Recovery Of PSII Research Articles

Photosynthetic characteristics of subtending leaves and their relationships with soybean pod development under heat, drought and combined stresses

AbstractSoybean is usually grown under rain‐fed conditions, and long‐term drought stress often occurs with short‐term heat stress. This study aimed to investigate changes in the photosynthetic characteristics of subtending leaves and their relationships with pod development under short‐term heat, long‐term drought and their combined stresses. Short‐term heat stress lasted for 5 days began at R5 stage, and long‐term drought stress lasted from R5 stage until maturity, respectively. No significant effect was observed on pod development under short‐term heat stress alone due to unaffected net photosynthetic rate after treatment and PSII recovery after the heat stress release. Except that the reduction of sucrose content had been brought forward from 12 to 5 days after treatment under combined stresses, application of combined stresses caused similar responses to long‐term drought stress alone on the photosynthetic characteristics of subtending leaves and pod development, but more pronounced under combined stresses. Reduced pod weight and seed weight per pod under long‐term drought stress alone or combined stress due to a decrease in the net photosynthetic rate and production of sucrose and starch, especially after 19 days of treatment. Findings from this study demonstrate that under combined stress, long‐term drought stress had a dominant effect on the photosynthetic performance of subtending leaves and pod development over short‐term heat stress; moreover, even short‐term heat stress also exacerbates the negative effects of long‐term drought stress.

Limiting steps and the contribution of alternative electron flow pathways in the recovery of the photosynthetic functions after freezing-induced desiccation of Haberlea rhodopensis

Haberlea rhodopensis Friv. is unique with its ability to survive desiccation to an air-dry state during periods of extreme drought and freezing temperatures. To understand its survival strategies, it is important to examine the protective mechanisms not only during desiccation but also during rehydration. We investigated the involvement of alternative cyclic electron pathways during the recovery of photosynthetic functions after freezing-induced desiccation. Using electron transport inhibitors, the role of PGR5-dependent and NDH-dependent PSI-cyclic electron flows and plastid terminal oxidase were assessed during rehydration of desiccated leaves. Recovery of PSII and PSI, the capacity of PSI-driven cyclic electron flow, the redox state of plastoquinone pool, and the intersystem electron pool were analyzed. Data showed that the effect of alternative flows is more pronounced in the first hours of rehydration. In addition, the NDH-dependent cyclic pathway played a more determining role in the recovery of PSI than in the recovery of PSII.

UV Light Acclimation Capacity of Leaf Photosynthetic and Photochemical Behaviour in Near-isohydric and Anisohydric Grapevines in Hot and Dry Environments

The photosynthetic and photochemical adaptation of grapevine leaves to high UV radiation, under hot and dry summer conditions, was investigated in near-isohydric Cannonau (syn. Grenache) and nearanisohydric Bovale grande (syn. Carignan). From pea-size stage until harvest, vines with mild to moderate water deficit were subjected to UV-blocking treatment (–UV) and compared to a control exposed to sunlight (C). Canopy light and thermal microclimate, growth and density, maximum leaf gas exchange, primary photochemistry (PSII) and phenols were monitored. Average increments in canopy temperature under –UV tunnels during day-time and night-time were 3.3°C and 0.8°C in Bovale grande and 2.6°C and 1.1°C in Cannonau. Cultivars reached similar leaf area, intrinsic water-use efficiency and stem water potential under C and –UV. Cannonau showed lower stomatal conductance, maximum net assimilation and transpiration rates, but also faster recovery of PSII under heat and moderate water stress. UV radiation induced a stronger and longer impact on leaf assimilation, but the duration of elevated temperatures under −UV induced higher photoinhibition and lower photochemical efficiency. A similar degree of correlation between leaf temperature and gas exchange was found among cultivars and treatments. In Cannonau, leaf anthocyanin decreased due to heat-induced long-lasting PSII photoinactivation under C. Conversely, Bovale grande showed higher phenolic content stability, thus higher photoprotection and recovery of PSII functional units. Agronomical practices affecting leaf phenolic accumulation influence canopy acclimation to heat and high sunlight. Vineyard management must avoid excessive canopy sun exposure and duration of elevated temperatures to favour high assimilation, while reducing photoinactivation and heat damage.

Comparative chloroplast proteome analysis of exogenously supplied trehalose to wheat seedlings under heat stress

The aim of our study was to investigate the underlying molecular mechanisms of exogenously supplied trehalose affecting wheat photosynthesis under heat stress. The amount of ATP synthase (ATPase), oxygen-evolving enhancer protein (OEE), PsbP, Rubisco, chloroplast fructose-bisphosphate aldolase (FBPA), and ferredoxin-NADP(H) oxidoreductase (FNR) were downregulated, while PSI reaction center subunits were upregulated under heat stress. However, in the trehalose-pretreated groups, the amount of FNR, cytochrome b6f complex, PSI reaction center subunits, ATPase, FBPA, and Rubisco were upregulated under normal growth conditions and heat stress. Besides, during the recovery period, the upregulation in CAB, PsbP, OEE2, and ATPase suggested that trehalose pretreatment might help to the recovery of PSII and PSI. These results indicate that trehalose pretreatment effectively regulates the levels of the photosynthesis-related proteins and relieves the damage of heat stress to wheat chloroplast.

Antioxidant protection and PSII regulation mitigate photo-oxidative stress induced by drought followed by high light in cashew plants

We tested the hypothesis that effective antioxidant and photoprotective mechanisms are able to avoid photodamage induced by prolonged water deficit (WD) followed by high light (HL). We employed cashew plants (Anacardium occidentale L.), a semiarid adapted species, as a model plant. WD-plants exposed to HL did not show alterations in maximum quantum efficiency of photosystem II (Fv/Fm), cellular integrity, H2O2 and thiobarbituric acid reactive species (TBARS) contents, evidencing that they did not suffered photoinhibition and oxidative stress. These responses were associated with increases in ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities, glutathione (GSH) oxidation and ascorbate (ASC) synthesis. This effective oxidative protection occurred in parallel to strong decrease in photosystem II and I (PSII and PSI) activities, increase in heat dissipation (qE), which was related to enhancement in cyclic electron flux. These favorable photoprotective changes were associated with efficient water use in response to HL, all contributing to avoid excess energy in chloroplasts of drought-exposed leaves. These protective features were associated with a tight regulation in D1 protein accumulation during HL, contributing to avoid reactive oxygen species over-accumulation and a subsequent effective PSII recovery during darkness. Our results indicate that cashew plants are able to avoid photoinhibition, tolerating extreme conditions of drought combined with HL. The displayed mechanisms involve essentially integrated responses to balance energy input and output, avoiding oxidative stress.

Leaf vs. inflorescence: differences in photosynthetic activity of grapevine

Using measures of gas exchange and photosynthetic chain activity, we found some differences between grapevine inflorescence and leaf in terms of photosynthetic activity and photosynthesis regulations. Generally, the leaf showed the higher net photosynthesis (P N) and lower dark respiration than that of the inflorescence until the beginning of the flowering process. The lower (and negative) P N indicated prevailing respiration over photosynthesis and could result from a higher metabolic activity rather than from a lower activity of the photosynthetic apparatus. Considerable differences were observed between both organs in the functioning and regulation of PSI and PSII. Indeed, in our conditions, the quantum yield efficiency and electron transport rate of PSI and PSII were higher in the inflorescence compared to that of the leaf; nevertheless, protective regulatory mechanisms of the photosynthetic chain were clearly more efficient in the leaf. This was in accordance with the major function of this organ in grapevine, but it highlighted also that inflorescence seems to be implied in the whole carbon balance of plant. During inflorescence development, the global PSII activity decreased and PSI regulation tended to be similar to the leaf, where photosynthetic activity and regulations remained more stable. Finally, during flowering, cyclic electron flow (CEF) around PSI was activated in parallel to the decline in the thylakoid linear electron flow. Inflorescence CEF was double compared to the leaf; it might contribute to photoprotection, could promote ATP synthesis and the recovery of PSII.

Evaluation of Aegilops tauschii and Aegilops speltoides for acquired thermotolerance: Implications in wheat breeding programmes

Severe and frequent heat waves are predicted in the near future having dramatic and far-reaching ecological and social impact. The aim of this study was to examine acquired thermotolerance of two Aegilops species: Aegilops tauschii and Aegilops speltoides and study their potential adaptive mechanisms. The effect of two episodes of high heat stress (45 °C/12 h) with a day of recovery period was investigated on their physiology. As compared to A. speltoides, A. tauschii suffered less inhibition of photosystem II efficiency and net photosynthetic rate (Pn). Although A. tauschii showed nearly complete recovery of PSII, the adverse effect was more pronounced in A. speltoides. Measurement of the minimum fluorescence (Fo) versus temperature curves revealed a higher inflection temperature of Fo for A. tauschii than A. speltoides, reflecting greater thermo stability of the photosynthetic apparatus. Absorbed light energy distribution revealed that A. speltoides showed increased steady state fluorescence and a lower absorbed light allocated to photosynthetic chemistry (ɸPSII) relative to A. tauschii. However, A. tauschii showed higher ability to scavenge free radicals as compared to A. speltoides. This was further validated by higher expression of ascorbate peroxidase gene. These results suggest that A. tauschii showed faster recovery and a better thermostability of its photosynthetic apparatus under severe stress conditions along with a better regulation of energy channeling of PSII complexes to minimize oxidative damage and thus retains greater capability of carbon assimilation. These factors aid in imparting a greater heat tolerance to A. tauschii as compared to A. speltoides and thus make it a better candidate for alien species introgression in wheat breeding programs for thermotolerance in wheat.

Faster recovery of a diatom from UV damage under ocean acidification

Diatoms are the most important group of primary producers in marine ecosystems. As oceanic pH declines and increased stratification leads to the upper mixing layer becoming shallower, diatoms are interactively affected by both lower pH and higher average exposures to solar ultraviolet radiation. The photochemical yields of a model diatom, Phaeodactylum tricornutum, were inhibited by ultraviolet radiation under both growth and excess light levels, while the functional absorbance cross sections of the remaining photosystem II increased. Cells grown under ocean acidification (OA) were less affected during UV exposure. The recovery of PSII under low photosynthetically active radiation was much faster than in the dark, indicating that photosynthetic processes were essential for the full recovery of photosystem II. This light dependent recovery required de novo synthesized protein. Cells grown under ocean acidification recovered faster, possibly attributable to higher CO2 availability for the Calvin cycle producing more resources for repair. The lower UV inhibition combined with higher recovery rate under ocean acidification could benefit species such as P.tricornutum, and change their competitiveness in the future ocean.

Short-term antioxidative responses of 15 microalgae exposed to excessive irradiance including ultraviolet radiation

Short-term photosensitivity and oxidative stress responses were compared for three groups of marine microalgae: Antarctic microalgae, temperate diatoms and temperate flagellates. In total, 15 low-light-acclimated species were exposed to simulated surface irradiance including ultraviolet radiation (SSI). Photosensitivity was assessed as the rate of recovery of Fv/Fm in the hours following SSI treatment. Before, during and after the SSI treatment, oxidative stress responses were assessed by following xanthophyll content and cycling, and activities of superoxide dismutase, ascorbate peroxidase and glutathione reductase, and glutathione redox status. When acclimated to low irradiance, antioxidant levels were not group specific. Superoxide dismutase activity was positively correlated with cell size, whereas in general, ascorbate peroxidase activity appeared to be lower and glutathione redox status appeared to be higher in the Antarctic than in the temperate species. After SSI exposure, the strong inhibition of PSII was followed by variable rates of recovery, although four species remained photosynthetically inactive. SSI tolerance appeared unrelated to geographic or taxonomic background, or to cell size. PSII recovery was enhanced in species with decreasing superoxide dismutase activity, glutathione redox status and increased xanthophyll cycle activity. We conclude that antioxidant responses are highly species specific and not related to the geographic or taxonomic background. Furthermore, xanthophyll cycling seems more important than antioxidants. Finally, it can be hypothesized that glutathione could function as a stress sensor and response regulator.

Photosynthesis and photoinhibition of two green macroalgae with contrasting habitats

We evaluated light-related traits ofUlva pertusa Kjellman from the intertidal and upper subtidal zones, and ofUmbraulva japonica (Holmes) Bae & Lee (formerlyUlva japonica) within its upper growth limit of 10 m, in Korean coastal regions.U. pertusa showed significantly higher maximum photosynthetic rates, photosynthetic efficiency, saturating irradiances, and total pigment contents. However, green light-use efficiency at limiting irradiances was notably higher inU. japonica, possibly due to the presence of green light absorbing pigments like siphonaxanthin and siphonein. Non-enzymatic antioxidation capacity as determined by DPPH (α,α-diphenyl-β-picrylhydrazyl) radical scavenging rate was markedly higher inU. pertusa (50.49%) than inU. japonica (8.85%). Both species showed a substantial decrease in optimal photosystem (PS) II efficiency (Fv/ Fm) with increasing PAR doses despite the degree of photoinhibition being more marked inU. japonica. After 24 h in dim light,U. pertusa rapidly and almost fully recovered Fv/ Fm within an hour whileU. japonica exhibited slow and incomplete PSII recovery over the full recovery period. A significant depression in photosynthetic activity with monochromatic UV-B radiation was observed inU. japonica only, followed by slight recovery in dim light. Light use efficiency and high irradiance tolerance may be important ecological axes along which the niche separation of green foliose algae occurs in Korean coastal waters.

광 스트레스에 대한 고추 잎의 광합성 반응과 광 보호 메커니즘

Photoinhibition and photoprotection of PSII in the leaves of hot pepper (Capsicum annuum L.) grown in Hoagland solution and Tap water were compared. Though changes in the rates of <TEX>$O_2$</TEX> evolution as a function of photon fluence rate (PFR) were comparable, the rates of respiration in the dark was 3 times higher in the Hoagland solution grown leaves than in the Tap-water grown ones. Compared to Hoagland solution grown plane, PSIIs of Tap water grown pepper leaves were more susceptible to photoinhibitory light treatment. In order to inactivate functional PSII to the same extents, Hoagland solution grown plants required almost 2-fold high light <TEX>$(1600{\mu}molm^{-2}s^-)$</TEX> treatment than those of Tap water <TEX>$(900{\mu}molm^{-2}s^-)$</TEX>. Interestingly, the remaining fraction of PSII in Hoagland grown pepper was able to survive under prolonged illumination in the presence of lincomycin, which probably means that the growth condition of plant seemed to have an effect on the recovery of PSII from light stress. When PSII was severly photoinactivated at a chilling temperature, recovery was observed only if the residual functional PSII were not inhibited with DCMU, Nigericin and MV during recovery. In conclusion, PSIIs grown in the Hoagland solution was more resistant to excess light than in the Tap water grown one and the recovery of PSII from photodamage was more efficient in Hoagland grown pepper leaves than Tap water grown one, which means that the increased dark respiration may play a important role in the protection of PSII from photoinhibition by helping repair photosynthetic proteins (in particular, the D1 protein of PSII) degraded by photoinhibition.

Dissection of photodamage at low temperature and repair in darkness suggests the existence of an intermediate form of photodamaged photosystem II.

Irradiation of the photosynthetic machinery with strong light induces damage to the photosystem II complex (PSII), and this phenomenon is referred to as photodamage. In an attempt to characterize the mechanism of photodamage to PSII, we examined the events associated with photodamage by monitoring the phenomenon in Synechocystis sp. PCC 6803 at a low temperature. After the activity of PSII had been reduced to 10% of the original activity by exposure of Synechocystis cells to strong light at 10 degrees C, recovery was allowed to proceed at 34 degrees C in darkness. Under these conditions, approximately 50% of the activity of PSII was restored within 60 min. The recovery in darkness did not require protein synthesis, as demonstrated by Western blotting analysis and a radiolabeling experiment with [(35)S]methionine. We also observed a similar recovery of PSII in darkness in isolated thylakoid membranes. Our findings, together with those of other studies, suggest the presence of an intermediate form of photodamaged PSII that is generated prior to the formation of photodamaged PSII.

Comparative analysis of photosensitivity in photosystem I donor and acceptor side mutants of Chlamydomonas reinhardtii

ABSTRACTElectron input from plastocyanin into photosystem I (PSI) is slowed down in the Chlamydomonas reinhardtii mutants affected at the donor side (PsaF or PsaB, lumenal loop j) of PSI. In contrast, electron exit from PSI to ferredoxin is diminished in the PSI acceptor side PsaC mutants K35E and FB1. Although, the electron transfer reactions are diminished to a similar extent in both type of mutants, the PsaC mutants K35E and FB1 are more light‐sensitive than the PsaF‐deficient strain 3bF or the PsaB mutants E613N and W627F. To assess the differential photosensitivity of donor and acceptor side mutants fluorescence transients, gross oxygen evolution and uptake, PSII photo‐inhibition and rate of recovery were measured as well as NADP+ photoreduction. The NADP+ photoreduction measurements indicated that the donor side is limiting the reduction rate. In contrast, measurements of gross oxygen evolution and uptake showed that the reducing side limits linear electron transfer. However, under high light, donor and acceptor side mutations lead to PSII photo‐inhibition and to a diminished rate of PSII recovery, cause lipid peroxidation and result in a decrease in the levels of PSI and PSII. The wild type is not affected under the same conditions. These responses are most pronounced in the PsaC‐K35E and PsaB‐W627F mutants, and they correlate with the light sensitivity of these strains. The correlation between limitation of electron transfer through PSI and the formation of reactive oxygen species as a cause for the light‐sensitivity is discussed.

Recovery from low-temperature photoinhibition is related to dephosphorylation of phosphorylated CP29 rather than zeaxanthin epoxidation in rice leaves

During recovery from chilling-induced photoinhibition in rice leaves, we compared the reactivation kinetics of PSII photochemical efficiency (Fv/Fm) with that of zeaxanthin (Z) epoxidation and the dephosphorylation of CP34 (i.e., the phosphorylated form of CP29). The latter two processes were kinetically similar to the slow increase in Fv/Fm measured in our control leaves. However, the rate of Z epoxidation was significantly retarded by an epoxidase inhibitor, 5 mM salicylaldoxime (SA), without any significant changes in the processes of PSII reactivation and CP34 dephosphorylation. When chilled leaves were incubated at 10°C in the dark, both reactivation and dephosphorylation were significantly blocked, but Z epoxidation was not. Finally, we observed that the kinetics of CP34 dephosphorylation matched very well with those of PSII recovery in two rice cultivars with different chilling sensitivities. These results suggest that PSII reactivation from low-temperature photoinhibition is more closely related to CP34 dephosphorylation than to Z epoxidation.

Chloroplast-encoded small polypeptide, PsbT, is required for the efficient recovery of photodamaged photosystem II

PsbT is a small hydrophobic PSII subunit that is encoded by chloroplast genome. Although psbT-deletion mutant of Chlamydomonas reinhardtii (?psbT) can grow photoautotrophycally under moderate light condition, the growth of this mutant is severely impaired by strong light illumination. Since the primary target of photoinhibition is PSII, PsbT may protect PSII against excess light and/or may be involved in the repair of photodamaged PSII. In this report, we studied photoinhibition and repair process of PSII in wild-type and ?psbT cells. In the strong light, the steady-state level of PSII proteins was constant in wild-type but slowly decreased in ?psbT. It was also found that O2 evolution was more severely photoinactivated in ?psbT. However, the both strains showed the same rates of photodegradation of PSII proteins and photoinactivation of O2 evolution. Subsequently, we have studied recovery of PSII after partial photoinactivation and found that the recovery was much slower in ?psbT than in wild-type. Since pulse-labeling experiments of chloroplast-encoded proteins revealed that the protein synthesis is not damaged by the photoinhibition treatment, it is concluded that PsbT is involved in the post-translational step(s) of the recovery process of photodamaged PSII. Localization and function of PsbT in PSII core complex is also discussed.