Recovery Of Photosynthesis Research Articles

APPARENT LIGHT REQUIREMENT FOR ACTIVATION OF PHOTOSYNTHESIS UPON REHYDRATION OF DESICCATED BEACHROCK MICROBIAL MATS1

Photosynthetic electron transport of beachrock microbial mats growing in the intertidal zone of Heron Island (Great Barrier Reef, Australia) was investigated with a pulse amplitude modulation chl fluorometer providing four different excitation wavelengths for preferential excitation of the major algal groups (cyanobacteria, green algae, diatoms/dinoflagellates). A new type of fiberoptic emitter‐detector unit (PHYTO‐EDF) was used to measure chl fluorescence at the sample surface. Fluorescence signals mainly originated from cyanobacteria, which could be almost selectively assessed by 640‐nm excitation. Even after desiccation for long time periods under full sunlight, beachrock showed rapid recovery of photosynthesis after rehydration in the light (t1/2∼ 15 min). However, when rehydrated in the dark, the quantum yield of energy conversion of PSII remained zero over extended periods of time. Parallel measurements of O2concentration with an oxygen microoptode revealed zero oxygen concentration in the surface layer of rehydrated beachrock in the dark. Upon illumination, O2concentration increased in parallel with PSII quantum yield and decreased again to zero in the dark. It is proposed that oxygen is required for preventing complete dark reduction of the PSII acceptor pools via the NADPH‐dehydrogenase/chlororespiration pathway. This hypothesis is supported by the observation that PSII quantum yield could be partially induced in the dark by flushing with molecular oxygen.Abbreviations:EDF, emitter‐detector unit; Fo, fluor‐escence yield of dark‐adapted sample; Fm, maximal fluorescence yield measured during saturation pulse; Fv, variable fluorescence yield; LED, light‐emitting diode; PAM, pulse amplitude modulation; PQ, plastoquinone

Effects of solar UV radiation on photosynthesis of the marine angiosperm Posidonia oceanica from southern Spain

The effects of solar irradiance on the photosynthesis of the marine angiosperm Posido- nia oceanica L. Delile were investigated by means of pulse amplitude-modulated (PAM) fluorescence in the Natural Park of Cabo de Gata-Nijar, southern Spain. The study was conducted in 2 different seasons, summer (September 1996) and winter (February 1997). Daily variation in the effective quan- tum yield (∆F/Fm') was determined in plants growing at 2.5 m and in plants transferred from 15 m to 0.5 and 2.5 m depth. Three different experimental designs were conducted: (1) Incubation of shoots under 3 different solar radiation treatments using cut-off UV filters: full solar radiation (PAR + UV-A + UV-B), solar radiation without UV-B (PAR + UV-A) and solar radiation without UV (PAR); (2) short- term exposure (30 min) to high solar irradiance (photoinhibitory phase) under all treatments followed by transfer fo the plants to low irradiance for 4 h (recovery phase); (3) Preincubation of plants for 4 d under the 3 cited treatments followed by short-term exposure (30 min) to high solar irradiance under PAR + UV-A + UV-B, PAR + UV-A and PAR. A significant decrease in ∆ F/Fm' occurred from dawn to noon (18% in September and only 6% in February), followed by total recovery during the afternoon in both seasons. The highest decrease in ∆F/Fm' occurred in shoots illuminated with PAR + UV-A radi- ation. This decrease was more pronounced in winter than in summer, and was substantially higher in plants transferred from deep (15 m) to shallow water than in plants harvested at 2.5 m. Moreover, the recovery in the afternoon was higher in plants incubated at 2.5 m than in those transferred from 15 m to shallow waters. In the second set of experiments, short exposure (30 min) of plants collected from 2.5 m confirmed that inhibition under PAR + UV-A was higher than under PAR + UV-A + UV-B. In general, full recovery after exposure to high solar irradiance (PAR + UVA + UV-B) occurred only in PAR-treated plants in September. Finally, when shoots of P. oceanica were preincubated for 4 d under PAR, PAR + UV-A or PAR + UV-A + UV-B and then submitted to full solar irradiance at the water sur- face, the greatest reduction in ∆ F/Fm' was seen in plants grown under PAR, while the lowest occurred in PAR + UV-A + UV-B pretreated plants in both seasons. Recovery was higher in PAR + UV-A + UV- B pretreated plants. UV solar irradiance also affected both maximal electron transport rate (ETR) and the initial slope of the ETR-irradiance curves. P. oceanica seems to be well acclimated to high solar irradiance, showing a high capacity for recovery. Solar UV-B might be involved in the impairment and recovery of photosynthesis, since removal of UV-B promoted higher inhibition by solar irradi- ance. The absence of UV under high PAR for several days resulted in a partial loss of the capacity for photoprotection. We conclude that UV radiation could act in the natural habitat as a trigger for the induction of photoprotective mechanisms against high solar irradiance. The ecological implication of the beneficial role of UV-B in well-acclimated marine plants to high irradiance is discussed.

Cold acclimation of Arabidopsis thaliana results in incomplete recovery of photosynthetic capacity, associated with an increased reduction of the chloroplast stroma.

The effects of short-term cold stress and long-term cold acclimation on the light reactions of photosynthesis were examined in vivo to assess their contributions to photosynthetic acclimation to low temperature in Arabidopsis thaliana (L.) Heynh.. All photosynthetic measurements were made at the temperature of exposure: 23 degrees C for non-acclimated plants and 5 degrees C for cold-stressed and cold-acclimated plants. Three-day cold-stress treatments at 5 degrees C inhibited light-saturated rates of CO2 assimilation and O2 evolution by approximately 75%. The 3-day exposure to 5 degrees C also increased the proportion of reduced QA by 50%, decreased the yield of PSII electron transport by 65% and decreased PSI activity by 31%. In contrast, long-term cold acclimation resulted in a strong but incomplete recovery of light-saturated photosynthesis at 5 degrees C. The rates of light-saturated CO2 and O2 gas exchange and the in vivo yield of PSII activity under light-saturating conditions were only 35-40% lower, and the relative redox state of QA only 20% lower, at 5 degrees C after cold acclimation than in controls at 23 degrees C. PSI activity showed full recovery during long-term cold acclimation. Neither short-term cold stress nor long-term cold acclimation of Arabidopsis was associated with a limitation in ATP, and both treatments resulted in an increase in the ATP/NADPH ratio. This increase in ATP/NADPH was associated with an inhibition of PSI cyclic electron transport but there was no apparent change in the Mehler reaction activity in either cold-stressed or cold-acclimated leaves. Cold acclimation also resulted in an increase in the reduction state of the stroma, as indicated by an increased total activity and activation state of NADP-dependent malate dehydrogenase, and increased light-dependent activities of the major regulatory enzymes of the oxidative pentose-phosphate pathway. We suggest that the photosynthetic capacity during cold stress as well as cold acclimation is altered by limitations at the level of consumption of reducing power in carbon metabolism.

ABA Increases the Desiccation Tolerance of Photosynthesis in the Afromontane Understorey Moss Atrichum androgynum

The effect of pretreatment with abscisic acid (ABA) on the physiology of the moss Atrichum androgynum during a desiccation–rehydration cycle was examined. During rehydration following desiccation for 16h, net CO2fixation recovered much more slowly than photosystem II (PSII) activity, conditions conducive to the formation of reactive oxygen species (ROS) in the photosynthetic apparatus. Pretreatment with ABA increased the rate of recovery of photosynthesis and PSII activity, and also doubled non-photochemical quenching (NPQ). Increased NPQ activity will reduce ROS formation, and may explain in part how ABA hardens the moss to desiccation. In ABA-pretreated, but not untreated mosses, desiccation significantly increased the concentration of soluble sugars. Sugar accumulation may promote vitrification of the cytoplasm and protect membranes during desiccation. Starch concentrations in freshly collected A. androgynum were only approx. 40mg g−1dry mass; they rose slightly during desiccation but were only slightly affected by ABA pretreatment. ABA did not reduce chlorophyll breakdown during desiccation.

Protecting cotton photosynthesis during moderate chilling at high light intensity by increasing chloroplastic antioxidant enzyme activity.

This study examined the effect of increasing chloroplastic superoxide dismutase (SOD), ascorbate peroxidase (APX), or glutathione reductase (GR) activity via plant transformation of cotton on the initial recovery of photosynthesis following exposures to 10 degrees C and high photon flux density (PFD). Growing wild-type or non-expressing segregate plants (controls) and transformants at two PFDs (600 micromol m(-2) s(-1) and full sun) resulted in a range of total antioxidant enzyme activities. Total SOD activities above that for control leaves grown in full sun did not substantially improve the recoveries of CO(2)-saturated photosynthesis, especially for stress treatments lasting more than 1 h, while elevated APX or GR activity did improve recoveries after 1-3 h of the chilling treatment. No synergistic effects were noted when the activities of more than one antioxidant enzyme were elevated in transgenic hybrids. Although these results suggest that the protection of photosynthesis can be realized by reducing either superoxide or H(2)O(2) levels, thereby reducing the possibility of hydroxyl radical formation, the situation is complicated, since elevated APX or GR activity can improve recoveries even when additional SOD activity has no effect. In conclusion, to enhance the protection of photosynthesis using stroma-targeted antioxidant enzymes, enhancing metabolism associated with H(2)O(2) is more effective than enhancing the capacity for superoxide scavenging. Although small, the improvement in the protection of photosynthetic capacity may be sufficient to improve cotton yield in temperate regions with large diurnal temperature fluctuations.

Effect of growth conditions on isoprene emission and other thermotolerance‐enhancing compounds

AbstractIsoprene is emitted from the leaves of many plants in a light‐dependent and temperature‐sensitive manner. Plants lose a large fraction of photo‐assimilated carbon as isoprene but may benefit from improved recovery of photosynthesis following high‐temperature episodes. The capacity for isoprene emission of plants in natural conditions (assessed as the rate of isoprene emission under standard conditions) varies with weather. Temperature‐controlled greenhouses were used to study the role of temperature and light in influencing the capacity of oak leaves for isoprene synthesis. A comparison was made between the capacity for isoprene emission and the accumulation of other compounds suggested to increase thermotolerance of photosynthesis under two growth temperatures and two growth light intensities. It was found that the capacity for isoprene emission was increased by high temperature or high light. Xanthophyll cycle intermediates increased in high light, but not in high temperature, and the chloroplast small heat‐shock protein was not expressed in any of the growth conditions. Thus, of the three thermotolerance‐enhancing compounds studied, isoprene was the only one induced by the temperature used in this study.

Photosynthetic electron transport adjustments in overwintering Scots pine (Pinus sylvestris L.).

As shown before [C. Ottander et al. (1995) Planta 197:176-183], there is a severe inhibition of the photosystem (PS) II photochemical efficiency of Scots pine (Pinus sylvestris L.) during the winter. In contrast, the in vivo PSI photochemistry is less inhibited during winter as shown by in vivo measurements of deltaA820/A820 (P700+). There was also an enhanced cyclic electron transfer around PSI in winter-stressed needles as indicated by 4-fold faster reduction kinetics of P700+. The differential functional stability of PSII and PSI was accompanied by a 3.7-fold higher intersystem electron pool size, and a 5-fold increase in the stromal electron pool available for P700+ reduction. There was also a strong reduction of the QB band in the thermoluminescence glow curve and markedly slower Q-A re-oxidation in needles of winter pine, indicating an inhibition of electron transfer between QA and QB. The data presented indicate that the plastoquinone pool is largely reduced in winter pine, and that this reduced state is likely to be of metabolic rather than photochemical origin. The retention of PSI photochemistry, and the suggested metabolic reduction of the plastoquinone pool in winter stressed needles of Scots pine are discussed in terms of the need for enhanced photoprotection of the needles during the winter and the role of metabolically supplied energy for the recovery of photosynthesis from winter stress in evergreens.

Propanil activity, uptake and metabolism in resistant Echinochloa spp. biotypes

Field resistance of Echinochloa spp. to propanil has been previously reported in Costa Rica, Colombia and Arkansas (USA). In this study, the mechanism of resistance was investigated in three resistant (R) and three susceptible (S) biotypes. The shoot fresh weight reduction in pot‐grown plants from a post‐emergence spray of propanil at 2.44 kg a.i. ha−1 on biotypes R/S from Costa Rica, Colombia and Arkansas was 35/98%, 25/79% and 20/82% respectively. In vitro chlorophyll fluorescence data from leaf tissue incubated in propanil showed that photosynthesis was inhibited in all biotypes, indicating that the propanil‐binding site and enzyme were not altered. After transfer to herbicide‐free solution, photosynthesis recovered only in resistant biotypes, indicating that the mechanism of resistance was caused by enhanced metabolism of the herbicide. Simultaneous treatment with fenitrothion, an aryl acylamidase inhibitor, prevented the recovery of photosynthesis in leaf tissue in two resistant biotypes. In contrast, the cytochrome P450 mono‐oxygenase inhibitor, 1‐aminobenzotriazole, did not prevent recovery from propanil in leaf tissue. Application of 14C‐propanil to the second leaf of intact Echinochloa plants showed that c. 90% of the radioactivity remained in the treated leaf for up to 72 h after application. No major differences in translocation between R and S biotype plants were found. TLC analysis of tissue extracts from the treated leaves showed substantially less radioactivity associated with propanil, present after 72 h in rice or in the three R biotypes, compared with S biotypes.

Water Stress and Day-To-Day Variation in Apparent Photosynthetic Acclimation of Field-Grown Soybeans to Elevated Carbon Dioxide Concentration

Midday measurements of single leaf gas exchange rates of upper canopy leaves of soybeans grown in the field at 350 (AC) and 700 (EC) µmol(CO2) mol-1 in open topped chambers sometimes indicated up to 50 % higher net photosynthetic rates (PN) measured at EC in plants grown at AC compared to EC. On other days mean PN were nearly identical in the two growth [CO2] treatments. There was no seasonal pattern to the variable photosynthetic responses of soybean to growth [CO2]. Even on days with significantly lower PN in the plants grown at EC, there was no reduction in ribulose-1,5-bisphosphate carboxylase/oxygenase, chlorophyll, or soluble protein contents per unit of leaf area. Over three years, gas exchange evidence of acclimation occurred on days when either soil was dry or the water vapor pressure deficit was high (n = 12 d) and did not occur on days after rain or on days with low water vapor pressure deficit (n = 9 d). On days when photosynthetic acclimation was evident, midday leaf water potentials were consistently 0.2 to 0.3 MPa lower for the plants grown at EC than at AC. This suggested that greater susceptibility to water stress in plants grown at EC cause the apparent photosynthetic acclimation. In other experiments, plants were grown in well-watered pots in field chambers and removed to the laboratory early in the morning for gas exchange measurements. In these experiments, the amount of photosynthetic acclimation evident in the gas exchange measurements increased with the maximum water vapor pressure deficit on the day prior to the measurements, indicating a lag in the recovery of photosynthesis from water stress. The apparent increase in susceptibility to water stress in soybean plants grown at EC is opposite to that observed in some other species, where photosynthetic acclimation was evident under wet but not dry conditions, and may be related to the observation that hydraulic conductance is reduced in soybeans when grown at EC. The day-to-day variation in photosynthetic acclimation observed here may account for some of the conflicting results in the literature concerning the existence of acclimation to EC in field-grown plants.

Effect of Benzylaminopurine on Rehydration of Bean Plants after Water Stress

The possibility of improving the recovery of plant photosynthesis after water stress by cytokinin-induced stimulation of stomatal opening or delay of leaf senescence was tested. The 6-benzylaminopurine (BAP) in concentrations 1 and 10 μM was applied to the substrate (sand + nutrient solution) or sprayed on primary leaves of 14-d-old Phaseolus vulgaris L. plants sufficiently supplied with water or water-stressed for 4 d. The later ones having relative water content decreased to 69 % were fully rehydrated during the following three days. Parameters of photosynthesis and water relations were measured in primary leaves of 7-, 10-, 14-, and 17-d-old plants. Application of 1 μM BAP slightly delayed leaf senescence: in 17-d-old control plants, net photosynthetic rate (PN) and chlorophyll (Chl) content, and when sprayed on leaves also some of Chl a fluorescence kinetic parameters of BAP-treated leaves were slightly higher than those of untreated leaves. Both types of application of 1 μM BAP slightly improved recovery of plants during rehydration after water stress in terms of increased gad, gab and PN, i.e., parameters which were markedly decreased by mild water stress. However, contents of Chl a, Chl b and carotenoids and parameters of Chl a fluorescence kinetic were not markedly affected by mild water stress and after rehydration were not stimulated by 1 μM BAP. 10 μM BAP had mostly negative effects on the parameters measured.

Longevity of dry Myrothamnus flabellifolius in simulated field conditions

This study tested the length of time the desiccation-tolerantMyrothamnus flabellifolius could remain in the dry stateunder simulated field conditions, without losing viability. Dry plants werekeptat 50% relative humidity, with a 25/16 °Cday/night temperature regime and a daylight intensity of 1200μmol·m−2 s−1. At threemonthly intervals plants were rehydrated and the ability to resume respiration,photosynthesis, protein synthesis and transcription was assessed and changes insubcellular organisation and plant growth regulator content (zeatin, zeatinriboside and abscisic acid) monitored. Plants survived in the dry state foronly one year. The ability toresurrect metabolism in existing leaves was lost after nine months, after whichsurvival occurred due to regrowth of new leaves from meristems. There waslittleevidence of subcellular damage in leaf tissues of plants kept dry for up to sixmonths. These plants recovered respiration before the onset of translation (andthus repair) suggesting considerable subcellular protection of this metabolismagainst desiccation damage. Furthermore, full recovery of metabolism, includingphotosynthesis, occurred before the onset of transcription in these plants.Somesubcellular damage occurred in plants maintained dry for nine months. There wasincreased electrolyte leakage indicative of membrane damage which was repairedwith the onset of protein synthesis. Since this repair occurred before theonsetof transcription, it is likely that stored mRNA, present in the dry leaves, wasused. Recovery of photosynthesis in plants dried for 9 months was delayed untilthe onset of transcription. We propose that mRNA for recovery of thismetabolismis not stored during drying and thus damage to the photosynthetic apparatus canbe repaired only upon de novo transcription of the genome.In leaves that were able to resurrect, cytokinin content increasedtransiently just prior to onset of chlorophyll biosynthesis and translation,andabscisic acid content increased just prior to the onset of transcription. Theseplant growth regulator changes did not occur in leaves which did not resurrect,but we cannot distinguish whether it was the lack of signal, or the extent oftissue damage (or both) which prevented the repair and recovery of metabolisminthese tissues.

Patterns of desiccation tolerance and recovery in bryophytes

The study of desiccation tolerance in bryophytes avoids thecomplications of higher-plant vascular systems and complex leaf structures, butremains a multifaceted problem. Some of the pertinent questions have at leastpartial analogues in seed biology – events during a drying-rewettingcyclewith processes in seed maturation and germination, and the gradual loss ofviability on prolonged desiccation, and the relation of this to intensity ofdesiccation and temperature, with parallel questions in seed storage. Pastresearch on bryophyte desiccation tolerance is briefly reviewed. Evidence ispresented from chlorophyll-fluorescence measurements and experiments withmetabolic inhibitors that recovery of photosynthesis in bryophytes followingdesiccation depends mainly on rapid reactivation of pre-existing structures andinvolves only limited de novo protein synthesis. Followinginitial recovery, protein synthesis is demonstrably essential to themaintenanceof photosynthetic function in the light, but the rate of maintenance turnoverinthe dark appears to be slow. Factors leading to long-term desiccation damagearediverse; indications are that desiccation tolerant species often survive bestinthe range −100 to −200 MPa.

What does chlorophyll do over winter?

Chlorophyll in leaves of evergreens exposed to high-light levels in winter is at risk from damage caused by photo-oxidation under those conditions. This degradation would have the effect of removing some of the competitive advantage that might otherwise accrue to evergreen trees, which are able to photosynthesize before their deciduous neighbours have developed leaves. In a study of leaves of the evergreen snow gum (Eucalyptus pauciflora), Adam Gilmore and Marilyn Ball found that chlorophyll is present as special complexes during winter [Proc. Natl. Acad. Sci. U. S. A. (2000) 97, 11098–11101]. These pigment–protein complexes have an increased capacity to dissipate heat, which in turn reduces the danger of photo-oxidative bleaching and leads to rapid recovery of photosynthesis in the spring.

Photosynthetic acclimation to solar UV radiation of marine red algae from the warm-temperate coast of southern Spain: A review

A review is presented of the physiological mechanisms developed bybenthic macroalgae to cope with deleterious wavelengths, particularly UVradiation. Photoinhibition of photosynthesis, is a photoprotectivemechanism in various species studied in southern Spain. Incubations inoutdoor systems and transplantation experiments under natural radiationallowed to led to understanding of some the photoprotective strategies usedby red algae. Under conditions of enhanced UV-B radiation, algae inshallow sites show marked photoinhibition and rapid recovery ofphotosynthesis (dynamic photoinhibition), whereas algae from deeperlocations can suffer photodamage (chronic photoinhibition). Theexpression of this photoprotective strategy by intertidal species representsan efficient physiological adaptation to tolerate deleterious irradiance, whenlow tides coincide with the onset of solar radiation. Subtidal species canbe also exposed to high doses of UV radiation. This is particularly evidentin clear, Mediterranean waters, where light (including UV-B) can reach to10 m depth. The implications of photoacclimation processes formacroalgal ecology in warm-temperate littorals and the possibleconsequences for outdoor cultivation are outlined in terms of environmentalUV variability.

Rapid recovery of photosystems on rewetting desiccation-tolerant mosses: chlorophyll fluorescence and inhibitor experiments.

In the mosses Racomitrium lanuginosum, Anomodon viticulosus and Rhytidiadelphus loreus, after a few days air dry, F:(v)/F:(m) reached, within the first minute of remoistening in the dark, two-thirds or more of the value attained after 40 min. A fast initial phase of recovery was completed within 10-20 min after which further change was slow. Initial recovery of Phi(PSII) in the light was somewhat slower, but was generally substantially complete within a similar time. Remoistening with 0.3 mM cycloheximide (CHX) or 3 mM dithiothreitol (DTT) made little difference to this short-term (40 min) recovery of either F:(v)/F:(m) or Phi(PSII); 3 mM chloramphenicol (CMP) had little effect on recovery of F:(v)/F:(m), but resulted in substantial (though not total) depression of Phi(PSII) and (14)CO(2) uptake. Effects of the protein-synthesis inhibitors and DTT were much more clearly apparent in longer-term experiments (>20 h) but only in the light. In the dark, the three inhibitors had at most only slight effects over periods of 60-100 h. In the light, CMP-treated samples of all three species showed a progressive decline of dark-adapted F:(v)/F:(m), falling to zero within 1-5 d (possibly due to blocking of the turnover of the D1 protein of PSII) and accelerated by DTT. CHX-treated samples showed a similar but slower decline. In the shade-adapted and relatively desiccation-sensitive Rhytidiadelphus loreus, slow recovery of F:(v)/F:(m) continued in the dark even in the presence of CMP and CHX for much of the 142 h of the experiment. The results indicate that in desiccation-tolerant bryophytes recovery of photosynthesis after periods of a few days air dry requires only limited chloroplast protein synthesis and is substantially independent of protein synthesis in the cytoplasm.

Protection and storage of chlorophyll in overwintering evergreens.

How evergreen species store and protect chlorophyll during exposure to high light in winter remains unexplained. This study reveals that the evergreen snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) stores and protects its chlorophylls by forming special complexes that are unique to the winter-acclimated state. Our in vivo spectral and kinetic characterizations reveal a prominent component of the chlorophyll fluorescence spectrum around 715 nm at 77 K. This band coincides structurally with a loss of chlorophyll and an increase in energy-dissipating carotenoids. Functionally, the band coincides with an increased capacity to dissipate excess light energy, absorbed by the chlorophylls, as heat without intrathylakoid acidification. The increased heat dissipation helps protect the chlorophylls from photo-oxidative bleaching and thereby facilitates rapid recovery of photosynthesis in spring.

Efeitos da deficiência de água em plantas de Euterpe oleracea (açaí)

The water stress resistance of young acai plants Euterpe oleracea Mart. was determined in four-month-old plants, cultivated in punched black plastic bags filled with 7 kg of soil. The experiment was carried out in a greenhouse and irrigation was suspended until the photosynthesis drops to zero, after this moment daily irrigation was applied. During this dehydration period photosynthesis, stomatal conductance, and transpiration were measured daily. Small reductions in stomatal conductance, transpiration and photosynthesis were observed from the tenth day and became more pronounced with time. The photosynthetic rate fell to half of its maximum value when the water potential reached -1.9 MPa. After withholding irrigation for 61 days, the water stress was severe enough to reduce photosynthesis to zero; at this time the leaf water potential was -2.5 MPa. Recovery of photosynthesis, stomatal conductance and transpiration started one day after rewatering and reached values similar to the control after two weeks of daily irrigation (rehydration). An osmotic potential reduction was calculated from leaf pressure-volume data, as a result the estimated osmotic adjustment was 0.29 MPa at zero turgor and 0.36 MPa at full turgor. Such osmotic adjustment indicates an acai adaptation mechanism to moderate water stress.

PH regulation in apoplastic and cytoplasmic cell compartments of leaves.

The regulation of pH in the apoplast, cytosol and chloroplasts of intact leaves was studied by means of fluorescent pH indicators and as a response of photosynthesis to acid stress. The apoplastic pH increased under anaerobiosis. Aeration reversed this effect. Apoplastic responses to CO2, HCl or NH3 differed considerably. Whereas HCl and ammonia caused rapid acidification or alkalinization, the return to initial pH values was slow after cessation of fumigation. Addition of CO2 either did not produce the acidification expected on the basis of known apoplastic buffering or even caused some alkalinization. Removal of CO2 shifted the apoplastic pH into the alkaline range before the pH returned to initial steady-state levels. In the presence of vanadate, the alkaline shift was absent and the apoplastic pH returned slowly to the initial level when CO2 was removed from the atmosphere. In contrast to the response of the apoplast, anaerobiosis acidified the cytosol or, in some species, had little effect on its pH. Acidification was rapidly reversed upon re-admission of oxygen. The CO2-dependent pH changes were very fast in the cytosol. Considerable alkalinization was observed after removal of CO2 under aerobic, but not under anaerobic conditions. Rates of the re-entry of protons into the cytosol during recovery from CO2 stress increased in the presence of oxygen with the length of previous exposure to high CO2. Effective pH regulation in the chloroplasts was indicated by the recovery of photosynthesis after the transient inhibition of photosynthetic electron flow when CO2 was increased from 0.038% to 16% in air. As photosynthesis became inhibited under high CO2, reduction of the electron transport chain increased transiently. The time required for recovery of photosynthesis from inhibition during persistent CO2 stress was similar to the time required for establishing steady-state pH values in the cytosol under acid stress. The high capacity of leaf cells for the rapid re-attainment of pH homeostasis in the apoplast and the cytoplasm under acid or alkaline stress suggested the rapid activation or deactivation of membrane-localised proton-transporting enzymes and corresponding ion channel regulation for co-transport of anions or counter-transport of cations together with proton fluxes. Acidification of the cytoplasm appeared to activate energy-dependent proton export primarily into the vacuoles whereas apoplastic alkalinization resulted in the pumping of protons into the apoplast. Proton export rates from the cytosol into the apoplast after anaerobiosis were about 100 nmol (m2 leaf area)(-1) s(-1) or less. Proton export under acid stress into the vacuole was about 1200 nmol m(-2) s(-1). The kinetics of pH responses to the addition or withdrawal of CO2 indicated the presence of carbonic anhydrase in the cytosol, but not in the apoplast.

Photoinhibition of natural phytoplankton assemblages in Lake Erie exposed to solar ultraviolet radiation

Photoinhibition was examined in natural assemblages of phytoplankton from Lake Erie exposed to ambient solar radiation. The impacts on photosynthesis of photosynthetically active radiation (400-700 nm) (PAR), ultraviolet-A radiation (320-400 nm) (UVA), and ultraviolet-B radiation (295-320 nm) (UVB) were assessed at three sites on the lake using pulse amplitude modulated chlorophyll fluorescence. Short exposures (<= 30 min) to sunlight containing UVB (1.8-4.4 mmol·m-2) resulted in the rapid loss of up to 60% of photosystem II efficiency (in the dark-adapted state) (Fv/Fm) and quantum yield (in the light-adapted state) (ΔF/F'm). Exposure to UVA (46-105 mmol·m-2) generally diminished Fv/Fm and, to a lesser extent, ΔF/F'm. Short exposures to PAR (733-1588 mmol·m-2) had no significant effects on electron transport. Recovery from UVA- or UVB-induced photoinhibition was complete for Fv/Fm and 90% complete for ΔF/F'm after 2 h in low light. The results indicate that exposures of phytoplankton to surface radiation need only be short in duration to cause substantial UV inhibition of photosynthesis. However, depending on the kinetics of mixing of the water column, recovery of photosynthesis is possible if there is sufficient time for repair of UV damage. Future elevated levels of solar UVB due to ozone depletion could significantly inhibit primary production in mesotrophic lakes such as Lake Erie.

Involvement of solar UV-B radiation in recovery of inhibited photosynthesis in the brown alga Dictyota dichotoma (Hudson) Lamouroux

Daily variations of photosynthetic performance of the brown alga Dictyota dichotoma (Hudson) Lamouroux (Dictyotales) (estimated by Fv/Fm ratio and net O2-based photosynthesis) have been determined under full-spectrum solar radiation (PAR+UV-A+UV-B), solar radiation depleted of ultraviolet-B (PAR+UV-A) and solar radiation depleted of total ultraviolet (photosynthetically active radiation, PAR). In the daily course, the photosynthetic response of the alga is inhibited by the three solar radiation conditions at midday, but recovers during the afternoon to those values measured in the early morning only under PAR + UV-A + UV-B, or PAR alone. Under solar radiation depleted of UV-B, the recovery of photosynthesis is significantly lower than in the other two solar radiation conditions. The relative electron transport rate (ETR) as a function of PAR is calculated for the three radiation treatments, at noon after 2 h exposure. The ETR is strongly inhibited under these conditions. In the PAR and PAR + UV-A + UV-B treatments, photosynthesis recovers after 1.5 h in low irradiance (< 10 μmol photons m−2 s−1) and attains values measured in the early morning. In the absence of UV-B, the inhibitory effect is persistent, visible as a low initial slope and lower ETR at higher levels of irradiance. These data suggest that UV-B may be involved both in the impairment and the recovery of photosynthesis of D. dichotoma.