Changes in PS II heterogeneity in response to osmotic and ionic stress in wheat leaves (Triticum aestivum)

Three-state mathematical model for the assessment of DCMU-treated photosystem II heterogeneity

Photosystem II (PSII) is one of the main pigment-protein complexes of photosynthesis which is highly sensitive to unfavorable environmental factors. The heterogeneity of PSII properties is essential for the resistance of autotrophic organisms to stress factors. Assessment of the PSII heterogeneity may be used in environmental monitoring for on-line detection of contamination of the environment. We propose an approach to assess PSII oxygen-evolving complex and light-harvesting antenna heterogeneity that is based on mathematical modeling of the shape of chlorophyll a fluorescence rise of 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated samples. The hierarchy of characteristic times of the processes considered in the model makes it possible to reduce the model to a system of three ordinary differential equations. The analytic solution of the reduced three-state model is expressed as a sum of two exponential functions, and it exactly reproduces the solution of the complete system within the time range from microseconds to hundreds of milliseconds. The combination of several such models for reaction centers with different properties made it possible to use it as an instrument to study PSII heterogeneity. PSII heterogeneity was studied for Chlamydomonas at different intensities of actinic light, for Scenedesmus under short-term heating, and for Chlorella grown in nitrate-enriched and nitrate-depleted media.

Resistance of Antarctic moss Sanionia uncinata to photoinhibition: chlorophyll fluorescence analysis of samples from the western and eastern coasts of the Antarctic Peninsula

Interspecific differences in sensitivity of the Antarctic moss Sanionia uncinata from King George Island (KGI) and James Ross Island (JRI) to photoinhibitory treatment were studied in laboratory conditions using chlorophyll fluorescence techniques. Slow (Kautsky) and fast (OJIP) kinetics were used for the measurements. Samples were exposed to a short-term (60 min) photoinhibitory treatment (PIT, 2000 μmol·m^-2 ·s^-1 PAR). The photoinhibitory treatment (PIT) led to photoinhibition which was indicated by the decrease in F_V /F_M and Φ_PSII in KGI but not in JRI samples. However, this decrease was small and full recovery was reached 90 min after PIT termination. Non-photochemical quenching (NPQ) was activated during the PIT, and rapidly relaxed during recovery. Early stages of photoinhibition showed a drop in F_V /F_M and Φ_PSII to minimum values within the first 10 s of the PIT, with their subsequent increase apparent within fast (0-5 min PIT) and slow (5-50 min PIT) phases of adjustment. The PIT caused a decrease in the performance index (Pi_Abs), photosynthetic electron transport per reaction centre (RC) (ET₀ /RC). The PIT induced an increase in thermal dissipation per RC (DI₀ /RC), effectivity of thermal dissipation (Phi_D₀ ), absorption per RC (ABS/RC) and trapping rate per RC (TR₀ /RC). In conclusion, PIT led to only slight photoinhibition followed by fast recovery in S. uncinata from KGI and JRI, since F_V /F_M and Φ_PSII returned to pre-photoinhibitory conditions. Therefore, S. uncinata might be considered resistant to photoinhibition even in the wet state. The KGI samples showed higher resistance to photoinhibition than the JRI samples.

Regulation of Photosystem II Heterogeneity and Photochemistry in Two Cultivars of C4 Crop Sugarcane Under Chilling Stress

In subtropical regions, chilling stress is one of the major constraints for sugarcane cultivation, which hampers yield and sugar production. Two recently released sugarcane cultivars, moderately chilling tolerant Guitang 49 and chilling tolerant Guitang 28, were selected. The experiments were conducted in the controlled environment, and seedlings were exposed to optimum (25°C/15°C), chilling (10°C/5°C), and recovery (25°C/15°C) temperature conditions. PSII heterogeneity was studied in terms of reducing side and antenna size heterogeneity. Under chilling, reducing side heterogeneity resulted in increased number of Q_B non-reducing centers, whereas antenna side heterogeneity resulted in enhanced number of inactive β centers in both cultivars, but the magnitude of change was higher in Guitang 49 than Guitang 28. Furthermore, in both cultivars, quantum efficiency of PSII, status of water splitting complex, and performance index were adversely affected by chilling, along with reduction in net photosynthesis rate and nighttime respiration and alterations in leaf optical properties. The extents of negative effect on these parameters were larger in Guitang 49 than in Guitang 28. These results reveal a clear differentiation in PSII heterogeneity between differentially chilling tolerant cultivars. Based on our studies, it is concluded that PSII heterogeneity can be used as an additional non-invasive and novel technique for evaluating any type of environmental stress in plants.

Amplification of abiotic stress tolerance potential in rice seedlings with a low dose of UV-B seed priming

UV-B radiation is a major abiotic stress factor that adversely affects the growth and productivity of crop plants including rice (Oryza sativa L.). However, on the other hand, lower doses of UV-B radiation applied to seeds can have a priming effect on plants emerging from it. In this study, seeds of O. sativa var. kanchana were primed with UV-B radiation (6 kJ m-2) and were further subjected to NaCl, polyethylene glycol 6000 (PEG) and UV-B stress. The effects of UV-B priming in imparting NaCl, PEG and UV-B stress tolerance to rice seedlings were analysed through various photosynthetic features and antioxidative mechanisms. PSI and PSII activity levels as well as chl a fluorescence were found to be significantly higher in the UV-B primed and unstressed seedlings. When stress (NaCl, PEG and high UV-B) was imposed, increased PSI and PSII activity levels, chl a fluorescence and metabolite accumulation (proline, total phenolics and sugar) as well as nonenzymatic (ascorbate and glutathione) and enzymatic (superoxide dismutase, catalase, ascorbate peroxidase) antioxidants were recorded in UV-B primed and NaCl-stressed plants followed by UV-B primed and UV-B-stressed plants, and primed and PEG-stressed, compared with unprimed and stressed conditions. The results indicate that UV-B priming in rice seedlings effectively enhances the NaCl stress tolerance potential in rice to a greater extent than UV-B and PEG stress tolerance potential. The cost-effectiveness of UV-B seed priming is predominantly clear from the differing tolerance responses of rice seedlings exposed to different stress conditions.

Photomodified fluoranthene exerts more harmful effects as compared to intact fluoranthene by inhibiting growth and photosynthetic processes in wheat

Polycyclic aromatic hydrocarbons (PAHs) are toxic to plants. Exposure of PAHs to sunlight can result in the formation of photomodified PAHs, which are generally more toxic than the parent compounds. In the present study, biochemical and physiological effects of intact fluoranthene (FLT) and photomodified FLT (P.FLT) in wheat were investigated. Our results demonstrated that the treatment of FLT and P.FLT, inhibited germination, seedling growth and pigment content. However, inhibition observed on primary photochemistry and photosystem II heterogeneity was more prominent in P.FLT treated plants. Both FLT and P.FLT affected the levels of antioxidant enzymes such as POD, SOD, CAT and GR to similar extent. Greater inhibitory effects of P.FLT than FLT may be ascribed to its effects on growth parameters and photochemistry rather than on the activities of various antioxidant enzymes.

Effect of photosystem I inactivation on chlorophyll a fluorescence induction in wheat leaves: Does activity of photosystem I play any role in OJIP rise?

Interpretation of the fast chlorophyll a fluorescence induction is still a subject of continuing discussion. One of the contentious issues is the influence of photosystem I (PSI) activity on the kinetics of the thermal JIP-phase of OJIP rise. To demonstrate this influence, we realized a series of measurements in wheat leaves subjected to PSI photoinactivation by the sequence of red saturation pulses (15,000 μmol photons m(-2) s(-1) for 0.3 s, every 10 s) applied in darkness. Such a treatment led to a moderate decrease of maximum quantum efficiency of PSII (by ~8%), but a strong decrease of the number of oxidizable PSI (by ~55%), which considerably limited linear electron transport and CO2 assimilation. Surprisingly, the PSI photoinactivation had low effects on OJIP kinetics of variable fluorescence. In particular, the amplitude of variable fluorescence of IP-step (ΔVIP), which has been considered to be a measure of PSI content, was not decreased, despite the low content of photooxidizable PSI. On the other hand, the slower relaxation of chlorophyll fluorescence after saturation pulse as well as the results of the double-hit method suggest that PSI inactivation treatment led to an increase of the fraction of QB-nonreducing PSII reaction centers. Our results somewhat challenge the mainstream interpretations of JIP-thermal phase, and at least suggest that the IP amplitude cannot serve to estimate reliably the PSI content or the PSI to PSII ratio. Moreover, these results recommend the use of the novel method of PSI inactivation, which might help clarify some important issues needed for the correct understanding of the OJIP fluorescence rise.

Assessment of phytotoxicity of anthracene in soybean (Glycine max) with a quick method of chlorophyll fluorescence

A decrease in photosynthetic efficiency may indicate the toxic effects of environmental pollutants on higher plants. Measurement of chlorophyll (Chl) a fluorescence to assess the performance of photosystem II (PSII) was used as an bioindicator of toxicity of the polycyclic aromatic hydrocarbon (PAH) anthracene (ANT) in soybean plants. The results revealed that ANT treatment caused a reduction in quantum yield of PSII, damage to the oxygen evolving complex, as well as a significant reduction in performance index of PSII. However, change in performance index was more prominent, and it seems that the performance index is a more sensitive parameter to environmental contaminants. Moreover, a change in heterogeneity of PSII was also observed. The number of active reaction centres decreased with increasing concentration of ANT, as secondary plastoquinone reducing centres were converted into non-reducing centres, and PSIIα centres were converted into PSIIβ and PSIIγ centres. The influence of ANT on PSII heterogeneity could be an important reason for reductions in the PSII performance.

Photosystem I shows a higher tolerance to sorbitol-induced osmotic stress than photosystem II in the intertidal macro-algae Ulva prolifera (Chlorophyta)

The photosynthetic performance of the desiccation-tolerant, intertidal macro-algae Ulva prolifera was significantly affected by sorbitol-induced osmotic stress. Our results showed that photosynthetic activity decreased significantly with increases in sorbitol concentration. Although the partial activity of both photosystem I (PS I) and photosystem II (PS II) was able to recover after 30 min of rehydration, the activity of PS II decreased more rapidly than PS I. At 4 M sorbitol concentration, the activity of PS II was almost 0 while that of PS I was still at about one third of normal levels. Following prolonged treatment with 1 and 2 M sorbitol, the activity of PS I and PS II decreased slowly, suggesting that the effects of moderate concentrations of sorbitol on PS I and PS II were gradual. Interestingly, an increase in non-photochemical quenching occurred under these conditions in response to moderate osmotic stress, whereas it declined significantly under severe osmotic stress. These results suggest that photoprotection in U. prolifera could also be induced by moderate osmotic stress. In addition, the oxidation of PS I was significantly affected by osmotic stress. P700(+) in the thalli treated with high concentrations of sorbitol could still be reduced, as PS II was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), but it could not be fully oxidized. This observation may be caused by the higher quantum yield of non-photochemical energy dissipation in PS I due to acceptor-side limitation (Y(NA)) during rehydration in seawater containing DCMU.

Photosynthetic proton and electron transport in wheat leaves under prolonged moderate drought stress

In conditions of long-lasting moderate drought stress, we have studied the photoprotective responses in leaves of wheat (Triticum aestivum L., cv. Katya) related to the photosynthetic electron and proton transport. The dark-interval relaxation kinetics of electrochromic bandshift (ECS) indicated a decrease of electric and an increase of osmotic component of the proton motive force in drought stressed leaves, but neither the total proton motive force (pmf) nor the thylakoid proton conductance (gH(+)) were affected. We observed the enhanced protection against overreduction of PSI acceptor side in leaves of drought stressed plants. This was obviously achieved by the rapid buildup of transthylakoid pH gradient at relatively low light intensities, directly associated to the steep increase of NPQ and the down-regulation of linear electron transport. It was further accompanied by the steep increase of redox poise at PSII acceptor side and PSI donor side. The early responses related to thylakoid lumen acidification in drought-stressed leaves could be associated with the activity of an enhanced fraction of PSI not involved in linear electron flow, which may have led to enhanced cyclic electron pathway even in relatively low light intensities, as well as to the drought-induced decrease of IP-amplitude in fast chlorophyll fluorescence kinetics.

Alteration in PS II heterogeneity under the influence of polycyclic aromatic hydrocarbon (fluoranthene) in wheat leaves (Triticum aestivum)

Polycyclic aromatic hydrocarbon, fluoranthene causes significant inhibitory effects on photosynthetic processes, particularly photosystem II. PS II exhibits heterogeneity with regards to antenna size, connectivity and ability to transfer electron beyond QA. This is the first report describing effects of FLT on PS II heterogeneity. FLT affects antenna heterogeneity by converting active α center into inactive β and γ centers. At the same time in the presence of FLT number of QB-non reducing center also increase. Such study can prove to be a biomarker for early detection to stress tolerance and toxicity of environment pollutants.

Drought tolerance in modern and wild wheat

The genus Triticum includes bread (Triticum aestivum) and durum wheat (Triticum durum) and constitutes a major source for human food consumption. Drought is currently the leading threat on world's food supply, limiting crop yield, and is complicated since drought tolerance is a quantitative trait with a complex phenotype affected by the plant's developmental stage. Drought tolerance is crucial to stabilize and increase food production since domestication has limited the genetic diversity of crops including wild wheat, leading to cultivated species, adapted to artificial environments, and lost tolerance to drought stress. Improvement for drought tolerance can be achieved by the introduction of drought-grelated genes and QTLs to modern wheat cultivars. Therefore, identification of candidate molecules or loci involved in drought tolerance is necessary, which is undertaken by "omics" studies and QTL mapping. In this sense, wild counterparts of modern varieties, specifically wild emmer wheat (T. dicoccoides), which are highly tolerant to drought, hold a great potential. Prior to their introgression to modern wheat cultivars, drought related candidate genes are first characterized at the molecular level, and their function is confirmed via transgenic studies. After integration of the tolerance loci, specific environment targeted field trials are performed coupled with extensive analysis of morphological and physiological characteristics of developed cultivars, to assess their performance under drought conditions and their possible contributions to yield in certain regions. This paper focuses on recent advances on drought related gene/QTL identification, studies on drought related molecular pathways, and current efforts on improvement of wheat cultivars for drought tolerance.