires further investigation to provide theoretical guidelines for the light environments used in industrial crop production. In this study, we tested the effects of different LEDs (white, W; blue, B; green, G; yellow, Y; and red, R) with the same photon flux density (300 μmol/m²·s) on the growth, development, photosynthesis, chlorophyll fluorescence characteristics, leaf structure, and chloroplast ultrastructure of Welsh onion (Allium fistulosum L.) plants.

RESULTS

Plants in the W and B treatments had significantly higher height, leaf area, and fresh weight than those in the other treatments. The photosynthetic pigment content and net photosynthetic rate (P_n) in the W treatment were significantly higher than those in the monochromatic light treatments, the transpiration rate (E) and stomatal conductance (G_s) were the highest in the B treatment, and the intercellular CO₂ concentration (C_i) was the highest in the Y treatment. The non-photochemical quenching coefficient (NPQ) was the highest in the Y treatment, but the other chlorophyll fluorescence characteristics differed among treatments in the following order: W > B > R > G > Y. This includes the maximum photochemical efficiency of photosystem II (PSII) under dark adaptation (Fv/Fm), maximum photochemical efficiency of PSII under light adaptation (Fv'/Fm'), photochemical quenching coefficient (qP), actual photochemical efficiency (ΦPSII), and apparent electron transport rate (ETR). Finally, the leaf structure and chloroplast ultrastructure showed the most complete development in the B treatment.

CONCLUSIONS

White and blue light significantly improved the photosynthetic efficiency of Welsh onions, whereas yellow light reduced the photosynthetic efficiency.

Adding Far-Red to Red-Blue Light-Emitting Diode Light Promotes Yield of Lettuce at Different Planting Densities

The economic viability and energy use of vertical farms strongly depend on the efficiency of the use of light. Increasing far-red radiation (FR, 700-800 nm) relative to photosynthetically active radiation (PAR, 400-700 nm) may induce shade avoidance responses including stem elongation and leaf expansion, which would benefit light interception, and FR might even be photosynthetically active when used in combination with PAR. The aims of this study are to investigate the interaction between FR and planting density and to quantify the underlying components of the FR effects on growth. Lettuce (Lactuca sativa cv. Expertise RZ) was grown in a climate chamber under two FR treatments (0 or 52 μmol m^-2 s^-1) and three planting densities (23, 37, and 51 plants m^-2). PAR of 89% red and 11% blue was kept at 218 μmol m^-2 s^-1. Adding FR increased plant dry weight after 4 weeks by 46-77% (largest effect at lowest planting density) and leaf area by 58-75% (largest effect at middle planting density). Radiation use efficiency (RUE: plant dry weight per unit of incident radiation, 400-800 nm) increased by 17-42% and incident light use efficiency (LUE_inc: plant dry weight per unit of incident PAR, 400-700 nm) increased by 46-77% by adding FR; the largest FR effects were observed at the lowest planting density. Intercepted light use efficiency (LUE_int: plant dry weight per unit of intercepted PAR) increased by adding FR (8-23%). Neither specific leaf area nor net leaf photosynthetic rate was influenced by FR. We conclude that supplemental FR increased plant biomass production mainly by faster leaf area expansion, which increased light interception. The effects of FR on plant dry weight are stronger at low than at high planting density. Additionally, an increased LUE_int may contribute to the increased biomass production.

Molecular Evolution and Interaction of Membrane Transport and Photoreception in Plants

Light is a vital regulator that controls physiological and cellular responses to regulate plant growth, development, yield, and quality. Light is the driving force for electron and ion transport in the thylakoid membrane and other membranes of plant cells. In different plant species and cell types, light activates photoreceptors, thereby modulating plasma membrane transport. Plants maximize their growth and photosynthesis by facilitating the coordinated regulation of ion channels, pumps, and co-transporters across membranes to fine-tune nutrient uptake. The signal-transducing functions associated with membrane transporters, pumps, and channels impart a complex array of mechanisms to regulate plant responses to light. The identification of light responsive membrane transport components and understanding of their potential interaction with photoreceptors will elucidate how light-activated signaling pathways optimize plant growth, production, and nutrition to the prevailing environmental changes. This review summarizes the mechanisms underlying the physiological and molecular regulations of light-induced membrane transport and their potential interaction with photoreceptors in a plant evolutionary and nutrition context. It will shed new light on plant ecological conservation as well as agricultural production and crop quality, bringing potential nutrition and health benefits to humans and animals.

Time course of the photochemical reflectance index during photosynthetic induction: its relationship with the photochemical yield of photosystem II

Time courses of photochemical reflectance index (PRI) of an attached cucumber leaf during a dark-light transition were compared with those of photochemical yields of photosystem II (Y_II ) to discuss the feasibility of PRI imaging for estimating the efficiency of photosynthetic light use. Y_II and PRI were simultaneously evaluated with a pulse-amplitude modulation chlorophyll fluorometer and a low-cost imaging system consisting of digital cameras and band-pass filters, respectively. Y_II decreased immediately after the transition and then increased under various photon flux densities. Although PRI exhibited delayed time courses with respect to Y_II under low light conditions, PRI decreased monotonically under high light conditions. There was no correlation between Y_II and the changes in PRI (ΔPRI) immediately after the transition but Y_II was correlated with ΔPRI under the steady-state photosynthesis. These results indicate that the use of PRI to estimate Y_II under fluctuating light based on the regression obtained at steady state can overestimate Y_II . The imaging system was also applied to evaluate the spatial PRI distribution within a leaf. While PRI of leaf areas that remained untreated, or had been treated with H₂ O again, first dropped and then rose under low light and monotonically decreased under high light conditions, leaf areas treated with inhibitor (dichlorophenyl dimethylurea) did not exhibit any changes. It is likely that the inhibitor suppressed lumen acidification, which triggers a decrease in PRI. It was suggested that Y_II of leaves with malfunctions in the photosynthetic electron transport can be overestimated by the PRI-based estimation.

Ozone-climate interactions and effects on solar ultraviolet radiation

This report assesses the effects of stratospheric ozone depletion and anticipated ozone recovery on the intensity of ultraviolet (UV) radiation at the Earth's surface. Interactions between changes in ozone and changes in climate, as well as their effects on UV radiation, are also considered. These evaluations focus mainly on new knowledge gained from research conducted during the last four years. Furthermore, drivers of changes in UV radiation other than ozone are discussed and their relative importance is assessed. The most important of these factors, namely clouds, aerosols and surface reflectivity, are related to changes in climate, and some of their effects on short- and long-term variations of UV radiation have already been identified from measurements. Finally, projected future developments in stratospheric ozone, climate, and other factors affecting UV radiation have been used to estimate changes in solar UV radiation from the present to the end of the 21^st century. New instruments and methods have been assessed with respect to their ability to provide useful and accurate information for monitoring solar UV radiation at the Earth's surface and for determining relevant exposures of humans. Evidence since the last assessment reconfirms that systematic and accurate long-term measurements of UV radiation and stratospheric ozone are essential for assessing the effectiveness of the Montreal Protocol and its Amendments and adjustments. Finally, we have assessed aspects of UV radiation related to biological effects and human health, as well as implications for UV radiation from possible solar radiation management (geoengineering) methods to mitigate climate change.

Diffuse light affects the contents of vitamin C, phenolic compounds and free amino acids in lettuce plants

Enhancement of the diffuse solar radiation to which lettuce plants were exposed clearly affected the vitamin C content and the quantitative and qualitative patterns of phenolic compounds and free amino acids (AA) in the leaves. Although the enhanced level of diffuse light was detrimental to the contents of vitamin C and total phenolic compounds, lowering them by 10-46% and 8-11%, respectively, the content of di-caffeoyltartaric acid increased from 0.26 ± 0.19 to 0.52 ± 0.10 μmol 100 g-1 f.w. for plants harvested in summer. The effect of diffuse light on AA depended on the total amount of global radiation incident on the plants. Considering the lowest amount of global radiation, the enhanced diffuse light increased the AA content from 766 ± 89 to 849 ± 90 μmol 100 g-1 f.w. By contrast, under the highest level of global radiation, diffuse light decreased the amount of AA from 990 ± 16 to 830 ± 76 μmol 100 g-1 f.w.

Heterologous synthesis of chlorophyll b in Nannochloropsis salina enhances growth and lipid production by increasing photosynthetic efficiency

BACKGROUND

Chlorophylls play important roles in photosynthesis, and thus are critical for growth and related metabolic pathways in photosynthetic organisms. They are particularly important in microalgae, emerging as the next generation feedstock for biomass and biofuels. Nannochloropsis are industrial microalgae for these purposes, but are peculiar in that they lack accessory chlorophylls. In addition, the localization of heterologous proteins to the chloroplast of Nannochloropsis has not been fully studied, due to the secondary plastid surrounded by four membranes. This study addressed questions of correct localization and functional benefits of heterologous expression of chlorophyllide a oxygenase from Chlamydomonas (CrCAO) in Nannochloropsis.

RESULTS

We cloned CrCAO from Chlamydomonas, which catalyzes oxidation of Chla producing Chlb, and overexpressed it in N. salina to reveal effects of the heterologous Chlb for photosynthesis, growth, and lipid production. For correct localization of CrCAO into the secondary plastid in N. salina, we added the signal-recognition sequence and the transit peptide (cloned from an endogenous chloroplast-localized protein) to the N terminus of CrCAO. We obtained two transformants that expressed CrCAO and produced Chlb. They showed improved growth under medium light (90 μmol/m²/s) conditions, and their photosynthetic efficiency was increased compared to WT. They also showed increased expression of certain photosynthetic proteins, accompanied by an increased maximum electron-transfer rate up to 15.8% and quantum yields up to 17%, likely supporting the faster growth. This improved growth resulted in increased biomass production, and more importantly lipid productivity particularly with medium light.

CONCLUSIONS

We demonstrated beneficial effects of heterologous expression of CrCAO in Chlb-less organism N. salina, where the newly produced Chlb enhanced photosynthesis and growth. Accordingly, transformants showed improved production of biomass and lipids, important traits of microalgae from the industrial perspectives. Our transformants are the first Nannochloropsis cells that produced Chlb in the whole evolutionary path. We also succeeded in delivering a heterologous protein into the secondary plastid for the first time in Nannochloropsis. Taken together, our data showed that manipulation of photosynthetic pigments, including Chlb, can be employed in genetic improvements of microalgae for production of biofuels and other biomaterials.

The Impacts of Fluctuating Light on Crop Performance

Rapidly changing light conditions can reduce carbon gain and productivity in field crops because photosynthetic responses to light fluctuations are not instantaneous. Plant responses to fluctuating light occur across levels of organizational complexity from entire canopies to the biochemistry of a single reaction and across orders of magnitude of time. Although light availability and variation at the top of the canopy are largely dependent on the solar angle and degree of cloudiness, lower crop canopies rely more heavily on light in the form of sunflecks, the quantity of which depends mostly on canopy structure but also may be affected by wind. The ability of leaf photosynthesis to respond rapidly to these variations in light intensity is restricted by the relatively slow opening/closing of stomata, activation/deactivation of C3 cycle enzymes, and up-regulation/down-regulation of photoprotective processes. The metabolic complexity of C4 photosynthesis creates the apparently contradictory possibilities that C4 photosynthesis may be both more and less resilient than C3 to dynamic light regimes, depending on the frequency at which these light fluctuations occur. We review the current understanding of the underlying mechanisms of these limitations to photosynthesis in fluctuating light that have shown promise in improving the response times of photosynthesis-related processes to changes in light intensity.

A three-dimensional spatial mapping approach to quantify fine-scale heterogeneity among leaves within canopies

PREMISE OF THE STUDY

The three-dimensional structure of tree canopies creates environmental heterogeneity, which can differentially influence the chemistry, morphology, physiology, and/or phenology of leaves. Previous studies that subdivide canopy leaves into broad categories (i.e., "upper/lower") fail to capture the differences in microenvironments experienced by leaves throughout the three-dimensional space of a canopy.

METHODS

We use a three-dimensional spatial mapping approach based on spherical polar coordinates to examine the fine-scale spatial distributions of photosynthetically active radiation (PAR) and the concentration of ultraviolet (UV)-absorbing compounds (A300) among leaves within the canopies of black mangroves (Avicennia germinans).

RESULTS

Linear regressions revealed that interior leaves received less PAR and produced fewer UV-absorbing compounds than leaves on the exterior of the canopy. By allocating more UV-absorbing compounds to the leaves on the exterior of the canopy, black mangroves may be maximizing UV-protection while minimizing biosynthesis of UV-absorbing compounds.

DISCUSSION

Three-dimensional spatial mapping provides an inexpensive and portable method to detect fine-scale differences in environmental and biological traits within canopies. We used it to understand the relationship between PAR and A300, but the same approach can also be used to identify traits associated with the spatial distribution of herbivores, pollinators, and pathogens.

Excess Diffuse Light Absorption in Upper Mesophyll Limits CO2 Drawdown and Depresses Photosynthesis

In agricultural and natural systems, diffuse light can enhance plant primary productivity due to deeper penetration into and greater irradiance of the entire canopy. However, for individual sun-grown leaves from three species, photosynthesis is actually less efficient under diffuse compared with direct light. Despite its potential impact on canopy-level productivity, the mechanism for this leaf-level diffuse light photosynthetic depression effect is unknown. Here, we investigate if the spatial distribution of light absorption relative to electron transport capacity in sun- and shade-grown sunflower (Helianthus annuus) leaves underlies its previously observed diffuse light photosynthetic depression. Using a new one-dimensional porous medium finite element gas-exchange model parameterized with light absorption profiles, we found that weaker penetration of diffuse versus direct light into the mesophyll of sun-grown sunflower leaves led to a more heterogenous saturation of electron transport capacity and lowered its CO2 concentration drawdown capacity in the intercellular airspace and chloroplast stroma. This decoupling of light availability from photosynthetic capacity under diffuse light is sufficient to generate an 11% decline in photosynthesis in sun-grown but not shade-grown leaves, primarily because thin shade-grown leaves similarly distribute diffuse and direct light throughout the mesophyll. Finally, we illustrate how diffuse light photosynthetic depression could overcome enhancement in canopies with low light extinction coefficients and/or leaf area, pointing toward a novel direction for future research.

Phenotypic plasticity to altered apical bud temperature in Cucumis sativus: more leaves-smaller leaves and vice versa

Many studies investigated temperature effects on leaf initiation and expansion by relating these processes to air temperature or the temperature of a specific organ (e.g. leaf temperature). In reality plant temperature is hardly ever equal to air temperature or spatially uniform. Apical bud temperature (T_bud ), for example, may greatly differ from the temperature of the rest of the plant (T_plant ) dependent on the environment. Recent research in Cucumis sativus showed that T_bud influences leaf initiation independent of T_plant . These findings trigger the question if such spatial temperature differences also influence leaf expansion and plant phenotype. In a 28 day study, we maintained temperature differences between T_bud and T_plant ranging from -7 to +8 °C using a custom-made bud temperature control system. Leaf expansion did not only depend on leaf temperature but also on the difference between bud and leaf temperature. Differences between T_bud and T_plant considerably influenced vertical leaf area distribution over the shoot: increasing T_bud beyond T_plant resulted in more and smaller leaves, while decreasing T_bud below T_plant resulted in less and larger leaves. The trade-off between leaf number and leaf area resulted in phenotypic alterations that cannot be predicted, for example, by crop models, when assuming plant temperature uniformity.

A unique approach to demonstrating that apical bud temperature specifically determines leaf initiation rate in the dicot Cucumis sativus

MAIN CONCLUSION

Leaf initiation rate is largely determined by the apical bud temperature even when apical bud temperature largely deviates from the temperature of other plant organs. We have long known that the rate of leaf initiation (LIR) is highly sensitive to temperature, but previous studies in dicots have not rigorously demonstrated that apical bud temperature controls LIR independent of other plant organs temperature. Many models assume that apical bud and leaf temperature are the same. In some environments, the temperature of the apical bud, where leaf initiation occurs, may differ by several degrees Celsius from the temperature of other plant organs. In a 28-days study, we maintained temperature differences between the apical bud and the rest of the individual Cucumis sativus plants from -7 to +8 °C by enclosing the apical buds in transparent, temperature-controlled, flow-through, spheres. Our results demonstrate that LIR was completely determined by apical bud temperature independent of other plant organs temperature. These results emphasize the need to measure or model apical bud temperatures in dicots to improve the prediction of crop development rates in simulation models.

Effects of Diffuse Light on Radiation Use Efficiency of Two Anthurium Cultivars Depend on the Response of Stomatal Conductance to Dynamic Light Intensity

The stimulating effect of diffuse light on radiation use efficiency (RUE) of crops is often explained by the more homogeneous spatial light distribution, while rarely considering differences in temporal light distribution at leaf level. This study investigated whether diffuse light effects on crop RUE can be explained by dynamic responses of leaf photosynthesis to temporal changes of photosynthetic photon flux density (PPFD). Two Anthurium andreanum cultivars ('Pink Champion' and 'Royal Champion') were grown in two glasshouses covered by clear (control) and diffuse glass, with similar light transmission. On clear days, diffusing the light resulted in less temporal fluctuations of PPFD. Stomatal conductance (g s) varied strongly in response to transient PPFD in 'Royal Champion,' whereas it remained relatively constant in 'Pink Champion.' Instantaneous net leaf photosynthesis (P n) in both cultivars approached steady state P n in diffuse light treatment. In control treatment this only occurred in 'Pink Champion.' These cultivar differences were reflected by a higher RUE (8%) in 'Royal Champion' in diffuse light treatment compared with control, whereas no effect on RUE was observed in 'Pink Champion.' We conclude that the stimulating effect of diffuse light on RUE depends on the stomatal response to temporal PPFD fluctuations, which response is cultivar dependent.

Leaf Morphology, Photosynthetic Performance, Chlorophyll Fluorescence, Stomatal Development of Lettuce (Lactuca sativa L.) Exposed to Different Ratios of Red Light to Blue Light

Red and blue light are both vital factors for plant growth and development. We examined how different ratios of red light to blue light (R/B) provided by light-emitting diodes affected photosynthetic performance by investigating parameters related to photosynthesis, including leaf morphology, photosynthetic rate, chlorophyll fluorescence, stomatal development, light response curve, and nitrogen content. In this study, lettuce plants (Lactuca sativa L.) were exposed to 200 μmol⋅m(-2)⋅s(-1) irradiance for a 16 h⋅d(-1) photoperiod under the following six treatments: monochromatic red light (R), monochromatic blue light (B) and the mixture of R and B with different R/B ratios of 12, 8, 4, and 1. Leaf photosynthetic capacity (A max) and photosynthetic rate (P n) increased with decreasing R/B ratio until 1, associated with increased stomatal conductance, along with significant increase in stomatal density and slight decrease in stomatal size. P n and A max under B treatment had 7.6 and 11.8% reduction in comparison with those under R/B = 1 treatment, respectively. The effective quantum yield of PSII and the efficiency of excitation captured by open PSII center were also significantly lower under B treatment than those under the other treatments. However, shoot dry weight increased with increasing R/B ratio with the greatest value under R/B = 12 treatment. The increase of shoot dry weight was mainly caused by increasing leaf area and leaf number, but no significant difference was observed between R and R/B = 12 treatments. Based on the above results, we conclude that quantitative B could promote photosynthetic performance or growth by stimulating morphological and physiological responses, yet there was no positive correlation between P n and shoot dry weight accumulation.

Advantages of diffuse light for horticultural production and perspectives for further research

Plants use diffuse light more efficiently than direct light, which is well established due to diffuse light penetrates deeper into the canopy and photosynthetic rate of a single leaf shows a non-linear response to the light flux density. Diffuse light also results in a more even horizontal and temporal light distribution in the canopy, which plays substantial role for crop photosynthesis enhancement as well as production improvement. Here we show some of the recent findings about the effect of diffuse light on light distribution over the canopy and its direct and indirect effects on crop photosynthesis and plant growth, and suggest some perspectives for further research which could strengthen the scientific understanding of diffuse light modulate plant processes and its application in horticultural production.