Osmotic relations during elongation growth in hypocotyls of Helianthus annum L.

The relationship between growth, change in cell osmotic pressure and accumulation of osmotic solutes was investigated in hypocotyls of sunflower (Helianthus annum L.) seedlings. During growth in darkness the osmotic pressure decreased by 50% between days 2 and 6 after sowing. After irradiation of dark-grown seedlings with continuous white light (WL) an inhibition of hypocotyl growth was measured, but the osmotic pressure of the growing cells was not lower than in the dark-grown control. Growth in darkness and after WL irradiation was accompanied by an increase in the amount of osmotic substances (soluble sugars) which was proportional to the increase in length of the organ. During growth in continuous WL the cell osmotic pressure decreased by 45 % between days 2 and 6 after sowing. The transfer of WL-grown seedlings to darkness (“re-etiolation”) resulted in a rapid acceleration of hypocotyl growth, but the cell osmotic pressure was the same as that of the WL grown control. Growth in continuous WL was accompanied by a corresponding accumulation of osmotic substances (soluble sugars). The transition from WL to darkness resulted in an enhanced accumulation of osmotica and an increase in cell-wall extensibility. The results indicate that the relative maintenance of cell osmotic pressure during rapid hypocotyl growth in darkness is caused by an enhanced accumulation of soluble sugars into the growing cells of the organ.     

Exogenous Hexoses Cause Quantitative Changes of Pigment and Glycerolipid Composition in Filamentous Cyanobacteria

Cyanobacteria Spirulina platensis and Nostoc linckia were grown in the presence of 5 mM and 50 mM glucose or 5 mM mannose, non-metabolisable glucose analogue that effectively triggers the repression of photosynthesis. Glucose evoked active cyanobacterial growth but chlorophyll (Chl) content decreased to some extent and porphyrins were excreted. The content of monogalactosyldiacylglycerol decreased in glucose-grown cyanobacteria and that of phosphatidylglycerol increased substantially. Mannose inhibited cyanobacteria growth as well as Chl synthesis, however, phosphatidylglycerol contents were higher than in respective control samples. In cyanobacterial cells glucose may not only inhibit photosynthetic processes, but also cause structural transformations of membranes which may be necessary for the activity of respiratory electron transport chain components under heterotrophic conditions.     

Myxococcal predation of the cyanobacterium Phormidium luridum in aqueous environments

Two strains of Myxococcus xanthus, and a strain of Myxococcus fulvus were compared with respect to their ability to entrap and lyse trichomes of the cyanobacterium Phormidium luridum var. olivaceae. All of these isolates form colonial aggregates and spherules in either axenic culture with a tryptone-salts medium or in a mixed culture with viable cyanobacterial cells as the sole source of nutrients. Light microscopy showed evidence of swarming activity on the surface of all three myxococci with the accompanying formation of fruiting structures. Extended incubation of mixed cultures showed the myxococci to be capable of long-term control of the cyanobacterial population with predator-prey population cycling occurring on average every 9 days. Serial transfer of mixed cultures into either fresh autotrophic medium or cyanobacterial cultures of 10⁷ per ml showed the persistence of predatory activity. Myxococcal densities were shown to return repeatedly to initial virulent levels. Predator inoculum levels could be reduced to 50 cells per 100 ml in a cyanobacterial culture of 10⁷ per ml. These in vitro data enhance the potential of the myxococcus predatory colony as a biological control agent for in situ cyanobacteria.     

Rapid and selective extraction of phycocyanin from Spirulina platensis with ultrasonic cell disruption

A study was conducted on the efficiency of phycocyanin extraction from Spirulina platensis (Arthrospira platensis) cells disrupted by ultrasonic irradiation. Extraction followed first-order kinetics with respect to the length of time for irradiation. The first-order rate constant increased linearly with the output of ultrasonic irradiation. In order to extract phycocyanin there was an appropriate range of ultrasonic frequency, f_u. In addition the most important finding is that the purity of phycocyanin in its crude extract depended on f_u. For example, phycocyanin was extracted with higher purity at f_u = 28 kHz than at f_u = 20 kHz. It is suggested that rapid and selective extraction of phycocyanin from S. platensis may be possible if an optimized ultrasonic application is developed for a given suspension.     

Growth and antioxidant system of the cyanobacterium Synechococcus elongatus in response to microcystin-RR

Microcystins, one type of the cyanobacterial toxins, show a broad range of hazardous effects on other organisms. Most of the researches on the toxic effects of microcystins have involved in animals and higher plants. Little work, however, has been done on evaluating the mechanisms of microcystin toxicity on algae. In this study, the toxicological effects of microcystin-RR (MC-RR) on the cyanobacterium Synechococcus elongatus were investigated. For this purpose, six physio-biochemical parameters (cell optical density, reactive oxygen species (ROS), malondialdehyde (MDA), glutathione (GSH), glutathione peroxidase (GSH-Px) and glutathione S-transferase (GST)) were tested in algal cells when exposed to 100 g^–1 microcystin-RR. The results showed that the growth of Synechococcus elongatus (expressed as optical density) was significantly inhibited compared with the control. At the same time, the treated algae exhibited a pronounced increase in production of ROS and MDA after 6 days exposure to microcystin-RR. Significant changes in GSH levels and GSH-Px, GSH activities were also detected in algal cells, with higher values being observed in the toxin treated algae after 6 days exposure. GST activities in the treated algae exhibited a decline after exposure and rapid augmentation on day 3, thereafter, they kept at a high level when compared to the control group. GSH contents and GSH-Px activities were also significantly raised in the toxin-treated algae cells from day 3, but they showed a sharp decrease on day 4, which was the onward of cell proliferation. These results suggested that oxidative stress manifested by elevated ROS levels and MDA contents might be responsible for the toxicity of microcystin to Synechococcus elongatus and the algal cells could improve their antioxidant ability through the enhancement of enzymatic and non-enzymatic preventive substances.     

Microwave effects on energy metabolism of rat brain

Rat brain was exposed to 591-MHz, continuous-wave (CW) microwaves at 13.8 or 5.0 mW/cm² to determine the effect on nicotinamide adenine dinucleotide, reduced (NADH), adenosine triphosphate (ATP) and creatine phosphate (CP) levels. On initiation of the in vivo microwave exposures, fluorimetrically determined NADH rapidly increased to a maximum of 4.0%–12.5% above pre-exposure control levels at one-half minute, then decreased slowly to 2% above control at three minutes, finally increasing slowly to 5% above control level at five minutes. ATP and CP assays were performed on sham- and microwave-exposed brain at each exposure time. At 13.8 mW/cm², brain CP level was decreased an average of 39.4%, 41.1%, 18.2%, 13.1%, and 36.4% of control at exposure points one-half, one, two three, and five minutes, respectively, and brain ATP concentration was decreased an average of 25.2%, 15.2%, 17.8%, 7.4%, and 11.2% of control at the corresponding exposure periods. ATP and CP levels of rat brain exposed to 591-MHz cw microwaves at 5 mW/cm² for one-half and one minute were decreased significantly below control levels at these exposure times, but were not significantly different from the 13.8 mW/cm² exposures. For all exposures, rectal temperature remained constant. Heat loss through the skull aperture caused brain temperature to decrease during the five-minute exposures. This decrease was the same in magnitude for experimental and control subjects. Changes in NADH, ATP, and CP levels during microwave exposure cannot be attributed to general tissue hyperthermia. The data support the hypothesis that microwave exposure inhibits mitochondrial electron transport chain function, which results in decreased ATP and CP levels in brain.     

In vitro characterization of salt stress effects and the selection of salt tolerant plants in rice (Oryza sativa L.)

Summary The response of plant cells to salt stress was studied on embryo derived calli of rice (Oryza sativa L.) in order to identify cellular phenotypes associated with the stress. The feasability of selecting salt tolerant callus and its subsequent regeneration to plants was also studied. Callus was grown on agar-solidified media containing 0%, 1% and 2% (w/v) NaCl for 24 days. Parameters such as fresh weight, dry weight, soluble protein and proline content were measured. The callus growth decreased markedly with increasing NaCl concentration in the medium. The proline content was enhanced several fold in salt stressed calli. A prolonged exposure of callus to the salt environment led to discolouration and arrested growth in the majority of the calli and only a small number of callus cells maintained healthy and stable growth. These variants were subcultured every three weeks for a period of four months onto medium containing 1% NaCl to identify tolerant lines. At the end of the third cell passage, the tolerant calli were transferred to regeneration medium to regenerate plants. The regeneration frequency in the salt-selected lines was enhanced when compared to unselected lines.     

Evidence of phytochrome involvement in the entrainment of the circadian rhythm of carbon dioxide metabolism in Bryophyllum

The rhythm of carbon dioxide output in Bryophyllum leaves was entrained on exposure to 0.25 h of white light every 24 h. Entrainment also occurred on similar exposure to monochromatic radiation in spectral bands centred at 660 nm and, to a lesser extent, at 730 nm, but a band centred at 450 nm was without effect. A skeleton irradiation programme comprising two 0.25-h exposures to white light per 24 h also entrained the rhythm when the intervening dark periods were either 7.5 h and 16 h, or 10.5 h and 13 h. The rhythm disappeared when the two exposures were separated by 11.5-h and 12-h dark periods. Regular 0.25-h exposures to red light separated by 11.75-h periods of darkness also resulted in loss of the rhythm. Red/far-red reversibility was observed in irradiation schedules having either one or two exposures to red light daily. In the latter case, far-red reversal of the effects of one of the exposures to red light resulted in entrainment of the rhythm by the other, instead of abolition of the rhythm. The occurrence of distinct red/far-red reversibility suggests strongly that phytochrome is the pigment involved in entrainment of this rhythm by cycles of light and darkness.Abbreviation LD light-dark rhythm     

Antioxidative response of Lemna minor plants exposed to thallium(I)-acetate

The physiological effects of thallium(I)-acetate on the duckweed Lemna minor after 1-, 4-, 7- and 14-d exposure were analyzed. High bioaccumulation of Tl (221 mg kg⁻¹ dry wt at 2.0 μM Tl-acetate) caused an inhibition of plant growth. After 14-d exposure, 0.2, 0.5, 1.0 and 2.0 μM Tl-acetate reduced the frond-number growth rate by 21.1%, 39.4%, 66% and 83.1%, respectively. Tl-acetate also induced a modulation of the antioxidative response by depleting the ascorbate content and affecting the antioxidative enzymes activities. Superoxide dismutase showed a continuous increase of activity (31–67%) after Tl-acetate exposure. Other antioxidative enzymes displayed a biphasic response to both the concentration and the exposure period. Exposure up to 7 d decreased the catalase activity (up to 40%) in plants treated with higher Tl-acetate concentrations. In contrast, 14-d exposure increased the activity of the enzyme (≥90%). Short-term exposure increased ascorbate peroxidase activity (13–41%), except in plants exposed to the highest Tl-acetate concentration. However, 14-d exposure decreased the enzyme activity at all concentrations tested (38–60%). Although pyrogallol peroxidase activity increased (up to 26%) during 4-d exposure, longer exposures to the highest two concentrations decreased the activity of the enzyme (25–48%). In general, short-term exposure to Tl-acetate activated the antioxidant capacity, which resulted in recovery of the frond-number growth rates in Tl-treated plants. In spite of the activation of the antioxidative response during short-term exposure, higher Tl-acetate concentrations increased the hydrogen peroxide level (up to 45%) and induced marked oxidative damage to lipids, proteins and DNA. Longer exposure induced a decline of the antioxidative response, and plants showed the symptoms of oxidative damage even at lower Tl-acetate concentrations. The genotoxic effect was evaluated by an alkaline version of the cellular and acellular Comet assay, which revealed an indirect genotoxic effect of Tl-acetate, suggesting oxidatively induced damage to DNA.     

Bioactivity of intra and extracellular substances from cyanobacteria and lactic acid bacteria on “wood blue stain” fungi

Cyanobacteria (photoautotrophic prokariota) have potential for the control of pathogenic bacteria and fungi. The effect of intra and extracellular products from cyanobacterial strains on the growth of fungi isolated from “wood blue stain,” was tested. Extracellular products were obtained by concentration and sterilization of the culture medium where cyanobacteria were grown. Cyanobacterial substances promoted or inhibited fungal growth according to the fungal and cyanobacterial strains tested. Extracellular products from Nostoc muscorum 79a and the methanolic extract from Microchaete tenera 84b biomass inhibited growth of Sphaeropsis sapinea 2157 (64.7 and 775.6%, respectively). Extracellular products of Nostoc piscinale 59 and biomass methanolic extract from N. muscorum 79a produced the highest growth promotion of Trichoderma boningii 452 (105.0%) and T. viride 993 (136.7%). Extracellular products of the heterotrophic lactic acid bacterium Streptococcus termophilus were also tested and strongly inhibited (64–92%) all the fungal strains. The tested fungi have different sensitivity to the bioactive substances present in the biomass and/or the culture medium of the studied cyanobacteria and lactic acid bacterium. N. muscorum 79a, M. tenera 84b, and S. termophilus have potential to control the wood blue stain fungi by a friendly environmental alternative.     

Growth and ultrastructure of the marine unicellular alga <Emphasis Type="Italic">Dunaliella salina</Emphasis> (Chlorophyta) after chronic selenium intoxication

The effects of selenium (0.01, 0.5, 1, 5 and 10 mg/liter) on the growth and ultrastructure of the microalga Dunaliella salina were investigated following its transfer into clean water. Selenium concentrations of 5 and 10 mg/liter were toxic to D. salina, and reinoculation of microalga into clean water did not prevent it from total mortality. When reinoculated from medium with 0.01 mg Se/liter, the cell population density of D. salina was restored in 14 days. The number of ultrastructural alterations in cells was the same as in the control, while the excretory activity of microalga between days 4 and 10 of this experiment was higher. Cell population growth of D. salina transferred from 0.5 and 1 mg Se/liter was lower than in the control. No ultrastructural defects were observed in microalga reinoculated from medium with a selenium concentration of 0.5 mg/liter and the excretion level corresponded to that at 0.01 mg/liter. Various types of ultrastructural damage were found in microalga from medium with 1 mg Se/liter, which was previously reported to be threshold for D. salina; however, the number of cell injuries decreased with increasing time in clean medium. Excretory activity was decreased at the beginning of experiment; but after 7 days, it was restored to the control level. Though there were no ultrastructural alterations in microalgal cells from medium with 0.5 mg Se/liter, we assume that they had molecular defects that could inhibit the cell population growth. The study of microalgae following their reinoculation from medium containing toxicants into clean medium can be a useful method for evaluating algal survival after toxic exposure.     

Effects of nifedipine,verapamil, and diltiazem on tip growth inFunaria hygrometrica

Protonemata ofFunaria hygrometrica Sibth. were treated with nifedipine, verapamil, or diltiazem. Responses to each of the drugs were, on the one hand, reduction of growth rate and tip cell length and, on the other hand, formation of apical swellings in caulonema tip cells and of anomalously oriented separation walls between main filaments and young side branches. The first effect is regarded as a more general expression of inhibition while the second complex of effects is attributed to perturbations in directed vesicle transport. Replacement of drug-containing media by normal Knop agar demonstrated the reversibility of inhibitor action: growth parameters were comparable to those of control protonemata within a few hours. A fast reaction, the formation of subapical vacoules, occurred within minutes of drug application and was only observed with verapamil and diltiazem. In connection with this process, rapid migrations of chloroplasts took place, but examination of the microtubule cytoskeleton in such cells by indirect immunofluorescence with a monoclonal antibody against tubulin showed an intact microtubule network. callose deposits in tip cells treated with verapamil. They were polarly distributed and started to appear in cell apices about 2h after the beginning of verapamil application. Two mechanisms of action for the tested inhibitors are discussed: (i) perturbations of membrane permeability by interference with one or more of the cell's Ca²⁺-transport systems, and (ii) a more indirect mechanism affecting vesicle transport via the microfilament system.     

Response of the estuarine cyanobacterium <Emphasis Type="Italic">Lyngbya aestuarii</Emphasis> to UV-B radiation

Growth response and changes in the spectral properties of methanolic extract of an estuarine cyanobacterium, Lyngbya aestuarii Agardh, to UV-B radiation were studied. Increase in growth accompanied by increase in chlorophyll a, protein and carbohydrate content was observed up to 12 h of UV-B irradiation followed by a decline with further increase in the duration of UV exposure. Carotenoid content progressively increased with the UV-B dose. The organism synthesized, to a significant extent, mycosporine amino acid-like substances (MAAs) upon UV-B exposure. The cells in the trichome became coiled followed by formation of small bundles as a response to UV-B radiation. SDS protein profile of the UV irradiated cells showed repression of 20 and 22 kDa proteins. However, irradiation with UV-B for 6–24 h led to overproduction of 84, 73, 60, 46, 40, 37 KDa proteins, possibly conferring protection to the organism from UV-B. UV irradiated cells cultured in florescent light for up to 7 days showed revival from UV damage of the pigments and macromolecular contents, suggesting existence of a repair mechanism in the organism.     

Toxicity and biodegradation of fluometuron by selected cyanobacterial species

The Biodegradation capabilities of six selected cyanobacterial species for fluometuron, a phenylurea herbicide, as well as its inhibitory effect on chlorophyll a content were investigated. The selected species (three strains of Microcystis aeruginosa, Anabaena cylindrica, A. flos-aquae and A. spiroides) were subjected to three elevated concentrations of fluometuron (0.14, 0.7 and 1.4 mg/ml) for different exposure times (1–5 days). Results revealed that biodegradation of fluometuron is species-dependent and positively correlated with the exposure time, reaching maximum efficiency after 5 days at all the investigated concentrations. All the species tested showed generally great ability to degrade the compound even at the highest concentration with specific variations among them. Biodegradation efficiencies of fluometuron by the selected species were in the following ranges; 39.2–99.9; 87.5–100; and 93.2–100 at 0.14; 0.7 and 1.4 mg fluometuron/ml respectively. It was noticed that the gradual increase in the pesticide concentration enhances its biodegradability by the selected algal species. Variations according to species as well as exposure time were discussed. The highest fluometuron concentration (1.4 mg/l) showed the highest inhibition of chlorophyll a content in the tested species and toxicity was also species- and time-dependent.     

Effects of copper on phagocytosis inTetrahymena

Summary Addition of copper, corresponding to 100 ppm, to the normal 2% proteose peptone medium is tolerated byTetrahymena. This concentration of copper stimulates phagocytosis to a maximum value which is reached gradually during the first 1 hour exposure, and which is maintained during continuous exposures. Cell proliferation is resumed after a lag period, although at a decreased rate. Cells exposed to copper contain small refractile granules, previously proposed to represent an ion-regulating system; the number of granules remains constant in proliferating cells. Higher concentrations of copper also resulted in an elevated rate of phagocytosis but at the same time cell mortality was observed; this lack of transition between inhibited phagocytosis and cell mortality may be ascribed to the physiological role of copper. The high amount of organic matter in the growth medium protects against the toxic effects of copper, thus in the absence of organic matterTetrahymena tolerated only a 100-fold lower concentration of copper than that tolerated in the growth medium. However, cells which had initiated granule formation (for example for regulation of calcium) prior to starvation and exposure to copper, were more resistant to copper than cells which had not yet activated this mechanism, perhaps because of the low capacity of starved cells for protein synthesis.     

Electrohydrodynamic effects on two species of insects with economic importance to stored food products

An electrohydrodynamic (EHD) system which generated air ions within a strong electric field was used to study responses of stored-product insectsTribolium confusum (du Val) andPlodia interpunctella (Hübner). Larval mortality of both species generally increased with increased exposure time to ions of either polarity. The larvae and pupae ofT. confusum suffered a higher mortality rate than the adults. The insects initially exhibited distinct avoiding motions away from regions of high towards low fluxes of air ions of both polarity. Insects moved vigorously, tumbled, flipped, curled up, and aggregated when the EHD system was turned on. The control insects not exposed to air ions survived and showed a total absence of such behaviour. For bipolar exposures, the insects occupied the neutral zone where the effects were minimal due to cancellation of the fields. Prolonged exposures of more than 20 min produced a quiescent state. EHD-enhanced mass transfer of the liquid component from physical objects established in fluid mechanics was invoked as a possible cause for insect mortality and avoiding behaviour to ions. Body fluid losses increased linearly with time of exposure (R ²0.97) for all biological stages of insect growth. The larvae and pupae ofT. confusum lost 12 and 15% of their body fluids, respectively, after 80 min of exposure to negative air ions. Fluid losses of such a magnitude are likely to have contributed to insect fatality.     

In vitro lead-induced cell toxicity and cytoprotective activity of fetal calf serum in human fibroblasts

The underlying mechanisms by which lead ions produce their deleterious effects prior to the onset of clinical symptoms are incompletely understood. This study aimed to assess lead-induced cell toxicity mechanisms by focusing on the effects of the metal on cell growth, DNA synthesis, cellular ATP, intracellular hexosaminidase activity and lysosomal function, and examine the possible cytoprotective role of fetal calf serum (FCS). Several human dermal cultured fibroblast lines were exposed to Pb (400 M) for 1–6 days with 2, 5, and 10% FCS. The earliest toxic effect of Pb was significant inhibition of DNA synthesis after 24 h direct exposure; this harmful effect was not progressive during the first 3 days, but worsened clearly on the 4th day regardless of the FCS concentration. A time-dependent depletion of intracellular ATP content was also caused by ionic lead, thereby compromising the cell energy charge which precedes cell death. Fibroblast growth was progressively and significantly inhibited from day 2 onwards; the greatest noxious effect was observed in the presence of 2% FCS: 49% reduction in cell proliferation after 5 days. Lead salts produced loss of cell adhesion to the culture dish which worsened from the 2nd day and was more pronounced when FCS in growth medium was decreased. Toxic actions on lysosomal membrane integrity provoked a decrease in neutral red uptake (NRU) which was exposure time-dependent and more marked with 2% FCS. In contrast, increased relative NRU (to 20% at 4 days), suggestive of endocytosis-induced lysosome enlargement, was observed in Pb-exposed cells. Intracellular hexosaminidase activity was not negatively affected until 5 days after exposure to Pb salts. FCS had a significant cytoprotective effect on Pb-induced toxicity.     

Evaluation of low humidity treatments to control Acarus farris (Acari: Acaridae) in Cabrales cheese

Laboratory experiments were conducted to establish the best combination of relative humidity (r.h.) and time of exposure to be applied on Cabrales cheeses infested with Acarus farris. Laboratory assays revealed that less than 30 h were required to obtain 90% mortality of mites at 50% and 60% r.h. Males were more susceptible than females to low relative humidity, since their lethal period values (LP) were lower than those obtained for females at the same relative humidity. Moreover, the response within sexes to low moisture treatments changed as exposure time increased, since the LP50 obtained for each sex at 50 and 60% r.h. were statistically different whereas the LP90 showed no significant differences within sexes. Accordingly, two modifications of the traditional maturing process were established to assess the efficacy of low moisture treatments to control A. farris on infested cheeses. The first modification consisted of one single exposure of 48 h at 50% r.h. and the second one consisted of two exposures of 48 h at 50% r.h. separated by a time interval of 15 days. No significant differences in final population density were observed for both low moisture treatments compared to control cheeses. Therefore, low humidity treatments are not effective to control A. farris in Cabrales cheese, despite the good results obtained in laboratory assays.     

绞股蓝提取液对蛋白核小球藻的化感效应及影响机理研究

以中草药植物绞股蓝[Gynostemma pentaphyllum(Thunb.) Makino]为化感供体材料,研究其不同浓度的提取液(0、5、10、25、50 g/L)对蛋白核小球藻(Chlorella pyrenoidesa)生长及生理生化特征的化感效应。结果表明:(1)绞股蓝提取液对蛋白核小球藻生长均具有抑制作用,其抑制作用随提取液质量浓度增大和培养时间延长均呈增强趋势,且25 g/L绞股蓝提取液培养15 d时的抑制率达到最大(79.41%)。(2)各浓度绞股蓝处理组蛋白核小球藻细胞内的叶绿素a含量均低于对照组,且随着提取液浓度升高以及处理时间延长叶绿素a含量较对照的降低量越多,表明蛋白核小球藻光合作用受到的影响也越大。(3)绞股蓝处理组蛋白核小球藻细胞的膜透性(吸光度OD_(264))显著高于对照,且膜透性随着提取液浓度增大而增强;高浓度提取液处理下,藻细胞内部的可溶性蛋白质(OD_(280))及核酸(OD_(260))含量均显著高于对照,且随着处理时间延长,细胞膜透性增大,细胞内部的可溶性蛋白质及核酸向胞外渗透增多。研究发现,绞股蓝提取液能够抑制蛋白核小球藻生长,并随着提取液质量浓度增大而增强;绞股蓝提取液能促进藻细胞叶绿素分解、增加细胞膜透性,引起可溶性蛋白质和核酸向胞外渗透量升高,导致藻细胞结构受损,代谢功能紊乱,从而达到化感抑制作用。