Maximum Electron Transport Rate Research Articles

Efficient Heat Dissipation and Cyclic Electron Flow Confer Daily Air Exposure Tolerance in the Intertidal Seagrass Halophila beccarii Asch.

Seagrasses inhabiting the intertidal zone experience periodically repeated cycles of air exposure and rehydration. However, little is known about the photoprotective mechanisms in photosystem (PS)II and PSI, as well as changes in carbon utilization upon air exposure. The photoprotective processes upon air exposure in Halophila beccarii Asch., an endangered seagrass species, were examined using the Dual-PAM-100 and non-invasive micro-test technology. The results showed that air exposure enhanced non-photochemical quenching (NPQ) in both PSII and PSI, with a maximum increase in NPQ and Y(ND) (which represents the fraction of overall P700 that is oxidized in a given state) of 23 and 57%, respectively, resulting in intensive thermal energy dissipation of excess optical energy. Moreover, cyclic electron transport driven by PSI (CEF) was upregulated, reflected by a 50 and 22% increase in CEF and maximum electron transport rate in PSI to compensate for the abolished linear electron transport with significant decreases in pmfLEF (the proton motive force [pmf]) attributable solely to proton translocation by linear electron flow [LEF]). Additionally, H+ fluxes in mesophyll cells decreased steadily with increased air exposure time, exhibiting a maximum decrease of six-fold, indicating air exposure modified carbon utilization by decreasing the proton pump influxes. These findings indicate that efficient heat dissipation and CEF confer daily air exposure tolerance to the intertidal seagrass H. beccarii and provide new insights into the photoprotective mechanisms of intertidal seagrasses. This study also helps explain the extensive distribution of H. beccarii in intertidal zones.

Response of a rice-field cyanobacterium Anabaena sp. HKAR-7 upon exposure to ultraviolet-B radiation and ammonium chloride

Interactive effects of ultraviolet radiation (UVR), photosynthetically active radiation (PAR) and exogenously supplied ammonium chloride (NH4Cl) was studied in the rice-field cyanobacterium Anabaena sp. HKAR-7. The cyanobacterium was cultured under varying NH4Cl concentrations i.e., 0, 50, 200, 500, 1000 and 5000 μM and 200 μM (concentration) was found to be optimum for the growth of the cyanobacterium. Detrimental effects of UV-B exposure were observed on photosynthetic pigments such as chlorophyll a (Chl a), carotenoids and phycocyanin (PC). However, damage to these pigments was less in the cyanobacterial samples supplemented with NH4Cl. Contents of Chl a and PC in cyanobacterial cells decreased upon UV-B exposure but decrement was less in the samples supplemented with NH4Cl. Upon UV-B exposure, carotenoids content enhanced initially (till 15 days) during the course of treatment (21 days) but significant decrease (in carotenoids content) was observed in later phase of the experiment. From the results of photosynthetic activity, maximum quantum efficiency of PSII (Fv/Fm) and maximum electron transport rate (ETRmax), it could be concluded that exogenous supplementation of NH4Cl (optimum concentration) helped in protecting the cyanobacterial cells from highly energetic UVR to certain extent. Another interesting observation was significantly higher levels of biosynthesis and accumulation of mycosporine-like amino acids (MAAs) in the cyanobacterial cells supplemented with NH4Cl in comparison to non-supplemented cells. The purified MAA was identified to be phorphyra-334 as evidenced by UV/VIS absorption spectra, high performance liquid chromatography (HPLC) and electrospray ionization-mass spectrometry (ESI–MS).

Analysis of leaf photosynthetic rates of hydroponically-grown paprika (Capsicum annuum L.) plants according to vertical position with multivariable photosynthesis models

The photosynthetic rates of leaves depend on the vertical position and cultivation conditions. However, few models have been proposed to express photosynthesis according to leaf position, and there was a lack of quantitative analysis between physiological indicators and model parameters. The objectives of this study were to analyze the leaf photosynthetic characteristics of paprika plants according to leaf vertical position using photosynthesis models, and to analyze the relationship between the total nitrogen content and the photosynthetic model parameters. Leaf photosynthetic rates at different vertical positions were measured under varying light intensities and CO2 concentrations in triplicate. Rectangular hyperbola and FvCB (Farquhar, von Caemmerer, and Berry) models were selected, calibrated, and validated as multivariable photosynthesis models. Total nitrogen contents and SPAD values were measured at each leaf position and the coefficients of the photosynthetic rate models were compared. The R2 values for the rectangular hyperbola and FvCB models were 0.86 and 0.91, and the RMSE values were 4.651 and 2.104, respectively. Total nitrogen content linearly increased with increasing vertical leaf position and it was linearly related to the maximum carboxylation capacity and maximum electron transport rate, estimated in the FvCB model. In this study, the FvCB model was considered more suitable for expressing the relationship between total nitrogen contents and plant’s physiological responses according to the vertical position of leaves. The vertical leaf photosynthetic rate models established in this study will contribute to determining optimal environmental conditions for maximizing crop photosynthesis and establish the criteria for precise CO2 enrichment in greenhouses.

Effects of Nitrogen Limitation on Phytoplankton Physiology in the Western Arctic Ocean in Summer

AbstractPhytoplankton in the Arctic Ocean are subject to nitrogen limitation in the summer, however, how severely the nitrogen stress affects phytoplankton physiology remains largely unknown. In the summers of 2015–2018, we examined the distribution of phytoplankton photophysiological properties across two contrasting regions of the Arctic Ocean with distinctly different levels of nitrogen availability in the upper water column. We quantified the extent of nitrogen stress using a highly sensitive fluorescence induction and relaxation system to obtain continuous underway measurements and via discrete sample analyses of phytoplankton physiology, as well as nutrient enrichment incubations. The results revealed vast regions in the Chukchi Sea where phytoplankton photosynthesis was severely nitrogen‐stressed. Thereby, the maximum quantum yield of photochemistry in photosystem II showed only a small decrease (12 ± 9%) relative to its nutrient‐replete values, while the maximum photosynthetic electron transport rates under saturating irradiance were impaired to a greater extent (40 ± 17%). This phytoplankton photosynthesis response is indicative of a severe nitrogen limitation, which results in dramatic reduction in growth and net primary production rates. Nutrient enrichment incubations also revealed a marked increase in large‐size phytoplankton growth (>10 μm) after the nitrogen stress was alleviated, suggesting that the larger cells were more susceptible to nitrogen stress. These results are important for understanding how regional nitrogen fluxes control variability in the primary production and phytoplankton community structure and how these processes might change with rapid climate changes in the Arctic Ocean.

Elevated CO2 concentration alleviates UVR-induced inhibition of photosynthetic light reactions and growth in an intertidal red macroalga

The commercially important red macroalga Pyropia (formerly Porphyra) yezoensis is, in its natural intertidal environment, subjected to high levels of both photosynthetically active and ultraviolet radiation (PAR and UVR, respectively). In the present work, we investigated the effects of a plausibly increased global CO2 concentration on quantum yields of photosystems II (PSII) and I (PSI), as well as photosynthetic and growth rates of P. yezoensis grown under natural solar irradiance regimes with or without the presence of UV-A and/or UV-B. Our results showed that the high-CO2 treatment (~1000 μbar, which also caused a drop of 0.3 pH units in the seawater) significantly increased both CO2 assimilation rates (by 35%) and growth (by 18%), as compared with ambient air of ~400 μbar CO2. The inhibition of growth by UV-A (by 26%) was reduced to 15% by high-CO2 concentration, while the inhibition by UV-B remained at ~6% under both CO2 concentrations. Homologous results were also found for the maximal relative photosynthetic electron transport rates (rETRmax), the maximum quantum yield of PSII (Fv/Fm), as well as the midday decrease in effective quantum yield of PSII (YII) and concomitant increased non-photochemical quenching (NPQ). A two-way ANOVA analysis showed an interaction between CO2 concentration and irradiance quality, reflecting that UVR-induced inhibition of both growth and YII were alleviated under the high-CO2 treatment. Contrary to PSII, the effective quantum yield of PSI (YI) showed higher values under high-CO2 condition, and was not significantly affected by the presence of UVR, indicating that it was well protected from this radiation. Both the elevated CO2 concentration and presence of UVR significantly induced UV-absorbing compounds. These results suggest that future increasing CO2 conditions will be beneficial for photosynthesis and growth of P. yezoensis even if UVR should remain at high levels.

The combined effects of water and nitrogen on the relationship between a native hemiparasite and its invasive host.

Stem hemiparasites are dependent on their hosts for water and nitrogen. Most studies, however, have assessed the influence of one factor on parasite:host associations, thus limiting our mechanistic understanding of their performance in nature. We investigated the combined effects of water and nitrogen (N) availability on both a host (Ulex europaeus) and its parasite (Cassytha pubescens). Parasite infection significantly decreased host shoot biomass and shoot:root ratio more severely in high water than low water, irrespective of N supply. Parasite stem [N] was significantly higher in high water than low water treatments, regardless of N supply, but parasite biomass did not vary among treatments. Irrespective of water and N supply, infected plants had significantly lower total, root and nodule biomass, predawn and midday quantum yields, maximum electron transport rates, water potentials and nitrogen concentration [N]. Parasite δ13 C was significantly higher than that of the host. Our results suggested that stem hemiparasites can better extract resources from hosts when water availability is high, resulting in a greater impact on the host under these conditions. When hemiparasitic plants are being investigated as a biocontrol for invasive weeds, they may be more effective in wetter habitats than in drier ones.

Short-term effects of increased CO2, nitrate and temperature on photosynthetic activity in Ulva rigida (Chlorophyta) estimated by different pulse amplitude modulated fluorometers and oxygen evolution.

Short-term effects of pCO2 (700-380 ppm; High carbon (HC) and Low carbon (LC), respectively) and nitrate content (50-5 µM; High nitrogen (HN) and Low nitrogen (LN), respectively on photosynthesis were investigated in Ulva rigida (Chlorophyta) under solar radiation (in-situ) and in the laboratory under artificial light (ex-situ). After six days of incubation at ambient temperature (AT), algae were subjected to a 4 °C temperature increase (AT+4 °C) for 3 d. Both in-situ and ex-situ maximal electron transport rate (ETRmax) and in situ gross photosynthesis (GP), measured by O2 evolution, presented highest values under HCHN, and lowest under HCLN, across all measuring systems. Maximal quantum yield (Fv/Fm), and ETRmax of photosystem (PS) II [ETR(II)max] and PSI [ETR(I)max], decreased under HCLN at AT+4 °C. Ex situ ETR was higher than in situ ETR. At noon, Fv/Fm decreased (indicating photoinhibition), whereas ETR(II)max and maximal non-photochemical quenching (NPQmax) increased. ETR(II)max decreased under AT+ 4 °C in contrast to Fv/Fm, photosynthetic efficiency (α ETR) and saturated irradiance (EK). Thus, U. rigida exhibited a decrease in photosynthesis under acidification, changing LN, and AT+4 °C. These results emphasize the importance of studying the interaction between environmental parameters using in-situ versus ex-situ conditions, when aiming to evaluate the impact of global change on marine macroalgae.

Drought stress strengthens the link between chlorophyll fluorescence parameters and photosynthetic traits.

Chlorophyll fluorescence (ChlF) has been used to understand photosynthesis and its response to climate change, particularly with satellite-based data. However, it remains unclear how the ChlF ratio and photosynthesis are linked at the leaf level under drought stress. Here, we examined the link between ChlF ratio and photosynthesis at the leaf level by measuring photosynthetic traits, such as net CO2 assimilation rate (An), the maximum carboxylation rate of Rubisco (Vcmax), the maximum rate of electron transport (Jmax), stomatal conductance (gs) and total chlorophyll content (Chlt). The ChlF ratio of the leaf level such as maximum quantum efficiency of PSII (Fv/Fm) is based on fluorescence kinetics. ChlF intensity ratio (LD685/LD740) based on spectrum analysis was obtained. We found that a combination of the stomatal limitation, non-stomatal limitation, and Chlt regulated leaf photosynthesis under drought stress, while Jmax and Chlt governed the ChlF ratio. A significant link between the ChlF ratio and An was found under drought stress while no significant correlation in the control, which indicated that drought stress strengthens the link between the ChlF ratio and photosynthetic traits. These results suggest that the ChlF ratio can be a powerful tool to track photosynthetic traits of terrestrial ecosystems under drought stress.

Rejuvenating soybean (Glycine max L.) growth and development through slight shading stress

The impact of increased shading stress on agronomic traits, photosynthetic performance and antioxidants activities in leaves of two soybeans cultivars (D16 and E93) was studied. Soybean seedlings were grown in pots and exposed to no shade (S0), slight shade (S1), moderate shade (S2), and heavy shade (S3). Our findings showed that under the S3 in both cultivars, leaf fresh weight (LFW), specific leaf area (SLA) and leaf thickness decreased significantly, accompanied by a reduction in photochemical parameters including the maximum quantum yield (Fv/Fm) and electron transport rate (ETR). Furthermore, compared to S0, S1 significantly increased the ETR, sucrose content and the activity of catalase (CAT) in both D16 and E93 cultivars while S2 and S3 decreased the activity. However, under all treatments of shading stress, the antioxidant activities of superoxide dismutase (SOD) and peroxidase (POD) were lowered in both cultivars. Such morphological and physiological plasticity to adapt S1 compensates for the decrease in biomass and leads to seed weight compared to that obtained with an amount of normal light. Through configuring the space in the intercropping systems, S1 could be helpful for optimum growth and yield. Redesigning photosynthesis through S1 for the intercropping systems could be a smart approach.

The role of morphological changes in algae adaptation to nutrient stress at the single-cell level

Individual cell heterogeneity within a population can be critical to its peculiar function and fate. Conventional algal cell-based assays mainly analyze the average responses from a population of algal cells. Therefore, the mechanisms through which changes in population characteristics are driven by the behavior of single algal cells are still not well understood. Algal cells may modulate their physiology and metabolism by changing their morphology in response to environmental stress. In this study, an algal single-cell culture and analysis system was developed to investigate the potential role of morphological changes by algal cells during adaptation to nutrient stress based on a microwell array chip. The surface-to-volume ratio of Microcystis aeruginosa (M. aeruginosa) and the volume of Scenedesmus obliquus (S. obliquus) significantly increased with increasing culture time under nutrient stress. The eccentricity of M. aeruginosa and S. obliquus gradually increased and decreased, respectively, with increasing culture time, indicating that the morphology of M. aeruginosa and S. obliquus became increasingly irregular and regular, respectively, under nutrient stress. There were significant correlations between the morphological characteristics and physiological characteristics of M. aeruginosa and S. obliquus under nutrient stress. In M. aeruginosa, an increased surface-to-volume ratio facilitated a high specific fluorescence intensity, specific Raman intensity, and maximum electron transport rate. In S. obliquus, increased cell volume enhanced nutrient absorption, which facilitated a higher specific growth rate. M. aeruginosa and S. obliquus adopted different adaptation strategies in response to nutrient stress based on morphological changes. These findings facilitate the development of management strategies for controlling harmful cyanobacterial blooms.

Light and irrigation effects on chlorophyll fluorescence depend on seedling provenance in Magnolia pugana endemic of Mexico

In riparian forests, light and water are two important factors for plant acclimation to different environments. We evaluated the effect of different light and irrigation regimes on the effective quantum yield of photosystem II (фPSII), the maximum quantum efficiency of photosystem II (Fv/Fm), and the electron transfer rate (ETR) of Magnolia pugana seedlings that come from wild and cultivated plants, under greenhouse conditions. Rapid light curves (RLCs) were performed to obtain the maximum apparent electron transport rate and the saturating photosynthetic photon flux density for PSII. Effective quantum yield of PSII was higher at afternoon for cultivated and wild provenances, but in the latter decreased in longest water deficit treatment at noon. The combined water deficit and high light stress in the morning decreased фPSII; however, фPSII recovered in the afternoon, suggesting dynamic photoinhibition. Furthermore, фPSII of wild and cultivated seedlings in all irrigation levels showed dynamic photoinhibition as well. The electron transfer rate was higher for seedlings grown under high light than in shade. ETR results suggest that wild seedlings showed greater tolerance to water deficit conditions than cultivated ones. Rapid light curves indicate that photosystem II of M. pugana seedlings grown in high light conditions requires higher light intensities to reach saturation. In conclusion, wild and cultivated populations of Magnolia pugana seedlings showed photochemical adjustments to tolerate water deficit and light stress. This study offers new insights into the physiology of endangered plants and valuable guidance for conservation efforts designed at protecting in situ and ex situ wild Magnolia species.

Monitoring biochemical limitations to photosynthesis in N and P-limited radiata pine using plant functional traits quantified from hyperspectral imagery

The prediction of carbon uptake by forests across fertility gradients requires accurate characterisation of how biochemical limitations to photosynthesis respond to variation in key elements such as nitrogen (N) and phosphorus (P). Over the last decade, proxies for chlorophyll and photosynthetic activity have been extracted from hyperspectral imagery and used to predict important photosynthetic variables such as the maximal rate of carboxylation (Vcmax) and electron transport (Jmax). However, little research has investigated the generality of these relationships within the nitrogen (N) and phosphorus (P) limiting phases, which are characterised by mass based foliage ratios of N:P ≤ 10 for N limitations and N:P > 10 for P limitations.Using measurements obtained from one year old Pinus radiata D. Don grown under a factorial range of N and P treatments this research examined relationships between photosynthetic capacity (Vcmax, Jmax) and measured N, P and chlorophyll (Chla+b). Using functional traits quantified from hyperspectral imagery we then examined the strength and generality of relationships between photosynthetic variables and Photochemical Reflectance Index (PRI), Sun-Induced Chlorophyll Fluorescence (SIF) and chlorophyll a + b derived by radiative transfer model inversion.There were significant (P < .001) and strong relationships between photosynthetic variables and both N (R2 = 0.82 for Vcmax; R2 = 0.87 for Jmax) and Chla+b (R2 = 0.85 for Vcmax; R2 = 0.86 for Jmax) within the N limiting phase that were weak (R2 < 0.02) and insignificant within the P limiting phase. Similarly, there were significant (P < .05) positive relationships between P and photosynthetic variables (R2 = 0.50 for Vcmax; R2 = 0.58 for Jmax) within the P limiting phase that were insignificant and weak (R2 < 0.33) within the N limiting phase.Predictions of photosynthetic variables using Chla+b estimated by model inversion were significant (P < .001), positive and strong (R2 = 0.64 for Vcmax; R2 = 0.63 for Jmax) within the N limiting phase but insignificant and weak (R2 < 0.05) within the P limiting phase. In contrast, both SIF and PRI exhibited moderate to strong positive correlations with photosynthetic variables within both the N and P limiting phases. These results suggest that quantified SIF and PRI from hyperspectral images may have greater generality in predicting biochemical limitations to photosynthesis than proxies for N and chlorophyll a + b, particularly under high foliage N content, when P is limiting.

Quantifying key model parameters for wheat leaf gas exchange under different environmental conditions

The maximum carboxylation rate of Rubisco (Vcmax) and maximum rate of electron transport (Jmax) for the biochemical photosynthetic model, and the slope (m) of the Ball-Berry stomatal conductance model influence gas exchange estimates between plants and the atmosphere. However, there is limited data on the variation of these three parameters for annual crops under different environmental conditions. Gas exchange measurements of light and CO2 response curves on leaves of winter wheat and spring wheat were conducted during the wheat growing season under different environmental conditions. There were no significant differences for Vcmax, Jmax or m between the two wheat types. The seasonal variation of Vcmax, Jmax and m for spring wheat was not pronounced, except a rapid decrease for Vcmax and Jmax at the end of growing season. Vcmax and Jmax show no significant changes during soil drying until light saturated stomatal conductance (gssat) was smaller than 0.15 mol m−2 s−1. Meanwhile, there was a significant difference in m during two different water supply conditions separated by gssat at 0.15 mol m−2 s−1. Furthermore, the misestimation of Vcmax and Jmax had great impacts on the net photosynthesis rate simulation, whereas, the underestimation of m resulted in underestimated stomatal conductance and transpiration rate and an overestimation of water use efficiency. Our work demonstrates that the impact of severe environmental conditions and specific growing stages on the variation of key model parameters should be taken into account for simulating gas exchange between plants and the atmosphere. Meanwhile, modification of m and Vcmax (and Jmax) successively based on water stress severity might be adopted to simulate gas exchange between plants and the atmosphere under drought.

Mechanisms underlying leaf photosynthetic acclimation to warming and elevated CO2 as inferred from least-cost optimality theory.

The mechanisms responsible for photosynthetic acclimation are not well understood, effectively limiting predictability under future conditions. Least-cost optimality theory can be used to predict the acclimation of photosynthetic capacity based on the assumption that plants maximize carbon uptake while minimizing the associated costs. Here, we use this theory as a null model in combination with multiple datasets of C3 plant photosynthetic traits to elucidate the mechanisms underlying photosynthetic acclimation to elevated temperature and carbon dioxide (CO2 ). The model-data comparison showed that leaves decrease the ratio of the maximum rate of electron transport to the maximum rate of Rubisco carboxylation (Jmax /Vcmax ) under higher temperatures. The comparison also indicated that resources used for Rubisco and electron transport are reduced under both elevated temperature and CO2 . Finally, our analysis suggested that plants underinvest in electron transport relative to carboxylation under elevated CO2 , limiting potential leaf-level photosynthesis under future CO2 concentrations. Altogether, our results show that acclimation to temperature and CO2 is primarily related to resource conservation at the leaf level. Under future, warmer, high CO2 conditions, plants are therefore likely to use less nutrients for leaf-level photosynthesis, which may impact whole-plant to ecosystem functioning.

The effect of plant extracts on growth and photosynthetic fluorescence characteristics of Microcystis flos-aquae.

The cyanobacteria Microcystis flos-aquae can cause harmful algal blooms in waterbodies, which threaten the normal functioning of aquatic ecosystems and human health. Some plant extracts are considered as promising algaecides. In this study, the effects of ten plant extracts (Cinnamomum camphora, Ginkgo biloba, Firmiana platanifolia, Salix babylonica, Euphorbia humifusa, Erigeron annuus, Solidago canadensis, Alternanthera philoxeroides, Thalia dealbata and Eichhornia crassipes) against M. flos-aquae were investigated. The results showed that all ten plant extracts had a significant inhibitory effect on M. flos-aquae growth after 96 h (P < 0.01). The inhibition rates of S. babylonica, E. humifusa, S. canadensis and A. philoxeroides were over 70.00%. Furthermore, the E. humifusa extract had the best inhibitory effect on the photosynthesis of M. flos-aquae, with the effective quantum yield of photosystem II and maximal relative electron transport rate decreasing by 97.50% and 97.00%, respectively, after 96 h. Additionally, the E. humifusa extract was found to be non-toxic to non-target organisms such as Brachydanio rerio and Vallisneria spiralis within 96 h. This study contributes to the existing knowledge and data of freshwater cyanobacteria blooms, and provides insights for their control and the restoration of freshwater systems affected by cyanobacteria blooms.

Low electric current density enhances the calcification rate of the colonial Stony Coral Galaxea fascicularis

This study assessed the effects of different electric current densities on the calcification and photosynthetic physiology of a massive, colonial stony coral species. Coral survivorship, calcification rate, and photosynthetic parameters (zooxanthellae density and chlorophyll fluorescence) were measured to determine the physiological and photosynthetic performance of Galaxea fascicularis after exposure to different electric current densities. After 60 days of treatment, survival of G. fascicularis was higher at low electric current density (10 mA m−2) than in the control and at high electric current density (100 mA m−2). Electric current did not alter zooxanthellae densities, but did enhance the photosynthetic processes (e.g. maximum electron transport rates (rETRm), and maximum quantum yield (Fv/Fm)) within 45 days. Coral calcification rate increased in response to low electric current density up to 30 days, but the effects of the electric current were not significant after a longer period of treatment. Overall, our results indicated that mineral accretion technology with appropriate electric current density conditions was somewhat helpful in enhancing coral growth, and thus this technique may be used in the restoration and management of massive stony coral reefs.

Potassium and Magnesium Mediate the Light and CO2 Photosynthetic Responses of Grapevines.

Potassium (K) and magnesium (Mg) deficiency are common stresses that can impact on grape yield and quality, but their effects on photosynthesis have received little attention. Understanding the diffusional and biochemical limitations to photosynthetic constraints will help to guide improvements in cultural practices. Accordingly, the photosynthetic response of Vitis vinifera cvs. Shiraz and Chardonnay to K or Mg deficiency was assessed under hydroponic conditions using miniature low-nutrient-reserve vines. Photosynthesis was at least partly reduced by a decline in stomatal conductance. Light and CO2-saturated photosynthesis, maximum rate of ribulose 1.5 bisphospate (RuBP) carboxylation (Vcmax) and maximum rate of electron transport (Jmax) all decreased under K and Mg deficiency. Likewise, chlorophyll fluorescence and electron transport were lower under both nutrient deficiencies while dark respiration increased. K deficiency drastically reduced shoot biomass in both cultivars, while root biomass was greatly reduced under both Mg and K deficiency. Taken together, these results indicate that the decrease in biomass was likely due to both stomatal and biochemical limitations in photosynthesis. Optimising photosynthesis through adequate nutrition will thus support increases in biomass with carry-on positive effects on crop yields.

Loss of Function of Fatty Acid Desaturase 7 in Tomato Enhances Photosynthetic Carbon Fixation Efficiency.

Fatty Acid Desaturase 7 (FAD7) generates polyunsaturated fatty acids, promoting the desaturation of chloroplast membranes; it also provides an essential precursor for the synthesis of jasmonic acid (JA), a phytohormone that can influence plant growth, development, and primary metabolism. This study examined the effects of spr2, a null mutation in SlFAD7, on the growth, morphology, and photosynthetic traits of tomato, Solanum lycopersicum. Although the spr2 mutant had a lower density of stomata than wild type plants, the two genotypes had comparable stomatal conductance, transpiration rates, and intracellular CO2 levels; in addition, spr2 had significantly thinner leaf blades, which may help maintain normal levels of CO2 diffusion despite the lower number of stomata. Surprisingly, spr2 also had significantly higher carbon assimilation (A) and maximum quantum efficiency of PSII (Fv/Fm) than wild type plants at both of the light intensities tested here (220 or 440 µmol m−2 s−1), despite having lower levels of chlorophyll than wild type plants under low light (220 µmol m−2 s−1). Furthermore, CO2 response curves indicated higher in vivo Rubisco activity (Vcmax) in spr2 compared to wild type plants, as well as an enhanced maximum rate of electron transport used in the regeneration of ribulose-1,5-bisphosphate (Jmax). These data indicate that loss of function of FAD7 can enhance the efficiency of both light-dependent and light-independent reactions in photosynthesis. Consistent with this, the spr2 mutant also displayed enhanced growth, with significantly more leaves and a more compact growth habit. In contrast to spr2, another tomato mutant impaired in JA synthesis (acx1) showed no enhancements in growth or photosynthetic efficiency, suggesting that the enhancements observed in spr2 are independent of the effects of this mutation on JA synthesis. These data demonstrate that loss of function of FAD7 can enhance photosynthesis and growth, potentially through its impacts on the chloroplast membranes.

High Photosynthetic Rates in a Solanum pennellii Chromosome 2 QTL Is Explained by Biochemical and Photochemical Changes.

Enhanced photosynthesis is strictly associated with to productivity and it can be accomplished by genetic approaches through identification of genetic variation. By using a Solanum pennellii introgression lines (ILs) population, it was previously verified that, under normal (CO2), IL 2–5 and 2–6 display increased photosynthetic rates by up to 20% in comparison with their parental background (M82). However, the physiological mechanisms involved in the enhanced CO2 assimilation exhibited by these lines remained unknown, precluding their use for further biotechnological applications. Thereby, here we attempted to uncover the physiological factors involved in the upregulation of photosynthesis in ILs 2–5 and 2–6 under normal (CO2) as well as under elevated (CO2). The results provide evidence for increased biochemical capacity (higher maximum carboxylation velocity and maximum electron transport rate) in plants from IL 2–5 and 2–6, whereas the diffusive components (stomatal and mesophyll conductances) were unaltered in these ILs in comparison to M82. Our analyses revealed that the higher photosynthetic rate observed in these ILs was associated with higher levels of starch as well as total protein levels, specially increased RuBisCO content. Further analyses performed in plants under high (CO2) confirmed that biochemical properties are involved in genetic variation on chromosome 2 related to enhanced photosynthesis.

Low pH in simulated acid rain promotes the toxicity of copper on the physiological performance in Sargassum horneri

In order to investigate the interactive effects of simulated acid rain and copper (Cu) on macroalgae, Sargassum horneri was cultured under pH of 8.2, 5.6, 4.5 and Cu concentrations of 0, 25, 100 μg L-1. Under the medium concentration of Cu, moderate pH mitigated the toxicity of Cu on S. horneri, while the low pH and high Cu concentration reduced the contents of chlorophyll (Chl) a and carotenoids. Furthermore, pH had no interaction with Cu on maximal Chl fluorescence or electron transport efficiency, while effective PSII quantum yield, maximum electron transport rate, and saturating irradiance were obviously affected by pH and Cu, and the interactive effects were significant. Additionally, with the increasing Cu concentration and decreasing pH, nonphotochemical quenching and regulated energy dissipation dropped sharply, while nonregulated energy dissipation increased. Therefore, the high concentration of Cu and low pH had synergistic effects on the photosynthetic performance and growth of S. horneri.