Role of light in the response of PSII photochemistry to salt stress in the cyanobacterium Spirulina platensis

The role of light in the effect of salt stress on PSII photochemistry in the cyanobacterium Spirulina platensis grown at 50 micromol m(-2) s(-1) was investigated. The time-course of changes in PSII photochemistry in response to high salinity (0.8 M NaCl) incubated in the dark and at 30, 50 and 100 micromol m(-2) s(-1) was composed of two phases. The first phase, which was independent of light, was characterized by a rapid decrease (20-50%) in the maximal efficiency of PSII photochemistry (F:(v)/F:(m)), the efficiency of excitation energy capture by open PSII reaction centres (F(1)(v)/F(1)(m)), photochemical quenching (q(P)), and the quantum yield of PSII electron transport (Phi(PSII)) in the first 15 min, followed by a recovery of up to about 86-92% of their initial levels after 4 h of incubation. The second phase took place after 4 h, in which a further decline in the above parameters occurred only in the light but not in the dark, reaching levels as low as 32-56% of their initial levels after 12 h. Moreover, the higher incubation light intensity, the greater the decrease in the above parameters. At the same time, Q(B)-non-reducing PSII reaction centres increased significantly in the first 15 min and then recovered to the initial level during the first phase, but increased again in the light in the second phase. Photosynthetic oxygen evolution activity decreased sharply by 70% in the first 5 min, and then kept largely constant until 12 h. The changes in oxygen evolution activity were independent of light intensity during both phases.     

Non-photochemical loss in PSII in high- and low-light-grown leaves of Vicia faba quantified by several fluorescence parameters including L(NP), F0/F'm, a novel parameter

Using the expression of fluorescence originated from the PSII open reaction center in the light by Oxborough and Baker (1997), we obtained a formula that expresses relationships between the quantum efficiency of PSII photochemistry in the dark (Phi(m)= F(v)/F(m)) and in the light Phi'(m)=F'(v)/F'(m):Phi'(m)=Phi(m)+L(NP), where L(NP)(=F(0)/F'(m)) denotes the quantum yield of light induced non-photochemical losses (heat dissipation and fluorescence de-excitation) in PSII. Using L(NP) and other conventional fluorescence parameters, we conducted quenching analyses with leaves of broad bean plants (Vicia faba L.) grown at 700 (high light; HL) and 80 mumol photons m(-2) s(-1) (low light; LL). We also examined whether behavior of q(0) quenching (q(0)=1-F'(0)/F(0)) is related to the reaction center quenching. When the actinic light (AL) was strong, Stern-Volmer quenching [NPQ=(F(m)-F'(m))/F'(m)] and L(NP) increased rapidly and then decreased slowly in HL leaves, while, in LL leaves, they increased slowly. It is probable that rapid formation of a large proton gradient was responsible for sharp rises in both parameters in HL leaves. The steady-state 'excess' parameter [Phi(Ex)= (1 - qP) Phi(m)/(Phi(m)+ L(NP))], fraction of energy migrating to closed PSII centers, increased with the photon flux density of AL in LL leaves. In contrast, in HL leaves, Phi(Ex) did not increase markedly. The examination of the relationship between Phi(Ex) and L(NP) obtained at various AL revealed that in LL leaves the increase in (1 - qP) with the increase in AL prevailed, while, in HL leaves, the increase in L(NP) suppressed the increase in (1 - qP). Using the difference between L(NP) and L(D) (Phi(ND)= L(NP)- L(D), where L(D)= F(0)/F(m)), q(0) and qN (=1-F'(v)/F(v)) were calculated without using measured F'(0). The relationships between q(0) and qN thus obtained for various AL levels were almost identical for both HL and LL leaves, implying no difference in the fluorescence origin between the HL and LL leaves. Usefulness of these equations expressing non-photochemical loss is discussed.     

Low temperature enhances photosynthetic down-regulation in French bean (Phaseolus vulgaris L.) plants

The mechanisms of photosynthetic adaptation to different combinations of temperature and irradiance during growth, and especially the consequences of exposure to high light (2000 micro mol m(-2) s(-1) PPFD) for 5 min, simulating natural sunflecks, was studied in bean plants (Phaseolus vulgaris L.). A protocol using only short (3 min) dark pre-treatment was introduced to maximize the amount of replication possible in studies of chlorophyll fluorescence. High light at low temperature (10 degrees C) significantly down-regulated photosynthetic electron transport capacity [as measured by the efficiency of photosystem II (PSII)], with the protective acclimation allowing the simulated sunflecks to be used more effectively for photosynthesis by plants grown in low light. The greater energy dissipation by thermal processes (lower F(v)'/F(m)' ratio) at low temperature was related to increased xanthophyll de-epoxidation and to the fact that photosynthetic carbon fixation was more limiting at low than at high temperatures. A key objective was to investigate the role of photorespiration in acclimation to irradiance and temperature by comparing the effect of normal (21 kPa) and low (1.5 kPa) O(2) concentrations. Low [O(2)] decreased F(v)/F(m) and the efficiency of PSII (Phi(PSII)), related to greater PSII down-regulation in cold pre-treated plants, but minimized further inhibition by the mild 'sunfleck' treatment used. Results support the hypothesis that photorespiration provides a 'safety-valve' for excess energy.     

High-light stress and photoprotection in Umbilicaria antarctica monitored by chlorophyll fluorescence imaging and changes in zeaxanthin and glutathione

The effect of high light on spatial distribution of chlorophyll (Chl) fluorescence parameters over a lichen thallus (Umbilicaria antarctica) was investigated by imaging of Chl fluorescence parameters before and after exposure to high light (1500 micro mol m (-2) s (-1), 30 min at 5 degrees C). False colour images of F (V)/F (M) and Phi (II) distribution, taken over thallus with 0.1 mm (2) resolution, showed that maximum F (V)/F (M) and Phi (II) values were located close to the thallus centre. Minimum values were typical for thallus margins. After exposure to high light, a differential response of F (V)/F (M) and Phi (II) was found. The marginal thallus part exhibited a loss of photosynthetic activity, manifested as a lack of Chl fluorescence signal, and close-to-centre parts showed a different extent of F (V)/F (M) and Phi (II) decrease. Subsequent recovery in the dark led to a gradual return of F (V)/F (M) and Phi (II) to their initial values. Fast (30 min) and slow (1 - 22 h) phase of recovery were distinguished, suggesting a sufficient capacity of photoprotective mechanisms in U. antarctica to cope with low-temperature photoinhibition. Glutathione and xanthophyll cycle pigments were analyzed by HPLC. High light led to an increase in oxidized glutathione (GSSG), and a conversion of violaxanthin to zeaxanthin, expressed as their de-epoxidation state (DEPS). The responses of GSSG and DEPS were reversible during subsequent recovery in the dark. GSSG and DEPS were highly correlated to non-photochemical quenching (NPQ), indicating involvement of these antioxidants in the resistance of U. antarctica to high-light stress. Heterogeneity of Chl fluorescence parameters over the thallus and differential response to high light are discussed in relation to thallus anatomy and intrathalline distribution of the symbiotic alga Trebouxia sp.     

Salinity treatment shows no effects on photosystem II photochemistry,but increases the resistance of photosystem II to heat stress in halophyte Suaeda salsa

Photosynthetic gas exchange, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, and the polyphasic fluorescence transients were used to evaluate PSII photochemistry in the halophyte Suaeda salsa exposed to a combination of high salinity (100-400 mM NaCl) and heat stress (35-47.5 degrees C, air temperature). CO(2) assimilation rate increased slightly with increasing salt concentration up to 300 mM NaCl and showed no decrease even at 400 mM NaCl. Salinity treatment showed neither effects on the maximal efficiency of PSII photochemistry (F(v)/F(m)), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, nor effects on the efficiency of excitation energy capture by open PSII reaction centres (F(v)'/F(m)') and the actual PSII effciency (Phi(PSII)), photochemical quenching (q(P)), and non-photochemical quenching (q(N)) in light-adapted leaves. The results indicate that high salinity had no effects on PSII photochemistry either in a dark-adapted state or in a light-adapted state. With increasing temperature, CO(2) assimilation rate decreased significantly and no net CO(2) assimilation was observed at 47.5 degrees C. Salinity treatment had no effect on the response of CO(2) assimilation to high temperature when temperature was below 40 degrees C. At 45 degrees C, CO(2) assimilation rate in control plants decreased to zero, but the salt-adapted plants still maintained some CO(2) assimilation capacity. On the other hand, the responses of PSII photochemistry to heat stress was modified by salinity treatment. When temperature was above 35 degrees C, the declines in F(v)/F(m), Phi(PSII), F(v)'/F(m)', and q(P) were smaller in salt-adapted leaves compared to control leaves. This increased thermostability was independent of the degree of salinity, since no significant changes in the above-described fluorescence parameters were observed among the plants treated with different concentrations of NaCl. During heat stress, a very clear K step as a specific indicator of damage to the O(2)-evolving complex in the polyphasic fluorescence transients appeared in control plants, but did not get pronounced in salt-adapted plants. In addition, a greater increase in the ratio (F(i)-F(o))/(F(p)-F(o)) which is an expression of the proportion of the Q(B)-non-reducing PSII centres was observed in control plants rather than in salt-adapted plants. The results suggest that the increased thermostability of PSII seems to be associated with the increased resistance of the O(2)-evolving complex and the reaction centres of PSII to high temperature.     

田间大豆叶片成长过程中的光合特性及光破坏防御机制

田间大豆叶片在成长进程中光饱和光合速率持续提高,但气孔导度的增加明显滞后.尽管叶片在成长初期就具有较高的最大光化学效率,但是仍略低于发育成熟的叶片.随着叶片的成长,光下叶片光系统Ⅱ实际效率增加;非光化学猝灭下降.幼叶叶黄素总量与叶绿素之比较高,随着叶面积的增加该比值下降,在光下,幼叶的脱环氧化程度较高.因此认为大豆叶片成长初期就能够有效地进行光化学调节;在叶片生长过程中依赖叶黄素循环的热耗散机制迅速建立起来有效抵御强光的破坏.     

Up-regulation of the mitochondrial alternative oxidase pathway enhances photosynthetic electron transport under drought conditions

The aim of this study was to explore the role of the mitochondrial alternative oxidase (AOX) in the protection of photosynthesis during drought in wheat leaves. The relative water contents of water-replete and drought-exposed wheat plants were 97.2+/-0.3 and 75+/-2, respectively. Drought increased the amount of leaf AOX protein and also enhanced the rate of AOX-dependent O(2) uptake by the respiratory electron transport chain. The amount of the reduced, active form of the AOX protein was specifically increased by drought. The AOX inhibitor salicylhydroxamic acid (1 mM; SHAM) inhibited 70% of AOX activity in vivo in both water-replete and drought-exposed plants. Plants treated with SHAM were then exposed to low (100), high (350), or excess light (800 mumol photons m(-2) s(-1)) for 90 min. SHAM did not modify chlorophyll a fluorescence quenching parameters in water-replete controls after any of these treatments. However, while the maximal quantum yield of photosystem II (PSII) electron transport (F(v)/F(m)) was not affected by SHAM, the immediate quantum yield of PSII electron transport (Phi(PSII)) and photochemical quenching (qP) were gradually reduced by increasing irradiance in SHAM-treated drought-exposed plants, the decrease being most pronounced at the highest irradiance. Non-photochemical quenching (NPQ) reached near maximum levels in plants subjected to drought at high irradiance. However, a combination of drought and low light caused an intermediate increase in NPQ, which attained higher values when AOX was inhibited. Taken together, these results show that up-regulation of the respiratory AOX pathway protects the photosynthetic electron transport chain from the harmful effects of excess light.     

Monitoring Migration and Measuring Biomass in Benthic Biofilms: The Effects of Dark/far-red Adaptation and Vertical Migration on Fluorescence Measurements

Pulse modulated fluorescence has increasingly been used as an ecological tool to examine changes in the vertical distribution of microphytobenthic cells within the upper layers of estuarine sediments (most often using the minimum fluorescence yield F(o)) as well as to indicate the health of the community (using the maximum PS II quantum efficiency F(v)/F(m)). However, the practicalities of in situ measurements, often dictates that short dark adaptation periods must be used ( approximately 15 min). The use of far-red light as an alternative to dark adaptation was investigated in natural migratory microphytobenthic biofilms and artificial non-migratory biofilms. Prolonged periods of darkness ( approximately 24 h) were not adequate to achieve 'true' measurements of F(o) and F(v)/F(m), which require complete oxidation of Q(A) and full reversal of non-photochemical quenching (NPQ). In some instances, stable values were only achieved using far-red light. Prolonged exposure to dark/far-red light led to a downwards migration of cells in natural assemblages, as seen by a reduction in both F(o) and the maximum fluorescence yield (F(m)). In non-migratory biofilms, F(m) increased in the dark and far-red treatments, indicating a reversal of NPQ, whereas F(o) decreased in far-red light but increased in the dark. It is suggested that far-red light and darkness differentially affected the balance between NPQ reversal and Q(A) oxidation that lead to the measured F(o) yield. The use of far-red light as an alternative to dark adaptation is discussed and the implications of short (e.g., 15 min) dark adaptation times used in situ are discussed with reference to the vertical migration of cells within sediment biofilms.     

Inactivation of a plastid evolutionary conserved gene affects PSII electron transport, life span and fitness of tobacco plants

Chloroplasts contain a plastoquinone-NADH-oxidoreductase (Ndh) complex involved in protection against stress and the maintenance of cyclic electron flow. Inactivation of the Ndh complex delays the development of leaf senescence symptoms. Chlorophyll a fluorescence measurements, blue native gel electrophoresis, immunodetection and other techniques were employed to study tobacco (Nicotiana tabacum) Ndh-defective mutants (DeltandhF). The DeltandhF mutants compared with wild-type plants presented: (i) higher photosystem II : photosystem I (PSII : PSI) ratios; (ii) similar or higher levels of ascorbate, carotenoids, thylakoid peroxidase and superoxide dismutase, yield (Phi(PSII)) and maximal photochemical efficiency of PSII levels (F(v)/F(m)) than wild-type plant leaves of the same age; (iii) lower values of nonphotochemical quenching yield (Phi(NPQ)), but not at very high light intensities or during induced leaf senescence; (iv) a similar decrease of antioxidants during senescence; (v) no significant differences in the total foliar area and apical growth rate; and (vi) a production of viable seeds significantly higher than wild-type plants. These results suggest that the Ndh complex is involved in one of the redundant mechanisms that play a safety role in photosynthesis under stress, which has been conserved during evolution, but that its deletion increases fitness when plants are grown under favourable controlled conditions.     

Response of the diatom Phaeodactylum tricornutum to photooxidative stress resulting from high light exposure

The response of microalgae to photooxidative stress resulting from high light exposure is a well-studied phenomenon. However, direct analyses of photosystem II (PSII) D1 protein (the main target of photoinhibition) in diatoms are scarce. In this study, the response of the diatom model species Phaeodactylum tricornutum to short-term exposure to high light was examined and the levels of D1 protein determined immunochemically. Low light (LL) acclimated cells (40 μmol photons m(-2) s(-1)) subjected to high light (HL, 1,250 μmol photons m(-2) s(-1)) showed rapid induction of non-photochemical quenching (NPQ) and ca. 20-fold increase in diatoxanthin (DT) concentration. This resulted from the conversion of diadinoxanthin (DD) to DT through the activation of the DD-cycle. D1 protein levels under LL decreased about 30% after 1 h of the addition of lincomycin (LINC), a chloroplast protein synthesis inhibitor, showing significant D1 degradation and repair under low irradiance. Exposure to HL lead to a 3.2-fold increase in D1 degradation rate, whereas average D1 repair rate was 1.3-x higher under HL than LL, leading to decreased levels of D1 protein under HL. There were significant effects of both HL and LINC on P. tricornutum maximum quantum yield of PSII (F(v)/F(m)), showing a reduction of active PSII reaction centres. Partial recovery of F(v)/F(m) in the dark demonstrates the photosynthetic resilience of this diatom to changes in the light regime. P. tricornutum showed high allocation of total protein to D1 and an active D1-repair cycle to limit photoinhibition.     

Allelochemical stress inhibits growth, leaf water relations, PSII photochemistry, non-photochemical fluorescence quenching, and heat energy dissipation in three C3 perennial species

In this study, the effect of two allelochemicals, benzoxazolin-2(3H)-one (BOA) and cinnamic acid (CA), on different physiological and morphological characteristics of 1-month-old C(3) plant species (Dactylis glomerata, Lolium perenne, and Rumex acetosa) was analysed. BOA inhibited the shoot length of D. glomerata, L. perenne, and R. acetosa by 49%, 19%, and 19% of the control. The root length of D. glomerata, L. perenne, and R. acetosa growing in the presence of 1.5 mM BOA and CA was decreased compared with the control. Both allelochemicals (BOA, CA) inhibited leaf osmotic potential (LOP) in L. perenne and D. glomerata. In L. perenne, F(v)/F(m) decreased after treatment with BOA (1.5 mM) while CA (1.5 mM) also significantly reduced F(v)/F(m) in L. perenne. Both allelochemicals decreased ΦPSII in D. glomerata and L. perenne within 24 h of treatment, while in R. acetosa, ΦPSII levels decreased by 72 h following treatment with BOA and CA. There was a decrease in qP and NPQ on the first, fourth, fifth, and sixth days after treatment with BOA in D. glomerata, while both allelochemicals reduced the qP level in R. acetosa. There was a gradual decrease in the fraction of light absorbed by PSII allocated to PSII photochemistry (P) in R. acetosa treated with BOA and CA. The P values in D. glomerata were reduced by both allelochemicals and the portion of absorbed photon energy that was thermally dissipated (D) in D. glomerata and L. perenne was decreased by BOA and CA. Photon energy absorbed by PSII antennae and trapped by 'closed' PSII reaction centres (E) was decreased after CA exposure in D. glomerata. BOA and CA (1.5 mM concentration) decreased the leaf protein contents in all three perennial species. This study provides new understanding of the physiological and biochemical mechanisms of action of BOA and CA in one perennial dicotyledon and two perennial grasses. The acquisition of such knowledge may ultimately provide a rational and scientific basis for the design of safe and effective herbicides.     

Suppression of host photosynthesis by the parasitic plant Rhinanthus minor

BACKGROUND AND AIMS: Parasitism is well understood to have wide-ranging deleterious effects on host performance in species thus far characterized. Photosynthetic performance reductions have been noted in the Striga-Zea mays association; however, no such information exists for facultative hemiparasitic plants and their hosts, nor are the effects of host species understood. METHODS: Chlorophyll fluorimetry was used to study the effects of parasitism by the hemiparasite Rhinanthus minor on the grass Phleum bertolinii and the forb Plantago lanceolata, and the effects of host species on the photosynthetic apparatus of R. minor. KEY RESULTS: Parasitism by Rhinanthus led to a significant decrease in the host, and total (host + parasite) biomass in Phleum; however, in Plantago, no significant repression of growth was noted. Maximum quantum yield (F(v)/F(m)) was reduced in parasitized Plantago, relative to control plants, but not in Phleum. F(v)/F(m) was significantly lower in R. minor parasitizing Phleum than Plantago, suggesting Phleum to be a superior host to Plantago for R. minor. Steady-state quantum yield (Phi(PSII)) was significantly depressed in parasitized Phleum, but only at low irradiances in Plantago. Phi(PSII) was very low for R. minor grown on Plantago, but not Phleum. CONCLUSIONS: Shown here is the first evidence of the suppression of host photosynthesis by a facultative hemiparasitic plant, which has significant effects on total biomass production. Host identity is a significant factor in parasite success, with the forb Plantago lanceolata exhibiting apparent chemical as well as previously identified physical defences to parasitism. It is proposed that the electron transport rate (as denoted by Phi(PSII)) represents the limiting factor for biomass accumulation in this system, and that Plantago is able to suppress the growth of Rhinanthus by suppressing the electron transport rate.     

3-D cell-level chlorophyll fluorescence imaging of ozone-injured sunflower leaves using a new passive light microscope system

A passive light microscope system has been developed, capable of reconstructing an extended-focus 3-D cell-level image of chlorophyll fluorescence and Phi(PSII) of intact attached leaves using a limited number of focal plane images of chlorophyll fluorescence. Using this system, the relationships between the depth of the mesophyll cells in spongy tissue and the intensity of the chlorophyll fluorescence and the Phi(PSII) were investigated in sunflower leaves exposed to 300 ppb ozone for 12 h at a PPFD of 300 micromol m(-2) s(-1) actinic light. After ozone exposure, fluorescence intensity (F) largely decreased in the cells just under the epidermal cells (within approximately 20 microm of the epidermal cells), but the sites where fluorescence intensity decreased had no relationship to the position of the stomata. By contrast, the distribution of Phi(PSII) showed no change after the ozone exposure. These findings suggest that ozone-induced inhibition occurs in the cells just under the epidermal cells by reducing the light absorption of the chloroplasts, while the operating quantum efficiency of PSII photochemistry is maintained.     

Impairment of the photosynthetic apparatus by oxidative stress induced by photosensitization reaction of protoporphyrin IX

Treatment with the herbicide acifluorfen-sodium (AF-Na), an inhibitor of protoporphyrinogen oxidase, caused an accumulation of protoporphyrin IX (Proto IX) , light-induced necrotic spots on the cucumber cotyledon within 12-24 h, and photobleaching after 48-72 h of light exposure. Proto IX-sensitized and singlet oxygen ((1)O(2))-mediated oxidative stress caused by AF-Na treatment impaired photosystem I (PSI), photosystem II (PSII) and whole chain electron transport reactions. As compared to controls, the F(v)/F(m) (variable to maximal chlorophyll a fluorescence) ratio of treated samples was reduced. The PSII electron donor NH(2)OH failed to restore the F(v)/F(m) ratio suggesting that the reduction of F(v)/F(m) reflects the loss of reaction center functions. This explanation is further supported by the practically near-similar loss of PSI and PSII activities. As revealed from the light saturation curve (rate of oxygen evolution as a function of light intensity), the reduction of PSII activity was both due to the reduction in the quantum yield at limiting light intensities and impairment of light-saturated electron transport. In treated cotyledons both the Q (due to recombination of Q(A)(-) with S(2)) and B (due to recombination of Q(B)(-) with S(2)/S(3)) band of thermoluminescence decreased by 50% suggesting a loss of active PSII reaction centers. In both the control and treated samples, the thermoluminescence yield of B band exhibited a periodicity of 4 suggesting normal functioning of the S states in centers that were still active. The low temperature (77 K) fluorescence emission spectra revealed that the F(695) band (that originates in CP-47) increased probably due to reduced energy transfer from the CP47 to the reaction center. These demonstrated an overall damage to the PSI and PSII reaction centers by (1)O(2) produced in response to photosensitization reaction of protoporphyrin IX in AF-Na-treated cucumber seedlings.     

Characterization of photosynthesis of flag leaves in a wheat hybrid and its parents grown under field conditions

Two wheat cultivars, one with high yield and the other with a high tolerance against oxidative stress, were compared with a hybrid of these two cultivars by investigating their photosynthetic characteristics of flag leaves. From the beginning of flowering to the 17th day, CO(2) assimilation rate (P(max)) was maintained and there were no appreciable differences between the hybrid and its parents. P(max) showed no decrease at noon compared to that in the morning. From the 20th to the 30th day of flowering, P(max) decreased significantly, and this decrease was significantly less in the hybrid than in its parents. The actual photosystem II (PSII) efficiency (Phi(PSII)) and the maximal efficiency of PSII photochemistry (F(v)/F(m)) showed a significant decrease only on the 30th day after anthesis; this decline was much less marked in the hybrid relative to its parents, both in the morning and at noon. A decrease occurred in Phi(PSII) and F(v)/F(m) at noon when compared to that in the morning, but this decrease was less marked in the hybrid than in its parents. Rubisco activity decreased significantly from the 13th day of flowering and was higher in the morning than at noon both in the hybrid and its parents. However, the hybrid always showed a higher value of Rubisco activity. The activities of posphoenolpyruvate carboxylase and pyruvate phosphate dikinase showed similar changes to those in Rubisco activity, particularly from the 20th to 30th day. The results of this study suggest that the higher photosynthetic capacity of the flag leaf in the hybrid can help to accumulate more dry material, and may be the physiological basis for higher yield over its parents.     

Desiccation tolerance in the moss Polytrichum formosum: physiological and fine-structural changes during desiccation and recovery

BACKGROUND AND AIMS: This study explores basic physiological features and time relations of recovery of photosynthetic activity and CO(2) uptake following rehydration of a desiccation-tolerant moss in relation to the full temporal sequence of cytological changes associated with recovery to the normal hydrated state. It seeks reconciliation of the apparently conflicting published physiological and cytological evidence on recovery from desiccation in bryophytes. METHODS: Observations were made of water-stress responses and recovery using infrared gas analysis and modulated chlorophyll fluorescence, and of structural and ultrastructural changes by light and transmission electron microscopy. KEY RESULTS: Net CO(2) uptake fell to zero at approx. 40 % RWC, paralleling the fluorescence parameter PhiPSII at 200 micromol m(-2) s(-1) PPFD. On re-wetting the moss after 9-18 d desiccation, the initially negative net CO(2) uptake became positive 10-30 min after re-wetting, restoring a net carbon balance after approx. 0.3-1 h. The parameter F(v)/F(m) reached approx. 80 % of its pre-desiccation value within approx. 10 min of re-wetting. In the presence of the protein-synthesis inhibitors chloramphenicol and cycloheximide, recovery of F(v)/F(m) (and CO(2) exchange) proceeded normally in the dark, but declined rapidly in the light. Though initial recovery was rapid, both net CO(2) uptake and F(v)/F(m) required approx. 24 h to recover completely to pre-desiccation values. The fixation protocols produced neither swelling of tissues nor plasmolysis. Thylakoids, grana and mitochondrial cristae remained intact throughout the drying-re-wetting cycle, but there were striking changes in the form of the organelles, especially the chloroplasts, which had prominent lobes and lamellar extensions in the normally hydrated state, but rounded off when desiccated, returning slowly to their normal state within approx. 24 h of re-wetting. Sub-cellular events during desiccation and re-wetting were generally similar to those seen in published data from the pteridophyte Selaginella lepidophylla. CONCLUSIONS: Initial recovery of respiration and photosynthesis (as of protein synthesis) is very rapid, and independent of protein synthesis, suggesting physical reactivation of systems conserved intact through desiccation and rehydration, but full recovery takes approx. 24 h. This is consistent with the cytological evidence, which shows the thylakoids and cristae remaining intact through the whole course of dehydration and rehydration. Substantial and co-ordinated changes in other cell components, which must affect spatial relationships of organelles and metabolic systems, return to normal on a time span similar to full recovery of photosynthesis. Comparison of the present data with recently published results suggests a significant role for the cytoskeleton in desiccation responses.     

Leaf movements and photoinhibition in relation to water stress in field-grown beans

Photoinhibition in plants depends on the extent of light energy being absorbed in excess of what can be used in photochemistry and is expected to increase as environmental constraints limit CO2 assimilation. Water stress induces the closure of stomata, limiting carbon availability at the carboxylation sites in the chloroplasts and, therefore, resulting in an excessive excitation of the photosynthetic apparatus, particularly photosystem II (PSII). Mechanisms have evolved in plants in order to protect against photoinhibition, such as non-photochemical energy dissipation, chlorophyll concentration changes, chloroplast movements, increases in the capacity for scavenging the active oxygen species, and leaf movement or paraheliotropism, avoiding direct exposure to sun. In beans (Phaseolus vulgaris L.), paraheliotropism seems to be an important feature of the plant to avoid photoinhibition. The extent of the leaf movement is increased as the water potential drops, reducing light interception and maintaining a high proportion of open PSII reaction centres. Photoinhibition in water-stressed beans, measured as the capacity to recover F(v)/F(m), is not higher than in well-watered plants and leaf temperature is maintained below the ambient, despite the closure of stomata. Bean leaves restrained from moving, increase leaf temperature and reduce qP, the content of D1 protein and the capacity to recover F(v)/F(m) after dark adaptation, the extent of such changes being higher in water-stressed plants. Data are presented suggesting that even though protective under water stress, paraheliotropism, by reducing light interception, affects the capacity to maintain high CO2 assimilation rates throughout the day in well-watered plants.     

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.     

Energy Transfer from Phycobilisomes to Photosystems of Nostoc flagelliforme Born. et Flah. During the Rewetting Course and Its Physiological Significance

During the non-frost season, the condensation of dew makes Nostocflagelliforme Born. et Flah., a highly drought-tolerant terrestrial cyanobacterium, frequently undergo rehydration-dehydration.Rehydration begins in the dark at night. After rewetting in the dark, photochemical activity and the structure of photosystem (PS)II were not recovered at all; the structure of PSI, energy transfer in phycobilisomes, and energy transfer from phycobilisomes to PSI were recovered within 5 min, as in the light. The recovery of energy transfer from phycobilisomes to PSII was light dependent and energy transfer from phycobilisomes to PSII was only partially recovered in the dark. These results suggest that the two-trigger control (water and light) of photosynthetic recovery may make IV, flagelliforme avoid unnecessary energy consumption and, at the same time, the partial recovery of energy transfer from phycobilisomes to PSII in the dark could help N. flagelliforme accumulate more photosynthetic products during the transient period of rehydration-dehydration.     

Changes in the redox potential of primary and secondary electron-accepting quinones in photosystem II confer increased resistance to photoinhibition in low-temperature-acclimated Arabidopsis

Exposure of control (non-hardened) Arabidopsis leaves for 2 h at high irradiance at 5 degrees C resulted in a 55% decrease in photosystem II (PSII) photochemical efficiency as indicated by F(v)/F(m). In contrast, cold-acclimated leaves exposed to the same conditions showed only a 22% decrease in F(v)/F(m). Thermoluminescence was used to assess the possible role(s) of PSII recombination events in this differential resistance to photoinhibition. Thermoluminescence measurements of PSII revealed that S(2)Q(A)(-) recombination was shifted to higher temperatures, whereas the characteristic temperature of the S(2)Q(B)(-) recombination was shifted to lower temperatures in cold-acclimated plants. These shifts in recombination temperatures indicate higher activation energy for the S(2)Q(A)(-) redox pair and lower activation energy for the S(2)Q(B)(-) redox pair. This results in an increase in the free-energy gap between P680(+)Q(A)(-) and P680(+)Pheo(-) and a narrowing of the free energy gap between primary and secondary electron-accepting quinones in PSII electron acceptors. We propose that these effects result in an increased population of reduced primary electron-accepting quinone in PSII, facilitating non-radiative P680(+)Q(A)(-) radical pair recombination. Enhanced reaction center quenching was confirmed using in vivo chlorophyll fluorescence-quenching analysis. The enhanced dissipation of excess light energy within the reaction center of PSII, in part, accounts for the observed increase in resistance to high-light stress in cold-acclimated Arabidopsis plants.