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81.
82.
State of the art and research needs for the impact category eutrophication are discussed. Eutrophication is a difficult impact
category because it includes emissions to both air and water — both subject to different environmental mechanisms — as well
as impacts occurring in different types of terrestrial and aquatic ecosystems. The possible fate processes are complex and
include transportation between different ecosystems. In some recent approaches, transportation modelling of air emissions
has been included. However, in general, the used characterisation methods do not integrate fate modelling, which is a limitation.
The definition of the impact indicator needs further research, too. The inclusion of other nutrients than those typically
considered should also be investigated. 相似文献
83.
Seasonal pattern of accumulation and effects of low temperatures on storage compounds in Trifolium repens 总被引:1,自引:0,他引:1
V. Bouchart J. H. Macduff A. Ourry M. M. Svenning A. P. Gay J. C. Simon J. Boucaud 《Physiologia plantarum》1998,104(1):65-74
The seasonal pattern of concentrations of nitrogen, starch and vegetative storage protein (VSP) in stolons of Trifolium repens L. grown in the field were studied. Two different genotypes, cv. Aran and cv. Rivendel, differing in their morphology (stolon thickness and branching rate) but with similar growth rates, were used. Maximum concentrations of starch were found in summer whereas hydrolysis of starch took place throughout winter, suggesting that C storage is more important for winter survival than for promotion of early spring growth. On the other hand, VSP and nitrogen accumulated in autumn and early winter and then decreased when growth was resumed during early spring. For both cultivars, an inverse relationship was found between VSP concentration in stolons and mean air temperature, suggesting that VSP accumulation may be triggered by low temperature. Further experiments with plants grown under different regimes of temperature and daylength, suggested that VSP synthesis is stimulated by low root temperatures, with a slight synergistic effect of short daylength.
The effects of root temperature on growth, N2 fixation, NH4 + uptake and N allocation within Trifolium repens L., were studied under controlled conditions. The shoot growth rate was greatly reduced when root temperatures were lowered from 12 to 6°C, while the rate of stolon growth was less affected. Low root temperatures inhibited N2 fixation more than it did NH4 + uptake, but the relative allocation of N to stolons was increased. Lowering root temperature also increased the accumulation of VSP in stolons. These results are discussed in terms of the mechanism associated with low temperature stimulation of VSP accumulation and its coupling with changes in the source/sink relations for allocation of N, between growth and storage. 相似文献
The effects of root temperature on growth, N
84.
Soybean is the most important leguminous crop in Brazil and the nitrogen required for plant growth is supplied byBradyrhizobium bacteria through the symbiotic relation established by the inoculation process. Since 1992, two new strains, CPAC 7 and CPAC
15, which have been shown to increase yields in several field experiments, have been recommended in Brazilian commercial inoculants.
CPAC 15 is a natural variant of theB. elkanii SEMIA 566 strain, and was isolated after several years of adaptation to a Brazilian Cerrado soil, while CPAC 7 is a variant
ofB. japonicum strain CB 1809, selected under laboratory conditions for higher nodulation and yield. The comparison between parental and
variant strains, under greenhouse conditions, showed that both CPAC 15 and CPAC 7 increased N2 fixation rates in relation to the parental strains. The better performance of CPAC 15 was related to an increase in nodule
efficiency (mg N2 fixed mg-1 nodule) while with CPAC 7 the higher N2 fixation rates were due to increased nodulation. Both CPAC 15 and CPAC 7 increased nodule occupancy, when co-inoculated at
a ratio of 1:1 withB. elkanii 29w, in relation to their parental strains. Variant strains also differed from parental in their ability to increase numbers
of root hairs (Hai phenotype) either when inoculated onto plants, or when supernatants of bacteria exposed to seed exudates
were used as inoculants. This results lead to the hypothesis that a modification in some of the “common” nodulation genes
had occurred. However, the increase in Hai phenotype with CPAC 7 was dependent on the soybean cultivar, indicating a possible
alteration in some genotypic specific nodulation gene. Apparently, there were no differences in Nod metabolites produced by
strains CPAC 15 and SEMIA 566, but a more detailed chemical analysis would be required to rule out subtle differences. On
the contrary, significant differences were found between CPAC 7 and the parental strain CP 1809, in the profile of Nod metabolites.
Consequently, it may be possible that diffusable molecules, responsible for Hai phenotype, would be related to nodulation
ability, competiviveness, and N2 fixation, resulting in the higher yields that have been associated with CPAC 7 and CPAC 15. For the CPAC 7 strain, the increase
in Hai phenotype could be atributed to the differences found in the Nod molecules. Consequently, a high degree of physiological
and genetic variability can result from the adaptation of rhizobial strains to the soil. Also, this variability can be found
under laboratory conditions, when searching single colonies with specific properties. ei]Section editor: R O D Dixon 相似文献
85.
Translocation - A key factor limiting the efficiency of nitrogen fixation in legume nodules 总被引:2,自引:0,他引:2
John G. Streeter 《Physiologia plantarum》1993,87(4):616-623
A hypothesis is presented that the availability of water for export of nitrogenous products from legume nodules is a major factor limiting the efficiency of symbiotic nitrogen fixation. Water for export of solutes in the xylem probably depends largely on the import of water and reduced carbon in the phloeum, and one function of respiration may be to dispose of reduced carbon in order to increase the supply of water. A second hypothesis presented is that control of gas diffusion in soybean nodules is largely restricted to the cortex nearby the vascular bundles, thus making possible the linkage of solute balances in xylem and phloem with resistance to diffusion. These concepts are used in a re-examination of literature on manipulations of nodules and nodulated plants such as lowering of light levels, water stress, defoliation, stem girdling, and alteration of oxygen supply. The concept of translocation as a major factor limiting efficiency of symbiotic fixation is consistent with the failure of superior rhizobial isolates to improve N input significantly, and this limitation could also prevent exploitation of superior bacterial symbionts in the future 相似文献
86.
Petter Nilsen 《Plant and Soil》1995,172(1):73-85
A field lysimeter study was established with the aim of investigating the effect of nitrogen availability upon drought strain in Norway spruce trees. Forest soil (Typic Udipsamment) was filled in lysimeters 1 m in diameter and 1 m deep. Small trees of Norway spruce from five different clones were planted in the lysimeters. Roofs under the canopy of the trees ensured full control of water and nutrient input. Three levels of nitrogen were given to the trees during five years; ambient rainwater, and five and fifteen times this N concentration, respectively. Additional N was given as NH4NO3 in irrigation water. Mean annual N-addition during the five years corresponded to 5, 27 and 82 kg per ha and year for the three treatments, respectively. During the third and fifth growth season drought was artificially induced. In addition to a watered control, two levels of drought were applied, representing water deprivation for 2 and 3 months, respectively, in 1990 and 3 and 4 months, respectively in 1992. A higher water consumption in the nitrogen fertilized trees during the droughts resulted in a significantly lower pre-dawn shoot water potential compared to the trees receiving ambient rain N. The interaction between drought and nitrogen fertilization was clear also for photosynthesis and transpiration. A decrease in height- and diameter increment caused by drought was most pronounced in the 82 kg N ha–1 yr–1 treatment. A water strain integral showed a strong positive correlation to the needle biomass of the trees. Foliar concentrations of several nutrients decreased significantly with increasing drought strain in the trees. Concentration of potassium and boron were especially low and visual symptoms of deficiency occurred. 相似文献
87.
Biological nitrogen fixation (BNF) has an assured place in agriculture, mainly as a source of nitrogen for legumes. Legumes are currently grown mostly as a source of vegetable oil and as food for humans and animals, but not as nitrogen source.Other crops with BNF capability may be eventually be developed eventually. Such crops will also need mineral fertilizers to maintain a good status of soil nutrients, but their possible effects to the environment is also a concern. Fertilizers, however, will remain a necessary and sustainable input to agriculture to feed the present and increasing human population. It is not a case of whether BNF is better or worse than mineral fertilizers because both plays an important role in agriculture. 相似文献
88.
89.
Nitrogenase activity in composting horse bedding and leaves 总被引:1,自引:1,他引:0
To date little or no nitrogen fixation has been found in composting materials. However, some materials have high C:N ratios and thus might be expected to support nitrogen fixation. We examined windrows of composting horse bedding and leaves for nitrogenase activity via acetylene reduction and used carbon monoxide controls to differentiate ethylene evolved by nitrogenase from that evolved by endogenous processes. In both piles temperatures were substantially elevated and partial pressures of oxygen greatly reduced thus indicating vigorous microbial activity. In the horse bedding, there were only low rates of ethylene evolution and little of this was due to nitrogen fixation. In contrast, much higher rates of ethylene evolution were measured in the leaf pile and 94% of this was due to nitrogenase activity. We estimate that the leaf pile fixed approximately 0.062 mg N/g leaves during a 5-week period. 相似文献
90.
Leaf senescence and nutrient remobilisation in barley and wheat 总被引:2,自引:0,他引:2