Implications of alternative electron sinks in increased resistance of PSII and PSI photochemistry to high light stress in cold-acclimated Arabidopsis thaliana.

Exposure of control (non-hardened) Arabidopsis leaves to high light stress at 5 °C resulted in a decrease of both photosystem II (PSII) (45 %) and Photosystem I (PSI) (35 %) photochemical efficiencies compared to non-treated plants. In contrast, cold-acclimated (CA) leaves exhibited only 35 and 22 % decrease of PSII and PSI photochemistry, respectively, under the same conditions. This was accompanied by an accelerated rate of P700(+) re-reduction, indicating an up-regulation of PSI-dependent cyclic electron transport (CET). Interestingly, the expression of the NDH-H gene and the relative abundance of the Ndh-H polypeptide, representing the NDH-complex, decreased as a result of exposure to low temperatures. This indicates that the NDH-dependent CET pathway cannot be involved and the overall stimulation of CET in CA plants is due to up-regulation of the ferredoxin-plastoquinone reductase, antimycin A-sensitive CET pathway. The lower abundance of NDH complex also implies lower activity of the chlororespiratory pathway in CA plants, although the expression level and overall abundance of the other well-characterized component involved in chlororespiration, the plastid terminal oxidase (PTOX), was up-regulated at low temperatures. This suggests increased PTOX-mediated alternative electron flow to oxygen in plants exposed to low temperatures. Indeed, the estimated proportion of O(2)-dependent linear electron transport not utilized in carbon assimilation and not directed to photorespiration was twofold higher in CA Arabidopsis. The possible involvement of alternative electron transport pathways in inducing greater resistance of both PSII and PSI to high light stress in CA plants is discussed.

Chlorophyll fluorescence analysis of cyanobacterial photosynthesis and acclimation.

Cyanobacteria are ecologically important photosynthetic prokaryotes that also serve as popular model organisms for studies of photosynthesis and gene regulation. Both molecular and ecological studies of cyanobacteria benefit from real-time information on photosynthesis and acclimation. Monitoring in vivo chlorophyll fluorescence can provide noninvasive measures of photosynthetic physiology in a wide range of cyanobacteria and cyanolichens and requires only small samples. Cyanobacterial fluorescence patterns are distinct from those of plants, because of key structural and functional properties of cyanobacteria. These include significant fluorescence emission from the light-harvesting phycobiliproteins; large and rapid changes in fluorescence yield (state transitions) which depend on metabolic and environmental conditions; and flexible, overlapping respiratory and photosynthetic electron transport chains. The fluorescence parameters FV/FM, FV'/FM',qp,qN, NPQ, and phiPS II were originally developed to extract information from the fluorescence signals of higher plants. In this review, we consider how the special properties of cyanobacteria can be accommodated and used to extract biologically useful information from cyanobacterial in vivo chlorophyll fluorescence signals. We describe how the pattern of fluorescence yield versus light intensity can be used to predict the acclimated light level for a cyanobacterial population, giving information valuable for both laboratory and field studies of acclimation processes. The size of the change in fluorescence yield during dark-to-light transitions can provide information on respiration and the iron status of the cyanobacteria. Finally, fluorescence parameters can be used to estimate the electron transport rate at the acclimated growth light intensity.

Low-Temperature Effects on Photosynthesis and Correlation with Freezing Tolerance in Spring and Winter Cultivars of Wheat and Rye.

Winter cultivars of rye (Secale cereale L., cv Musketeer) and wheat (Triticum aestivum L. cvs Kharkov and Monopol), but not a spring cultivar of wheat (Glenlea), grown at cold-hardening temperatures showed, at high irradiances, a higher proportion of oxidized to reduced primary, stable quinone receptor (QA) than did the same cultivars grown under nonhardening conditions. In addition, there was a positive correlation between the effects of low-growth temperature on this increased proportion of oxidized QA, and a concomitant increase in the capacity for photosynthesis, and LT50, the temperature at which 50% of the seedlings are killed, in cultivars showing different freezing tolerances. This suggests that low-temperature modulation of the photosynthetic apparatus may be an important factor during the induction of freezing resistance in cereals. Finally, the control of photosystem II photochemistry by nonphotochemical quenching of excitation energy was identical for nonhardened and cold-hardened winter rye. However, examination of measuring temperature effects per se revealed that, irrespective of growth temperature, nonphotochemical quenching exerted a stronger control on photosystem II photochemistry at 10[deg] C rather than at 20[deg] C.

Effect of Cold Hardening on the Components of Respiratory Decarboxylation in the Light and in the Dark in Leaves of Winter Rye.

In the dark, all decarboxylation reactions are associated with the oxidase reactions of mitochondrial electron transport. In the light, photorespiration is also active in photosynthetic cells. In winter rye (Secale cereale L.), cold hardening resulted in a 2-fold increase in the rate of dark respiratory CO2 release from leaves compared with nonhardened (NH) controls. However, in the light, NH and cold-hardened (CH) leaves had comparable rates of oxidase decarboxylation and total intracellular decarboxylation. Furthermore, whereas CH leaves showed similar rates of total oxidase decarboxylation in the dark and light, NH leaves showed a 2-fold increase in total oxidase activity in the light compared with the dark. Light suppressed oxidase decarboxylation of end products of photosynthesis 2-fold in NH leaves and 3-fold in CH leaves in air. However, in high-CO2, light did not suppress the oxidase decarboxylation of end products. Thus, the decrease in oxidase decarboxylation of end products observed in the light and in air reflected glycolate-cycle-related inhibition of tricarboxylic acid cycle activity. We also showed that the glycolate cycle was involved in the decarboxylation of the end products of photosynthesis in both NH and CH leaves, suggesting a flow of fixed carbon out of the starch pool in the light.

Cold Hardening of Spring and Winter Wheat and Rape Results in Differential Effects on Growth, Carbon Metabolism, and Carbohydrate Content.

The effect of long-term (months) exposure to low temperature (5[deg]C) on growth, photosynthesis, and carbon metabolism was studied in spring and winter cultivars of wheat (Triticum aestivum) and rape (Brassica napus). Cold-grown winter rape and winter wheat maintained higher net assimilation rates and higher in situ CO2 exchange rates than the respective cold-grown spring cultivars. In particular, the relative growth rate of spring rape declined over time at low temperature, and this was associated with a 92% loss in in situ CO2 exchange rates. Associated with the high photosynthetic rates of cold-grown winter cultivars was a 2-fold increase per unit of protein in both stromal and cytosolic fructose-1,6-bisphosphatase activity and a 1.5- to 2-fold increase in sucrose-phosphate synthase activity. Neither spring cultivar increased enzyme activity on a per unit of protein basis. We suggest that the recovery of photosynthetic capacity at low temperature and the regulation of enzymatic activity represent acclimation in winter cultivars. This allow these overwintering herbaceous annuals to maximize the production of sugars with possible cryoprotective function and to accumulate sufficient carbohydrate storage reserve to support basal metabolism and regrowth in the spring.

Effects of a Short-Term Shift to Low Temperature and of Long-Term Cold Hardening on Photosynthesis and Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase and Sucrose Phosphate Synthase Activity in Leaves of Winter Rye (Secale cereale L.).

The effect of a short-term (hours) shift to low temperature (5[deg]C) and long-term (months) cold hardening on photosynthesis and carbon metabolism was studied in winter rye (Secale cereale L. cv Musketeer). Cold-hardened plants grown at 5[deg]C exhibited 25% higher in situ CO2 exchange rates than nonhardened plants grown at 24[deg]C. Cold-hardened plants maintained these high rates throughout the day, in contrast to nonhardened plants, which showed a gradual decline in photosynthesis after 3 h. Associated with the increase in photosynthetic capacity following cold hardening was an increase in ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose phosphate synthase activity and 3- to 4-fold increases in the pools of associated metabolites. Leaves of nonhardened plants shifted overnight to 5[deg]C required 9 h in the light at 5[deg]C before maximum rates of photosynthesis were reached. The gradual increase in photosynthesis in leaves shifted to 5[deg]C was correlated with a sharp decline in the 3-phosphoglycerate/triose phosphate ratio and by an increase in the ribulose bisphosphate/3-phosphoglycerate ratio, indicating the gradual easing of aninorganic phosphate-mediated feedback inhibition on photo-synthesis. We suggest that the strong recovery of photosynthesis in winter rye following cold hardening indicates that the buildup of photosynthetic enzymes, as well as those involved in sucrose synthesis, is an adaptive response that enables these plants to maximize the production of sugars that have both cryoprotective and storage functions that are critical to the performance of these cultivars during over-wintering.

Accumulation of Zeaxanthin in Abscisic Acid-Deficient Mutants of Arabidopsis Does Not Affect Chlorophyll Fluorescence Quenching or Sensitivity to Photoinhibition in Vivo.

Abscisic acid (ABA)-deficient mutants of Arabidopsis do not synthesize the epoxy-xanthophylls antheraxanthin, violaxanthin, or neoxanthin. However, thylakoid membranes from these mutants contain 3-fold more zeaxanthin than wild-type plants. This increase in zeaxanthin occurs as a stoichiometric replacement of the missing violaxanthin and neoxanthin within the pigment-protein complexes of both photosystem I and photosystem II (PSII). The retention of zeaxanthin in the dark by ABA-deficient mutants sensitizes the leaves to the development of nonphotochemical quenching (NPQ) during the first 2 to 4 min following a dark-light transition. However, the increase in pool size does not result in any increase in steady-state NPQ. When we exposed wild-type and ABA-deficient mutants leaves to twice growth irradiance, the mutants developed lower maximal NPQ but suffered similar photoinhibition to wildtype, measured both as a decline in the ratio of variable to maximal fluorescence and as a loss of functional PSII centers from oxygen flash yield measurements. These results suggest that only a few of the zeaxanthin molecules present within the light-harvesting antenna of PSII may be involved in NPQ and neither the accumulation of a large pool of zeaxanthin within the antenna of PSII nor an increase in conversion of violaxanthin to zeaxanthin will necessarily enhance photoprotective energy dissipation.

Novel amplification of non-photochemical chlorophyll fluorescence quenching following viral infection in Chlorella.

In higher plants non-photochemical dissipation of excess light, trapped by the pigment pool of photosystem II, prevents photodamage to the photosynthetic apparatus. We report here that an algal virus infecting Chlorella strain Pbi induces non-photochemical quenching of photosystem II fluorescence, indicating enhanced loss of absorbed light energy from photosystem II. This phenomenon occurs soon after the establishment of the virus infection cycle and is observed at low irradiance (20 micromol quanta m-2 s-1). At low light, infection associated non-photochemical quenching is not linked to extensive conversion of violaxanthin to antheraxanthin and zeaxanthin. However, such conversion occurs rapidly (2-10 min) in infected cells under conditions of high irradiance (100-300 micromol quanta m-2 s-1). Under similar conditions uninfected Chlorella cells do not display significant changes in non-photochemical quenching.

The zinc, copper, and manganese status of children with malabsorption syndromes and inborn errors of metabolism.

The zinc, copper, and manganese status of seven children with malabsorption syndromes of varied etiology (MVE) and 12 with inborn errors of metabolism (IEM) receiving semi-synthetic diets was investigated using serum and hair trace element concentrations, dietary trace element intakes, and anthropometric measurements as the principal indices. The hair zinc levels of both test groups and hair manganese levels of the IEM group were significantly lower (p<0.05) than those of their respective healthy controls matched by age, sex, and geographic location, despite comparable dietary zinc and manganese intakes in test and control subjects. Four subjects from the malabsorption and five from the inborn errors group had hair zinc levels below 100 µg/g (range 30-88 µg/g). Of these nine subjects, serum zinc levels were determined for six, and five were less than normal (range 64-74 µg/dL). In contrast, the copper status of the MVE and IEM subjects, as indicated by hair and dietary copper levels, was not lower than the controls. Mean serum copper levels were 136±30 and 171±40 µg/dL for the IEM and MVE groups, respectively. Levels for the MVE subjects were higher than published normal values. The suboptimal zinc and manganese status observed in some of these test subjects probably arose from malabsorption and decreased availability of dietary zinc and manganese. However, the zinc depletion was not severe enough to result in linear growth retardation.

High temporal resolution tracing of photosynthate carbon from the tree canopy to forest soil microorganisms.

Half of the biological activity in forest soils is supported by recent tree photosynthate, but no study has traced in detail this flux of carbon from the canopy to soil microorganisms in the field. Using (13)CO(2), we pulse-labelled over 1.5 h a 50-m(2) patch of 4-m-tall boreal Pinus sylvestris forest in a 200-m(3) chamber. Tracer levels peaked after 24 h in soluble carbohydrates in the phloem at a height of 0.3 m, after 2-4 d in soil respiratory efflux, after 4-7 d in ectomycorrhizal roots, and after 2-4 d in soil microbial cytoplasm. Carbon in the active pool in needles, in soluble carbohydrates in phloem and in soil respiratory efflux had half-lives of 22, 17 and 35 h, respectively. Carbon in soil microbial cytoplasm had a half-life of 280 h, while the carbon in ectomycorrhizal root tips turned over much more slowly. Simultaneous labelling of the soil with (15)NH(+)(4) showed that the ectomycorrhizal roots, which were the strongest sinks for photosynthate, were also the most active sinks for soil nitrogen. These observations highlight the close temporal coupling between tree canopy photosynthesis and a significant fraction of soil activity in forests.

Low growth temperature effects a differential inhibition of photosynthesis in spring and winter wheat.

In vivo room temperature chlorophyll a fluorescence coupled with CO(2) and O(2) exchange was measured to determine photosynthetic limitation(s) for spring and winter wheat (Triticum aestivum L.) grown at cold-hardening temperatures (5 degrees C/5 degrees C, day/night). Plants of comparable physiological stage, but grown at nonhardening temperatures (20 degrees C/16 degrees C, day/night) were used in comparison. Winter wheat cultivars grown at 5 degrees C had light-saturated rates of CO(2) exchange and apparent photon yields for CO(2) exchange and O(2) evolution that were equal to or greater than those of winter cultivars grown at 20 degrees C. In contrast, spring wheat cultivars grown at 5 degrees C showed 35% lower apparent photon yields for CO(2) exchange and 25% lower light-saturated rates of CO(2) exchange compared to 20 degrees C grown controls. The lower CO(2) exchange capacity is not associated with a lower efficiency of photosystem II activity measured as either the apparent photon yield for O(2) evolution, the ratio of variable to maximal fluorescence, or the level of reduced primary quinone electron acceptor maintained at steady-state photosynthesis, and is most likely associated with carbon metabolism. The lower CO(2) exchange capacity of the spring cultivars developed following long-term exposure to low temperature and did not occur following over-night exposure of nonhardened plants to 5 degrees C.

Effect of cold hardening on sensitivity of winter and spring wheat leaves to short-term photoinhibition and recovery of photosynthesis.

Photoinhibition of photosynthesis and its recovery were studied in wheat (Triticum aestivum L.) leaves grown at nonhardening (20 degrees C) and cold-hardening (5 degrees C) temperatures. Cold-hardened wheat leaves were less susceptible to photoinhibition at 5 degrees C than nonhardened leaves, and the winter cultivars, Kharkov and Monopol, were less susceptible than the spring cultivar, Glenlea. The presence of chloramphenicol, a chloroplastic protein synthesis inhibitor, increased the susceptibility to photoinhibition, but cold-hardened leaves still remained less susceptible to photoinhibition than nonhardened leaves. Recovery at 50 mumol m(-2) s(-1) photosynthetic photon flux density and 20 degrees C was at least biphasic, with a fast and a slow phase in all cultivars. Cold-hardened leaves recovered maximum fluorescence and maximum variable fluorescence in the dark-adapted state during the fast phase at a rate of 42% h(-1) compared with 22% h(-1) for nonhardened leaves. The slow phase occurred at similar rates (2% h(-1)) in cold-hardened and nonhardened leaves. Full recovery required up to 30 h. Fast-recovery phase was not reduced by either lowering the recovery temperature to 5 degrees C or by the presence of chloramphenicol. Slow-recovery phase was inhibited by both treatments. Hence, the fast phase of recovery does not require de novo chloroplast protein synthesis. In addition, only approximately 60% of the photochemical efficiency lost through photoinhibition at 5 degrees C was associated with lost [(14)C]atrazine binding and, hence, with damage to the secondary quinone electron acceptor for photosystem II-binding site. We conclude that the decrease in susceptibility to photoinhibition exhibited following cold hardening of winter and spring cultivars is not due to an increased capacity for repair of photoinhibitory damage at 5 degrees C but reflects intrinsic properties of the cold-hardened photosynthetic apparatus. A model to account for the fast component of recovery is discussed.

The role of inorganic phosphate in the development of freezing tolerance and the acclimatization of photosynthesis to low temperature is revealed by the pho mutants of Arabidopsis thaliana.

Low temperature inhibits sucrose synthesis, leading to a phosphate-limitation of photosynthesis. We have used the Arabidopsis pho1-2 and pho2-1 mutants with decreased and increased shoot phosphate, respectively, to investigate whether low phosphate triggers cold acclimatization of photosynthetic carbon metabolism. Wild-type Arabidopsis, pho1-2 and pho2-1 were grown at 23 degrees C and transferred to 5 degrees C to investigate acclimatization in pre-existing leaves and in new leaves developing at 5 degrees C. The development of frost tolerance and the accumulation of proline and sugars was unaltered or improved in pho1-2, and impaired in pho2-1. Sucrose phosphate synthase and cytoplasmic fructose-1,6-bisphosphatase activity and protein increase after transfer to 5 degrees C. This increase was accentuated in pho1-2 and attenuated in pho2-1. RBCS and LHCB2 transcript levels decrease in pre-formed wild-type leaves after transfer to 5 degrees C and recover in new leaves that develop at 5 degrees C. The initial decrease was attenuated in pho1-2, and accentuated in pho2-1, where the recovery in new leaves was also suppressed. Rubisco activity increased in wild-type leaves that developed at 5 degrees C. This increase was accentuated in pho1-2 and absent in pho2-1. NADP-glyceraldehyde-3-phosphate dehydrogenase, plastidic fructose-1,6-bisphosphatase and aldolase activity increase relative to phosphoglycerate kinase, transketolase and phosphoribulokinase in wild-type leaves at 5 degrees C. This shift was accentuated in pho1-2 and reversed in pho2-1. Transcript levels for COR genes increase transiently 1 day after transfer to 5 degrees C but were very low in leaves that developed at 5 degrees C in wild-type Arabidopsis, pho1-2 and pho2-1. We conclude that low phosphate plays an important role in triggering cold acclimatization of leaves, leading in particular to an increase of Rubisco expression, changes in other Calvin cycle enzymes to minimize sequestration of phosphate in metabolites, and increased expression of sucrose biosynthesis enzymes.

Development of Arabidopsis thaliana leaves at low temperatures releases the suppression of photosynthesis and photosynthetic gene expression despite the accumulation of soluble carbohydrates.

Arabidopsis thaliana plants were grown at 23 degrees C and changes in carbohydrate metabolism, photosynthesis and photosynthetic gene expression were studied after the plants were shifted to 5 degrees C. The responses of leaves shifted to 5 degrees C after development at 23 degrees C are compared to leaves that developed at 5 degrees C. Shifting warm developed leaves to 5 degrees C lead to a severe suppression of photosynthesis that correlated with a rapid and sustained accumulation of hexose phosphates and soluble sugars. Associated with the suppression of photosynthesis and the accumulation of soluble sugars was a reduction in the amount of transcript for genes encoding photosynthetic proteins (cab and rbcS). In contrast, leaves that developed at 5 degrees C showed an increase in photosynthesis and control levels of photosynthetic gene expression. This recovery occurred even though leaves that developed at 5 degrees C maintained large pools of soluble sugars. Leaves that developed at 5 degrees C also showed a strong upregulation of the cytosolic pathway for soluble sugar synthesis but not of the chloroplastic pathway for starch synthesis. This was shown at the level of both enzyme activity and the amount of transcript. Thus, development of Arabidopsis leaves at 5 degrees C resulted in metabolic changes that enabled them to produce and accumulate large soluble sugar pools without any associated suppression of photosynthesis or photosynthetic gene expression. These changes were also associated with enhanced freezing tolerance. We suggest that this reprogramming of carbohydrate metabolism associated with development at low temperature is essential to the development of full freezing tolerance and for winter survival of over-wintering herbaceous annuals.

Prevalence of respiratory symptoms, bronchial hyperresponsiveness and atopy in schoolchildren living in the Villawood area of Sydney.

Respiratory symptoms, bronchial hyperresponsiveness (BHR) and atopic status were measured in 1,217 schoolchildren, aged 8 to 12 years, living in the Villawood area of the city of Sydney. The findings are compared to those from studies previously conducted, using identical protocols, in the inland town of Wagga Wagga and in the coastal town in Belmont, NSW. There was a higher prevalence of respiratory symptoms in Villawood and Wagga Wagga (40%) than in Belmont (29%). The prevalence of BHR was 15% in Villawood and Belmont and 20% in Wagga Wagga. However, the distribution of severity of BHR was similar in each study town, reflecting the same pattern of responsiveness. The percentage of children who were atopic was higher in Villawood (44%) than in inland Wagga Wagga (39%) or coastal Belmont (40%). Children in the three areas differed in their reactions to the predominant allergen groups. There were more children who were reactive to grass pollens in the inland area and more children who were reactive to house dust mites in the coastal area. Children in Villawood had a high prevalence of reactivity both to house dust mites and to grass pollens. The Villawood children who were born in Australia had a higher prevalence of respiratory symptoms, of BHR and of atopy than the foreign-born children.

Phosphate status affects the gene expression, protein content and enzymatic activity of UDP-glucose pyrophosphorylase in wild-type and pho mutants of Arabidopsis.

The effects of inorganic phosphate (Pi) deficiency on the expression of the UDP-glucose pyrophosphorylase (UGPase) gene (Ugp), involved in sucrose synthesis/metabolism, and on carbohydrate status were investigated in different tissues of Arabidopsis thaliana (L.) Heynh. For leaves, a decrease in internal Pi status caused by growth of plants on a medium lacking Pi (-P conditions) led to an increase in the overall content of glucose and starch, but had little effect on sucrose content. The Pi deficiency also led to an increased carbohydrate content in stems/flowers, but not in roots. The expression of Ugp was upregulated in both leaves and roots, but not in stems/flowers. The effects of Pi status on Ugp expression were confirmed using leaves of both pho1-2 and pho2-1 mutants of Arabidopsis (Pi-deficient and Pi-accumulating, respectively) and by feeding the leaves with D-mannose, which acts as a sink for Pi. The Pi-status-dependent changes in Ugp expression followed the same patterns as those of ApS, a gene encoding the small subunit of ADP-glucose pyrophosphorylase, a key enzyme of starch synthesis. The changes in Ugp mRNA levels, depending on internal Pi status, were generally correlated with changes in UGPase protein content and enzymatic activity. This was demonstrated both for wild-type plants grown under Pi-deficiency and for Pi mutants. The data suggest that, under Pi-deficiency, UGPase represents a transcriptionally regulated step in sucrose synthesis/metabolism, involved in homeostatic mechanisms readjusting the nutritional status of a plant under Pi-stress conditions.

Effect of long-term photoinhibition on growth and photosynthesis of cold-hardened spring and winter wheat.

The effect of repeated exposure to high light (1200 µmol · m(-2) · s(-1) photosynthetic photon flux density, PPFD) at 5° C was examined in attached leaves of cold-grown spring (cv. Katepwa) and winter (cv. Kharkov) wheat (Triticum aestivum L.) over an eight-week period. Under these conditions, Kharkov winter wheat exhibited a daily reduction of 24% in FV/FM (the ratio of variable to maximal fluorescence in the dark-adapted state), in contrast to 41% for cold-grown Katepwa spring wheat. Both cultivars were able to recover from this daily suppression of FV/FM such that the leaves exhibited an average morning FV/FM of 0.651 ± 0.004. Fluorescence measurements made under steady-state conditions as a function of irradiance from 60 to 2000 µmol · m(-2) · s(-1) indicated that the yield of photosystem II (PSII) electron transport under light-saturating conditions was the same for photoinhibited and control cold-grown plants, regardless of cultivar. Repeated daily exposure to high light at low temperature did not increase resistance to short-term photoinhibition, although zeaxanthin levels increased by three- to fourfold. In addition, both cultivars increased the rate of dry-matter accumulation, relative to control plants maintained at 5° C and 250 µmol · m(-2) · s(-1) PPFD (10% and 28% for Katepwa and Kharkov, respectively), despite exhibiting suppressed fv/fm and reduced photon yields for O2 evolution following daily high-light treatments. Thus, although photosynthetic efficiency is suppressed by a longterm, photoinhibitory treatment, light-saturated rates of photosynthesis are sufficiently high during the high-light treatment to offset any reduction in photochemical efficiency of PSII. We suggest that in these cold-tolerant plants, photoinhibition of PSII may represent a longterm, stable, down-regulation of photochemistry to match the overall photosynthetic demand for ATP and reducing equivalents.

Reduced levels of cytochrome b 6/f in transgenic tobacco increases the excitation pressure on Photosystem II without increasing sensitivity to photoinhibition in vivo.

We have examined tobacco transformed with an antisense construct against the Rieske-FeS subunit of the cytochromeb 6 f complex, containing only 15 to 20% of the wild-type level of cytochrome f. The anti-Rieske-FeS leaves had a comparable chlorophyll and Photosystem II reaction center stoichiometry and a comparable carotenoid profile to the wild-type, with differences of less than 10% on a leaf area basis. When exposed to high irradiance, the anti-Rieske-FeS leaves showed a greatly increased closure of Photosystem II and a much reduced capacity to develop non-photochemical quenching compared with wild-type. However, contrary to our expectations, the anti-Rieske-FeS leaves were not more susceptible to photoinhibition than were wild-type leaves. Further, when we regulated the irradiance so that the excitation pressure on photosystem II was equivalent in both the anti-Rieske-FeS and wild-type leaves, the anti-Rieske-FeS leaves experienced much less photoinhibition than wild-type. The evidence from the anti-Rieske-FeS tobacco suggests that rapid photoinactivation of Photosystem II in vivo only occurs when closure of Photosystem II coincides with lumen acidification. These results suggest that the model of photoinhibition in vivo occurring principally because of limitations to electron withdrawal from photosystem II does not explain photoinhibition in these transgenic tobacco leaves, and we need to re-evaluate the twinned concepts of photoinhibition and photoprotection.

Photosynthesis, photoinhibition and low temperature acclimation in cold tolerant plants.

Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep QA oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO2 fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.

Two functionally distinct forms of the photosystem II reaction-center protein D1 in the cyanobacterium Synechococcus sp. PCC 7942.

The cyanobacterium Synechococcus sp. PCC 7942 possesses a small psbA multigene family that codes for two distinct forms of the photosystem II reaction-center protein D1 (D1:1 and D1:2). We showed previously that the normally predominant D1 form (D1:1) was rapidly replaced with the alternative D1:2 when cells adapted to a photon irradiance of 50 mumol.m-2.s-1 are shifted to 500 mumol.m-2.s-1 and that this interchange was readily reversible once cells were allowed to recover under the original growth conditions. By using the psbA inactivation mutants R2S2C3 and R2K1 (which synthesize only D1:1 and D1:2, respectively), we showed that this interchange between D1 forms was essential for limiting the degree of photoinhibition as well as enabling a rapid recovery of photosynthesis. In this report, we have extended these findings by examining whether any intrinsic functional differences exist between the two D1 forms that may afford increased resistance to photoinhibition. Initial studies on the rate of D1 degradation at three photon irradiances (50, 200, and 500 mumol.m-2.s-1) showed that the rates of degradation for both D1 forms increase with increasing photon flux density but that there was no significant difference between D1:1 and D1:2. Analysis of light-response curves for oxygen evolution for the mutants R2S2C3 and R2K1 revealed that cells with photosystem II reaction centers containing D1:2 have a higher apparent quantum yield (approximately 25%) than cells possessing D1:1. Further studies using chlorophyll a fluorescence measurements confirmed that R2K1 has a higher photochemical yield than R2S2C3; that is, a more efficient conversion of excitation energy from photon absorption into photochemistry. We believe that the higher photochemical efficiency of reaction centers containing D1:2 is causally related to the preferential induction of D1:2 at high light and thus may be an integral component of the protection mechanism within Synechococcus sp. PCC 7942 against photoinhibition.