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991.
Soil pH is commonly measured in water (pHw) or 0.01 M CaCl2 (pHCa). The need to convert between these methods has led to the publication of linear, quadratic and cubic polynomial relationships for limited suites of soils. Concerns over the applicability of such relationships when mapping a wide range of soils and pH led to the establishment of a database of pHW and pHCa values on each of 7894 samples from soil survey and field experimental sites in Queensland. The relationship between pHW and pHCa across all soils was investigated and preliminary results examining the effect of soil depth and soil type on the relationship are presented.For all soils and depths, a linear regression accounted for 93.2% of the variation but did not predict pHCa well at very high or low pHW values. The inclusion of second and third powers of pHW accounted for significantly more of the variation (R2=0.94) in pHCa and the resultant curve matched the data better at high and low pH.Analysis of surface, sub-surface and subsoil groupings did not reveal any appreciable differences in the relationship between pHW and pHCa attributable to depth. In contrast, differences in the relationship were evident between soil types. Generally, the mildly leached soils had linear relationships, while the weathered soils were distinctly curvilinear at low pH. 相似文献
992.
The metabolic significance of Se in plants is not well documented, though the presence of many selenoenzymes in bacteria and
the essentiality of Se in higher animals is established. Since germination is an active process in plant growth and metabolism,
the effect of Se was investigated in germinatingVigna radiata L, a nonaccumulating Sedeficient legume. Growth and protein were enhanced in seedlings supplemented with selenium (Se) as
sodium selenite in the medium up to 1 μg/mL. The pattern of uptake of75Se in the differentiating tissues and the subcellular distribution were investigated. The percentage of incorporation of75Se was greater in the mitochondria at the lowest level (0.5 μg/mL) of Se supplementation compared to higher levels of Se exposure.
Proteins precipitated from the postmitochondrial supernatant fractions, when separated by means of polyacrylamide gel electrophoresis
(PAGE), indicated a major selenoprotein in the seedlings germinated at 2.0 μg/mL Se. In seedlings grown with supplemented
Se, enhanced respiratory control ratio and succinate dehydrogenase activity were observed in the mitochondria of tissues,
indicative of a role for Se in mitochondrial membrane functions. 相似文献
993.
Daniel I. Arnon 《Photosynthesis research》1995,46(1-2):47-71
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)
- QA and QB
specialized membrane-bound forms of PQ
- PC
plastocyanin
- Fd
ferredoxin
- BISC
FAFB, 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-559HP-cytochrome b-559LP, 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. 相似文献
994.
Photoinhibition in the green alga Dunaliella salina is accompanied by the formation of inactive Photosystem II reaction centers. In SDS-PAGE analysis, the latter appear as 160 kD complexes. These complexes are structurally stable, enough to withstand re-electrophoresis of excised gel slices from the 160 kD region. Western blot analyses with specific polyclonal antibodies raised against the D1 or D2 reaction center proteins provided evidence for the presence of both of these polypeptides in the re-electrophoresed 160 kD complex. Incubation of excised gel slices from the 160 kD region, under aerobic conditions at 4°C for a prolonged period of time, caused a break-up of the 160 kD complex into a 52 kD D1-containing and 80 and 26 kD D2-containing pieces. Western blot analysis with polyclonal antibodies raised against the apoproteins of CPI (reaction center proteins of PS I) did not show cross-reaction either with the 160 kD complex or with the 52, 80 and 26 kD pieces. The results show the presence of both D1 and D2 in the 160 kD complex and strengthen the notion of a higher molecular weight D1- and D2-containing complex that forms upon disassembly of photodamaged PS II units.Abbreviations Chl
chlorophyll
- PS II
Photosystem II
- D1
the 32 kD reaction center protein of PS II, encoded by the chloroplast psbA gene
- D2
the 34 kD reaction center protein of PS II, encoded by the chloroplast psbD gene
- CPI
the 82 and 83 kD reaction center proteins of PS I, encoded by the chloroplast psaA and psaB genes
- HL
high light
- LL
low light
This publication is dedicated to the memory of the late Professor Daniel Arnon, whom the first author will fondly remember for his many accounts of past scientific discovery and debate. 相似文献
995.
996.
R. Hedrich O. Moran F. Conti H. Busch D. Becker F. Gambale I. Dreyer A. Küch K. Neuwinger K. Palme 《European biophysics journal : EBJ》1995,24(2):107-115
We have investigated the electrophysiological basis of potassium inward rectification of the KAT1 gene product from Arabidopsis thaliana expressed in Xenopus oocytes and of functionally related K+ channels in the plasma membrane of guard and root cells from Vicia faba and Zea mays. The whole-cell currents passed by these channels activate, following steps to membrane potentials more negative than –100 mV, with half activation times of tens of milliseconds. This voltage dependence was unaffected by the removal of cytoplasmic magnesium. Consequently, unlike inward rectifier channels of animals, inward rectification of plant potassium channels is an intrinsic property of the channel protein itself. We also found that the activation kinetics of KAT1 were modulated by external pH. Decreasing the pH in the range 8.5 to 4.5 hastened activation and shifted the steady state activation curve by 19 mV per pH unit. This indicates that the activity of these K+ channels and the activity of the plasma membrane H+-ATPase may not only be coordinated by membrane potential but also by pH. The instantaneous current-voltage relationship, on the other hand, did not depend on pH, indicating that H+ do not block the channel. In addition to sensitivity towards protons, the channels showed a high affinity voltage dependent block in the presence of cesium, but were less sensitive to barium. Recordings from membrane patches of KAT1 injected oocytes in symmetric, Mg2+-free, 100 mM-K+, solutions allowed measurements of the current-voltage relation of single open KAT1 channels with a unitary conductance of 5 pS. We conclude that the inward rectification of the currents mediated by the KAT1 gene product, or the related endogenous channels of plant cells, results from voltage-modulated structural changes within the channel proteins. The voltage-sensing or the gating-structures appear to interact with a titratable acidic residue exposed to the extracellular medium.
Correspondence to: R. Hedrich 相似文献
997.
Raffaello Romeo Giuseppe Arena Luigi Mons Scolaro Maria Rosario Plutino 《Inorganica chimica acta》1995,240(1-2):81-92
The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen) (CH3) (Me2SO)]+ by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) mixture. The starting complex and the reaction products were characterized either as solids or in solution by their IR and 1H NMR spectra. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sequence of reactivity observed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en) (NH3)Cl]+ used as standard). The difference of reactivity between the first (t-BuNH2) and the last (SeCN−) members of the series spans five orders of magnitude. The value measured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series of protic and dipolar aprotic solvents, using chloride ion as nucleophile, the rate of formation of [Pt (phen) (CH3)Cl] is dominated by the extent of solvation of Cl−, as measured by its values of the Gibbs molar energy of transfer ΔtG0. Conductivity measurements at 25°C in dichloromethane were fitted to the Fuoss equation and the values of the dissociation constants Kd for the ion pairs were calculated as follows: 2.27 × 10−5 M for Bu4NCl, 2.75 × 10−5 M for Bu4NSCN and 17.05 × 10−5 M for [Pt(phen) (CH3) (Me2SO)]PF6. The pseudo-first-order rate constants kobs for the reactions with Bu4NCl, Bu4NBr, Bu4NSCN and Bu4NI showed a curvilinear dependence on the concentration of the salt which levels off very soon (at concentrations higher than 0.005 M the kinetics are zero order in [Bu4NX]). On addition of the inert electrolyte Bu4NPF6 the rates slow down and the kinetics follow the rate law kobs = kKip[Bu4NX]/[Bu4NPF6] + Kip[Bu4NX]). These findings fit well with a reaction scheme which involves a pre-equilibrium Kip between ion pairs, followed by unimolecular substitution within the contact ion pair [Pt(phen) (CH3) (Me2SO)X]ip. Values of the equilibrium constants Kip for ion-pair exchange and of the internal substitution rates k were derived. The latter showed that the discrimination in reactivity between Cl−, Br−, SCN− and I− is greatly reduced with respect to aqueous solutions. The reason behind this may be desolvation of the ions coupled to the fact that a contact ion pair is already at a certain distance along the reaction coordinate in the direction of the transition state. Applications of the special salt effect and of ion pairing to synthesis are discussed. 相似文献
998.
The kinetics of substitution reactions of [η-CpFe(CO)3]PF6 with PPh3 in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH)4, 4-Ph, 3-Me, 2,3-(CH)4, 2,6-Me2, 2-Me), the reactions yeild the same product [η5-CpFe(CO)2PPh3]PF6, according to a second-order rate law that is first order in concentrations of [η5-CpFe(CO)3]PF6 and of R-PyO but zero order in PPh3 concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: ΔH≠ = 13.4 ± 0.4 kcal mol−1, ΔS≠ = −19.1 ± 1.3 cal k−1 mol−1 for 4-PhPyO; ΔH≠ = 12.3 ± 0.3 kcal mol−1, ΔS≠ = 24.7 ±1.0 cal K−1 mol−1 for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N-oxide basicities, but decrease with increasing 17O chemical shifts of the N-oxides. Electronic and steric factors contributing to the reactivity of pyridine N-oxides have been quantitatively assessed. 相似文献
999.
Macroscopic instantaneous and time-dependent currents have been measured in the vacuolar membrane of Beta vulgaris using a patch clamp configuration analogous to whole cell mode. At low cytosolic Ca2+ and in the absence of Mg2+, only an instantaneous current was observed. This current is carried predominantly by cations (PKPCl 71, pnapcl 41 and arginine is also conducted). The instantaneous current can be activated by ATP4– (e.g., ATP-activated mean K+ current density was –20 mA.m–2 at a membrane voltage of –20 mV) and by increasing cytosolic pH and Mg2+ (raising Mg2+ from 0 to 0.4 mm induced a mean current density increase of –7 mA.m–2 at –20 mV). Such current can be activated by simultaneous addition of putative in vivo concentrations of ATP4–/MgATP/Mg
free
2+
(in the presence of bafilomycin to inhibit the vacuolar ATPase) and further modulated by cytosolic pH. With vacuolar K+ concentration greater than that of the cytosol, activation of the instantaneous current would mediate vacuolar K+ release over the range of physiological membrane voltage. It is argued that the ATP4–-activated current, in addition to acting as a K+ mobilization pathway, could provide a counter-ion (shunt) conductance, allowing the two electrogenic H+ pumps which reside in the vacuolar membrane to acidify the vacuolar lumen.A separate time-dependent current, which was not observed at low Ca2+ concentrations (less than 500 nm) could also be elicited by addition of Mg2+ at the cytoplasmic membrane face. This current was stimulated by increasing cytoplasmic pH.The authors are grateful to the BBSRC for financial support (Grant PG87/529) and to the Royal Society (University Research Fellowship to J.M.D.). We thank C. Abbott, K. Partridge and J. Robinson for plant cultivation; A. Amtmann, A. Bertl, D. Gradmann and G. Thiel for helpful discussion. 相似文献
1000.