Functional characterization of the corticular photosynthetic apparatus in grapevine

A comparative analysis of functional characteristics of the grapevine leaf photosynthetic apparatus (LPA) and corticular photosynthetic apparatus (CPA) in chlorenchyma tissues of first-year lignified vine was performed. Obtained results demonstrate significant differences between the functional properties of the CPA and the LPA. CPA contains an increased proportion (about 2/3) of Q_B-non-reducing centers of photosystem II (PSII) that is confirmed by elevated O-J phase in fluorescence kinetics, high PSIIβ content, and slower Q_A^—• reoxidation. CPA and LPA use different strategies to utilize absorbed light energy and to protect itself against excessive light. CPA dissipates a significant proportion of absorbed light energy as heat (regulated and non-regulated dissipation), and only a smaller part of the excitation energy is used in the dark stages of photosynthesis. The rate constant of photoinhibition and fluorescence quenching due to photoinhibition in CPA is almost three times higher than in LPA, while high-energy state fluorescence quenching value is twice lower. The saturation of vine chlorenchyma tissue with water increases the CPA tolerance to photoinhibition and promotes the ability to restore the photosynthetic activity after photoinhibition. The electron microscopy analysis confirmed the presence of intact plastids in vine chlorenchyma tissue, the interior space of plastids is filled with large starch grains while bands of stacked thylakoid membranes are mainly localized on the periphery. Analyzes showed that corticular plastids are specialized organelles combining features of chloroplasts, amyloplasts and gerontoplasts. Distinct structural organization of photosynthetic membranes and microenvironment predetermine distinctive functional properties of CPA.     

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.     

Probing corticular photosynthesis through in vivo chlorophyll fluorescence measurements: evidence that high internal CO levels suppress electron flow and increase the risk of photoinhibition

Twigs of many woody plants possess chlorenchyma under a well-developed periderm which lacks stomata and impedes both gas diffusion and light penetration. The so-called corticular photosynthesis, occurring in the shade and under extremely high CO(2) concentrations, was probed in this study through in vivo chlorophyll fluorescence measurements. Field comparisons between twigs and corresponding leaves in five species indicated that both the dark- and light-adapted PSII photochemical efficiencies are considerably lower in twigs at all incident photon fluence rates, in spite of the significant attenuation of solar radiation by the periderm. Light saturation curves for linear electron transport rates (corrected according to the actual light intensities reaching twig chlorenchyma) were compatible with a shade-acclimated photosynthetic machinery, showing very low maximum electron transport rates (at approximately 5% of the corresponding leaf values) and threshold irradiances for light saturation. However, removing periderms from twig segments (i.e. relieving the twig interior form the high CO(2) partial pressures) considerably improved the light-adapted (but not the dark-adapted) PSII photochemical efficiency, allowing the assumption that the high internal CO(2) levels may interfere with the smooth functioning of photosynthesis. Indeed, laboratory experiments with twig segments equilibrated under various CO(2) levels (0.036-20%), resulted in a progressive decrease of light-adapted PSII photochemical yield, with the values obtained at 20% CO(2) being similar to those obtained with intact twigs in the field. Further experiments indicated that high CO(2) combined with high light suppressed the development of a photoprotective non-photochemical quenching through a reduction of its fast relaxing component, accompanied by a higher risk of photoinhibition. It is suggested that high internal CO(2) concentrations in twigs impede photosynthesis possibly through acidification of protoplasm and impairment of the pH-dependent high energy state quenching followed by reduction in the efficiency of heat dissipation.     

Maximum and effective PSII yields in the cortex of the main stem of young <Emphasis Type="Italic">Prunus cerasus</Emphasis> trees: effects of seasons and exposure

Seasonal changes in chlorophyll fluorescence parameters of corticular chlorenchyma in the main trunk of Prunus cerasus were followed in the field under ambient temperature and light conditions during bright days. Concomitantly, measurements of periderm light transmittance also allowed the calculation of linear electron transport rates along PSII. Pre-dawn PSII photochemical efficiency was high during late spring, summer and early autumn, but low during winter in the North-facing, permanently shaded, side and extremely low in the South-facing, exposed side. Corresponding mid-day PSII effective yield and linear electron transport rates peaked at late spring and early summer with the exposed side always displaying lower values for effective yield, but higher values for electron transport rate. However, corticular electron transport rates were more than sixfold lower compared to leaves. Non-photochemical quenching was higher in the exposed side throughout the year while peak values appeared at early autumn. Although a photoinhibitory damage during winter can be claimed, we may note that Mediterranean winter temperatures are mild, while the light reaching the trunk photosynthetic tissues is very low (maximum at 30 and 280 μmol m⁻² s⁻¹ in the shaded and the exposed side, respectively) to be considered as photoinhibitory. Based on recent findings for the retention of PSI activity and a concomitant inhibition of PSII under low temperatures in leaves, together with an adequate cyclic electron flow found in bark chlorenchyma, we suggest a temperature-dependent adaptive adjustment in the relative rates of PSI over PSII activity, possibly linked to seasonally changing needs for metabolic energy supply.     

Characterization of the photosynthetic apparatus in cortical bark chlorenchyma of Scots pine

Winter-induced inhibition of photosynthesis in Scots pine (Pinus sylvestris L.) needles is accompanied by a 65% reduction of the maximum photochemical efficiency of photosystem II (PSII), measured as F _v/F _m, but relatively stable photosystem I (PSI) activity. In contrast, the photochemical efficiency of PSII in bark chlorenchyma of Scots pine twigs was shown to be well preserved, while PSI capacity was severely decreased. Low-temperature (77 K) chlorophyll fluorescence measurements also revealed lower relative fluorescence intensity emitted from PSI in bark chlorenchyma compared to needles regardless of the growing season. Nondenaturating SDS-PAGE analysis of the chlorophyll–protein complexes also revealed much lower abundance of LHCI and the CPI band related to light harvesting and the core complex of PSI, respectively, in bark chlorenchyma. These changes were associated with a 38% reduction in the total amount of chlorophyll in the bark chlorenchyma relative to winter needles, but the Chl a/b ratio and carotenoid composition were similar in the two tissues. As distinct from winter pine needles exhibiting ATP/ADP ratio of 11.3, the total adenylate content in winter bark chlorenchyma was 2.5-fold higher and the estimated ATP/ADP ratio was 20.7. The photochemical efficiency of PSII in needles attached to the twig recovered significantly faster (28–30 h) then in detached needles. Fluorescence quenching analysis revealed a high reduction state of Q _A and the PQ-pool in the green bark tissue. The role of bark chlorenchyma and its photochemical performance during the recovery of photosynthesis from winter stress in Scots pine is discussed.     

Antitranspirant functions of stem periderms and their influence on corticular photosynthesis under drought stress

Transpiration and photosynthesis of current-year stems and adult leaves of different deciduous tree species were investigated to estimate their probable influence on carbon balance. Peridermal transpiration of young stems was found to be rather small as compared to the transpiration of leaves (stem/leaf like 1/5–1/20). A characteristic that was mainly attributable to the lower peridermal conductance to water and CO₂, which made up only 8–28% of stomatal conductance. Water vapour conductance was significantly lower in stems, but also non-responsive to PAR, which led to a comparatively higher water use efficiency (WUE, ratio assimilation/transpiration). Thus, although corticular photosynthesis reached only 11–37% of leaf photosynthesis, it may be a means of improving the carbon balance of stems under limited water availability. The influence of drought stress on primary photosynthetic reactions was also studied. Under simulated drought conditions the drying time needed to provoke a 50% reduction (t ₅₀) in dark- and light-adapted PSII efficiency (Fv/Fm, ΔF/Fm′) was up to ten times higher in stems than in leaves. Nevertheless, up to a relative water deficit (RWD) of around 40–50% dark-adapted PSII efficiency of leaves and stems was rather insensitive to dehydration, showing that the efficiency of open PS II reaction centres is not impaired. Thus, it may be concluded that in stems as well as in leaves the primary site of drought damage is at the level of dark enzyme reactions and not within PSII. However, enduring severe drought caused photoinhibitory damage to the photosynthetic apparatus of leaves and stems; thereby RWD₅₀ values (= RWD needed to provoke a 50% reduction in Fv/Fm ad ΔF/Fm′) were comparably lower in stems as compared to leaves, indicating a possibly higher drought sensitivity of the cortex chlorenchyma.     

Effect of Grapevine Leafroll on the Photosynthesis of Field Grown Grapevine Plants (Vitis vinifera L. cv. Lagrein)

We have studied the effect of grapevine leafroll infection on some features of the thylakoids from field grown grapevine (Vitis vinifera L.) leaves. Changes in photosynthetic pigments, soluble proteins, ribulose‐1,5‐bisphosphate carboxylase (RuBP), nitrate reductase, photosynthetic activities and thylakoid membrane proteins were investigated. The level of total chlorophyll (Chl) and carotenoids were reduced in virus‐infected leaves. Similar results were also observed for soluble proteins and RuBP case activity. The in vivo nitrate reductase activity was significantly reduced in infected leaves. Virus infection considerably decreased leaf net photosynthetic rate (P_n), stomatal conductance (g_s) and transpiration rate (E) in grapevine leaves. When various photosynthetic activities were followed in isolated thylakoids, virus infection caused marked inhibition of whole chain and photosystem (PS) II activity while the inhibition of PSI activity was only marginal. The artificial exogenous electron donors, diphenyl carbazide and hydroxylamine (NH₂OH) significantly restored the loss of PSII activity in infected leaves. The same results were obtained when F_v/F_m was evaluated by Chl fluorescence measurements. The marked loss of PSII activity in infected leaves could be due to the loss of 47, 43, 33, 28–25, 23 and 17 kDa polypeptides. It is concluded that virus infection inactivates the donor side of PSII. This conclusion was confirmed by immunological studies showing that the content of the 33 kDa protein of the water‐splitting complex was diminished significantly in infected leaves.     

Effect of Reducing Nitric Oxide in <Emphasis Type="Italic">Rumex</Emphasis> K-1 Leaves on the Photoprotection of Photosystem II Under High Temperature with Strong Light

The purpose of this study was to explore the effect of reducing nitric oxide (NO) in Rumex K-1 leaves on the photoprotection of photosystem II (PSII) under high temperature with strong light. Reducing the content of NO in Rumex K-1 leaves significantly aggravated the PSII photoinhibition and net degradation of D1 protein under high temperature with strong light, but not under high temperature in the darkness. The reduction of NO remarkably inhibited the electron transport of PSII in the leaves under high temperature and strong light, which resulted in an increase in excitation pressure and an over-accumulation of reactive oxygen species (ROS). The over-accumulation of ROS further damaged PSII. However, when the synthesis of D1 protein was inhibited, the D1 protein content and PSII activity were no longer influenced by reducing NO content in the leaves. The reduction of NO in leaves decreased the activities of ROS scavenger enzymes after treatment with high temperature and strong light for 2 h, which enhanced the over-accumulation of ROS to damage photosynthetic apparatus severely. All of these results suggest that NO was involved in the synthesis of D1 protein. Maintaining physiologically appropriate NO content in leaves will alleviate net degradation of D1 protein under high temperature with strong light to keep photosynthetic electrons flowing smoothly, which mitigates the accumulation of ROS in photosystems to avoid damage to the photosynthetic apparatus. Therefore, NO plays an important role in maintaining higher PSII photosynthetic performance under high temperature with strong light.     

Loss of the precise control of photosynthesis and increased yield of non‐radiative dissipation of excitation energy after mild heat treatment of barley leaves

The after effects of a short exposure of intact barley leaves to moderately elevated temperature (40°C, 5 min) on the induction transients and the irradiance dependencies of photosynthesis and chlorophyll fluorescence are presented. This mild heat treatment strongly reduced the oscillations in the rate of photosynthesis and in the yield of chlorophyll fluorescence. However, only a 25% irreversible inhibition of maximum photosynthetic capacity of photosystem II (PSII) measured by oxygen evolution was produced and the intrinsic quantum yield of PSII measured by the chlorophyll fluorescence ratio (F_m‐ F_o)/F_m decreased by only 15%. In contrast, the above treatment increased radiationless dissipation processes in PSII by a factor of two. In heat‐treated leaves, photosynthesis was not saturated even by strong light. Both ΔpH‐dependent quenching of excitons in PSII (including formation of zeaxanthin) and state 1/state 2 transition were found to be stimulated. Heat exposure enhanced the control of PSII activity by PSI, as evidenced by a significant increase in the quenching effect of far‐red light on the maximum yield of chlorophyll fluorescence. It was deduced that after mild heat treatment, the photosynthetic apparatus in leaves lacks the precise coordinating control of electron transport and carbon metabolism owing to the inability of PSII to support electron transport at a level adequate for carbon metabolism. This effect was not related to the small irreversible thermal damage to PSII, but was rather due to a significant increase in non‐photochemical quenching of excitation energy.     

Photosynthetic acclimation of Tradescantia albiflora to growth irradiance: Lack of adjustment of light-harvesting components and its consequences

The photosynthetic acclimation of Tradescantia albiflora (Kunth), a trailing ground species naturally occurring in the deep shade of rainforests, was studied in relation to growth irradiance (glasshouse; direct light and 1 to 4 layers of shade cloth, giving 100 to 1.4% relative growth irradiance). Contrary to other irradiance studies of higher plants grown in natural habitats or controlled light environments, the chlorophyll a/b ratios of Tradescantia leaves were low (∼2.2) and constant. Acclimation to growth irradiance caused no changes in the relative amounts of specific Chl-proteins or the numbers of photosystem I (PSI) and PSII reaction centres on a chlorophyll basis, indicating that the light-harvesting antenna sizes of PSII and PSI, as well as the photosystem stoichiometry, were independent of growth irradiance. However, the amount of cytochrome f and ATP synthase on a chlorophyll basis increased with increasing the relative growth irradiance from 1.4 to 35%, showing acclimation of electron transport and photophosphorylation capacity. The photosynthetic capacity and ribulose 1, 5-bisphosphate carboxylase (EC 4.1.1.39) activity also increased with increase of the growth irradiance to 35%. Beyond that, the inflexible PSII/PSI stoichiometry and shade-type photosystem II/light-harvesting units in Tradescaniia are a disadvantage for long-term exposure to high irradiance since the leaves are more prone to photoinhibition.     

Effects of prolonged freezing stress on the photosynthetic apparatus of moderately hardy leaves as assayed by chlorophyll fluorescence kinetics

Abstract Moderately frost-hardy leaves of the wintergreen broadleaf woody shrubs Pyracantha coccinea and Ligustrum ovalifolium and the winter annual herb Spinacia oleracea were subjected to extended freezing stress up to 15 d at temperatures 2–8°C above the mean lethal temperature (LT₅₀). After thawing, the fast kinetics of in vivo chlorophyll fluorescence of photosystem II (PSII) and the potential of linear photosynthetic electron transport of isolated thylakoid membranes was measured at room temperature. The lower the minimum freezing temperature and the longer the time of exposure, the greater was the suppression of the fluorescence signals of the leaves and decrease of the electron transport capacity of the thylakoid membranes. The pattern of inactivation of PSII -mediated electron flow, i.e. inhibition of photoreaction to photochemistry and/or electron donation to the photochemical reaction, during long-term freezing at temperatures somewhat above the LT₅₀ of the leaves was similar to that observed earlier after relatively brief exposure of leaves and isolated thylakoid membranes to more severe freezing stress. As injury occurred during freezing in complete darkness, it is likely that prolonged winter stress under natural environmental conditions causes changes in the photosynthetic apparatus of moderately hardy leaves which are not due to photoinhibition.     

Response of the photosynthetic apparatus of cotton (Gossypium hirsutum) to the onset of drought stress under field conditions studied by gas-exchange analysis and chlorophyll fluorescence imaging.

The functioning of the photosynthetic apparatus of cotton (Gossypium hirsutum) grown during the onset of water limitation was studied by gas-exchange and chlorophyll fluorescence to better understand the adaptation mechanisms of the photosynthetic apparatus to drought conditions. For this, cotton was grown in the field in Central Asia under well-irrigated and moderately drought-stressed conditions. The light and CO(2) responses of photosynthesis (A(G)), stomatal conductance (g(s)) and various chlorophyll fluorescence parameters were determined simultaneously. Furthermore, chlorophyll fluorescence images were taken from leaves to study the spatial pattern of photosystem II (PSII) efficiency and non-photochemical quenching parameters. Under low and moderate light intensity, the onset of drought stress caused an increase in the operating quantum efficiency of PSII photochemistry (varphi(PSII)) which indicated increased photorespiration since photosynthesis was hardly affected by water limitation. The increase in varphi(PSII) was caused by an increase of the efficiency of open PSII reaction centers (F(v)'/F(m)') and by a decrease of the basal non-photochemical quenching (varphi(NO)). Using a chlorophyll fluorescence imaging system a low spatial heterogeneity of varphi(PSII) was revealed under both irrigation treatments. The increased rate of photorespiration in plants during the onset of drought stress can be seen as an acclimation process to avoid an over-excitation of PSII under more severe drought conditions.     

Carotenoid composition of peridermal twigs does not fully conform to a shade acclimation hypothesis

The photosynthetic pigments of twigs in five tree and shrub species possessing chlorenchyma under a well developed, stomata-less, and highly photon absorptive periderm were analysed and compared to those of the corresponding canopy leaves. We asked whether the unavoidable shade acclimation of corticular chlorenchyma results in photosynthetic pigment complements typically found in shade leaves. As expected, chlorophyll (Chl) a/b ratios in twigs were consistently low. However, carotenoid (Car) analysis did not confirm the initial hypothesis, since twigs generally contained increased Chl-based pool sizes of the xanthophyll cycle components. The contents of photo-selective neoxanthin and lutein were high as well. Yet, β-carotene content was extraordinarily low. In addition, twigs retained high pre-dawn ratios of the deepoxidized antheraxanthin and zeaxanthin, although environmental conditions were not pre-disposing for such a state. The unexpected Car composition allows the conclusion that other micro-environmental conditions within twigs (hypoxia, increased red to blue photon ratios, and extremely high CO₂ concentrations) are more important than shade in shaping the Car profiles.     

Tissue specific protochlorophyll(ide) forms in dark-forced shoots of grapevine (<Emphasis Type="Italic">Vitis vinifera</Emphasis>L.)

Cuttings of grapevine (Vitis vinifera L. cv. Chardonnay) were dark-forced at least three weeks. Pigment contents, 77 K fluorescence emission, excitation spectra of the leaves, petioles, stems, transmission electron micrographs of the etioplasts from leaves, the chlorenchyma tissues of the stems were analysed. The dark-grown leaves, stems contained 8 to 10, 3 to 5 μg/g fresh weight protochlorophyllide, its esters, respectively. HPLC analysis showed that the molar ratio of the unesterified, esterified pigments was 7:3 in the shoot developed in darkness. The dark-forced leaves contained carotenoids identified as: neoxanthin, violaxanthin, antheraxanthin, lutein, β-carotene. Detailed analyses of the fluorescence spectra proved that all tissues of the dark-forced shoots had protochlorophyllide or protochlorophyll forms with emission maxima at 628, 636, 644, 655, 669 nm. The 628, 636 nm emitting forms were present in all parts of the dark-forced shoot, but dominated in the stems, which may indicate an organ specificity of the etioplast development. Variations in the distribution of the pigment forms were even found in the different tissues of the stem. The subepidermal layers were more abundant in the 655 nm form than the parenchyma cells of the inner part of the cortex, the pith. In the latter cells, the plastid differentiation stopped in intermediary stages between proplastids, etioplasts. The plastids in the subepidermal layers had developed prolamellar body structures, which were similar to those of etiolated leaves. The results highlight the importance of organ-, tissue specificity of plastid differentiation for chlorophyll biosynthesis, greening of different plant organs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Structural and functional aspects of the Nuphar lutea (L.) Smith heterophylly: Ultrastructure and photosynthesis

The ultrastructure and functional characteristics of the photosynthetic apparatus of floating and submersed leaves of the heterophyllous plant Nuphar lutea (L.) Smith have been examined. Differences have been revealed in mesophyll cell chloroplasts, content of pigments, and chlorophyll fluorescence parameters between floating and submersed leaves and submersed leaves at different depths. A sharp decline in the PSII (photosystem II) efficiency of submersed leaves when exposed to an actinic light intensity of more than 60 ??mol m^?2 s^?1 has been noted. The described differences may be considered as an adaptation mechanism of submersed leaves to life in an aquatic environment with a reduced light intensity and changed light spectral composition.     

Carbon isotopic composition of legumes with photosynthetic stems from mediterranean and desert habitats

The carbon isotopic compositions of leaves and stems of woody legumes growing in coastal mediterranean and inland desert sites in California were compared. The overall goal was to determine what factors were most associated with the carbon isotope composition of photosynthetic stems in these habitats. The carbon isotope signature (d13C) of photosynthetic stems was less negative than that of leaves on the same plants by an average of 1.51 ± 0.42 ;pp. The d13C of bark (cortical chlorenchyma and epidermis) was more negative than that of wood (vascular tissue and pith) from the same plant for all species studied on all dates. Desert woody legumes had a higher d13C (less negative) and a lower intercellular CO2 concentration (Ci ) (for both photosynthetic tissues) than that of woody legumes from mediterranean climate sites. Differences in the d13C of stems among sites could be entirely accounted for by differences among site air temperatures. Thus, the d13C composition of stems did not indicate a difference in whole-plant integrated water use efficiency (WUE) among sites. In contrast, stems on all plants had a lower stem Ci and a higher d13C than leaves on the same plant, indicating that photosynthetic stems improve long-term, whole-plant water use efficiency in a diversity of species.     

Acclimation of Arabidopsis thaliana to the light environment: regulation of chloroplast composition

The regulation by light of the composition of the photosynthetic apparatus was investigated in Arabidopsis thaliana (L.) Heynh. cv. Landsberg erecta. When grown in high- and low-irradiance white light, wild-type plants and photomorphogenic mutants showed large differences in their maximum photosynthetic rate and chlorophyll a/b ratios; such changes were abolished by growth in red light. Photosystem I (PSI) and PSII levels were measured in wild-type plants grown under a range of light environments; the results indicate that regulation of photosystem stoichiometry involves the specific detection of blue light. Supplementing red growth lights with low levels of blue light led to large increases in PSII content, while further increases in blue irradiance had the opposite effect; this latter response was abolished by the hy4 mutation, which affects certain events controlled by a blue-light receptor. Mutants defective in the phytochrome photoreceptors retained regulation of photosystem stoichiometry. We discuss the results in terms of two separate responses controlled by blue-light receptors: a blue-high-fluence response which controls photosystem stoichiometry; and a blue-low-fluence response necessary for activation of such control. Variation in the irradiance of the red growth light revealed that the blue-high-fluence response is attenuated by red light; this may be evidence that photosystem stoichiometry is controlled not only by photoreceptors, but also by photosynthetic metabolism.Abbreviations BHF blue-high-fluence - BLF blue-low-fluence - Chl chlorophyll - FR far-red light - LHCII light-harvesting complex of PSII - P_max maximum photosynthetic rate - R red light - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase This work was supported by Natural Environment Research Council Grant No. GR3/7571A. We would like to thank H. Smith (Botany Department, University of Leicester) and E. Murchie (INRA, Versailles) for helpful discussions.     

定向种植对夏玉米群体内光环境及叶片光合性能的影响

为改善玉米群体内光环境,进一步提高玉米单株光合能力以获得高产,本研究以郑单958为试验材料,通过设置种子定向入土方式,研究了定向有序种植条件下群体内光分布特征,以及单株玉米穗位叶花后光合性能,并借助快速叶绿素荧光动力学曲线分析了与叶片光合性能有密切联系的光系统Ⅱ(PSⅡ)的性能特征.结果表明:叶片不同朝向显著改变夏玉米群体内穗位叶处光合有效辐射截获量,朝南处理(S)平均比朝北处理(N)高271.8%.不同朝向的叶片对高光与弱光的利用能力差异显著,朝南处理饱和光强下净光合速率(Pn)显著升高,表明其高光强利用能力显著提升;而朝北处理(N)表观量子效率(α)则随生育期推进显著增加,有利于叶片适应长期弱光环境.生育前期朝南处理PSⅡ电子供体侧和受体侧性能显著提高,进而改善了PSⅡ反应中心性能(PIABS)和荧光光化学淬灭系数(ψo),电子在电子传递链中转移效率(φEo)的提高增强了电子由PSⅡ向光系统Ⅰ(PSⅠ)的传递性能.生育前期叶片性能呈现出朝南>朝东>朝西>朝北的趋势.但成熟末期朝南处理对强光的利用效率显著降低,朝北处理在生育后期表现出较强的弱光利用能力,表观量子效率显著升高,花后40dPn与PSⅡ性能均表现为朝北>朝西>朝东>朝南的趋势.总体上,朝南与朝东处理群体内光环境改善显著,群体穗位层截获光合有效辐射较多,光合能力和干物质生产能力增强,有利于夏玉米产量提高.     

Cyanobacterial Acclimation to Photosystem I or Photosystem II Light

The organization and function of the photochemical apparatus of Synechococcus 6301 was investigated in cells grown under yellow and red light regimes. Broadband yellow illumination is absorbed preferentially by the phycobilisome (PBS) whereas red light is absorbed primarily by the chlorophyll (Chl) pigment beds. Since PBSs are associated exclusively with photosystem II (PSII) and most of the Chl with photosystem I (PSI), it follows that yellow and red light regimes will create an imbalance of light absorption by the two photosystems. The cause and effect relationship between light quality and photosystem stoichiometry in Synechococcus was investigated. Cells grown under red light compensated for the excitation imbalance by synthesis/assembly of more PBS-PSII complexes resulting in high PSII/PSI = 0.71 and high bilin/Chl = 1.30. The adjustment of the photosystem stoichiometry in red light-grown cells was necessary and sufficient to establish an overall balanced absorption of red light by PSII and PSI. Cells grown under yellow light compensated for this excitation imbalance by assembly of more PSI complexes, resulting in low PSII/PSI = 0.27 and low bilin/Chl = 0.42. This adjustment of the photosystem stoichiometry in yellow light-grown cells was necessary but not quite sufficient to balance the absorption of yellow light by the PBS and the Chl pigment beds. A novel excitation quenching process was identified in yellow light-grown cells which dissipated approximately 40% of the PBS excitation, thus preventing over-excitation of PSII under yellow light conditions. It is hypothesized that State transitions in O₂ evolving photosynthetic organisms may serve as the signal for change in the stoichiometry of photochemical complexes in response to light quality conditions.     

田间条件下棉花幼叶光合特性及光保护机制

通过比较棉花(Gossypium hirsutum)幼叶和完全展开叶气体交换参数及叶绿素荧光特性的差异, 探讨高光强下幼叶的光抑制程度及明确光保护机制间的协调机理。在田间自然条件下, 以棉花刚展平的幼嫩叶片(幼叶)和面积已达到最大的完全展开叶片为研究对象, 通过测定不同发育阶段叶片气体交换参数及叶绿素a荧光参数的变化, 并运用Dual-PAM100对不同发育阶段的叶片进行快速光响应曲线的拟合。结果表明: 幼叶和完全展开叶片在光合、荧光特性方面表现出明显的差异。与完全展开叶相比, 较低的叶绿素(Chl)含量和气孔导度(G_s)是幼叶较低净光合速率(P_n)的限制因素, 从而直接导致其光系统II (PSII)实际光化学效率(Φ_PSII)和光化学猝灭系数(q_P)的降低。在1800 μmol·m^-2·s^-1光强以下, 完全展开叶具有较强的围绕PSI循环的电子流(CEF), 有利于合成ATP, 是其具有较高光合能力的原因之一。相同光强下, 幼叶较低的光饱和点(LSP)更易受光抑制, 但其PSII原初光化学效率(F_v/F_m)的日变化幅度显著小于完全展开叶, 说明强光下幼叶通过类胡萝卜素(Car)猝灭单线态氧、光呼吸(P_r)、热耗散(NPQ)以及PSI-CEF等光保护机制能有效地耗散过剩的光能, 从而避免其光合机构发生光抑制。