Photosynthetic Electron Transport Rate Research Articles

Rice (Oryza sativa L.) exhibits differential responses to salt stress and salt shock in electron transfer efficiency

The study of crop response to soil salinity has traditionally focused on the effects of varying salt concentrations, with less attention given to the methodologies of salt application. This research aimed to elucidate the differential impacts of two distinct salinity exposure methods on the photosynthetic characteristics of rice. Plants subjected to a gradual increase in salinity experience salt stress, whereas those immediately exposed to high salinity levels encounter a salt shock condition. Our findings revealed that rice plants accumulated comparable levels of shoot Na+ under both salt stress and salt shock at 4 and 10 days after the onset of stress. No significant differences were observed in the steady-state photosynthetic rate and electron transport rate (ETR). In contrast, salt stress markedly reduced the relative variable fluorescence at the ‘J’ step (Vj), thereby enhancing the probability of an exciton transferring an electron beyond the primary quinone electron acceptor (QA−) (ΨEo). This led to a notable increase in the activity of leaf photosystem II (PSII), as evidenced by the Performance Index on absorption basis (PIabs), which was twice as high as that of the control group. This mechanism is believed to facilitate the activation of alternative metabolic pathways. Additionally, kinetic measurements of light potential indicated that salt stress improved the initial induction of ETR after a brief exposure to high photosynthetically active radiation (PAR) compared to the control. This effect was not observed in the salt shock treatment group. Our results suggest that plants exhibit distinct physiological responses to salt stress versus salt shock, and the potential positive influence on leaf photosynthetic activity during salt stress may have been previously underestimated.

Transcriptomics combined with physiology reveals the effect of different light qualities on the development of Fritillaria cirrhosa in the Qinghai Tibet Plateau

Fritillaria cirrhosa, a high-altitude plant susceptible to environmental conditions, commonly grows beneath bushes and is categorized as shade-tolerant. The development and secondary metabolite synthesis of F. cirrhosa are significantly influenced by light quality. This study investigated the physiological responses and molecular mechanisms of F. cirrhosa to different light qualities using colored agricultural film treatments. The results demonstrated that the number of sprout tumbles in F. cirrhosa was significantly reduced by the different colored film treatments. The quality of the fruits exhibited a notable improvement under the blue film (BF) treatment compared to the other groups (P < 0.05). The guanosine content showed a substantial increase of 440.55% under the BF treatment relative to the white film (WF), and the uracil content significantly increased by 171.27% in BF and by 296.38% in the green film (GF). Leaf thickness was considerably higher under the GF treatment than under the other conditions. F. cirrhosa demonstrated enhanced photosynthesis when exposed to BF and GF (P < 0.05). The transcriptome results indicated 75 differentially expressed genes (DEGs) involved in photosynthesis-related pathways that regulated photosynthetic utilization efficiency by encoding photosynthetic antenna proteins (Lhca and Lhcb), photosynthetic electron transporters (PsbE, PsbP, and PetA), and carbon fixation enzymes (PPC, PPDK, and rbcL). Most these genes were upregulated in BF and GF, improving the light energy conversion efficiency, photosynthetic electron transport rate, and CO2 assimilation ability in F. cirrhosa. Notably, 44 DEGs involved in flavonoid metabolic pathways (PAL, HCT, 4CL, CHS, CHI, and ANS) showed higher expression levels in WF and yellow film (YF), differing in the regulation of the photosynthetic pathway. This suggested that F. cirrhosa can utilize nutrients produced by photosynthesis more efficiently under BF and GF, and does not require excessive amounts of flavonoids for maintaining cell homeostasis. These results offer valuable insights into the transcriptional changes and molecular processes occurring in F. cirrhosa in response to different light qualities and provide a basis for research on the mechanisms of photo regulation, cultivation, and ecological adaptation in F. cirrhosa.

The combination of elevated CO2 and warmer temperature reduces photosynthetic capacity without diluting leaf N concentration in Amur linden (Tilia amurensis)

Abstract Elevated CO2 and warmer temperatures represent the future environmental conditions in the context of global change. A good understanding of plant response to their combined effects is, therefore, critically important for predicting future plant performance. This study investigated the photosynthetic acclimation of Amur linden (Tilia amurensis) seedlings (current year, about 60 cm tall), a shade-tolerant tree species in the temperate broadleaf deciduous forest, to the combination of current CO2 concentration and temperature (CC) and the combination of the predicted future CO2 concentration and temperature (FC). The results show that FC promoted aboveground growth, but reduced photosynthetic capacity (Vcmax: maximum rate of RuBP carboxylation and Jmax: maximum photosynthetic electron transport rate). However, the photosynthetic rate measured under the corresponding growth CO2 concentration was still higher under FC than under CC. FC depressed the photosynthetic limiting transition point (Ci-t, An-t) from Rubisco carboxylation to RuBP regeneration, i.e. An-t decreased without a change in Ci-t. FC did not change leaf N concentration but increased the total leaf N content per tree and photosynthetic nitrogen utilization efficiency. This suggests that N utilization, rather than photosynthetic capacity, may play an important role in the acclimation of the species to future climatic conditions. This study provides new insights into the photosynthetic acclimation of Amur linden and can be used to predict its possible performance under future climatic conditions.

Mitigating Salinity Stress in Pomegranate: Effects of Pseudomonas fluorescens and Glomus mosseae on Stress Responses of Red Angel and Wonderful Cultivars

Salinity stress is a major challenge in modern pomegranate orchards. The application of biofertilizers is a promising strategy to mitigate the adverse effects of salinity stress and enhance the performance in pomegranates. This experiment was conducted in 2021 to investigate the effects of salinity stress (control, 4, and 8 dS/m) and inoculation with Pseudomonas fluorescens and Glomus mosseae, either individually or in combination (Pf+Gm), on Red Angel and Wonderful pomegranate cultivars. The results revealed a significant difference in the percentage of symbiosis and colonization among the biofertilizer treatments. Pf+Gm exhibited the highest percentages of symbiosis and colonization, followed by P. fluorescens. While salinity levels did not significantly affect symbiosis percentage, they had a detrimental effect on colonization percentage. Furthermore, biofertilizer treatments significantly increased leaf density, stem growth, total chlorophyll, and total anthocyanin, while reducing ion leakage and salinity damage index. In comparison to the non-inoculated control, the Pf+Gm treatment resulted in a 53.2 % decrease in ion leakage for the Red Angel cultivar and a 39.9 % decrease for the Wonderful cultivar. Salinity had a significant effect on proline production, with the Red Angel cultivar exhibiting the highest proline content (2.09 µmol/g) in response to salinity stress of 8 ds/m. However, biofertilizer treatments increased proline content in both cultivars. In Red Angel, the Pf+Gm treatment resulted in the highest proline production in the leaves (5.82 µmol/g), which was 123 % higher than that of the non-inoculated control. Additionally, the transpiration rate decreased with increasing salinity levels, and biofertilizer treatments had varying effects on photosynthetic gas exchange and electron transport rate. The interaction effect of biofertilizer treatments and salinity on CO2 absorption was highly significant, with Pf+Gm treatment exhibiting the highest absorption (8.51 and 8.61 µmol/m2/s in Red Angel and Wonderful, respectively). These findings suggest that applied biofertilizer treatments, especially Pf+Gm, can be an effective strategy for alleviating the negative effects of salinity stress on pomegranate cultivation.

The importance of species-specific and temperature-sensitive parameterisation of A/Ci models: A case study using cotton (Gossypium hirsutum L.) and the automated 'OptiFitACi' R-package.

Leaf gas exchange measurements are an important tool for inferring a plant's photosynthetic biochemistry. In most cases, the responses of photosynthetic CO2 assimilation to variable intercellular CO2 concentrations (A/Ci response curves) are used to model the maximum (potential) rate of carboxylation by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, Vcmax) and the rate of photosynthetic electron transport at a given incident photosynthetically active radiation flux density (PAR; JPAR). The standard Farquhar-von Caemmerer-Berry model is often used with default parameters of Rubisco kinetic values and mesophyll conductance to CO2 (gm) derived from tobacco that may be inapplicable across species. To study the significance of using such parameters for other species, here we measured the temperature responses of key in vitro Rubisco catalytic properties and gm in cotton (Gossypium hirsutum cv. Sicot 71) and derived Vcmax and J2000 (JPAR at 2000 µmol m-2 s-1 PAR) from cotton A/Ci curves incrementally measured at 15°C-40°C using cotton and other species-specific sets of input parameters with our new automated fitting R package 'OptiFitACi'. Notably, parameterisation by a set of tobacco parameters produced unrealistic J2000:Vcmax ratio of <1 at 25°C, two- to three-fold higher estimates of Vcmax above 15°C, up to 2.3-fold higher estimates of J2000 and more variable estimates of Vcmax and J2000, for our cotton data compared to model parameterisation with cotton-derived values. We determined that errors arise when using a gm,25 of 2.3 mol m-2 s-1 MPa-1 or less and Rubisco CO2-affinities in 21% O2 (KC 21%O2) at 25°C outside the range of 46-63 Pa to model A/Ci responses in cotton. We show how the A/Ci modelling capabilities of 'OptiFitACi' serves as a robust, user-friendly, and flexible extension of 'plantecophys' by providing simplified temperature-sensitivity and species-specificity parameterisation capabilities to reduce variability when modelling Vcmax and J2000.

Effects of Exogenous Melatonin on the Growth and Physiological Characteristics of Ginkgo biloba L. under Salinity Stress Conditions

Ginkgo (Ginkgo biloba L.) is a cherished relic among plants, commonly planted as a street tree. However, it faces cultivation challenges due to escalating soil salinization and widespread snowmelt application. Therefore, this study used 4-year-old Ginkgo seedlings to investigate how exogenous melatonin at varying concentrations affects seedling growth and physiology under salinity stress. The results revealed that appropriate melatonin concentrations (0.02, 0.1 mmol·L−1) significantly mitigated leaf yellowing under different NaCl stress levels. Furthermore, they increased ground diameter, current-year branch growth, relative water concentration, free proline, and soluble sugars in leaves. Melatonin also reduced electrolyte exudation rates, flavonoids, and malonic dialdehyde concentration, while enhancing peroxidase and superoxide dismutase activities. This led to reduced chlorophyll content, photosynthetic rate, stomatal conductance, and transpiration rate, stabilizing intercellular CO2 concentration, preserving photosynthetic structures, and enhancing photosynthetic rates. Additionally, the decline in the photosynthetic electron transport rate, the effective photochemical quantum yield of PSII, and the potential efficiency of primary conversion of light energy of PSII was alleviated. Minimal fluorescence and the non-photochemical quenching coefficient also improved. However, high melatonin concentration (0.5 mmol·L−1) exacerbated salinity stress. After analyzing composite scores, the 0.02 mmol·L−1 melatonin treatment was most effective in alleviating NaCl stress, while the 0.5 mmol·L−1 treatment intensified physiological stress under 200 mmol·L−1 NaCl stress. Principal component analysis and correlation analysis identified seven physiological indicators (photosynthetic rate, transpiration rate, photosynthetic electron transport rate, minimal fluorescence, superoxide dismutase, free proline, and chlorophyll a) and three growth indicators (ground diameter, branch length, and current-year branch thickness) as key markers for rapid salinity stress assessment in Ginkgo. These findings are crucial for addressing challenges associated with snowmelt’s impact on roadside Ginkgo trees, expanding planting areas, and breeding exceptional salt-tolerant Ginkgo varieties.

Effect of copper oxide and zinc oxide nanoparticles on photosynthesis and physiology of Raphanus sativus L. under salinity stress

The study evaluates the impact of two metal oxide nanoparticles: copper oxide (CuO) and zinc oxide (ZnO) on the growth and physiology of Raphanus sativus L. (radish) under salinity stress. Fifteen days old seedlings of R. sativus were subjected to different concentrations of salt stress (0 mM, 150 mM, and 300 mM NaCl) alone and in interaction with 100 mgL−1 metal oxide nanoparticle treatments (CuO and ZnO NPs via foliar spray) for 15 days. The results confirmed the severe effects of salinity stress on the growth and physiology of radish plants by decreasing nutrient uptake, leaf area, and photosystems photochemistry and by increasing proline accumulation, anthocyanin, flavonoids content, and antioxidant enzyme activities which is directly linked to increased oxidative stress. The foliar application of CuO and ZnO NPs alleviated the adverse effects of salt stress on radish plants, as indicated by improving these attributes. Foliar spray of ZnO NPs was found efficient in improving the leaf area, photosynthetic electron transport rate, the PSII quantum yield, proton conductance and mineral content in radish plants under NaCl stress. Besides, ZnO NPs decreased the NaCl-induced oxidative stress by declining proline, anthocyanin, and flavonoids contents and enzymatic activities such as superoxide dismutase (SOD), ascorbate peroxidase (APX) and guaiacol peroxidase (GOPX). Thus, our study revealed that ZnO NPs are more effective and have beneficial effects over CuO NPs in promoting growth and reducing the adverse effects of NaCl stress in radish plants.

Fluorescence-based primary productivity estimates are influenced by non-photochemical quenching dynamics in Arctic phytoplankton.

Chlorophyll fluorescence-based estimates of primary productivity typically include dark or low-light pre-treatments to relax non-photochemical quenching (NPQ), a process that influences the relationship between PSII photochemistry and fluorescence yields. The time-scales of NPQ relaxation vary significantly between phytoplankton taxa and across environmental conditions, creating uncertainty in field-based productivity measurements derived from fluorescence. To address this practical challenge, we used fast repetition rate fluorometry to characterize NPQ relaxation kinetics in Arctic Ocean phytoplankton assemblages across a range of hydrographic regimes. Applying numerical fits to our data, we derived NPQ relaxation life times, and determined the relative contributions of various quenching components to the total NPQ signature across the different assemblages. Relaxation kinetics were best described as a combination of fast-, intermediate- and slow-relaxing processes, operating on time-scales of seconds, minutes, and hours, respectively. Across sampling locations and depths, total fluorescence quenching was dominated by the intermediate quenching component. Our results demonstrated an average NPQ relaxation life time of 20 ± 1.9 min, with faster relaxation among high light acclimated surface samples relative to lowlight acclimated sub-surface samples. We also used our results to examine the influence of NPQ relaxation on estimates of photosynthetic electron transport rates (ETR), testing the commonly held assumption that NPQ exerts proportional effects on light absorption (PSII functional absorption cross section, σPSII) and photochemical quantum efficiency (FV/FM). This assumption was violated in a number of phytoplankton assemblages that showed a significant decoupling of σPSII and FV/FM during NPQ relaxation, and an associated variability in ETR estimates. Decoupling of σPSII and FV/FM was most prevalent in samples displaying symptoms photoinhibition. Our results provide insights into the mechanisms and kinetics of NPQ in Arctic phytoplankton assemblages, with important implications for the use of FRRF to derive non-invasive, high-resolution estimates of photosynthetic activity in polar marine waters.

Photosynthesis in newly developed leaves of heat-tolerant wheat acclimates to long-term nocturnal warming.

We examined photosynthetic traits of pre-existing and newly developed flag leaves of four wheat genotypes grown in controlled-environment experiments. In newly developed leaves, acclimation of the maximum rate of net CO2 assimilation (An) to warm nights (i.e. increased An) was associated with increased capacity of Rubisco carboxylation and photosynthetic electron transport, with Rubisco activation state probably contributing to increased Rubisco activity. Metabolite profiling linked acclimation of An to greater accumulation of monosaccharides and saturated fatty acids in leaves; these changes suggest roles for osmotic adjustment of leaf turgor pressure and maintenance of cell membrane integrity. By contrast, where An decreased under warm nights, the decline was related to lower stomatal conductance and rates of photosynthetic electron transport. Decreases in An occurred despite higher basal PSII thermal stability in all genotypes exposed to warm nights: Tcrit of 45-46.5 °C in non-acclimated versus 43.8-45 °C in acclimated leaves. Pre-existing leaves showed no change in An-temperature response curves, except for an elite heat-tolerant genotype. These findings illustrate the impact of night-time warming on the ability of wheat plants to photosynthesize during the day, thereby contributing to explain the impact of global warming on crop productivity.

Expression of the plastocyanin gene PETE2 in Camelina sativa improves seed yield and salt tolerance

Plastocyanin functions as an electron carrier in the photosynthetic electron transport chain, located at the thylakoid membrane. In several species, endogenous plastocyanin levels are correlated with the photosynthetic electron transport rate. Overexpression of plastocyanin genes in Arabidopsis thaliana increases plant size, but this phenomenon has not been observed in crop species. Here, we investigated the effects of heterologous expression of a gene encoding a plastocyanin isoform from Arabidopsis, AtPETE2, in the oil seed crop Camelina sativa under standard growth conditions and under salt stress. AtPETE2 heterologous expression enhanced photosynthetic activity in Camelina, accelerating plant development and improving seed yield under standard growth conditions. Additionally, CsPETE2 from Camelina was induced by salt stress and AtPETE2 expression lines had larger primary roots and more lateral roots than the wild type. AtPETE2 expression lines also had larger seeds and higher total seed yield under long-term salt stress compared with non-transgenic Camelina. Our results demonstrate that increased plastocyanin levels in Camelina can enhance photosynthesis and productivity, as well as tolerance to osmotic and salt stresses. Heterologous expression of plastocyanin may be a useful strategy to mitigate crop stress in saline soils.

Elucidating the molecular mechanisms of exogenous melatonin for improving heat tolerance in Solanum tuberosum L. seedlings

In the era of climate change, the rise in temperature can adversely impact the potato growth, yield and even threaten food security. Temperature both as high and long-term have always retarded growth and development of potatoes. Based on this, overall study provides experimental proof of potato (Solanum tuberosum L.) heat tolerance mechanisms. Plant hormone melatonin (MT) elicits and improves potato tolerance against high temperature, but scarce information is present about detailed mechanisms. Therefore, introduction of exogenous MT in potato seedlings (Lishu 6) together with high-temperature stress was investigated to find the physiological and molecular mechanisms. Temperature stress results in wilting of basal leaves and inhibits photosynthesis in potato seedlings. Interestingly, we found distinct effects of introduced MT with increase in the actual photosynthetic quantum yield (ΦPSII), apparent photosynthetic electron transport rate (ETR), chlorophyll content, intercellular CO2 concentration (Ci), net photosynthetic rate (Pn), photochemical quenching (qP), and transpiration rate (Tr). Further, hydrogen peroxide (H2O2) content and activities of antioxidant enzymes in potato seedling leaves reduced after introduction of exogenous MT. Through transcriptome analysis, we found that during temperature stress, MT significantly upregulated the expression of genes involved in photosynthesis and antioxidant enzymes, and plant hormone signal transduction. Importantly, it also augmented antioxidant capacity in potato seedlings and alleviated the damage at high temperature. In summary, we provide evidence that identifying factors could modulate the potential interaction of temperature stress with introduced MT to improve the heat tolerance of potato seedlings for maximum function.

Effects of Shading on the Growth and Photosynthetic Fluorescence Characteristics of Castanopsis hystrix Seedlings of Top Community-Building Species in Southern Subtropical China

Castanopsis hystrix is a major community-building species in the top communities of southern subtropical China, with a high natural regeneration capacity. However, excessive logging and the introduction of exotic tree species have substantially reduced the area of natural forest patches of Castanopsis hystrix, and seedling regeneration is essential for the long-term continuation of Castanopsis hystrix populations. To explore the effects of light intensity on the seedling emergence and early growth of Castanopsis hystrix, shading experiments were conducted under four shading treatments (0%, 40%, 60%, and 80%). The growth, biomass accumulation, and distribution, the quality index of seedlings, the morphology and structure of the root systems of seedlings, and the leaf chlorophyll content and chlorophyll fluorescence properties of seedlings under different shading treatments were analyzed. The results displayed the following: (1) Shade intensity impacts growth of Castanopsis hystrix seedlings and biomass allocation, with optimal results observed at 60% shade, leading to the promotion of organic matter production in leaves and the limitation of stem growth. (2) Using a multi-indicator composite index, it was determined that seedling quality for Castanopsis hystrix peaks at 60% shade intensity. (3) Shade significantly impacts the morphology and structure of Castanopsis hystrix’s root system, with most root characteristics peaking at 60% shade, indicating a substantial increase in root development compared to no-shade conditions. (4) The D-values indicated the most suitable shade intensity for seedling growth to be 60%, suggesting that Castanopsis hystrix seedlings are sensitive to light and excessive light can be detrimental to their growth. (5) The 60% shade treatment showed the maximum values of chlorophyll fluorescence characteristics and photochemical activity, with variations in energy conversion efficiency and dissipation reflected in parameters like photochemical burst coefficient (qP), photochemical burst coefficient (qN), the actual photometric yield of PSII under light acclimation (YII), and the maximum photosynthetic electron transport rate in photoinhibition (ETR), thereby supporting seedling growth and maintaining the normal function of photosynthetic organs. In conclusion, 60% shade treatment can effectively improve the growth and photosynthetic characteristics of Castanopsis hystrix seedlings and promote the accumulation of nutrient elements, ultimately promoting their growth.

Effect of high salinity and of priming of non-germinated seeds by UV-C light on photosynthesis of lettuce plants grown in a controlled soilless system.

High salinity results in a decrease in plant photosynthesis and crop productivity. The aim of the present study was to evaluate the effect of UV-C priming treatments of lettuce seeds on photosynthesis of plants grown at high salinity. Non-primed and primed seeds were grown in an hydroponic system, with a standard nutrient solution, either supplemented with 100 mM NaCl (high salinity), or not (control). Considering that leaf and root K+ concentrations remained constant and that chlorophyll fluorescence parameters and root growth were not affected negatively in the high salinity treatment, we conclude that the latter was at the origin of a moderate stress only. A substantial decrease in leaf net photosynthetic assimilation (Anet) was however observed as a consequence of stomatal and non-stomatal limitations in the high salinity treatment. This decrease in Anet translated into a decrease in growth parameters; it may be attributed partially to the high salinity-associated increase in leaf concentration in abscisic acid and decrease in stomatal conductance. Priming by UV-C light resulted in an increase in total photosynthetic electron transport rate and Anet in the leaves of plants grown at high salinity. The increase of the latter translated into a moderate increase in growth parameters. It is hypothesized that the positive effect of UV-C priming on Anet and growth of the aerial part of lettuce plants grown at high salinity, is mainly due to its stimulating effect on leaf concentration in salicylic acid. Even though leaf cytokinins' concentration was higher in plants from primed seeds, maintenance of the cytokinins-to-abscisic acid ratio also supports the idea that UV-C priming resulted in protection of plants exposed to high salinity.

Long-term CO2 Enrichment Increases Biomass but Results in Rapid Physiological Acclimation of Petunia and Pansy

Although crops often respond immediately to enriched CO2 concentrations (e.g., increased photosynthesis), this initial response is often not sustained throughout production, thus reducing the benefit of this input. For horticulture species, the timing and extent of these acclimation responses are still widely uncertain. Therefore, the objective of this research was to determine species-specific acclimation responses to elevated CO2 concentrations for pansy (Viola ×wittrockiana ‘Matrix Blue Blotch Improved’) and petunia (Petunia ×hybrida ‘Dreams Midnight’). Seedlings were transplanted to 11.5-cm pots and placed in growth chambers with air temperature, relative humidity, and radiation intensity setpoints of 21 °C, 55%, and 250 μmol⋅m−2⋅s−1, respectively. Carbon dioxide treatments were established using the two growth chambers with setpoints of either 400 (ambient) or 1000 μmol⋅mol−1 (elevated) maintained during a 16-hour photoperiod. In addition to data collected through destructive harvest, the rate of photosynthesis (A) in response to increasing internal leaf CO2 concentration (A-Ci) and at the operating CO2 concentration (A-Ca) were measured weekly with a portable leaf photosynthesis system at saturating [A-Ci (1000 μmol⋅m−2⋅s−1)] or production [A-Ca (250 μmol⋅m−2⋅s−1)] radiation intensities. For both pansy and petunia, elevated CO2 produced greater total shoot dry mass than ambient CO2 after 4 weeks. However, the decreased maximum rate of photosynthetic electron transport, maximum rate of Rubisco carboxylase, and triose phosphate utilization rate of both species were also observed under elevated CO2. Similarly, A measured at 400 and 1000 μmol⋅mol−1 was reduced for both pansy and petunia grown under the elevated compared with ambient CO2 concentration based on A-Ca responses after 7 days, indicating quick physiological acclimation to this input. These results provide information regarding the timing and extent of physiological acclimation in response to elevated CO2 concentrations. However, because of physiological acclimation potentially occurring within 7 days of treatment initiation, additional research is necessary to develop species-specific recommendations for controlled environment production.

Faster induction of photosynthesis increases biomass and grain yield in glasshouse-grown transgenic Sorghum bicolor overexpressing Rieske FeS.

Sorghum is one of the most important crops providing food and feed in many of the world's harsher environments. Sorghum utilizes the C4 pathway of photosynthesis in which a biochemical carbon-concentrating mechanism results in high CO2 assimilation rates. Overexpressing the Rieske FeSsubunit of the Cytochrome b6 f complex was previously shown to increase the rate of photosynthetic electron transport and stimulate CO2 assimilation in the model C4 plant Setaria viridis. To test whether productivity of C4 crops could be improved by Rieske overexpression, we created transgenic Sorghum bicolor Tx430 plants with increased Rieske content. The transgenic plants showed no marked changes in abundances of other photosynthetic proteins or chlorophyll content. The steady-state rates of electron transport and CO2 assimilation did not differ between the plants with increased Rieske abundance and control plants, suggesting that Cytochrome b6 f is not the only factor limiting electron transport in sorghum at high light and high CO2 . However, faster responses of non-photochemical quenching as well as an elevated quantumyield of Photosystem II and an increased CO2 assimilation rate were observed from the plants overexpressing Rieske during the photosyntheticinduction, a process of activation of photosynthesis upon the dark-light transition. Asaconsequence,sorghum with increased Rieske content produced more biomass and grain when grown in glasshouse conditions. Our results indicate that increasing Rieske content has potential to boost productivity of sorghum and other C4 crops by improving the efficiency of light utilization and conversion to biomass through the faster induction of photosynthesis.

Photosynthetic electron transport in a tropical moss Hyophila involuta

Despite their global distribution moss physiology is not well understood, particularly in tropical environments. Photosynthetic Electron Transport Rate (ETR) of Hyophila involuta was measured using PAM technology. The plants were growing in a heavily shaded habitat with irregular sunflecks of full sunlight. Two models were used for estimating photosynthetic electron transport (ETR), the Waiting-in-Line and Eilers-Peeters models. Both take photoinhibition into account and gave similar results. The population studied was growing on bricks and stonework and so periodically dried out. The same species is sometimes found in waterfall environments where it is semi-aquatic. ETRmax was low at dawn, increased to an early morning maxima and then decreased during the course of the day, partially reversed by rainstorms. Mid-morning (10:30 solar time): Eopt ≈ 521 ± 77 µmol photon m–2 s–1 which is higher than might be expected in a shaded sun-fleck environment, ETRmax ≈ 28.3 ± 2.5 µmol e- m–2 s–1 on a projected surface area basis or ≈ 57.7 ± 5.1 (µmol e- g–1 Chl a s–1), photosynthetic efficiency (Alpha, α0) ≈ 0.301 (e- photon–1 g–1 Chl a). Chl a content of Hyophila was ≈ 453 mg Chl a m–2 with Chl b/a ≈ 0.371. The O2 electrode-based respiration rate was R = 6.78 ± 0.698 µmol O2 g–1 Chl a s–1, the ETRmax is roughly equivalent to Gross photosynthesis (Pg) ≈ 14.4 ± 1.28 µmol O2 g–1 Chl a s–1 (4 e-/ O2), so the optimum Pg/R ratio is about 2.12 ± 0.289 and net photosynthesis (Pn) is ≈ 7.62 ± 1.46 µmol O2 g–1 Chl a s–1. Thus even under optimum irradiance net photosynthesis was actually rather low. pH experiments showed that Hyophila used both CO2 & HCO3- inorganic carbon sources, suggesting that a concentrating mechanism (CCM) is present. Hyophila is partially homiochlorophyllous, recovering well from desiccation after 2h of lighted rehydration and recovers more after 24h but lost ≈30% of its Chl a. The moss retained its chlorophyll content if desiccated in the dark. The moss lost ≈ 2/3 of its Chl a during the monsoonal dry season.

Physiological and Molecular Responses of Wheat to Low Light Intensity

Here we document physiological and molecular attributes of three wheat cultivars (ZM9023, YM158 and FM1228) under low light intensity with advanced technologies, including non-standard quantitative technology and quantitative proteomics technology. We found lower dry matter accumulation of YM158 compared with ZM 9023 and FM1228 under low light intensities due to up-regulation of photosynthetic parameters electron transport rate (ETR), Y(II), Fv/Fm, Chl (a + b) of YM158 and down-regulation of Chl a/b. ETR, Y(II) and Fv/Fm significantly decreased between ZM9023 and FM1228. The ETR between PSII and PSI of YM158 increased, while light use efficiency (LUE) of ZM9023 and FM1228 decreased. We found that YM158 had greater propensity to adapt to low light compared with ZM9023, as the former was able to increase photochemical electron transfer rate, enhance photosystem activity, and increase the light energy under low light. This meant that the YM158 flag leaf has stronger regulatory mechanism under low light environment. Through proteomic analysis, we found LHC protein (LHCB1, LHCB4, LHCA2, LHCA3) for YH158 was significantly up-regulated, while the PSII subunit protein of FM1228 and ZM9023 b559 subunit protein were down-regulated. We also documented enhanced light use efficiency (LUE) due to higher light capture pigment protein complex (LHC), photosystem II (PSII), PSI and cytochrome B6F-related proteins, with dry matter accumulation being positively correlated with Fv/Fm, ETR, and ΦPS(II), and negatively correlated with initial fluorescence F0. We suggest that Fv/Fm, ETR, and ΦPS(II) could be considered in shade tolerance screening to facilitate wheat breeding.

Stimulative Effects of Lupinus sp. and Melilotus albus Underseed on the Photosynthetic Performance of Maize (Zea mays) in Two Intercropping Systems

In order to evaluate influential mechanisms for photosynthetic processes on the yields of an intercropping system composed of maize (Zea mays), Lupinus sp. and Melilotus albus, three treatments were designed and conducted in southern Moravia (Czech Republic) in the form of agronomy trials. The treatments included sole maize (SM), maize with Lupinus sp. (ML) and maize with field melilot (MM). The photosynthetic processes of Zea mays were monitored using several chlorophyll fluorescence techniques on the three treatments for 20 days in the late summer season. An analysis of fast chlorophyll fluorescence transients (OJIP) showed that the capacity of photochemical photosynthetic reactions in photosystem II (FV/FM), as well as the photosynthetic electron transport rate (ET0/RC), declined in response to a four-day episode of extremely warm days with full sunshine. Similarly, the performance index (PI), an indicator of general plant vitality, declined. The episode activated protective mechanisms in photosystem II (PSII), which resulted in an increase of thermal dissipation. For the majority of Z. mays photosynthetic parameters, their values decreased for particular treatments in the following order: MM, ML, SM. The MM and ML intercropping systems had a positive effect on the primary photosynthetic parameters in Z. mays.

Canopy development, leaf traits and yield in high-altitude Andean maize under contrasting plant densities in Argentina

Summary In highlands, the increase in altitude results in a drastic decrease in temperature (T) that delays phenological development of maize, decreasing light interception during the cycle. This could be partially overcome by increasing plant density, but information is scarce for designing specific management options. The objective of this work was to describe changes in canopy development, photosynthetic performance, biomass and yield of maize grown at contrasting plant densities (5.7 plants m−2, locally used, and 8.7 plants m−2, 50% higher). Three experiments were carried out in two high-altitude environments within the Argentinean Andean region, Hornillos (HOR, 2380 masl, 2019–20 and 2020–21) and El Rosal (ERO, 3350 masl, 2019–20), and complementary data were obtained from samplings in 8 farmer’s fields (from 2400 to 3400 masl, 2022–23). In the experiments, mean T during the first 150 days of the cycle was 33% lower at ERO, which implied 39 extra days but 25% shorter thermal time to achieve silking. The higher plant density significantly increased leaf area index and light interception at ERO, whereas at HOR, this was only evident during the second season. At the leaf level, plants grown at ERO had thicker leaves with higher chlorophyll (+36%) and nitrogen (40%) content. Photosynthetic electron transport rate at full irradiance was +20% higher at ERO but significantly varied throughout the day with lowest values in the morning, which was not observed at HOR and was not related to light intensity or stomatal conductance. At HOR, the increase in plant density did not improve light interception, nor yield in 2019–20 (with average yields of 6356 kg ha−1) but it did improve both in 2020–21 when generally lower yields were attained (4821 kg ha−1). Across farmer’s fields, increasing densities consistently reduced yield per plant (r2 = 0.57***) but improved yield per area basis, which was maximised at 10 pl m−2 as a result of a steady increase in kernel number m−2 (up to 15 pl m−2). Thus, in these high-altitude environments, increasing plant density beyond recommended (6 pl m−2) is a promising approach for improving yield, with major penalties of supra-optimum densities being related to kernel weight. Further work is needed to explore the effect of different factors limiting kernel growth, over plant density responses.

Temperature-light shapes the nutritional strategy of a mixotrophic green alga, Picochlorum sp. GLMF1 (Trebouxiophyceae)

Constitutive mixotrophs (CM), combining phagotrophy with inherent phototrophy, are critical primary producers and consumers in marine pelagic food webs. However, how mixotrophs, especially obligate CM, balance phototrophy and phagotrophy in response to changing environmental conditions remains equivocal. Here, we examined the growth performance, photosynthesis traits, and phagotrophic activities of a mixotrophic green alga, Picochlorum sp. GLMF1, over a wide range of temperature-irradiance combinations. The growth of Picochlorum sp. exhibited a unimodal thermal response under all the experimental light intensities. The optimal growth temperature decreased dramatically when the irradiance exceeded 80 μmol m-2 s-1, whereas the maximum growth rate kept a relatively constant and high value (around 1.1 d-1). This might be attributed to the trade-off between photosynthesis and ingestion under various light and temperature conditions. We found that the photosynthetic electron transport rate (ETR) and chlorophyll a specific inorganic carbon uptake rate increased with increasing irradiance. The maximum ETR decreased from 22.26 ± 1.93 μmol e m-2 s-1 at 22°C to 8.96 ± 0.42 μmol e m-2 s-1 at 31°C, suggesting a significant constraint of photosynthesis at high temperature. By contrast, ingestion increased with increasing temperature in most cases, and was significantly lower under high irradiance conditions. Combining the contributions of phototrophic and phagotrophic processes to total carbon acquisition, Picochlorum sp. showed a flexible nutritional strategy in response to temperature and light. Increasing light favors photosynthesis, whereas warming stimulates phagotrophy. Such complementation allows them to flourish under a wide range of temperature and irradiance. Our study adds knowledge to the nutritional strategy of mixotrophic green algae, with significant implications for their ecological role in marine food webs under projected climate change.