Photochromic pigments in akinetes and pigment characteristics of akinetes in comparison with vegetative cells of Anabaena variabilis

Since akinete germination is triggered by light and the action spectrum for this process has features in common with the spectra of the two photochromic pigments, phycochromes b and d, a search was made for the presence of these phycochromes in akinetes of the blue-green alga. Anabaena variabilis Kützing. Allophycocyanin-B was also looked for, since the action spectrum for akinete germination points to a possible participation of this pigment too. Isoelectric focusing was used for purification of the pigments. The different fractions were investigated for phycochromes b and d by measuring the absorbance difference spectra: for phycochrome b. 500 nm irradiated minus 570 nm irradiated, and for phycochrome d, 650 nm irradiated minus 610 nm irradiated. For determination of allophycocyanin-B. fourth derivative analysis of absorption spectra was made for some of the fractions from the isoelectric focusing column. Phycochrome b was also assayed for by measuring in vivo absorption difference spectra. The assays were positive for all three pigments. The complete photosynthetic pigment systems were also studied by in vivo fluorescence measurements on both akinetes and vegetative cells of Anabaena variabilis. Fluorescence emission and excitation spectra at selected emission wavelengths were measured at room temperature and liquid nitrogen temperature. The energy transfer from phycoerythrocyanin to phycocyanin is very efficient under all conditions, as is the energy transfer from phycocyanin to allophycocyanin at room temperature. At low temperature, however, phycocyanin is partly decoupled from allophycocyanin, particularly in the akinetes; the energy transfer from allophycocyanin to chlorophyll a is less efficient at low temperature in both types of cells, but especially in akinetes. Delayed light emission was measured for both types of cells and found to be very weak in akinetes compared to vegetative cells. From this study it would seem that akinetes lack an active photosystem II, although the 691 nm peak in the 570 nm excited low temperature fluorescence emission spectrum proves the presence of photosystem II chlorophyll, and also its energetic connection to the phycobilisomes.     

PHOTOSYNTHETIC CHARACTERIZATION OF DEVELOPING AND MATURE AKINETES OF APHANIZOMENON OVALISPORUM (CYANOPROKARYOTA)1

Akinetes, differentiated resting cells produced by many species of filamentous, heterocystous cyanobacteria, enable the organism to survive adverse conditions, such as cold winters and dry seasons, and to maintain germination capabilities until the onset of suitable conditions for vegetative growth. Mature akinetes maintain a limited level of metabolic activities, including photosynthesis. In the present study, we have characterized changes in the photosynthetic apparatus of vegetative cells and akinetes of the cyanobacterium Aphanizomenon ovalisporum Forti (Nostocales) during their development and maturation. Photosynthetic variable fluorescence was measured by microscope‐PAM (pulse‐amplitude‐modulated) fluorometry, and the fundamental composition of the photosynthetic apparatus was evaluated by fluorescence and immunological techniques. Vegetative cells and akinetes from samples of Aphanizomenon trichomes from akinete‐induced cultures at various ages demonstrated a gradual reduction, with age, in the maximal photosynthetic quantum yield in both cell types. However, the maximal quantum yield of akinetes declined slightly faster than that of their adjacent vegetative cells. Mature akinetes isolated from 6‐ to 8‐week‐old akinete‐induced cultures maintained only residual photosynthetic activity, as indicated by very low values of maximal photosynthetic quantum yields. Based on 77 K fluorescence emission data and immunodetection of PSI and PSII polypeptides, we concluded that the ratio of PSI to PSII reaction centers in mature akinetes is slightly higher than the ratio estimated for exponentially grown vegetative cells. Furthermore, the cellular abundance of these protein complexes substantially increased in akinetes relative to exponentially grown vegetative cells, presumably due to considerable increase in the biovolume of akinetes.     

PHYSIOLOGICAL CHANGES DURING GERMINATION OF PITHOPHORA OEDOGONIA (CHLOROPHYCEAE) AKINETES1

Akinetes of Pithophora oedogonia (Mont.) Wittrock, formed in stationary phase cultures, were induced to germinate by transfer to fresh media. Changes in various physiological parameters were monitored during the first 20 d of germination. Ungerminated akinetes contained a greater percentage of lipid, starch, and dry matter and exhibited lower photosynthetic rates than did vegetative cells. Germination consisted of four phases. Phase I (day-0 to day-1) was marked by a rapid increase in respiratory rates. Phase II (day-2 to day-6) involved protrusion of the germination tube and was insensitive to cyanide, a respiratory inhibitor, and simazine, a photosynthetic inhibitor. Phase III (day-7 to day-14) was marked by rapid elongation of the germination tube, increasing photosynthetic rate, increasing chlorophyll and water content and declining levels of lipid and starch. Germination tube elongation during phase III was not inhibited by simazine, but was cyanide sensitive. Phase IV (day-15 to day-20) was characterized by a reduction in respiratory rate and an abrupt increase in the ratio of photosynthesis to respiration. Germination tube elongation during phase IV was inhibited by simazine. The data indicate that germination in Pithophora oedogonia akinetes consists of an extensive period (phases I, II and III) during which reserve materials are respired. Utilization of internal food reserves apparently permits akinetes to germinate and supports the initial growth of the germination tube in light limited microenvironments.     

Studies on the mechanism of light-dependent germination of akinetes of blue-green algae

This paper deals with the role of light in the germination of akinetes of Anabaena azollae. The two maxima action spectra are situated at 385 and 615 nm and the stimulation of the germination process by photosynthate was confirmed. The photoreceptor absorbing at 385 nm was identified as a flavin and that at 615 nm as a phytochrome. A model is suggested for the mode of action of light in the germination of akinetes of blue-green algae.C. Tsui     

Growth of Anacystis in the Presence of Thiosulphate and its Consequences for the Architecture of the Photosynthetic Apparatus

With the aim of obtaining information on the degree of flexibility maintained in cyanobacteria in context with their phylogenetic position, Anacystis was grown in the presence of thiosulphate, oxidized in a photosystem I (PSI) dependent reaction (K_M 7.4 × 10^?3 M thiosulfate). Besides DBMIB, only o-phenanthroline and p-hydroxymercuribenzoate blocked thiosulphate-dependent PSI activity to some extent; iodonitrothymol, DCMU and cyanide had no influence. Growth of Anacystis in the presence of thiosulphate induced a reorganization of the photosynthetic apparatus characterized by a shift in the PSII/PSI ratio in favor of PSI, comparable to low light conditions. Capability for oxygenic photosynthesis never completely disappeared; structural elements of PSII were retained in the membrane to a certain degree. The antenna pigment system signalled high light under conditions of thiosulphate oxidation as judged from the ratio of phycocyanin to chlorophyll. Besides a shift in the ratio of PSII to PSI components, the polypeptide pattern of thylakoids from thiosulphate grown cells shows several additional components compared to the controls and, moreover, higher concentrations of some polypeptides present in the controls, particularly a M_r 41000 polypeptide. The process of thiosulphate oxidation appears bound to the thylakoid membrane.     

Environmental influences on akinete germination and development in Nodularia spumigena (Cyanobacteriaceae), isolated from the Gippsland Lakes, Victoria, Australia

This study was carried out to investigate the genesis of N. spumigena blooms by specifically studying the effects of environmental variables (salinity, nitrogen, phosphorus and light) on the germination of N. spumigena akinetes. Optimal conditions for maximum germination and germling growth were determined by exposing akinetes to a range of salinities and nutrient (nitrogen and phosphorus) concentrations under two different irradiances. At pre-determined time periods, treatments were sampled and the percent germination and length of germlings assessed. The results indicated that akinete germination and germling growth were optimal at salinities from 5 to 25 and significantly reduced outside this range. A positive correlation in germination was observed with increasing nutrient (phosphorus and nitrate) concentration. Similarly, germling growth increased with increasing concentrations of both nutrients. Irradiance significantly influenced both germination and growth during salinity experiments, whereas in nutrient addition experiments, irradiance had no effect on germination; however, growth was significantly influenced during phosphorus addition experiments. Consequently, salinity and light appeared to be most critical in the germination process for N. spumigena akinetes, with phosphorus most important for germling growth. The study showed that N. spumigena may be able to germinate under environmental conditions outside its optimal range, but the growth of the germling is significantly reduced, which in turn suggests that its ability to form a bloom outside its optimal environmental conditions would also be greatly reduced.     

Effects of UV-light andγ-rays on the survival,akinete formation and akinete germination inStigeoclonium pascheri

The damage produced by UV light to any of the three different stages of the life cycle of the parent generation of the green algaS. pascheri, i.e. akinetes, germinating akinetes and vegetative cells remained up to the stage of germination of akinetes of the first generation and no deleterious effect was reported thereafter. Lower doses of γ-rays (25–75 Gy) increased the percentage germination of akinetes and germinating akinetes of parent generation. The percentage germination of akinetes, germinating akinetes, survival of colonies originated from vegetative cells and sporulation of cells of the parent generation decreased with increasing doses from 100 to 300 Gy. The γ-induced effect to any of the three different stages was not transferred to the subsequent stage of algal generation.     

Developmental regulation of methylammonium (ammonium) transport activity in the cyanobacterium Anabaena doliolum

Abstract Whiel heterocystous vegetative filaments of A. doliolum exhibited biphasic pattern of methylammonium (ammonium) uptake, its populations of akinetes deficient in chlorophylla, phycocyanin, oxygenic photosynthesis and aerobic nitrogenase activity showed monophasic uptake pattern with no evidence for second phase activity. Such akinetes with monophasic uptake pattern of methylammonium became biphasic by developing second phase activity at a stage during their germination at which oxygenic photosynthesis also developed. It is suggested that first phase activity of the methylammonium uptake process corresponds to plasma membrane regulated uptake activity leading to one methylammonium pool and that second phase activity of the metylammonium uptake process corresponds to the thylakoid membrane regulated uptake activity leading to the other methylammonium pool.     

Fluorescence changes accompanying short-term light adaptations in photosystem I and photosystem II of the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 and phycobiliprotein-impaired mutants: State 1/State 2 transitions and carotenoid-induced quenching of phycobilisomes

The features of the two types of short-term light-adaptations of photosynthetic apparatus, State 1/State 2 transitions, and non-photochemical fluorescence quenching of phycobilisomes (PBS) by orange carotene-protein (OCP) were compared in the cyanobacterium Synechocystis sp. PCC 6803 wild type, CK pigment mutant lacking phycocyanin, and PAL mutant totally devoid of phycobiliproteins. The permanent presence of PBS-specific peaks in the in situ action spectra of photosystem I (PSI) and photosystem II (PSII), as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 690 nm (PSII) and 725 nm (PSI) showed that PBS are constitutive antenna complexes of both photosystems. The mutant strains compensated the lack of phycobiliproteins by higher PSII content and by intensification of photosynthetic linear electron transfer. The detectable changes of energy migration from PBS to the PSI and PSII in the Synechocystis wild type and the CK mutant in State 1 and State 2 according to the fluorescence excitation spectra measurements were not registered. The constant level of fluorescence emission of PSI during State 1/State 2 transitions and simultaneous increase of chlorophyll fluorescence emission of PSII in State 1 in Synechocystis PAL mutant allowed to propose that spillover is an unlikely mechanism of state transitions. Blue–green light absorbed by OCP diminished the rout of energy from PBS to PSI while energy migration from PBS to PSII was less influenced. Therefore, the main role of OCP-induced quenching of PBS is the limitation of PSI activity and cyclic electron transport under relatively high light conditions.     

Effect of partial or complete elimination of light‐harvesting complexes on the surface electric properties and the functions of cyanobacterial photosynthetic membranes

Influence of the modification of the cyanobacterial light‐harvesting complex [i.e. phycobilisomes (PBS)] on the surface electric properties and the functions of photosynthetic membranes was investigated. We used four PBS mutant strains of Synechocystis sp. PCC6803 as follows: PAL (PBS‐less), CK (phycocyanin‐less), BE (PSII‐PBS‐less) and PSI‐less/apcE^? (PSI‐less with detached PBS). Modifications of the PBS content lead to changes in the cell morphology and surface electric properties of the thylakoid membranes as well as in their functions, such as photosynthetic oxygen‐evolving activity, P700 kinetics and energy transfer between the pigment–protein complexes. Data reveal that the complete elimination of PBS in the PAL mutant causes a slight decrease in the electric dipole moments of the thylakoid membranes, whereas significant perturbations of the surface charges were registered in the membranes without assembled PBS–PSII macrocomplex (BE mutant) or PSI complex (PSI‐less mutant). These observations correlate with the detected alterations in the membrane structural organization. Using a polarographic oxygen rate electrode, we showed that the ratio of the fast to the slow oxygen‐evolving PSII centers depends on the partial or complete elimination of light‐harvesting complexes, as the slow operating PSII centers dominate in the PBS‐less mutant and in the mutant with detached PBS.     

Changes in the photosynthetic apparatus in the cyanobacterium Synechocystis sp. PCC 6714 following light-to-dark and dark-to-light transitions

The photosynthetic apparatus of Synechocystis sp. PCC 6714 cells grown chemoheterotrophically (dark with glucose as a carbon source) and photoautotrophically (light in a mineral medium) were compared. Dark-grown cells show a decrease in phycocyanin content and an even greater decrease in chlorophyll content with respect to light-grown cells. Analysis of fluorescence emission spectra at 77 K and at 20 °C, of dark- and light-grown cells, and of phycobilisomes isolated from both types of cells, indicated that in darkness the phycobiliproteins were assembled in functional phycobilisomes (PBS). The dark synthesized PBS, however, were unable to transfer their excitation energy to PS II chlorophyll. Upon illumination of dark-grown cells, recovery of photosynthetic activity, pigment content and energy transfer between PBS and PS II was achieved in 24–48 h according to various steps. For O₂ evolution the initial step was independent of protein synthesis, but the later steps needed de novo synthesis. Concerning recovery of PBS to PS II energy transfer, light seems to be necessary, but neither PS II functioning nor de novo protein synthesis were required. Similarly, light, rather than functional PS II, was important for the recovery of an efficient energy transfer in nitrate-starved cells upon readdition of nitrate. In addition, it has been shown that normal phycobilisomes could accumulate in a Synechocystis sp. PCC 6803 mutant deficient in Photosystem II activity.Abbreviations APC allophycocyanin - CAP chloroamphenicol - Chl chlorophyll - DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea - CP-47 chlorophyll-binding Photosystem II protein of 47 kDa - EF exoplasmic face - PBS phycobilisome - PC phycocyanin - PS Photosystem     

Acclimation to low ultraviolet-B radiation increases photosystem I abundance and cyclic electron transfer with enhanced photosynthesis and growth in the cyanobacterium Nostoc sphaeroides

Ultraviolet-B radiation is known to harm most photosynthetic organisms with the exception of several studies of photosynthetic eukaryotes in which UV-B showed positive effects. In this study, we investigated the effect of acclimation to low UV-B radiation on growth and photosynthesis of the cyanobacterium Nostoc sphaeroides. Exposure to 0.08 W m⁻² UV-B plus low visible light for 14 d significantly increased the growth rate and biomass production by 16% and 30%, respectively, compared with those under visible light alone. The UV-B acclimated cells showed an approximately 50% increase in photosynthetic efficiency (α) and photosynthetic capacity (P_max), a higher PSI/PSII fluorescence ratio, an increase in PSI content and consequently enhanced cyclic electron flow, relative to those of non-acclimated cells. Both the primary quinone-type acceptor and plastoquinone pool re-oxidation were up-regulated in the UV-B acclimated cells. In parallel, the UV-B acclimated colonies maintained a higher rate of D1 protein synthesis following exposure to elevated intensity of UV-B or visible light, thus functionally mitigating photoinhibition. The present data provide novel insight into photosynthetic acclimation to low UV-B radiation and suggest that UV-B may act as a positive ecological factor for the productivity of some photosynthetic prokaryotes, especially during twilight periods or in shaded environments.     

Iron stress responses in the cyanobacterium Synechococcus sp. PCC7942

In the present study, we describe the sequential events by which the cyanobacterium Synechococcus sp. PCC 7942 adapts to iron deficiency. In doing so, we have tried to elucidate both short and long-term acclimation to low iron stress in order to understand how the photosynthetic apparatus adjusts to low iron conditions. Our results show that after an initial step, where CP43' is induced and where ferredoxin is partly replaced by flavodoxin, the photosynthetic unit starts to undergo major rearrangements. All measured components of Photosystem I (PSI), PSII and cytochrome (Cyt) ƒ decrease relative to chlorophyll (Chl) a . The photochemical efficiencies of the two photosystems also decline during this phase of acclimation. The well-known drop in phycobilisome content measured as phycocyanin (PC)/Chl was not due to an increased degradation, but rather to a decreased rate of synthesis. The largest effects of iron deficiency were observed on PSI, the most iron-rich structure of the photosynthetic apparatus. In the light of the recent discovery of an iron deficiency induced CP43' ring around PSI a possible dual function of this protein as both an antenna and a quencher is discussed. We also describe the time course of a blue shift in the low temperature Chl emission peak around 715 nm, which originates in PSI. The shift might reflect the disassembly and/or degradation of PSI during iron deficiency and, as a consequence, PSI might under these conditions be found predominantly in a monomeric form. We suggest that the observed functional and compositional alterations represent cellular acclimation enabling growth and development under iron deficiency, and that growth ceases when the acclimation capacity is exhausted. However, the cells remain viable even after growth has ceased, since they resumed growth once iron was added back to the culture.     

The composition and structure of photosystem I‐associated antenna from Cyanidioschyzon merolae

Red algae contain two types of light‐harvesting antenna systems, the phycobilisomes and chlorophyll a binding polypeptides (termed Lhcr), which expand the light‐harvesting capacity of the photosynthetic reaction centers. In this study, photosystem I (PSI) and its associated light‐harvesting proteins were isolated from the red alga Cyanidioschyzon merolae. The structural and functional properties of the largest PSI particles observed were investigated by biochemical characterization, mass spectrometry, fluorescence emission and excitation spectroscopy, and transmission electron microscopy. Our data provide strong evidence for a stable PSI complex in red algae that possesses two distinct types of functional peripheral light‐harvesting antenna complex, comprising both Lhcr and a PSI‐linked phycobilisome sub‐complex. We conclude that the PSI antennae system of red algae represents an evolutionary intermediate between the prokaryotic cyanobacteria and other eukaryotes, such as green algae and vascular plants.     

INORGANIC CARBON REPLETION CONSTRAINS STEADY‐STATE LIGHT ACCLIMATION IN THE CYANOBACTERIUM SYNECHOCOCCUS ELONGATUS1

Cyanobacteria show high metabolic plasticity by re‐allocating macromolecular resources in response to variations in both environmental inorganic carbon (Ci) and light. We grew cultures of the picoplanktonic cyanobacterium Synechococcus elongatus Nägeli across a 50‐fold range of growth irradiance at either a dissolved [Ci] <0.1 mM, sufficient to induce strongly the carbon‐concentrating mechanism (CCM) or a dissolved [Ci] of ～4 mM, sufficient to strongly induce the CCM to basal constitutive activity. There was no detectable growth cost of acclimation to low Ci across the entire range of irradiance and growth was nearly light saturated at 50 l mol photons·m^?2·s^?1. Cells acclimated to low Ci significantly re‐allocated macromolecular resources to support their CCM, while maintaining near homeostatis of metabolic flux per unit photosynthetic complex. Changing growth irradiance also drove re‐organization of the photosynthetic machinery to balance excitation flux and metabolic demands, but flux per complex varied widely across the range of tolerable growth irradiances. Across the range of growth irradiance, low Ci cells had significantly less phycocyanin than high Ci cells, which corresponded to a lower PSII absorbance capacity. Furthermore, low Ci cells maintained more PSI per cell^?1 than high Ci cells under high growth irradiance. Low Ci cells could therefore maintain more of their PSII reaction centers open at high growth irradiance than could high Ci cells, which experienced a significant PSII closure. Thus, acclimation to growth under high available Ci actually constrained acclimation to high light by restricting electron transport downstream from PSII in S. elongatus.     

Environmental influences on akinete germination of Anabaena circinalis and implications for management of cyanobacterial blooms

Certain cyanobacteria, including the noxious bloom-forming species Anabaena circinalis Rabenhorst, produce thick-walled reproductive structures (akinetes) which may serve as a resting stage and ensure survival during adverse growth conditions. The effect of certain environmental variables (temperature, salinity and desiccation) on akinete germination of A. circinalis was investigated under laboratory conditions, to determine the conditions under which germination was inhibited. The overall aims were to provide a broader understanding of the life history and ecology of this species and to assess suppression of akinete germination as a potential management strategy for control of cyanobacterial blooms in the lower Murray River, Australia. The results indicated a marked threshold of temperature and salinity tolerance for germination of A. circinalis, but the latter was not within a range that could be successfully manipulated in a natural ecosystem. However, it was found that desiccation of akinetes for moderately short periods can significantly impair their capacity to germinate. It is, therefore, speculated that allowing periodic drying of shallow wetlands adjacent to the Murray River and in other areas may reduce the size of the inoculum for population growth by reducing viability of akinetes in surface sediments.     

Regulation of Mitochondrial Function and Biogenesis in Cucumber (Cucumis sativus L.) Cotyledons during Early Seedling Growth

The aim of this work was to characterize the respiratory metabolism of the greening cotyledons of cucumber (Cucumis sativus L.) during early seedling growth and to investigate how this is integrated with changes in mitochondrial biogenesis and function. In light-grown cotyledons, lipid mobilization extended from germination to 6 days postimbibition, reaching a maximum at 3 to 4 days postimbibition. The rate of dark oxygen uptake reached a maximum at 2 days postimbibition in dark-grown and 3 days postimbibition in light-grown cotyledons. Development of photosynthetic capacity occurred from 4 to 7 days postimbibition. In dark-grown cotyledons, lipid mobilization extended beyond 7 days postimbibition, and there was no greening or acquisition of photosynthetic competence. Measurements of mitochondrial function indicated that the respiratory capacity of the tissue changed such that during lipid mobilization there was a much greater capacity for the operation of the nondecarboxylating portion of the citric acid cycle (succinate to oxaloacetate), whereas during the development of photosynthetic function the activity of the remainder of the cycle (oxaloacetate to succinate) was induced. Comparison of the maximum capacities for mitochondrial substrate oxidations in vitro with the rates of in vivo substrate oxidations, predicted from the rate of lipid breakdown, indicated that mitochondria in this tissue operate at or below state 4 rates, suggesting limitation by both availability of ADP and substrate.     

Interferometric studies of the dynamics of hydration and dry matter content during light-dependent germination of the Anabaena variabilis Kützing akinetes]

Calculations following interference-microscopical measurements performed on akinetes (A), heterocyts (H), and "vegetative" cells (F) of the Cyanobacterium (blue-green alga) Anabaena variabilis resulted in significant higher values of mean absolute dry matter content of the akinetes (2.06 . 10(-10) g; as compared to 0.46 . 10(-10) g and 0.31 . 10(-10) g for H and F, respectively). tthe water content of these resting cells (63%) was significantly lower than in the other two types of cells (H: 85%, F: 77%). Light exposition of the akinetes in fresh nutrition medium (i. e., conditions allowing germination within 30--50 h) resulted in a decrease of the relative dry matter content so that already in the period preceding the outgrowth of the germling the water content of the vegetative cells was achieved. Simultaneously their volume increased by the uptake of water; whereas the absolute content of dry matter remained constant or was even temporarily diminished during the first period. Only in the second period the values increased in some cases and then remained constant up to germination. The increased dry matter content, however, was not a precondition necessary for the germination of the akinetes. In darkness under otherwise unaltered conditions the values remained unchanged or, after a light period, came back to the initial level. The results demonstrate that formation and germination of the resting cells of Cyanobacteria as well are connected with an alteration in the hydratation level, i. e., in cells which continuously are kept under water saturated conditions. This increase by hydratation during the germination period is, as the germination process itself, strictly controlled by light.     

Viability of dried vegetative cells and the formation and germination of reproductive structures inPithophora œdogonia, Cladophora glomerata andRhizoclonium hieroglyphicum under water stress

All dried vegetative cells ofPithophora œdogonia died within 1 h, while those ofCladophora glomerata andRhizoclonium hieroglyphicum retain viability to some extent for 1 and 8 d, respectively, under similar storage conditions. The viability of dried vegetative cells of eitherC. glomerata orR. hieroglyphicum decreased more or less equally when stored either at 20 °C. in light or dark or at 12 °C in dark, but was lost rapidly and drastically when stored at 0 °C in dark. Both dried and wet akinetes ofP. œdogonia were equally more viable when stored at 20 °C in dark than in light, but they lost germination ability when stored either at 12 or 0 °C in dark; this might be either due to loss of viability or dormancy induction at low temperatures. The water stress imposed by growing vegetative filaments either on highly agarized media, in NaCl-supplemented liquid media or in media undergoing progressive air-drying to complete dryness did not induce, but reduced akinete formation inP. œdogonia, decreased zoosporangium formation inC. glomerata andR. hieroglyphicum, decreased or totally suppressed akinete germination inP. œdogonia and zoospore germination inC. glomerata andR. hieroglyphicum. Akinetes ofP. œdogonia formed under water stress were equally viable, while zoosporangia ofC. glomerata andR. hieroglyphicum formed under water stress were comparatively less viable than those formed without any water stress. Akinete germination inP. œdogonia and zoospore germination inC. glomerata andR. hieroglyphicum were comparatively more sensitive to water stress than the formation of akinetes and zoosporangia. The akinete germination inP. œdogonia was more sensitive to water stress than zoospore germination inC. glomerata andR. hieroglyphicum and it might be either due to their large size, thick wall or dense content.     

Consequences of photosystem‐I damage and repair on photosynthesis and carbon use in Arabidopsis thaliana

Natural growth environments commonly include fluctuating conditions that can disrupt the photosynthetic energy balance and induce photoinhibition through inactivation of the photosynthetic apparatus. Photosystem II (PSII) photoinhibition is efficiently reversed by the PSII repair cycle, whereas photoinhibited photosystem I (PSI) recovers much more slowly. In the current study, treatment of the Arabidopsis thaliana mutant proton gradient regulation 5 (pgr5) with excess light was used to compromise PSI functionality in order to investigate the impact of photoinhibition and subsequent recovery on photosynthesis and carbon metabolism. The negative impact of PSI photoinhibition on CO₂ fixation was especially deleterious under low irradiance. Impaired starch accumulation after PSI photoinhibition was reflected in reduced respiration in the dark, but this was not attributed to impaired sugar synthesis. Normal chloroplast and mitochondrial metabolisms were shown to recover despite the persistence of substantial PSI photoinhibition for several days. The results of this study indicate that the recovery of PSI function involves the reorganization of the light‐harvesting antennae, and suggest a pool of surplus PSI that can be recruited to support photosynthesis under demanding conditions.