Far-Red Light Accelerates Photosynthesis in the Low-Light Phases of Fluctuating Light

The crosstalk of far-red energy and signaling defines the regulation of photosynthesis, growth, and flowering in tomatoes

This study investigated the effect of light intensity and signaling on the regulation of far-red (FR)-induced alteration in photosynthesis. The low (LL: 440 μmol m-2 s-1) and high (HL: 1135 μmol m-2 s-1) intensity of white light with or without FR (LLFR: 545 μmol m-2 s-1 including 115 μmol m-2 s-1; HLFR: 1254 μmol m-2 s-1 + 140 μmol m-2 s-1) was applied on the tomato cultivar (Solanum Lycopersicon cv. Moneymaker) and mutants of phytochrome A (phyA) and phytochrome B (phyB1, and phyB2). Both light intensity and FR affected plant morphological traits, leaf biomass, and flowering time. Irrespective of genotype, flowering was delayed by LLFR and accelerated by HLFR compared to the corresponding light intensity without FR. In LLFR, a reduced energy flux through the electron transfer chain along with a reduced energy dissipation per reaction center improved the maximum quantum yield of PSII, irrespective of genotype. HLFR increased net photosynthesis and gas exchange properties in a genotype-dependent manner. FR-dependent regulation of hormones was affected by light signaling. It appeared that PHYB affected the levels of abscisic acid and salicylic acid while PHYA took part in the regulation of CK in FR-exposed plants. Overall, light intensity and signaling of FR influenced plants' photosynthesis and growth by altering electron transport, gas exchange, and changes in the level of endogenous hormones.

The Effects of Light Spectrum and Intensity, Seeding Density, and Fertilization on Biomass, Morphology, and Resource Use Efficiency in Three Species of Brassicaceae Microgreens

Light is a critical component of indoor plant cultivation, as different wavelengths can influence both the physiology and morphology of plants. Furthermore, fertilization and seeding density can also potentially interact with the light recipe to affect production outcomes. However, maximizing production is an ongoing research topic, and it is often divested from resource use efficiencies. In this study, three species of microgreens-kohlrabi; mustard; and radish-were grown under five light recipes; with and without fertilizer; and at two seeding densities. We found that the different light recipes had significant effects on biomass accumulation. More specifically, we found that Far-Red light was significantly positively associated with biomass accumulation, as well as improvements in height, leaf area, and leaf weight. We also found a less strong but positive correlation with increasing amounts of Green light and biomass. Red light was negatively associated with biomass accumulation, and Blue light showed a concave downward response. We found that fertilizer improved biomass by a factor of 1.60 across species and that using a high seeding density was 37% more spatially productive. Overall, we found that it was primarily the main effects that explained microgreen production variation, and there were very few instances of significant interactions between light recipe, fertilization, and seeding density. To contextualize the cost of producing these microgreens, we also measured resource use efficiencies and found that the cheaper 24-volt LEDs at a high seeding density with fertilizer were the most efficient production environment for biomass. Therefore, this study has shown that, even with a short growing period of only four days, there was a significant influence of light recipe, fertilization, and seeding density that can change morphology, biomass accumulation, and resource input costs.

Seed priming with light quality and Cyperus rotundus L. extract modulate the germination and initial growth of Moringa oleifera Lam. seedlings

Abstract Improving plant germination is essential to guarantee better quality seedlings. Thus, this research aimed to evaluate whether the seed priming with light quality (LIQ) and the aqueous extract of Cyperus rotundus (AEC) tuber could modulate the germination and initial growth of Moringa oleifera L. seedlings. The experimental design was a completely randomized in the 4x4 factorial scheme, composed of four LIQ conditions (white, blue, red, and distant red light) and four AEC concentrations (0, 25, 50 and 100%). Seed priming with red light reduced the average emergence time, while blue, red, and extreme red lights associated with 50% of aqueous extract of C. rotundus increased shoot initial length and photosynthetic pigment accumulation. Seed priming with blue light resulted in seedlings with a shorter final shoot length. However, application of 100% of aqueous extract of C. rotundus reversed this. The white light in combination with concentrations of 50 and 100% of AEC promoted a higher relative shoot growth rate of seedlings. The research revealed that seed priming with light quality and aqueous extracts of C. rotundus tubers modulates the germination and initial growth of M. oleifera seedlings. More work needs to be done to determine the responsible compounds in AEC that is responsible for priming growth as phytohormones.

Far-Red Light Mediated Carbohydrate Concentration Changes in Leaves of Sweet Basil, a Stachyose Translocating Plant

Photosynthetic active radiation (PAR) refers to photons between 400 and 700 nm. These photons drive photosynthesis, providing carbohydrates for plant metabolism and development. Far-red radiation (FR, 701-750 nm) is excluded in this definition because no FR is absorbed by the plant photosynthetic pigments. However, including FR in the light spectrum provides substantial benefits for biomass production and resource-use efficiency. We investigated the effects of continuous FR addition and end-of-day additional FR to a broad white light spectrum (BW) on carbohydrate concentrations in the top and bottom leaves of sweet basil (Ocimum basilicum L.), a species that produces the raffinose family oligosaccharides raffinose and stachyose and preferentially uses the latter as transport sugar. Glucose, fructose, sucrose, raffinose, and starch concentrations increased significantly in top and bottom leaves with the addition of FR light. The increased carbohydrate pools under FR light treatments are associated with more efficient stachyose production and potentially improved phloem loading through increased sucrose homeostasis in intermediary cells. The combination of a high biomass yield, increased resource-use efficiency, and increased carbohydrate concentration in leaves in response to the addition of FR light offers opportunities for commercial plant production in controlled growth environments.

RGA1 alleviates low-light-repressed pollen tube elongation by improving the metabolism and allocation of sugars and energy

Low-light stress compromises photosynthetic and energy efficiency and leads to spikelet sterility; however, the effect of low-light stress on pollen tube elongation in the pistil remains poorly understood. The gene RGA1, which encodes a Gα-subunit of the heterotrimeric G-protein, enhanced low-light tolerance at anthesis by preventing the cessation of pollen tube elongation in the pistil of rice plants. In this process, marked increases in the activities of acid invertase (INV), sucrose synthase (SUS) and mitochondrial respiratory electron transport chain complexes, as well as the relative expression levels of SUTs (sucrose transporter), SWEETs (sugars will eventually be exported transporters), SUSs, INVs, CINs (cell-wall INV 1), SnRK1A (sucrose-nonfermenting 1-related kinase 1) and SnRK1B, were observed in OE-1 plants. Accordingly, notable increases in contents of ATP and ATPase were presented in OE-1 plants under low-light conditions, while they were decreased in d1 plants. Importantly, INV and ATPase activators (sucrose and Na₂ SO₃ , respectively) increased spikelet fertility by improving the energy status in the pistil under low-light conditions, and the ATPase inhibitor Na₂ VO₄ induced spikelet sterility and decreased ATPase activity. These results suggest that RGA1 could alleviate the low-light stress-induced impairment of pollen tube elongation to increase spikelet fertility by promoting sucrose unloading in the pistil and improving the metabolism and allocation of energy.

Photosynthesis in sun and shade: the surprising importance of far-red photons

The current definition of photosynthetically active radiation includes only photons from 400 up to 700 nm, despite evidence of the synergistic interaction between far-red photons and shorter-wavelength photons. The synergy between far-red and shorter-wavelength photons has not been studied in sunlight under natural conditions. We used a filter to remove photons above 700 nm to quantify the effects on photosynthesis in diverse species under full sun, medium light intensity and vegetation shade. Far-red photons (701 to 750 nm) in sunlight are used efficiently for photosynthesis. This is especially important for leaves in vegetation shade, where far-red photons can be > 50% of the total incident photons between 400 and 750 nm. Far-red photons accounted for 24-25% of leaf gross photosynthesis (P_gross ) in a C₃ and a C₄ species when sunlight was filtered through a leaf, and 10-14% of leaf P_gross in a tree and an understory species in deep shade. Accounting for the photosynthetic activity of far-red photons is critical for accurate measurement and modeling of photosynthesis at single leaf, canopy and ecosystem scales. This, in turn, is crucial in understanding crop productivity, the global carbon cycle and climate change impacts on agriculture and ecosystems.

Green Tea Catechins, (-)-Catechin Gallate, and (-)-Gallocatechin Gallate are Potent Inhibitors of ABA-Induced Stomatal Closure

Stomatal movement is indispensable for plant growth and survival in response to environmental stimuli. Cytosolic Ca²⁺ elevation plays a crucial role in ABA-induced stomatal closure during drought stress; however, to what extent the Ca²⁺ movement across the plasma membrane from the apoplast to the cytosol contributes to this process still needs clarification. Here the authors identify (-)-catechin gallate (CG) and (-)-gallocatechin gallate (GCG), components of green tea, as inhibitors of voltage-dependent K⁺ channels which regulate K⁺ fluxes in Arabidopsis thaliana guard cells. In Arabidopsis guard cells CG/GCG prevent ABA-induced: i) membrane depolarization; ii) activation of Ca²⁺ permeable cation (I_Ca ) channels; and iii) cytosolic Ca²⁺ transients. In whole Arabidopsis plants co-treatment with CG/GCG and ABA suppressed ABA-induced stomatal closure and surface temperature increase. Similar to ABA, CG/GCG inhibited stomatal closure is elicited by the elicitor peptide, flg22 but has no impact on dark-induced stomatal closure or light- and fusicoccin-induced stomatal opening, suggesting that the inhibitory effect of CG/GCG is associated with Ca²⁺ -related signaling pathways. This study further supports the crucial role of I_Ca channels of the plasma membrane in ABA-induced stomatal closure. Moreover, CG and GCG represent a new tool for the study of abiotic or biotic stress-induced signal transduction pathways.

Spectral light distribution affects photosynthesis, leaf reflective indices, antioxidant activity and growth of Vanillaplanifolia

Vanilla planifolia is an obligate sciophyte (shade plant) with crassulacean acid metabolism (CAM) photosynthesis. Plants were grown for 12 months under black, blue, green, or red photoselective shade netting (PSN) to alter the spectral light distribution impacting the plants. Light wavelengths were measured in each treatment and plants were assessed for photosynthetic characteristics, leaf chlorophyll index (LCI), maximum quantum yield of photosystem II, leaf reflectance indices, leaf area, growth, antioxidant enzymes, lipid peroxidation, reactive oxygen species (ROS), and osmolyte content. Plants grown under red PSN had a higher quantity of red and far-red light and had greater nocturnal net CO₂ assimilation (NocA), leaf area and leaf dry weight than plants in the other treatments. Plants grown under blue PSN had a higher quantity of blue light, resulting in a higher LCI and maximum quantum yield than plants in the other treatments. Plants grown under the red and blue PSN had increased leaf spectral reflectance indices compared to plants in the other treatments, which resulted in the highest levels of antioxidant scavenging enzymes, ascorbic acid (AsA), proline, and glycine betaine, and the lowest levels of H₂O₂. These findings demonstrate that increasing light in the red and far-red or blue portions of the spectrum by using PSN alters the photosynthetic and/or antioxidant responses of V. planifolia and increasing red and far-red light by using red PSN can also accelerate plant growth, possibly due to higher photosynthesis.

Modulations in Chlorophyll a Fluorescence Based on Intensity and Spectral Variations of Light

Photosynthetic efficiency is significantly affected by both qualitative and quantitative changes during light exposure. The properties of light have a profound effect on electron transport and energy absorption in photochemical reactions. In addition, fluctuations in light intensity and variations in the spectrum can lead to a decrease in photosystem II efficiency. These features necessitate the use of a simple and suitable tool called chlorophyll a fluorescence to study photosynthetic reactions as a function of the aforementioned variables. This research implies that chlorophyll a fluorescence data can be used to determine precise light conditions that help photoautotrophic organisms optimally function.

Electron transport and photosynthetic performance in Fragaria × ananassa Duch. acclimated to the solar spectrum modified by a spectrum conversion film

Functional films have been used in greenhouses to improve the light environment for plant growth. Among them, a spectrum conversion film converting the green light of incident sunlight into red light has been reported to increase the crop productivity. However, the results are not always consistent, and the reasons for the improvement are not fully understood. The objectives of this study were to reveal the cumulative effects of a green-to-red spectrum conversion film (SCF) on the electron transport and photosynthetic performance of Fragaria × ananassa Duch. The photosynthetic efficiency, leaf optical properties, chlorophyll content, chlorophyll fluorescence, growth, and fruit qualities when the plant was grown under a transparent polyethylene film (PE) and SCF were evaluated. The sunlight modified by SCF did not change the leaf optical properties and chlorophyll content but significantly increased the chlorophyll fluorescence parameters related to reduction end electron acceptors at PSI acceptor side and the efficiency of electron transport. Without an increase in nonphotochemical quenching, the effective quantum yields of PSII and PSI of leaves grown under SCF were significantly higher than those parameters when grown under PE. Forty eight days after transplanting, the photosynthetic efficiency and photosynthetic rates of leaves and whole plants increased significantly under SCF compared to PE. The vegetative growth was not affected by SCF, but the fruit weight, sweetness, acidity, and firmness under SCF were significantly improved. These results indicated that sunlight modified by SCF stimulates electron flow and improves photosynthetic capacity and fruit quality of Fragaria × ananassa Duch.

The Responses of Light Reaction of Photosynthesis to Dynamic Sunflecks in a Typically Shade-Tolerant Species Panax notoginseng

Light is highly heterogeneous in natural conditions, and plants need to evolve a series of strategies to acclimate the dynamic light since it is immobile. The present study aimed to elucidate the response of light reaction of photosynthesis to dynamic sunflecks in a shade-tolerant species Panax notoginseng and to examine the regulatory mechanisms involved in an adaptation to the simulated sunflecks. When P. notoginseng was exposed to the simulated sunflecks, non-photochemical quenching (NPQ) increased rapidly to the maximum value. Moreover, in response to the simulated sunflecks, there was a rapid increase in light-dependent heat dissipation quantum efficiency of photosystem II (PSII) (Φ_NPQ), while the maximum quantum yield of PSII under light (F _v'/F _m') declined. The relatively high fluorescence and constitutive heat dissipation quantum efficiency of PSII (Φ_f,d) in the plants exposed to transient high light (400, 800, and 1,600 μmol m^-2 s^-1) was accompanied by the low effective photochemical quantum yield of PSII (Φ_PSII) after the dark recovery for 15 min, whereas the plants exposed to transient low light (50 μmol m^-2 s^-1) has been shown to lead to significant elevation in Φ_PSII after darkness recovery. Furthermore, PSII fluorescence and constitutive heat dissipation electron transfer rate (J _f,d) was increased with the intensity of the simulated sunflecks, the residual absorbed energy used for the non-net carboxylative processes (J _NC) was decreased when the response of electron transfer rate of NPQ pathway of PSII (J _NPQ) to transient low light is restricted. In addition, the acceptor-side limitation of PSI [Y(NA)] was increased, while the donor-side limitation of photosystems I (PSI) [Y(ND)] was decreased at transient high light conditions accompanied with active cyclic electron flow (CEF). Meanwhile, when the leaves were exposed to transient high light, the xanthophyll cycle (V cycle) was activated and subsequently, the J _NPQ began to increase. The de-epoxidation state [(Z + A)/(V + A + Z)] was strongly correlated with NPQ in response to the sunflecks. In the present study, a rapid engagement of lutein epoxide (Lx) after the low intensity of sunfleck together with the lower NPQ contributed to an elevation in the maximum photochemical quantum efficiency of PSII under the light. The analysis based on the correlation between the CEF and electron flow devoted to Ribulose-1, 5-bisphosphate (RuBP) oxygenation (J _O) indicated that at a high light intensity of sunflecks, the electron flow largely devoted to RuBP oxygenation would contribute to the operation of the CEF. Overall, photorespiration plays an important role in regulating the CEF of the shade-tolerant species, such as P. notoginseng in response to transient high light, whereas active Lx cycle together with the decelerated NPQ may be an effective mechanism of elevating the maximum photochemical quantum efficiency of PSII under light exposure to transient low light.

An insight into spectral composition of light available for photosynthesis via remotely assessed absorption coefficient at leaf and canopy levels

Non-invasive comparative analysis of the spectral composition of energy absorbed by crop species at leaf and plant levels was carried out using the absorption coefficient retrieved from leaf and plant reflectance as an informative metric. In leaves of three species with contrasting leaf structures and photosynthetic pathways (maize, soybean, and rice), the blue, green, and red fractions of leaf absorption coefficients were 48, 20, and 32%, respectively. The fraction of green light in the total budget of light absorbed at the plant level was higher than at the leaf level approaching the size of the red fraction (24% green vs. 25.5% red) and surpassing it inside the canopy. The plant absorption coefficient in the far-red region (700-750 nm) was significant reaching 7-10% of the absorption coefficient in green or red regions. The spectral composition of the absorbed light in the three species was virtually the same. Fractions of light in absorbed PAR remained almost invariant during growing season over a wide range of plant chlorophyll content. Fractions of absorption coefficient in the green, red, and far-red were in accord with published results of quantum yield for CO2 fixation on an absorbed light basis. The role of green and far-red light in photosynthesis was demonstrated in simple experiments in natural conditions. The results show the potential for using leaf and plant absorption coefficients retrieved from reflectance to quantify photosynthesis in each spectral range.

Effects of Artificially Reproduced Fluctuations in Sunlight Spectral Distribution on the Net Photosynthetic Rate of Cucumber Leaves

The effects of photosynthetic photon flux density (PPFD) fluctuations in sunlight have already been investigated; however, the spectral photon flux density distribution (SPD) has hardly been considered. Here, sunlight SPD fluctuations recorded for 200 min in October in Tokyo, Japan were artificially reproduced using an LED-artificial sunlight source system. The net photosynthetic rate (P n) of cucumber leaves under reproduced sunlight was measured and compared with the P n estimated from a steady-state PPFD-P n curve for the same leaves. The measured and estimated P n agreed except when the PPFD was low, where the measured P n was lower than the estimated P n. The ratio of measured P n to estimated P n was 0.94-0.95 for PPFD ranges of 300-700 μmol m-2 s-1, while the value was 0.98-0.99 for 900-1,300 μmol m-2 s-1, and the overall ratio was 0.97. This 3% reduction in the measured P n compared with the P n estimated from a steady-state PPFD-P n curve was significantly smaller than the approximately 20-30% reduction reported in previous experimental and simulation studies. This result suggests that the loss of integral net photosynthetic gain under fluctuating sunlight can vary among days with different fluctuation patterns or may be non-significant when fluctuations in both PPFD and relative SPD of sunlight are taken into consideration.

LEDs Make It Resilient: Effects on Plant Growth and Defense

Light spectral composition influences plant growth and metabolism, and has important consequences for interactions with plant-feeding arthropods and their natural enemies. In greenhouse horticulture, light spectral composition can be precisely manipulated by light-emitting diodes (LEDs), and LEDs are already used to optimize crop production and quality. However, because light quality also modulates plant secondary metabolism and defense, it is important to understand the underlying mechanisms in the context of the growth-defense trade-off. We review the effects of the spectral composition of supplemental light currently used, or potentially used, in greenhouse horticulture on the mechanisms underlying plant growth and defense. This information is important for exploring opportunities to optimize crop performance and pest management, and thus for developing resilient crop-production systems.

Seasonal Efficiency of Supplemental LED Lighting on Growth and Photomorphogenesis of Sweet Basil

For decisions on supplemental lighting a quantitative knowledge of the plants' responses to light under varying conditions is fundamental. In this study, we developed light dose-response curves of growth and morphological traits for Ocimum basilicum L. and examined the effects of light color (blue, red, and white plus far-red) and natural environment (season) on these curves. Four greenhouse experiments were conducted throughout the year to determine the efficiencies of the light regimes on growth and their effects on plant morphology. A special aspect was the photosynthetic efficiency of far-red light. Linear and monomolecular relationships were found for the relationships between plant traits and supplemental light dose. Traits related to biomass productivity increased linearly with light dose whereas some morphological characters showed a saturation behavior. Red light and white plus far-red light were more efficient in plant dry weight production than blue light, and the plants adapted differently to the light qualities: higher biomass under red light was related to a plant architecture more favorable for light capture, i.e., taller plants and bigger leaves. White plus far-red light, on the other hand, increased leaf mass per area (LMA) and light use efficiency (LUE). Blue light resulted in lowest plant light interception and LUE. Considering photosynthetic effects of near-infrared light (PPFD800, 400-800 nm) instead of photosynthetic photon flux density (PPFD700, 400-700 nm) led to strongly reduced efficiencies. Traits related to photosynthesis such as dry weight, LMA and LUE were particularly affected by PPFD800. There were no interactions between the efficiencies of the different light colors and the seasons. Efficiencies of all light regimes were significantly lower during summer compared to spring and winter. Higher dry weight production during summer compared to winter and spring were a consequence of increased light interception rather than changes in LUE. The observed differences in seasonal efficiencies were directly linked to the amount of natural light present as indicated by changes in the ratio of supplemental to natural light.

Perspectives on improving light distribution and light use efficiency in crop canopies

Plant stands in nature differ markedly from most seen in modern agriculture. In a dense mixed stand, plants must vie for resources, including light, for greater survival and fitness. Competitive advantages over surrounding plants improve fitness of the individual, thus maintaining the competitive traits in the gene pool. In contrast, monoculture crop production strives to increase output at the stand level and thus benefits from cooperation to increase yield of the community. In choosing plants with higher yields to propagate and grow for food, humans may have inadvertently selected the best competitors rather than the best cooperators. Here, we discuss how this selection for competitiveness has led to overinvestment in characteristics that increase light interception and, consequently, sub-optimal light use efficiency in crop fields that constrains yield improvement. Decades of crop canopy modeling research have provided potential strategies for improving light distribution in crop canopies, and we review the current progress of these strategies, including balancing light distribution through reducing pigment concentration. Based on recent research revealing red-shifted photosynthetic pigments in algae and photosynthetic bacteria, we also discuss potential strategies for optimizing light interception and use through introducing alternative pigment types in crops. These strategies for improving light distribution and expanding the wavelengths of light beyond those traditionally defined for photosynthesis in plant canopies may have large implications for improving crop yield and closing the yield gap.

Dissecting the Genotypic Variation of Growth Responses to Far-Red Radiation in Tomato

The recent development of light-emitting diodes (LEDs) and their application in modern horticulture stimulated studies demonstrating that additional far-red (FR) radiation (700-800 nm) increases plant dry mass. This effect of FR has been explained by improved photosynthesis and/or plant architecture. However, the genotypic variation in this response is largely unknown. Here, we aim to explore and explain the genotypic variation in growth responses to additional FR. We expected the genotypic variation in the responses of plant dry mass to additional FR. Further, we hypothesized that a significant improvement of both net assimilation rate (NAR) and leaf area ratio (LAR) is responsible for a strong dry mass increase under additional FR, while some genotypes respond only marginally or even negatively in NAR or LAR under FR, thus resulting in a weak FR effect on plant dry mass. To test these hypotheses, we grew 33 different tomato genotypes for 21 days with 0, 25, or 100 μmol m^-2 s^-1 of FR added to a common white + red LED background lighting of 150 μmol m^-2 s^-1. Genotypes responded similarly with respect to plant height, stem dry mass, and shoot:root ratio; i.e., they all increased with increasing FR. However, the response of total plant dry mass varied among genotypes. We categorized the genotypes into three groups (strongly, moderately, and weakly responding groups) based on their relative response in total plant dry mass to FR. Growth component analysis revealed that the strongly responding genotypes increased strongly in NAR rather than LAR. The weakly responding genotypes, however, showed a substantial increase in LAR but not NAR. The increase in LAR was due to the increase in specific leaf area. Leaf mass fraction, which is the other component of LAR, decreased with FR and did not differ between groups. In conclusion, tomato genotypes that increased strongly in NAR in response to FR were able to achieve a more substantial increase in dry mass than did other genotypes. This is the first study to explain the differences in growth responses of a large number of tomato genotypes toward FR in their light environment.

Steady-state stomatal responses of C3 and C4 species to blue light fraction: Interactions with CO2 concentration

Blue light induced stomatal opening has been studied by applying a short pulse (~5 to 60 s) of blue light to a background of saturating photosynthetic red photons, but little is known about steady-state stomatal responses. Here we report stomatal responses to blue light at high and low CO₂ concentrations. Steady-state stomatal conductance (g_s ) of C₃ plants increased asymptotically with increasing blue light to a maximum at 20% blue (120 μmol m^-2 s^-1 ). This response was consistent from 200 to 800 μmol mol^-1 atmospheric CO₂ (C_a ). In contrast, blue light induced only a transient stomatal opening (~5 min) in C₄ species above a C_a of 400 μmol mol^-1 . Steady-state g_s of C₄ plants generally decreased with increasing blue intensity. The net photosynthetic rate of all species decreased above 20% blue because blue photons have lower quantum yield (moles carbon fixed per mole photons absorbed) than red photons. Our findings indicate that photosynthesis, rather than a blue light signal, plays a dominant role in stomatal regulation in C₄ species. Additionally, we found that blue light affected only stomata on the illuminated side of the leaf. Contrary to widely held belief, the blue light-induced stomatal opening minimally enhanced photosynthesis and consistently decreased water use efficiency.

Artifleck: The Study of Artifactual Responses to Light Flecks with Inappropriate Leaves

Methods in sunfleck research commonly employ the use of experimental leaves which were constructed in homogeneous light. These experimental organs may behave unnaturally when they are challenged with fluctuating light. Photosynthetic responses to heterogeneous light and leaf macronutrient relations were determined for Cycas micronesica, Glycine max, and Zea mays leaves that were grown in homogeneous shade, heterogeneous shade, or full sun. The speed of priming where one light fleck increased the photosynthesis during a subsequent light fleck was greatest for the leaves grown in heterogeneous shade. The rate of induction and the ultimate steady-state photosynthesis were greater for the leaves that were grown in heterogeneous shade versus the leaves grown in homogeneous shade. The leaf mass per area, macronutrient concentration, and macronutrient stoichiometry were also influenced by the shade treatments. The amplitude and direction in which the three developmental light treatments influenced the response variables were not universal among the three model species. The results indicate that the historical practice of using experimental leaves which were constructed under homogeneous light to study leaf responses to fluctuating light may produce artifacts that generate dubious interpretations.

Increased stomatal conductance induces rapid changes to photosynthetic rate in response to naturally fluctuating light conditions in rice

A close correlation between stomatal conductance and the steady-state photosynthetic rate has been observed for diverse plant species under various environmental conditions. However, it remains unclear whether stomatal conductance is a major limiting factor for the photosynthetic rate under naturally fluctuating light conditions. We analysed a SLAC1 knockout rice line to examine the role of stomatal conductance in photosynthetic responses to fluctuating light. SLAC1 encodes a stomatal anion channel that regulates stomatal closure. Long exposures to weak light before treatments with strong light increased the photosynthetic induction time required for plants to reach a steady-state photosynthetic rate and also induced stomatal limitation of photosynthesis by restricting the diffusion of CO₂ into leaves. The slac1 mutant exhibited a significantly higher rate of stomatal opening after an increase in irradiance than wild-type plants, leading to a higher rate of photosynthetic induction. Under natural conditions, in which irradiance levels are highly variable, the stomata of the slac1 mutant remained open to ensure efficient photosynthetic reaction. These observations reveal that stomatal conductance is important for regulating photosynthesis in rice plants in the natural environment with fluctuating light.

Photosynthetic Acclimation to Fluctuating Irradiance in Plants

Unlike the short-term responses of photosynthesis to fluctuating irradiance, the long-term response (i.e., acclimation) at the chloroplast, leaf, and plant level has received less attention so far. The ability of plants to acclimate to irradiance fluctuations and the speed at which this acclimation occurs are potential limitations to plant growth under field conditions, and therefore this process deserves closer study. In the first section of this review, we look at the sources of natural irradiance fluctuations, their effects on short-term photosynthesis, and the interaction of these effects with circadian rhythms. This is followed by an overview of the mechanisms that are involved in acclimation to fluctuating (or changes of) irradiance. We highlight the chain of events leading to acclimation: retrograde signaling, systemic acquired acclimation (SAA), gene transcription, and changes in protein abundance. We also review how fluctuating irradiance is applied in experiments and highlight the fact that they are significantly slower than natural fluctuations in the field, although the technology to achieve realistic fluctuations exists. Finally, we review published data on the effects of growing plants under fluctuating irradiance on different plant traits, across studies, spatial scales, and species. We show that, when plants are grown under fluctuating irradiance, the chlorophyll a/b ratio and plant biomass decrease, specific leaf area increases, and photosynthetic capacity as well as root/shoot ratio are, on average, unaffected.

Spectrum of Light as a Determinant of Plant Functioning: A Historical Perspective

The significance of the spectral composition of light for growth and other physiological functions of plants moved to the focus of "plant science" soon after the discovery of photosynthesis, if not earlier. The research in this field recently intensified due to the explosive development of computer-controlled systems for artificial illumination and documenting photosynthetic activity. The progress is also substantiated by recent insights into the molecular mechanisms of photo-regulation of assorted physiological functions in plants mediated by photoreceptors and other pigment systems. The spectral balance of solar radiation can vary significantly, affecting the functioning and development of plants. Its effects are evident on the macroscale (e.g., in individual plants growing under the forest canopy) as well as on the meso- or microscale (e.g., mutual shading of leaf cell layers and chloroplasts). The diversity of the observable effects of light spectrum variation arises through (i) the triggering of different photoreceptors, (ii) the non-uniform efficiency of spectral components in driving photosynthesis, and (iii) a variable depth of penetration of spectral components into the leaf. We depict the effects of these factors using the spectral dependence of chloroplast photorelocation movements interlinked with the changes in light penetration into (light capture by) the leaf and the photosynthetic capacity. In this review, we unfold the history of the research on the photocontrol effects and put it in the broader context of photosynthesis efficiency and photoprotection under stress caused by a high intensity of light.