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51.
The reactions of singlet oxygen (1O2) with cis and trans butenes-1,1,1-d3, at—80°C in Freon-11, show a product isotope effect (kH/kD) of 1.38 and 1.25 respectively. Isomerization of the starting materials or formation of dioxetanes were not observed during the course of the photooxygenation. Together with the isotope effects on the reactions of tetramethylethylene-d6 isomers with singlet oxygen, these results require the reversible formation of a perepoxide or charge transfer intermediate. 相似文献
52.
J. D. ACKERMAN 《Biological journal of the Linnean Society. Linnean Society of London》1983,20(3):301-314
Seasonal and geographic relationships, and host pollinator specificities are examined for indications of interdependency in the orchid-euglossine bee interaction. The orchids are dependent on the bees for pollination, and their flowering seasonality corresponds well with peak activity of their pollinators. However, there is little evidence that the bees are dependent on these fragrance hosts. The orchids tap the majority of euglossine species and individuals for pollinator services during any given season, but most of those bee species that temporarily lack orchid fragrance hosts persist in the area, continually emerge from nests, and seek floral fragrance compounds. Pollinator specificity occurs in less than half of the orchids, and host specificity is rare. Geographic distributions of nearly all orchid-pollinator pairs are not mutually inclusive. Moreover, nearly a third of the local male euglossine bee species censused are not pollinators of any fragrance orchids in the area. Local alternative fragrance sources occur. The orchid-male euglossine bee interaction does not appear to represent a mutually obligatory relationship. The orchids may have exploited a preexisting behavioural phenomenon of the bees, and reciprocal evolutionary responses may not have occurred. 相似文献
53.
The control of incompatibility in distylous Pulmonaria affinis Jordan (Boraginaceae) 总被引:1,自引:0,他引:1
A.J. RICHARDS JEAN MITCHELL 《Botanical journal of the Linnean Society. Linnean Society of London》1990,104(4):369-380
In P. affinis, pin pollen is shorter on average than thrum pollen. Pins have more coronal hooks on stigmatic papillae than thrums, but gaps between papillae are relatively smaller for thrums. Pin stigmas receive more pollen than thrum stigmas. Thrum stigmas receive more (dissortive) pin pollen, but pin stigmas are assortively pollinated. Pollen only germinates when trapped below papilla coronas. On thrum stigmas, most trapped pollen is pin. Pollen germination is better on thrum stigmas than pin stigmas, and thrum stigmas show a close relationship between numbers of legitimate pollen grains, numbers of germinating grains, and numbers of pollen tubes in the style. There is no inhibition of illegitimate pollen germination. Illegitimate pollen tubes are inhibited in the style. Incompatibility operates by a combination of dissortive pollination, dissortive pollen trapping, and stylar pollen tube inhibition. All heteromorphic features differing between pins and thrums are implicated in the inhibition of within-morph fertilization in thrums. 相似文献
54.
High light treatments were given to attached leaves of pumpkin (Cucurbita pepo L.) at room temperature and at 1°C where the diffusion- and enzyme-dependent repair processes of Photosystem II are at a minimum. After treatments, electron transfer activities and fluorescence induction were measured from thylakoids isolated from the treated leaves. When the photoinhibition treatment was given at 1°C, the Photosystem II electron transfer assays that were designed to require electron transfer to the plastoquinone pool showed greater inhibition than electron transfer from H2O to paraphenyl-benzoquinone, which measures all PS II centers. When the light treatment was given at room temperature, electron transfer from H2O to paraphenyl-benzoquinone was inhibited more than whole-chain electron transfer. Variable fluorescence measured in the presence of ferricyanide decreased only during room-temperature treatments. These results suggest that reaction centers of one pool of Photosystem II, non-QB-PS II, replace photoinhibited reaction centers at room temperature, while no replacement occurs at 1°C. A simulation of photoinhibition at 1°C supports this conclusion.Abbreviations BSA
bovine serum albumin
- Chl
chlorophyll
- DCMU
3-(3,4,-dichlorophenyl)-1,1,-dimethylurea
- DCPIP
dichlorophenol-indophenol (2,6-dichloro-4((4-hydroxyphenyl)imino)-2,5-cyclohexadien-1-one)
- DPC
diphenyl carbazide (2,2-diphenylcarbonic dihydrazide)
- FeCN
ferricyanide (hexacyanoferrate(III))
- app
apparent quantum yield of photosynthetic oxygen evolution
- MV
methyl viologen (1,1-dimethyl-4,4-bipyridinium dichloride)
- PPBQ
phenyl-p-benzoquinone
- PPFD
photosynthetic photon flux density
- PQ
pool plastoquinone
- QB
secondary quinone acceptor of PS II
- RT
room temperature
- WC
whole chain electron transfer 相似文献
55.
Mark O. Johnston Daniel J. Schoen 《Evolution; international journal of organic evolution》1996,50(4):1478-1491
The relation between inbreeding depression and rate of self-fertilization was studied in nine natural populations of the annual genus Amsinckia. The study included two clades (phylogenetic lineages) in which small-flowered, homostylous populations or species are believed to have evolved from large-flowered, heterostylous, self-compatible ones. In one lineage the small-flowered species is tetraploid with disomic inheritance. Rates of self-fertilization were 25% to 55% in the four large-flowered, heterostylous populations; 72% in a large-flowered but homostylous population; and greater than 99.5% in the four small-flowered, homostylous populations, which produce seed autonomously. When present, inbreeding depression occurred in the fertility but not the survival components of fitness. Using a cumulative fitness measure incorporating both survival and fertility (flower number), we found inbreeding depression to be lower in the four very highly self-fertilizing populations than in the five intermediate ones. The Spearman rank correlation between inbreeding depression and selfing rate for the nine populations was –0.50, but was not statistically significant (P = 0.12). Inbreeding depression was greater in the two tetraploid populations than in the very highly self-fertilizing, diploid ones. Phenotypic stability of progeny from self-fertilization tended to be higher in populations with lower inbreeding depression. We conclude that levels of self-fertilization and inbreeding depression in Amsinckia are determined more by other factors than by each other. Estimates of mutation rates and dominance coefficients of deleterious alleles, obtained from a companion study of the four highly self-fertilizing populations, suggest that a strong relationship may not be expected. We discuss the relationship of the present results to current theory of the coevolution of self-fertilization and inbreeding depression. 相似文献
56.
Allison A. Snow Timothy P. Spira 《Evolution; international journal of organic evolution》1996,50(5):1866-1870
The stigmas of animal-pollinated flowers often capture more pollen than is needed to fertilize all available ovules, and mixed-donor pollen loads are probably common. When this is the case, variation in average pollen-tube growth rates can potentially affect the number of seeds sired by a given plant. Despite considerable interest in effects of postpollination processes on male fitness, little is known about the extent of variation in pollen performance among plants from natural populations. To examine this question in Hibiscus moscheutos (rose mallow), we conducted mixed-donor hand-pollination experiments with 39 pollen donors bearing distinctive isozyme markers. Pairs of competing donors were compared on sets of 11 to 15 recipient plants per pair. These donors often differed in the proportions of seeds they sired, with the maximum deviation from an expected ratio of 50:50 being 68:32. Furthermore, three intensively studied plants exhibited consistent trends in relative pollen performance when each was tested against (1) the same three competitors, and (2) groups of 14 competitors chosen at random from the study population. In a separate experiment, we investigated the effects of salinity stress and high soil nutrients on pollen performance. These environmental factors had anticipated effects on leaf production, flower production, and petal length, but style length and (most importantly) the number of seeds sired relative to a standard pollen donor were not affected. In summary, this study provides the strongest evidence to date that pollen-tube competitive ability varies among coexisting plants and may be an important component of male fitness in plants. 相似文献
57.
Luísa M. D. R. S. Martins Armando J. L. Pombeiro Richard A. Henderson 《Inorganica chimica acta》1996,250(1-2):311-315
The kinetics of the displacement reactions of the bromide ligands of trans-[FeBr2(depe)2] (depe = Et2PCH2CH2PEt2) by the organonitrile NCCH2C6H4OMe-4, in tetrahydrofuran (either in the absence or in the presence of added Br−), to give the corresponding mono- and dinitrile complexes trans-[FeBr(NCCH2C6H4OMe-4)(depe)2]+ and trans-[Fe(NCCH2C6H4OMe-4)2(depe)2]2+, have been investigated by stopped-flow spectrophotometry. The substitution reaction occurs by a mechanism involving rate-limiting dissociation of bromo ligands to form the unsaturated intermediates [FeBr(depe)2]+ (k1 = 1.52 ± 0.02 s−1) and [Fe(NCR)(depe)2]2+ (k3 = 0.063 ± 0.008 s−1) which add the nitrile ligand to form those nitrile complexes. The competition between the nitrile and Br− for such metal centres has also been investigated and a stronger inhibiting effect of added Br− is observed for the substitution of the second bromo ligand relative to the first one. The kinetic data are rationalized in terms of π-electronic effects of these unsaturated metal centres and of the bromide and nitrile ligands. 相似文献
58.
59.
60.
Substituted enzyme (or ping-pong) mechanisms usually involve enzymes that exist in two forms that alternate during the catalytic
reaction. A method is described here for determining the position of the equilibrium of a half reaction in a ping-pong enzyme
mechanism that is based on the kinetics of the burst reaction which occurs upon addition of reactants that recycle the enzyme
from one form to another. The theoretical basis for the analysis is developed, and the method is applied to the half reaction
of the aldimine form of aspartate transaminase with difluoro-oxaloacetate.
Special issue dedicated to Herman Bachelard 相似文献