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1.
E. A. C. MacRobbie 《Plant biology (Stuttgart, Germany)》1988,101(2):140-148
Opening and closing of the stomatal pore is associated with very large changes in K-salt accumulation in stomatal guard cells. This review discusses the ionic relations of guard cells in relation to the general pattern of transport processes in plant cells, in plasmalemma and tonoplast, involving primary active transport of protons, proton-linked secondary active transport, and a number of gated ion channels. The evidence available suggests that the initiation of stomatal opening is regulated through the uptake mechanisms, whereas initiation of stomatal closing is regulated by control of ion efflux at the plasmalemma, and of fluxes to and from the vacuole. In response to a closing signal there are large transient increases in efflux of both Cl? (or Br?) and Rb+ (K+) at the plasmalemma, with also a probable increase in anion flux from vacuole to cytoplasm and decrease in anion flux from cytoplasm to vacuole. A speculative hypothetical sequence of events is discussed, by which the primary response to a closing signal is an increase in Ca2+ influx at the plasmalemma, producing depolarisation and increase in cytoplasmic Ca2+. The consequent opening of Ca2+-sensitive Cl? channels, and voltage-sensitive K+ channels (also Ca2+-sensitive?) in the plasmalemma, and of a Ca2+-sensitive nonspecific channel in the tonoplast, could produce the flux effects identified by the tracer work; this speculation is also consistent with the Ca2+-sensitivity of the response to closing signals and with evidence from patch clamping that such channels exist in at least some plant cells, though not yet all shown in guard cells. 相似文献
2.
Calcineurin,a Type 2B Protein Phosphatase,Modulates the Ca2+-Permeable Slow Vacuolar Ion Channel of Stomatal Guard Cells 总被引:3,自引:1,他引:2
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The slowly activating vacuolar (SV) channel of plant vacuoles is gated open by cytosolic free Ca2+ and by cytosol-positive potentials. Using vacuoles isolated from broad bean guard cell protoplasts, SV-mediated currents could be measured in the whole-vacuole configuration of a patch clamp as the time-dependent increase in current at cytosol-positive voltages. Time-dependent deactivation of the SV currents when changing from activating to nonactivating voltages (tail currents) was used to calculate the selectivity of the channel to Ca2+ and Cl- with respect to K+. Changing the equilibrium potential for each permeant ion (Ca2+, Cl-, and K+) at least once for individual vacuoles allowed the relative permeabilities (P) of each of these ions to be calculated in a single experiment. The resulting Pca:Pcl:Pk ratio was close to 3:0.1:1. In accord with its characterization as a weakly selective Ca2+ channel, the SV-mediated current density decreased with increasing Ca2+ activity in the vacuole lumen. SV currents were potently modulated by the Ca2+-dependent, calmodulin-stimulated protein phosphatase 2B (calcineurin). At low concentrations ([less than or equal to]0.4 units per mL), calcineurin stimulated SV currents by ~60%, whereas at higher concentrations the phosphatase was inhibitory, reaching ~90% inhibition at 3 units per mL. Bovine calmodulin had no direct effect on SV-mediated currents, although calcineurin stimulated by exogenous calmodulin inhibited SV currents at all concentrations tested with half-maximal inhibition for calcineurin at 0.16 units per mL. The inhibitory effect of calcineurin could be blocked by the pyrethroid deltamethrin, indicating inhibition of SV channels by calcineurin via dephosphorylation. A model is discussed in which vacuolar Ca2+ release through SV channels is subject to both positive feedforward and negative feedback control through cytosolic Ca2+ and dephosphorylation, respectively. 相似文献
3.
Visualizing Changes in Cytosolic-Free Ca2+ during the Response of Stomatal Guard Cells to Abscisic Acid
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In this paper, we report the results of a detailed investigation into abscisic acid (ABA)[mdash]stimulated elevations of guard cell cytosolic-free Ca2+ ([Ca2+]cyt). Fluorescence ratio photometry and ratio imaging techniques were used to investigate this phenomenon. Guard cells of open and closed (opened to 10 to 12 [mu]m before treatment with ABA) stomata were microinjected with the fluorescent Ca2+ indicator Indo-1. Resting [Ca2+]cyt ranged from 50 to 350 nM. ABA (100 nM) stimulated an increase in [Ca2+]cyt in 68 and 81% of guard cells microinjected in the open and closed configuration, respectively. All stomata were observed to close in response to ABA. Increases ranged from 100 to 750 nM above the resting concentration and were arbitrarily grouped into five "classes." ABA-stimulated increases in [Ca2+]cyt were not uniformly distributed across the cytosol of guard cells. Rapid transient increases in [Ca2+]cyt were also observed in the guard cells of stomata microinjected in the closed configuration. We concluded that the ABA-induced turgor loss in guard cells is a Ca2+-dependent process. 相似文献
4.
5.
茉莉酸甲酯诱导保卫细胞气孔关闭的信号转导机制 总被引:1,自引:0,他引:1
气孔是由植物器官表面成对的保卫细胞围成的小孔,气孔运动控制气体交换,与植物逆境应答和生长发育等生物学过程密切相关,受多种因子调控,茉莉酸甲酯(MeJA)是其中之一。与ABA类似,MeJA也可诱导气孔关闭,但是其机理尚不清楚。该文综述了近年来MeJA调控气孔运动的信号转导机制进展,包括Ca2+、胞质pH、活性氧和NO等第二信使对气孔开闭的影响以及COI1、JAR1、RCN1和TGG1/2等信号组分之间的调控关系,并讨论了保卫细胞中MeJA与ABA信号途径的相互作用。 相似文献
6.
Here the regulatory role of CO during stomatal movement In Vicla faba L. was surveyed. Results Indicated that, like hydrogen peroxide (H2O2), CO donor Hematin induced stomatal closure in dose- and time-dependent manners. These responses were also proven by the addition of gaseous CO aqueous solution with different concentrations, showing the first time that CO and H2O2 exhibit the similar regulation role in the atomatal movement. Moreover, our data showed that ascorbic acid (ASA, an important reducing substrate for H2O2 removal) and diphenylene iodonium (DPI, an inhibitor of the H2O2-generating enzyme NADPH oxidase) not only reversed stomatal closure by CO, but also suppressed the H2O2 fluorescence induced by CO, implying that CO induced-atomatal closure probably involves H2O2 signal. Additionally, the CO/NO scavenger hemoglobin (Hb) and CO specific synthetic inhibitor ZnPPIX, ASA and DPI reversed the darkness-induced stomatal closure and H2O2 fluorescence. These results show that, perhaps like H2O2, the levels of CO in guard cells of V. faba are higher In the dark than in light, HO-1 and NADPH oxidase are the enzyme systems responsible for generating endogenous CO and H2O2 in darkness respectively, and that CO is involved in darkness-induced H2O2 synthesis in V. faba guard cells. 相似文献
7.
H.M. Staines W. Chang J.C. Ellory T. Tiffert K. Kirk V.L. Lew 《The Journal of membrane biology》1999,172(1):13-24
Previous reports have indicated that Plasmodium falciparum-infected red cells (pRBC) have an increased Ca2+ permeability. The magnitude of the increase is greater than that normally required to activate the Ca2+-dependent K+ channel (K
Ca
channel) of the red cell membrane. However, there is evidence that this channel remains inactive in pRBC. To clarify this
discrepancy, we have reassessed both the functional status of the K
Ca
channel and the Ca2+ permeability properties of pRBC. For pRBC suspended in media containing Ca2+, K
Ca
channel activation was elicited by treatment with the Ca2+ ionophore A23187. In the absence of ionophore the channel remained inactive. In contrast to previous claims, the unidirectional
influx of Ca2+ into pRBC in which the Ca2+ pump was inhibited by vanadate was found to be within the normal range (30–55 μmol (1013 cells · hr)−1), provided the cells were suspended in glucose-containing media. However, for pRBC in glucose-free media the Ca2+ influx increased to over 1 mmol (1013 cells · hr)−1, almost an order of magnitude higher than that seen in uninfected erythrocytes under equivalent conditions. The pathway responsible
for the enhanced influx of Ca2+ into glucose-deprived pRBC was expressed at approximately 30 hr post-invasion, and was inhibited by Ni2+. Possible roles for this pathway in pRBC are considered.
Received: 12 May 1999/Revised: 8 July 1999 相似文献
8.
过氧化氢在水杨酸诱导的蚕豆气孔关闭中的作用 总被引:9,自引:0,他引:9
许多植物病原菌可通过气孔进入叶片组织,因此减小气孔开度有利于提高植物的抗性。我们通过表皮条分析和激光扫描共聚显微镜得到的证据表明在保卫细胞中过氧化氢可能是水杨酸信号的中间环节。SA可以浓度依赖的方式诱导气孔关闭(图1A),H2O2也有类似的作用(图1B)。100μmol/L的水杨酸诱导的气孔关闭作用可明显地被20U/ml的过氧化氢酶或10μmol/L的Vc逆转,但CAT和Vc单独处理时诱导气孔开放的作用很微弱。单细胞中基于荧光探针DCFH的时间进程实验表明直接外加(图版I)或显微注射100μmol/L的SA均可诱导保卫细胞中H2O2产生,但以显微注射双蒸水作为对照时对DCFH荧光无影响(图版II)。这些结果暗示了植物被病原菌感染时可能通过产生H2O2导致气孔关闭而阻止病原菌继续通过气孔侵入。 相似文献
9.
Ca2+在茉莉酸甲酯诱导拟南芥气孔关闭中的信号转导作用 总被引:8,自引:0,他引:8
以拟南芥叶片下表皮为材料 ,分别用表皮生物分析法和激光扫描共聚焦显微镜成像技术 ,研究茉莉酸甲酯 (JA Me)促进气孔关闭过程中胞质Ca2 浓度的变化及其与气孔关闭的关系。结果表明 ,10 - 7到 10 - 3mol L的JA Me处理能促进拟南芥叶片的气孔关闭 ,其中 ,10 - 5mol L是最适浓度。用 10 - 5mol L的JA Me处理5min ,胞质Ca2 浓度从静息态的 10 5nmol L增加到 332 0nmol L ;质膜Ca2 通道阻断剂LaCl3和Ca2 螯合剂EGTA均能明显地降低JA Me对气孔关闭的促进作用。由此推测 ,胞质Ca2 可能是JA Me促进气孔关闭的重要信号转导因子 相似文献
10.
Milan Brumen Aleš Fajmut Andrej Dobovišek Etienne Roux 《Journal of biological physics》2005,31(3-4):515-524
The action of different agonists such as acetylcholine on the membrane of airway smooth muscle cells may induce cytosolic
Ca2+ oscillations which can be a part of the Ca2+ signalling pathway, eventually leading to cell contraction. The aim of the present study is to present a mathematical model
of the possible effect of the initial Ca2+ distribution within the cell on the form and frequency of induced Ca2+ oscillations. It takes into account intracellular Ca2+ stores such as sarcoplasmic reticulum and cytosolic proteins as well as Ca2+ exchange across the plasma membrane. We are able to demonstrate a closer agreement of model predictions with observed Ca2+ traces for a significantly wider range of parameter values, as was previously reported. We show also that the total cellular
Ca2+ content is an important system parameter especially because of the content in sarcoplasmic reticulum. At a total Ca2+ increase of about 20%, the oscillation frequency increases by 25%; also, damped oscillations become sustained. Cases are
indicated in which such a situation could occur. 相似文献
11.
以拟南芥为材料,利用药理学实验,结合分光光度法和激光共聚焦显微技术,研究了Ca2+在硫化氢(H2S)诱导拟南芥气孔关闭过程中的作用及其与过氧化氢(H2O2)的关系。结果表明: H2S诱导气孔关闭, Ca2+螯合剂EGTA和质膜Ca2+通道阻断剂硝苯地平(Nif)能不同程度抑制H2S诱导的气孔关闭,而内质网钙泵阻断剂毒胡萝卜素(Thaps)对H2S的作用无显著影响。由此推测, Ca2+参与调节H2S诱导的拟南芥气孔关闭过程,且胞质中Ca2+来源于胞外Ca2+的内流。另外, H2S诱导拟南芥叶片NADPH氧化酶基因AtRBOHD和AtRBOHF以及细胞壁过氧化物酶基因AtPRX34表达增强,促进叶片和保卫细胞中H2O2积累, EGTA对此起抑制作用,而外源CaCl2处理上调AtRBOHD、AtRBOHF和AtPRX34的表达。表明Ca2+可能位于H2O2上游参与H2S诱导的拟南芥气孔关闭过程。 相似文献
12.
Calcium-Activated K+ Channels and Calcium-Induced Calcium Release by Slow Vacuolar Ion Channels in Guard Cell Vacuoles Implicated in the Control of Stomatal Closure 总被引:9,自引:4,他引:9
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Stomatal closing requires the efflux of K+ from the large vacuolar organelle into the cytosol and across the plasma membrane of guard cells. More than 90% of the K+ released from guard cells during stomatal closure originates from the guard cell vacuole. However, the corresponding molecular mechanisms for the release of K+ from guard cell vacuoles have remained unknown. Rises in the cytoplasmic Ca2+ concentration have been shown to trigger ion efflux from guard cells, resulting in stomatal closure. Here, we report a novel type of largely voltage-independent K+-selective ion channel in the vacuolar membrane of guard cells that is activated by physiological increases in the cytoplasmic Ca2+ concentration. These vacuolar K+ (VK) channels had a single channel conductance of 70 pS with 100 mM KCI on both sides of the membrane and were highly selective for K+ over NH4+ and Rb+. Na+, Li+, and Cs+ were not measurably permeant. The Ca2+, voltage, and pH dependences, high selectivity for K+, and high density of VK channels in the vacuolar membrane of guard cells suggest a central role for these K+ channels in the initiation and control of K+ release from the vacuole to the cytoplasm required for stomatal closure. The activation of K+-selective VK channels can shift the vacuolar membrane to more positive potentials on the cytoplasmic side, sufficient to activate previously described slow vacuolar cation channels (SV-type). Analysis of the ionic selectivity of SV channels demonstrated a Ca2+ over K+ selectivity (permeability ratio for Ca2+ to K+ of ~3:1) of these channels in broad bean guard cells and red beet vacuoles, suggesting that SV channels play an important role in Ca2+-induced Ca2+ release from the vacuole during stomatal closure. A model is presented suggesting that the interaction of VK and SV channel activities is crucial in regulating vacuolar K+ and Ca2+ release during stomatal closure. Furthermore, the possibility that the ubiquitous SV channels may represent a general mechanism for Ca2+-induced Ca2+ release from higher plant vacuoles is discussed. 相似文献
13.
Responses of Photosynthetic Electron Transport in Stomatal Guard Cells and Mesophyll Cells in Intact Leaves to Light, CO2, and Humidity
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High-resolution images of the chlorophyll fluorescence parameter Fq'/Fm' from attached leaves of commelina (Commelina communis) and tradescantia (Tradescantia albiflora) were used to compare the responses of photosynthetic electron transport in stomatal guard cell chloroplasts and underlying mesophyll cells to key environmental variables. Fq'/Fm' estimates the quantum efficiency of photosystem II photochemistry and provides a relative measure of the quantum efficiency of non-cyclic photosynthetic electron transport. Over a range of light intensities, values of Fq'/Fm' were 20% to 30% lower in guard cell chloroplasts than in mesophyll cells, and there was a close linear relationship between the values for the two cell types. The responses of Fq'/Fm' of guard and mesophyll cells to changes of CO2 and O2 concentration were very similar. There were similar reductions of Fq'/Fm' of guard and mesophyll cells over a wide range of CO2 concentrations when the ambient oxygen concentration was decreased from 21% to 2%, suggesting that both cell types have similar proportions of photosynthetic electron transport used by Rubisco activity. When stomata closed after a pulse of dry air, Fq'/Fm' of both guard cell and mesophyll showed the same response; with a marked decline when ambient CO2 was low, but no change when ambient CO2 was high. This indicates that photosynthetic electron transport in guard cell chloroplasts responds to internal, not ambient, CO2 concentration. 相似文献
14.
It has been suggested that neighbouring cells in higher plantsco-ordinate their direction of growth by sensing the electricalpolarities of their neighbours. The present work sets out toexamine the role played by calcium in the response of theirindividual cells to externally-applied fields. The transcellular currents of cultured tobacco cells were investigatedwith a vibrating probe before and after the application of anartificial electric current with a density of 250 µA cm2,giving a potential difference of approximately 3 mV across thecell. When calcium was omitted from the experimental medium,the externally-applied current had little effect on either thedirection or magnitude of the cells' own transcellular currents.When 01 mM calcium was present, the external current repolarizedthe cells so that their own currents tended to flow in the samedirection as the current applied. This was due to a large localizedincrease in inward current in the region nearest the positiveelectrode, with the outward current being more evenly spread. Adding cobalt ions (a Ca2+ -channel blocker) in the presenceof external calcium had little immediate effect on the transcellularcurrents themselves, but they lost their ability to change inresponse to the artificially applied current. This suggeststhat the cells may have detected the applied current by enhancedcalcium ingress through calcium channels in the most hyperpolarizedregion of the membrane. An hypothesis is presented which proposesthat asymmetric calcium entry results in the electrical polarizationof cells by a mechanism involving both the opening of calcium-gatedion channels and the lateral movement of electrophoreticallymobile channels. Key words: Cell-culture, cobalt, plant electrophysiology, polarity, vibrating-probe 相似文献
15.
16.
The effects of abscisic acid (ABA) on the size of the apertureof stomata on epidermal strips of Vicia faba were studied inincubation media with different pH values. The osmotic potentialof guard cells, as determined by the limiting plasmolysis method,was higher at pH 4.0 than at pH 6.0, although the size of thestomatal apertures was almost identical at both pH values. AtpH 4.0, ABA effectively caused stomatal closure but had onlya small effect on the osmotic potential, whereas, at pH 6.0,ABA significantly increased the osmotic potential. ABA promotedthe efflux of Cl and malate from epidermal strips intothe incubation medium, an effect which was more marked at pH6.0, with a concomitant efflux of K+ to balance the charge onthe exported anions. From these results, it is suggested thatABA may cause an increase in the elastic modulus of the cellwalls of guard cells.
3 Present address: Nagano Prefectural Vegetable and OrnamentalCrops Experimental Station, 2206 Oomuro, Matsusiro-machi, Nagano381-12, Japan (Received September 30, 1986; Accepted January 9, 1987) 相似文献
17.
This study examined [Ca2+]i oscillations in the human salivary gland cell lines, HSY and HSG. Relatively low concentrations of carbachol (CCh) induced
oscillatory, and higher [CCh] induced sustained, steady-state increases in [Ca2+]i and K
Ca currents in both cell types. Low IP3, but not thapsigargin (Tg), induced [Ca2+]i oscillations, whereas Tg blocked CCh-stimulated [Ca2+]i oscillations in both cell types. Unlike in HSG cells, removal of extracellular Ca2+ from HSY cells (i) did not affect CCh-stimulated [Ca2+]i oscillations or internal Ca2+ store refill, and (ii) converted high [CCh]-induced steady-state increase in [Ca2+]i into oscillations. CCh- or thapsigargin-induced Ca2+ influx was higher in HSY, than in HSG, cells. Importantly, HSY cells displayed relatively higher levels of sarcoendoplasmic
reticulum Ca2+ pump (SERCA) and inositoltrisphosphate receptors (IP3Rs) than HSG cells.
These data demonstrate that [Ca2+]i oscillations in both HSY and HSG cells are primarily determined by the uptake of Ca2+ from, and release of Ca2+ into, the cytosol by the SERCA and IP3R activities, respectively. In HSY cells, Ca2+ influx does not acutely contribute to this process, although it determines the steady-state increase in [Ca2+]i. In HSG cells, [Ca2+]i oscillations directly depend on Ca2+ influx; Ca2+ coming into the cell is rapidly taken up into the store and then released into the cytosol. We suggest that the differences
in the mechanism of [Ca2+]i oscillations HSY and HSG cells is related to their respective abilities to recycle internal Ca2+ stores.
Received: 30 October 2000/Revised: 26 February 2001 相似文献
18.
Cytosolic Concentration of Ca2+ Regulates the Plasma Membrane H+-ATPase in Guard Cells of Fava Bean 总被引:10,自引:1,他引:10
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Opening of the stomata is driven by the light-activated plasma membrane proton pumping ATPase, although the activation and inactivation mechanism of the enzyme is not known. In this study, we show that the H+-ATPase in guard cells is reversibly inhibited by Ca2+ at physiological concentrations. Isolated microsomal membranes of guard cell protoplasts from fava bean exhibited vanadate-sensitive, ATP-dependent proton pumping. The activity was inhibited almost completely by 1 [mu]M Ca2+ with a half-inhibitory concentration at 0.3 [mu]M and was restored immediately by the addition of 1,2-bis(2-aminophenoxy)ethane N,N,N[prime],N[prime]-tetraacetic acid, a calcium chelating reagent. Similar reversible inhibition by Ca2+ was shown by the generation of electrical potential in the membranes. Activity of ATP hydrolysis was inhibited similarly by Ca2+ in the same membrane preparations. The addition of 1,2-bis(2-aminophenoxy)ethane N,N,N[prime],N[prime]-tetraacetic acid and EGTA, Ca2+ chelators, to epidermal peels of fava bean induced stomatal opening in the dark, and the opening was suppressed by vanadate. This suggests that the lowered cytosolic Ca2+ activated the proton pump in vivo and that the activated pump elicited stomatal opening. Inhibition of H+-ATPase by Ca2+ may depolarize the membrane potential and could be a key step in the process of stomatal closing through activation of the anion channels. Furthermore, similar inhibition of the proton pumping and ATP hydrolysis by Ca2+ was found in isolated plasma membranes of mesophyll cells of fava bean. These results suggest that Ca2+ regulates the activity of plasma membrane H+-ATPases in higher plant cells, thereby modulating stomatal movement and other cellular processes in plants. 相似文献
19.
Phototropins (phot1 and phot2) are blue light (BL) receptorsthat mediate responses including phototropism, chloroplast movementand stomatal opening, and increased cytosolic Ca2+. BL absorbedby phototropins activates plasma membrane H+-ATPase in guardcells, resulting in membrane hyperpolarization, and drives K+uptake and stomatal opening. However, it is unclear whetherthe phototropin-mediated Ca2+ increase activates the H+-ATPase.Here, we determined cytosolic Ca2+ concentrations in guard cellprotoplasts (GCPs) from Arabidopsis transformed with aequorin.Cytosolic Ca2+ increased rapidly in response to BL in GCPs fromboth the wild type and phot1 phot2 double mutants, but was mostlysuppressed by an inhibitor of photosynthetic electron flow (DCMU).With depleted external K+, we observed another slower Ca2+ increase,which was phototropin- dependent. Fusicoccin, a H+-ATPase activator,mimicked the effect of BL. The slow Ca2+ increase thus appearsto result from membrane hyperpolarization. The slow Ca2+ increasewas suppressed by external K+ and was restored by blockers ofinward-rectifying K+ channels, CsCl and tetraethylammonium,suggesting the preferential uptake of K+ over Ca2+. Such efficientK+ uptake in response to BL was not found in mesophyll cells.Both the fast and the slow Ca2+ increases were inhibited byCa2+ channel blockers (CoCl2 and LaCl3) and a chelating agent(EGTA). These results indicate that the phototropin-mediatedCa2+ increase was not observed prior to H+-ATPase activationin guard cells and that Ca2+ entered guard cells via Ca2+ channelsthrough photosynthesis and phototropin-mediated membrane hyperpolarization. 相似文献
20.
Induction of Stomatal Closure by Vanadate or a Light/Dark Transition Involves Ca2+-Calmodulin-Dependent Protein Phosphorylations 总被引:1,自引:1,他引:0
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Previous studies indicate that a continual source of adenosine 5[prime]-triphosphate is required for both opening and closing of stomata. However, vanadate (Na3VO4 at 500 [mu]M) as well as a light/dark transition induced stomatal closing in epidermal peels of Commelina communis L., showing that the stoppage or even the decrease of the activity of the plasma membrane H+-adenosine 5[prime]-triphosphatase is sufficient to induce stomatal closure. Furthermore, stomatal closing in response to Na3VO4 or a light/dark transition was suppressed by inhibitors of metabolism (10 [mu]M carbonyl cyanide m-chlorophenylhydrazone) and of protein kinases (20 [mu]M 1-[5-iodonaphthalene-1-sulfonyl]-1H-hexa-hydro-1,4-diaz-epine), calmodulin antagonists (20 [mu]M N-[6-aminohexyl]-5-chloro-1-naphthalenesulfonamide), and the anion channel blocker 5-nitro-2,3-phenylpropyllamino benzoic acid (50 [mu]M). These data suggest that the slow, outward rectifying anion channel, whose opening would be related to the membrane potential, and at least one step requiring a protein phosphorylation by a Ca2+-calmodulin-dependent protein kinase of the myosin light chain kinase type might be implicated in the induction of stomatal closing by vanadate or a light/dark transition. 相似文献