CAROTENOID COMPOSITION OF MARINE RED ALGAE1

Photosynthetic organisms possess carotenoids that function either as accessory, photoprotective, or structural pigments. Therefore, the carotenoid profile provides information about certain photoacclimation and photoprotection responses. Carotenoids are also important chemosystematic markers because specific enzymes mediate each step of carotenoid biosynthesis. For red algae, diverse and often contradictory carotenoid compositions have been reported. As a consequence, it is difficult to infer the physiological importance of carotenoids in Rhodophyta. To characterize the relationship between carotenoid composition, rhodophycean phylogeny, and the presence of potentially photoprotective pigments, we analyzed the carotenoid composition of 65 subtropical species from 12 orders and 18 rhodophyte families. Our results showed that red algae do not present a unique carotenoid profile. However, a common profile was observed up to the level of order, with exception of the Ceramiales and the Corallinales. The main difference between profiles is related to the xanthophyll that represents the major carotenoid. In some species lutein is the major carotenoid while in others it is substituted by zeaxanthin or antheraxanthin. The presence of this epoxy carotenoid together with the presence of violaxanthin that are xanthophyll cycle (XC)‐related pigments was found in four of the 12 analyzed orders. The carotenoid pigment profiles are discussed in relation to Rhodophyta phylogeny, and it is suggested that the xanthophyll cycle‐related pigments appeared early in the evolution of eukaryotic phototrophs.     

Violaxanthin cycle pigment de-epoxidation and thermal dissipation of light energy in three boreal species of evergreen conifer plants

We studied carotenoid composition and chlorophyll fluorescence in two-year-old needles from Siberian spruce (Picea obovata (L.) Karst.), Siberian fir (Abies sibirica L.), and common juniper (Juniperus communis L.). The highest values of maximum PSII photochemical activity (F _v/F _m) equaling 0.82–0.85 were observed in July–September. The decrease in F _v/F _m in December–March was more pronounced in juniper (down to 0.15) than in spruce and fir (0.45–0.50). In May, we observed a nearly complete recovery in maximum PSII photochemical activity in fir and spruce (0.72–0.77), while in juniper, the F _v/F _m value was notably lower (0.65–0.67). The amount of thermal dissipation of energy absorbed by PSII LHC did not exceed 30% in summer and equaled 60–90% in winter and early spring. The carotenoid pool consisted mainly of xanthophylls, among which lutein (70%), neoxanthin (7–10%), and a violaxanthin cycle (VXC) component — violaxanthin (3–15%) were constantly present. The accumulation of two other VXC pigments—zeaxanthin and antheraxanthin, was noted in December–March. In July, these xanthophylls were not identified. We discovered a direct connection between VXC pigment de-epoxidation level and light energy thermal dissipation in boreal conifer leaves. Such association reflects the non-species-specific character of the mechanism for quenching zeaxanthin-dependent nonphotochemical chlorophyll fluorescence in PSII LHC in winter and spring.     

Close relationship between the state of the xanthophyll cycle pigments and photosystem II efficiency during recovery from winter stress

The potential involvement of the xanthophyll cycle in photoprotection of overwintering evergreen plants was investigated. Leaves from five evergreen species. Pseudotsuga menziesii, Pinus panderosa, Euonyums kiautschovicus. Mahonia repens and Malva neglecta, were collected from the field predawn during winter and transferred to the laboratory where chlorophyll fluorescence emission as well as the chlorophyll and carotenoid composition were ascertained periodically for 4.5 days. Leaves and needles from all species were found to have retained large amounts of the xanthophyll cycle pigments zeaxanthin and antheraxanthin, and they exhibited sustained low values of the intrinsic efficiency of photosystem II (PSII; measured as the ratio of variable to maximal fluorescence, F_v/F_m) upon collection. The increase in PSII efficiency was biphasic, with a rapid phase (requiring several hours) and a slow phase (requiring several days). Changes in the conversion state of the xanthophyll cycle were found to correlate with increases in PSII efficiency in both phases, with the latter phase involving large increases in both F_m (maximal fluorescence) and F_o (minimal fluorescence) throughout the period of recovery. The relationship between F_m quenching (expressed as nonphotochemical or Stern-Volmer quenching [NPQ] of F_m, i.e. F_m/ F_m–1) and F_o quenching (F_o/F_o–1) was linear, as expected for changes in xanthophyll cycle-dependent energy dissipation in the antenna complexes. Furthermore, the relationship between F_v/F_m and NPQ during recovery followed the theoretical relationship predicted for changes in the rate constant for energy dissipation in the antenna complexes. This fit between the theoretical relationship and the actual data indicates that all changes in NPQ or F_v/F_m can be accounted for by changes in this rate constant. The results suggest a role for the photoprotective xanthophyll cycle-dependent dissipation process in the lowered efficiency of PSII observed in coldstressed evergreen plants in the field.     

PSII photochemistry, thermal energy dissipation, and the xanthophyll cycle in Kalanchoë daigremontiana exposed to a combination of water stress and high light

Kalanchoë daigremontiana, a CAM plant grown in a greenhouse, was subjected to severe water stress. The changes in photosystem II (PSII) photochemistry were investigated in water‐stressed leaves. To separate water stress effects from photoinhibition, water stress was imposed at low irradiance (daily peak PFD 150 μmol m^?2 s^?1). There were no significant changes in the maximal efficiency of PSII photochemistry (F_v/F_m), the traditional fluorescence induction kinetics (OIP) and the polyphasic fluorescence induction kinetics (OJIP), suggesting that water stress had no direct effects on the primary PSII photochemistry in dark‐adapted leaves. However, PSII photochemistry in light‐adapted leaves was modified in water‐stressed plants. This was shown by the decrease in the actual PSII efficiency (Φ_PSII), the efficiency of excitation energy capture by open PSII centres (F_v′/F_m′), and photochemical quenching (q_P), as well as a significant increase in non‐photochemical quenching (NPQ) in particular at high PFDs. In addition, photoinhibition and the xanthophyll cycle were investigated in water‐stressed leaves when exposed to 50% full sunlight and full sunlight. At midday, water stress induced a substantial decrease in F_v/F_m which was reversible. Such a decrease was greater at higher irradiance. Similar results were observed in Φ_PSII, q_P, and F_v′/F_m′. On the other hand, water stress induced a significant increase in NPQ and the level of zeaxanthin via the de‐epoxidation of violaxanthin and their increases were greater at higher irradiance. The results suggest that water stress led to increased susceptibility to photoinhibition which was attributed to a photoprotective process but not to a photodamage process. Such a photoprotection was associated with the enhanced formation of zeaxanthin via de‐epoxidation of violaxanthin. The results also suggest that thermal dissipation of excess energy associated with the xanthophyll cycle may be an important adaptive mechanism to help protect the photosynthetic apparatus from photoinhibitory damage for CAM plants normally growing in arid and semi‐arid areas where they are subjected to a combination of water stress and high light.     

Cellular responses to elevated light levels in Fucus spiralis embryos during the first days after fertilization

Cellular responses of 1‐, 2‐ and 4‐d‐old Fucus spiralis embryos subjected to a single dose of elevated photosynthetically active photon flux density (PPFD), with or without ultraviolet (UV) radiation, were investigated by measuring the effects on the effective quantum yield of photosystem II (ΔF / F_m′) and intracellular production of active oxygen species (AOS). Production of AOS was determined by the in vivo conversion of 5‐(and‐6)‐chloromethyl‐2′,7′‐dichlorodihydrofluorescein diacetate (CM‐DCFH₂‐DA) to the fluorescent compound dichlorofluorescein (DCF) using confocal laser scan microscopy (CLSM) and image analysis. The role of xanthophyll cycle pigments in photoprotection was also assessed. A rapid decline in ΔF / F_m′ was observed under all elevated light conditions. A correlation was found between non‐photochemical quenching and the de‐epoxidation ratio zeaxanthin/(zeaxanthin + violoxanthin). Active oxygen formation increased with PPFD and was higher in older embryos and when UVB was present. Two photoinhibition responses were recognized: (i) a rapid decline of the PSII yield due to the violoxanthin–zeaxanthin cycle (photoprotection), and (ii) a slower second‐phase decline, correlated with active oxygen production. Electron transport rate (ETR) increased with embryo age, and was correlated with AOS production. As a result of enhanced AOS production, there was a slow recovery of the PSII yield, in particular with increased effective UV dose. In general, embryos were able to recover from the imposed light conditions, but UVB had a more damaging effect. Overall, our data suggest that under natural conditions, embryos of F. spiralis are susceptible to elevated light levels, and that UVB radiation is an important stress factor.     

Analysis of xanthophyll cycle carotenoids and chlorophyll fluorescence in light intensity-dependent chlorophyll-deficient mutants of wheat and barley

Three light intensity-dependent Chl b-deficient mutants, two in wheat and one in barley, were analyzed for their xanthophyll cycle carotenoids and Chl fluorescence characteristics under two different growth PFDs (30 versus 600 mol photons·m^–2 s^–1 incident light). Mutants grown under low light possessed lower levels of total Chls and carotenoids per unit leaf area compared to wild type plants, but the relative proportions of the two did not vary markedly between strains. In contrast, mutants grown under high light had much lower levels of Chl, leading to markedly greater carotenoid to Chl ratios in the mutants when compared to wild type. Under low light conditions the carotenoids of the xanthophyll cycle comprised approximately 15% of the total carotenoids in all strains; under high light the xanthophyll cycle pool increased to over 30% of the total carotenoids in wild type plants and to over 50% of the total carotenoids in the three mutant strains. Whereas the xanthophyll cycle remained fairly epoxidized in all plants grown under low light, plants grown under high light exhibited a considerable degree of conversion of the xanthophyll cycle into antheraxanthin and zeaxanthin during the diurnal cycle, with almost complete conversion (over 90%) occurring only in the mutants. 50 to 95% of the xanthophyll cycle was retained as antheraxanthin and zeaxanthin overnight in these mutants which also exhibited sustained depressions in PS II photochemical efficiency (F_v/F_m), which may have resulted from a sustained high level of photoprotective energy dissipation activity. The relatively larger xanthophyll cycle pool in the Chl b-deficient mutant could result in part from the reported concentration of the xanthophyll cycle in the inner antenna complexes, given that the Chl b-deficient mutants are deficient in the peripheral LHC-II complexes.Abbreviations A antheraxanthin - Chl chlorophyll - F_o and F_m minimal yield (at open PS II reaction centers) and maximal yield (at closed centers) of chlorophyll fluorescence in darkness - F level of fluorescence during illumination with photosynthetically active radiation - F_m maximal yield (at closed centers) of chlorophyll fluorescence during illumination with photosynthetically active radiation - (F_m–F)/F_m actual efficiency of PS II during illumination with photosynthetically active radiation - F_v/F_m+(F_m–F_o)/F_m intrinsic efficiency of PS II in darkness - LHC_II light-harvesting chlorophyll-protein complex of Photosystem II - PFD photon flux density (between 400 and 700 nm) - PS I Photosystem I - PS II Photosystem II - V violaxanthin - Z zeaxanthin     

Seasonal Differences in Photochemical Efficiency and Chlorophyll and Carotenoid Contents in Six Mediterranean Shrub Species under Field Conditions

The effects of summer and winter stress on the chlorophyll and carotenoid contents and photosystem 2 efficiency were examined in six Mediterranean scrub species. These six species belong to two different plant functional types: drought semi-deciduous (Halimium halimifolium L., Rosmarinus officinalis L., Erica scoparia L.) and evergreen sclerophylls (Juniperus phoenicea L., Pistacia lentiscus L., Myrtus communis L.). Two sites with different water availability were chosen. In the xerophytic site, despite they belong to two different functional types, R. officinalis and J. phoenicea showed a similar response. These were the most affected species in summer. H. halimifolium showed optimal values of F_v/F_m and non-significant seasonal changes in xanthophyll content. In the mesic site, E. scoparia and M. communis were apparently the most affected species by winter climatic conditions. P. lentiscus presented a pattern similar to H. halimifolium, except for elevated F₀ values. In all the studied species, lutein plus zeaxanthin content was negatively correlated with F_v/F_m in summer and with leaf water potential, thus indicating that the thermal dissipation of energy was a general pattern for all species. Under stress, plant response is more species-specific than dependent on its functional type.     

Effects of N supply on the accumulation of photosynthetic pigments and photoprotectors in <Emphasis Type="Italic">Gracilaria tenuistipitata</Emphasis> (Rhodophyta) cultured under UV radiation

We have studied the effects of nitrate supply under photosynthetic active radiation (PAR) plus ultraviolet radiation (UVR) exposure on photosynthetic pigments (chlorophyll a and carotenoids), photoprotective UV screen mycosporine-like amino acids (MAAs), and photosynthetic parameters, including the maximum quantum yield (F _v/F _m) and electron transport rate (ETR) on the red agarophyte Gracilaria tenuistipitata. Apical tips of G. tenuistipitata were cultivated under ten different concentrations of NO₃⁻ for 7 days. It has been shown that G. tenuistipitata cultured under laboratory conditions has the ability to accumulate high amounts of MAAs following a nitrate concentration-dependent manner under PAR + UVR. Two MAAs were identified, shinorine and porphyra-334. The relative concentration of the first increased under high concentrations of nitrate, while the second one decreased. The presence of antheraxanthin is reported for the first time in this macroalgae, which also contains zeaxanthin, lutein, and β-carotene. The accumulation of pigments, photoprotective compounds, and photosynthetic parameters of G. tenuistipitata is directly related to N availability. All variables decreased under low N supplies and reached constant maximum values with supplements higher than 0.5 mM NO₃⁻. Our results suggest a high potential to acclimation and photoprotection against stress factors (including high PAR and UVR) directly related to N availability for G. tenuistipitata.     

Strategies for severe drought survival and recovery in a Pyrenean relict species

In the context of future climate change new habitats will be threatened and unique species will be forced to develop different strategies to survive. Saxifraga longifolia Lapeyr. is an endemic species from the Pyrenees with a very particular habitat. We explored the capacity and strategies of S. longifolia plants to face different severities of drought stress under both natural conditions and controlled water stress followed by a re-watering period of 20 days. Our results showed a role for abscisic acid (ABA), salicylic acid (SA) and cytokinins (CKs) in plant survival from drought stress, and as the stress increased, ABA lost significance and SA appeared to be more associated with the response mechanisms. Moreover, photo-oxidative stress markers revealed that both xanthophyll cycles played a photoprotection role with a stronger participation of the lutein epoxide cycle as the stress was more intense. Severe drought decreased the maximum efficiency of photosystem II (F_v/F_m) below 0.45, being this the limit to survive upon rewatering. Overall, our results proved different strategies of S. longifolia plants to cope with drought stress and suggested a F_v/F_m threshold to predict plant survival in high-mountain environments.     

Xanthophyll cycle and light stress in nature: uniform response to excess direct sunlight among higher plant species

Photosystem II (PS II) efficiency, nonphotochemical fluorescence quenching, and xanthophyll cycle composition were determined in situ in the natural environment at midday in (i) a range of differently angled sun leaves ofEuonymus kiautschovicus Loesener and (ii) in sun leaves of a wide range of different plant species, including trees, shrubs, and herbs. Very different degrees of light stress were experienced by these leaves (i) in response to different levels of incident photon flux densities at similar photosynthetic capacities amongEuonymus leaves and (ii) as a result of very different photosynthetic capacities among species at similar incident photon flux densities (that were equivalent to full sunlight). ForEuonymus as well as the interspecific comparison all data fell on one single, close relationship for changes in intrinsic PSII efficiency, nonphotochemical fluorescence quenching, or the levels of zeaxanthin + antheraxanthin in leaves, respectively, as a function of the actual level of light stress. Thus, the same conversion state of the xanthophyll cycle and the same level of energy dissipation were observed for a given degree of light stress independent of species or conditions causing the light stress. Since all increases in thermal energy dissipation were associated with increases in the levels of zeaxanthin + antheraxanthin in these leaves, there was thus no indication of any form of xanthophyll cycle-independent energy dissipation in any of the twenty-four species or varieties of plants examined in their natural environment. It is also concluded that transient diurnal changes in intrinsic PSII efficiency in nature are caused by changes in the efficiency with which excitation energy is delivered from the antennae to PSII centers, and are thus likely to be purely photoprotective. Consequently, the possibility of quantifying the allocation of absorbed light into PSII photochemistry versus energy dissipation in the antennae from changes in intrinsic PSII efficiency is explored.Abbreviations A antheraxanthin - F actual level of fluorescence - F_a, F _o minimal fluorescence in the absence, presence of thylakoid energization - F_m, F _m maximal fluorescence in the absence, presence of thylakoid energization - F_m, - F)/F _m actual PSII efficiency ( = percent of absorbed light utilized in PSII photochemistry) - F_v/F_m, F _v /F_m/ PSII efficiency of open centers in the absence, presence of thylakoid energization - NPQ nonphotochemical fluorescence quenching - F_m/F _m - 1; q_p quenching coefficient for photochemical quenching - V violaxanthin - Z zeaxanthin     

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.     

Changes of fluorescence and xanthophyll pigments during dehydration in the resurrection plantSelaginella lepidophylla in low and medium light intensities

The changes in photosynthetic efficiency and photosynthetic pigments during dehydration of the resurrection plantSelaginella lepidophylla (from the Chiuhahuan desert, S.W. Texas, USA) were examined under different light conditions. Changes in the photosynthetic efficiency were deduced from chlorophyll a fluorescence measurements (F_o, F_m, and F_v) and pigment changes were measured by HPLC analysis. A small decrease in F_v/F_m was seen in hydrated stems in high light (650 μmol photons·m⁻²·s⁻¹) but not in low light (50 μmol photons·m⁻²·s⁻¹). However, a pronounced decline in F_v/F_m was observed during dehydration in both light treatments, after one to two hours of dehydration. A rise in F_o was observed only after six to ten hours of dehydration. Concomitant with the decrease in photosynthetic efficiency during dehydration a rise in the xanthophyll zeaxanthin was observed, even in low-light treatments. The increase in zeaxanthin can be related to previously observed photoprotective non-photochemical quenching of fluorescence in dehydrating stems ofS. lepidophylla. We hypothesize that under dehydrating conditions even low light levels become excessive and zeaxanthin-related photoprotection is engaged. We speculate that these processes, as well as stem curling and self shading (Eickmeier et al. 1992), serve to minimize photoinhibitory damage toS. lepidophylla during the process of dehydration.     

Operation of the xanthophyll cycle in the seagrass Zostera marina in response to variable irradiance

Changes in the photobiology and photosynthetic pigments of the seagrass Zostera marina from Chesapeake Bay (USA) were examined under a range of natural and manipulated irradiance regimes. Photosynthetic activity was assessed using chlorophyll-a fluorescence, and photosynthetic pigments were measured by HPLC. Large changes in the violaxanthin, zeaxanthin, and antheraxanthin content were concomitant with the modulation of non-photochemical quenching (NPQ). Photokinetics (F_v/F_m, rapid light curves (RLC), and non-photochemical quenching) varied as a result of oscillating irradiance and were highly correlated to xanthophyll pigment content. Zeaxanthin and antheraxanthin concentrations increased under elevated light conditions, while violaxanthin increased in darkened conditions. Unusually high concentrations of antheraxanthin were found in Z. marina under a wide range of light conditions, and this was associated with the partial conversion of violaxanthin to zeaxanthin. These results support the idea that xanthophyll intermediate pigments induce a photoprotective response during exposure to high irradiances in this seagrass.     

Photosynthetic Characteristics and the Ratios of Chlorophyll, β-Carotene,and the Components of the Xanthophyll Cycle Upon a Sudden Increase in Growth Light Regime in Several Plant Species*

Among three species, Gossypium hirsutum, Rhizophora mangle, and Monstera deliciosa, which were transferred from low to high growth PFD, only small decreases in the efficiency of photochemical energy conversion were observed in those plants which exhibited an increase in photosynthetic capacity. Leaves of plants which showed no increase in photosynthetic capacity experienced a continuous decrease in photochemical efficiency, accompanied by a more pronounced loss of chlorophyll than that observed in the former group. In all species marked increases in the xanthophyll/β-carotene ratio resulted from small increases in lutein, and several-fold increases in the sum of the three components of the xanthophyll cycle, zeaxanthin, antheraxanthin, and violaxanthin. A strong increase in the level of zeaxanthin was only partially matched by a decrease of violaxanthin to zero, and was further paralleled by a decrease in β-carotene. Antiparallel changes in the sum of zeaxanthin + antheraxanthin + violaxanthin and β-carotene between morning and evening were observed in all species. These diel changes were overlaid on a net increase in β-carotene as well as total carotenoid content in those plants in which photosynthetic capacity increased. In those, however, which exhibited no photosynthetic acclimation upon transfer to high light, a decrease in both β-carotene and total carotenoid content was observed. Rhizophora mangle grown at 100 % seawater exhibited a particularly high capacity for increasing the level of zeaxanthin in response to high light.     

Transgenic tobacco with suppressed zeaxanthin formation is susceptible to stress-induced photoinhibition

Tobacco (Nicotiana tabacum cv. Xanthi) transformed with the antisense construct of tobacco violaxanthin de-epoxidase was analyzed for responses in growth chambers to both short and long-term stress treatments. Following a short-term (2 or 3 h) high-light treatment, antisense plants had a greater reduction in F_v/F_m relative to wild-type, indicating a greater susceptibility to photoinhibition. The responses of antisense plants to long-term stress were examined in two separate experiments, one with high light alone and the other wherein high light and water stress were combined. In the light-stress experiment, plants were grown at 1300 mol photons m^–2 s^–1 under a 12 h photoperiod. In the light and water-stress experiment, plants were grown under moderately high light of 900 mol photons m^–2 s^–1, under a 16 h photoperiod, in combination with water stress. Both conditions caused formation of high antheraxanthin and zeaxanthin levels in wild-type plants but not in antisense plants. In both cases, antisense plants showed significant reductions in F_v/F_m and total leaf-pigment content relative to wild-type. The data demonstrate a critical photoprotective function of the xanthophyll cycle-dependent energy dissipation in tobacco exposed suddenly to high amounts of excess light over extended times.This revised version was published online in October 2005 with corrections to the Cover Date.     

Photosynthesis and protective mechanisms during ageing in transgenic tobacco leaves with over-expressed cytokinin oxidase/dehydrogenase and thus lowered cytokinin content

The content of cytokinins (CKs), the plant inhibitors of the final phase of plant development, senescence, is effectively controlled by irreversible degradation catalysed by cytokinin oxidase/dehydrogenase (CKX). In transgenic tobacco, denoted as AtCKX, with over-expressed CKX causing lowered CK content, we investigated changes in the time courses of chlorophyll (Chl) and xanthophyll (violaxanthin, antheraxanthin, zeaxanthin, neoxanthin, and lutein) contents. We also determined parameters of slow Chl fluorescence kinetics such as minimum Chl fluorescence yield in the darkadapted state F₀, maximum quantum yield of PS2 photochemistry (F_v/F_m), maximum ratio of quantum yields of photochemical and concurrent non-photochemical processes in photosystem 2 (PS2), F_v/F₀, non-photochemical quenching (NPQ), and effective quantum yield of photochemical energy conversion in PS2 (Φ₂). We used three different developmental leaf stages, old, mature, and young, and compared this with time courses of these characteristics in leaves with natural CK levels. The parameters F_v/F_m, F_v/F₀, and Φ₂ were unchanged during ageing in AtCKX plants in contrast to control ones where a significant decrease in old leaves was found. In control plants F₀ increased during ageing, but in the oldest leaf a considerable decrease was observed. This could indicate progressive damage to PS2 reaction centres and then detachment and rapid degradation of Chl. This is in agreement with time course of Chl content. NPQ decreased with age and was similar in both plant types. We observed a decline of xanthophyll contents in the oldest leaves in both plant types, but the contents were enhanced in AtCKX compared to control plants, especially of neoxanthin. The higher xanthophyll contents in the transgenic plants contribute to a better photoprotection and the fluorescence parameters indicated that photosynthetic apparatus was in better condition compared to control and it consequently postponed the onset of leaf senescence.     

Contribution of the UV component of natural sunlight to photoinhibition of photosynthesis in six species of subtidal brown and red seaweeds

Field‐collected specimens of three species of Laminaria and three species of subtidal red algae (Delesseria sanguinea, Plocamium cartilagineum and Phyllophora pseudoceranoides) were exposed to natural summer sunlight on Helgoland (southern North Sea) for up to 4 h at 15 °C. Dark‐adapted variable fluorescence (F_v : F_m) was measured immediately after these treatments, and following 6, 24 and 48 h of recovery in moderate irradiances of white light. The response of plants to the full spectrum of natural sunlight was compared with that to PAR alone, UV‐A + visible, UV‐A + UV‐B, or UV‐A alone. The F_v : F_m values of all species were reduced to minimal values after 4 h in all of these treatments, but those of the more resistant species (Laminaria spp. and P. pseudoceranoides) were higher after shorter exposures to UV radiation alone than to PAR with or without UV. The recovery of F_v : F_m in all species was also more rapid in the two treatments that contained UV radiation alone than in those that included PAR. These results suggest that it is the high irradiances of PAR in natural sunlight which are responsible for the photoinhibition of photosynthesis of subtidal seaweeds and that the current ambient irradiances of UV radiation (either UV‐B or UV‐A) in northern temperate latitudes would not contribute significantly to this photoinhibition.     

LIGHT DEPENDENCY OF PHOTOSYNTHETIC RECOVERY DURING WETTING AND THE ACCLIMATION OF PHOTOSYNTHETIC APPARATUS TO LIGHT FLUCTUATION IN A TERRESTRIAL CYANOBACTERIUM NOSTOC COMMUNE1

The PSII photochemical activity in a terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault during rewetting was undetectable in the dark but was immediately recognized in the light. The maximum quantum yield of PSII (F_v/F_m) during rewetting in the light rose to 85% of the maximum within ～30 min and slowly reached the maximum within 6 h, while with rewetting in the darkness for 6 h and then exposure to light the recovery of F_v/F_m required only ～3 min. These results suggested that recovery of photochemical activity might depend on two processes, light dependence and light independence, and the activation of photosynthetic recovery in the initial phase was severely light dependent. The inhibitor experiments showed that the recovery of F_v/F_m was not affected by chloramphenicol (CMP), but severely inhibited by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) in the light, suggesting that the light‐dependent recovery of photochemical activity did not require de novo protein synthesis but required activation of PSII associated with electron flow to plastoquinone. Furthermore, the test indicated that the lower light intensity and the red light were of benefit to its activation of photochemical activity. In an outdoor experiment of diurnal changes of photochemical activity, our results showed that PSII photochemical activity was sensitive to light fluctuation, and the nonphotochemical quenching (NPQ) was rapidly enhanced at noon. Furthermore, the test suggested that the repair of PSII by de novo protein synthesis played an important role in the acclimation of photosynthetic apparatus to high light, and the heavily cloudy day was more beneficial for maintaining high photochemical activity.     

Nocturnally retained zeaxanthin does not remain engaged in a state primed for energy dissipation during the summer in two Yucca species growing in the Mojave Desert

Differently oriented leaves of Yucca schidigera and Yucca brevifolia were characterized in the Mojave Desert with respect to photosystem II and xanthophyll cycle activity during three different seasons, including the hot and dry summer, the relatively cold winter, and the mild spring season. Photosynthetic utilization of a high percentage of the light absorbed in PSII was observed in all leaves only during the spring, whereas very high levels of photoprotective, thermal energy dissipation were employed both in the summer and the winter season in all exposed leaves of both species. Both during the summer and the winter season, when energy dissipation levels were high diurnally, xanthophyll cycle pools (relative to either Chl or other carotenoids) were higher relative to the spring, and a nocturnal retention of high levels of zeaxanthin and antheraxanthin (Z + A) occurred in all exposed leaves of both species. Although this nocturnal retention of Z + A was associated with nocturnal maintenance of a low PSII efficiency (F_v/F_m) on a cold winter night, pre‐dawn F_v/F_m was high in (Z + A)‐retaining leaves following a warm summer night. This indicates nocturnal engagement of Z + A in a state primed for energy dissipation throughout the cold winter night – while high levels of retained Z + A were not engaged for energy dissipation prior to sunrise on a warm summer morning. Possible mechanisms for a lack of sustained engagement of retained Z + A for energy dissipation at elevated temperatures are discussed.