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91.
An experiment is described that provides evidence for an exchange mechanism to explain the increase in ribosomal gene number that occurs during bobbed magnification. We show that bobbed and bobbed-lethal alleles do not magnify in closed X chromosomes, but that a spontaneous ring opening restores normal magnification. The results provide strong evidence that the elementary magnifying event is unequal sister chromatid exchange, and can be interpreted in the framework of an inducible rDNA-specific recombination system as the basis of ribosomal gene magnification.
Correspondence to: S.A. Endow at the above address 相似文献
92.
S. T. Lovett T. J. Gluckman P. J. Simon V. A. Sutera P. T. Drapkin 《Molecular & general genetics : MGG》1994,245(3):294-300
We have examined the influence of proximity on the efficiency of recombination between repeated DNA sequences in Escherichia coli. Our experiments have employed a plasmid-based assay to detect deletions between direct repeats of 100 bp. The rate of deletion of the juxtaposed direct repeats was reasonably high at 6 × 10–5 per cell. A comparison of recA+ and recA mutant strains showed that these deletion events are primarily the result of recA-independent recombination at these homologous sequences. Random restriction fragments of yeast or E. coli genomic DNA were used to separate the two repeats. Deletion rates decreased over two orders of magnitude with increasing separation of up to 7 kb. There was a surprisingly strong effect of even short sequence separations, with insertions of a few hundred base pairs exhibiting 10-fold reductions of deletion rates. No effect of recA on the efficiency of deletion was observed at any distance between repeats. 相似文献
93.
Van de Geijn S. C. Vos J. Groenwold J. Goudriaan J. Leffelaar P. A. 《Plant and Soil》1994,161(2):275-287
A research facility is described for the integrated study of soil-root-shoot-atmosphere relationships in crops. The Wageningen Rhizolab has been in use since 1990, and consists of two rows, each with eight below-ground compartments aligned along a corridor. A rain shelter automatically covers the experimental area at the start of rainfall. Compartments are 125 cm × 125 cm and 200 cm deep. Each compartment has a separate drip irrigation system. Crop canopy photosynthesis, respiration, and transpiration can be measured simultaneously and continuously on four out of eight compartments at a time. Each compartment can be filled with a selected soil material (repacked soil) and is accessible from the corridor over its full depth. Multiple sensors for measuring soil moisture status, electrical conductivity, temperature, soil respiration, trace gases and oxygen are installed in spatial patterns in accordance with the requirements of the experiments. Sensors are connected to control and data-acquisition devices. Likewise, provisions have been made to sample manually the soil solution and soil atmosphere. Root observation tubes (minirhizotrons) are installed horizontally at depth intervals ranging from 5 cm (upper soil layers) to 25 cm (below 1 m). The facility is at present in use to study growth and development of vegetation (crops) in relation to drought, nutrient status, soil-borne diseases, and underground root competition. One important application is the study of elevated CO2 concentration and climate change and the way they affect crops and their carbon economy. Growth and development of field grown vegetables and winter cover crops are also evaluated. The common aspect of those studies is to gain a better understanding of crop growth under varying environmental conditions, and to collect datasets that may help to improve mechanistic crop growth simulation models that can address suboptimal growth conditions. 相似文献
94.
Simultaneous extraction of soil phytoavailable cadmium,copper, and lead by chelating resin membrane 总被引:2,自引:0,他引:2
Cadmium, copper, and lead were extracted from suspensions of contaminated soils using metal chelating exchange resin membranes.
Nine soils with widely varying properties and Cd, Cu and Pb levels were tested. Soil suspensions made up with 4 g in 40 mL
deionized water were equilibrated with 5 cm2 Ca-saturated Chelex exchange resin membrane which was retained inside a polypropylene bag and shaken at 150 rpm for 24 hrs.
Resin membrane extractable Cd, Cu and Pb of the soils were correlated with Cd, Cu, and Pb uptake by young wheat seedlings
grown in these soils and compared with soil Cd, Cu, and Pb extracted by 0.1 M HCl, 0.01 M CaCl2, and 0.005 M Diethylenetriamine pentaacetic acid (DTPA). The amounts of Cd, Cu and Pb extracted by the Ca-saturated Chelex membrane from
all tested soils correlated well with those absorbed by young wheat seedlings. The Ca-saturated Chelex membrane extractable
Cd, Cu and Pb of the soil had the strongest correlation with plant uptake Cd, Cu and Pb among the extraction methods we tested.
It was demonstrated that the Ca-saturated Chelex membrane extraction is an appropriate method in simultaneously estimating
Cd, Cu and Pb phytoavailability of soil and is applicable to a wide range of soils. 相似文献
95.
纵坑切梢小蠹蛀梢期空间分布 总被引:10,自引:1,他引:9
在昆明地区,纵坑切梢小蠹(Tomicus piniperda)成虫蛀梢多集中在蛀干木附近。 种群密度以蛀干木为中心向周围呈指数递减,散布半径约30m。在蛀梢过程中,该种群逐渐向新区扩张。在树冠内,纵坑切梢小蠹主要分布在4-10轮枝上。第7轮枝虫口百分率最高。6-7轮枝受害率最大。 树冠上层受害较其下层严重。从树冠水平层次考察,树冠外层虫量相对集中,约为树冠中、内层虫量之和。 树冠内层虫量最少。纵坑切梢小蠹在树冠内的种群分布系由梢径、种群密度、蛀梢行为、降落方式、光照等因素综合影响的结果。 相似文献
96.
The severity of the incidence of the fungal disease, potato scab, varies with different soil groups at the same soil pH. At a soil pH of 5.3, potato scab is easily controlled in soils of western Hokkaido (soil group A) by simply decreasing soil pH, but in soils from eastern Hokkaido (soil group B) it is not so easily controlled. The difference appears to be due to higher levels and exchangeable aluminium in Group A.Addition of sufficient aluminium or ferrous sulfate to a group B soil decreased the incidence of potato scab in a field experiment. Higher levels of aluminium sulfate depressed crop production. It is concluded that aluminium ions control the incidence of potato scab in acid soils. It is suggested that, in soils with low exchange acidity Y1, potato scab can be controlled by adding sufficient aluminium to increase their exchange acidity Y1 to above 7–8. 相似文献
97.
Refixation of xylem sap CO2 in Populus deltoides 总被引:1,自引:0,他引:1
Vascular plants have respiring tissues which are perfused by the transpiration stream, allowing solubilization of respiratory CO2 in the xylem sap. The transpiration stream could provide a conduit for the internal delivery of respiratory CO2 to leaves. Trees have large amounts of respiring tissues in the root systems and stems, and may have elevated levels of CO2 in the xylem sap which could be delivered to and refixed by the leaves. Xylem sap from the shoots of three Populus deltoides trees had mean dissolved inorganic carbon concentrations (CO2+H2CO3+HCO?3) ranging from 0. 5 to 0. 9 mM. When excised leaves were allowed to transpire 1 mM[14C]NaHCO3, 99. 6% of the label was fixed in the light. Seventy-seven percent of the label was fixed in major veins and the remainder was fixed in the minor veins. Autoradiography confirmed that label was confined to the vasculature. In the dark, approximately 80% of the transpired label escaped the leaf, the remainder was fixed in the major veins, slightly elevating dark respiration measurements. This indicates that the vascular tissue in P. deltoides leaves is supplied with a carbon source distinct from the atmospheric source fixed by interveinal lamina. However, the contribution of CO2 delivered to the leaves in the transpiration stream and fixed in the veins was only 0. 5% of atmospheric CO2 uptake. In the light 90% of the label was found in sugar, starch and protein, a pattern similar to that found for atmospheric uptake of[14C]CO2. Compared with leaves labelled in the light, leaves labelled in the dark had more label in organic acid, amino acid and protein and less label in sugar and starch. After a 5-s pulse the majority of the label fed to petioles in both the light and the dark was found in malate. The majority of the label was found in malate at 120 s in the dark; only 2% of the label was found in phosphorylated compounds at 120 s. The proportion of label found in phosphorylated compounds increased from 17% at 5 s to 80% at 120 s in the light. This suggests that CO2 delivered to leaves in the light via the transpiration stream is fixed in the veins, a small portion through dark fixation into malate, the remainder by C-3 photosynthesis. 相似文献
98.
A. M. Coimbra K. G. Ferreira P. Fernandes H. G. Ferreira 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(3):196-202
Electrical potential differences between the haemolymph and the extrapallial fluid, and between the haemolymph and the mantle cavity fluid, and ionic concentrations of calcium in the haemolymph and in extrapallial fluid were measured in vivo in Anodonta cygnea. The electrochemical potential of ionic calcium in the haemolymph is clearly above the electrochemical potential of ionic calcium in the environment and is very nearly in equilibrium with that of the extrapallial fluid. Simultaneous measurements of carbon dioxide partial pressure and pH in the extrapallial fluid showed that in this compartment ionic calcium is clearly above saturation. It is proposed that calcium deposition is regulated through the secretion of the organic matrix and by controlling the pH and the carbon dioxide partial pressure of the extrapallial fluid. An estimation of the minimum positive balance of calcium required to sustain shell growth together with the electrophysiological characterization of the mantle cavity epithelium showed that this tissue is not the route of entry of calcium into the animal.Abbreviations BW
body weight
- DW
dry weight
- EEPF-S
chemical potential difference
- EPF
extrapallial fluid
- Gtot
total conductance
- Isc
short-circuit current
- Ksp
solubility product
- MCE
mantle cavity epithelium
- MCF
mantle cavity fluid
- OME
outer mantle epithelium
- PCO2
partial pressure of carbon dioxide
- PVC
Poly(vinyl chloride)
- S
shell
- SEM
standard error of mean
-
V
ic
intracellular electrical potential
-
V
oc
open-circuit voltage 相似文献
99.
R. W. Wilson E. W. Taylor 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(3):239-246
Rainbow trout, Oncorhynchus mykiss, acclimated to 33% sea water (12 mg·ml-1 salinity) experienced significant (10 meq·1-1) increases in plasma [Na+] and [Cl-] within 5 h of exposure to 6.3 mol copper·1-1 indicating severe impairment of branchial ionoregulatory capacity. All plasma ion levels subsequently stabilised once the transbranchial [Na+] gradient was reduced to zero. The similar ionic strength of the external medium and their body fluids appeared to protect trout maintained in 33% sea water from further ionoregulatory stress and any secondary physiological disturbances during exposure to copper. Despite three- and fourfold greater transbranchial [Na+] and [Cl-] gradients, trout acclimated to full-strength sea water (35 mg·ml-1 salinity) suffered no major changes in plasma Na+, Cl-, K+, or Ca2+, blood gases or haematology during 24 h exposure to 6.3 mol copper·1-1. This reduction in toxicity in full strength sea water cannot be explained by differences in copper speciation. We suggest that during acute exposure to waterborne copper, active NaCl extrusion is unaffected due to the basolateral location of the gill Na+/K+-ATPase, but that ionoregulatory disturbances can occur due to gill permeability changes secondary to the displacement of surface-bound Ca2+. However, in full strength sea water the three-fold higher ambient [Ca2+] and [Mg2+] appear to be sufficient to prevent any detrimental permeability changes in the presence of 6.3 mol copper·1-1. Plasma [NH
+
4
] and [HCO
-
3
] were both significantly elevated during exposure to copper, indicating that some aspects of gill ion transport (specifically the apical Na+/NH
+
4
and Cl-/HCO
-
3
exchanges involved in acid/base regulation and nitrogenous waste excretion) are vulnerable to inhibition in the presence of waterborne copper.Abbreviations
C
aO2
arterial oxygen content
- Hb
haemoglobin
- Hct
haematocrit
- MABP
mean arterial blood pressure
- MCHC
mean cell haemoglobin content
-
MO2
rate of oxygen consumption
-
P
a
CO2
arterial carbon dioxide tension
-
P
aO2
arterial oxygen partial pressure
- S
salinity
- SW
sea water
-
T
Amm
total ammonia (=NH3+NH
+
4
)
-
T
CO
2
total carbon dioxide
- TEP
transepithelial potential
- TOC
total organic carbon
- %Hb-O2
percentage of haemoglobin saturated with oxygen 相似文献
100.
Field investigations of ammonia exchange between barley plants and the atmosphere. I. Concentration profiles and flux densities of ammonia 总被引:5,自引:2,他引:3
J. K. SCHJOERRING A. KYLLINGSBAEK J. V. MORTENSEN S. BYSKOV-NIELSEN 《Plant, cell & environment》1993,16(2):161-167
The exchange of ammonia between the atmosphere and the canopy of spring barley crops growing at three levels of nitrogen application (medium N, high N and excessive N) was studied over two consecutive growing seasons by use of micrometeorological techniques. In most cases, ammonia was emitted from the canopy to the atmosphere. The emission started around 2 weeks before anthesis, and peaked about or shortly after anthesis. The volatilization of ammonia only took place in the daytime. During the night-time, atmospheric ammonia was frequently aborbed by the canopy. Occasionally, plants in the medium and high N treatments also absorbed ammonia from the atmosphere during the daytime. Daytime absorption of ammonia never occurred in the excessive N canopy. The loss of ammonia from the canopy amounted in both years to 0.5–1.5 kg NH3-N ha?1 and increased with the N status of the canopy. In agreement with the small losses of ammonia, the content of 15N-labelled nitrogen in the plants did not decline during the grain-filling period. The experimental years were characterized by very favourable conditions for grain dry matter formation, and for re-utilization of nitrogen mobilized from leaves and stems. Consequently, a very high part of the nitrogen in the mature plants was located in grain dry matter (80–84% in 1989; 74–80% in 1990). The efficient re-utilization of nitrogen may have reduced the volatilization of ammonia. 相似文献