Multi‐wavelength Pulse Amplitude Modulated fluorometry (Phyto‐PAM) reveals differential effects of ultraviolet radiation on the photosynthetic physiology of phytoplankton pigment groups

相似文献   

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.     

Contrasting effects of high‐intensity photosynthetically active radiation on two bloom‐forming dinoflagellates

While light limitation can inhibit bloom formation in dinoflagellates, the potential for high‐intensity photosynthetically active radiation (PAR) to inhibit blooms by causing stress or damage has not been well‐studied. We measured the effects of high‐intensity PAR on the bloom‐forming dinoflagellates Alexandrium fundyense and Heterocapsa rotundata. Various physiological parameters (photosynthetic efficiency F_v/F_m, cell permeability, dimethylsulfoniopropionate [DMSP], cell volume, and chlorophyll‐a content) were measured before and after exposure to high‐intensity natural sunlight in short‐term light stress experiments. In addition, photosynthesis‐irradiance (P‐E) responses were compared for cells grown at different light levels to assess the capacity for photophysiological acclimation in each species. Experiments revealed distinct species‐specific responses to high PAR. While high light decreased F_v/F_m in both species, A. fundyense showed little additional evidence of light stress in short‐term experiments, although increased membrane permeability and intracellular DMSP indicated a response to handling. P‐E responses further indicated a high light‐adapted species with Chl‐a inversely proportional to growth irradiance and no evidence of photoinhibition; reduced maximum per‐cell photosynthesis rates suggest a trade‐off between photoprotection and C fixation in high light‐acclimated cells. Heterocapsa rotundata cells, in contrast, swelled in response to high light and sometimes lysed in short‐term experiments, releasing DMSP. P‐E responses confirmed a low light‐adapted species with high photosynthetic efficiencies associated with trade‐offs in the form of substantial photoinhibition and a lack of plasticity in Chl‐a content. These contrasting responses illustrate that high light constrains dinoflagellate community composition through species‐specific stress effects, with consequences for bloom formation and ecological interactions within the plankton.     

Temperature dependence of UV radiation effects in Arctic and temperate isolates of three red macrophytes

The temperature dependence of UV effects was studied for Arctic and temperate isolates of the red macrophytes Palmaria palmata, Coccotylus truncatus and Phycodrys rubens. The effects of daily repeated artificial ultraviolet B and A radiation (UVBR: 280–320?nm, UVAR: 320–400?nm) treatments were examined for all isolates at 6, 12 and 18?°C by measuring growth, optimal quantum yield of PSII (F_v/F_m) and cyclobutane-pyrimidine dimer (CPD) accumulation. Furthermore, possible ecotypic differences in UV sensitivity between Arctic and temperate isolates were evaluated. Large species-specific differences in UV sensitivity were observed for all parameters: the lower subtidal species C. truncatus and P. rubens were highly sensitive to the UV treatments, whereas P. palmata, which predominantly occurs in the upper subtidal zone, was not affected by these treatments. Only minor differences were found between Arctic and temperate isolates, suggesting that no differences in UV sensitivity have evolved in these species. Relative growth rates were temperature-dependent, whereas species-specific UV effects on growth rates were relatively independent of temperature. In contrast, the species-specific decrease in F_v/F_m and its subsequent recovery were temperature-dependent in all species. UV effects on F_v/F_m were lower at 12 and 18?°C compared with 6?°C. In addition, UV effects on F_v/F_m decreased in the course of the experiment at all temperatures, indicating acclimation to the UV treatments. CPDs accumulated during the experiment in both isolates of P. rubens, whereas CPD concentrations remained low for the other two species. CPD accumulation appeared to be independent of temperature. The results suggest that summer temperatures occurring in temperate regions facilitate repair of UV-induced damage and acclimation to UV radiation in these algae compared with Arctic temperatures. Because the differences in UV effects on F_v/F_m, growth and CPD accumulation were relatively small over a broad range of temperatures, it was concluded that the influence of temperature on UV effects is small in these species.     

Photosynthetic performance,DNA damage and repair in gametes of the endemic Antarctic brown alga Ascoseira mirabilis exposed to ultraviolet radiation

Abstract Stress physiology on the reproductive cells of Antarctic macroalgae remained unstudied. Ascoseira mirabilis is endemic to the Antarctic region, an isolated ecosystem exposed to extreme environmental conditions. Moreover, stratospheric ozone depletion leads to increasing ultraviolet radiation (280–400 nm) at the earth's surface, thus it is necessary to investigate the capacity of reproductive cells to cope with different UV irradiances. This study is aimed to investigate the impact of exposure to different spectral irradiance on the photosynthetic performance, DNA damage and gamete morphology of the A. mirabilis. Gametangia, gametes and zygotes of the upper sublittoral brown alga A. mirabilis were exposed to photosynthetically active radiation (PAR = P; 400–700 nm), P + UV‐A radiation (UV‐A, 320–400 nm) and P + UV‐A + UV‐B radiation (UV‐B, 280–320 nm). Rapid photosynthesis versus irradiance curves of freshly released propagules were measured. Photosynthetic efficiencies and DNA damage (in terms of cyclobutane pyrimidine dimers) were determined after 1, 2, 4 and 8 h exposure as well as after 2 days of recovery in dim white light. Saturation irradiance (I_k) in freshly released propagules was 52 μmol photons m⁻² s⁻¹. Exposure for 1 h under 22 μmol photons m⁻² s⁻¹ of PAR significantly reduced the optimum quantum yield (F_v/F_m), suggesting that propagules are low light adapted. Furthermore, UVR significantly contributed to the photoinhibition of photosynthesis. Increasing dose as a function of exposure time additionally exacerbated the effects of different light treatments. The amount of DNA damage increased with the UV‐B dose but an efficient repair mechanism was observed in gametes pre‐exposed to a dose lower than 5.8 × 10³ J m⁻² of UV‐B. The results of this study demonstrate the negative impact of UV‐B radiation. However, gametes of A. mirabilis are capable of photosynthetic recovery and DNA repair when the stress factor is removed. This capacity was observed to be dependent on the fitness of the parental sporophyte.     

Photosynthesis and photoinhibition of two green macroalgae with contrasting habitats

We evaluated light-related traits ofUlva pertusa Kjellman from the intertidal and upper subtidal zones, and ofUmbraulva japonica (Holmes) Bae & Lee (formerlyUlva japonica) within its upper growth limit of 10 m, in Korean coastal regions.U. pertusa showed significantly higher maximum photosynthetic rates, photosynthetic efficiency, saturating irradiances, and total pigment contents. However, green light-use efficiency at limiting irradiances was notably higher inU. japonica, possibly due to the presence of green light absorbing pigments like siphonaxanthin and siphonein. Non-enzymatic antioxidation capacity as determined by DPPH (α,α-diphenyl-β-picrylhydrazyl) radical scavenging rate was markedly higher inU. pertusa (50.49%) than inU. japonica (8.85%). Both species showed a substantial decrease in optimal photosystem (PS) II efficiency (F_v/ F_m) with increasing PAR doses despite the degree of photoinhibition being more marked inU. japonica. After 24 h in dim light,U. pertusa rapidly and almost fully recovered F_v/ F_m within an hour whileU. japonica exhibited slow and incomplete PSII recovery over the full recovery period. A significant depression in photosynthetic activity with monochromatic UV-B radiation was observed inU. japonica only, followed by slight recovery in dim light. Light use efficiency and high irradiance tolerance may be important ecological axes along which the niche separation of green foliose algae occurs in Korean coastal waters.     

Photoinhibition of photosynthesis in Minquartia guianensis and Swietenia macrophylla inferred by monitoring the initial fluorescence

We assessed the effect of the exposure to full sunlight (5, 35, and 120 min, i.e. T₅, T₃₅, and T₁₂₀) on fluorescence parameters of two young tropical trees, Swietenia macrophylla, a gap-demanding species, and Minquartia guianensis, a shade tolerant species. Fluorescence parameters (F₀, F_m, F_v/F_m) were recorded before treatments and after the transition to low irradiance (LI). Recovery from photoinhibition (measured as F_v/F_m) was monitored for 24 h at LI. In Swietenia, an almost complete restoration of the F_v/F_m values occurred in T₅ and T₃₅ plants, when a rise in F₀ was observed after the transition to LI. This was inferred as indicative of dynamic photoinhibition. T₁₂₀ led to a decline in F₀ in Minquartia, but not in Swietenia. The plants of both species were unable to recovery from photoinhibition after 24 h at LI, when F₀ declined or remained unchanged. This was interpreted as indicative of chronic photoinhibition. Compared with Swietenia, Minquartia was more susceptible to photoinhibition, as indicated by lower F_v/F_m values.     

Photosynthetic characteristics and UV stress tolerance of Antarctic seaweeds along the depth gradient

The photosynthetic characteristics through P-E curves and the effect of UV radiation on photosynthesis (measured as rapid adjustment of photochemistry, F _v/F _m) and DNA damage (as formation of CPDs) were studied in field specimens of green, red and brown algae collected from the eulittoral and sublittoral zone of Fildes Peninsula (King George Island, Antarctic). The content of phenolic compounds (phlorotannins) and the antioxidant activity were also studied in seven brown algae from 0 to 40 m depth. The results indicated that photosynthetic efficiency (α) was high and did not vary between different species and depths, while irradiances for saturation (E _k) averaged 55 μmol m^?2 s^?1 in subtidal and 120 μmol m^?2 s^?1 in eulittoral species. The studied species exhibited notable short-term UV tolerance along the vertical zonation. In intertidal and shallow water species, decreases in F _v/F _m by UV radiation were between 0 and 18 %, while in sublittoral algae, decreases in F _v/F _m varied between 3 and 35 % relative to PAR treatment. In all species, recovery was high averaging 84–100 %. The formation of CPDs increased (15–150 %) under UV exposure, with the highest DNA damage found in some subtidal species. Phlorotannin content varied between 29 mg g^?1 DW in Ascoseira mirabilis from 8 m depth and 156 mg g^?1 DW in Desmarestia menziesii from 17 m depth. In general, phlorotannin concentrations were constitutively high in deeper sublittoral brown algae, which were correlated with higher antioxidant activities of algal extracts and low decreases in photosynthesis. UV radiation caused a strong decrease in phlorotannin content in the deep-water Himantothallus grandifolius, whereas in D. menziesii and Desmarestia anceps, induction of the synthesis of phlorotannins by UV radiation was observed. The antioxidant activity was in general less affected by UV radiation.     

High resistance to low-temperature photoinhibition in two alpine, snowbank species

At high elevation, the combination of low temperature and high solar irradiance may be particularly conducive to the low‐temperature photoinhibition of photosynthesis (LTP). Microclimate and photosynthesis were measured in situ in Caltha leptosepala and Erythronium grandiflorum, alpine perennials that may experience even lower temperatures and higher sunlight (PFD, photon flux density) than other alpine plants due to their proximity to snowbanks. Light‐saturated CO₂ assimilation (A_sat) and chlorophyll a fluorescence were measured for naturally frosted and non‐frosted plants that also experienced either natural or experimentally‐lowered sunlight. Following several hours of full sunlight exposure, A_sat in both species was not different in leaves that experienced either frostless nights, shading (ca. 1200 versus 2000 μmol m^?2 s^?1 PFD), or the combination of frostless nights and shade, compared to leaves that experienced frost followed by full sunlight. However, increases in the maximum efficiency of photosystem II photochemistry occurred following either frostless nights (change in F_v/F_m=5.3%; P<0.001), experimental shade (4.0%; P<0.1), or the combination of frostless nights and shade (8.4%; P<0.001) in C. leptosepala. Corresponding increases in F_v/F_m were less in E. grandiflorum (1.8% following frostless nights, P<0.05; 3.7% with shade, P<0.05; and 5.1% with both, P<0.001). Plants of E. grandiflorum in the process of emerging through snowbanks had a 10% increase in F_v/F_m with experimental shading (P<0.05). In both species, depressions in F_v/F_m that resulted from natural frost and high sunlight exposure recovered fully by sunset the same day, and depressed F_v/F_m was associated with greater non‐photochemical quenching. Thus, only slight and reversible LTP was apparent, and both species appeared well‐adapted for maintaining carbon gain on days following frost and high sunlight exposure.     

Response of two Antarctic bryophytes to stratospheric ozone depletion

We report a study which measured changes to the radiative environment arising from stratospheric O₃ depletion at Rothera Point on the western Antarctic Peninsula (67°S, 68°W) and subsequent associations between these changes and the pigmentation and maximum quantum yield of photochemistry (F_v/F_m) of two Antarctic bryophytes, the liverwort Cephaloziella varians and the moss Sanionia uncinata. We found a strong relationship between O₃ column depth and the ratio of UV‐B to PAR irradiance (F_UV‐B/F_PAR) recorded at ground level. Weaker, but significant, associations were also found between O₃ column depth and noon irradiances and daily doses of unweighted and biologically effective UV‐B radiation received at ground level. Regression analyses indicated that F_UV‐B/F_PAR and daily dose of unweighted UV‐B were best predictors for concentrations of total carotenoids and UV‐B screening pigments extracted from bryophyte tissues. Concentrations of these pigments were loosely but significantly positively associated with O₃‐dependent irradiance parameters. HPLC analyses of carotenoids also suggested that both species increased the synthesis of neoxanthin during periods of O₃ depletion. Violaxanthin, lutein, zeaxanthin and b,bββ‐carotene concentrations were also apparently influenced by O₃ reduction, but not consistently across both bryophyte species. Concentrations of chlorophylls a and b were apparently unaffected by O₃ depletion. No direct associations between F_v/F_m and O₃‐dependent irradiance parameters were found. However stepwise multiple regression analyses suggested that the production of UV‐B screening pigments conferred protection from elevated F_UV‐B/F_PAR on F_v/F_m in both species and that carotenoids conferred protection on F_v/F_m in Sanionia. Our data suggest that changes to the radiative environment associated with stratospheric O₃ depletion influence the pigmentation of two Antarctic bryophytes, but that F_v/F_m is unaffected, at least in part because of rapid synthesis of protective pigments.     

Acclimation of tropical tree seedlings to excessive light in simulated tree-fall gaps

Acclimation to periodic high‐light stress was studied in tree seedlings from a neotropical forest. Seedlings of several pioneer and late‐succession species were cultivated under simulated tree‐fall gap conditions; they were placed under frames covered with shade cloth with apertures of different widths that permitted defined periods of daily leaf exposure to direct sunlight. During direct sun exposure, all plants exhibited a marked reversible decline in potential photosystem II (PSII) efficiency, determined by means of the ratio of variable to maximum Chl a fluorescence (F_v/F_m). The decline in F_v/F_m under full sunlight was much stronger in late‐succession than in pioneer species. For each gap size, all species exhibited a similar degree of de‐epoxidation of violaxanthin in direct sunlight and similar pool sizes of xanthophyll cycle pigments. Pool sizes increased with increasing gap size. Pioneer plants possessed high levels of β‐carotene that also increased with gap size, whereas α‐carotene decreased. In contrast to late‐succession plants, pioneer plants were capable of adjusting their Chl a/b ratio to a high value in wide gaps. The content of extractable UV‐B‐absorbing compounds was highest in the plants acclimated to large gaps and did not depend on the successional status of the plants. The results demonstrate a better performance of pioneer species under high‐light conditions as compared with late‐succession plants, manifested by reduced photoinhibition of PSII in pioneer species. This was not related to increased pool size and turnover of xanthophyll cycle pigments, nor to higher contents of UV‐B‐absorbing substances. High β‐carotene levels and increased Chl a/b ratios, i.e. reduced size of the Chl a and b binding antennae, may contribute to photoprotection in pioneer species.     

Solar UV-B radiation, associated with ozone depletion, inhibits the Antarctic terrestrial microalga, Stichococcus bacillaris

This study shows that increased UV-B arising from stratospheric ozone depletion over Antarctica reduced cell viability and the maximum quantum yield of photochemistry (F _v/F _m) in a unicellular terrestrial microalga, Stichococcus bacillaris. In the UV waveband, rates of F _v/F _m decline increased with decreasing wavelength. Photosynthetically active radiation (PAR) also reduced F _v/F _m, though less than UV radiation. Further experiments under different ozone column thicknesses showed a significantly greater decline in cell viability and F _v/F _m under ozone depletion compared with non-depleted conditions. The inhibitory effects of ambient solar radiation suggest that S. bacillaris is unlikely to inhabit soil surfaces, but colonises shaded areas beneath soil surface particles. During periods of ozone depletion, increases in the ratio of UV-B:PAR may reduce the thickness of the sub-surface zone where light conditions are suitable for colonisation by this alga.     

Water bird guilds and their feeding connections in the Bodrogzug, Hungary

Thalli of the intertidal Phaeophyte Fucus spiralis L. and the subtidal Chlorophyte Ulva olivascens Dangeard were exposed to artificial UV-A, UV-B and photosynthetically active radiation (PAR) by combination of PAR + UV-A + UV-B (PAB), PAR + UV-A (PA) and PAR (P) treatments. UV-A enhanced photosynthesis and stimulated carbonic anhydrase (CA) and nitrate reductase (NR) in F. spiralis whilst PAR only had an inhibitory effect in this species. U. olivascens suffered chronic photoinhibition in all the treatments as evidenced by reduced maxima photosynthesis (P_max) and photosynthetic efficiency (α). Non stimulatory effect was observed upon CA and NR in this species. Our results showed that artificial UV radiation triggered opposite responses in both species. We suggest that differences shown by both species might be related to their location in the rocky shore and their ability to sense UV. We propose that the ratio UV:PAR acts as an environmental signal involved in the control of photosynthesis as shown by pronounced inhibition in samples exposed to only PAR. We also suggest that UV-regulated photosynthesis would be related to carbon (C) and nitrogen (N) cycles, regulating feedback processes that control C and N assimilation.     

Gap size effects on photoinhibition in understorey saplings in tropical rainforest

The sudden increase in irradiance after canopy disturbance in primary forest together with the accompanying increase in leaf temperatures is known to cause photoinhibition in shade acclimated foliage of understorey plants. We hypothesized that there is species specific variation among understorey saplings in the magnitude of photoinhibition in response to gap creation, which is related to their requirement for overstorey disturbance. Eleven more or less circular gaps were created varying in size from 60 up to 1459 m². Photoinhibition was assessed by determining predawn and midday F_v/F_m using chlorophyll fluorescence at two occasions during the first 3 weeks after creation of the gaps. The light environment was assessed using hemispherical photography. Five species that occurred in sufficient numbers in the understorey after gap creation were measured. They all showed an increase of photoinhibition with increasing gap size. Variation in exposure to direct sunlight within gaps contributed also to variation in photoinhibition. Dynamic photoinhibition, the overnight increase in F_v/F_m, was about 20% of total photoinhibition as measured at midday. The species responded quantitatively different. Oxandra asbeckii was most sensitive as evident from a decrease of predawn F_v/F_m from 0.79 in the understorey of undisturbed forest to 0.70 in the smallest and further to 0.41 in the largest gaps. Catostemma fragrans, the least sensitive species showed hardly any photoinhibition in the smallest gaps and less in the largest ones, whereas Lecythis concertiflora, Licania heteromorpha, and Chlorocardium rodiei had intermediate responses. Species rank order in sensitivity to photoinhibition was maintained across the whole range of gap sizes. The relationship between sensitivity to photoinhibition and species-specific gap size preference for regeneration is discussed.     

Photosynthetic Fluorescence Induction and Oxygen Production in Corallinacean Algae Measured on Site

Photoinhibition of photosynthesis was measured in two Mediterranean Corallinaceae, Jania rubens and Corallina mediterranea, using pulse-amplitude modulation (PAM) fluorescence and oxygen production on site. Both algae were found to be adapted to low irradiances of solar radiation and easily inhibited by exposure to excessive radiation. Both algae were impaired even in their natural habitat under overhanging rocks which protected them from direct solar radiation, except for a few hours in the early morning. Recovery from photoinhibition of both the photosynthetic quantum yield, defined as F_v′/F_m′, and oxygen production took several hours and was not complete. Judging from both parameters indicated above, Jania seems to be even more sensitive than Corallina, even though the former alga was found in more exposed habitats.     

EFFECTS OF UV-B-INDUCED DNA DAMAGE AND PHOTOINHIBITION ON GROWTH OF TEMPERATE MARINE RED MACROPHYTES: HABITAT-RELATED DIFFERENCES IN UV-B TOLERANCE

The sensitivity to UV-B radiation (UVBR: 280–315 nm) was tested for littoral (Palmaria palmata[L.] O. Kuntze, Chondrus crispus Stackhouse) and sublittoral (Phyllophora pseudoceranoides S. G. Gmelin, Rhodymenia pseudopalmata[Lamouroux] Silva, Phycodrys rubens[L.] Batt, Polyneura hilliae[Greville] Kylin) red macrophytes from Brittany, France. Algal fragments were subjected to daily repeated exposures of artificial UVBR that were realistic for springtime solar UVBR at the water surface in Brittany. Growth, DNA damage, photoinhibition, and UV-absorbing compounds were monitored during 2 weeks of PAR + UV-A radiation (UVAR) + UVBR, whereas PAR + UVAR and PAR treatments were used as controls. The littoral species showed a higher UV tolerance than the sublittoral species. After 2 weeks, growth of P. palmata and C. crispus was not significantly affected by UVBR, and DNA damage, measured as the number of cyclobutane-pyrimidine dimers per 10⁶ nucleotides, was negligible. Photoinhibition, determined as the decline in optimal quantum yield, was low and decreased during the course of the experiment, coinciding with the production of UV-absorbing compounds in these species. In contrast, no UV-absorbing compounds were induced in the sublittoral species. Growth rates of P. pseudoceranoides and R. pseudopalmata were reduced by 40% compared with the PAR treatment. Additionally, constant levels of DNA damage and pronounced photoinhibition were observed after the UVBR treatments. Growth was completely halted for Phycodrys rubens and Polyneura hilliae, whereas DNA damage accumulated in the course of the experiment. Because Phycodrys rubens and Polyneura hilliae showed the same degree of photoinhibition as the other sublittoral species, it appears that the accumulation of DNA damage may have been responsible for the complete inhibition of growth. The results suggest an important role of DNA repair pathways in determining the UV sensitivity in red macrophytes.     

Susceptibility to photoinhibition of three deciduous broadleaf tree species with different successional traits raised under various light regimes

The susceptibility to photoinhibition of tree species from three different successional stages were examined using chlorophyll fluorescence and gas exchange techniques. The three deciduous broadleaf tree species were Betula platyphylla var. japonica, pioneer and early successional, Quercus mongolica, intermediate shade‐tolerant and mid‐successional, and Acer mono, shade‐tolerant and late successional. Tree seedlings were raised under three light regimes: full sunlight (open), 10% full sun, and 5% full sun. Susceptibility to photoinhibition was assessed on the basis of the recovery kinetics of the ratio of vaviable to maximum fluorescence (F_v/F_m) of detached leaf discs exposed to about 2000 μmol m^?1 s^?1 photon flux density (PFD) for 2 h under controlled conditions (25 to 28 °C, fully hydrated). Differences in susceptibility to photodamage among species were not significant in the open and 10% full sun treatments. But in 5% full sun, B. platyphylla sustained a significantly greater photodamage than other species, probably associated with having the lowest photosynthetic capacity indicated by light‐saturated photosynthetic rate (B. platyphylla, 9·87, 5·85 and 2·82; Q. mongolica, 8·05, 6·28 and 4·41; A. mono, 7·93, 6·11 and 5·08 μmol CO₂ m^?1 s^?1for open, 10% and 5% full sun, respectively). To simulate a gap formation and assess its complex effects including high temperature and water stress in addition to strong light on the susceptibility to photoinhibition, we examined photoinhibition in the field by means of monitoring ΔF/F′_m on the first day of transfer to natural daylight. Compared with ΔF/F′_m in AM, the lower ΔF/F′_m in PM responding to lower PFD following high PFD around noon indicated that photoinhibition occurred in plants grown in 10 and 5% full sun. The diurnal changes of ΔF/F′_m showed that Q. mongolica grown in 5% full sun was less susceptible to photoinhibition than A. mono although they showed little differences both in photosynthetic capacity in intact leaves and susceptibility to photoinhibition based on leaf disc measurements. These results suggest that shade‐grown Q. mongolica had a higher tolerance for additional stresses such as high temperature and water stress in the field, possibly due to their lower plasticity in leaf anatomy to low light environment.     

Susceptibility of green leaves and green flower petals of CAM orchid <Emphasis Type="Italic">Dendrobium</Emphasis> cv. Burana Jade to high irradiance under natural tropical conditions

Photosynthetic rates of green leaves (GL) and green flower petals (GFP) of the CAM plant Dendrobium cv. Burana Jade and their sensitivities to different growth irradiances were studied in shade-grown plants over a period of 4 weeks. Maximal photosynthetic O₂ evolution rates and CAM acidities [dawn/dusk fluctuations in titratable acidity] were higher in leaves exposed to intermediate sunlight [a maximal photosynthetic photon flux density (PPFD) of 500–600 μmol m⁻² s⁻¹] than in leaves grown under full sunlight (a maximal PPFD of 1 000–1 200 μmol m⁻² s⁻¹) and shade (a maximal PPFD of 200–250 μmol m⁻² s⁻¹). However, these two parameters of GFP were highest in plants grown under the shade and lowest in full sun-grown plants. Both GL and GFP of plants exposed to full sunlight had lower predawn F_v/F_m [dark adapted ratio of variable to maximal fluorescence (the maximal photosystem 2 yield without actinic irradiation)] than those of shade-grown plants. When exposed to intermediate sunlight, however, there were no significant changes in predawn F_v/F_m in GL whereas a significant decrease in predawn F_v/F_m was found in GFP of the same plant. GFP exposed to full sunlight exhibited a greater decrease in predawn F_v/F_m compared to those exposed to intermediate sunlight. The patterns of changes in total chlorophyll (Chl) content of GL and GFP were similar to those of F_v/F_m. Although midday F_v/F_m fluctuated with prevailing irradiance, changes of midday F_v/F_m after exposure to different growth irradiances were similar to those of predawn F_v/F_m in both GL and GFP. The decreases in predawn and midday F_v/F_m were much more pronounced in GFP than in GL under full sunlight, indicating greater sensitivity in GFP to high irradiance (HI). In the laboratory, electron transport rate and photochemical and non-photochemical quenching of Chl fluorescence were also determined under different irradiances. All results indicated that GFP are more susceptible to HI than GL. Although the GFP of Dendrobium cv. Burana Jade require a lower amount of radiant energy for photosynthesis and this plant is usually grown in the shade, is not necessarily a shade plant.     

Diurnal changes in the photochemical efficiency of the symbiotic dinoflagellates (Dinophyceae) of corals: photoprotection, photoinactivation and the relationship to coral bleaching

The photochemical efficiency of symbiotic dinoflagellates within the tissues of two reef‐building corals in response to normal and excess irradiance at water temperatures < 30 °C were investigated using pulse amplitude modulated (PAM) chlorophyll fluorescence techniques. Dark‐adapted F_v/F_m showed clear diurnal changes, decreasing to a low at solar noon and increasing in the afternoon. However, F_v/F_m also drifted downwards at night or in prolonged darkness, and increased rapidly during the early morning twilight. This parameter also increased when the oxygen concentration of the water holding the corals was increased. Such changes have not been described previously, and most probably reflect state transitions associated with PQ pool reduction via chlororespiration. These unusual characteristics may be a feature of an endosymbiotic environment, reflective of the well‐documented night‐time tissue hypoxia that occurs in corals. F_v/F_m decreased to 0·25 in response to full sunlight in shade‐acclimated (shade) colonies of Stylophora pistillata, which is considerably lower than in light‐acclimated (sun) colonies. In sun colonies, the reversible decrease in F_v/F_m was caused by a lowering of F_m and F_o suggesting photoprotection and no lasting damage. The decrease in F_v/F_m, however, was caused by a decrease in F_m and an increase in F_o in shade colonies suggesting photoinactivation and long‐term cumulative photoinhibition. Shade colonies rapidly lost their symbiotic algae (bleached) during exposure to full sunlight. This study is consistent with the hypothesis that excess light leads to chronic damage of symbiotic dinoflagellates and their eventual removal from reef‐building corals. It is significant that this can occur with high light conditions alone.     

Research note: Irradiance of photosynthetically active radiation determines ultraviolet-susceptibility of photosynthesis in Ulva lactuca L.(Chlorophyta)

The respective ratio of photosynthetically active to ultraviolet radiation is of crucial importance to results obtained in ultraviolet (UV)‐research on photoautotrophic organisms. Specimens of the green macroalga Ulva lactuca L. were exposed to a constant irradiance of UV‐radiation at increasing irradiances of photosynthetically active radiation (PAR). The effects of experimental irradiance and spectral composition on photoinhibition of photosynthesis and its recovery were monitored by chlorophyll fluorescence measurements and the activity of the xanthophyll cycle was assessed by high performance liquid chromatography‐(HPLC) based pigment analysis. Results indicate a UV‐induced delay in recovery from PAR‐induced photoinhibition and a deceleration of violaxanthin conversion within the xanthophyll cycle due to the presence of UV‐radiation. Also the concentration of the protective pigment lutein increased considerably and could be indicative of the existence of an additional light‐protective mechanism, as, for example, the lutein‐epoxid cycle in Ulva. In total, results clearly show that the extent of UV‐induced inhibition of photosynthesis to be found in UV‐exposure experiments is highly dependent on the irradiance of background photosynthetically active radiation: with increasing irradiance of PAR the UV‐effects were diminished. Exemplified by the green algae Ulva lactuca this study demonstrates the crucial importance of the ratios of PAR:UV applied in UV‐research, particularly when conducting laboratory experiments in an ecological context.