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951.
Stream valleys as wetlands 总被引:2,自引:2,他引:0
Wetlands as a component of discharge areas have been reduced to mere stream channels in the larger part of The Netherlands.
The original shape and condition are described with help of data from the past and from some small remnants. The typical Dutch
type of running water is the lowland stream, fed predominantly by rain and groundwater. Its nature is nutrient-poor in the
upper course and eutrophying gradually downstream. The chemical composition of streamwater, the vegetation in the floodplain
and the flora and fauna in the stream reflect this gradient. Some attention is paid to the amount and cycling of minerals
and nutrients. It is shown that the input of nutrients is not a limiting factor for primary production, but under natural
conditions light is limiting. Threats to stream wetlands are the composition of the precipitation, fertilization of the discharge
area and water management measures such as draining and canalization. As a result perspectives for a qualitative recovery
are unfavourable. Restoration of the shape of streams, however, is more prospective and several restoration programs are in
execution. 相似文献
952.
Stomatal control by both [ABA] in the xylem sap and leaf water status: a test of a model for draughted or ABA-fed field-grown maize 总被引:1,自引:1,他引:0
A model of maize stomatal behaviour has been developed, in which stomatal conductance is linked to the concentration of abscisic acid ([ABA]) in the xylem sap, with a sensitivity dependent upon the leaf water potential (Ψ1). It was tested against two alternative hypotheses, namely that stomatal sensitivity to xylem [ABA] would be linked to the leaf-to-air vapour pressure difference (VPD), or to the flux of ABA into the leaf. Stomatal conductance (gs) was studied: (1) in field-grown plants whose xylem [ABA] and Ψ1 depended on soil water status and evaporative demand; (2) in field-grown plants fed with ABA solutions such that xylem [ABA] was artificially raised, thereby decreasing gs and increasing Ψ1 and leaf-to-air VPD; and (3) in ABA-fed detached leaves exposed to varying evaporative demands, but with a constant and high Ψ1. The same relationships between gs, xylem [ABA] and Ψ1, showing lower stomatal sensitivity to [ABA] at high Ψ1, applied whether variations in xylem [ABA] were due to natural increase or to feeding, and whether variations in Ψ1, were due to changes in evaporative demand or to the increased Ψ1 observed in ABA-fed plants. Conversely, neither the leaf-to-air VPD nor the ABA flux into the leaf accounted for the observed changes in stomatal sensitivity to xylem [ABA]. The model, using parameters calculated from previous field data and the detached-leaf data, was tested against the observations of both ABA-fed and droughted plants in the field. It accounted with reasonable accuracy for changes in gs (r2 ranging from 0.77 to 0.81). These results support the view that modelling of stomatal behaviour requires consideration of both chemical and hydraulic aspects of root-to-shoot communication. 相似文献
953.
Studies on energy status and mitochondrial respiration during growth and senescence of mung bean cotyledons 总被引:2,自引:0,他引:2
Several characteristics of mitochondrial respiration and energy status have been studied during growth and senescence of mung bean ( Phaseolus radiatus L.) cotyledons. The results showed that mitochondrial oxygen consumption, respiratory control, ADP:O ratios, and energy charge changed in the cotyledons during germination and growth of the seedlings. The respiration rate of intact cotyledons approximately reflected the trend of the oxidative activities of the isolated mitochondria. An increase was observed in both whole cotyledon respiration and mitochondrial oxygen uptake at the onset of senescence of mung bean cotyledons (day 3 after germination), which thereafter declined gradually. The capacity and activity of the alternative pathway increased markedly in mitochondria isolated from senescent cotyledons. After the onset of senescence, the mung bean cotyledon mitochondria exhibited a decrease both in the respiratory control ratios and ADP:O ratios, and the cotyledons exhibited a gradual decline in energy charge. All these results showed an irreversible deterioration of energy conservation in mung bean cotyledons. The role(s) of the alternative pathway in senescent mung bean cotyledons is discussed. 相似文献
954.
Lipid composition of plasma membranes isolated from sunflower seedlings grown under water-stress 总被引:5,自引:0,他引:5
Flavia Navari-Izzo Mike Frank Quartacci Donato Melfi Riccardo Izzo 《Physiologia plantarum》1993,87(4):508-514
Plasma membranes were isolated by aqueous two-phase-partitioning from sunflower ( Helianthus annuus cv. Isabel) seedlings grown both under field irrigation and dryland conditions. Water-stressed plants showed a decrease in the leaf water potential and in the osmotic potential at full turgor, with the turgor pressure remaining at positive values. Dryland conditions also induced a reduction in the bulk modulus of elasticity. Plasma membranes of irrigated plants were characterized by high contents of phospholipids (68% of total lipids), free sterols (15. 7%) and glycolipids (9. 1%), mainly glycosphingolipids and steryl glycosides. Diacylglycerols, triacylglycerols and free fatty acids were also present. The major phospholipids were phosphatidylcholine and phosphatidylethanolamine with smaller amounts of phosphatidylinositol and phosphatidylglycerol. Following water stress, the plasma membranes showed a reduction of about 24 and 31% in total lipids and phospholipids, respectively. Also the amounts of glycolipids and diacylglycerols decreased significantly upon water stress. There was no change in free fatty acids, however, and triacylglycerols and free sterols increased. As a consequence, the free sterol to phospholipid molar ratio increased from 0. 4 to 0. 7 under water deficit conditions. The ratio of phosphatidylcholine to phosphatidylethanolamine increased from 1. 1 (control plants) to 1. 6 (water-stressed plants), while phosphatidic acid rose to 4% of total phospholipids. Dehydration did not result in any substantial change in the unsaturation level of the individual lipid classes, however. The results show that dryland conditions resulted in a marked alteration in the lipid composition of the sunflower leaf plasma membrane 相似文献
955.
Michael M. Mullin 《Global Change Biology》1998,4(1):115-119
Samples from the southern California sector of the California Current System were examined to test for interannual changes in winter–spring abundance of the planktonic copepod, Calanus pacificus, coincident with the 1992–93 and 1958–59 El Niños, each evaluated relative to immediately preceding years, and for interdecadal change (the early 1990s relative to the late 1950s). Calanus was anomalously rare in both of the El Niño periods, as was macrozooplankton, but (unlike macrozooplanktonic biomass) was not rarer in the early 1990s than in the late 1950s. The El Niño anomalies in Calanus’s abundance and macrozooplanktonic biomass were not spatially correlated on the mesoscale, implying that different proximate ecological causes may dominate at this scale. 相似文献
956.
cDNA encoding the full-length hKv1.3 lymphocyte channel and a C-terminal truncated (Δ459-523) form that lacks the putative
PKA Ser468 phosphorylation site were stably transfected in human embryonic kidney (HEK) 293 cells. Immunostaining of the transfected
cells revealed a distribution at the plasma membrane that was uniform in the case of the full-length channel whereas clustering
was observed in the case of the truncated channel. Some staining within the cell cytoplasm was found in both instances, suggesting
an active process of biosynthesis. Analyses of the K+ current by the patch-clamp technique in the whole cell configuration showed that depolarizing steps to 40 mV from a holding
potential (HP) of −80 mV elicited an outward current of 2 to 10 nA. The current threshold was positive to −40 mV and the current
amplitude increased in a voltage-dependent manner. The parameters of activation were −5.7 and −9.9 mV (slope factor) and −35
mV (half activation, V
0.5) in the case of the full-length and truncated channels, respectively. The characteristics of the inactivation were 14.2 and
24.6 mV (slope factor) and −17.3 and −39.0 mV (V
0.5) for the full-length and truncated channels, respectively. The activation time constant of the full-length channel for potentials
ranging from −30 to 40 mV decreased from 18 to 12 msec whereas the inactivation time constant decreased from 6600 msec at
−30 mV to 1800 msec at 40 mV. The unit current amplitude measured in cells bathing in 140 mm KCl was 1.3 ± 0.1 pA at 40 mV, the unit conductance, 34.5 pS and the zero current voltage, 0 mV. Both forms of the channels
were inhibited by TEA, 4-AP, Ni2+ and charybdotoxin. In contrast to the native (Jurkat) lymphocyte Kv1.3 channel that is fully inhibited by PKA and PKC, the
addition of TPA resulted in 34.6 ± 7.3% and 38.7 ± 9.4% inhibition of the full-length and the truncated channels, respectively.
8-BrcAMP induced a 39.4 ± 5.4% inhibition of the full-length channel but had no effect (8.6 ± 8.3%) on the truncated channel.
Cell dialysis with alkaline phosphatase had no effects, suggesting that the decreased sensitivity of the transfected channels
to PKA and PKC was not due to an already phosphorylated channel. Patch extract experiments suggested that the hKv1.3 channel
was partially sensitive to PKA and PKC. Cotransfecting the Kvβ1.2 subunit resulted in a decrease in the value of the time
constant of inactivation of the full-length channel but did not modify its sensitivity to PKA and PKC. The cotransfected Kvβ2
subunit had no effects. Our results indicate that the hKv1.3 lymphocyte channel retains its electrophysiological characteristics
when transfected in the Kvβ-negative HEK 293 cell line but its sensitivity to modulation by PKA and PKC is significantly reduced.
Received: 18 June 1997/Revised: 7 October 1997 相似文献
957.
Osamu Katano 《Oecologia》1996,106(2):199-205
The foraging tactics of dark chub, Zacco temmincki were classified into sit-and-wait where fish remained stationary in running water, stop where fish remained stationary in still water, and cruising where fish moved widely near the water surface. Sit-and-wait foragers primarily fed upon invertebrates that were carried by the current or fell onto the water surface, whereas cruising foragers fed upon algae and invertebrates that fell onto the water surface. Stopping individuals only fed occasionally. The percentage of sit-and-wait foraging increased as current velocity increased, whereas the percentage of cruising foraging was negatively affected by both current velocity and fish density. Larger individuals and males employed sit-and-wait foraging more frequently than smaller individuals and females. Hom range was large for cruising foragers. Aggressive interactions were frequent among sit-and-wait foragers. Although behavioural patterns of dark chub markedly differed between running and still waters, the gut content and growth rate did not differ significantly between the two habitats. 相似文献
958.
959.
Primary cultures containing a high percentage of lactotrophs were obtained by dissociating the pituitary of rats following
14–18 days of lactation. Lactotrophs with a distinctive appearance were recorded after 1–35 days in vitro and identified by
immunocytochemical staining for prolactin. Whole-cell voltage clamp measurements in isotonic KCl solution from a holding potential
of −40 mV revealed the presence of inward-rectifying K currents with a time-dependent, Na+-independent inactivation at potentials negative to −60 mV. The time for complete inactivation was strikingly different between
lactotrophs, varying between 1 sec and more than 5 sec at −120 mV, and was not related to time in culture. The reversal potential
shifted 59 mV (25°C) for a tenfold change in external K+ concentration, demonstrating the selectivity of the channel for K+ over Na+. The inward-rectifying K current was blocked by 5 mm Ba2+ and partially blocked by 10 mm TEA. Chloramine-T (1 and 2 mm) produced a total block of the inward-rectifying K current in lactotrophs. Thyrotropin-releasing hormone (500 nm) significantly reduced the inward-rectifying K current in about half of the lactotrophs. This current is similar to the inward-rectifying
K current previously characterized in clonal somatomammotrophic pituitary cells (GH3B6). The variability of the rate of inactivation of this current in lactotrophs and its responsiveness to TRH is discussed.
Received: 28 September 1995/Revised: 11 December 1995 相似文献
960.
Biosynthesis of proteoglycans by isolated rabbit glomeruli 总被引:8,自引:0,他引:8
J L Stow E F Glasgow C J Handley V C Hascall 《Archives of biochemistry and biophysics》1983,225(2):950-957
Isolated rabbit glomeruli were incubated in vitro with 35SO4 in order to analyze the proteoglycans synthesized. Proteoglycans extracted with 4 M guanidine HCl from whole isolated glomeruli and from purified glomerular basement membrane (GBM) were analyzed by gel filtration chromatography. Two types of sulfated proteoglycans were found to be synthesized by rabbit glomeruli and these contained either heparan sulfate or chondroitin/dermatan sulfate glycosaminoglycan chains. These glycosaminoglycans were characterized by their sensitivity to selective degradation by nitrous acid or chondroitinase ABC, respectively. The major proteoglycan extracted from the whole glomeruli was a chondroitin/dermatan sulfate species (75%), while purified GBM contained mostly heparan sulfate (70%). The glycosaminoglycan chains were estimated to be about 12,000 molecular weight which is consistent with previous estimates for similar molecules extracted from the rat GBM. 相似文献