Structural and functional characteristics of photosynthetic apparatus of chlorophyll-containing grape vine tissue

Photosynthesis in tissues under periderm of woody stems and shoots of perennial plants occurs in environment that is very different from the internal environment of leaf chloroplasts. These tissues are characterized by high CO₂ and low O₂ concentrations, more acidic surroundings, besides that only light which have passed through periderm reaches photosynthetic antennas. In contrast to leaves of deciduous plants chlorenchyma tissues of wintering plant organs are exposed to temperature fluctuations during all seasons, that is why the photosynthetic apparatus of woody stems has to be able to adapt to a wide range of environmental temperatures. In order to reveal unique features, which enable photosynthetic apparatus of chlorenchyma cells in woody plant organs to implement biological functions under different light and temperature conditions, we studied photosynthetic tissues of stem cortex in grapevine (Vitis vinifera L.) under normal conditions and after exposure to suboptimal temperatures and high light intensity. Comparative analysis of photosynthetic pigment composition and low-temperature chlorophyll fluorescence emission spectrum of leaves, young shoots and chlorenchyma of lignified shoots revealed relatively high level of chlorophyll b and carotenoids, and high photosystem II (PSII) to photosystem I (PSI) ratio in woody shoots. Analysis of parameters of variable chlorophyll fluorescence revealed high PSII activity in grapevine shoot cortex and demonstrated improved freeze tolerance and higher sensitivity to light of photosynthetic apparatus in grape vine in comparison to leaves. It was shown for the first time that photosynthetic apparatus in chlorenchyma cells of vine undergoes so-called “state-transition”–fast rearrangements leading to redistribution of energy between photosystems. Analysis of fatty acid (FA) compositions of lipids in examined tissues showed that the FA unsaturation index in green tissue of vine is lower than in leaves. A distinct feature of FA compositions of lipids in vine cortex was relatively high level of linoleic acid.     

Reaction centre quenching of excess light energy and photoprotection of photosystem II

In addition to the energy dissipation of excess light occurring in PSII antenna via the xanthophyll cycle, there is mounting evidence of a zeaxanthin-independent pathway for non-photochemical quenching based within the PSII reaction centre (reaction centre quenching) that may also play a significant role in photoprotection. It has been demonstrated that acclimation of higher plants, green algae and cyanobacteria to low temperature or high light conditions which potentially induce an imbalance between energy supply and energy utilization is accompanied by the development of higher reduction state of Q_A and higher resistance to photoinhibition (Huner et al., 1998). Although this is a fundamental feature of all photoautotrophs, and the acquisition of increased tolerance to photoinhibition has been ascribed to growth and development under high PSII excitation pressure, the precise mechanism controlling the redox state of Q_A and its physiological significance in developing higher resistance to photoinhibition has not been fully elucidated. In this review we summarize recent data indicating that the increased resistance to high light in a broad spectrum of photosynthetic organisms acclimated to high excitation pressure conditions is associated with an increase probability for alternative non-radiative P680⁺Q_A ^- radical pair recombination pathway for energy dissipation within the reaction centre of PSII. The various molecular mechanisms that could account for non-photochemical quenching through PSII reaction centre are also discussed.     

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.     

Cold-hardening-induced resistance to photoinhibition of photosynthesis in winter rye is dependent upon an increased capacity for photosynthesis

Analyses of chlorophyll fluorescence and photosynthetic oxygen evolution were conducted to understand why cold-hardened winter rye (Secale cereale L.) is more resistant to photoinhibition of photosynthesis than is non-hardened winter rye. Under similar light and temperature conditions, leaves of cold-hardened rye were able to keep a larger fraction of the PS II reaction centres in an open configuration, i.e. a higher ratio of oxidized to reduced Q_A (the primary, stable quinone acceptor of PSII), than leaves of non-hardened rye. Three fold-higher photon fluence rates were required for cold-hardened leaves than for non-hardened leaves in order to establish the same proportion of oxidized to reduced Q_A. This ability of cold-hardened rye fully accounted for its higher resistance to photoinhibition; under similar redox states of q_a cold-hardened and non-hardened leaves of winter rye exhibited similar sensitivities to photoinhibition. Under given light and temperature conditions, it was the higher capacity for light-saturated photosynthesis in cold-hardened than in non-hardened leaves, which was responsible for maintaining a higher proportion of oxidized to reduced Q_A. This higher capacity for photosynthesis of cold-hardened leaves also explained the increased resistance of photosynthesis to photoinhibition upon cold-hardening.Abbreviations F_m and F'_m fluorescence when all PSII reaction centres are closed in dark- and light-acclimated leaves, respectively - F_o and F'_o fluorescence when all PSII reaction centres are open in darkness and steady-state light, respectively - F_v variable fluorescence (F'_m-F'_o) under steady-state light conditions - F_v/F_m the ratio of variable to maximum fluorescence as an expression of the maximum photochemical yield of PSII in dark-acclimated leaves - Q_A the primary, stable, quinone electron acceptor of PSII - q_N non-photochemical quenching of fluorescence due to high energy state (pH) - q_p photochemical quenching of fluorescence - RH cold-hardened rye - RNH non-hardened rye This work was supported by a Natural Sciences and Engineering Research Council of Canada (NSERCC) Operating Grant to N.P.A.H. G.Ö. was supported by an NSERCC International Exchange Award and by the Swedish Natural Science Research Council.     

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.     

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.     

A nuclear mutant of Arabidopsis thaliana selected for enhanced sensitivity to light-chill stress is altered in PSII electron transport activity

Chilling in the light imposes a considerable level of stress on the photosynthetic apparatus, resulting in a decrease of photosystem II activity and the quenching of maximum and variable fluorescence. We selected in a fah - 1 mutagenized population of Arabidopsis thaliana , which permits a direct visible evaluation of the intensity of chlorophyll (Chl) fluorescence, a monogenic recessive nuclear mutant hypersensitive to photoinhibition induced by light and cold. The major phenotypic trait of the mutant is the appearance of chlorotic areas on developed leaves. Photochemical analyses indicate that the mutant is hypersensitive to photoinhibition in excess light in the cold but also at room temperature. The susceptibility to photoinhibition is a consequence of perturbations in photochemistry already present in unstressed plants. Such perturbations result in a greater fraction of the primary acceptor Q_A remaining in the reduced state even at low light fluxes. From estimates of the number of total and functional PSII units and measurements of PSII quantum yield and Q_A⁻ reoxidation kinetics, the basic lesion of the mutant seems restricted to PSII photochemistry likely affecting the rate of electron transport from Q_A⁻ to Q_B.     

Internal CO(2) Supply during Photosynthesis of Sun and Shade Grown CAM Plants in Relation to Photoinhibition

Leaves of Kalanchoë pinnata were exposed in the dark to air (allowing the fixation of CO₂ into malic acid) or 2% O₂, 0% CO₂ (preventing malic acid accumulation). They were then exposed to bright light in the presence or absence of external CO₂ and light dependent inhibition of photosynthetic properties assessed by changes in 77 K fluorescence from photosystem II (PSII), light response curves and quantum yields of O₂ exchange, rates of electron transport from H₂O through Q_B (secondary electron acceptor from the PSII reaction center) in isolated thylakoids, and numbers of functional PSII centers in intact leaf discs. Sun leaves of K. pinnata experienced greater photoinhibition when exposed to high light in the absence of CO₂ if malic acid accumulation had been prevented during the previous dark period. Shade leaves experienced a high degree of photoinhibition when exposed to high light regardless of whether malic acid had been allowed to accumulate in the previous dark period or not. Quantum yields were depressed to a greater degree than was 77 K fluorescence from PSII following photoinhibition.     

Evidence for active cyclic electron flow in twig chlorenchyma in the presence of an extremely deficient linear electron transport activity

Fast and slow chlorophyll fluorescence induction curves at high and low actinic visible light, post-illumination changes in fluorescence yield and reflectance changes at 820 nm induced by far-red light were used to characterize the state of PSII and PSI and their electron transport capabilities in chlorophyllous twig cortices of Eleagnus angustifolius L., while corresponding leaves served as controls. Twigs displayed low dark-adapted PSII photochemical efficiencies and particularly low linear electron transport rates when illuminated. In addition, their PSII population was characterized by a high proportion of inactive, non-Q_B-reducing centers and an incomplete quenching of fluorescence during the slow induction phase. It is suggested that PSII in twigs is an inefficient electron donor to PSI and/or the reductive pentose phosphate cycle. Yet, in spite of this apparent PSII deficiency, pools of intermediate electron carriers and potential PSI activity were more than sufficient to support the observed linear electron transport rates. Moreover, the rate of PSI reduction upon far-red/dark transitions and the magnitude of fluorescence yield increase upon white light/dark transitions were compatible with an efficient electron flow to PSI from stromal donors in the absence of PSII activity. We conclude that corticular chlorenchyma may be actively engaged in cyclic at the expense of a linear electron flow and discuss the possible physiological significance of this finding in conjunction with the particular microenvironmental conditions encountered within twigs.     

Modulation of photosynthetic electron transport in the absence of terminal electron acceptors: Characterization of the rbcL deletion mutant of tobacco

Tobacco rbcL deletion mutant, which lacks the key enzyme Rubisco for photosynthetic carbon assimilation, was characterized with respect to thylakoid functional properties and protein composition. The ΔrbcL plants showed an enhanced capacity for dissipation of light energy by non-photochemical quenching which was accompanied by low photochemical quenching and low overall photosynthetic electron transport rate. Flash-induced fluorescence relaxation and thermoluminescence measurements revealed a slow electron transfer and decreased redox gap between Q_A and Q_B, whereas the donor side function of the Photosystem II (PSII) complex was not affected. The 77 K fluorescence emission spectrum of ΔrbcL plant thylakoids implied a presence of free light harvesting complexes. Mutant plants also had a low amount of photooxidisible P700 and an increased ratio of PSII to Photosystem I (PSI). On the other hand, an elevated level of plastid terminal oxidase and the lack of F₀ ‘dark rise’ in fluorescence measurements suggest an enhanced plastid terminal oxidase-mediated electron flow to O₂ in ΔrbcL thylakoids. Modified electron transfer routes together with flexible dissipation of excitation energy through PSII probably have a crucial role in protection of PSI from irreversible protein damage in the ΔrbcL mutant under growth conditions. This protective capacity was rapidly exceeded in ΔrbcL mutant when the light level was elevated resulting in severe degradation of PSI complexes.     

Thermostability and photostability of photosystem II of the resurrection plant Haberlea rhodopensis studied by chlorophyll fluorescence

The stability of PSII in leaves of the resurrection plant Haberlea rhodopensis to high temperature and high light intensities was studied by means of chlorophyll fluorescence measurements. The photochemical efficiency of PSII in well-hydrated Haberlea leaves was not significantly influenced by temperatures up to 40 degrees C. Fo reached a maximum at 50 degrees C, which is connected with blocking of electron transport in reaction center II. The intrinsic efficiency of PSII photochemistry, monitored as Fv/Fm was less vulnerable to heat stress than the quantum yield of PSII electron transport under illumination (phiPSII). The reduction of phiPSII values was mainly due to a decrease in the proportion of open PSII centers (qP). Haberlea rhodopensis was very sensitive to photoinhibition. The light intensity of 120 micromol m(-2) s(-1) sharply decreased the quantum yield of PSII photochemistry and it was almost fully inhibited at 350 micromol m(-2) s(-1). As could be expected decreased photochemical efficiency of PSII was accompanied by increased proportion of thermal energy dissipation, which is considered as a protective effect regulating the light energy distribution in PSII. When differentiating between the three components of qN it was evident that the energy-dependent quenching, qE, was prevailing over photoinhibitory quenching, qI, and the quenching related to state 1-state 2 transitions, qT, at all light intensities at 25 degrees C. However, the qE values declined with increasing temperature and light intensities. The qI was higher than qE at 40 degrees C and it was the major part of qN at 45 degrees C, indicating a progressing photoinhibition of the photosynthetic apparatus.     

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。     

‘Birth defects’ of photosystem II make it highly susceptible to photodamage during chloroplast biogenesis

High solar flux is known to diminish photosynthetic growth rates, reducing biomass productivity and lowering disease tolerance. Photosystem II (PSII) of plants is susceptible to photodamage (also known as photoinactivation) in strong light, resulting in severe loss of water oxidation capacity and destruction of the water‐oxidizing complex (WOC). The repair of damaged PSIIs comes at a high energy cost and requires de novo biosynthesis of damaged PSII subunits, reassembly of the WOC inorganic cofactors and membrane remodeling. Employing membrane‐inlet mass spectrometry and O₂‐polarography under flashing light conditions, we demonstrate that newly synthesized PSII complexes are far more susceptible to photodamage than are mature PSII complexes. We examined these ‘PSII birth defects’ in barley seedlings and plastids (etiochloroplasts and chloroplasts) isolated at various times during de‐etiolation as chloroplast development begins and matures in synchronization with thylakoid membrane biogenesis and grana membrane formation. We show that the degree of PSII photodamage decreases simultaneously with biogenesis of the PSII turnover efficiency measured by O₂‐polarography, and with grana membrane stacking, as determined by electron microscopy. Our data from fluorescence, Q_B‐inhibitor binding, and thermoluminescence studies indicate that the decline of the high‐light susceptibility of PSII to photodamage is coincident with appearance of electron transfer capability Q_A^? → Q_B during de‐etiolation. This rate depends in turn on the downstream clearing of electrons upon buildup of the complete linear electron transfer chain and the formation of stacked grana membranes capable of longer‐range energy transfer.     

Photoprotection in plants: a new light on photosystem II damage

Sunlight damages photosynthetic machinery, primarily photosystem II (PSII), and causes photoinhibition that can limit plant photosynthetic activity, growth and productivity. The extent of photoinhibition is associated with a balance between the rate of photodamage and its repair. Recent studies have shown that light absorption by the manganese cluster in the oxygen-evolving complex of PSII causes primary photodamage, whereas excess light absorbed by light-harvesting complexes acts to cause inhibition of the PSII repair process chiefly through the generation of reactive oxygen species. As we review here, PSII photodamage and the inhibition of repair are therefore alleviated by photoprotection mechanisms associated with avoiding light absorption by the manganese cluster and successfully consuming or dissipating the light energy absorbed by photosynthetic pigments, respectively.     

Effects of shading on photosynthetic characteristics and chlorophyll fluorescence parameters in leaves of Hydrangea macrophylla

Aims The objectives were to investigate the effects of different light intensities on photosynthetic characteristics and chlorophyll fluorescence parameters, to clarify the physiological responses and photo-protective mechanisms of Hydrangea macrophylla to changes in light regimes in view of the distribution of energy absorbed and photosynthetic characteristics.Methods Three light regimes including natural and shade (shading rate 50% and 75% of natural light) were applied to plants for 60 days. After the treatment, the gas-exchange, chlorophyll a fluorescence and photosynthesis-light curves were measured by a portable leaf gas exchange system (LI-6400).Important findings The results showed that the weak light intensity treatment reduced dark respiration rate, light compensation point and light saturation point of plant, but increased apparent quantum yield, suggesting that plants had the physiological strategy to utilize the weakening light by reducing respiration. The net photosynthetic rate, intercellular CO₂ concentration, transpiration rate and water use efficiency of plants grown below 50% of natural light showed significant difference compared with natural and shading rate 75% of natural light. There were significant difference between natural and shade treatments in the maximal quantum efficiency of PSII (F_v/F_m), as indicated that it was significantly less at full light than that at 50% of natural light. Initial fluorescence intensity (F_o) of plants was higher at full light than that at 50% of natural light, suggesting that photoinhibition occurred in natural light. The non-photochemical quenching (NQP) decreased with the aggravation of shade stress, indicating that shading decreased the efficiency of photochemical reaction by reducing the fraction of incident light in photochemical energy utilization and decreased thermal dissipation through regulating energy distribution in photosystem II (PSII) in the leaves of Hydrangea macrophylla. In general, the 70% of incident light in photochemical energy utilization was distributed to thermal dissipation, 20% was distributed to non-regulated energy dissipation and 4% was distributed to effective photochemical reaction. In conclusion, responses of plants to increased irradiance are governed by strategy: to utilize a high fraction of incident light in photochemistry and regulate energy dissipation in PSII and weaken the accumulation of excess excitation energy in PSII to protect the photosynthetic apparatus in the leaves of H. macrophylla under saturated radiation.     

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.     

Reversible Redox-Dependent Inactivation of Photosystem II as Photosynthesis Regulation in Chlorella

Inactivation of photosystem II (PSII) in the alga Chlorella pyrenoidosa Chick induced by photoinhibition (high light illumination at an intensity 10 times higher than photosynthesis-saturating light) or by incubation at a supraoptimum temperature (41°C) in darkness, resulted in a decrease in the relative yield of variable fluorescence due to a selective suppression of the slow phase of its rise. This indicates that low-activity PSII complexes, with a low efficiency of Q_A ^– formation are inactivated first. We suppose that the transition of normal PSII complexes to a low-activity state precedes the complete loss of their photochemical activity. The existence of some common stages of PSII inactivation, when induced by photoinhibition or incubation at supraoptimum temperature in darkness, is discussed. We suggest a scheme of the sequential stages in the regulation of photosynthetic light reactions involving a reversible redox-dependent PSII inactivation.     

General trait relationships in stems: a study on the performance and interrelationships of several functional and structural parameters involved in corticular photosynthesis

We addressed corticular photosynthesis, focusing on parameters of underlying dark and light reactions as well as structural differentiation. To unveil general stem traits and underlying principles that may be valid across several tree species, CO₂ exchange rates and chlorophyll‐fluorescence parameters were measured in current‐year to 3‐year‐old stems of five deciduous tree species (including climax and pioneer species). Across species, dark CO₂ efflux rates (R_d) of stems exhibited a common regression relationship with photosynthetic rates (A) and light‐adapted quantum efficiency of photosystem II (PSII) (ΔF/Fm′), a pattern analogous to leaf trait correlations. Furthermore, A and ΔF/Fm′ were closely interrelated to each other. Consistent correlations of stem structure and function were also assessed among species. Changes in tissue structure during ageing significantly affected several stem functional parameters. Stem CO₂ efflux during the dark and corticular photosynthetic rates declined with increasing stem age as well as light‐adapted quantum efficiency of PSII. Furthermore, a strong relationship between stem R_d and peridermal PFD‐transmittance (T) as well as between R_d and total bark chlorophyll was evident. Consistent results were found for the relationships between corticular photosynthesis (or primary photosynthetic reactions like ΔF/Fm′) and selected structural traits. The found correlation patterns among functional and/or structural traits of stems and their concordance with leaf trait relationships may aid in identifying underlying mechanisms and scaling relationships that link traits to plant and ecosystem function.     

Low-temperature modulation of the redox properties of the acceptor side of photosystem II: photoprotection through reaction centre quenching of excess energy

Although it has been well established that acclimation to low growth temperatures is strongly correlated with an increased proportion of reduced Q_A in all photosynthetic groups, the precise mechanism controlling the redox state of Q_A and its physiological significance in developing cold tolerance in photoautotrophs has not been fully elucidated. Our recent thermoluminescence (TL) measurements of the acceptor site of PSII have revealed that short‐term exposure of the cyanobacterium Synechococcus sp. PCC 7942 to cold stress, overwintering of Scots pine (Pinus sylvestris L.), and acclimation of Arabidopsis plants to low growth temperatures, all caused a substantial shift in the characteristic T_M of S₂Q_B^– recombination to lower temperatures. These changes were accompanied by much lower overall TL emission, restricted electron transfer between Q_A and Q_B, and in Arabidopsis by a shift of the S₂Q_A^–‐related peak to higher temperatures. The shifts in recombination temperatures are indicative of a lower activation energy for the S₂Q_B^– redox pair and a higher activation energy for the S₂Q_A^– redox pair. This results in an increase in the free‐energy gap between P680⁺Q_A^– and P680⁺Pheo^– and a narrowing of the free energy gap between Q_A and Q_B electron acceptors. We propose that these effects result in an increased population of reduced Q_A (Q_A^–), facilitating non‐radiative P680⁺Q_A^– radical pair recombination within the PSII reaction centre. The proposed reaction centre quenching could be an important protective mechanism in cyanobacteria in which antenna and zeaxanthin cycle‐dependent quenching are not present. In herbaceous plants, the enhanced capacity for dissipation of excess light energy via PSII reaction centre quenching following cold acclimation may complement their capacity for increased utilization of absorbed light through CO₂ assimilation and carbon metabolism. During overwintering of evergreens, when photosynthesis is inhibited, PSII reaction centre quenching may complement non‐photochemical quenching within the light‐harvesting antenna when zeaxanthin cycle‐dependent energy quenching is thermodynamically restricted by low temperatures. We suggest that PSII reaction centre quenching is a significant mechanism enabling cold‐acclimated organisms to acquire increased resistance to high light.     

田间条件下海岛棉和陆地棉花铃期叶片光保护的机制

研究海岛棉(Gossypium barbadense)和陆地棉(G. hirsutum)两个棉花栽培种的光合作用特性, 探讨两个栽培种光合机构的光抑制以及防御保护机制, 以期为新疆棉花高光效品种选育和高产高效栽培实践提供理论基础。在新疆生态气候条件下, 系统测定了海岛棉和陆地棉的叶片运动、叶片接受光量子通量密度(PFD)、叶片温度、叶绿素荧光参数、气体交换参数和光呼吸速率的日变化。研究结果表明: 陆地棉叶片的“横向日性”较强而海岛棉较弱, 导致海岛棉叶片接受PFD较低, 但其叶片温度较高。海岛棉叶片的光合速率和气孔导度均显著低于陆地棉。在8:00-10:00 (北京时间, 下同)海岛棉叶片的光呼吸速率略低于陆地棉, 其余时间段海岛棉和陆地棉叶片的光呼吸速率相似。不同栽培种间, 叶片的最大光化学效率和实际光化学效率的日变化均无明显差异。除14:00-16:00以外, 海岛棉叶片的表观电子传递速率和光化学猝灭系数均显著低于陆地棉。8:00以后, 海岛棉叶片的非光化学猝灭显著高于陆地棉。因此, 在新疆生态气候条件下, 海岛棉和陆地棉叶片“横向日性”运动能力和气孔导度的差异导致叶片所处的光温环境不同, 同时造成海岛棉叶片的碳同化能力较低。为阻止光合电子传递链的过度还原, 减轻光合机构的光抑制, 陆地棉叶片主要通过光合机构的电子流途径耗散激发能, 而海岛棉叶片通过热耗散途径和相对较高的光呼吸能力来耗散激发能。