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31.
Allosteric and isosteric modifiers of NADH binding to cytoplasmic malic dehydrogenase 总被引:1,自引:0,他引:1
M Cassman 《Biochemical and biophysical research communications》1973,53(2):666-672
The binding of NADH to cytoplasmic malic dehydrogenase is shown to be affected by a number of added ligands. One class of ligands appear to be analogs of a substrate for the enzyme, -malate. These alter the binding constant for NADH without affecting the cooperativity of binding. In contrast, fructose-1,6-diphosphate behaves as an allosteric inhibitor at low enzyme concentrations, apparently by shifting the monomer-dimer equilibrium of the protein to the cooperatively binding dimer. The significance of these results are discussed in terms of a proposed regulatory function for the enzyme. 相似文献
32.
Crop rotation and residue management effects on carbon sequestration,nitrogen cycling and productivity of irrigated rice systems 总被引:31,自引:0,他引:31
Witt C. Cassman K.G. Olk D.C. Biker U. Liboon S.P. Samson M.I. Ottow J.C.G. 《Plant and Soil》2000,225(1-2):263-278
The effects of soil aeration, N fertilizer, and crop residue management on crop performance, soil N supply, organic carbon
(C) and nitrogen (N) content were evaluated in two annual double-crop systems for a 2-year period (1994–1995). In the maize-rice
(M-R) rotation, maize (Zea mays, L.) was grown in aerated soil in the dry season (DS) followed by rice (Oriza sativa, L.) grown in flooded soil in the wet season (WS). In the continuous rice system (R-R), rice was grown in flooded soil in
both the DS and WS. Subplot treatments within cropping-system main plots were N fertilizer rates, including a control without
applied N. In the second year, sub-subplot treatments with early or late crop residue incorporation were initiated after the
1995 DS maize or rice crop. Soil N supply and plant N uptake of 1995 WS rice were sensitive to the timing of residue incorporation.
Early residue corporation improved the congruence between soil N supply and crop demand although the size of this effect was
influenced by the amount and quality of incorporated residue. Grain yields were 13-20% greater with early compared to late
residue incorporation in R-R treatments without applied N or with moderate rates of applied N. Although substitution of maize
for rice in the DS greatly reduced the amount of time soils remained submerged, the direct effects of crop rotation on plant
growth and N uptake in the WS rice crops were small. However, replacement of DS rice by maize caused a reduction in soil C
and N sequestration due to a 33–41% increase in the estimated amount of mineralized C and less N input from biological N fixation
during the DS maize crop. As a result, there was 11–12% more C sequestration and 5–12% more N accumulation in soils continuously
cropped with rice than in the M-R rotation with the greater amounts sequestered in N-fertilized treatments. These results
document the capacity of continuous, irrigated rice systems to sequester C and N during relatively short time periods.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
33.
34.
Sedimentation equilibrium studies on glutamic dehydrogenase 总被引:3,自引:0,他引:3
35.
At a global scale, cereal yields and fertilizer N consumption have increased in a near-linear fashion during the past 40 years and are highly correlated with one another. However, large differences exist in historical trends of N fertilizer usage and nitrogen use efficiency (NUE) among regions, countries, and crops. The reasons for these differences must be understood to estimate future N fertilizer requirements. Global nitrogen needs will depend on: (i) changes in cropped cereal area and the associated yield increases required to meet increasing cereal demand from population and income growth, and (ii) changes in NUE at the farm level. Our analysis indicates that the anticipated 38% increase in global cereal demand by 2025 can be met by a 30% increase in N use on cereals, provided that the steady decline in cereal harvest area is halted and the yield response to applied N can be increased by 20%. If losses of cereal cropping area continue at the rate of the past 20 years (?0.33% per year) and NUE cannot be increased substantially, a 60% increase in global N use on cereals would be required to meet cereal demand. Interventions to increase NUE and reduce N losses to the environment must be accomplished at the farm-or field-scale through a combination of improved technologies and carefully crafted local policies that contribute to the adoption of improved N management; uniform regional or national directives are unlikey to be effective at both sustaining yield increases and improving NUE. Examples from several countries show that increases in NUE at rates of 1% per year or more can be achieved if adequate investments are made in research and extension. Failure to arrest the decrease in cereal crop area and to improve NUE in the world’s most important agricultural systems will likely cause severe damage to environmental services at local, regional, and global scales due to a large increase in reactive N load in the environment. 相似文献
36.
37.
Acidification of Soil in a Dry Land Winter Wheat-sorghum/corn-fallow Rotation in the Semiarid U.S. Great Plains 总被引:4,自引:0,他引:4
David D. Tarkalson José O. Payero Gary W. Hergert Kenneth G. Cassman 《Plant and Soil》2006,283(1-2):367-379
Soil pH is decreasing in many soils in the semiarid Great Plains of the United States under dry land no-till (NT) cropping
systems. This study was conducted to determine the rate of acidification and the causes of the acidification of a soil cropped
to a winter wheat (Triticum aestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench]/corn (Zea mays L.)-fallow rotation (W-S/C-F) under NT. The study was conducted from 1989 to 2003 on soil with a long-term history of either
continuous NT management [NT(LT)] (1962–2003) or conventional tillage (CT) (1962–1988) then converted to NT [NT(C)] (1989–2003).
Nitrogen was applied as ammonium nitrate (AN) at a rate of 23 kg N ha−1 in 1989 and as urea ammonium nitrate (UAN) at an average annual rate of 50 kg N ha−1 from 1990 to 2003 for both NT treatments. Soil samples were collected at depth increments of 0–5, 5–10, 10–15, and 15–30 cm
in the spring of 1989 and 2003. Acidification rates for the NT(LT) and NT(C) treatments were 1.13 and 1.48 kmol H+ ha−1 yr−1 in the 0–30 cm depth, respectively. The amount of CaCO3 needed to neutralize the acidification is 57 and 74 kg ha−1 yr−1 for the NT(LT) and NT(C) treatments, respectively. A proton budget estimated by the Helyar and Porter [1989, Soil Acidity
and Plant Growth, Academic Press] method indicated that NO3− leaching from the 30 cm depth was a primary cause of long-term acidification in this soil. Nitrate leaching accounted for
59 and 66% of the H+ from the acid causing factors for NT(LT) and NT(C) treatments, respectively. The addition of crop residues to the soil neutralized
62 and 47% of the acidity produced from the leaching of NO3−, and 37 and 31% of the acid resulting from NO3− leaching and the other acid-causing constituents for the NT(LT) and NT(C) treatments, respectively. These results document
that surface soils in dry land W-S/C-F rotations under NT are acidifying under current management practices. Improved management
to increase nitrogen uptake efficiency from applied fertilizer would help reduce the rate of acidification. The addition of
lime materials to prevent negative impacts on grain yields may be necessary in the future under current management practices.
A contribution of the university of Nebraska Agricultural Research Division, Lincoln, NE 68583. Journal series No. 15120 相似文献
38.
Nitrogen and yield potential of irrigated rice 总被引:7,自引:1,他引:6
Yield potential of modern rice varieties and implications for N management were evaluated in a series of field studies that provided data for validation of an eco-physiological simulation model for rice. We tested the hypothesis that N was the major factor limiting yield potential of irrigated rice. The simulation model ORYZA1 was used to evaluate the observed yield differences between varieties grown with different N management and in different environments. The model explained differences in yield of the treatments resonably well on the basis of differences in radiation, temperature, leaf N content and variety coefficients for phenological development. It was demonstrated by the model and experimental data that yield levels of 6 t ha-1 in the wet season and 10 t ha-1 in the dry season can be obtained in the tropics with the current short duration varieties only when the N supply from soil and fertilizer is adequately maintained at key growth stages. Yield probabilities for rice crops were simulated for different environments using long term weather data at two Philippine sites. 相似文献
39.
Partial 18S rRNA sequences of five chelicerate arthropods plus a
crustacean, myriapod, insect, chordate, echinoderm, annelid, and
platyhelminth were compared. The sequence data were used to infer phylogeny
by using a maximum-parsimony method, an evolutionary-distance method, and
the evolutionary-parsimony method. The phylogenetic inferences generated by
maximum-parsimony and distance methods support both monophyly of the
Arthropoda and monophyly of the Chelicerata within the Arthropoda. These
results are congruent with phylogenies based on rigorous cladistic analyses
of morphological characters. Results support the inclusion of the
Arthropoda within a spiralian or protostome coelomate clade that is the
sister group of a deuterostome clade, refuting the hypothesis that the
arthropods represent the "primitive" sister group of a protostome coelomate
clade. Bootstrap analyses and consideration of all trees within 1% of the
length of the most parsimonious tree suggest that relationships between the
nonchelicerate arthropods and relationships within the chelicerate clade
cannot be reliably inferred with the partial 18S rRNA sequence data. With
the evolutionary-parsimony method, support for monophyly of the Arthropoda
is found in the majority of the combinations analyzed if the coelomates are
used as "outgroups." Monophyly of the Chelicerata is supported in most
combinations assessed. Our analyses also indicate that the
evolutionary-parsimony method, like distance and parsimony, may be biased
by taxa with long branches. We suggest that a previous study's inference of
the Arthropoda as paraphyletic may be the result of (a) having two few
arthropod taxa available for analysis and (b) including long-branched taxa.
相似文献
40.
Cereal area and nitrogen use efficiency are drivers of future nitrogen fertilizer consumption 总被引:2,自引:0,他引:2
At a global scale, cereal yields and fertilizer N consumption have increased in a near-linear fashion during the past 40 years and are highly correlated with one another. However,large differences exist in historical trends of N fertilizer usage and nitrogen use efficiency (NUE)among regions, countries, and crops. The reasons for these differences must be understood to estimate future N fertilizer requirements. Global nitrogen needs will depend on: (i) changes in cropped cereal area and the associated yield increases required to meet increasing cereal demand from population and income growth, and (ii) changes in NUE at the farm level. Our analysis indicates that the anticipated 38% increase in global cereal demand by 2025 can be met by a 30% increase in N use on cereals, provided that the steady decline in cereal harvest area is halted and the yield response to applied N can be increased by 20%. If losses of cereal cropping area continue at the rate of the past 20 years (-0.33% per year) and NUE cannot be increased substantially, a 60% increase in global N use on cereals would be required to meet cereal demand. Interventions to increase NUE and reduce N losses to the environment must be accomplished at the farm- or field-scale through a combination of improved technologies and carefully crafted local policies that contribute to the adoption of improved N management; uniform regional or national directives are unlikey to be effective at both sustaining yield increases and improving NUE. Examples from several countries show that increases in NUE at rates of 1% per year or more can be achieved if adequate investments are made in research and extension. Failure to arrest the decrease in cereal crop area and to improve NUE in the world's most important agricultural systems will likely cause severe damage to environmental services at local, regional, and global scales due to a large increase in reactive N load in the environment. 相似文献