Diurnal changes in phosphoenolpyruvate carboxylase and pyruvate, orthophosphate dikinase properties in the natural environment: interplay of light and temperature in a C4 thermophile

Activities of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) were measured in leaf extracts of field grown Amaranthus paniculatus L. (C₄) during a natural diurnal irradiance and temperature pattern. Enzyme assays were run at both fixed (30°C) and the corresponding leaf temperature at the time of harvest. Light activation of PEP carboxylase (PEPCase) at fixed assay temperatures was expressed as a decrease in S_0–5 (PEP) after a threshold (> 330 μmol m^–2 s^–1) photon fluence rate was surpassed at noon. Earlier in the morning, increase in apparent enzyme affinity for PEP was observed when the assay was run at leaf temperature, indicating a physiologically meaningfull effect of temperature on S^0.5 (PEP). The 3.3-fold increase in PEPCase activity at low PEP and fixed assay temperature between the minimal and maximal irradiance and temperature hours of the day, became 12.8-, 11.5- and 7.4-fold when assays were run at the corresponding leaf temperature during three diurnal cycles with respective temperature differences (max minus min) of 9.0, 8.3 and 7.4°C. The extent of malate inhibition was the same for both day and night forms of PEPCase assayed at 35°C, but increased considerably with night enzyme at 25°C. The results indicate that light increases the apparent affinity of PEPCase for PEP and that at lower temperatures malate becomes more inhibitory. Pyruvate orthophosphate dikinase activity started to increase immediately after sunrise and the 10-fold increase at fixed temperature became 14.8-, 14.2- and 13.1-fold when assays were run at the above leaf temperatures. This indicates that the light effect predominates with pyruvate, orthophosphate dikinase, while with phosphoenolpyravate carboxylase, light and temperature co-operate to increase the day enzyme activities.     

Assessing the impact of ozone on photosynthesis of European beech (Fergus sylvatica L.) in environmental chambers

An exposure — response study with proportionalto-ambient ozone levels was conducted in closed chambers on 3-year-old European beech (Fagus sylvatica L.) of montane origin. The fumigation started in April 1990 and lasted for a single growing season. Climate data and ozone concentrations monitored at an experimental station of the Institute for Applied Plant Biology, Schönenbuch, Switzerland were simulated in the exposure chambers 12 days later (1*O₃). To test exposure-response relations three additional treatments were applied, subambient (0.2*O₃) and two proportionally increased ozone treatments (1.5*O₃ and 2*O₃). The photosynthetic behaviour of the trees in August revealed the light reactions to be less affected than parameters which are related to the dark reactions of photosynthesis. Assimilation (A₃₅₀), apparent carboxylation efficiency (CE), and maximum photosynthetic capacity (A₂₅₀₀) were reduced with increasing ozone concentration. For the ozone response of CE and A₂₅₀₀ Critical Levels were calculated.     

高压静电场分离水稻、油菜及芝麻种子对萌发期生物效应的影响

落入高压静电场内的水稻、芝麻及油菜种子,在场的作用下,即产生沿电场方向的位移,在同一跌落高度下,重量基本相同的种子,其分离距离产生很大的差异,且分离距离与种子的发芽率及发芽势的改善程度存在着较为明显的相关关系,研究中证实,种子活力强度得到提高,其α淀粉酶活性、蛋白酶活性、脂肪酶活性及电导率得到明显的改善。     

Manipulating photosynthesis

The levels of individual photosynthetic proteins can be independently decreased by theAgrobacterium-mediated transformation of plants with antisens RNA constructs. Protocols for the introduction of such constructs intoAgrobacterium, theAgrobacterium-mediated transformation of tobacco leaf disks, and the screening and analysis of the transgenic plants produced are described.     

矮嵩草草甸植物群落的光合特性研究

 矮嵩草草甸植物群落的光合、暗呼吸和土壤呼吸的研究表明：光合作用的日变化在6月份接近平坦型,7、8月份呈午间降低型。矮嵩草草甸植物群落的光合作用受较低的光合面积指数及冠层叶片的受光势态的影响,存在着明显的光饱和现象,光补偿点及光饱和点相对于全日照光合有效辐射均较低,接近于单叶的光响应特性。裸露地面的土壤呼吸和植物与土壤体系的暗呼吸不仅与温度有关,而且与土壤水分状况和降雨量也有密切联系。影响草甸群落光合特性的主要因素有：高原地区强烈的太阳辐射,较低的光合面积指数和植物根系与土壤紧密结合所形成的草结皮层结构。     

Human Interindividual Variability in Cancer Risks—Technical and Management Challenges

Existing risk assessment procedures for carcinogens are intended to be “conservative” in the uncertainty dimension—giving estimates that are expected to be higher than true risks for typical people. However, these procedures do not consider the likely variability in susceptibility among individual people. This paper updates previous estimates of the likely extent of this variability for metabolically activated, genetically-acting carcinogens based on recent information on human interindividual variability in metabolic activation, detoxification, and DNA repair. The resulting expected skewness of cancer risk distributions is estimated using Monte Carlo simulations of both variability and uncertainty.

Some risk management implications are:

1. When evaluating the fairness of a particular risk distribution, managers need to gain familiarity with a three-dimensional characterization—X level of risk, for the Yth percentile individual (addressing variability) with Z degree of confidence (addressing uncertainty).

2. To the extent that variability distributions are skewed (e.g., with a long tail extending to high values) population mean risks will tend to exceed risks for median individuals. Together with the skewness in uncertainty distributions, this implies that “expected value” estimates of aggregate population risks—the estimates of interest for cost benefit analyses—are likely to be closer to traditional upper confidence limit risk estimates than has often been assumed in the past.

Effect of daily photon receipt on the susceptibility of dwarf bean (Phaseolus vulgaris L.) leaves to photoinhibition of photosynthesis

Bean (Phaseolus vulgaris L.) plants were grown at two light periods of 8 and 13 h with a similar photon flux density (PFD) giving a daily photon receipt (DPR) of 17.9 and 38.2 mol · m–2, respectively. Shoot growth and leaf area development were followed at regular intervals and diurnal whole-plant photosynthesis measured. Single mature trifoliate leaves were exposed to photoinhibitory treatments at PFDs of 800 and 1400 mol · m–2 · s–1 and at temperatures of 12 and 20°C. Chlorophyll fluorescence and photon yields were measured at regular intervals throughout each treatment. Plants grown in 13 h had significantly greater leaf areas than those grown in 8 h. There were no differences in maximum rates of photosynthesis, photon yields and only minor but significant differences in F_v/F_m for plants in the two treatments, showing photosynthetic characteristics were dependent on PFD but not DPR. A significant decline in photosynthesis and F_v/F_m occurred over the 13-h but little change in photosynthesis for plants in the 8 h, indicating some feedback inhibition of photosynthesis was occurring. Plants grown in 8 h were consistently more susceptible to photoinhibition of photosynthesis at all treatments than 13-h plants. Nevertheless, photoinhibition was exacerbated by increases in PFD, and by decreases in temperature for leaves from both treatments. However, for plants from the 8-h day, exposing leaves to 12°C and 1400 mol · m–2 · s–1 caused photo-oxidation and severe bleaching but no visible damage on leaves from 13-h-grown plants. Closure of the photosystem II reaction-centre pool was partially correlated with increasing extents of photoinhibition but the relationship was similar for plants from both treatments. There remains no clear explanation for their wide differences in susceptibility to photoinhibition.Abbreviations and Symbols DPR daily photon receipt - F₀ and F_m initial and maximal fluorescence - F_v/F_m fluorescence ratio in dark-treated leaves - F/F_m intrinsic efficiency of PSII during illumination - PFD photon flux density - _i photon yield (incident basis) - _psi quantum yield of PSII electron transport - P_max maximum rate of photosynthesis - q_N non-photochemical quenching coefficient - q_P photochemical quenching coefficient Many thanks to my colleague William Laing who spent a considerable effort in developing the programme to run the photosynthesis apparatus. I am also indebted to one reviewer with whom I corresponded to resolve some issues in the paper. This project was funded by the New Zealand Foundation for Research, Science and Technology.     

Divergent pathways of photosynthetic electron transfer: The autonomous oxygenic and anoxygenic photosystems

The aim of this article is to assemble and integrate, from a personal perspective of a research participant, seldom examined evidence that is incompatible with some basic tenets of photosynthetic electron transport, the cornerstone of which is the Z scheme. The nonconforming evidence pertaining to the mode of ferredoxin reduction and the role of the copper redox protein, plastocyanin, indicates that contrary to the Z scheme ferredoxin is reduced in two experimentally distinguishable ways: oxygenically by PS II (renamed the oxygenic photosystem), without the participation of PS I, and anoxygenically by PS I (renamed the anoxygenic photosystem). It also indicates that plastocyanin is not only, as the Z scheme asserts, the electron donor to the reaction center chlorophyll of PS I (P700) but also to the reaction center chlorophyll of PS II (P680). Other unconventional findings include evidence that the fully functional oxygenic photosystem, when operating separately from the anoxygenic photosystem, reduces plastoquinone to plastoquinol and subsequently oxidizes plastoquinol by two pathways acting in concert: one being the universally recognized DBMIB-sensitive pathway via the Rieske iron-sulfur center of the cytochrome bf complex and the other, a hitherto unrecognized, DBMIB-insensitive electron transport pathway around P680 that centers on cytochrome b-559. These nonconforming findings form the basis of an alternate hypothesis of photosynthetic electron transport that modifies and complements the Z scheme.Abbreviations PS photosystem - PQ oxidized plastoquinone - PQH₂ reduced plastoquinone (plastoquinol) - Q_A and Q_B specialized membrane-bound forms of PQ - PC plastocyanin - Fd ferredoxin - BISC F_AF_B, membrane-bound iron-sulfur centers of PS I - DBM1B 2,5-dibromo-3-methyl-6-isopropyl-n-benzoquinone (dibromothymoquinone) - DNP-INT dinitrophenol ether of iodonitrothymol - NADP⁺ NADPH, oxidized and reduced forms of nicotinamide adenine dinucleotide phosphate - FCCP carbonylcyanide-p-trifluoromethoxyphenyl-hydrazone - CCCP carbonyl cyanide-3-chlorophenylhydrazone - SF 6847 2,6,-di-(t-butyl)-4-(2,2-dicyanovinyl) phenol - diuron (DCMU) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EPR electron paramagnetic resonance - DCIP 2,6-dichloro-phenolindophenol - UHDBT 5-(n-undecyl)-6-hydroxy-4-7-dioxobenzothiazole; cytochrome b-559_HP-cytochrome b-559_LP, high- and low potential states of cytochrome b-559 - oxygenic reductions reductions in which water is the electron donor - BBY PS II preparation made according to Berthold et al. (1981) Dedicated to Professor Achim Trebst on his 65th birthday.Based in part on lecture in Advanced Course on Trends in Photosynthesis Research, Palma de Mallorca, Spain, September 18, 1990.