Green light is similarly effective in promoting plant biomass as red/blue light - a meta-analysis.

Whether green light promotes or represses plant growth is an unresolved but important question, warranting a global meta-analysis of published data. We collected 136 datasets from 48 publications on 17 crop species, and calculated the green light effect for a range of plant traits. For each trait the effect was calculated as the ratio between the trait value attained under a red/blue background light plus green, divided by the value attained under the background light only, both having the same light intensity. Generally, green light strongly increased intrinsic water use efficiency (15%), the shoot-to-root ratio (13%), and decreased stomatal conductance (-15%). Moreover, green light increased fresh weight to a small extent (4%), but not plant dry weight, resulting in a reduced dry matter content (-2%). Hence, green light is similarly effective at increasing biomass as red and blue light. Green light also showed to increase leaf area (7%) and specific leaf area (4%; i.e., thinner leaves). Furthermore, effects of green light were species-dependent, with positive effects on biomass for lettuce and microgreens, and negative effects in basil and tomato. Our data suggest that future research should focus on the role of green light in modulating water loss, its putative role as a shade signal, and the causes for its species-specific effects on crop biomass.

Short-term salt stress reduces photosynthetic oscillations under triose phosphate utilization limitation in tomato.

Triose phosphate utilization (TPU) limitation is one of the three biochemical limitations of photosynthetic CO2 assimilation rate in C3 plants. Under TPU limitation, abrupt and large transitions in light intensity cause damped oscillations in photosynthesis. When plants are salt-stressed, photosynthesis is often down-regulated particularly under dynamic light intensity, but how salt stress affects TPU-related dynamic photosynthesis is still unknown. To elucidate this, tomato (Solanum lycopersicum) was grown with and without sodium chloride (NaCl, 100 mM) stress for 13 d. Under high CO2 partial pressure, rapid increases in light intensity caused profound photosynthetic oscillations. Salt stress reduced photosynthetic oscillations in leaves initially under both low- and high-light conditions and reduced the duration of oscillations by about 2 min. Besides, salt stress increased the threshold for CO2 partial pressure at which oscillations occurred. Salt stress increased TPU capacity without affecting Rubisco carboxylation and electron transport capacity, indicating the up-regulation of end-product synthesis capacity in photosynthesis. Thus salt stress may reduce photosynthetic oscillations by decreasing leaf internal CO2 partial pressure and/or increasing TPU capacity. Our results provide new insights into how salt stress modulates dynamic photosynthesis as controlled by CO2 availability and end-product synthesis.

Rapid spatial assessment of leaf-absorbed irradiance.

Image-based high-throughput phenotyping promises the rapid determination of functional traits in large plant populations. However, interpretation of some traits - such as those related to photosynthesis or transpiration rates - is only meaningful if the irradiance absorbed by the measured leaves is known, which can differ greatly between different parts of the same plant and within canopies. No feasible method currently exists to rapidly measure absorbed irradiance in three-dimensional plants and canopies. We developed a method and protocols to derive absorbed irradiance at any visible part of a canopy with a thermal camera, by fitting a leaf energy balance model to transient changes in leaf temperature. Leaves were exposed to short light pulses (30 s) that were not long enough to trigger stomatal opening but strong enough to induce transient changes in leaf temperature that was proportional to the absorbed irradiance. The method was successfully validated against point measurements of absorbed irradiance in plant species with relatively simple architecture (sweet pepper, cucumber, tomato, and lettuce). Once calibrated, the model was used to produce absorbed irradiance maps from thermograms. Our method opens new avenues for the interpretation of plant responses derived from imaging techniques and can be adapted to existing high-throughput phenotyping platforms.

Petunia as a model for MYB transcription factor action under salt stress.

Salinity is a current and growing problem, affecting crops worldwide by reducing yields and product quality. Plants have different mechanisms to adapt to salinity; some crops are highly studied, and their salinity tolerance mechanisms are widely known. However, there are other crops with commercial importance that still need characterization of their molecular mechanisms. Usually, transcription factors are in charge of the regulation of complex processes such as the response to salinity. MYB-TFs are a family of transcription factors that regulate various processes in plant development, and both central and specialized metabolism. MYB-TFs have been studied extensively as mediators of specialized metabolism, and some are master regulators. The influence of MYB-TFs on highly orchestrated mechanisms, such as salinity tolerance, is an attractive research target. The versatility of petunia as a model species has allowed for advances to be made in multiple fields: metabolomic pathways, quality traits, stress resistance, and signal transduction. It has the potential to be the link between horticultural crops and lab models, making it useful in translating discoveries related to the MYB-TF pathways into other crops. We present a phylogenetic tree made with Petunia axillaris and Petunia inflata R2R3-MYB subfamily sequences, which could be used to find functional conservation between different species. This work could set the foundations to improve salinity resistance in other commercial crops in later studies.

Diurnal decline in photosynthesis and stomatal conductance in several tropical species.

Photosynthesis (A) and stomatal conductance (gs) change diurnally due to internal signals, but the effects of diurnal rhythms on dynamic photosynthetic behavior are understudied. We examined diurnal changes in A and gs in ten tropical species: across species, there was a tendency for A and gs to decline diurnally when these were repeatedly measured under either steady-state or fluctuating irradiance conditions. We then examined in more detail the irradiance-induced kinetics of gas exchange in a C3 and C4 crop species each, namely fig (Ficus carica) and sugarcane (Saccharum officinarum). During the day, fig showed significantly slower photosynthetic induction and lower gs, as well as a slower gs increase, in the afternoon than in the morning and noon. Sugarcane showed a reduction in steady-state A reached under high irradiance and slower gs increase as well as lower gs reached under high irradiance, but no changes in the rate of photosynthetic induction, in the afternoon, compared to morning and noon. These reductions in the afternoon were not reverted by a dark treatment in the middle of the day, suggesting that the decrease was not proportional to diurnal time-integrated carbon fixation. Repeated exposure to light- and shadeflecks (1000 and 50 µmol m-2 s-1, lasting 20 min each) revealed fundamental differences in stomatal regulation between species: in fig, stomata opened and closed slowly, and their opening became progressively slower under a series of lightflecks, whereas sugarcane showed much faster stomatal opening than closure that was unchanged during the course of the day. Our results highlight that steady-state rates and irradiance-induced kinetics of photosynthesis and stomatal movement change diurnally in most species studied, and that they do so differently in fig and sugarcane.

Corrigendum: Variation of photosynthetic induction in major horticultural crops is mostly driven by differences in stomatal traits.

[This corrects the article DOI: 10.3389/fpls.2022.860229.].

A small dynamic leaf-level model predicting photosynthesis in greenhouse tomatoes.

The conversion of supplemental greenhouse light energy into biomass is not always optimal. Recent trends in global energy prices and discussions on climate change highlight the need to reduce our energy footprint associated with the use of supplemental light in greenhouse crop production. This can be achieved by implementing "smart" lighting regimens which in turn rely on a good understanding of how fluctuating light influences photosynthetic physiology. Here, a simple fit-for-purpose dynamic model is presented. It accurately predicts net leaf photosynthesis under natural fluctuating light. It comprises two ordinary differential equations predicting: 1) the total stomatal conductance to CO2 diffusion and 2) the CO2 concentration inside a leaf. It contains elements of the Farquhar-von Caemmerer-Berry model and the successful incorporation of this model suggests that for tomato (Solanum lycopersicum L.), it is sufficient to assume that Rubisco remains activated despite rapid fluctuations in irradiance. Furthermore, predictions of the net photosynthetic rate under both 400ppm and enriched 800ppm ambient CO2 concentrations indicate a strong correlation between the dynamic rate of photosynthesis and the rate of electron transport. Finally, we are able to indicate whether dynamic photosynthesis is Rubisco or electron transport rate limited.

A simple system for phenotyping of plant transpiration and stomatal conductance response to drought.

Plant breeding for increased crop water use efficiency or drought stress resistance requires methods to quickly assess the transpiration rate (E) and stomatal conductance (gs) of a large number of individual plants. Several methods to measure E and gs exist, each of which has its own advantages and shortcomings. To add to this toolbox, we developed a method that uses whole-plant thermal imaging in a controlled environment, where aerial humidity is changed rapidly to induce changes in E that are reflected in changes in leaf temperature. This approach is based on a simplified energy balance equation, without the need for a reference material or complicated calculations. To test this concept, we built a double-sided, perforated, open-top plexiglass chamber that was supplied with air at a high flow rate (35 L min-1) and whose relative humidity (RH) could be switched rapidly. Measurements included air and leaf temperature as well as RH. Using several well-watered and drought stressed genotypes of Arabidopsis thaliana that were exposed to multiple cycles in RH (30-50 % and back), we showed that leaf temperature as measured in our system correlated well with E and gs measured in a commercial gas exchange system. Our results demonstrate that, at least within a given species, the differences in leaf temperature under several RH can be used as a proxy for E and gs. Given that this method is fairly quick, noninvasive and remote, we envision that it could be upscaled for work within rapid plant phenotyping systems.

Light acclimation interacts with thylakoid ion transport to govern the dynamics of photosynthesis in Arabidopsis.

Understanding photosynthesis in natural, dynamic light environments requires knowledge of long-term acclimation, short-term responses, and their mechanistic interactions. To approach the latter, we systematically determined and characterized light-environmental effects on thylakoid ion transport-mediated short-term responses during light fluctuations. For this, Arabidopsis thaliana wild-type and mutants of the Cl- channel VCCN1 and the K+ exchange antiporter KEA3 were grown under eight different light environments and characterized for photosynthesis-associated parameters and factors in steady state and during light fluctuations. For a detailed characterization of selected light conditions, we monitored ion flux dynamics at unprecedented high temporal resolution by a modified spectroscopy approach. Our analyses reveal that daily light intensity sculpts photosynthetic capacity as a main acclimatory driver with positive and negative effects on the function of KEA3 and VCCN1 during high-light phases, respectively. Fluctuations in light intensity boost the accumulation of the photoprotective pigment zeaxanthin (Zx). We show that KEA3 suppresses Zx accumulation during the day, which together with its direct proton transport activity accelerates photosynthetic transition to lower light intensities. In summary, both light-environment factors, intensity and variability, modulate the function of thylakoid ion transport in dynamic photosynthesis with distinct effects on lumen pH, Zx accumulation, photoprotection, and photosynthetic efficiency.

Ultrasound Pulse Emission Spectroscopy Method to Characterize Xylem Conduits in Plant Stems.

Although it is well known that plants emit acoustic pulses under drought stress, the exact origin of the waveform of these ultrasound pulses has remained elusive. Here, we present evidence for a correlation between the characteristics of the waveform of these pulses and the dimensions of xylem conduits in plants. Using a model that relates the resonant vibrations of a vessel to its dimension and viscoelasticity, we extract the xylem radii from the waveforms of ultrasound pulses and show that these are correlated and in good agreement with optical microscopy. We demonstrate the versatility of the method by applying it to shoots of ten different vascular plant species. In particular, for Hydrangea quercifolia, we further extract vessel element lengths with our model and compare them with scanning electron cryomicroscopy. The ultrasonic, noninvasive characterization of internal conduit dimensions enables a breakthrough in speed and accuracy in plant phenotyping and stress detection.

Variation of Photosynthetic Induction in Major Horticultural Crops Is Mostly Driven by Differences in Stomatal Traits.

Under natural conditions, irradiance frequently fluctuates, causing net photosynthesis rate (A) to respond slowly and reducing the yields. We quantified the genotypic variation of photosynthetic induction in 19 genotypes among the following six horticultural crops: basil, chrysanthemum, cucumber, lettuce, tomato, and rose. Kinetics of photosynthetic induction and the stomatal opening were measured by exposing shade-adapted leaves (50 µmol m-2 s-1) to a high irradiance (1000 µmol m-2 s-1) until A reached a steady state. Rubisco activation rate was estimated by the kinetics of carboxylation capacity, which was quantified using dynamic A vs. [CO2] curves. Generally, variations in photosynthetic induction kinetics were larger between crops and smaller between cultivars of the same crop. Time until reaching 20-90% of full A induction varied by 40-60% across genotypes, and this was driven by a variation in the stomatal opening rather than Rubisco activation kinetics. Stomatal conductance kinetics were partly determined by differences in the stomatal size and density; species with densely packed, smaller stomata (e.g., cucumber) tended to open their stomata faster, adapting stomatal conductance more rapidly and efficiently than species with larger but fewer stomata (e.g., chrysanthemum). We conclude that manipulating stomatal traits may speed up photosynthetic induction and growth of horticultural crops under natural irradiance fluctuations.

NaCl affects photosynthetic and stomatal dynamics by osmotic effects and reduces photosynthetic capacity by ionic effects in tomato.

NaCl stress affects stomatal behavior and photosynthesis by a combination of osmotic and ionic components, but it is unknown how these components affect stomatal and photosynthetic dynamics. Tomato (Solanum lycopersicum) plants were grown in a reference nutrient solution [control; electrical conductivity (EC)=2.3 dS m-1], a solution containing additional macronutrients (osmotic effect; EC=12.6 dS m-1), or a solution with additional 100 mM NaCl (osmotic and ionic effects; EC=12.8 dS m-1). Steady-state and dynamic photosynthesis, and leaf biochemistry, were characterized throughout leaf development. The osmotic effect decreased steady-state stomatal conductance while speeding up stomatal responses to light intensity shifts. After 19 d of treatment, photosynthetic induction was reduced by the osmotic effect, which was attributable to lower initial stomatal conductance due to faster stomatal closing under low light. Ionic effects of NaCl were barely observed in dynamic stomatal and photosynthetic behavior, but led to a reduction in leaf photosynthetic capacity, CO2 carboxylation rate, and stomatal conductance in old leaves after 26 d of treatment. With increasing leaf age, rates of light-triggered stomatal movement and photosynthetic induction decreased across treatments. We conclude that NaCl impacts dynamic stomatal and photosynthetic kinetics by osmotic effects and reduces photosynthetic capacity by ionic effects.

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.

LED and HPS Supplementary Light Differentially Affect Gas Exchange in Tomato Leaves.

Using light emitting diodes (LED) instead of conventionally used high pressure sodium (HPS) lamps as a supplemental light source in greenhouses results in a higher efficacy (µmol light per J electricity) and makes it possible to customize the light spectrum. To explore the effects of LED and HPS on gas exchange, thermal relations, photosynthesis, and water status of young tomato plants, seven genotypes were grown in a greenhouse under LED (95% red, 5% blue) or HPS lamps in four experiments differing in the fraction of lamp light over natural light. HPS lights emit a broader spectrum of red (40%), green-yellow (50%), blue (5%), and far-red (5%) and a substantial amount of infrared radiation (heat). Young tomato plants grown under LED showed lower leaf temperature and higher stomatal density, stomatal conductance (gs) and transpiration rate (E) than plants grown under HPS; this may be due to the different supplemental light spectrum. The young plants grown under LED tended to have increased photosynthetic capacity. Furthermore, the water stress indices CWSI and IG, which were obtained using thermal imaging, were positively correlated with gas exchange-derived gs and E, putting forward the use of thermal imaging for the phenotyping of transpiration. Under LED light, photosynthetic gas exchange was generally increased, which agreed with the water stress indices. The extent of this increase was genotype-dependent. All differences between LED and HPS were smaller in the experiments where the fraction of lamp light over natural light was smaller.

High Stomatal Conductance in the Tomato Flacca Mutant Allows for Faster Photosynthetic Induction.

Due to their slow movement and closure upon shade, partially closed stomata can be a substantial limitation to photosynthesis in variable light intensities. The abscisic acid deficient flacca mutant in tomato (Solanum lycopersicum) displays very high stomatal conductance (gs ). We aimed to determine to what extent this substantially increased gs affects the rate of photosynthetic induction. Steady-state and dynamic photosynthesis characteristics were measured in flacca and wildtype leaves, by the use of simultaneous gas exchange and chlorophyll fluorometry. The steady-state response of photosynthesis to CO2, maximum quantum efficiency of photosystem II photochemistry (Fv/Fm ), as well as mesophyll conductance to CO2 diffusion were not significantly different between genotypes, suggesting similar photosynthetic biochemistry, photoprotective capacity, and internal CO2 permeability. When leaves adapted to shade (50 µmol m-2 s-1) at 400 µbar CO2 partial pressure and high humidity (7 mbar leaf-to-air vapour pressure deficit, VPD) were exposed to high irradiance (1500 µmol m-2 s-1), photosynthetic induction was faster in flacca compared to wildtype leaves, and this was attributable to high initial gs in flacca (~0.6 mol m-2 s-1): in flacca, the times to reach 50 (t50 ) and 90% (t90 ) of full photosynthetic induction were 91 and 46% of wildtype values, respectively. Low humidity (15 mbar VPD) reduced gs and slowed down photosynthetic induction in the wildtype, while no change was observed in flacca; under low humidity, t50 was 63% and t90 was 36% of wildtype levels in flacca. Photosynthetic induction in low CO2 partial pressure (200 µbar) increased gs in the wildtype (but not in flacca), and revealed no differences in the rate of photosynthetic induction between genotypes. Effects of higher gs in flacca were also visible in transients of photosystem II operating efficiency and non-photochemical quenching. Our results show that at ambient CO2 partial pressure, wildtype gs is a substantial limitation to the rate of photosynthetic induction, which flacca overcomes by keeping its stomata open at all times, and it does so at the cost of reduced water use efficiency.

Salt stress and fluctuating light have separate effects on photosynthetic acclimation, but interactively affect biomass.

In nature, soil salinity and fluctuating light (FL) often occur concomitantly. However, it is unknown whether salt stress interacts with FL on leaf photosynthesis, architecture, biochemistry, pigmentation, mineral concentrations, as well as whole-plant biomass. To elucidate this, tomato (Solanum lycopersicum) seedlings were grown under constant light (C, 200 µmol m-2 s-1 ) or FL (5-650 µmol m-2 s-1 ), in combination with no (0 mM NaCl) or moderate (80 mM NaCl) salinity, for 14 days, at identical photoperiods and daily light integrals. FL and salt stress had separate effects on leaf anatomy, biochemistry and photosynthetic capacity: FL reduced leaf thickness as well as nitrogen, chlorophyll and carotenoid contents per unit leaf area, but rarely affected steady-state and dynamic photosynthetic properties along with abundance of key proteins in the electron transport chain. Salt stress, meanwhile, mainly disorganized chloroplast grana stacking, reduced stomatal density, size and aperture as well as photosynthetic capacity. Plant biomass was affected interactively by light regime and salt stress: FL reduced biomass in salt stressed plants by 17%, but it did not affect biomass of non-stressed plants. Our results stress the importance of considering FL when inferring effects of salt-stress on photosynthesis and productivity under fluctuating light intensities.

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.

Growth under Fluctuating Light Reveals Large Trait Variation in a Panel of Arabidopsis Accessions.

The capacity of photoautotrophs to fix carbon depends on the efficiency of the conversion of light energy into chemical potential by photosynthesis. In nature, light input into photosynthesis can change very rapidly and dramatically. To analyze how genetic variation in Arabidopsis thaliana affects photosynthesis and growth under dynamic light conditions, 36 randomly chosen natural accessions were grown under uniform and fluctuating light intensities. After 14 days of growth under uniform or fluctuating light regimes, maximum photosystem II quantum efficiency (Fv/Fm) was determined, photosystem II operating efficiency (ΦPSII) and non-photochemical quenching (NPQ) were measured in low light, and projected leaf area (PLA) as well as the number of visible leaves were estimated. Our data show that ΦPSII and PLA were decreased and NPQ was increased, while Fv/Fm and number of visible leaves were unaffected, in most accessions grown under fluctuating compared to uniform light. There were large changes between accessions for most of these parameters, which, however, were not correlated with genomic variation. Fast growing accessions under uniform light showed the largest growth reductions under fluctuating light, which correlated strongly with a reduction in ΦPSII, suggesting that, under fluctuating light, photosynthesis controls growth and not vice versa.

Growth under Fluctuating Light Reveals Large Trait Variation in a Panel of Arabidopsis Accessions.

The capacity of photoautotrophs to fix carbon depends on the efficiency of the conversion of light energy into chemical potential by photosynthesis. In nature, light input into photosynthesis can change very rapidly and dramatically. To analyze how genetic variation in Arabidopsis thaliana affects photosynthesis and growth under dynamic light conditions, 36 randomly chosen natural accessions were grown under uniform and fluctuating light intensities. After 14 days of growth under uniform or fluctuating light regimes, maximum photosystem II quantum efficiency (Fv/Fm) was determined, photosystem II operating efficiency (ΦPSII) and non-photochemical quenching (NPQ) were measured in low light, and projected leaf area (PLA) as well as the number of visible leaves were estimated. Our data show that ΦPSII and PLA were decreased and NPQ was increased, while Fv/Fm and number of visible leaves were unaffected, in most accessions grown under fluctuating compared to uniform light. There were large changes between accessions for most of these parameters, which, however, were not correlated with genomic variation. Fast growing accessions under uniform light showed the largest growth reductions under fluctuating light, which correlated strongly with a reduction in ΦPSII, suggesting that, under fluctuating light, photosynthesis controls growth and not vice versa.