Photoinhibition and recovery in tropical plant species: response to disturbance

Disturbance or rainforest is often followed by mass mortality of understorey seedlings. Transitions of shade grown plants to full sunlight can cause reductions in the efficiency with which light is used in photosynthesis, called photoinhibition. In order to assess the influence of photoinhibition on mortality and growth after rainforest disturbance this study examined photoinhibition in both simulated and real forest disturbances in northern Papua New Guinea. In an experiment simulating rainforest disturbance, exposure of shade-grown plants to full sunlight resulted in abrupt decreases in the chlorophyll fluorescence parameter F _v/F _m that is characteristic of photoinhibition. However, in the well-watered plants used in these experiments there were no fatalities during 3 weeks after exposure to full sunlight. Thus, it is unlikely that photoinhibition, alone, is responsible for seedling fatalities after rainforest disturbances, but more likely that fatalities are due to photoinhibition in conjunction with other environmental stress. There were differences between the response of species to the simulated disturbance that concurred with their preferred habitats. For example, species form the genus Barringtonia, which is commonly found in shaded understorey environments, underwent greater reductions in F _v/F _m and were slower to recover than species that usually inhabit high solar radiation environments. The extent of photoinhibition and the rate of recovery appeared to be dependent on avoidance of direct solar radiation by altering leaf angles and on increasing maximum photosynthetic rates. A field survey of photoinhibition in man-made rainforest gaps corroborated the findings of the simulated disturbance experiment showing that plant species commonly found in shaded environments showed a greater degree of photoinhibition in forest gaps at midday than those species which are classified as species that benefit from gaps or specialist gap inhabitors.     

Diurnal changes in photochemical efficiency and xanthophyll concentrations in shallow water reef corals : evidence for photoinhibition and photoprotection

 Diurnal patterns of photoinhibition have been identified in seven species of shallow water reef corals from the Andaman Sea, off the west coast of Thailand, using pulse amplitude fluorometry. Photochemical efficiency (F_v/F_m) and quantum yield (ΔF/F_m∑) of symbiotic dinoflagellates within the corals declined after dawn to reach a minimum between midday and early afternoon, recovering to former dawn levels by early evening. Parallel studies on the xanthophylls diadinoxanthin (Dn) and diatoxanthin (Dt), and their inter-conversion, also revealed a strong diurnal pattern as well as inverse correlations between the xanthophyll ratio Dt/(Dn+Dt) and F_v/F_m and ΔF/F_m′. These findings suggest a photoprotective function for these pigments. Accepted: 18 March 1999     

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.     

Inhibition of photosynthetic reactions under water stress: interaction with light level

When the shrub Nerium oleander L., growing under full natural daylight outdoors, was subjected to water stress, stomatal conductance declined, and so did non-stomatal components of photosynthesis, including the CO₂-saturated rate of CO₂ uptake by intact leaves and the activity of electron transport by chloroplasts isolated from stressed plants. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics determined at 77 K (-196°C) for the upper leaf surface and from isolated chloroplasts. The maximum (F _M) and the variable (F _V) fluorescence yield at 692 nm were strongly quenched but there was little effect on the instantaneous (F _O) fluorescence. There was a concomitant quenching of the maximum and variable fluorescence at 734 nm. These results indicate an inactivation of the primary photochemistry associated with photosystem II. The lower, naturally shaded surfaces of the same leaves were much less affected than the upper surfaces and water-stress treatment of plants kept in deep shade had little or no effect on the fluorescence characteristics of either surface, or of chloroplasts isolated from the water-stressed leaves. The effects of subjecting N. oleander plants, growing in full daylight, to water stress are indistinguishable from those resulting when plants, grown under a lower light regime, are exposed to full daylight (photoinhibition). Both kinds of stress evidently cause an inactivation of the primary photochemistry associated with photosystem II. The results indicate that water stress predisposes the leaves to photoinhibition. Recovery from this inhibition, following restoration of favorable water relations, is very slow, indicating that photoinhibition is an important component of the damage to the photosynthetic system that takes place when plants are exposed to water stress in the field. The underlying causes of this water-stress-induced susceptibility to photoinhibition are unknown; stomatal closure or elevated leaf temperature cannot explain the increased susceptibility.Abbreviations and symbols Chl chlorophyll - PFD photon flux area density - PSI, PSII photosystem I, II - F _M, F _O, F _V maximum, instantaneous, variable fluorescence emission - leaf water potential C.I.W.-D.P.B. Publication No. 775     

Operation of the xanthophyll cycle in higher plants in response to diurnal changes in incident sunlight

Changes in the carotenoid composition of leaves in response to diurnal changes in sunlight were determined in the crop species Helianthus annuus L. (sunflower), Cucurbita pepo L. (pumpkin), and Cucumls sativus L. (cucumber), in the diaheliotropic mesophyte Malva neglecta Wallr., and in the perennial shrub Euonymus kiautschovicus Loesner. Large daily changes were observed in the relative proportions of the components of the xanthophyll cycle, violaxanthin (V), antheraxanthin (A), and zeaxanthin (Z) in plants grown in full sunlight. In all leaves large amounts of Z were formed at peak irradiance, with the changes in Z content closely following changes in incident photon flux density (PFD) over the course of the day. All leaves also contained large total pools of the three xanthophyll-cycle components. However, the extent to which the V pool present at dawn became de-epoxidized during the day varied widely among leaves, from a 27% decrease in M. neglecta to a 90% decrease in E. kiautschovicus. The largest amounts of Z and the lowest amounts of V at peak irradiance (full sunlight) were observed in the species with the lower rates of photosynthesis (particularly in E. kiautschovicus and pumpkin), and smaller amounts of Z and a lesser decrease in V content were found at peak irradiance in those species with the higher rates of photosynthesis (particularly in M. neglecta and sunflower). In all species some Z was present in the leaves prior to sunrise. Furthermore, in individuals of sunflower, pumpkin, and cucumber grown at 85% of full sunlight and transferred to full sunlight, a further increase in the already large pool of the xanthophyll-cycle pigments occurred over the course of 1 d.Abbreviations A antheraxanthin - -Car, -Car - and -carotene - EPS epoxidation state - PFD photon flux density, between 400 and 700 nm - V violaxanthin - Z zeaxanthin This work was supported by the U.S. Department of Agriculture, Competitive Research Grants Office, award No. 90-37130-5422, and a Faculty Development Award from the University of Colorado to W.W. Adams III.     

Oxygen-dependent electron transport and protection from photoinhibition in leaves of tropical tree species

The roles of photorespiration and the Mehlerperoxidase pathway in sustaining electron transport and protection from photoinhibition were studied in outer canopy leaves of two species of tropical trees: the drought-deciduous Pseudobombax septenatum (Jacq.) Dug. and the evergreen Ficus insipida Willd. Ficus had a higher photosynthetic capacity than Pseudobombax and also a greater capacity for light-dependent electron transport under photorespiratory conditions (in the absence of CO₂). As a consequence, in the absence of CO₂, Ficus was able to maintain a largely oxidized electron-transport chain at higher photon flux densities than Pseudobombax. Under the same light conditions, photoinhibition (reduction in F_v/F_m) was always greater in Pseudobombax than Ficus, was increased when leaves were exposed to 2% O₂ in nitrogen compared to 21% O₂ in CO₂-free air, but was not increased by the absence of CO₂. Rates of electron transport due to the Mehler-peroxidase pathway (assessed in 2% O₂ in nitrogen) ranged between 16–40 mol · m^–2·s^–1 in both species. As the dry season approached and Pseudobombax neared leaf senescence there was a decline in the capacity for photorespiratory flux to maintain electron transport in Pseudobombax, but not in Ficus. Ratios of light-dependent electron transport to net CO₂ fixation for Pseudobombax, Ficus and two other species in the field, Luehea seemannii Tr. & Planch, and Didymopanax morototoni (Aubl.) Dec. & Planch., ranged from 6.2 (Ficus) to 16.7 (Pseudobombax). High in-situ rates of photorespiration combined with the decreased capacity of Pseudobombax for photorespiratory flux as the dry season approached indicates a decreased capacity to protect against photooxidative damage. This may contribute to the promotion of leaf senescence in Pseudobombax during the transition from wet to dry season.Abbreviations F_v/F_m ratio of variable to maximum chlorophyll a fluorescence - NPQ nonphotochemical fluorescence quenching - PFD photon flux density - Q_A primary electron acceptor of PSII This research was supported by a grant from the Mellon Foundation. We thank Monica Mejia and Juan Posada for assisting with the fluorescence measurements and Aurelio Virgo for assisting with the field CO₂-exchange measurements.     

Reversible photoinhibition of unhardened and cold-acclimated spinach leaves at chilling temperatures

The photoinhibition of photosynthesis at chilling temperatures was investigated in cold-acclimated and unhardened (acclimated to +18° C) spinach (Spinacia oleracea L.) leaves. In unhardened leaves, reversible photoinhibition caused by exposure to moderate light at +4° C was based on reduced activity of photosystem (PS) II. This is shown by determination of quantum yield and capacity of electron transport in thylakoids isolated subsequent to photoinhibition and recovery treatments. The activity of PSII declined to approximately the same extent as the quantum yield of photosynthesis of photoinhibited leaves whereas PSI activity was only marginally affected. Leaves from plants acclimated to cold either in the field or in a growth chamber (+1° C), were considerably less susceptible to the light treatment. Only relatively high light levels led to photoinhibition, characterized by quenching of variable chlorophyll a fluorescence (F_V) and slight inhibition of PSII-driven electron transport. Fluorescence data obtained at 77 K indicated that the photoinhibition of cold-acclimated leaves (like that of the unhardened ones) was related to increased thermal energy dissipation. But in contrast to the unhardened leaves, 77 K fluorescence of cold-acclimated leaves did not reveal a relative increase of PSI excitation. High-light-treated, cold-acclimated leaves showed increased rates of dark respiration and a higher light compensation point. The photoinhibitory fluorescence quenching was fully reversible in low light levels both at +18° C and +4° C; the recovery was much faster than in unhardened leaves. Reversible photoinhibition is discussed as a protective mechanism against excess light based on transformation of PSII reaction centers to fluorescence quenchers.Abbreviations F_O initial fluorescence - F_M maximal fluorescence - F_V devariable fluorescence (f_m-f_o) - PFD photon flux density - PS photosystem - SD standard deviation The authors thank the Deutsche Forschungsgemeinschaft and the Academy of Finland for financial support.     

Kinetic evidence for involvement of two cytochrome b-563 hemes in photosynthetic electron transport

The effect of 2-(n-heptyl)-4-hydroxyquinoline N-oxide (HQNO) on the kinetics of cytochrome b-563 and cytochrome c² turnovers following single-turnover flashes was measured in isolated heterocysts. Low concentrations of HQNO (below 3 μM) blocked reoxidation of cytochrome b-563, whereas higher concentrations (above 5 μM) resulted in additional inhibition of cytochrome b-563 oxidation and also inhibited reduction of cytochrome b-563 and cytochrome c. Similar effects on cytochrome b-563 reduction and reoxidation were obtained with a combination of 5 μM HQNO and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (1–7 μM). In HQNO-inhibited heterocysts, cytochrome c reduction following a flash occurred in three phases with half-times of 0.5, 2.8 and 45 ms. The second phase nearly equalled the cytochrome b-563 reduction in half-time and magnitude. In the presence of HQNO, the reoxidation of cytochrome b-563 following two closely spaced actinic flashes displayed biphasic kinetics. The two phases correspond to reoxidation of cytochrome b-563 in which one or both of the cytochrome b-563 hemes in the cytochrome b–f complex are reduced. These results are interpreted in terms of a Q-loop in which HQNO, at low concentrations, blocks the site of rapid cytochrome b-563 reoxidation and at higher concentrations, also inhibits the site of electron donation by plastoquinol to the cytochrome b-f complex.     

Photosynthesis and phosphorylation of light-harvesting chlorophyll a/b—protein in intact chloroplasts: Effects of uncouplers

Protein phosphorylation in isolated, intact pea chloroplasts was measured during the onset of CO₂-dependent O₂ evolution. Total incorporation of ³²P (from ³²P_i) into the light-harvesting chlorophyll a/b—protein was found to be less sensitive than O₂ evolution to inhibition by the uncouplers FCCP and NH₄C1 It is concluded that changes in the rate of ATP synthesis cannot affect protein phosphorylation without also affecting the rate of CO₂-fixation in this system. The ATP/ADP ratio is therefore unlikely to regulate photosynthetic protein phosphorylation under normal physiology conditions.     

Rapid proton release accompanying photosynthetic electron transport in intact cells of Rhodopseudomonas capsulata

The release of protons from intact cells of Rhodopseudomonas capsulata after either 4μs flashes or during brief periods of continuous illumination has been measured with the indicator, cresol red. The half-time for H⁺-release after a flash was 35 ms and the extent, 1H⁺ per 134 bacteriochlorophyll. Myxothiazol completely inhibited the flash-induced H⁺-release and antimycin A reduced it by 37%. The proton-releasing reaction is discussed with reference to the protonmotive Q-cycle. During continuous illumination the rapid phase of H⁺ release is followed by a lag and then by another period of acidification, suggesting that other protolytic reactions may be in operation.     

Cyanogen-induced gamma-glutamyl to imidazole cross-link in carbonic anhydrase. A unique mode of inhibition

The release of protons from intact cells of Rhodopseudomonas capsulata after either 4μs flashes or during brief periods of continuous illumination has been measured with the indicator, cresol red. The half-time for H⁺-release after a flash was 35 ms and the extent, 1H⁺ per 134 bacteriochlorophyll. Myxothiazol completely inhibited the flash-induced H⁺-release and antimycin A reduced it by 37%. The proton-releasing reaction is discussed with reference to the protonmotive Q-cycle. During continuous illumination the rapid phase of H⁺ release is followed by a lag and then by another period of acidification, suggesting that other protolytic reactions may be in operation.     

Light-dependent proton transport into mesophyll vacuoles of leaves of C3 plants as revealed by pH-indicating fluorescent dyes: a reappraisal

Esculin, a pH-sensitive fluorescent dye, was used to indicate light-dependent pH changes in leaves of Spinacia oleracea L. and Pelargonium zonale L. Shortly after its introduction into the leaves via the transpiration stream, esculin was localized mainly in the symplasm. An increase in its blue fluorescence on illumination with red actinic light indicated that the cytosolic pH had increased. A similar light-dependent alkalinization was seen when the green fluorescence of pyranine was used to monitor changes in the cytosolic pH. After esculin had been transferred into the vacuoles, a light-dependent vacuolar acidification was indicated by a decrease in its blue fluorescence. Since the pK of esculin is close to neutrality, it is suitable as an indicator of proton transport into vacuoles provided the vacuolar sap is only moderately acidic. In leaf cells with very acidic vacuoles, esculin therefore responds only to cytosolic pH changes as long as it remains in the cytosol. The observations made with esculin after it had entered the vacuoles confirmed earlier conclusions on light-dependent proton transport into the vacuoles of mesophyll cells. Previous measurements had been made with 5-carboxy-2,7-dichlorofluoresceine (CDCF), which has a pK of 4.8. In contrast to esculin, CDCF can, in principle, record pH changes in very acidic vacuoles. However, earlier conclusions made on the basis of observed CDCF fluorescence are now recognized to have no unambiguous basis because new measurements, reported here, show that CDCF fluorescence is influenced not only by pH changes but also by changes in light scattering. The latter are, like pH changes, light-dependent and originate from the thylakoid system of chloroplasts. They indicate both the formation of a large transthylakoid proton gradient and the dissipation of excess light energy as heat. Decreased green fluorescence of leaves which had been fed CDCF may therefore, depending on conditions, indicate vacuolar acidification or the dissipation of excess light energy absorbed by the pigment system of chloroplasts, or both. Pyranine fluorescence was found to be much less influenced by light scattering than CDCF fluorescence.Abbreviations CDCF 5-(and 6-)carboxy-2,7-dichlorofluoresceine - P₇₀₀ primary donor of PS I - PFD photon flux density - Q_A primary quinone acceptor of PS II - Q_P, Q_N photochemical, non-photochemical quenching of chlorophyll fluorescence, respectively This work was supported by the Deutsche Forschungsgemeinschaft within the framework of the research of the Sonderforschungsbereich 251 of the University of Würzburg. We are grateful to Drs. U. Schreiber and K.-J. Dietz and to Mrs. B. Hollenbach (all from our Institute) for discussions.     

Physiological ecology of photosynthesis of five sympatric species of Velloziaceae in the rupestrian fields of Serra do Cipó, Minas Gerais, Brazil

Comparative ecophysiology of photosynthesis of five sympatric Velloziaceae species (Liliidae, Monocotyledonae) in the savanna- or cerrado-like rupestrian fields of Serra do Cipó, southeastern Brazil (19°17′S; 43°33′W, 1170 m a.s.l.), was assessed by field measurements of chlorophyll fluorescence parameters on 3 consecutive days in October 2002. The species investigated co-occurred in a plot of 50 m×50 m. The chosen species were distinguished by different architectures, mainly tallness of the plants. They were Vellozia variabilis Mart. ex Shult. f.>Vellozia glabra J.C. Mikan>Vellozia nivea L.B.Sm. et Ayensu=Vellozia alata L.B.Sm.>Barbacenia involucrata L.B.Sm. In the open highly sun exposed habitat there was no photoinhibition in any of the five species being not reversible within 30 min at any time during the day including the afternoon. There was a slightly but not statistically significant better instantaneous performance of the taller species. Significant differences showing a higher intrinsic photosynthetic capacity of the taller as compared to the smaller species were revealed by photosynthetic parameters extracted from light dependence curves obtained in the morning and at midday. Light dependence curves of all five species showed a significant and substantial drop of intrinsic photosynthetic capacity when measured in the afternoon as compared to the morning and midday. As this cannot be explained by a build up of photoinhibition during the day this is a novel observation made here with the Velloziaceae.     

Stomatal aperture,photosythesis and water fluxes in mesophyll cells as affected by the abscission of leaves. Simultaneous measurements of gas exchange,light scattering and chlorphyll fluorescence

Carbon dioxide exchange, transpiration, chlorophyll fluorescence and light scattering of leaves of Lycopersicom esculentum, Helianthus annuus and Arbutus unedo were measured simultaneously before and after abscission of leaves. Scattering of a weak green measuring beam was used to monitor water fluxes across the thylakoid membranes of the mesophyll. When leaves were cut under water, stomata initially closed partially and then occasionally exhibited distinct regulatory oscillations. As stomata closed, light scattering decreased indicating water influx into the mesophyll. Stomatal oscillations were accompanied, with small but noticeable phase shifts, by oscillations of water fluxes at the thylakoid level. These fluxes could be distinguished from the water fluxes accompanying light-dependent ion pumping across the thylakoids by the concomitant chlorophyll fluorescence signals. The latter record energy-dependent ion fluxes in addition to redox changes of the electron-transport chain. As stomata closed partially after cutting a leaf under water, photosynthesis decreased. In Arbutus unedo and Helianthus annuus leaves, transient stomatal closure was insufficient to account for transient inhibition of photosynthesis which appeared to be brought about by transfer of an inhibitory solute through the petiole into the mesophyll. This solute also stimulated respiration in the dark. When leaves were cut in air, stomata opened transiently (Iwanoff effect) before wilting enforced closure. Photosynthesis followed the stomatal responses, increasing during opening and decreasing during closure.Dedicated to Professor H. Ullrich on the occasion of his 85th birthday     

An assessment of the relationship between chlorophyll a fluorescence and CO2 gas exchange from field measurements on a moss and lichen

The relationship between CO₂ exchange and relative electron-transport rate through photosystem II (ETR, measured using chlorophyll a fluorescence) was determined for a moss and a green algal lichen, photobiont probably Trebouxia sp., in the field in Antarctica. Net photosynthesis (NP) and dark respiration (DR) were measured over temperatures from zero to 25 °C and gross photosynthesis (GP) calculated (GP = NP + DR). The strong response of DR to temperature in these organisms resulted in substantial changes in CO₂ exchange rates. The moss Bryum argenteum Hedw. showed a strong, linear relationship between GP and ETR. This was an unexpected result since mosses are C₃ plants and, in higher plants, this group normally has a curvilinear GP versus ETR relationship. It is suggested that suppression of DR in the light might be involved. The lichen, Umbilicaria aprina Nyl., had nonlinear relationships between ETR and GP that were different at each measurement temperature. In some cases the lowest ETR was at the higher CO₂ exchange rates. It is suggested that these relationships are the result of strong quenching mechanisms that are inversely proportional to GP. The results support a growing impression that the relationships between ETR and CO₂ exchange are complex in these organisms and different from those found for higher plants. Received: 24 November 1997 / Accepted 2 May 1998     

The function of three main call types in common cuckoo

Acoustic signals play a key role in shaping the relationships in birds. Common cuckoos Cuculus canorus are known to produce various call types, but the function of these calls has only been studied recently. Here, we used a combination of field recordings (conducted in 2017) and playback experiments (conducted in 2018) to investigate the functional significance of common cuckoo calls. We found significant differences in the characteristics between male two‐element “cu‐coo” and three‐element “cu‐cu‐coo” calls, with these two call types being used in different contexts. The three‐element male “cu‐cu‐coo” calls were associated with females emitting their “bubbling” call. Playback experiments revealed that both males and females exhibit stronger responses to playing female “bubbling” calls than with the calls of the Eurasian sparrowhawk (Accipter nisus) serving as a control, suggesting a significant intraspecific communication function for this call type. However, we did not find any evidence to support mate attraction in male calls, as females were not stimulated by playback of male calls compared with sparrowhawk calls in the control group.     

Comparison of the effect of excessive light on chlorophyll fluorescence (77K) and photon yield of O2 evolution in leaves of higher plants

High-light treatments (1750–2000 mol photons m^–2 · s^–1) of leaves from a number of higher-plant species invariably resulted in quenching of the maximum 77K chlorophyll fluorescence at both 692 and 734 nm (F _M, 692 and F _M, 734). The response of instantaneous fluorescence at 692 nm (F _O, 692) was complex. In leaves of some species F _O, 692 increased dramatically in others it was quenched, and in others yet it showed no marked, consistent change. Regardless of the response of F _O, 692 an apparently linear relationship was obtained between the ratio of variable to maximum fluorescence (F _V/F _M, 692) and the photon yield of O₂ evolution, indicating that photoinhibition affects these two variables to approximately the same extent. Treatment of leaves in a CO₂–free gas stream containing 2% O₂ and 98% N₂ under weak light (100 mol · m^–2 · s^–1) resulted in a general and fully reversible quenching of 77K fluorescence at 692 and 734 nm. In this case both F _O, 692 and F _M, 692 were invariably quenched, indicating that the quenching was caused by an increased non-radiative energy dissipation in the pigment bed. We propose that high-light treatments can have at least two different, concurrent effects on 77K fluorescence in leaves. One results from damage to the photosystem II (PSII) reaction-center complex and leads to a rise in F _O, 692; the other results from an increased non-radiative energy dissipation and leads to quenching of both F _O, 692 and F _M, 692 This general quenching had a much longer relaxation time than reported for pH-dependent quenching in algae and chloroplasts. Sun leaves, whose F _V/F _M, 692 ratios were little affected by high-light exposure in normal air, suffered pronounced photoinhibition when the exposure was made under conditions that prevent photosynthetic gas exchange (2% O₂, 0% CO₂). However, they were still less susceptible than shade leaves, indicating that the higher capacity for energy dissipation via photosynthesis is not the only cause of their lower susceptibility. The rate constant for recovery from photoinhibition was much higher in mature sun leaves than in mature shade leaves, indicating that differences in the capacity for continuous repair may in part account for the difference in their susceptibility to photoinhibition.Abbreviations and symbols kDa kilodalton - LHC-II light-harvesting chlorophyll-protein complex - PFD photon flux density (photon fluence rate) - PSI, PSII photosystem I, II - F _O, F _M, F _V instantaneous, maximum, variable fluorescence emission - absorptance - _a photon yield of O₂ evolution (absorbed light) C.I.W.-D.P.B. Publication No. 925     

几种南亚热带木本植物光合作用对生长光强的响应

分别将马尾松（Pinus massoniana）、黧蒴（Castanopsis fissa）、荷木（Schima superba）、黄果厚壳桂（Cryptocarya concinna）的幼苗置于100％自然光和32％自然光下生长6个月,测定它们的光强-光合反应曲线和叶绿素荧光的某些参数。结果表明,在100％光下,马尾松有最高的最大光合速率（Pmax）、光饱和点（LSP）、光补偿点（LCP）、暗呼吸（Rd）、表观量子效率（AQY）和总电子传递速率（JF）,光化学猝灭（qP）也最大。而黄果厚壳棒有最大的分配到光呼吸的电子流比率（JO/JF）。100％光下AQY的大小顺序为：马尾松〉黧蒴〉荷木〉黄果厚壳桂,32％光下AQY的顺序则相反。这说明群落早期演替的先锋树种马尾松属于强阳生性树种,具有适应强光的特点,而处于群落演替项级阶段的优势种黄果厚壳梓则能更加充分利用低光生长环境中的光强,同时也可通过提高电子流向光呼吸分配的比例来避免自然光环境中强光的伤害。     

Photoadaptation of two members of the Chlorophyta (Scenedesmus and Chlorella) in laboratory and outdoor cultures: changes in chlorophyll fluorescence quenching and the xanthophyll cycle

The role of the xanthophyll cycle in the adaptation of two chlorococcal algae Scenedesmus quadricauda and Chlorella sorokiniana to high irradiance was studied under laboratory and outdoor conditions. We wished to elucidate whether the xanthophyll cycle plays a key role in dissipating the excesses of absorbed light, as in higher plants, and to characterise the relationship between chlorophyll fluorescence parameters and the content of xanthophyll-cycle pigments. The xanthophyll cycle was found to be operative in both species; however, its contribution to overall non-photochemical quenching (NPQ) could only be distinguished in Scenedesmus (15–20% of total NPQ). The Scenedesmus cultures showed a larger pool of xanthophyll-cycle pigments than Chlorella, and lower sensitivity to photoinhibition as judged from the reduction of maximum quantum yield of photosystem II. In general, both algae had a larger xanthophyll-cycle pool when grown outdoors than in laboratory cultures. Comparing the two species, Scenedesmus exhibited a higher capacity to adapt to high irradiance, due to an effective quenching mechanism and high photosynthetic capacity; in contrast, Chlorella represents a species with a larger antennae system, less-efficient quenching and lower photosynthetic performance. Non-photochemical quenching (NPQ) induced through the xanthophyll cycle can, to a limited extent, represent a regulatory factor in diluted algal cultures grown in outdoor solar photobioreactors, as well as in natural algal phytoplankton populations exposed transiently to high irradiance. However, it does not play an appreciable role in dense, well-mixed microalgal suspensions. Received: 6 August 1998 / Accepted: 12 February 1999