Nonphotosynthetic reduction of the intersystem electron transport chain of chloroplasts following heat stress. The pool size of stromal reductants

Impact of Sodium Nitroprusside on the Photosynthetic Performance of Maize and Sorghum

Nitric oxide (NO) is an important molecule in regulating plant growth, development and photosynthetic performance. This study investigates the impact of varying concentrations (0-300 µM) of sodium nitroprusside (SNP, a donor of NO) on the functions of the photosynthetic apparatus in sorghum (Sorghum bicolor L. Albanus) and maize (Zea mays L. Kerala) under physiological conditions. Analysis of the chlorophyll fluorescence signals (using PAM and the JIP-test) revealed an increased amount of open PSII reaction centers (qP increased), but it did not affect the number of active reaction centers per PSII antenna chlorophyll (RC/ABS). In addition, the smaller SNP concentrations (up to 150 μM) alleviated the interaction of QA with plastoquine in maize, while at 300 μM it predominates the electron recombination on QAQB-, with the oxidized S2 (or S3) states of oxygen evolving in complex ways in both studied plant species. At the same time, SNP application stimulated the electron flux-reducing end electron acceptors at the PSI acceptor side per reaction center (REo/RC increased up to 26%) and the probability of their reduction (φRo increased up to 20%). An increase in MDA (by about 30%) and H2O2 contents was registered only at the highest SNP concentration (300 µM). At this concentration, SNP differentially affected the amount of P700+ in studied plant species, i.e., it increased (by 10%) in maize but decreased (by 16%) in sorghum. The effects of SNP on the functions of the photosynthetic apparatus were accompanied by an increase in carotenoid content in both studied plants. Additionally, data revealed that SNP-induced changes in the photosynthetic apparatus differed between maize and sorghum, suggesting species specificity for SNP's impact on plants.

Cadmium Caused Different Toxicity to Photosystem I and Photosystem II of Freshwater Unicellular Algae Chlorella pyrenoidosa (Chlorophyta)

Heavy metals such as Cd pose environmental problems and threats to a variety of organisms. The effects of cadmium (Cd) on the growth and activities of photosystem I (PSI) and photosystem II (PSII) of Chlorella pyrenoidosa were studied. The growth rate of cells treated with 25 and 100 µM of Cd for longer than 48 h were significantly lower than the control, accompanying with the inhibition of photosynthesis. The result of quantum yields and electron transport rates (ETRs) in PSI and PSII showed that Cd had a more serious inhibition on PSII than on PSI. Cd decreased the efficiency of PSII to use the energy under high light with increasing Cd concentration. In contrast, the quantum yield of PSI did not show a significant difference among different Cd treatments. The activation of cyclic electron flow (CEF) and the inhibition of linear electron flow (LEF) due to Cd treatment were observed. The photochemical quantum yield of PSI and the tolerance of ETR of PSI to Cd treatments were due to the activation of CEF around PSI. The activation of CEF also played an important role in induction of non-photochemical quenching (NPQ). The binding features of Cd ions and photosystem particles showed that Cd was easier to combine with PSII than PSI, which may explain the different toxicity of Cd on PSII and PSI.

Assessment of the Photosynthetic Apparatus Functions by Chlorophyll Fluorescence and P700 Absorbance in C3 and C4 Plants under Physiological Conditions and under Salt Stress

Functions of the photosynthetic apparatus of C3 (Pisum sativum L.) and C4 (Zea mays L.) plants under physiological conditions and after treatment with different NaCl concentrations (0-200 mM) were investigated using chlorophyll a fluorescence (pulse-amplitude-modulated (PAM) and JIP test) and P700 photooxidation measurement. Data revealed lower density of the photosynthetic structures (RC/CSo), larger relative size of the plastoquinone (PQ) pool (N) and higher electron transport capacity and photosynthetic rate (parameter RFd) in C4 than in C3 plants. Furthermore, the differences were observed between the two studied species in the parameters characterizing the possibility of reduction in the photosystem (PSI) end acceptors (REo/RC, REo/CSo and δRo). Data revealed that NaCl treatment caused a decrease in the density of the photosynthetic structures and relative size of the PQ pool as well as decrease in the electron transport to the PSI end electron acceptors and the probability of their reduction as well as an increase in the thermal dissipation. The effects were stronger in pea than in maize. The enhanced energy losses after high salt treatment in maize were mainly from the increase in the regulated energy losses (ΦNPQ), while in pea from the increase in non-regulated energy losses (ΦNO). The reduction in the electron transport from QA to the PSI end electron acceptors influenced PSI activity. Analysis of the P700 photooxidation and its decay kinetics revealed an influence of two PSI populations in pea after treatment with 150 mM and 200 mM NaCl, while in maize the negligible changes were registered only at 200 mM NaCl. The experimental results clearly show less salt tolerance of pea than maize.

Heat stress-induced effects of photosystem I: an overview of structural and functional responses

Temperature is one of the main factors controlling the formation, development, and functional performance of the photosynthetic apparatus in all photoautotrophs (green plants, algae, and cyanobacteria) on Earth. The projected climate change scenarios predict increases in air temperature across Earth's biomes ranging from moderate (3-4 °C) to extreme (6-8 °C) by the year 2100 (IPCC in Climate change 2007: The physical science basis: summery for policymakers, IPCC WG1 Fourth Assessment Report 2007; Climate change 2014: Mitigation of Climate Change, IPCC WG3 Fifth Assessment Report 2014). In some areas, especially of the Northern hemisphere, even more extreme warm seasonal temperatures may occur, which possibly will cause significant negative effects on the development, growth, and yield of important agricultural crops. It is well documented that high temperatures can cause direct damages of the photosynthetic apparatus and photosystem II (PSII) is generally considered to be the primary target of heat-induced inactivation of photosynthesis. However, since photosystem I (PSI) is considered to determine the global amount of enthalpy in living systems (Nelson in Biochim Biophys Acta 1807:856-863, 2011; Photosynth Res 116:145-151, 2013), the effects of elevated temperatures on PSI might be of vital importance for regulating the photosynthetic response of all photoautotrophs in the changing environment. In this review, we summarize the experimental data that demonstrate the critical impact of heat-induced alterations on the structure, composition, and functional performance of PSI and their significant implications on photosynthesis under future climate change scenarios.

Differential Mechanisms of Photosynthetic Acclimation to Light and Low Temperature in Arabidopsis and the Extremophile Eutrema salsugineum

Photosynthetic organisms are able to sense energy imbalances brought about by the overexcitation of photosystem II (PSII) through the redox state of the photosynthetic electron transport chain, estimated as the chlorophyll fluorescence parameter 1-qL, also known as PSII excitation pressure. Plants employ a wide array of photoprotective processes that modulate photosynthesis to correct these energy imbalances. Low temperature and light are well established in their ability to modulate PSII excitation pressure. The acquisition of freezing tolerance requires growth and development a low temperature (cold acclimation) which predisposes the plant to photoinhibition. Thus, photosynthetic acclimation is essential for proper energy balancing during the cold acclimation process. Eutrema salsugineum (Thellungiella salsuginea) is an extremophile, a close relative of Arabidopsis thaliana, but possessing much higher constitutive levels of tolerance to abiotic stress. This comparative study aimed to characterize the photosynthetic properties of Arabidopsis (Columbia accession) and two accessions of Eutrema (Yukon and Shandong) isolated from contrasting geographical locations at cold acclimating and non-acclimating conditions. In addition, three different growth regimes were utilized that varied in temperature, photoperiod and irradiance which resulted in different levels of PSII excitation pressure. This study has shown that these accessions interact differentially to instantaneous (measuring) and long-term (acclimation) changes in PSII excitation pressure with regard to their photosynthetic behaviour. Eutrema accessions contained a higher amount of photosynthetic pigments, showed higher oxidation of P700 and possessed more resilient photoprotective mechanisms than that of Arabidopsis, perhaps through the prevention of PSI acceptor-limitation. Upon comparison of the two Eutrema accessions, Shandong demonstrated the greatest PSII operating efficiency (ΦPSII) and P700 oxidizing capacity, while Yukon showed greater growth plasticity to irradiance. Both of these Eutrema accessions are able to photosynthetically acclimate but do so by different mechanisms. The Shandong accessions demonstrate a stable response, favouring energy partitioning to photochemistry while the Yukon accession shows a more rapid response with partitioning to other (non-photochemical) strategies.

Effects of salinity on the photosynthetic apparatus of two Paulownia lines

The effects of soil salinity on the functional activity of photosynthetic apparatus and pigment composition of two Paulownia lines (Paulownia tomentosa x fortunei and Paulownia elongata x elongata) were investigated. PAM chlorophyll fluorescence measurements revealed that salinity leads to: (i) an increase of the photochemical quenching coefficient (qP) and the linear electron transport rate (ETR) in both lines of Paulownia, while the maximum quantum yield of the primary photochemistry of PSII in the dark adapted state (Fv/Fm) was unaffected; (ii) improved the efficiency of the photochemical energy conversion (ФPSII); (iii) an impact on the chlorophyll fluorescence decrease ratio (RFd), which correlates to the net CO2 assimilation rate; (iv) an impact on [Formula: see text] reoxidation. The analysis of the kinetics of P700(+) reduction upon turning off far-red irradiation revealed that salinization lead to a delay of the cyclic electron transport around PSI in both studied lines as the effect on this process is more pronounced in P. tomentosa x fortunei than in (in comparison with) P. elongata x elongata. The present experimental results suggested high salt tolerance of the studied lines Paulownia, but P. tomentosa x fortunei is more tolerant to salinity than P. elongata x elongata. Molecular mechanisms involved in the Paulownia response to the soil salinity are discussed.

How does iron deficiency disrupt the electron flow in photosystem I of lettuce leaves?

The changes observed photosystem I activity of lettuce plants exposed to iron deficiency were investigated. Photooxidation/reduction kinetics of P700 monitored as ΔA820 in the presence and absence of electron transport inhibitors and acceptors demonstrated that deprivation in iron decreased the population of active photo-oxidizable P700. In the complete absence of iron, the addition of plant inhibitors (DCMU and MV) could not recover the full PSI activity owing to the abolition of a part of P700 centers. In leaves with total iron deprivation (0μM Fe), only 15% of photo-oxidizable P700 remained. In addition, iron deficiency appeared to affect the pool size of NADP(+) as shown by the decline in the magnitude of the first phase of the photooxidation kinetics of P700 by FR-light. Concomitantly, chlorophyll content gradually declined with the iron concentration added to culture medium. In addition, pronounced changes were found in chlorophyll fluorescence spectra. Also, the global fluorescence intensity was affected. The above changes led to an increased rate of cyclic electron transport around PSI mainly supported by stromal reductants.

The physiological links of the increased photosystem II activity in moderately desiccated Porphyra haitanensis (Bangiales, Rhodophyta) to the cyclic electron flow during desiccation and re-hydration

Photosynthetic electron flow changed considerably during desiccation and re-hydration of the intertidal macroalgae Porphyra haitanensis. Activities of both photosystem (PSI) and photosystem (PSII) increased significantly at moderate desiccation levels. Whereas PSII activity was abolished at an absolute water content (AWC) <24 %, PSI remained active with progressive decreases in AWC to values as low as 16 %. This result suggested that cyclic electron flow around PSI was still active after inactivation of linear electron flow following severe desiccation. Moreover, the PSI activity was restored more rapidly than that of PSII upon re-hydration. Pretreatment of the blades with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) suppressed PSII activity following desiccation to an AWC of ~16 % AWC. Cyclic electron flow around PSI decreased markedly in blades pretreated with DCMU than in blades without pretreatment of DCMU during re-hydration in seawater containing DCMU. All results suggested that the activity of PSII under desiccation conditions plays an important role in the operation of cyclic electron flow during desiccation and its recovery during re-hydration. Therefore, we proposed the PSII activity during desiccation could eventually lead to the accumulation of NADPH, which could serve as electron donor for P700(+) and promote its recovery during re-hydration, thereby favoring the operation of cyclic electron flow.

The enhancement of cyclic electron flow around photosystem I improves the recovery of severely desiccated Porphyra yezoensis (Bangiales, Rhodophyta)

Porphyra yezoensis, a representative species of intertidal macro-algae, is able to withstand periodic desiccation at low tide but is submerged in seawater at high tide. In this study, changes in photosynthetic electron flow in P. yezoensis during desiccation and re-hydration were investigated. The results suggested that the cyclic electron flow around photosystem I (PSI) increased significantly during desiccation, continued to operate at times of severe desiccation, and showed greater tolerance to desiccation than the electron flow around PSII. In addition, PSI activity in desiccated blades recovered faster than PSII activity during re-hydration. Even though linear electron flow was suppressed by DCMU [3-(3',4'-dichlorophenyl)-1,1-dimethylurea], cyclic electron flow could still be restored. This process was insensitive to antimycin A and could be suppressed by dibromothymoquinone (DBMIB). The prolonged dark treatment of blades reduced the speed in which the cyclic electron flow around PSI recovered, suggesting that stromal reductants, including NAD(P)H, played an important role in the donation of electrons to PSI and were the main cause of the rapid recovery of cyclic electron flow in desiccated blades during re-hydration. These results suggested that cyclic electron flow in P. yezoensis played a significant physiological role during desiccation and re-hydration and may be one of the most important factors allowing P. yezoensis blades to adapt to intertidal environments.

Abolition of photosystem I cyclic electron flow in Arabidopsis thaliana following thermal-stress

Heat tolerance of Arabidopsis thaliana (WT) and its mutants, crr2-2, lacking NADPH-dehydrogenase (Ndh-pathway), and pgr5, deficient in proton gradient regulation and/or ferredoxin-quinone-reductase (FQR-pathway), was studied from 30 to 46°C. Chlorophyll fluorescence revealed that thermal damage to photosystem II (PSII) was maximal in WT plants following short-term exposure of leaves to moderate or high temperature stress. Thermal stress impaired the photosynthetic electron flow at oxidizing and reducing sides of PSII. This was deduced from the transformation of temperature dependent OJIP to OKP patterns, changes in the relative amplitudes of K-step fluorescence rise and F(v)/F(o) ratio. The amplitude of the K-peak that corresponds to the magnitude of damage to the oxygen evolving complex (OEC) in crr2-2 mutants was about 50% of that observed in WT plants exposed to 46°C. The damage to OEC in pgr5 mutants was relatively smaller and thus their PSII complexes were more heat tolerant. P700 oxidation-reduction kinetics following heat-stress revealed that photosystem I (PSI) complexes remained oxidizable either with 10-ms multiple turn-over flashes or far-red illumination but the complementary cyclic electron flow around PSI (CEF) was abolished in both mutants. With further increase in incubation temperature, CEF was fully suppressed even in WT. Thus, P700 turn-over was not enhanced following thermal stress. Furthermore, the experimental data predicts the onset of pseudocyclic electron transport with molecular oxygen as terminal acceptor in crr2-2 and pgr5 mutants but not in wild type Arabidopsis subjected to severe thermal-stress.

Acceleration of plastoquinone pool reduction by alternative pathways precedes a decrease in photosynthetic CO2 assimilation in preheated barley leaves

Heat stress causes inhibition of photosynthetic CO(2) assimilation, affects light photosynthetic reactions and accelerates alternative pathways of plastoquinone pool reduction (APPR). We have studied all these heat-sensitive processes after preheating to a broad range of physiological temperatures (24-46 degrees C) to explore a role of these alternative pathways during heat stress. Primarily, the effective quantum yield of PSII photochemistry was reduced (at 40 degrees C). This PSII downregulation was accompanied by the stimulation of APPR and preceded reduction of photosynthetic CO(2) assimilation by 2 degrees ; it occurred after preheating at 42 degrees C because of inhibition in Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) activation process. Thus, we suggest that the heat-induced stimulation of APPR is not associated with the heat-induced inhibition of Calvin cycle as it was reported for other types of stresses. A possible role of APPR in the compensation of PSII downregulation is briefly discussed.

Homologous recombination in plants is temperature and day-length dependent

Homologous recombination (HR) as a strand break repair mechanism was shown to be influenced by various factors. The balance of different vitamins, macro- and microelements, light spectrum, sodium chloride concentration as well as various environmental mutagens were shown to influence the frequency of HR. In this paper we analysed the influence of temperature (4, 22, and 32 degrees C) and day/night duration on the genome stability of plants. We analyzed the HR frequency in transgenic Arabidopsis thaliana plants carrying beta-glucuronidase based homologous recombination substrate. To find the recombination rate (RR), we related the HR frequency to the number of genomes present in plants grown under different conditions. The RR was also standardized to the transcription activity of the transgene. We found RR to be higher in plants grown at suboptimal temperatures (either 4 or 32 degrees C) as compared to plants grown at 22 degrees C. This negatively correlated with the plant metabolic rate and positively correlated with concentration of peroxide produced by plant. In contrast, the RR in plants grown at different day length (8-24 h) was the lowest in plants grown at the longest day (24 h) and highest in the plants grown at the shortest day (8 h). Over 15-fold difference in the RR between plants grown at the shortest and the longest day was observed. Such a difference in recombination rate was primarily due to the higher transgene activity and higher endoreduplication levels in plants grown at longer days. Our data suggests that even "moderate" changes of environmental conditions may have a significant effect on plant genome stability.

Photosynthetic response of transgenic soybean plants, containing an Arabidopsis P5CR gene, during heat and drought stress

The biochemical basis of heat/drought tolerance was investigated by comparing the response of antisense and sense transgenic soybean plants (containing the L-delta1-pyrroline-5-carboxylate reductase gene) with non-transgenic wild-type plants. The plants were subjected to a simultaneous drought and heat stress of 2 days, whereafter they were rewatered at 25 degrees C. During this time the sense plants only showed mild symptoms of stress compared to the antisense plants which were severely stressed. Upon stress, nicotinamide adenine dinucleotide phosphate (NADP+) levels decreased in antisense while it increased in sense plants. Recovery with respect to NADP+ levels was best in sense plants. Sense plants had the highest ability to accumulate proline during stress and to metabolise proline after rewatering. Analyses of the fast phase chlorophyll-a fluorescence transients showed dissociation of the oxygen-evolving complex (OEC) upon stress in all plants tested. In the sense plants, which best resisted the stress, OEC dissociation was bypassed by proline feeding electrons into photosystem 2 (PSII), maintaining an acceptable nicotinamide adenine dinucleotide hydrogen phosphate (NADPH) level, preventing further damage. Upon recovery, NADPH is consumed during oxidation of accumulated proline providing high Levels of NADP+ to act as electron acceptor to PSII, which indirectly may ameliorate the inhibition and/or the effect of uncoupling of the OEC.