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1.
A Model to Investigate the Effect of Evaporative Cooling on the Pattern of Evaporation in Sub-stomatal Cavities 总被引:2,自引:1,他引:1
A model has been developed to investigate the pattern of waterevaporation from sub-stomatal cavities. In this model equationsfor the flux of matter are coupled to equations for the fluxof heat. Failure to couple heat and mass transfer processesin previous models may have led to erroneous conclusions regardingthe pattern of evaporation in sub-stomatal cavities. Our model predicts that the coolest surfaces are likely to be0.10.4 °C cooler than the warmest surfaces. Evaporativecooling does alter the pattern of evaporation in sub-stomatalcavities, yet 63 to 80 per cent of all evaporation still occursfrom a peristomatal region occupying about 4 per cent of thetotal sub-stomatal surface area. stomata, sub-stomatal cavities, transpiration, peristomatal evaporation, evaporative cooling 相似文献
2.
The hydraulic conductances of leaves of a species which exhibitsstomatal responses to humidity (Nicotiana glauca) are significantlylower than the conductances in a species which does not exhibitsuch responses (Tradescantia virginiana). This difference couldat least partly account for their difference in stomatal responseto humidity. In both species, the hydraulic conductance betweenthe leaf bulk and its epidermis is much lower than the conductancein any other part of the pathway. The apparently conflictingresults, reported in recent literature, on the hydraulic conductancesand water pathways in leaves are reinterpreted, and shown tobe due to misinterpretation of results. The recently publishedcriticisms of a technique used to measure hydraulic conductivityare commented on and refuted. An examination of the factors that influence the water potentialat the sites of evaporation from the inner walls of the epidermisnear stomatal pores showed that the water potential at thesesites is lower than the bulk epidermal water potential. Thewater potential at these sites changes in a complex way as stomatalaperture changes. As it is reduced the ratio of: waterpotential at sites of evaporation on the inner walls of theepidermis near stomatal pores/bulk leaf water potentialincreases. The positive feedback effect of this phenomenon,which tends to keep stomatal water potential constant as thestomata close and therefore enhances closure, and two otherpassive positive feedback effects on the waterpotential at sites of evaporation near stomata that have beenreported in the literature are briefly discussed. Nicotiana glauca (Grah.), Tradescantia virginiana (L.), sub-stomatal cavities, peristomatal evaporation, stomata, humidity response, leaf hydraulic conductance, water potential 相似文献
3.
During transpiration, water vapour diffusing through the stomatamoves through air that has no mean motion: its diffusive inwardflux is balanced by a general outward mass flow. The effectis to accelerate outward diffusive flows (water vapour, CO2in respiration) and retard inward diffusive flows (air, CO2for assimilation) and alter the apparent ratio of diffusioncoefficients. The magnitude of the effect is calculated theoretically,and estimated practically for an experiment in sugar-beet leaves.For this particular case the error in estimating mesophyll diffusiveresistance is near 2 per cent, but for other conditions couldbe much larger. Estimated values of the apparent ratio of thediffusion coefficient of water vapour and carbon dioxide (truevalue = 1.59) ranged from 0.027 to 2.79, but could lieanywhere between and +. 相似文献
4.
Gas exchange in Clusia rosea has been measured under variousconditions of water status, light and leaf-air vapour pressuredeficit (w, mbar bar1) which produce daytime (C3), night-time(CAM) or 24 h uptake of CO2. At high light levels, at a w of6.6, well-watered plants utilized C3 photosynthesis while CAMand 24 h uptake occurred under lower light levels and with lowto normal water availability and differing w (13.5 and 3.4,respectively). CO2 uptake was highest, stomatal conductanceto water vapour (gH2o) lowest, and water use efficiency (WUE)highest in plants using C3 photosynthesis. This latter factis contrary to the accepted view that CAM is most water useefficient, i.e. it optimizes CO2 uptake with minimal water loss.It is suggested that the low CO2 uptake in CAM photosynthesismay be related not only to the higher w but also to the factthat Clusia species accumulate citrate which may originate fromß-carboxylation of fatty acids (i.e. an internal sourceof CO2) and does not contribute to night-time external CO2 assimilation.Curves of assimilation (A) versus internal partial pressureof CO2 (A/c1) for the three photosynthetic types, under atmosphericconditions, did not produce a single trend. The trends whichwere produced represent the supply function for the interaction,under differing modes of photosynthesis, of the two major enzymesystems involved in CAM. Key words: Clusia rosea, Crassulacean acid metabolism, C3 photosynthesis, internal CO2 concentration, 24 h carbon dioxide uptake, water use efficiency. 相似文献
5.
Elevated carbon dioxide affects the patterning of subsidiary cells in Tradescantia stomatal complexes 总被引:3,自引:0,他引:3
Boetsch John; Chin Jonathan; Ling Michael; Croxdale Judith 《Journal of experimental botany》1996,47(7):925-931
The influence of elevated CO2 concentration (670 ppm) on thestructure, distribution, and patterning of stomata in Tradescantialeaves was studied by making comparisons with plants grown atambient CO2. Extra subsidiary cells, beyond the normal complementof four per stoma, were associated with nearly half the stomatalcomplexes on leaves grown in elevated CO2. The extra cells sharedcharacteristics, such as pigmentation and expansion, with thetypical subsidiary cells. The position and shape of the extrasubsidiary cells in face view differed in the green and purplevarieties of Tradescantia. Substomatal cavities of complexeswith extra subsidiary cells appeared larger than those foundin control leaves. Stomatal frequency expressed on the basisof leaf area did not differ from the control. Stomatal frequencybased on cell counts (stomatal index) was greater in leavesgrown in CO2-enriched air when all subsidiary cells were countedas part of the stomatal complex. This difference was eliminatedwhen subsidiary cells were included in the count of epidermalcells, thereby evaluating the frequency of guard cell pairs.The extra subsidiary cells were, therefore, recruited from theepidermal cell population during development. Stomatal frequencyin plants grown at elevated temperature (29 C) was not significantlydifferent from that of the control (24 C). The linear aggregationsof stomata were similar in plants grown in ambient and elevatedCO2. Since enriched CO2 had no effect on the structure or patterningof guard cells, but resulted in the formation of additionalsubsidiary cells, it is likely that separate and independentevents pattern the two cell types. Plants grown at enrichedCO2 levels had significantly greater internode lengths, butleaf area and the time interval between the appearance of successiveleaves were similar to that of control plants. Porometric measurementsrevealed that stomatal conductance of plants grown under elevatedCO2 was lower than that of control leaves and those grown atelevated temperature. Tradescantia was capable of regulatingstomatal conductance in response to elevated CO2 without changingthe relative number of stomata present on the leaf. Key words: Elevated CO2, stomata, subsidiary cells, patterning 相似文献
6.
J.I.L. Morison M.T.F. Piedade E. Müller S.P. Long W.J. Junk M.B. Jones 《Oecologia》2000,125(3):400-411
Fluxes of CO2 and H2O vapour from dense stands of the C4 emergent macrophyte grass Echinochloa polystachya were measured by eddy covariance in both the low water (LW) and high water (HW, flooded) phases of the annual Amazon river cycle at Manaus, Brazil. Typical clear-sky midday CO2 uptake rates by the vegetation stand (including detritus, sediment or water surface) were 30 and 35 µmol CO2 (ground) m-2 s-1 in the LW and HW periods, respectively. A rectangular hyperbola model fitted the responses of "instantaneous" (20- or 30-min average) net CO2 exchange rates to incident photosynthetic photon flux densities (PFD) well. Stand evaporation rates were linearly related to PFD. The major difference in CO2 uptake rates between the two periods was the larger respiration flux during LW due to the CO2 efflux from sediment, roots and litter. Integrated 20- or 30-min fluxes were used to derive relationships between daily CO2 and H2O vapour fluxes and incident radiation. The daily CO2 fluxes were almost linearly related to incident radiation, but there was evidence of saturation at the highest daily radiation totals. Annual productivity estimated from the daily model in 1996-1997 agreed closely with that previously estimated for 1985-1986 from a leaf-scale photosynthetic model, but were some 15% less than those derived at that time from biomass harvests. Both CO2 uptake and water use efficiency were comparable with those found in fertilised maize fields in warm temperate conditions. 相似文献
7.
A Method for Separating Cuticular and Stomatal Components of Gas Exchange by Amphistomatous Leaves: II. EXPERIMENTAL SOLUTION 总被引:1,自引:1,他引:0
The apparent cuticular component of transpiration of stomatabearing leaf epidermis was estimated by restricting stomataldiffusion by mass flow of air in the opposite direction. Thiswas achieved by applying an air pressure gradient across theamphistomatous leaf. Some assumptions of the previously suggestedmethod (antrcek and Slav?k, 1990) were experimentally verifiedusing maize leaves. The technique makes possible a quantitativeestimation of cuticular water loss including that of the externalperistomatal (i.e. vapour not passing through the pores) andthe respective conductance when the stomata are partially open. In addition to the fact that the cuticular portion of the totalleaf vapour loss (i.e. relative cuticular transpiration) dependson stomatal opening, even the absolute value of apparent cuticulartranspiration was (1) increased by lower vapour pressure deficitand (2) decreased with closing stomata. These changes, inducedby variations in a vapour pressure deficit of 2.45?0.35 kPa,ranged between 0.66?0.14µg cm 2 s1. Theabsolute value of apparent cuticular transpiration changed onaverage by a factor of 2.3 due to stomata opening or closingwhich was induced by turning the light on or by exogenous ABAapplication. Possible interference by residual vapour diffusingthrough the stomatal pore was evaluated by the model application.An attempt was also made to assess the cuticular component ofCO2-uptake rate. Experimental results are discussed in contextwith the feedforward response of stomata to air humidity. Key words: Cuticular transpiration, cuticular CO2-uptake, feedforward response, maize 相似文献
8.
The rates of net photosynthesis by closed canopies of tomatoplants were measured at three CO2 concentrations and three humiditiesover a range of natural light flux densities. The data havebeen analysed using a model of canopy photosynthesis which allowsfor variation in leaf area index and other leaf and canopy characteristics.The model also deals explicitly with the effects of CO2 concentration,leaf conductance, and photorespiration on the leaf photochemicalefficiency, . The leaves were found to have a photochemicalefficiency in the absence of photorespiration, m, of 12?6 ?109 kg (CO2) J1. At a CO2 concentration of 0?73 ? 103 kg m3 (400vpm) the leaf photochemical efficiency, , and canopy light utilizationefficiency, c, were 18 per cent greater at a vapour pressuredeficit of 0?5 kPa than at 1?0 kPa. At a CO2 concentration of2?2 ? 103 kg m3 (1200 vpm) they were only 5 percent greater. 相似文献
9.
A Method for Separating Cuticular and Stomatal Components of Gas Exchange by Amphistomatous Leaves: I MODEL SOLUTION 总被引:1,自引:1,他引:0
A new technique for estimating the cuticular component of epidermalgas exchange by a stomatous leaf side is proposed. It is basedon the process of elimination of stomatal diffusion by mass(viscous) flow of air applying an air pressure gradient acrossthe leaf. This technique was designed to enable a reliable estimationof the cuticular component irrespective of stomatal opening. A model solution of diffusive and mass flow counteraction interms of general substance fluxes is presented. Water vapourloss and CO2 uptake by a model leaf was simulated by varyingboth stomatal diffusion resistance and viscous flow of air throughthe stomatal pores in physiologically and experimentally relevantranges. Depending on stomatal opening, elimination of the stomatalcontribution to epidermal vapour and CO2 exchange by the viscousflow of air ranged from small to practically complete. It supportsthe relevance of the procedure for cuticular vapour loss estimationunder conditions of partially open stomata. Modification of the model CO2-uptake patterns due to expectedchanges in intercellular CO2 concentration, , was evaluated. Net CO2 flux under the diffusive-viscous flowscounteraction is sensitive to the changes mentioned above. Nevertheless,the changes in , evaluated by a simple model, were too small to cause significant departures from the CO2-uptakeelimination curves by constant . The relevance of the method for the determination of cuticular CO2-uptakeis discussed. Key words: Cuticular transpiration, cuticular CO2-uptake, methods, diffusive-viscous flows counteraction, model 相似文献
10.
Photosynthesis and Diffusion Conductance of the Valencia Orange Fruit under Field Conditions 总被引:5,自引:1,他引:4
CO2 uptake and diffusion conductance of Valencia orange fruits(Citrus sinensis L. Osbeck) were measured in the field duringthe growing season of 1977/78 to ascertain if, as in the leaf,stomata control photosynthesis and transpiration under changingenvironmental conditions. Measurements were made on 15 yearold trees grown in a sandy loam soil and receiving either adry or a wet treatment. Fruit diffusive conductance was measuredwith a modified water vapour diffusion conductance meter andgross photosynthesis was measured with a 14CO2 uptake meter.Photosynthetically active radiation (PAR) was measured witha quantum sensor. Fruits exposed to light assimilated CO2 ata rate which was 2550% of that assimilated by leaves.The uptake was dependent on fruit size, PAR, chlorophyll content,and on diffusive conductance of the fruit epidermis. Epidermalconductance showed a diurnal trend which was similar in shapeto that of the leaf except in the late afternoon. Cuticularconductance of the fruit was calculated and ranged between 0.22and 0.30 mm s1. It was speculated that the CO2 uptakeby the fruit could support the growth of flavedo cell layerswhen exposed to light. Dry soil caused an increase in the 14CO2uptake by fruit possibly caused by the increased potential areaof the stomatal opening per unit of fruit surface area. 相似文献
11.
Willmer, C. M., Wilson, A. B. and Jones, H. G. 1988. Changingresponses of stomata to abscisic acid and CO2 as leaves andplants age.J. exp. Bot. 39: 401410. Stomatal conductances were measured in ageing leaves of Commelinacommunis L. as plants developed; stomatal responses to CO2 andabscisic acid (ABA) in epidermal strips of C. communis takenfrom ageing leaves of developing plants and in epidermal stripsfrom the same-aged leaves (the first fully-expanded leaf) ofdeveloping plants were also monitored. Stomatal behaviour wascorrelated with parallel measurements of photosynthesis andleaf ABA concentrations. Stomatal conductance in intact leavesdecreased from a maximum of 0-9 cm s 1 at full leaf expansionto zero about 30 d later when leaves were very senescent. Conductancesdeclined more slowly with age in unshaded leaves. Photosynthesisof leaf slices also declined with age from a maximum at fullleaf expansion until about 30 d later when no O2 exchange wasdetectable. Exogenously applied ABA (0.1 mol m 3) didnot affect respiration or photosynthesis. In epidermal stripstaken from ageing leaves the widest stomatal apertures occurredabout 10 d after full leaf expansion (just before floweringbegan) and then decreased with age; this decrease was less dramaticin unshaded leaves. The inhibitory effects of ABA on stomatalopening in epidermal strips decreased as leaves aged and wasgreater in the presence of CO2 than in its absence. When leaveswere almost fully-senescent stomata were still able to open.At this stage, guard cells remained healthy-looking with greenchloroplasts while mesophyll cells were senescing and theirchloroplasts were yellow. Similar data were obtained for stomatain epidermal strips taken from the same-aged leaves of ageingplants. The inhibitory effects of ABA on stomatal opening alsodecreased with plant age. In ageing leaves both free and conjugated ABA concentrationsremained low before increasing dramatically about 30 d afterfull leaf expansion when senescence was well advanced. Concentrationsof free and conjugated ABA remained similar to each other atall times. It is concluded that the restriction of stomatal movements inintact leaves as the leaves and plants age is due mainly toa fall in photosynthetic capacity of the leaves which affectsintracellular CO2 levels rather than to an inherent inabilityof the stomata to function normally. Since stomatal aperturein epidermal strips declines with plant and leaf age and stomatabecome less responsive to ABA (while endogenous leaf ABA levelsremain fairly constant until leaf senescence) it is suggestedthat some signal, other than ABA, is transmitted from the leafor other parts of the plant to the stomata and influences theirbehaviour. Key words: Abscisic acid, CO2, Commelina, leaf age, senescence, stomatal sensitivity 相似文献
12.
Stomatal Responses to Two Herbicidal Auxins 总被引:5,自引:0,他引:5
The effects of 1-naphthylacetic acid (NAA) and 2-naphthoxyaceticacid (NOXA) on stomatal opening on illumination of excised,turgid leaves of Stachytarpheta indica were investigated bymicroscopic examination of abaxial epidermises fixed in absoluteethanol. Both chemicals were effective in restricting, but notcompletely preventing, stomatal opening and suppressing starchhydrolysis and potassium accumulation in the guard cells. Therepressive effects were only partly reversed by CO2-free air.It is concluded that NAA and NOXA do not greatly affect passiveopening mediated by changes in the leaf water balance, but partlysupress photoactive opening by arresting starch hydrolysis andpotassium accumulation in the guard cells and partly by disturbingthe intercellular CO2 concentration. A possible link betweenstarch hydrolysis and potassium accumulation in the guard cellsis briefly mentioned. 相似文献
13.
Carbon dioxide and water vapour exchange rates were measuredon attached leaves of field-grown citrus trees. The exchangerates were measured continuously during several weeks in thespring of two successive years. These data confirmed the ratherlow rates of maximum CO2 exchange (611 µmol m2s1) by citrus leaves. However, the maximum rate was maintainedthrough the midday period on only about half the days. On theother days, characterized by high temperatures and high atmosphericwater vapour pressure deficits, pronounced midday depressionsin CO2 exchange rates were observed. Since midday transpirationremained stable at a constant rate even with increasing vapourpressure deficit, these results indicate that stomatal closurewas occurring. In fact, the data suggest tfiat specific, maximumtranspiration rates were associated with differing rootstocks.Thus, the rate of water supply to the leaves may be an importantfactor in determining the maximum transpiration rate, and therebymediating control of stomatal conductance and the resultantmidday depression in CO2 exchange rates. 相似文献
14.
The 13C values for epidermal and mesophyll tissues of two C3plants, Commelina communis and Tulipa gesneriana, and a CAMplant, Kalancho daigremontiana, were measured. The values forthe tissues of both C3 plants were similar. In young leavesof Kalancho, the epidermis and the mesophyll showed S13C valueswhich were nearly identical, and similar to those found in C3plants. However, markedly more negative values for epidermalcompared to mesophyll tissue, were obtained in the mature Kalancholeaf. This is consistent with the facts that the epidermis ina CAM leaf is formed when leaves engage in C3 photosynthesisand that subsequent dark CO2 fixation in guard cells or mesophyllcells makes only a small contribution to total epidermal carbon. (Received January 27, 1981; Accepted May 14, 1981) 相似文献
15.
The ontogenic changes in several component processes of photosynthesiswere measured in chickpeas. Gas exchange characteristics ofintact leaves were studied to analyse the effects of ambientconditions under which chickpeas are usually grown. The CO2assimilation rate per unit leaf area remained fairly high duringthe vegetative stage, reaching a peak at early pod-fill anddeclining subsequently throughout pod development. The intercellularCO2 partial pressure (C1) remained more or less constant (195µbar) during vegetative growth and the early stages ofseed-filling. With falling RWC and PAR interception, the stomatalconductance declined more rapidly than the CO2 assimilationrate resulting in a value of C1 less than that normally existingunder ambient conditions. From the A/C1-analysis, CO2 assimilationduring pod-filling appears to be limited by the RuBP-regenerationcapacity because the carboxylation efficiency and in vitro RuBPCaseactivity were initially unaffected. However, as leaves aged,the carboxylation efficiency and in vitro RuBPCase activitydecreased abruptly with increasing leaf temperatures above 30°C, and the C1 was greater than normally existing values(195 µbar), suggesting an increased mesophyll limitationof photosynthesis. It is suggested that a decline in the CO2assimilation rate of leaves during pod development and an acceleratedsenescence are induced by adverse ambient conditions, particularlyplant water stress and high leaf temperature. Key words: Cicer arietinum L., gas exchange, photosynthesis, ribulose-1,5-bisphosphate carboxylase 相似文献
16.
A system for measuring leaf gas exchange based on regulating vapour pressure difference 总被引:2,自引:0,他引:2
Waichi Agata Yoshinobu Kawamitsu Susumu Hakoyama Yasuo Shima 《Photosynthesis research》1986,9(3):345-357
A system for measurement of leaf gas exchange while regulating leaf to air vapour pressure difference has been developed; it comprises an assimilation chamber, leaf temperature controller, mass flow controller, dew point controller and personal computer. A relative humidity sensor and air and leaf temperature sensors, which are all used for regulating the vapour pressure difference, are mounted into the chamber. During the experiments, the computer continuously monitored the photosynthetic parameters and measurement conditions, so that accurate and intenstive measurements could be made.When measuring the light-response curve of CO2 assimilation for single leaves, in order to regulate the vapour pressure difference, the leaf temperature and relative humidity in the chamber were separately and simultaneously controlled by changing the air temperature around the leaf and varying the air flow rate through the chamber, respectively. When the vapour pressure difference was regulated, net CO2 assimilation, transpiration and leaf conductance for leaves of rice plant increased at high quantum flux density as compared with those values obtained when it was not regulated.When measuring the temperature-response curve of CO2 assimilation, the regulation of vapour pressure difference was manipulated by the feed-forward control of the dew point temperature in the inlet air stream. As the vapour pressure difference was regulated at 12 mbar, the maximum rate of and the optimum temperature for CO2 assimilation in rice leaves increased 5 molCO2 m–2 s–1 and 5°C, respectively, as compared with those values obtained when the vapour pressure difference took its own course. This was reasoned to be due to the increase in leaf conductance and the decrease in transpiration rate. In addition, these results confirmed that stomatal conductance essentially increases with increasing leaf temperature under constant vapour pressure difference conditions, in other words, when the influence of the vapour pressure difference is removed.This system may be used successfully to measure inter- and intra-specific differences and characteristics of leaf gas exchange in plants with a high degree of accuracy.Abbreviations A
CO2 assimilation rate
- Amax
Maximum rate of CO2 assimilation
- Aopt
Optimum teperature for CO2 assimilation
- CTWB
Controlled-temperature water bath
- DPC
Dew point controller
- E
Transpiration rate; gl, leaf conductance
- HCC
Humidity control circuit
- IRGA
Infrared gas analyzer
- LT
Leaf temperature
- LTC
Leaf temperature controller
- MFC
Mass flow controller
- QFD
Quantum flux density
- RH
Relative humidity
- RHC
Relative humidity controller
- VPD
Vapour pressure difference
- CO2
Difference of CO2 concentration between inlet and outlet air 相似文献
17.
de S. WAIDYANATHA U. P.; KEYS A. J.; WHITTINGHAM C. P. 《Journal of experimental botany》1975,26(1):15-26
When solutions of [14C]glycollate, glycine, serine, glycerate,or glucose were supplied to segments of wheat leaves throughtheir cut bases in the light, most of the 14C was incorporatedinto sucrose in air but in CO2-free air less sucrose was made.The synthesis of sucrose was decreased because metabolism ofserine was partly blocked. Sucrose synthesis from glucose andglycerate in CO2-free air was decreased but to a smaller extent;relatively more CO2 was evolved and serine accumulated. Theeffects of DCMU and light of different wavelengths on metabolismby leaves of L-[U-14C]serine confirmed that simultaneous photosyntheticassimilation of carbon was necessary for the conversion of serineto sucrose. Of various products of photosynthesis fed exogenouslyto the leaves -keto acids were the most effective in promotingphotosynthesis of sucrose and release of 14CO2 from 14C-labelledserine. This suggests that in CO2-free air the metabolism ofserine may be limited by a shortage of -keto acid acceptorsfor the amino group. In CO2-free air added glucose stimulatedproduction of CO2 and sucrose from D-[U-14C]- glycerate andno competitive effects were evident even though glucose is convertedrapidly to sucrose under these conditions. In addition to asupply of keto acid, photosynthesis may also provide substratesthat can be degraded and provide energy in the cytoplasm forthe conversion of glycerate to sugar and phosphates and sucrose. 相似文献
18.
Measurements of sustained rates of vapour loss via stomatalpores in epidermal strips show that these rates compare withtranspiration rates of intact leaves. The water supply pathwithin the epidermal tissue is thus capable of sustaining ahigh rate of evaporation from subsidiary and guard cells. 相似文献
19.
The carbon isotope discrimination ratio of the floral parts,leaves, and stems of barley and oat plants were measured todetermine if net CO2 fixation by PEP carboxylase (describedin these tissues by other workers) makes a significant contributionto total carbon fixation in these tissues. The 13C values rangedfrom 26.6 to 29.6% and are within the range normallyexpected for plants with the C3 pathway of photosynthesis inwhich autotrophic CO2 fixation proceeds via RuBP carboxylase.We conclude that PEP carboxylase does not make a substantialcontribution to autotrophic CO2 fixation in the floral partsof these C3 plants. 相似文献
20.
Responses of stomata to changes in humidity 总被引:24,自引:0,他引:24
Summary Large areas of the lower epidermis of full-grown leaves of Polypodium vulgare (and Valerianella locusta) are normally separated from the mesophyll by an extensive subepidermal airspace. Epidermal stripes were prepared for experiments to simulate these conditions in order to investigate stomatal reactions. They were placed with their inner surface in contact with an airspace of uniformly high humidity. The outer surface was treated with air of varying degrees of humidity. The stomatal reactions were observed by microscope and the opening of the guard cells determined photographically.Treatment of the outer side of the epidermis with dry air led to a rapid closing of the stomata, whilst moist air caused opening. This induction of opening and closing movements could be repeated up to 15 times with the same stoma by changing the degree of humidity. Neighbouring groups of stomata showed different apertures according to their individual humidity conditions. The degree of aperture of the stomata depended on the water potential of the ambient air and also on the humidity conditions in the subepidermal airspace.The cause of this stomatal behaviour could lie in the peristomatal transpiration. In this way, the guard cells are able to function as humidity sensors which measure the difference in water potential inside and outside the leaf. Their aperture thus is controlled by their individual transpiration conditions. This controlling mechanism could be very important for the water economy of plants. They would appear to be able to reduce their transpiration through an increase in diffusion resistance of the stomata during decreasing humidity in the ambient air, without changing the water status of the whole leaf. 相似文献