Reduction State Of QA Research Articles

Energy dissipation and antioxidant enzyme system protect photosystem II of sweet sorghum under drought stress

The effect of drought stress on energy dissipation and antioxidant enzyme system in two sweet sorghum inbred lines (M-81E and Roma) was investigated. Results showed that the germination indicator increased more in M-81E than that in Roma under rehydration. Under drought stress, both the maximal photochemical efficiency of PSII (Fv/Fm) and oxidoreductive activity (ΔI/I0) of Roma decreased more than those in M-81E. Relative to Fv/Fm, the ΔI/I0 decreased markedly, which indicated that PSI was more sensitive to drought stress than PSII. Increases in the reduction state of QA (1–qp), nonphotochemical quenching (NPQ) and minimal fluorescence yield of the dark-adapted state (F0) were greater in Roma than those in M-81E; meanwhile, the H2O2 content was lower in M-81E than that in Roma. Our results suggested that the photoinhibition might be related to the accumulation of reactive oxygen species (ROS). The antioxidant enzyme system and energy dissipation of M-81E could respectively increase drought tolerance by eliminating ROS and excess energy more efficiently than that of Roma.

Diurnal patterns of net photosynthetic rate, stomatal conductance, and chlorophyll fluorescence in leaves of field‐grown mungbean (Phaseolus radiatus) and millet (Setaria italica)

The diurnal variations of net photosynthetic rate (Pn), quantum yield of photochemical quenching (YP), and non‐photochemical quenching (YN) of photosystem II (PSII), as well as stomatal status were studied in leaves of field‐grown mungbean (Phaseolus radiatus, C3 crop) and millet (Setaria italica, C4 crop) in arid and semi‐arid regions of China. Although patterns of Pn in both crops were one‐peak curves, the time that Pn peaks occurred was 0600 h for mungbean and 1000 h for millet. YP was always higher, whereas YN was lower in mungbean leaves than in millet. By analysing the pool size of the primary electron acceptor of PSII (QA), it was indicated that millet leaves had a higher capacity to control or regulate the reduction state of QA, thus dissipating excessive energy under intense irradiance and high ambient temperature, and therefore advantageous to leaf photosynthesis in millet. Midday depression of stomatal conductance occurred in both crops, but played different roles on leaf photosynthesis in mungbean and millet. Additionally, stomatal regulation operated for much longer in millet leaves than in mungbean. In general, the present studies indicated that regulation of the QA reduction state and operation of stomata were two physiological factors contributing to millet being more adaptive than mungbean for cultivation in arid and semi‐arid regions.

Characterisation of the photosynthetic response of tobacco leaves to ozone: CO 2 assimilation and chlorophyll fluorescence

Summary The present paper reports on the characterisation of the photosynthetic responses to a single pulse of ozone (150 ppb for 5 hours) in the leaves of two tobacco cultivars displaying different degrees of sensitivity to O3 (BelB, O3-resistant and BelW3, O3-sensitive). In the BelW3 cultivars, O3 induced a decrease in the photosynthetic rate, but not in actual PSII efficiency at steady-state photosynthesis. The reduction state of QA did not change in these plants while a strong decrease in intrinsic PSII efficiency was observed. The quantum yield for photosynthetic CO2 assimilation decreased more than the actual PSII efficiency, suggesting the presence of a significant fraction of electron transport to molecular oxygen or the existence of some form of cyclic electron flow. O3-treated leaves reduced the excess of absorbed light (i.e. light that couldn't be used in photosynthesis) by dissipating a large part of the light absorbed by the PSII antenna as heath. In BelB, the CO2 assimilation rate did not change in the presence of the pollutant O3 and the only gas exchange parameter that changed was the stomatal conductance, which significantly increased. Some of the Chl fluorescence parameters changed after O3 fumigation, but returned to values similar to the controls when measured 24 hours after removing the stress. The only Chl fluorescence parameter that significantly increased during the recovery phase was the 1-qP

Chlorophyll a Fluorescence Emission, Xanthophyll Cycle Activity, and Net Photosynthetic Rate Responses to Ozone in Some Foliose and Fruticose Lichen Species

The lichens Parmelia quercina, Parmelia sulcata, Evernia prunastri, Hypogymnia physodes, and Anaptychia ciliaris were exposed to ozone (O3) in controlled environment cuvettes designed to maintain the lichens at optimal physiological activity during exposure. Measurements of gas exchange, modulated chlorophyll (Chl) fluorescence, and pigment analysis were conducted before and after exposure to 300 mm3 (O3) m-3, 4 h per d for 14 d. No changes in the efficiency of photosystem 2 (PS2) photochemistry, the reduction state of QA, or the electron flow through PS2, measured by Chl fluorescence, were detected in any of the five lichen species studied. Additionally, neither photosynthetic CO2 assimilation nor xanthophyll cycle activity or photosynthetic pigment concentration were affected by high O3 concentrations. Thus the studied lichen species have significant capacities to withstand oxidative stresses induced by high concentration of O3.

Xanthophyll cycle, light energy dissipation and electron transport in transgenic tobacco with reduced carbon assimilation capacity

The effects of reduced CO2 assimilation capacity on the leaf pigment composition and the dissipation of light energy were studied using transgenic tobacco (Nicotiana tabacum L. cv. W38). Two plant types were used: anti-SSu plants with reduced amounts of Rubisco and anti-GAPDH plants with reduced activity of chloroplast glycer-aldehyde 3-phosphate dehydrogenase. A moderate reduction in the photosynthetic capacity increased the de-epoxidation state of the xanthophyll-cycle pigments. In contrast, there was no large effect on the leaf pigment composition and the ratio of the xanthophyll cycle pigments to chlorophyll, and total carotenoids increased only in the most severe transgenic plants. The light induction of photosynthesis, fluorescence quenching and de-epoxida ion of the xanthophyll cycle pigments were also followed in wild-type and anti-SSu plants. Anti-SSu plants maintained high nonphotochemical quenching and increased xanthophyll de-epoxidation in the light but the reduction state of QA remained high. For both wild-type and anti-SSu plants, the electron transport rate estimated from chlorophyll a fluorescence appeared to be much higher than that required to support the observed rate of CO2 assimilation and photorespiration during the early phase of photosynthetic induction. However, the two estimates converged with the onset of steady-state photosynthesis.

Interactions of chronic exposure to elevated CO2 and O3 levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica)

Young trees of European beech (Fagus sylvatica) acclimated for one growing season to ambient (c. 367 μl l−1) or elevated CO2 levels (c. 660 μl l−1) were exposed during the subsequent year to combinations of the same CO2 regimes and ambient or twice‐ambient ozone (O3) levels (generated from the database of a rural site). By the end of June, before the development of macroscopic leaf injury, the raised O3 levels had not affected the light and dark reactions of photosynthesis. However, acclimation to elevated CO2 had resulted in lowered chlorophyll and nitrogen concentrations, whereas photosynthetic performance, examined over a wide range of parameters from light and dark reactions, remained unchanged or showed only slight reductions (e.g. apparent electron transport rate, ETR; apparent quantum yield of CO2 gas exchange, ΦCO2; apparent carboxylation efficiency, CE; and photosynthetic capacity at light and CO2 saturation, PC). In August, after the appearance of leaf necroses, plants grown under ambient CO2 and twice‐ambient O3 conditions declined in both the photosynthetic light reactions (optimum electron quantum yield, Fv/Fm, non‐photochemical energy quenching, NPQ, reduction state of QA, apparent electron quantum yield, ΦPSII, maximum electron transport rates) and the dark reactions as reflected by CE, ΦCO2, as well as the maximum CO2 uptake rate (i.e. PC). CE, ΦCO2 and PC were reduced by c. 75, 40 and 75%, respectively, relative to plants exposed to ambient CO2 and O3 levels. By contrast, plants exposed to twice‐ambient O3 and elevated CO2 levels maintained a photosynthetic performance similar to individuals grown either under ambient CO2 and ambient O3, or elevated CO2 and ambient O3 conditions. The long‐term exposure to elevated CO2 therefore tended to counteract adverse chronic effects of enhanced O3 levels on photosynthesis. Possible reasons for this compensatory effect in F. sylvatica are discussed.

Differential regulation of chloroplast gene expression in Chlamydomonas reinhardtii during photoacclimation: light stress transiently suppresses synthesis of the Rubisco LSU protein while enhancing synthesis of the PS II D1 protein.

Transfer of Chlamydomonas reinhardtii cells grown photoautotrophically in low light to higher light intensities has a dramatic transient effect on the differential expression of the two major chloroplast encoded photosynthetic proteins. Synthesis of the D1 protein of Photosystem II increases more than 10-fold during the first six hours in high light (HL), whereas synthesis of the large subunit (LSU) of Rubisco drops dramatically within 15 min and only gradually resumes at about 6 h. Synthesis of the chloroplast-encoded ATP synthase beta subunit, the nuclear-encoded Rubisco small subunit and the nuclear-encoded beta-tubulin is not noticeably affected. Up regulation of psbA mRNA translation accounts for a substantial fraction of the increased D1 synthesis, since accumulation of psbA mRNA increases 4.2- and 6.3-fold less than D1 synthesis at 6 and 18 h in HL. Down-regulation of LSU synthesis is not correlated with a reduction in the steady-state level of the rbcL transcript. Primer extension mapping of the 5' ends of the rbcL mRNAs reveals transcripts with start points located at -93 and -168 relative to the first translated ATG. Transfer of low light (LL)-grown cells to HL temporarily decreases the ratio of the -93 to -168 transcripts, but this ratio normalizes after 6 h in HL, coincident with the recovery in the synthesis of LSU. These several distinct effects of temporary light stress were correlated with a rapid, sustained increase in the reduction state of QA, a transient decline in photosynthetic efficiency, a less rapid drop in total chlorophyll content and a delay in cell division.

The effect of light levels on daily patterns of chlorophyll fluorescence and organic acid accumulation in the tropical CAM treeClusia hilariana

Sandy plains are characteristic of the coastal region of Brazil. We investigated the diel patterns of changes in organic acid levels, leaf conductance and chlorophylla fluorescence for sun-exposed and shaded plants ofClusia hilariana, one of the dominant woody species in the sandy coastal plains of northern Rio de Janeiro state. Both exposed and shaded plants showed a typical CAM pattern with considerable diel oscillations in organic acid levels. The degradation of both malic and citric acids during the midday stomatal closure period could lead to potential CO2 fixation rates of 28 μmol m-2 s-1 in exposed leaves. Moreover, exposed leaves exhibited large increases in total non-photochemical quenching (qN) accompanied by a substantial decrease in effective quantum yield during the course of the day. However, these potential high rates of CO2 fixation and the increases inqn of exposed plants were not enough to maintain the primary electron acceptor of photosystem II (qA) in a low reduction state, similar to that of shaded plants. As a result, there was a moderate increase in the reduction state of qA throughout the day. Most of the decline in photochemical efficiency of exposed leaves ofC. hilariana was reversible, as evidenced by the high levels of pre-dawn potential quantum yields (Fv/Fm) and their rapid recovery after sunset. However, the depletion of the organic acid pool in the afternoon resulted in an accentuated subsequent drop in Fv/Fm, suggesting that prolonged periods of water stress accompanied by high irradiance levels may expose plants ofC. hilariana in unprotected habitats to the danger of photoinhibition.