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Photosynthetic dynamics in chrysanthemum in response to single step increases and decreases in photon flux density 下载免费PDF全文
The time-course of CO2 assimilation rate and stomatal conductance to step changes in photosynthetic photon flux density (PPFD) was observed in Chrysanthemum × morifolium Ramat. `Fiesta'. When PPFD was increased from 200 to 600 micromoles per square meter per second, the rate of photosynthetic CO2 assimilation showed an initial rapid increase over the first minute followed by a slower increase over the next 12 to 38 minutes, with a faster response in low-light-grown plants. Leaves exposed to small step increases (100 micromoles per square meter per second) reached the new steady-state assimilation rate within a minute. Both stomatal and biochemical limitations played a role during photosynthetic induction, but carboxylation limitations seemed to predominate during the first 5 to 10 minutes. Stomatal control during the slow phase of induction was less important in low-light compared to high-light-grown plants. In response to step decreases in PPFD, photosynthetic rate decreased rapidly and a depression in CO2 assimilation prior to steady-state was observed. This CO2 assimilation `dip' was considerably larger for the large step (400 micromoles per square meter per second) than for the small step. The rapid photosynthetic response seems to be controlled by biochemical processes. High- and low-light-grown plants did not differ in their photosynthetic response to PPFD step decreases. 相似文献
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A mannitol:mannose 1-oxidoreductase was isolated from celeriac (Apium graveolens var. rapaceum) root tips by fractionation with (NH4)2SO4, followed by chromatography on a Fractogel DEAE column and then concentration with (NH4)2SO4. This newly discovered mannitol dehydrogenase catalyzes the NAD-dependent oxidation of mannitol to mannose, not mannitol to fructose. The sugar product of the enzyme reaction was identified by three independent HPLC systems and by an enzymatically linked system as being mannose and not fructose or glucose. Normal Michaelis--Menten kinetics were exhibited for both mannitol and NAD with Km values of 72 and 0.26 mM, respectively, at pH 9.0. The Vmax was 40.14 mumol/h/mg protein for mannitol synthesis and 0.8 mumol/h/mg protein for mannose synthesis at pH 9.0. In the polyol oxidizing reaction, the enzyme was very specific for mannitol with a low rate of oxidation of sorbitol. In the reverse reaction, the enzyme was specific for mannose. The enzyme was strongly inhibited by NADH and sensitive to alterations of NAD/NADH ratio. The enzyme is of physiological importance in that it is mainly localized in root tips (sink tissue) where it functions to convert mannitol into hexoses which are utilized to support root growth. Product determination and kinetic characterization were carried out on an enzyme preparation with a specific activity (SA) of 30.44 mumol/h/mg protein. Subsequently, the enzyme was further purified to a SA of 201 mumol/h/mg protein using an NAD affinity column. This paper apparently represents the first evidence of the existence of a mannitol:mannose 1-oxidoreductase and also the first evidence of the presence of a mannitol dehydrogenase in vascular plants. 相似文献
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The phylogeny of Greya Busck (Lepidoptera: Prodoxidae) was inferred from
nucleotide sequence variation across a 765-bp region in the cytochrome
oxidase I and II genes of the mitochondrial genome. Most parsimonious
relationships of 25 haplotypes from 16 Greya species and two outgroup
genera (Tetragma and Prodoxus) showed substantial congruence with the
species relationships indicated by morphological variation. Differences
between mitochondrial and morphological trees were found primarily in the
positions of two species, G. variabilis and G. pectinifera, and in the
branching order of the three major species groups in the genus. Conflicts
between the data sets were examined by comparing levels of homoplasy in
characters supporting alternative hypotheses. The phylogeny of Greya
species suggests that host-plant association at the family level and larval
feeding mode are conservative characters. Transition/transversion ratios
estimated by reconstruction of nucleotide substitutions on the phylogeny
had a range of 2.0-9.3, when different subsets of the phylogeny were used.
The decline of this ratio with the increase in maximum sequence divergence
among taxa indicates that transitions are masked by transversions along
deeper internodes or long branches of the phylogeny. Among transitions,
substitutions of A-->G and T-->C outnumbered their reciprocal
substitutions by 2-6 times, presumably because of the approximately 4:1
(77%) A+T-bias in nucleotide base composition. Of all transversions,
73%-80% were A<-->T substitutions, 85% of which occurred at third
positions of codons; these estimates did not decrease with an increase in
maximum sequence divergence of taxa included in the analysis. The high
frequency of A<-->T substitutions is either a reflection or an
explanation of the 92% A+T bias at third codon positions.
相似文献
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Purification of NAD-dependent mannitol dehydrogenase from celery suspension cultures. 总被引:5,自引:3,他引:2 下载免费PDF全文
Mannitol dehydrogenase, a mannitol:mannose 1-oxidoreductase, constitutes the first enzymatic step in the catabolism of mannitol in nonphotosynthetic tissues of celery (Apium graveolens L.). Endogenous regulation on the enzyme activity in response to environmental cues is critical in modulating tissue concentration of mannitol, which, importantly, contribute to stress tolerance of celery. The enzyme was purified to homogeneity from celery suspension cultures grown on D-mannitol as the carbon source. Mannitol dehydrogenase was purified 589-fold to a specific activity of 365 mumol h-1 mg-1 protein with a 37% yield of enzyme activity present in the crude extract. A highly efficient and simple purification protocol was developed involving polyethylene glycol fractionation, diethylaminoethyl-anion-exchange chromatography, and NAD-agarose affinity chromatography using NAD gradient elution. Sodium dodecylsulfate gel electrophoresis of the final preparation revealed a single 40-kD protein. The molecular mass of the native protein was determined to be approximately 43 kD, indicating that the enzyme is a monomer. Polyclonal antibodies raised against the enzyme inhibited enzymatic activity of purified mannitol dehydrogenase. Immunoblots of crude protein extracts from mannitol-grown celery cells and sink tissues of celery, celeriac, and parsley subjected to sodium dodecyl sulfate gel electrophoresis showed a single major immuno-reactive 40-kD protein. 相似文献
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