Epidermal UV-shielding and photosystem II adjustment in wild type and chlorina f2 mutant of barley during exposure to increased PAR and UV radiation

The wild-type barley (WT; Hordeum vulgare L.) and its chlorophyll (Chl) b-less mutant chlorina f2 (clo f2) grown under shaded conditions in a greenhouse were transferred to outdoor conditions in early June with predominantly bright sunny days. During 6 days following transfer of plants we monitored the content of photosynthetic pigments, functional state of photosystem II (PSII) by means of Chl fluorescence induction kinetics and epidermal UV-shielding efficiency using Chl fluorescence imaging technique. Clo f2 mutant was more sensitive to exposure to an enhanced natural solar irradiance than WT barley. Nevertheless, clo f2 as well as WT were able to cope with stressful outdoor conditions, as was documented by the recovery of Chl a content and the maximal photochemical efficiency of PSII (F_V/F_M) after an initial decline. This was due to the immediate carotenoid-mediated photoprotection, reflected by strongly increased total carotenoids content and thermal energy dissipation localized within light-harvesting complexes of PSII (assessed by non-photochemical quenching of minimal fluorescence level). The positive acclimation response was further documented by an enhanced light-saturated electron transport rate through PSII (ETR). Based on the ratios of blue- to UV-excited Chl fluorescence we found that for both WT and clo f2 epidermal UV-shielding increased clearly after transfer to outdoor conditions and reached a saturation level after 3 days. In comparison with WT, clo f2 exhibited lower ability to induce UV-shielding. The kinetics of UV-shielding development during the outdoor treatment was different for the particular leaf regions. We suggest that this is related to the different age and developmental stage of the tissue along the leaf blade. The complementarity of carotenoid-mediated photoprotection and UV-shielding in acclimation of the assimilatory apparatus to increased visible and UV radiation is discussed.     

Photosynthetic responses of sun- and shade-grown barley leaves to high light: is the lower PSII connectivity in shade leaves associated with protection against excess of light?

In this study, we have compared photosynthetic performance of barley leaves (Hordeum vulgare L.) grown under sun and shade light regimes during their entire growth period, under field conditions. Analyses were based on measurements of both slow and fast chlorophyll (Chl) a fluorescence kinetics, gas exchange, pigment composition; and of light incident on leaves during their growth. Both the shade and the sun barley leaves had similar Chl a/b and Chl/carotenoid ratios. The fluorescence induction analyses uncovered major functional differences between the sun and the shade leaves: lower connectivity among Photosystem II (PSII), decreased number of electron carriers, and limitations in electron transport between PSII and PSI in the shade leaves; but only low differences in the size of PSII antenna. We discuss the possible protective role of low connectivity between PSII units in shade leaves in keeping the excitation pressure at a lower, physiologically more acceptable level under high light conditions.     

The aba Mutant of Arabidopsis thaliana (L.) Heynh. Has Reduced Chlorophyll Fluorescence Yields and Reduced Thylakoid Stacking

It has been shown that the aba mutant of Arabidopsis thaliana (L.) Heynh. is impaired in epoxy-carotenoid biosynthesis and accumulates the epoxy-carotenoid precursor, zeaxanthin (C.D. Rock, J.A.D. Zeevaart [1991] Proc Natl Acad Sci USA 88: 7496-7499). In addition to providing conclusive evidence for the indirect pathway of abscisic acid biosynthesis from epoxy-carotenoids, the aba mutation offers a powerful means to study the function of xanthophylls (oxygenated carotenoids) in photosynthesis. We measured in vivo the chlorophyll (Chl) fluorescence parameters F_o (initial), F_m (maximum), F_v (variable = F_m − F_o), and t_½ (half-rise time of fluorescence induction) of wild-type (WT) and three allelic aba mutants. The mutant genotypes had significantly lower F_o and F_m values relative to those of WT. The F_v/F_m ratio and t_½, which are parameters affected by photochemical efficiency, photosystem II (PSII), and plastoquinone pool sizes, were similar in the aba alleles and WT. Because the aba genotypes accumulate high levels of zeaxanthin, which is involved in nonphotochemical quenching of Chl fluorescence, we propose that the reduced fluorescence yields in the aba genotypes are a consequence of the accumulated zeaxanthin. Measurement of PSII oxygen evolution rates in isolated thylakoid membranes of WT and aba-4 confirmed that quantum efficiency was not altered in aba-4 but indicated that the mutant had reduced PSII activity in vitro. Electron microscopy revealed an abnormal chloroplast ultrastructure in the aba plants: the mutants had significantly fewer thylakoid lamellae per granum stack but significantly more grana per chloroplast, as well as more chloroplasts per cell than WT. Immunoblot analysis established that aba-4 had normal levels of the Chl a/b-binding core polypeptide of PSII (CP29) and the PSII light-harvesting Chl a/b-binding complex. These results provide evidence for the role of zeaxanthin in nonphotochemical fluorescence quenching and suggest involvement of epoxy-carotenoids and/or zeaxanthin in thylakoid stacking and PSII activity.     

The interplay of anthocyanin biosynthesis and chlorophyll catabolism in senescing leaves and the question of photosystem II photoprotection

Fully exposed, senescing leaves of Cornus sanguinea and Parthenocissus quinquefolia display during autumn considerable variation in both anthocyanin and chlorophyll (Chl) concentrations. They were used in this study to test the hypothesis that anthocyanins may have a photoprotective function against photosystem II (PSII) photoinhibitory damage. The hypothesis could not be confirmed with field sampled leaves since maximum photochemical efficiency (F_v/F_m) of PSII was negatively correlated to anthocyanin concentration and the possible effects of anthocyanins were also confounded by a decrease in F_v/F_m with Chl loss. However, after short-term laboratory photoinhibitory trials, the percent decrease of F_v/F_m was independent of Chl concentration. In this case, a slight alleviation of PSII damage with increasing anthocyanins was observed in P. quinquefolia, while a similar trend in C. sanguinea was not statistically significant. It is inferred that the assumed photoprotection, if addressed to PSII, may be of limited advantage and only under adverse environmental conditions.     

Effects of salt stress on photosystem II efficiency and CO2 assimilation of two Syrian barley landraces

The photosynthetic activity of two Syrian barley landraces, Arabi (A.) Aswad and A. Abiad, grown under 120 mM NaCl, was studied, using gas exchange and chlorophyll (Chl) a fluorescence transient (OJIP) measurements. Salt treatment of barley seedlings decreased both the rates of photosynthesis and photosystem II (PSII) activity, as evaluated from chlorophyll fluorescence data. However, the noted decrease was dependent on the duration of the salt treatment and the barley cultivar. Several parameters (e.g., light absorption flux per cross section of leaf; time to reach maximum chlorophyll a fluorescence intensity; plastoquinone pool size; yield of heat loss; rate of reaction center closure; and the so-called Performance Index), calculated and inferred from Chl fluorescence measurements, and related to PSII activity, were affected after 24 h of salt application, but these changes were much more pronounced after 7 days of salt treatment. Similar changes were found for measured gas exchange parameters: CO₂ uptake (photosynthetic) rate and stomatal conductance. The photosynthetic apparatus of the cultivar variety (c.v.) Arabi Aswad was found to be much more tolerant to salt treatment, compared with c.v. Arabi Abiad. After 7 days of salt treatment, the latter showed a very high value of the initial (minimal) fluorescence (F_o) and then essentially almost flat fluorescence transient curve; this result may be due to several causes that include structural changes as well as changes in the rate constants of different dissipative processes. The parameters that were most affected, by salt treatment, were: the time needed to reach the maximal chlorophyll fluorescence (F_m), and the inferred oxygen evolving complex activity (F_v/F_o, where F_v, is F_m − F_o), and the calculated Performance Index (PI_ABS) that depends on the efficiency and the yield of energy transfer and primary photochemistry. We suggest that the early reactions of the photosynthetic apparatus of barley plants could play a key role in their tolerance to salt stress. Further, we found that the first stage of salinity effect on photosynthesis of barley plants is related to stomatal conductance limitation rather than to PSII activity reduction. Thus, on the basis of our results on the two barley landraces, we recommend the use of a combination of gas exchange measurements along with the analysis of the OJIP fluorescence transient for the detection of salt stress-induced changes in plants.     

Drought tolerance of photosynthetic electron transport under CO2‐enriched and normal air in cereal species

The quantum yield of photosynthetic electron transport (ΦPSII), evaluated by means of chlorophyll (Chl) fluorescence analysis, has proven to be a useful screening test for drought tolerance in durum wheat (Triticum durum Desf.). To explore the potential of this parameter further in detecting drought-tolerant genotypes, three cereal species were studied; ΦPSII measurements were carried out under two different gas mixtures, at three points of the induction curve (to obtain the maximal ΦPSII and both the transient and steady-state actual ΦPSII), and at three different water stress levels (moderate, severe and drastic). The species investigated were durum and bread wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.); two cultivars per species, characterized by different levels of drought tolerance, were tested. The two gas mixtures used were normal air (21% O₂, 0.035% CO₂ in N₂) to monitor the whole photosynthetic process under physiological conditions, and CO₂ enriched-low O₂ air (1% O₂, 5% CO₂ in N₂) to monitor ΦPSII reduction under stress mainly related to Calvin cycle activity. When ΦPSII related to both assimilatory and non-assimilatory metabolism was evaluated, the cultivar differences observed under normal Air were more representative of the agronomic performance upon drought stress than under high CO₂-low O₂ air. Maximal ΦPSII showed no difference among either cultivars, gas mixtures or stress levels, the efficiency of excitation capture being highly resistant to drought. The ΦPSII evaluated during the transient yielded predictable values in respect of drought tolerance for durum wheat and barley cultivars, highlighting the key role of regulatory processes such as the Mehler peroxidase reaction and possibly also cyclic electron transport, in preventing overreduction under stress. The results clearly show that when Chl fluorescence analysis is used as a parameter in plant breeding, different experimental conditions should be used depending on the physiological mechanism that is bred or selected for.     

Leaf chloroplast ultrastructure and photosynthetic properties of a chlorophyll-deficient mutant of rice

Leaf chloroplast ultrastructure and photosynthetic properties of a natural, yellow-green leaf mutant (ygl1) of rice were characterized. Our results showed that chloroplast development was significantly delayed in the mutant leaves compared with the wild-type rice (WT). As leaves matured, more grana stacks formed concurrently with increasing leaf chlorophyll (Chl) content. Except for the lower intercellular CO₂ concentration, the ygl1 plants had a higher leaf net photosynthetic rate, stomatal conductance, and transpiration rate than those of the WT plants. Under equal amounts of Chl, the excitation energy of PSI and PSII was much stronger in the mutant than that in the WT. The ygl1 plants showed higher nonphotochemical quenching and lower photochemical quenching. They also exhibited higher actual photochemical efficiency of PSII with a higher electron transport rate. Under the light of 200 μmol(photon) m^?2 s^?1, the ygl1 mutant showed lesser deepoxidation of violaxanthin in the xanthophyll cycle than WT, but it increased substantially under strong light conditions. In conclusion, the photosynthetic machinery of the ygl1 remained stable during leaf development. The plants were less sensitive to photoinhibition compared with WT due to the active xanthophyll cycle. The ygl1 plants were efficient in both light harvesting and conversion of solar energy.     

Effect of Glomus mosseae on chlorophyll content, chlorophyll fluorescence parameters, and chloroplast ultrastructure of beach plum (Prunus maritima) under NaCl stress

The present study was undertaken to investigate the effect of Glomus mosseae on chlorophyll (Chl) content, Chl fluorescence parameters and chloroplast ultrastructure of beach plum seedlings under 2% NaCl stress. The results showed that compared to control, both Chl a and Chl b contents of NaCl + G. mosseae treatment were significantly lower during the salt stress, while Chl a/b ratio increased significantly. The increase of minimal fluorescence of darkadapted state (F₀), and the decrease of maximal fluorescence of dark-adapted state (F_m) and variable fluorescence (F_v) values were inhibited. The maximum quantum yield of PSII photochemistry (F_v/F_m), the maximum energy transformation potential of PSII photochemistry (F_v/F₀) and the effective quantum yield of PSII photochemistry (??_PSII) increased significantly, especially the latter two variables. The values of the photochemical quenching coefficient (q_P) and the nonphotochemical quenching (NPQ) were similar between G. mosseae inoculation and noninoculation. It could be concluded that G. mosseae inoculation could protect the photosystem II (PSII) of beach plum, enhance the efficiency of primary light energy conversion and improve the primitive response of photosynthesis under salinity stress. Meanwhile, G. mosseae inoculation was beneficial to maintain the integrity of thylakoid membrane and to protect the structure and function of chloroplast, which suggested that G. mosseae can alleviate the damage of NaCl stress to chloroplast.     

Fluorescence F 0 of photosystems II and I in developing C3 and C4 leaves,and implications on regulation of excitation balance

This work addresses the question of occurrence and function of photosystem II (PSII) in bundle sheath (BS) cells of leaves possessing NADP-malic enzyme-type C₄ photosynthesis (Zea mays). Although no requirement for PSII activity in the BS has been established, several component proteins of PSII have been detected in BS cells of developing maize leaves exhibiting O₂-insensitive photosynthesis. We used the basal fluorescence emissions of PSI (F _0I) and PSII (F _0II) as quantitative indicators of the respective relative photosystem densities. Chl fluorescence induction was measured simultaneously at 680 and 750 nm. In mature leaves, the F _m(680)/F ₀(680) ratio was 10.5 but less in immature leaves. We propose that the lower ratio was caused by the presence of a distinct non-variable component, F _c, emitting at 680 and 750 nm. After F _c was subtracted, the fluorescence of PSI (F _0I) was detected as a non-variable component at 750 nm and was undetectably low at 680 nm. Contents of Chls a and b were measured in addition to Chl fluorescence. The Chl b/(a + b) was relatively stable in developing sunflower leaves (0.25–0.26), but in maize it increased from 0.09 to 0.21 with leaf tissue age. In sunflower, the F _0I/(F _0I + F _0II) was 0.39 ± 0.01 independent of leaf age, but in maize, this parameter was 0.65 in young tissue of very low Chl content (20–50 mg m^?2) falling to a stable level of 0.53 ± 0.01 at Chl contents >100 mg m^?2. The values of F _0I/(F _0I + F _0II) showed that in sunflower, excitation was partitioned between PSII and PSI in a ratio of 2:1, but the same ratio was 1:1 in the C₄ plant. The latter is consistent with a PSII:PSI ratio of 2:1 in maize mesophyll cells and PSI only in BS cells (2:1:1 distribution). We suggest, moreover, that redox mediation of Chl synthesis, rather than protein accumulation, regulates photosystem assembly to ensure optimum excitation balance between functional PSII and PSI. Indeed, the apparent necessity for two Chls (a and b) may reside in their targeted functions in influencing accumulation of PSI and PSII, respectively, as opposed to their spectral differences.     

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F_o) and to markedly retard the light-induced rise of variable fluorescence (F_v). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F_o level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F_v at low concentration, while F_o was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F_o level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).     

Ecophysiological responses of Caragana korshinskii Kom. under extreme drought stress: Leaf abscission and stem survives

Caragana korshinskii Kom. is a perennial xerophytic shrub, well known for its ability to resist drought. In order to study ecophysiological responses of C. korshinskii under extreme drought stress and subsequent rehydration, diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem II as well as Chl content were analyzed. Plant responses to extreme drought included (1) leaf abscission and using stem for photosynthesis, (2) improved instantaneous water-use efficiency, (3) decreased photosynthetic rate and partly closed stomata owing to leaf abscission and low water status, (4) decreased maximum photochemical efficiency of photosystem II (PSII) (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of noncyclic electron transport of PSII, and Chl a and Chl b. Four days after rehydration, new leaves budded from stems. In the rewatered plants, the chloroplast function was restored, the gas exchange and Chl fluorescence returned to a similar level as control plant. The above result indicated that maintaining an active stem system after leaf abscission during extreme drought stress may be the foundation which engenders these mechanisms rapid regrowth for C. korshinskii in arid environment.     

两种生境条件下6种牧草叶绿素含量及荧光参数的比较

昆仑山前山牧场海拔较高, 策勒绿洲海拔相对较低, 两者生境差异较大。以昆仑山前山牧场和策勒绿洲边缘两种不同生境条件下生长的6种牧草: 冰草(Agropyron cristatum)、无芒雀麦(Bromus inermis)、矮生高羊茅(Festuca elata)、披碱草(Elymus dahuricus )、红豆草(Onobrychis pulchella)及和田大叶(Medicago sativa var. luxurians)为试验材料, 研究了不同生境条件下牧草叶片叶绿素含量及叶绿素荧光动力学参数的变化情况。结果显示: (1)在两种生境条件下, 昆仑山前山牧场生境生长的牧草叶绿素a、叶绿素b、总叶绿素的含量明显较高, 生长在策勒绿洲生境的牧草品种叶绿素a/b值较高; (2)昆仑山前山牧场生境牧草最大荧光、光系统II (PSII)最大光化学效率、PSII潜在活性和单位面积反应中心的数量的值明显高于策勒绿洲生境品种, 而初始荧光、单位反应中心吸收的光能、单位反应中心捕获的能量、单位反应中心耗散的能量、荧光诱导曲线初始斜率值则低于策勒绿洲生境品种。因此, 两种生境下环境因子发生了改变, 对牧草产生综合的胁迫作用; 策勒绿洲生境明显对牧草生长产生了抑制, 策勒绿洲生境牧草的色素含量降低以及PSII的机构遭到损坏, 导致反应中心一部分失活或裂解, 剩余有活性的反应中心的效率增加, 昆仑山生境则相对比较适宜牧草生长; 两种生境不同牧草叶绿素含量和叶绿素荧光参数的变化幅度不同。     

Influence of knockout of <Emphasis Type="Italic">At4g20990</Emphasis> gene encoding α-CA4 on photosystem II light-harvesting antenna in plants grown under different light intensities and day lengths

Effect of knockout of the At4g20990 gene encoding α-carbonic anhydrase 4 (α-CA4) in Arabidopsis thaliana in plants grown in low light (LL, 80 μmol photons m^?2 s^?1) or in high light (HL, 400 μmol photons m^?2 s^?1) under long (LD, 16 h) or short (SD, 8 h) day length was studied. In α-CA4 knockout plants, under all studied conditions, the non-photochemical quenching was lower; the decrease was more pronounced under HL. This pointed to α-CA4 implication in the processes leading to energy dissipation in PSII antenna. In this context the content of major antenna proteins Lhcb1 and Lhcb2 was lower in α-CA4 knockouts than in wild-type (WT) plants under all growth conditions. The expression level of lhcb2 gene was also lower in mutants grown under LD, LL and HL in comparison to WT. At the same time, this level was higher in mutants grown under SD, LL and it was the same under SD, HL. Overall, the data showed that the knockout of the At4g20990 gene affected both the contents of proteins of PSII light-harvesting complex and the expression level of genes encoding these proteins, with peculiarities dependent on day length. These data together with the fact of a decrease of non-photochemical quenching of leaf chlorophyll a fluorescence in α-CA4-mut as compared with that in WT plants implied that α-CA4 participates in acclimation of photosynthetic apparatus to light intensity, possibly playing important role in the photoprotection. The role of this CA can be especially important in plants growing under high illumination conditions.     

Light-dependent reversal of dark-chilling induced changes in chloroplast structure and arrangement of chlorophyll-protein complexes in bean thylakoid membranes

Changes in chloroplast structure and rearrangement of chlorophyll-protein (CP) complexes were investigated in detached leaves of bean (Phaseolus vulgaris L. cv. Eureka), a chilling-sensitive plant, during 5-day dark-chilling at 1 °C and subsequent 3-h photoactivation under white light (200 μmol photons m⁻² s⁻¹) at 22 °C. Although, no change in chlorophyll (Chl) content and Chl a/b ratio in all samples was observed, overall fluorescence intensity of fluorescence emission and excitation spectra of thylakoid membranes isolated from dark-chilled leaves decreased to about 50%, and remained after photoactivation at 70% of that of the control sample. Concomitantly, the ratio between fluorescence intensities of PSI and PSII (F736/F681) at 120 K increased 1.5-fold upon chilling, and was fully reversed after photoactivation. Moreover, chilling stress seems to induce a decrease of the relative contribution of LHCII fluorescence to the thylakoid emission spectra at 120 K, and an increase of that from LHCI and PSI, correlated with a decrease of stability of LHCI-PSI and LHCII trimers, shown by mild-denaturing electrophoresis. These effects were reversed to a large extent after photoactivation, with the exception of LHCII, which remained partly in the aggregated form. In view of these data, it is likely that dark-chilling stress induces partial disassembly of CP complexes, not completely restorable upon photoactivation. These data are further supported by confocal laser scanning fluorescence microscopy, which showed that regular grana arrangement observed in chloroplasts isolated from control leaves was destroyed by dark-chilling stress, and was partially reconstructed after photoactivation. In line with this, Chl a fluorescence spectra of leaf discs demonstrated that dark-chilling caused a decrease of the quantum yield PSII photochemistry (F_v/F_m) by almost 40% in 5 days. Complete restoration of the photochemical activity of PSII required 9 h post-chilling photoactivation, while only 3 h were needed to reconstruct thylakoid membrane organization and chloroplast structure. The latter demonstrated that the long-term dark-chilled bean leaves started to suffer from photoinhibition after transfer to moderate irradiance and temperature conditions, delaying the recovery of PSII photochemistry, independently of photo-induced reconstruction of PSII complexes.     

Relationship between photosynthetic pigments and chlorophyll fluorescence in soybean under varying phosphorus nutrition at ambient and elevated CO<Subscript>2</Subscript>

To assess the relationship between chlorophyll (Chl) fluorescence (CF) and photosynthetic pigments, soybean was grown under varying phosphorus (P) nutrition at ambient and elevated CO₂ (EC). The EC stimulated, but P deficiency decreased plant height, node numbers, and leaf area concomitantly with the rates of stem elongation, node addition, and leaf area expansion. Under P deficiency, CF parameters and pigments declined except that carotenoids (Car) were relatively stable indicating its role in photoprotection. The CF parameters were strongly related with Chl concentration but not with Chl a/b or Car. However, total Chl/Car showed the strongest association with CF parameters such as quantum efficiency and yield of photosystem II. This relationship was not affected by CO₂ treatment. The high correlation between CF and total Chl/Car underscores the significance of the quantification of both, Chl and Car concentrations, to understand the photochemistry and underlying processes of photoprotection and mechanisms of excess energy dissipation in a given environment.     

Response of Photosynthetic Apparatus of Spring Barley (Hordeum vulgare L.) to Combined Effect of Elevated CO2 Concentration and Different Growth Irradiance

The response of barley (Hordeum vulgare L. cv. Akcent) to various photosynthetic photon flux densities (PPFDs) and elevated [CO₂] [700 μmol (CO₂) mol⁻¹; EC] was studied by gas exchange, chlorophyll (Chl) a fluorescence, and pigment analysis. In comparison with barley grown under ambient [CO₂] [350 μmol (CO₂) mol⁻¹; AC] the EC acclimation resulted in a decrease in photosynthetic capacity, reduced stomatal conductance, and decreased total Chl content. The extent of acclimation depression of photosynthesis, the most pronounced for the plants grown at 730 μmol m⁻² s⁻¹ (PPFD₇₃₀), may be related to the degree of sink-limitation. The increased non-radiative dissipation of absorbed photon energy for all EC plants corresponded to the higher de-epoxidation state of xanthophylls only for PPFD₇₃₀ barley. Further, a pronounced decrease in photosystem 2 (PS2) photochemical efficiency (given as F_V/F_M) for EC plants grown at 730 and 1 200 μmol m⁻² s⁻¹ in comparison with AC barley was related to the reduced epoxidation of antheraxanthin and zeaxanthin back to violaxanthin in darkness. Thus the EC conditions sensitise the photosynthetic apparatus of high-irradiance acclimated barley plants (particularly PPFD₇₃₀) to the photoinactivation of PS2. This revised version was published online in August 2006 with corrections to the Cover Date.     

Peroxynitrite inhibits electron transport on the acceptor side of higher plant photosystem II

Peroxynitrite is a strong oxidant that has been proposed to form in chloroplasts. The interaction between peroxynitrite and photosystem II (PSII) has been investigated to determine whether this oxidant could be a hazard for PSII. Peroxynitrite is shown to inhibit oxygen evolution in PSII membranes in a dose-dependent manner. Analyses by PAM fluorimetry and EPR spectroscopy have demonstrated that the inhibition target of peroxynitrite is on the PSII acceptor side. In the presence of the herbicide DCMU, the chlorophyll (Chl) a fluorescence induction curve is inhibited by peroxynitrite, but the slow phase of the Chl a fluorescence decay does not change. EPR studies demonstrate that the Signal II_slow and Signal II_fast of peroxynitrite-treated Tris-washed PSII membranes are induced at room temperature, implying that the redox active tyrosines Y_Z and Y_D of PSII are not significantly nitrated. A featureless EPR signal with a g value of approximately 2.0043 ± 0.0003 and a line width of 10 ± 1 G is induced under continuous illumination in the presence of peroxynitrite. This new EPR signal corresponds with the semireduced plastoquinone Q_A in the absence of magnetic interaction with the non-heme Fe²⁺. We conclude that peroxynitrite impairs PSII electron transport in the Q_AFe²⁺ niche.     

Purification and characterization of a stable oxygen-evolving Photosystem II complex from a marine centric diatom, Chaetoceros gracilis

Oxygen-evolving Photosystem II particles (crude PSII) retaining a high oxygen-evolving activity have been prepared from a marine centric diatom, Chaetoceros gracilis (Nagao et al., 2007). The crude PSII, however, contained a large amount of fucoxanthin chlorophyll a/c-binding proteins (FCP). In this study, a purified PSII complex which was deprived of major components of FCP was isolated by one step of anion exchange chromatography from the crude PSII treated with Triton X-100. The purified PSII was still associated with the five extrinsic proteins of PsbO, PsbQ', PsbV, Psb31 and PsbU, and showed a high oxygen-evolving activity of 2135 μmol O₂ (mg Chl a)^− 1 h^− 1 in the presence of phenyl-p-benzoquinone which was virtually independent of the addition of CaCl₂. This activity is more than 2.5-fold higher than the activity of the crude PSII. The activity was completely inhibited by 3-(3,4)-dichlorophenyl-(1,1)-dimethylurea (DCMU). The purified PSII contained 42 molecules of Chl a, 2 molecules of diadinoxanthin and 2 molecules of Chl c on the basis of two molecules of pheophytin a, and showed typical absorption and fluorescence spectra similar to those of purified PSIIs from the other organisms. In this study, we also found that the crude PSII was significantly labile, as a significant inactivation of oxygen evolution, chlorophyll bleaching and degradation of PSII subunits were observed during incubation at 25 °C in the dark. In contrast, these inactivation, bleaching and degradation were scarcely detected in the purified PSII. Thus, we succeeded for the first time in preparation of a stable PSII from diatom cells.     

Latent manganese deficiency in barley can be diagnosed and remediated on the basis of chlorophyll a fluorescence measurements

Background and aims

Manganese (Mn) deficiency represents a major plant nutritional disorder in winter cereals. The deficiency frequently occurs latently and the lack of visual symptoms prevents timely remediation and cause significant yield reductions. These problems prompted us to investigate chlorophyll (Chl) a fluorescence as a tool for diagnosis of latent Mn deficiency.

Methods

Barley plants grown under controlled greenhouse conditions or in the field were exposed to different intensities of Mn deficiency. The responses were characterised by analysis of Chl a fluorescence, photosystem II (PSII) proteins and mineral elements.

Results

Analysis of the Chl a fluorescence induction kinetics (FIK) revealed distinct changes long before any visual symptoms of Mn deficiency were apparent. The changes were specific for Mn and did not occur in Mg, S, Fe or Cu deficient plants. The changes in Mn deficient plants were accompanied by a marked reduction of the D1 protein in PSII. Foliar Mn application fully restored PSII functionality, ensured winter survival, and increased grain yields under field conditions.

Conclusions

The efficiency and stability of PSII are markedly affected by latent Mn deficiency. Chlorophyll a fluorescence measurements constitute a powerful and valuable tool for diagnosis and remediation of latent Mn deficiency.