Light acclimation potential and xanthophyll cycle pigments in photoautotrophic suspension cells of Chenopodium rubrum

The present study investigates the light acclimation potential of photoautotrophic suspension culture cells of Chenopodium rubrum L. grown in 16 h light/8 h dark cycles. Typical features of sun/shade acclimation could be demonstrated in cultures grown at photon flux densities of 30 and 150 μmol m⁻² s⁻¹. Low light grown cells had lower chlorophyll a/b ratios, lower respiration rates and lower light compensation points than high light grown cells. Maximum photosynthetic rate per cell dry weight was highest in low light conditions, indicating that the cells did not enlarge their photosynthetic machinery upon exposure to high light. Transfer of cultures to 800 μmol m⁻² s⁻¹ caused photoinhibition as indicated by a decrease in photosynthetic efficiency and by the occurrence of a slowly reversible quenching of variable chlorophyll fluorescence. Extension of the photoinhibitory treatment over six light dark cycles did not result in further dramatic changes of these parameters, whereas the chlorophyll content per dry weight and the chlorophyll a/b ratio decreased. Measurements of photochemical quenching showed that the capability of the cells to dissipate excessive energy had increased during the acclimation process. The presence of the xanthophyll cycle pigments and the operation of the cycle could be demonstrated. In agreement with the putative photoprotective function of antheraxanthin and zeaxanthin these pigments could only be detected under photoinhibitory conditions. Prolonged photoinhibitory treatment resulted in increases in the xanthophyll pigment concentration but not of the potential to deepoxidate violaxanthin. The limited potential of the cells to accumulate zeaxanthin and antheraxanthin might indicate that the xanthophyll cycle is not the main factor determining their resistance to high light stress.     

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     

Application of heat stress in situ demonstrates a protective role of irradiation on photosynthetic performance in alpine plants

The impact of sublethal heat on photosynthetic performance, photosynthetic pigments and free radical scavenging activity was examined in three high mountain species, Rhododendron ferrugineum, Senecio incanus and Ranunculus glacialis using controlled in situ applications of heat stress, both in darkness and under natural solar irradiation. Heat treatments applied in the dark reversibly reduced photosynthetic performance and the maximum quantum efficiency of photosystem II (F_v/F_m), which remained impeded for several days when plants were exposed to natural light conditions subsequently to the heat treatment. In contrast, plants exposed to heat stress under natural irradiation were able to tolerate and recover from heat stress more readily. The critical temperature threshold for chlorophyll fluorescence was higher under illumination (T_c^′) than in the dark (T_c). Heat stress caused a significant de‐epoxidation of the xanthophyll cycle pigments both in the light and in the dark conditions. Total free radical scavenging activity was highest when heat stress was applied in the dark. This study demonstrates that, in the European Alps, heat waves can temporarily have a negative impact on photosynthesis and, importantly, that results obtained from experiments performed in darkness and/or on detached plant material may not reliably predict the impact of heat stress under field conditions.     

Iron deficiency-induced changes in the photosynthetic pigment composition of field-grown pear (Pyrus communis L) leaves

In this work we characterize the changes induced by iron deficiency in the pigment composition of pear (Pyrus communis L.) leaves grown under high light intensities in field conditions in Spain. Iron deficiency induced decreases in neoxanthin and β-carotene concomitantly with decreases in chlorophyll a, whereas lutein and carotenoids within the xanthophyll cycle were less affected. Iron deficiency caused major increases in the lutein/chlorophyll a and xanthophyll cycle pigments/chlorophyll a molar ratios. The chlorophyll a/chlorophyll b ratio increased in response to iron deficiency. The carotenoids within the xanthophyll cycle in iron-deficient and in iron-sufficient (control) leaves underwent epoxidations and de-epoxidations in response to ambient light conditions. In control leaves dark-adapted for several hours, most of the xanthophyll cycle pigment pool was in the epoxidated form vio-laxanthin, whereas iron-deficient leaves had significant amounts of zeaxanthin. Iron-deficient leaves also exhibited an increased non-photochemical quenching, supporting the possibility of a role for pigments within the xanthophyll cycle in photoprotection.     

Enhanced Employment of the Xanthophyll Cycle and Thermal Energy Dissipation in Spinach Exposed to High Light and N Stress

The involvement of the xanthophyll cycle in photoprotection of N-deficient spinach (Spinacia oleracea L. cv Nobel) was investigated. Spinach plants were fertilized with 14 mM nitrate (control, high N) versus 0.5 mM (low N) fertilizer, and grown under both high- and low-light conditions. Plants were characterized from measurements of photosynthetic oxygen exchange and chlorophyll fluorescence, as well as carotenoid and cholorophyll analysis. Compared with the high-N plants, the low-N plants showed a lower capacity for photosynthesis and a lower chlorophyll content, as well as a lower rate of photosystem II photosynthetic electron transport and a corresponding increase in thermal energy dissipation activity measured as nonphotochemical fluorescence quenching. The low-N plants displayed a greater fraction of the total xanthophyll cycle pool as zeaxanthin and antheraxanthin at midday, and an increase in the ratio of xanthophyll cycle pigments to total chlorophyll. These results indicate that under N limitation both the light-collecting system and the photosynthetic rate decrease. However, the increased dissipation of excess energy shows that there is excess light absorbed at midday. We conclude that spinach responds to N limitation by a combination of decreased light collection and increased thermal dissipation involving the xanthophyll cycle.     

Photoacclimation impacts the molecular features of photosystem supercomplexes in the centric diatom Thalassiosira pseudonana

In diatoms, light-harvesting processes take place in a specific group of proteins, called fucoxanthin chlorophyll a/c proteins (FCP). This group includes many members and represents the major characteristic of the diatom photosynthetic apparatus, with specific pigments bound (chlorophyll c, fucoxanthin, diadino- and diatoxanthin besides chlorophyll a). In thylakoids, FCP and photosystems (PS) form multimeric supercomplexes.In this study, we compared the biochemical properties of PS supercomplexes isolated from Thalassiosira pseudonana cells grown under low light or high light conditions, respectively. High light acclimation changed the molecular features of the PS and their ratio in thylakoids. In PSII, no obvious changes in polypeptide composition were observed, whereas for PSI changes in one specific group of FCP proteins were detected. As reported before, the amount of xanthophyll cycle pigments and their de-epoxidation ratio was increased in PSI under HL. In PSII, however, no additional xanthophyll cycle pigments occurred, but the de-epoxidation ratio was increased as well. This comparison suggests how mechanisms of photoprotection might take place within and in the proximity of the PS, which gives new insights into the capacity of diatoms to adapt to different conditions and in different environments.     

OXIDATIVE STRESS RESPONSES IN THE MARINE ANTARCTIC DIATOM CHAETOCEROS BREVIS (BACILLARIOPHYCEAE) DURING PHOTOACCLIMATION1

The enzyme superoxide dismutase (SOD) holds a key position in the microalgal antioxidant network. The present research focused on oxidative stress responses in the Antarctic diatom Chaetoceros brevis F. Schütt during transition to excess (including ultraviolet radiation [UVR]) and limiting irradiance conditions. Over a 4 d period, cellular responses of thiobarbituric acid reactive substances (TBARS, a general oxidative stress indicator), SOD activity, photosynthetic and xanthophyll cycle pigments, PSII efficiency, and growth were determined. In addition, oxidative responses were measured during a daily cycle. Changing irradiance conditions significantly affected growth rates of C. brevis. PSII efficiency decreased significantly during periodic excess irradiance and increased under low irradiance conditions. Transition to excess irradiance increased the ratio of xanthophyll to light‐harvesting pigments, whereas the opposite was observed for cultures transferred to low irradiance. This acclimation process was completed after 2 d in the new irradiance environment. SOD activity increased significantly after the first day regardless of the new irradiance environment but returned to preexposure values on the fourth day. We hypothesize that SOD activity may be temporarily elevated in C. brevis after irradiance shifts, thereby reducing oxidative stress when photoacclimation is in progress.     

ACCLIMATION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) TO PHOTON FLUX DENSITY1

Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymnesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons·m^?2·s^?1. Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d^?1 for high PFD grown cells compared with 0.38 d^?1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5‐fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark‐adapted maximal photochemical efficiency (F_v/F_m) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light‐adapted state, (F_v′/F_m′), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F_q′/F_m′) decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.     

PHOTOINHIBITION AND RECOVERY AFTER HIGH LIGHT STRESS IN DIFFERENT DEVELOPMENTAL AND LIFE-HISTORY STAGES OF LAMINARIA SACCHARINA (PHAEOPHYTA)1,2

The capacity to cope with high light stress was investigated in different life-history and developmental stages of Laminaria saccharina Lamour. sporophytes and gametophytes. Changes in photosynthetic efficiency and in the level of photoinhibition were measured by in vivo fluorescence changes of photosystem II. Pigment content was studied using high performance liquid chromatography. Additionally, the morphology of the various developmental stages during the life cycle was studied by light microscopy in relation to the photosynthetic parameters. High light stress (2 h, 500 μmol.m-².s^?1) induced photoinhibition of photosynthesis with fast kinetics in older sporophytes and gametophytes. In contrast, the absolute degree of photoinhibition after light stress was higher in younger than in older sporophytes. Photosynthesis recovered faster in older sporophytes and gametophytes compared to young sporophytes. In very young sporophytes, photosynthesis did not recover fully even after 12 h exposure to low light, indicating severe photodamage. Kinetics of recovery in old sporophytes and in gametophytes showed a fast and a slow phase, whereas younger sporophytes recovered only with a slow phase, The fast phase is indicative of a decline of the photoprotective process, whereas the slow phase indicates a recovery from photodamage. The capacity to cope with high light stress in Laminaria sporophytes increased with increasing age of the thalli. The gametophytes are less sensitive to high light stress and may be selected to endure unfavorable white light conditions. Investigation of the xanthophylls showed that the higher resistance to high light is not caused solely by a higher content of xanthophyll cycle pigments. Additionally, changes in the thallus structure during the development of the sporophytes seemed to cause a higher resistance to high light. The observed changes in the ability to cope with high light in the different life-history and developmental stages of Laminaria saccharina may influence the distribution of the species on the shore.     

Effect of abscisic acid on photosynthetic parameters during <Emphasis Type="Italic">ex vitro</Emphasis> transfer of micropropagated tobacco plantlets

The aim of this research was to determine whether exogenous abscisic acid (ABA) applied immediately after ex vitro transfer of in vitro grown plants can improve their acclimatization. Tobacco (Nicotiana tabacum L.) plantlets were transferred into pots with Perlite initially moistened either by water or 50 μM ABA solution and they were grown under low (LI) or high (HI) irradiance of 150 and 700 μmol m⁻² s⁻¹, respectively. Endogenous content of ABA in tobacco leaves increased considerably after ABA application and even more in plants grown under HI. Stomatal conductance, transpiration rate and net photosynthetic rate decreased considerably 1 d after ex vitro transfer and increased thereafter. The gas exchange parameters were further decreased by ABA application and so wilting of these plants was limited. Chlorophyll (a+b) and β-carotene contents were higher in ABA-treated plants, but the content of xanthophyll cycle pigments was not increased. However, the degree of xanthophyll cycle pigments deepoxidation was decreased what also suggested less stress in ABA-treated plants. No dramatic changes in most chlorophyll a fluorescence parameters after ex vitro transfer suggested that the plants did not suffer from restriction of electron transport or photosystem damage.     

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 responses in Phaeocystis antarctica towards varying light and iron conditions

The effects of iron limitation on photoacclimation to a dynamic light regime were studied in Phaeocystis antarctica. Batch cultures were grown under a sinusoidal light regime, mimicking vertical mixing, under both iron-sufficient and -limiting conditions. Iron-replete cells responded to changes in light intensity by rapid xanthophyll cycling. Maximum irradiance coincided with maximum ratios of diatoxanthin/diadinoxanthin (dt/dd). The maximum quantum yield of photosynthesis (F _v /F _m ) was negatively related to both irradiance and dt/dd. Full recovery of F _v /F _m by the end of the light period suggested successful photoacclimation. Iron-limited cells displayed characteristics of high light acclimation. The ratio of xanthophyll pigments to chlorophyll a was three times higher compared to iron-replete cells. Down-regulation of photosynthetic activity was moderated. It is argued that under iron limitation cells maintain a permanent state of high energy quenching to avoid photoinhibition during exposure to high irradiance. Iron-limited cells could maintain a high growth potential due to an increased absorption capacity as recorded by in vivo absorption, which balanced a decrease in F _v /F _m . The increase in the chlorophyll a-specific absorption cross section was related to an increase in carotenoid pigments and a reduction in the package effect. These experiments show that P. antarctica can acclimate successfully to conditions as they prevail in the Antarctic ocean, which may explain the success of this species.     

GROWTH AND PHOTOPROTECTION IN THREE DINOFLAGELLATES (INCLUDING TWO STRAINS OF ALEXANDRIUM TAMARENSE) AND ONE DIATOM EXPOSED TO FOUR WEEKS OF NATURAL AND ENHANCED ULTRAVIOLET‐B RADIATION1

Long‐term growth response to natural solar radiation with enhanced ultraviolet‐B (UVB) exposure was examined in two species of dinoflagellates [Alexandrium tamarense (M. Lebour) Balech, At, and Heterocapsa triquetra (Ehrenb.) F. Stein, Ht], including two strains of A. tamarense, one from Spain and another from UK, and one diatom species (Thalassiosira pseudonana Hasle et Heimdal). We examined whether variable photoprotection (mycosporine‐like amino acids [MAAs] and xanthophyll‐cycle pigments) affected photosynthetic performance, phytoplankton light absorption, and growth. Growth rate was significantly reduced under enhanced UVB for the UK strain of At and for Ht (both grew very little) as well as for the diatom (that maintained high growth rates), but there was no effect for the Spanish strain of At. MAA concentration was high in the dinoflagellates, but undetectable in the diatom, which instead used the xanthophyll cycle for photoprotection. The highest cell concentrations of MAAs and photoprotective pigments were observed in the UK strain of At, along with lowest growth rates and F_v/F_m, indicating high stress levels. In contrast, the Spanish strain showed progressive acclimation to the experimental conditions, with no significant difference in growth between treatments. Increase in total MAAs followed linearly the cumulative UVB of the preceding day, and both total and primary MAAs were maintained at higher constitutive levels in this strain. Acclimation to enhanced UVB in the diatom resulted in an increase in PSII activity and reduction in nonphotochemical quenching, indicating an increased resistance to photoinhibition after a few weeks. All four species showed increased phytoplankton light absorption under enhanced UVB. Large intrastrain differences suggest a need to consider more closely intraspecific variability in UV studies.     

Solar ultraviolet radiation affects the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase and the composition of photosynthetic and xanthophyll cycle pigments in the intertidal green alga Ulva lactuca L

The effect of solar UV radiation on the physiology of the intertidal green macroalga Ulva lactuca L. was investigated. A natural Ulva community at the shore of Helgoland was covered with screening foils, excluding UV-B or UV-B + UV-A from the solar spectrum. In the sampled material, changes in the activity and concentration of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), and the concentration of photosynthetic and xanthophyll cycle pigments were determined. Exclusion of UV radiation from the natural solar spectrum resulted in an elevated overall activity of Rubisco, related to an increase in its cellular concentration. Among the photosynthetic pigments, lutein concentration was substantially elevated under UV exclusion. In addition, marked UV effects on the xanthophyll cycle were found: exclusion of solar UV radiation (and particularly UV-B) resulted in an increased ratio of zeaxanthin concentration to the total xanthophyll content, indicating adverse effects of UV-B on the efficiency of photoprotection under high irradiances of photosynthetically active radiation. The results confirm a marked impact of present UV-B levels on macroalgal physiology under field conditions.     

The photoprotective role of epidermal anthocyanins and surface pubescence in young leaves of grapevine (Vitis vinifera)

BACKGROUND AND AIMS: Depending on cultivar, surfaces of young leaves of Vitis vinifera may be glabrous-green ('Soultanina') or transiently have anthocyanins ('Siriki') or pubescence ('Athiri'). A test is made of the hypothesis that anthocyanins and pubescence act as light screens affording a photoprotective advantage to the corresponding leaves, and an assessment is made of the magnitude of their effect. METHODS: Measurements were made on young leaves of the three cultivars in spring under field conditions. Photosynthetic gas-exchange and in vivo chlorophyll fluorescence were measured. Photosynthetic and photoprotective pigments were analysed by HPLC. KEY RESULTS: Compared with glabrous-green leaves, both anthocyanic and pubescent leaves had greater dark-adapted PSII photochemical efficiency and net photosynthesis. In leaves possessing either anthocyanins or pubescence, the ratio of xanthophyll cycle components to total chlorophyll, and mid-day de-epoxidation state of the xanthophyll cycle were considerably smaller, than in glabrous-green leaves. These differences were more evident in pubescent leaves, probably indicating that trichomes were more effective in decreasing light stress than anthocyanins in the epidermis. CONCLUSIONS: Light screens, especially in the form of pubescence, decrease the risk of photoinhibition whilst allowing leaves to maintain a smaller content of xanthophyll cycle components and depend less on xanthophyll cycle energy dissipation. This combination of photoprotective features, i.e. decreased photon flux to the photosynthetic apparatus and lower xanthophyll cycle utilization rates may be particularly advantageous under stressful conditions.     

Effects of ultraviolet (UV) exclusion on the seasonal concentration of photosynthetic and UV-screening pigments in Scots pine needles

The effects of ultraviolet (UV) radiation on the photosynthetic and UV‐screening pigments in needles of Scots pine (Pinus sylvestris L.) saplings were studied in a UV‐exclusion field chamber experiment in northern Finland (67°N) during 2001–2002. The chambers held filters that excluded both UVB and UVA, only UVB, transmitted all UV, or lacked filters. Analyses of control needles (no filter and polyethene filter) showed that the first changes to occur in spring (end of April) was an abrupt increase in the epoxidation state (EPS) of the xanthophyll cycle pigments, likely in relation with the beginning of the photosynthetic activity. The concentration of chlorophyll, lutein, neoxanthin, α‐carotene, β‐carotene, and the size of the xanthophyll cycle pool (violaxanthin+antheraxanthin+zeaxanthin=VAZ) changed only later when needles reached their summer photosynthesis state. Exclusion of UV radiation significantly affected the xanthophyll cycle but not the other photosynthetic pigments analysed. Interestingly, the effects on xanthophylls were dependent on the sampling date. Under UVA/B‐exclusion, the EPS was increased and VAZ pool size was unchanged in April, whereas EPS remained unchanged and the VAZ pool size was reduced in May and June. The existence of two sustained and active antenna modes during winter and summer could be an explanation for the specific UV‐exclusion effect in the different season. A high‐performance liquid chromatography analysis of soluble phenolics showed that the exclusion of UVA/B radiation caused a significant effect on five compounds out of 46 studied, without affecting the concentration of the total soluble phenolics. Under UVA/B‐exclusion, the concentration of three of them (secoisolariciresinol‐glucopyranoside, two unknown) was reduced while the concentration of dicoumaroyl‐astragalin and pinosylvin monomethylether was increased compared with both controls separately. In general, the exclusion of UVA/B caused a stronger effect than the exclusion of UVB on both photosynthetic and UV screening pigments. The effects of UV radiation on xanthophyll cycle pigments were season‐specific and detectable only under stressful spring conditions (freezing temperatures and high irradiance due to snow reflection). The effect on the xanthophyll cycle could be a direct consequence of UV treatments, or an indirect consequence of the changed flavonoid composition, or a combination of both.     

Chlorophyll a/b-binding proteins, pigment conversions, and early light-induced proteins in a chlorophyll b-less barley mutant.

Monospecific polyclonal antibodies have been raised against synthetic peptides derived from the primary sequences from different plant light-harvesting Chl a/b-binding (LHC) proteins. Together with other monospecific antibodies, these were used to quantify the levels of the 10 different LHC proteins in wild-type and chlorina f2 barley (Hordeum vulgare L.), grown under normal and intermittent light (ImL). Chlorina f2, grown under normal light, lacked Lhcb1 (type I LHC II) and Lhcb6 (CP24) and had reduced amounts of Lhcb2, Lhcb3 (types II and III LHC II), and Lhcb4 (CP 29). Chlorina f2 grown under ImL lacked all LHC proteins, whereas wild-type ImL plants contained Lhcb5 (CP 26) and a small amount of Lhcb2. The chlorina f2 ImL thylakoids were organized in large parallel arrays, but wild-type ImL thylakoids had appressed regions, indicating a possible role for Lhcb5 in grana stacking. Chlorina f2 grown under ImL contained considerable amounts of violaxanthin (2-3/reaction center), representing a pool of phototransformable xanthophyll cycle pigments not associated with LHC proteins. Chlorina f2 and the plants grown under ImL also contained early light-induced proteins (ELIPs) as monitored by western blotting. The levels of both ELIPs and xanthophyll cycle pigments increased during a 1 h of high light treatment, without accumulation of LHC proteins. These data are consistent with the hypothesis that ELIPs are pigment-binding proteins, and we suggest that ELIPs bind photoconvertible xanthophylls and replace "normal" LHC proteins under conditions of light stress.     

Strategies providing success in a variable habitat: III. Dynamic control of photosynthesis in Cladophora glomerata

Diurnal patterns of photosynthesis were studied in July and April populations of Cladophora glomerata (L.) Kütz. from open and from shaded sites. Summer samples exposed to full sunlight showed decreased efficiency of open photosystem II at noon, and only slight differences were found between samples that had grown at open or at shaded sites. Electron transport rate was limited at highest fluence rates in shade plants, and non‐photochemical quenching (NPQ) revealed faster regulation in samples from open sites. Daily course of de‐epoxidation was not linearly correlated with the course of NPQ. The comparison of samples from open and from shaded sites revealed a higher capacity of thermal energy dissipation and an increase in the total amount of xanthophyll‐cycle pigments (21%) in samples from open sites. In April, down‐regulation of the efficiency of open photosystem II was related to lower water temperature, and hence, increased excitation pressure. In April the pool size of xanthophyll‐cycle pigments was increased by 21% in comparison with summer and suggested higher levels of thermal energy dissipation via de‐epoxidized xanthophylls. In both, summer and spring the amount of xanthophyll‐cycle pigments was 20% higher in samples from open sites. Acclimation of C. glomerata to growth light conditions was further shown by experimental induction of NPQ, indicating NPQ increases of 23%, and increases of 77% in the reversible component of NPQ in open site samples. The effect of temperature on photosynthetic rate was non‐linear, and different optimum temperatures of electron transport rate and oxygen evolution were exhibited.     

Differential adaptation of two varieties of common bean to abiotic stress: II. Acclimation of photosynthesis

The photosynthetic characteristics of two contrasting varieties of common bean (Phaseolus vulgaris) have been determined. These varieties, Arroz and Orfeo, differ in their productivity under stress conditions, resistance to drought stress, and have distinctly different stomatal behaviour. When grown under conditions of high irradiance and high temperature, both varieties displayed evidence of photosynthetic acclimation at the chloroplast level-there was an increase in chlorophyll a/b ratio, a decreased content of Lhcb proteins, and an increased xanthophyll cycle pool size. Both varieties also showed reduced chlorophyll content on a leaf area basis and a decrease in leaf area. Both varieties showed an increase in leaf thickness but only Arroz showed the characteristic elongated palisade cells in the high light-grown plants; Orfeo instead had a larger number of smaller, rounded cells. Differences were found in stomatal development: whereas Arroz showed very little change in stomatal density, Orfeo exhibited a large increase, particularly on the upper leaf surface. It is suggested that these differences in leaf cell structure and stomatal density give rise to altered rates of photosynthesis and stomatal conductance. Whereas, Arroz had the same photosynthetic rate in plants grown at both low and high irradiance, Orfeo showed a higher photosynthetic capacity at high irradiance. It is suggested that the higher yield of Orfeo compared with Arroz under stress conditions can be explained, in part, by these cellular differences.     

Expression of a bacterial carotene hydroxylase gene (crtZ) enhances UV tolerance in tobacco

Carotenoids are essential components of the photosynthetic apparatus involved in plant photoprotection. To investigate the protective role of zeaxanthin under high light and UV stress we have increased the capacity for its biosynthesis in tobacco plants (Nicotiana tabacum L. cv. Samsun) by transformation with a heterologous carotenoid gene encoding -carotene hydroxylase (crtZ) from Erwinia uredovora under constitutive promoter control. This enzyme is responsible for the conversion of -carotene into zeaxanthin. Although the total pigment content of the transgenics was similar to control plants, the transformants synthesized zeaxanthin more rapidly and in larger quantities than controls upon transfer to high-intensity white light. Low-light-adapted tobacco plants were shown to be susceptible to UV exposure and therefore chosen for comparative analysis of wild-type and transgenics. Overall effects of UV irradiation were studied by measuring bioproductivity and pigment content. The UV exposed transformed plants maintained a higher biomass and a greater amount of photosynthetic pigments than controls. For revelation of direct effects, photosynthesis, pigment composition and chlorophyll fluorescence were examined immediately after UV treatment. Low-light-adapted plants of the crtZ transgenics showed less reduction in photosynthetic oxygen evolution and had higher chlorophyll fluorescence levels in comparison to control plants. After 1 h of high-light pre-illumination and subsequent UV exposure a greater amount of xanthophyll cycle pigments was retained in the transformants. In addition, the transgenic plants suffered less lipid peroxidation than the wild-type after treatment with the singlet-oxygen generator rose bengal. Our results indicate that an enhancement of zeaxanthin formation in the presence of a functional xanthophyll cycle contributes to UV stress protection and prevention of UV damage.