Xanthophyll cycle pool size and composition in relation to the nitrogen content of apple leaves

The objective of this study was to determine xanthophyll cycle pool size and composition in response to N status and their relationships to non-photochemical quenching in apple leaves. Bench-grafted Fuji/M.26 trees were fertilized with different N concentrations (0-20 mM) in a modified Hoagland's solution for 6 weeks to create a wide range of leaf N status (1-4.4 g m(-2)). Chlorophyll content, xanthophyll cycle pool size, lutein, total carotene, and neoxanthin on a leaf area basis all increased linearly with increasing leaf N. However, only the ratios of the xanthophyll cycle pool and of lutein to chlorophyll were higher in low N leaves than in high N leaves. Under high light at midday, both zeaxanthin (Z), expressed on a chlorophyll basis, and the percentage of the xanthophyll cycle pool present as Z, increased as leaf N decreased. Thermal dissipation of excitation energy, measured as non-photochemical quenching of chlorophyll fluorescence, was positively related to, whereas efficiency of excitation transfer and photosystem II quantum efficiency were negatively related to, Z, expressed on a chlorophyll basis or on a xanthophyll cycle pool basis. It is concluded that both xanthophyll cycle pool size (on a chlorophyll basis) and conversion of violaxanthin to zeaxanthin are enhanced in response to N limitation to dissipate excessive absorbed light under high irradiance.     

Effect of nitrogen limitation on foliar antioxidants in relationship to other metabolic characteristics

The long-term effect of limiting soil nitrogen (N) availability on foliar antioxidants, thermal energy dissipation, photosynthetic and respiratory electron transport, and carbohydrates was investigated in Spinacia oleracea L. Starch, sucrose, and glucose accumulated in leaves of N-limited spinach at predawn, consistent with a downregulation of chloroplast processes by whole-plant sink limitation in response to a limited supply of N-based macromolecules throughout the plant. On a leaf-area or dry-weight basis, levels of chlorophyll, carotenoid pools, photosynthetic electron transport capacity, as well as activities for the predominantly chloroplast-localized antioxidant enzymes ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2) were much lower in N-limited versus N-replete plants. When expressed on a chlorophyll basis, foliar levels of all of these parameters were similar in N-replete versus N-limited plants. However, on a total-protein basis, antioxidant enzyme activities were higher in N-limited plants. Nitrogen-limited spinach showed higher levels of thermal energy dissipation and of zeaxanthin and antheraxanthin at midday, as well as slightly higher ascorbate contents relative to chlorophyll. These results indicate that strong, long-term N limitation led not only to alterations in the balance between different processes but also to an overall downregulation of light collection, photosynthetic electron transport capacity, and chloroplast-based antioxidant enzymes. This is further supported by the finding that glucose-feeding of excised leaves led to strong concomitant decreases in photosynthetic electron transport capacity and ascorbate peroxidase activity. On a leaf-area basis, neither superoxide dismutase (EC 1.15.1.1) activity nor dark repiration rates showed a treatment effect. This indicates that overall mitochondrial electron transport activity does not decrease under long-term N limitation and is consistent with localization of an important fraction of foliar superoxide dismutase in mitochondria. Received: 19 March 1999 / Accepted: 13 April 1999     

Photoinhibition and photoprotection in senescent leaves of field-grown wheat plants

Photosynthesis, photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle in the senescent flag leaves of wheat (Triticum aestivum L.) plants grown in the field were investigated. Compared to the non-senescent leaves, photosynthetic capacity was significantly reduced in senescent flag leaves. The light intensity at which photosynthesis was saturated also declined significantly. The light response curves of PSII photochemistry indicate that a down-regulation of PSII photochemistry occurred in senescent leaves in particular at high light. The maximal efficiency of PSII photochemistry in senescent flag leaves decreased slightly when measured at predawn but substantially at midday, suggesting that PSII function was largely maintained and photoinhibition occurred in senescent leaves when exposed to high light. At midday, PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centers decreased considerably, while non-photochemical quenching increased significantly. Moreover, compared with the values at early morning, a greater decrease in CO₂ assimilation rate was observed at midday in senescent leaves than in control leaves. The levels of antheraxanthin and zeaxanthin via the de-epoxidation of violaxanthin increased in senescent flag leaves from predawn to midday. An increase in the xanthophyll cycle pigments relative to chlorophyll was observed in senescent flag leaves. The results suggest that the xanthophyll cycle was activated in senescent leaves due to the decrease in CO₂ assimilation capacity and the light intensity for saturation of photosynthesis and that the enhanced formation of antheraxanthin and zeaxanthin at high light may play an important role in the dissipation of excess light energy and help to protect photosynthetic apparatus from photodamage. Our results suggest that the well-known function of the xanthophyll cycle to safely dissipate excess excitation energy is also important for maintaining photosynthetic function during leaf senescence.     

Arabidopsis mutants define a central role for the xanthophyll cycle in the regulation of photosynthetic energy conversion.

A conserved regulatory mechanism protects plants against the potentially damaging effects of excessive light. Nearly all photosynthetic eukaryotes are able to dissipate excess absorbed light energy in a process that involves xanthophyll pigments. To dissect the role of xanthophylls in photoprotective energy dissipation in vivo, we isolated Arabidopsis xanthophyll cycle mutants by screening for altered nonphotochemical quenching of chlorophyll fluorescence. The npq1 mutants are unable to convert violaxanthin to zeaxanthin in excessive light, whereas the npq2 mutants accumulate zeaxanthin constitutively. The npq2 mutants are new alleles of aba1, the zeaxanthin epoxidase gene. The high levels of zeaxanthin in npq2 affected the kinetics of induction and relaxation but not the extent of nonphotochemical quenching. Genetic mapping, DNA sequencing, and complementation of npq1 demonstrated that this mutation affects the structural gene encoding violaxanthin deepoxidase. The npq1 mutant exhibited greatly reduced nonphotochemical quenching, demonstrating that violaxanthin deepoxidation is required for the bulk of rapidly reversible nonphotochemical quenching in Arabidopsis. Altered regulation of photosynthetic energy conversion in npq1 was associated with increased sensitivity to photoinhibition. These results, in conjunction with the analysis of npq mutants of Chlamydomonas, suggest that the role of the xanthophyll cycle in nonphotochemical quenching has been conserved, although different photosynthetic eukaryotes rely on the xanthophyll cycle to different extents for the dissipation of excess absorbed light energy.     

Effects of groundwater depth variation on photosynthesis and photoprotection of <Emphasis Type="Italic">Elaeagnus angustifolia</Emphasis> L.

Physiological and photosynthetic responses were investigated at three different depths of groundwater (DGW: 1.4, 2.4, and 3.4 m) in Elaeagnus angustifolia L., a locally adapted tree to the arid region in northwest China. Predawn leaf water potential and chlorophyll content declined gradually with the increasing DGW, whereas there was little effect on predawn variable-to-maximum chlorophyll fluorescence ratio F _v/F _m and leaf carotenoid compositions (xanthophyll cycle pool, neoxanthin, lutein, and β-carotene). Net photosynthetic rate (P _n), quantum yield of PSII electron transport (Φ_PSII), stomatal conductance (Gs), and intercellular CO₂ concentration (Ci) declined obviously; however, P _n decreased more than Φ_PSII at deeper DGW. The photoinhibition of PSII at all three DGW occurred at midday in summer and increased as DGW increased. The ΔpH-dependent thermal dissipation and the level of de-epoxidation of the xanthophyll cycle at all three DGW reached their maxima at midday with the increase of light intensity. However, the fraction of functional PSII and light intensity at deeper DGW (2.4, 3.4 m) showed a negative correlation. This correlation suggested that most of violaxanthin was converted into zeaxanthin at midday, and the reversible inactivation of partial PSII reaction centers took place at deeper DGW. These results together suggest that both the xanthophyll cycle-dependent thermal dissipation and the reversible inactivation of partial PSII might have played important roles in avoiding the excess light-induced energy damage in leaves of this tree species at deeper DGW.     

Photosynthesis impairment in cassava leaves in response to nitrogen deficiency

Plants of cassava (Manihot esculenta Crantz cv. Cigana Preta) grown in a sand root medium were watered with nutrient solutions containing either 3 mM nitrate (low N) or 12 mM nitrate (high N). Chlorophyll concentration, chlorophyll a/b ratio, stomatal conductance, photorespiration rate and net carbon assimilation rate (on an area and a mass basis, but not on a chlorophyll basis) all decreased in low-N plants as compared with high-N ones. By contrast, photosynthetic nitrogen-use efficiency increased in low-N plants. As indicated by chlorophyll a fluorescence data, these plants exhibited increases in both excitation pressure on Photosystem II and thermal energy dissipation, with a corresponding decrease in quantum yield of electron transport, when contrasted with high-N plants. This decrease paralleled an unchanged maximal Photosystem II photochemical efficiency, suggesting a down-regulation of the Photosystem II photochemistry. It is proposed that decline in biochemical capacity for carboxylation, rather than stomatal limitation or electron transport, were the major constraints associated to the reduced photosynthetic rates induced by nitrogen deficiency in cassava plants.     

The xanthophyll cycle and sustained thermal energy dissipation activity in Vinca minor and Euonymus kiautschovicus in winter

The influence of low temperature on the operation of the xanthophyll cycle and energy dissipation activity, as ascertained through measurements of chlorophyll fluorescence, was examined in two broad-leaved evergreen species, Vinca minor L. and Euonymus kiautschovicus Loessner. In leaves examined under laboratory conditions, energy dissipation activity developed more slowly at lower leaf temperatures, but the final, steady-state level of such activity was greater at lower temperatures where the rate of energy utilization (through photosynthetic electron transport) was much lower. The rate at which energy dissipation activity increased was similar to that of the de-epoxidation of violaxanthin to antheraxanthin and zea-xanthin at different temperatures. However, leaves in the field examined prior to sunrise on mornings following cold days and nights exhibited a retention of antheraxanthin and zeaxanthin that was associated with sustained decreases in photosystem II efficiency. We therefore suggest that this phenomenon of ‘photoinhibition’ in response to light and cold temperatures during the winter results from sustained photoprotective thermal energy dissipation associated with the xanthophyll cycle. Such retention of the de-epoxidized components of the xanthophyll cycle responded to day-to-day changes in temperature, being greatest on the coldest mornings (when photoprotective energy dissipation might be most required) and less on warmer mornings when photosynthesis could presumably proceed at higher rates.     

Both xanthophyll cycle-dependent thermal dissipation and the antioxidant system are up-regulated in grape (Vitis labrusca L cv Concord) leaves in response to N limitation

One-year-old grapevines (Vitis labrusca L. cv. Concord) were supplied with 0, 5, 10, 15, or 20 mM nitrogen (N) in a modified Hoagland's solution twice weekly for 4 weeks. As leaf N decreased in response to N limitation, leaf chlorophyll (Chl) decreased linearly whereas leaf absorptance declined curvilinearly. Compared with high N leaves, low N leaves had lower quantum efficiency of PSII as a result of both an increase in non-photochemical quenching (NPQ) and an increase in closure of PSII reaction centres at midday under high photon flux density (PFD). Both the xanthophyll cycle pool size on a Chl basis and the conversion of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z) at noon increased with decreasing leaf N. NPQ was closely related to A+Z expressed either on a Chl basis or as a percentage of the xanthophyll cycle pool. As leaf N increased, superoxide dismutase (SOD) activity on a Chl basis decreased linearly; activities of catalase (CAT) and glutathione reductase (GR) on a Chl basis increased linearly; activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR) and dehydroascorbate reductase (DHAR) expressed on the basis of Chl decreased rapidly first, then gradually reached a low level. In response to N limitation, the contents of ascorbate (AsA), dehydroascorbate (DAsA), reduced glutathione (GSH), and oxidized glutathione (GSSG) increased when expressed on a Chl basis, whereas the ratios of both AsA to DAsA and GSH to GSSG decreased. It is concluded that, in addition to decreasing light absorption by lowering Chl concentration, both xanthophyll cycle-dependent thermal energy dissipation and the antioxidant system are up-regulated to protect low N leaves from photo-oxidative damage under high light.     

Effects of Exogenous Putrescine on Chlorophyll Fluorescence Imaging and Heat Dissipation Capacity in Cucumber (Cucumis sativus L.) Under Salt Stress

The objective of this study was to identify the effects of exogenous putrescine on photosynthetic performance and heat dissipation capacity in cucumber seedlings under salt stress. The stress of 75 mM NaCl for 7 days caused a significant decrease in net photosynthetic rate (P _N ). The experiment employed a chlorophyll fluorescence imaging technique and demonstrated that the maximal quantum yield of photosystem II photochemistry (Fv/Fm) and the actual photochemical efficiency of photosystem II (ΦPSII) were reduced by salt stress. Moreover, salt stress markedly reduced the photochemical quenching coefficient (qP) and non-photochemical quenching coefficient (qN), and significantly increased non-regulated heat dissipation (ΦNO). However, stressed plants supplied with exogenous putrescine exhibited higher P _N and ΦPSII, which indicated that putrescine can alleviate the detrimental effects on photosynthesis induced by salt stress. Putrescine sprayed on stressed plants significantly enhanced the regulated energy dissipation (ΦNPQ) and decreased ΦNO. Application of exogenous putrescine also changed the levels of xanthophyll cycle components and further enhanced the de-epoxidation state of xanthophyll cycle pigments under salt stress. Under control conditions, putrescine exerted little influence on the photosynthetic parameters in cucumber leaves. In conclusion, the application of exogenous putrescine may improve the heat dissipation capacity by promoting the xanthophyll cycle to reduce the damage caused by excess excitation energy, thus enhancing the salt tolerance of cucumber seedlings.     

Characterization of photosynthetic pigment composition, photosystem II photochemistry and thermal energy dissipation during leaf senescence of wheat plants grown in the field

Photosynthetic pigment composition and photosystem II (PSII) photochemistry were characterized during the flag leaf senescence of wheat plants grown in the field. During leaf senescence, neoxanthin and beta-carotene decreased concomitantly with chlorophyll, whereas lutein and xanthophyll cycle pigments were less affected, leading to increases in lutein/chlorophyll and xanthophyll cycle pigments/chlorophyll ratios. The chlorophyll a/b ratio also increased. With the progression of senescence, the maximal efficiency of PSII photochemistry decreased only slightly in the early morning (low light conditions), but substantially at midday (high light conditions). Actual PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centres decreased significantly both early in the morning and at midday and such decreases were much greater at midday than in the early morning. At the same time, non-photochemical quenching, zeaxanthin and antheraxanthin contents at the expense of violaxanthin increased both early in the morning and at midday, with a greater increase at midday. The results in the present study suggest that a down-regulation of PSII occurred in senescent leaves and that the xanthophyll cycle plays a role in the protection of PSII from photoinhibitory damage in senescent leaves by dissipating excess excitation energy, particularly when exposed to high light.     

Effects of aluminum on light energy utilization and photoprotective systems in citrus leaves

* BACKGROUND AND AIMS: Under high photon flux, excitation energy may be in excess in aluminum (Al)-treated leaves, which use a smaller fraction of the absorbed light in electron transport due to decreased CO2 assimilation compared with normal leaves. The objectives of this study were to test the hypothesis that the antioxidant systems are up-regulated in Al-treated citrus leaves and correlate with protection from photoxidative damage, and to test whether xanthophyll cycle-dependent thermal energy dissipation is involved in dissipating excess excitation energy. * METHODS: 'Cleopatra' tangerine seedlings were fertilized and irrigated daily for 8 weeks with quarter-strength Hoagland's nutrient solution containing Al at a concentration of 0 or 2 mM from Al2(SO4)3.18H2O. Thereafter, leaf absorptance, chlorophyll (Chl) fluorescence, Al, pigments, antioxidant enzymes and metabolites were measured on fully expanded leaves. * KEY RESULTS: Compared with control leaves, energy was in excess in Al-treated leaves, which had smaller thermal energy dissipation, indicated by non-photochemical quenching (NPQ). In contrast, conversion of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z) at midday increased in both treatments, but especially in Al-treated leaves, although A + Z accounted for less 40 % of the total xanthophyll cycle pool in them. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR) and catalase (CAT), and concentrations of ascorbate (AsA), dehydroascorbate (DASA), reduced glutathione (GSH) and oxidized glutathione (GSSG) were higher in Al-treated than in control leaves. * CONCLUSIONS: These results corroborate the hypothesis that, compared with control leaves, antioxidant systems are up-regulated in Al-treated citrus leaves and protect from photoxidative damage, whereas thermal energy dissipation was decreased. Thus, antioxidant systems are more important than thermal energy dissipation in dissipating excess excitation energy in Al-treated citrus leaves.     

Antioxidants and xanthophyll cycle-dependent energy dissipation in Cucurbita pepo L. and Vinca major L. acclimated to four growth PPFDs in the field

The acclimation of photochemistry, xanthophyll cycle-dependent energy dissipation, and antioxidants was characterized in leaves of Cucurbita pepo L. and Vinca major L. that developed under photosynthetic photon flux densities (PPFDs) ranging from deep shade to full sunlight in the field. The predominant acclimatory response of leaf pigment composition was an increase in the xanthophyll cycle pool size with increasing growth PPFD. In both species, the estimated rate of thermal energy dissipation at midday increased with increasing PPFD and midday levels of zeaxanthin and antheraxanthin per chlorophyll were closely correlated with the levels of non-photochemical fluorescence quenching under all growth PPFD regimes. However, at full sunlight there appeared to be considerably higher levels of xanthophyll cycle dependent energy dissipation in V. major compared with pumpkin while estimated rates of photochemistry exhibited the reverse trend. Leaf activities of the antioxidant enzymes ascorbate peroxidase and superoxide dismutase, as well as ascorbate content, increased with increasing growth PPFD in both plant species. Activities/contents were higher under 100% full sunlight and increased more strongly from intermediate growth PPFDs to 100% full sunlight in V. major than in C. pepo. These patterns of acclimation are similar to those exhibited by xanthophyll cycle-dependent energy dissipation. The patterns of acclimation of glutathione reductase are discussed in the context of the multiple roles for reduced glutathione. Catalase acclimated in a manner consistent with its role in scavenging H2O2 generated via photorespiration and/or mitochondrial respiration. Leaf -tocopherol did not exhibit growth PPFD-dependent trends.     

Chlamydomonas Xanthophyll Cycle Mutants Identified by Video Imaging of Chlorophyll Fluorescence Quenching

The photosynthetic apparatus in plants is protected against oxidative damage by processes that dissipate excess absorbed light energy as heat within the light-harvesting complexes. This dissipation of excitation energy is measured as nonphotochemical quenching of chlorophyll fluorescence. Nonphotochemical quenching depends primarily on the [delta]pH that is generated by photosynthetic electron transport, and it is also correlated with the amounts of zeaxanthin and antheraxanthin that are formed from violaxanthin by the operation of the xanthophyll cycle. To perform a genetic dissection of nonphotochemical quenching, we have isolated npq mutants of Chlamydomonas by using a digital video-imaging system. In excessive light, the npq1 mutant is unable to convert violaxanthin to antheraxanthin and zeaxanthin; this reaction is catalyzed by violaxanthin de-epoxidase. The npq2 mutant appears to be defective in zeaxanthin epoxidase activity, because it accumulates zeaxanthin and completely lacks antheraxanthin and violaxanthin under all light conditions. Characterization of these mutants demonstrates that a component of nonphotochemical quenching that develops in vivo in Chlamydomonas depends on the accumulation of zeaxanthin and antheraxanthin via the xanthophyll cycle. However, observation of substantial, rapid, [delta]pH-dependent nonphotochemical quenching in the npq1 mutant demonstrates that the formation of zeaxanthin and antheraxanthin via violaxanthin de-epoxidase activity is not required for all [delta]pH-dependent nonphotochemical quenching in this alga. Furthermore, the xanthophyll cycle is not required for survival of Chlamydomonas in excessive light.     

Thermal energy dissipation and xanthophyll cycles beyond the Arabidopsis model

Thermal dissipation of excitation energy is a fundamental photoprotection mechanism in plants. Thermal energy dissipation is frequently estimated using the quenching of the chlorophyll fluorescence signal, termed non-photochemical quenching. Over the last two decades, great progress has been made in the understanding of the mechanism of thermal energy dissipation through the use of a few model plants, mainly Arabidopsis. Nonetheless, an emerging number of studies suggest that this model represents only one strategy among several different solutions for the environmental adjustment of thermal energy dissipation that have evolved among photosynthetic organisms in the course of evolution. In this review, a detailed analysis of three examples highlights the need to use models other than Arabidopsis: first, overwintering evergreens that develop a sustained form of thermal energy dissipation; second, desiccation tolerant plants that induce rapid thermal energy dissipation; and third, understorey plants in which a complementary lutein epoxide cycle modulates thermal energy dissipation. The three examples have in common a shift from a photosynthetically efficient state to a dissipative conformation, a strategy widely distributed among stress-tolerant evergreen perennials. Likewise, they show a distinct operation of the xanthophyll cycle. Expanding the list of model species beyond Arabidopsis will enhance our knowledge of these mechanisms and increase the synergy of the current studies now dispersed over a wide number of species.     

Mechanistic aspects of xanthophyll cycle-dependent photoprotection in higher plant chloroplasts and leaves

Higher plants must dissipate absorbed light energy that exceeds the photosynthetic capacity to avoid molecular damage to the pigments and proteins that comprise the photosynthetic apparatus. Described in this minireview is a current view of the biochemical, biophysical and bioenergetic aspects of the primary photoprotective mechanism responsible for dissipating excess excitation energy as heat from photosystem II (PSII). The photoprotective heat dissipation is measured as nonphotochemical quenching (NPQ) of the PSII chlorophyll a (Chl a) fluorescence. The NPQ mechanism is controlled by the trans-thylakoid membrane pH gradient (ΔpH) and the special xanthophyll cycle pigments. In the NPQ mechanism, the de-epoxidized endgroup moieties and the trans-thylakoid membrane orientations of antheraxanthin (A) and zeaxanthin (Z) strongly affect their interactions with protonated chlorophyll binding proteins (CPs) of the PSII inner antenna. The CP protonation sites and steps are influenced by proton domains sequestered within the proteo-lipid core of the thylakoid membrane. Xanthophyll cycle enrichment around the CPs may explain why changes in the peripheral PSII antenna size do not necessarily affect either the concentration of the xanthophyll cycle pigments on a per PSII unit basis or the NPQ mechanism. Recent time-resolved PSII Chi a fluorescence studies suggest the NPQ mechanism switches PSII units to an increased rate constant of heat dissipation in a series of steps that include xanthophyll de-epoxidation, CP-protonation and binding of the xanthophylls to the protonated CPs; the concerted process can be described with a simple two-step, pH-activation model. The xanthophyll cycle-dependent NPQ mechanism is profoundly influenced by temperatures suboptimal for photosynthesis via their effects on the trans-thylakoid membrane energy coupling system. Further, low temperature effects can be grouped into either short term (minutes to hours) or long term (days to seasonal) series of changes in the content and composition of the PSII pigment-proteins. This minireview concludes by briefly highlighting primary areas of future research interest regarding the NPQ mechanism.     

田间大豆叶片成长过程中的光合特性及光破坏防御机制

田间大豆叶片在成长进程中光饱和光合速率持续提高,但气孔导度的增加明显滞后.尽管叶片在成长初期就具有较高的最大光化学效率,但是仍略低于发育成熟的叶片.随着叶片的成长,光下叶片光系统Ⅱ实际效率增加;非光化学猝灭下降.幼叶叶黄素总量与叶绿素之比较高,随着叶面积的增加该比值下降,在光下,幼叶的脱环氧化程度较高.因此认为大豆叶片成长初期就能够有效地进行光化学调节;在叶片生长过程中依赖叶黄素循环的热耗散机制迅速建立起来有效抵御强光的破坏.     

Examination of chlorophyll fluorescence in sulfur-deprived cells of Chlamydomonas reinhardtii

Modulated fluorometry (PAM) was applied for probing the photosynthesis in cells of C. reinhardtii during sulfur deprivation. A significant (up to a fourfold) increase in chlorophyll fluorescence yield (parameters F(o) and F(m)) normalized to chlorophyll concentration was shown for deprived cells. An analysis of nonphotochemical quenching of chlorophyll fluorescence indicated a considerable modification of the energy deactivation pathways in PS II of sulfur-deprived cells. Thus, starved cells exhibited a lower deltapH-dependent quenching of excited states and a higher thermal dissipation of excess light energy in reaction centers of PS II, as well as the transition of the photosynthetic apparatus primarily to state 2. However, these changes cannot cause the elevation of chlorophyll fluorescence in the cells under sulfur limitation. The phenomenon observed may be due to a partial dissociation of light-harvesting complexes from reaction centers of PS II and/or dysfunction of the dissipative cycle in PS II with cytochrome b559 as an intermediate.     

叶黄素循环及其在光保护中的分子机理研究

植物的生命活动离不开充足的光照 ,但是当光照过强时 ,叶片吸收的光能超过了光合电子传递所需 ,过剩的光能便会对光合器官产生潜在的危害 ,引起光合作用的光抑制或光破坏。依赖于叶黄素循环的热耗散被认为是光保护的主要途径。本文着重介绍近年来有关植物叶黄素循环在酶学方面的分子调控、它的主要功能以及依赖于叶黄素循环的热耗散在光保护中的分子机理等 ,并对需进一步研究的问题作了探讨     

Resistance of spinach plants to seawater stress is correlated with higher activity of xanthophyll cycle and better maintenance of chlorophyll metabolism

The relationship between the activity of xanthophyll cycle and chlorophyll (Chl) metabolism was investigated using two cultivars, Helan No. 3 (seawater-tolerant cultivar) and Yuanye (seawater-sensitive cultivar), of spinach (Spinacia oleracea L.) plants cultured in Hoagland’s nutrient solution, with or without seawater (40%). The results showed that, in plants of two cultivars with seawater, the xanthophyll cycle seems to show a principal protection mechanism against photoinhibition under seawater stress. Furthermore, accumulation of reactive oxygen species (ROS) in chloroplasts of two cultivars was enhanced by seawater to lower the activity of porphobilinogen deaminase. Namely, the conversion of porphobilinogen into uroporphyrinogen III involved in Chl biosynthetic processes was inhibited by seawater. In Helan No. 3 spinach plants with seawater, higher activity of xanthophyll cycle in the leaves dissipated more excess light energy, which appeared to lower the levels of ROS in chloroplasts. As a consequence, the Chl biosynthesis in Helan No. 3 leaves with seawater showed only a weak inhibition and the activity of chlorophyllase (Chlase) was not affected by seawater stress. In contrast, a more pronounced accumulation of ROS in chloroplasts of Yuanye leaves, which possess lower xanthophyll cycle activity, severely inhibited Chl biosynthesis and remarkably enhanced the activity of Chlase, which aggravates the decomposition of Chl. These results suggest that higher activity of xanthophyll cycle in seawater-tolerant spinach plays a role in maintaining Chl metabolic processes, probably by decreasing the levels of ROS, when the plants are cultured in the nutrient solution with seawater (40%).