青藏高原春小麦叶片光合作用的光抑制及PSII反应中心光化学效率的恢复分析

在青海省都兰县香日德镇东盛村, 以中国科学院西北高原生物研究所培育的春小麦(Triticum aestivum)品种为材料, 主要采用调制叶绿素荧光分析手段, 研究了抽穗期旗叶光合作用的光抑制现象, 并分析了非光化学猝灭组分的光诱导和非光诱导耗散的量子产量变化。结果表明, 高原春小麦各品种间旗叶光合色素含量和比叶重存在差异; 全晴天3个典型时段准确暗适应20 min后的PSII最大光化学效率(F_v/F_m)的比较分析证实, 高原春小麦存在着光合作用的光抑制现象, F_v/F_m的降低是由于PSII反应中心的可逆失活; 稳态作用光下PSII有效光化学效率(F_v′/F_m′)易受持续强光胁迫的影响, 而PSII实际光化学效率(Φ_PSII)在各春小麦品种间的差异略为明显; 上下午间4个春小麦品种的光化学猝灭系数(q_P)和非光化学猝灭系数(NPQ)呈较一致的变化趋势, 显然q_P和NPQ既属品种的内禀特性, 又与强太阳光胁迫的累积密切相关; 非光化学猝灭组分中光诱导的PSII调节性能量耗散的量子产量(Φ_NPQ)所占比例较大, 下午时分Φ_NPQ的上调反映了高原春小麦对青藏高原持续强光胁迫的驯化适应。     

The development of antenna complexes of barley (Hordeum vulgare cv. Akcent) under different light conditions as judged from the analysis of 77 K chlorophyll a fluorescence spectra

Using 77 K chlorophyll a (Chl a) fluorescence spectra in vivo, the development was studied of Photosystems II (PS II) and I (PS I) during greening of barley under intermittent light followed by continuous light at low (LI, 50 μmol m⁻² s⁻¹) and high (HI, 1000 μmol m⁻² s⁻¹) irradiances. The greening at HI intermittent light was accompanied with significantly reduced fluorescence intensity from Chl b excitation for both PS II (F685) and PS I (F743), in comparison with LI plants, indicating that assembly of light-harvesting complexes (LHC) of both photosystems was affected to a similar degree. During greening at continuous HI, a slower increase of emission from Chl b excitation in PS II as compared with PS I was observed, indicating a preferred reduction in the accumulation of LHC II. The following characteristics of 77 K Chl a fluorescence spectra documented the photoprotective function of an elevated content of carotenoids in HI leaves: (1) a pronounced suppression of Soret region of excitation spectra (410–450 nm) in comparison with the red region (670–690 nm) during the early stage of greening indicated a strongly reduced excitation energy transfer from carotenoids to the Chl a fluorescing forms within PS I and PS II; (2) changes in the shape of the excitation band of Chl b and carotenoids (460–490 nm) during greening under continuous light confirmed that the energy transfer from carotenoids to Chl a within PS II remained lower as compared with the LI plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Measurements of manganese in thylakoids of Sinapis alba grown under high-light and low-light conditions

The manganese content of thylakoids and tissues was measured in leaves grown under high- and low-light conditions. Especially when grown in a nutrient medium enriched in manganese (20 M), the thylakoids contained large amounts of manganese, which could be removed by EDTA washing without impairment of the Hill reaction. The unremovable content of manganese was almost the same in thylakoids from plants grown in nutrient media of normal (2 M) and reduced (0.2 M) manganese content. Up to this limit of manganese content, Hill activity did not seem to be impaired. 1.2 atoms Mn per 100 molecules chlorophyll were found in low-light thylakoids and 1.6 atoms Mn in high-light thylakoids. This is similar to the behaviour of other electron transport components, the number of which is also decreased under low-light conditions. However, the decrease in the manganese content is not as striking as the decrease in, for example, the cytochrome f and ferredoxin content. This may be attributed to an invariable pool of manganese which is not involved in the oxygen evolving system. Alternatively, if all of our measured manganese is involved in electron transport to PS II, this could indicate that in low-light chloroplasts the ratio of PS II/PS I components may be somewhat increased.

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Zusammenfassung Der Mangangehalt von Thylakoiden und Gewebe aus Starklicht- und Schwachlichtblättern wurde untersucht. Besonders bei Pflanzen, welche unter erhöhtem Manganangebot (20 M) angezogen wurden, besaßen die Thylakoide sehr viel Mangan, welches durch Waschen mit EDTA entfernt werden konnte, ohne die Hill-Aktivität zu beeinträchtigen. In Thylakoiden aus Pflanzen, welche unter normalem (2 M) und reduziertem (0,2 M) Manganangebot gewachsen waren, unterschied sich der nicht entfernbare Mangangehalt nicht sehr. Dies scheint die untere Grenze des Mangangehalts zu sein, bis zu welchem die Hill-Aktivität noch nicht beeinträchtigt wird. Schwachlicht-Thylakoide besitzen 1,2 Atome Mn pro 100 Chlorophyllmoleküle, während Starklicht-Thylakoide 1,6 Atome Mn pro 100 Chlorophyllmoleküle enthalten. Dies gleicht dem Verhalten anderer Komponenten des Elektronentransports, welche ebenfalls im Starklicht vermehrt vorkommen. Die Unterschiede im Mangangehalt sind jedoch geringer als die Unterschiede im Gehalt von z.B. Cytochrom f und Ferredoxin. Dies könnte auf einen konstanten Anteil von Mangan zurückzuführen sein, welcher nicht am wasserspaltenden System beteiligt ist. Wenn jedoch das gesamte gemessene Mangan am Elektronentransport zum PS II beteiligt ist, könnte dies ein Hinweis sein, daß in Schwachlicht-Chloroplasten sich das Verhältnis der PS II-/PS I-Komponenten etwas vergrößert.

     

Exogenous ethanol treatment alleviates oxidative damage of Arabidopsis thaliana under conditions of high-light stress

Abiotic stresses, such as high light and salinity, are major factors that limit crop productivity and sustainability worldwide. Chemical priming is a promising strategy for improving the abiotic stress tolerance of plants. Recently, we discovered that ethanol enhances high-salinity stress tolerance in Arabidopsis thaliana and rice by detoxifying reactive oxygen species (ROS). However, the effect of ethanol on other abiotic stress responses is unclear. Therefore, we investigated the effect of ethanol on the high-light stress response. Measurement of chlorophyll fluorescence showed that ethanol mitigates photoinhibition under high-light stress. Staining with 3,3′-diaminobenzidine (DAB) showed that the accumulation of hydrogen peroxide (H₂O₂) was inhibited by ethanol under high-light stress conditions in A. thaliana. We found that ethanol increased the gene expressions and enzymatic activities of antioxidative enzymes, including ASCORBATE PEROXIDASE1 (AtAPX1), Catalase (AtCAT1 and AtCAT2). Moreover, the expression of flavonoid biosynthetic genes and anthocyanin contents were upregulated by ethanol treatment during exposure to high-light stress. These results imply that ethanol alleviates oxidative damage from high-light stress in A. thaliana by suppressing ROS accumulation. Our findings support the hypothesis that ethanol improves tolerance to multiple stresses in field-grown crops.     

Xanthophyll cycle components and capacity for non-radiative energy dissipation in sun and shade leaves ofLigustrum ovalifolium exposed to conditions limiting photosynthesis

The relationships between photosynthetic efficiency, non-radiative energy dissipation and carotenoid composition were studied in leaves ofLigustrum ovalifolium developed either under full sunlight or in the shade. Sun leaves contained a much greater pool of xanthophyll cycle components than shade leaves. The rate of non-radiative energy dissipation, measured as non-photochemical fluorescence quenching (NPQ), was strictly related to the deepoxidation state (DPS) of xanthophyll cycle components in both sun and shade leaves, indicating that zeaxanthin (Z) and antheraxanthin (A) are involved in the development of NPQ. Under extreme conditions of excessive energy, sun leaves showed higher maximum DPS than shade leaves. Therefore, sun leaves contained not only a greater pool of xanthophyll cycle components but also a higher proportion of violaxanthin (V) actually photoconvertible to A and Z, compared to shade leaves. Both these effects contributed to the higher NPQ in sun versus shade leaves. The amount of photoconvertible V was strongly related to chla/b ratio and inversely to leaf neoxanthin content. This evidence indicates that the amount of photoconvertible V may be dependent on the degree of thylakoid membrane appression and on the organization of chlorophyll-protein complexes, and possible explanations are discussed. Exposure to chilling temperatures caused a strong decline in the photon yield of photosynthesis and in the intrinsic efficiency of PS II photochemistry in sun leaves, but little effects in shade leaves. These effects were accompanied by increases in the pool of xanthophyll cycle components and in DPS, more pronounced in sun than in shade leaves. This corroborates the view that Z and A may play a photoprotective role under unfavorable conditions. In addition to the xanthophyll-related non-radiative energy dissipation, a slow relaxing component of NPQ, independent from A and Z concentrations, has been found in leaves exposed to low temperature and high light. This quenching component may be attributed either to other regulatory mechanism of PS II efficiency or to photoinactivation.Research supported by National Research Council of Italy, Special Project RAISA, Sub-Project 2, Paper N. 1587.     

Alterations of pigment composition and their interactions in response to different light conditions in the diatom Chaetoceros gracilis probed by time-resolved fluorescence spectroscopy

Maintenance of energy balance under changeable light conditions is an essential function of photosynthetic organisms to achieve efficient photochemical reactions. Among the photosynthetic organisms, diatoms possess light-harvesting fucoxanthin chlorophyll (Chl) a/c-binding protein (FCP) as peripheral antennas. However, how diatoms regulate excitation-energy distribution between FCP and the two photosystem cores during light adaptation is poorly understood. In this study, we examined spectroscopic properties of a marine diatom Chaetoceros gracilis adapted in the dark and at photosynthetic photon flux density at 30 and 300?μmol?photons?m^?2?s^?1. Absorption spectra at 77?K showed significant changes in the Soret region, and 77-K steady-state fluorescence spectra showed significant differences in the spectral shape and relative fluorescence intensity originating from both PSII and PSI, among the cells grown under different light conditions. These results suggest alterations of pigment composition and their interactions under the different light conditions. These alterations affected the excitation-energy dynamics monitored by picosecond time-resolved fluorescence analyses at 77?K significantly. The contributions of Chls having lower energy levels than the reaction center Chls in the two photosystems to the energy dynamics were clearly identified in the three cells but with presumably different roles. These findings provide insights into the regulatory mechanism of excitation-energy balance in diatoms under various light conditions.     

Reduced susceptibility to waterlogging together with high-light stress is related to increases in superoxide dismutase and catalase activities in sweet potato

We investigated the changes in antioxidative enzyme activities of two sweet potato cultivars under waterlogging and high-light conditions in the growth chamber. The activities of antioxidative enzymes were measured from leaf crude extract of sweet potato during the first five days of the treatments. Activities of superoxide dismutase and catalase were consistently increased in Taoyuan 1 sweet potato over time under waterlogging and high-light conditions. However, decreases in both superoxide dismutase and catalase activities were observed for cultivar Yongtsai under waterlogging and high-light conditions. Waterlogging, together with high-light intensity, impairs superoxide dismutase and catalase activities in the cultivar Yongtsai indicating its greater susceptibility to waterlogging and high-light stress. In contrast, the increase in activities of superoxide dismutase and catalase in Taoyuan 1 indicated its greater ability to detoxify reactive oxygen species during the treatment and ensured its reduced susceptibility to waterlogging and high-light stress. The activities of peroxidase may be inactivated by high-light treatment and, therefore, may not be associated with tolerance of sweet potato plants to waterlogging and high-light stress. Differences in susceptibility to waterlogging and high-light conditions in the leafy vegetable Yongtsai and storage root Taoyuan 1 are discussed.     

Light and nitrogen nutrition regulate apical control in Rosa hybrida L.

Apical control is defined as the inhibition of basal axillary bud outgrowth by an upper actively growing axillary axis, whose regulation is poorly understood yet differs markedly from the better-known apical dominance. We studied the regulation of apical control by environmental factors in decapitated Rosa hybrida in order to remove the apical hormonal influence and nutrient sink. In this plant model, all the buds along the main axis have a similar morphology and are able to burst in vitro. We concentrated on the involvement of light intensity and nitrate nutrition on bud break and axillary bud elongation in the primary axis pruned above the fifth leaf of each rose bush. We observed that apical control took place in low light (92 μmol m⁻² s⁻¹), where only the 2-apical buds grew out, both in low (0.25 mM) and high (12.25 mM) nitrate. In contrast, in high light (453 μmol m⁻² s⁻¹), the apical control only operates in low nitrate while all the buds along the stem grew out when the plant was supplied with a high level of nitrate. We found a decreasing photosynthetic activity from the top to the base of the plant concomitant with a light gradient along the stem. The quantity of sucrose, fructose, glucose and starch are higher in high light conditions in leaves and stem. The expression of the sucrose transporter RhSUC2 was higher in internodes and buds in this lighting condition, suggesting an increased capacity for sucrose transport. We propose that light intensity and nitrogen availability both contribute to the establishment of apical control.