Internal gas transport in Typha latifolia L. and Typha angustifolia L. 1. Humidity-induced pressurization
and convective throughflow
Malene Bendix, Troels Tornbjerg, Hans Brix* Department of Plant Ecology, Institute of Biological Sciences, University of Aarhus,
Nordlandsvej 68, DK-8240 Risskov, Denmark
Accepted 2 August 1994
Abs~a~
The internal gas transport in the shoots of the cattails, Typha latifolia L. and Typha angustifolia L., occurs principally via pressurized convective throughflow of gases. Static pressure differentials of up to 350 Pa relative to ambient for T. latifolia and 570 Pa for T. angustifolia were found to be generated mainly by humidity-induced diffusion at ambient temperatures of 15-25°C. Thermal transpiration did not contribute significantly to the internal pressurization. Convective gas flow rates of up to 8 cm 3 min- ~ for T. latifolia and 3.5 cm 3 min-~ for T. angustifolia were recorded from cut rhizomes. Internal pressuriza- tion and convective throughflow rates were highest at high ambient temperature and low ambient relative humidity. Light did not affect pressurization in T. latifolia, whereas pres- surization and convective gas flows were lower in the light than in the dark in T. angusti- folia, probably as a consequence of stomatal movements. A layer of closely packed meso- phyll cells located just below the palisade parenchyma of the leaves is probably the porous partition responsible for the pressurization, but stomata with tortuous pathways may also be involved. Under identical environmental conditions the ventilation capacity of T. an- gustifolia is about twice as high as that of T. latifolia indicating that root aeration of the former may be more efficient.
1. Introduction
Most vascular aquatic macrophytes have a lacunar system of intercellular air- spaces that acts as a gas transport pathway, aerating underground organs and ox-
76 M. Bendix et aL / Aquatic Botany 49 (1994) 75-89
idizing the rhizosphere (Armstrong, 1979, 1982; Brix, 1993 ). Transport of gases within the lacunal system can occur by molecular diffusion, where reciprocating concentration gradients of 02 and CO2 result in transport of 02 to the roots and of CO2 from the roots to the shoots (Armstrong, 1979; Brix, 1988, 1989 ). How- ever, in a number of emergent and floating-leaved species the diffusive gas trans- port is supplemented, and in many cases dominated, by throughflow convection (Armstrong et al., 1991; Brix et al., 1992).
Throughfiow convection is defined as a Poisseuille flow in a channel which is open at both ends and is driven by a pressure differential between the two ends. In emergent macrophytes throughflow convection can be initiated by pressuri- zation in one plant part, inducing a PoisseuiUe flow through the lacunal system of the plant towards a venting point at another emergent part of the plant. The pressure differential is generated by gradients in temperature and water vapour pressure between the internal gas spaces and the surrounding atmosphere or by gradients in wind velocity around the plant (Armstrong et al., 1991, 1992). Throughflow convection was first reported in the yellow water lily Nuphar lutea (L.) Sibth.&Sm., where air enters the youngest emergent leaves against a small pressure gradient, is vented through the petioles to the rhizome, and then back to the atmosphere through the petioles and the leaf blades of older leaves (Dacey, 1980, 1981 ). Most floating-leaved species tested since then possess a similar con- vective throughflow route (Dacey, 1987; Grosse and Mevi-Schiitz, 1987; Mevi- Schiitz and Grosse, 1988a,b; Grosse et al., 1991 ). Throughflow convection is, however, not restricted to floating-leaved species. Recent studies have docu- mented that the internal gas transport in several emergent species with cylindrical culms and linear leaves also is enhanced by throughflow convection (Armstrong and Armstrong, 1990a,b, 1991; Armstrong et al., 1991; Hwang and Morris, 1991; Brix et al., 1992 ).
The cattails, Typha latifolia L. and Typha angustifolia L., are perennial emer- gent macrophytes often dominating the upper littoral zone of eutrophic lakes and edging rivers in temperate and subtropical areas (e.g. GrSntved, 1954). Both spe- cies produce up to 3 m high linear leaves and an extensive system of horizontal rhizomes. Cattails are able to grow on organic, highly reduced sediments, as well as on acid sites with high concentrations of reduced metal ions in the interstitial water (Wenerick et al., 1989). This indicates that cattails possess an efficient mechanism for root-aeration. Constable et al. (1992), studying diurnal and sea- sonal variations in CO2 concentration in leaf gas spaces of T. latifolia, were una- ble to detect internal pressures over 10 Pa. However, Sebacher et al. ( 1985 ) de- tected internal static pressure differentials in green leaves of T. latifolia of up to 100 Pa and differences in the internal gas composition between young green leaves and old brown ones, indicating that T. latifolia pressurizes its leaves and venti- lates its root system by convective throughflow. Recent studies have confirmed that the gas transport in several species of cattails is accomplished by convective throughflow (Brix et al., 1992; Chanton et al., 1993; Stengel, 1993). However, little information is available as yet on the mechanisms of pressurization and the gas transport pathways within the plants.
M. Bendix et al. / Aquatic Botany 49 (1994,) 75-89 77
In this paper we document that T. latifolia and T. angustifolia enhance root aeration by internal pressurization and throughflow convection. We present data describing the dominant pressurization mechanism for both species and its de- pendency on ambient temperature, relative humidity and light. In a forthcoming paper, we evaluate the throughflow pathways and discuss the role of the ventilat- ing capacity for the zonation of the two species in the littoral zone (Tornbjerg et al., 1994).
2. Materials and methods
2.1. Plant material
In February 1992 horizontal rhizomes with dormant buds of T. latifolia and T. angustifolia were collected from natural wetlands near Aarhus, Denmark: a water reservoir near Hornslet (56°18'N, 10°25'E) and Lake Ulstrup (56°16'N, 10 ° 38'E). The rhizomes were planted in 101 buckets with coarse sand and prop- agated in a greenhouse supplemented with artificial light. The temperature in the greenhouse was maintained at approximately 20°C and the day length was kept at 16 h. After 2 months, 1.6-1.9 m high leafy shoots had been produced and ad- ventitious roots with fine laterals at their basal parts had developed.
2.2. Experimental
Single shoots with rhizomes were used in the experiments. Rhizomes were cut to a length of 10 cm for T. latifolia and 6 cm for T. angustifolia. They were con- nected to a PVC tube (inner diameter: 1.6 mm ) via pieces of silicone tubing with successively decreasing diameters for measurement of internal pressurization and convective gas flow. Care was taken to avoid water entering the lacunae of the cut rhizome before the PVC tube was fitted. The junction between the PVC tube and the rhizome was made gas-tight with silicone grease and a waterproof flexible sealing compound (Terostat 9). Old leaves were cut off at their base. To ensure that no air could escape and to avoid desiccation of the plant, these cut ends, along with the rhizome with the fitted tubing, were kept submerged in at least 10 cm of water during the experiment. The experiments were performed in a growth chamber (Weiss Umwelttechnik GMBH, Lindenstruth, Germany), in which temperature, relative humidity (r.h.) and light-intensity could be controlled. In order to evaluate the dependency of pressurization and gas flow on temperature, r.h. and light conditions, we exposed the plants to different combinations of tem- perature (15, 20 and 25°C), light (dark or 500 gmol m -2 s -~ PAR) and r.h. (30-100%). The light conditions varied vertically in the growth chamber from 200/zmol m -2 s-~ at the base of the leaves to > 500 gmol m -2 s-~ for the leaves closest to the light source. During each experimental run the plants were exposed to a 12 h dark period followed by a 12 h light period. During each 12 h period the r.h. was programmed to vary between 30 and 100%. The effects of the artificial
78 M. Bendix et al. / Aquatic Botany 49 (1994) 75-89
lights on the ability of the growth chamber to control the r.h. caused the total range in actual r.h. to be less in the light (40-75%) than in the dark (30-95%).
The temperature of the gases in the lacunae of the leaves (Ti) was measured with a needle shaped thermistor (YSI model 524, Yellow Springs Instruments Co., OH, USA). The temperature of the ambient air near the plant (Ta) was measured with a temperature probe (YSI model 401, Yellow Springs Instruments Co., OH, USA). The thermistors were calibrated to give similar readings (_+ 0.05 °C) in the temperature interval 5-35 °C. Relative humidity in the growth chamber was measured using a calibrated humidity probe (Model YA-100, Hy- gromer ®, Sensing element: C-80, Rotronic AG, Bassersdorf, Switzerland), and light intensity was measured with a photometer (Li-COR integrating Quantum/ Radiometer/Photometer, model LI-188; Sensor: Lambda, type: Quantum, Inst. Corp. ).
Static pressure differentials (AP s) between the plant and the ambient air were measured with an electronic pressure transducer (P-sensor, type PU-25, capacity 2.5 kPa, Burster Pr'dzisionsmesstechnik, Gernsbach, Germany), and convective flow rates were measured using a gas flow meter (TopTrack, model 822-1, Sierra Instruments, Carmel Valley, CA; capacity 0-10 cm 3 rain-~ ). The pressure trans- ducer and the gas flow meter were connected to the cut rhizome via a PVC tubing and a three-way magnetic valve. Thus, pressure and flow were measured alternately.
Signals from the pressure transducer, the gas flow meter, the temperature probes, the humidity probe and the three-way magnetic valve were collected and stored via a datalogger (HP3421A data ac0uisition/control unit, Hewlett Packard ). Data for temperature and the three-way magnetic valve (on/off) were collected once every 21 s. Data for static pressure differentials, gas flows and r.h. were collected ten times every 21 s and stored as means of the ten measurements. The valve was set to shift between the pressure transducer and the gas flow meter every 4 rain. After a valve shift data collection was delayed 25 s to ensure that steady state conditions of pressure differential and flow were reached.
2.3. Pressurization mechanisms
Internal pressurization in wetland plants can occur as a result of two purely physical processes, namely thermal transpiration and humidity-induced pressur- ization. The physical basis and theory of the two processes have been described in detail by Dacey ( 1981 ) Brix et al. (1992), and Schrtder et al. (1986). Both processes require a porous partition within the plant tissue, ideally with pore sizes in the Knudsen regime (i.e. < 0.1 #m). Thermal transpiration is the movement of gas from the colder to the warmer side of a porous partition when there is a gradient in temperature across the partition. The gas movement will result in a pressure buildup within the plant tissue when the plant tissue is warmer than the ambient air. The equilibrium pressure gradient induced under ideal conditions by thermal transpiration (JPt) Can be described by the following formula:
M. Bendix et al. / Aquatic Botany 49 (1994) 75-89 79
APt =Pa(T°i'ST; °'5 - 1 ) (1)
where Pa is the ambient pressure and Ti and Ta are the absolute temperatures (K) of the internal and the ambient air.
Humidity-induced diffusion is based on a difference in water vapour pressure between the lacunae of the leaves and the surrounding atmosphere, resulting in a pressure build-up within the lacunae. The humidity-induced pressure differential (3PI) at steady state ideally approximates the difference in water vapour pres- sure between the lacunae of the leaf (P,~) and the atmosphere (Pwa):
APw = P ~ -Pwa (2)
Thermal transpiration and humidity-induced diffusion operate simultane- ously. The potential steady state pressure differential resulting from the two in- dependently operating processes is thus:
APpot = APt + JPw ( 3 )
APt,/IPw and ,4Pvot were calculated from Eqs. 1-3 with Pa set nominally to 101.325 Kpa. The water vapour pressure of the ambient air was calculated from T~ and the r.h. of the ambient air. The water vapour pressure within the lacunae was calculated from Ti, assuming water saturated conditions. The Goff-Gratch for- mulation as published in Smithsonian Meteorological Tables (1963) was used for both calculations.
The pressurization efficiency (Ep; Pa Pa- ~ ), here defined as the actual static pressure differential as a fraction of the calculated potential pressure differential, was calculated from the slope of the regression between APs and APpot:
Ep =,aeszfP?,o[ (4)
The specific convective efficiency (Es; cm 3 rain -1 cm -2 Pa -~ ), here defined as the gas flow rate (F; em 3 min -1 ) per leaf surface area (A; cm 2) and per zfPpot (Pa) was calculated from the slope of the regression between FA - ~ and zlPpot:
Es = FA -IAp~,~ ( 5 )
The areas of the parts of the leaves which were exposed to the air and hence able to pressurize were measured using a rolling band leaf area measurer (LI- COR, Portable area meter, model LI-3000, Lambda Instruments Corporation, USA).
2. 4. Scanning electron microscopy
Sections of leaves of T. latifolia and T. angustifolia were cut with a razor blade and dehydrated in ethanol starting at 70% and increasing in steps of 5% during 3 days to 99% to minimize physical disruption of the tissues by the dehydration process. The sections were then critical point dried, mounted on aluminium hold- ers with conductive carbon cement, coated with platinum and studied in a scan- ning electron microscope (Jeol SM-840, Jeol Ltd., Tokyo, Japan) at 20 kV.
80 M. Bendix et aL / Aquatic Botany 49 (1994) 75-89
3. Results
Turning on the light in the growth chamber during the experiments increased the internal and ambient temperature by 3-5 °C (Fig. 1 a, e). The direct heating from the lamp and the variations in ambient temperatures affected the transpir- ation of the plants. In the dark the temperature difference between the leaf tissue and the ambient air (AT) changed from slightly positive (approximately 0.1 °C) at 15 ° C to negative ( - 0.2 to - 1 o C ) at 20 and 25 ° C owing to the transpirational cooling of the plant tissues. In the light the effect of transpirational cooling of the plant tissue is more pronounced. The leaf temperature of T. latifolia increased (and AT became less negative ) with increasing ambient r.h. during the light pe- riod (Fig. la).
The static internal pressure differentials and convective flow rates varied in- versely with the r.h. of the ambient air at all three ambient temperatures. The highest static pressure differentials (Fig. lc, g) and convective flow rates (Fig. ld, h) occurred in the periods of low external r.h. (Fig. lb, h). Within the varying humidity regime, static internal pressure differentials and convective flow rates increased with increasing temperatures of the ambient air. Changes in tempera- ture and r.h. induced a quick response in pressurization and convection in both species.
A linear relationship between the convective gas flow rate and the static inter- nal pressure differential at all experimental temperatures were observed (Fig. 2). Raising the temperature increased maximum pressurization and convection by the same relative proportion. Light and dark did not influence the relationship. Maximum static pressure differentials were 350 Pa for T. latifolia and 570 Pa for T. angustifolia, and maximum convective gas flow rates were 8 cm 3 min- ~ for T. latifolia and 3.5 cm 3 min-~ for T. angustifolia. The slope of the regression line between the convective gas flow rate and the static pressure differential was three times higher for T. latifolia than for T. angustifolia (0.021 vs. 0.007 cm 3 min -~ Pa- ~ ) owing to the larger leaf surface area of T. latifolia relative to T. angustifolia ( 1398 vs. 330 cm2).
The static pressure differentials (APs) were linearly dependent on the calcu- lated total pressurization potentials (APvo t), in both light and dark and at all am- bient temperatures. The slopes of the regression lines represent the pressurization efficiencies (Ep) of the plants under the given conditions. Ep of T. latifolia seemed to be independent of the ambient temperature, but was higher in the light com- pared with the dark (Table 1 ). For T. angustifolia Ep increased with increasing temperature in both light and dark, but was consistently two-fold lower in the light (Table 1 ).
The plots of measured zips vs. the calculated APt and APw show that humidity- induced diffusion was the dominant pressurization mechanism for both species (Fig. 3; only data at 20°C shown). The observed relationships between ziPs and APw were linear and generally similar to the relations between ziPs and APvo t at all three growth chamber temperatures. Maximum APs increased with temperature because of the higher water vapour pressure in the lacunae. There was no consis-
M. Bendix et aL / Aquatic Botany 49 (1994) 75-89 81
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, , / ' J r , ....... , - -
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Fig. 1. Growth chamber conditions and internal pressurization and gas flow of Typha latifolia (a-d) and Typha angustifolia (e-h) during incubations at three preset growth chamber temperatures ( 15, 20 and 25 ° C). The r.h. of the growth chamber was programmed to vary between 30 and 100% during each 12 h dark and light period. Dark and light periods (approximately 500/~mol m -e s - ~ PAR) are indicated by solid and open bars on the top of each subfigure. (a) and (e): Internal leaf temperature ( Ti ) and ambient air temperature (7",); (b) and (f): Measured relative air humidity (r.h.); (c) and (g): Internal static pressure differential (zfPs); (d) and (h): Convective gas flow rate out of cut rhizomes.
82 M. Bendix et al. /Aquatic Botany 49 (1994) 75-89
Table 1 Pressurization efficiencies (Ep; Pa Pa- 1 ) of Typha latifolia and Typha angustifolia during light and dark incubations at three ambient temperatures
Typha latifolia Typha angustifolia
15°C Light 0.12 0.10 Dark 0.12 0.21 20°C Light 0.20 0.13 Dark 0.12 0.25 25°C Light 0.13 0.14 Dark 0.11 0.28
tent relationship between AP, and AP t in either dark or light for any of the species at all three temperatures. Transpirational cooling at 20 and 25 °C caused AT to be low or negative (Fig. I a, e). So APt never contributed significantly to the total pressurization potential.
The specific convective efficiency (E,) increased slightly with temperature for T. angustifolia in both fight and dark, and was up to three times higher for T. angustifolia than for T. latifolia, especially in the dark (Table 2).
In the search for specific layers in the leaves with pore sizes in the Knudsen regime, the dimensions of stomata and intercellular spaces of the cell layers sep- arating the lacunae and the surrounding air were studied by scanning electron microscopy. The stomatal length is about 13/an for T. latifolia and 10 #m for T.
M. Bendix et aL / Aquatic Botany 49 (1994) 75-89 83
300
200
<3 loo
(a) o oo o
! i ! o = = ==== Vl
! _
i . J , " , , 0 500 1000 1500 2000
APt ,.APw (Pa)
4OO
300
.....,.,.. o
0 ;00 1000 1500 2000
APt , AP. (Pa)
Fig. 3. Static internal pressure differentials (JP,) at 20 °C plotted against the pressurization induced by thermal transpiration (4Pt; circles) and the humidity-induced pressurization (3/',; squares) for (a) Typha latifolia and (b) Typha angustifolia. Solid symbols represent the dark period; open sym- bols the light period. Data obtained during the initial 2 h of light and dark periods are not included.
angustifolia (Fig. 4a). The stomatal width is about 2/an for T. latifolia and 1 Izm for T. angustifolia. Thus, the dimensions of the stomata are not in the Knudsen regime. The cells of the palisade parenehyma are elliptical in form (Fig. 4b, c) and positioned with the long sides of the cells parallel to the direction of outward Poisscuille flow. Primary cell wall thickenings are not conspicuous. The intercel- lular spaces of the three to four layers of pa~sade parenchyma cells are of varying
84 M. Bendix et al. / Aquatic Botany 49 (1994) 75-89
Table 2 Specific convective efficiencies (Es; 10 -6 cm 3 min -~ cm -2 Pa -~ ) of Typha latifolia and Typha an- gustifolia during light and dark incubations at three ambient temperatures
15°C Light 2.44 2.24 Dark 1.57 4.58 20°C Light 3.17 3.18 Dark 1.51 4.71 25°C Light 2.10 3.48 Dark 1.36 5.50
Fig. 4. (a) Closed stomata of Typha angustifolia surrounded by epidermal cells (scale bar 10/zm). The crystalline structures covering the epidermis are due to the waxy nature of the cuticula. (b) Cross section through a young leaf of Typha angustifolia (scale bar 100/tm). E, epidermal cells; PP, palisade parenchyma; VB, vascular bundle; M, mesophyll cells below the palisade parenchyma; L, lacuna. (c) Palisade parenchyma cells of Typha latifolia leaf (scale bar 10/zm ). (d) Cross section of the meso- phyll cells just below the palisade parenchyma of Typha angustifolia (scale bar 10/zm ). Intercellular spaces are recognizable where more than two cells meet (indicated by arrow).
sizes, the smallest being about 1/an (Fig. 4c). The layer of mesophyll cells just below the palisade parenchyma consists of very closely packed round paren- chyma cells (Fig. 4b, d). Primary cell wall thickenings are conspicuous between
M. Bendix et al. / Aquatic Botany 49 (1994) 75-89 85
the cells. The intercellular spaces situated where more than two cells meet have diameters of 0.5 to 2/zm (Fig. 4d ). This layer of spherically mesophyll cells might be the layer with intercellular spaces closest to the Knudsen dimension. Between the layer shown and the lacuna are several similar layers of closely packed isodi- ametric parenchyma ceils but of increasing diameter towards the lacuna. The pal- isade parenchyma and the spongy parenchyma generally consist of more closely packed cells in the young leaves than in the older leaves, indicating that young and old tissue might differ in pressurization potential.
4. Discussion
Contradictory information has been presented in the literature concerning whether humidity-induced diffusion or thermal transpiration contributes most to the pressurization in plants. Dacey ( 1987 ) noted that humidity-induced dif- fusion under most circumstances seems to be the most important component for pressurization in the leaves of the lotus Nelumbo nucifera Gaertn. In contrast Mevi-Schiitz and Grosse (1988b) explained the pressurization in N. nucifera as being mainly influenced by thermal transpiration. Based on experiments with nucleopore membranes and in vitro experiments with Phragmites australis (Cav.) Trin. ex Steud., Armstrong and Armstrong ( 1991 ) concluded that humidity-in- duced diffusion is the most important pressurization mechanism in this species. Similarly, Hwang and Morris ( 1991 ), studying Spartina alterniflora Loisel.; Brix et al. (1992), studying a number of emergent macrophytes; and Sorrell and Boon (1994), studying Eleocharis sphacelata R. Br., concluded that humidity-induced diffusion is the major pressurization mechanism.
Temperature has a great influence on the relative contribution of humidity- induced diffusion and thermal transpiration to the potential static pressure dif- ferential (APpot). Within a natural ambient temperature range, the temperature difference between the plant tissue and the ambient air (AT) must be relatively high, even at high ambient r.h., to make the contribution of thermal transpiration (AP t) significant compared with the contribution of humidity-indueed diffusion (dPw). This is illustrated in Fig. 5, where the ratios between ztPw and APt for different external temperatures and relative humidities are shown for AT= 1 °C and AT= 10°C. The higher the ambient temperature, the larger is the contribu- tion to dPpot of ZIPw relative to APt, at any AT and external r.h.
In this study we document that humidity-induced diffusion is the dominant pressurization mechanism in both T. latifolia and T. angustifolia. In our experi- ments AT varied between - 2 ° C and 2 °C, and thermal transpiration therefore never contributed significantly to the pressurization. One explanation for the low ztT values seems to be transpirational cooling, which keeps the temperature of the plant tissue close to and in some cases below ambient. In the dark dTwas nega- tive because of transpirational cooling. In the light the radiant energy from the lamps increased the temperature of the plant tissue and consequently the rate of
86 M. Bendix et al. / Aquatic Botany 49 (1994) 75-89
Fig. 5. Calculated ratios of humidity-induced pressurization and the pressurization induced by ther- mal transpiration (APw APt - I ) at temperature differences between the leaf and the ambient air (AT) of 1 °C and 10°C and for ambient temperatures of 5, 15 and 25 ° C, as a function of external r.h.
transpiration, which in turn kept AT low. Negative values of AT have also been observed for E. sphacelata (Sorrell and Boon, 1994).
The porous partition responsible for pressurization is most likely located just below the palisade parenchyma of the leaves, as the diameter of the intercellular spaces of these layers are closest to the ideal Knudsen pore dimensions (0.5-2.0 /zm). Recently, Ingenhoff ( 1992 ) investigating anatomical sections made paral- lel to the surface of the leaves of T. latifolia, concluded that the most probable layer with intercellular spaces of Knudsen dimensions is the first layer of round parenchyma cells just below the palisade parenchyma. She found intercellular spaces in the range of 0.23 #In long and 0.09 #In wide to 0.55 #m long and 0.27 /an wide. In young leaves ofNuphar lutea a monolayer of cells, consisting of tightly packed isodiametdc cells situated between the palisade parenchyma and the la- cuna, with intercellular spaces near the Knudsen range (0.8-1.05 #m) has been identified (Schr6der et al., 1986). In older more differentiated leaves the inter- cellular spaces of the monolayer were too large to support pressurization. The existence of a specific layer of cells, having intercellular spaces of Knudsen di- mensions, may not be necessary for humidity-induced pressurization (Arm- strong and Armstrong, 1990a; Hwang and Morris, 1991 ). Larger pores with long tortuous pathways might be as effective in bringing about humidity-induced pres- surization as are shorter pores with diameters approaching the Knudsen range. Humidity-induced diffusion is dependent on the tissue between the lacunae and the atmosphere having a sufficiently high resistance to reduce outward Pois- seuille flow and make a pressurization sufficiently high to sustain a convection. Using nucleopore membranes Armstrong and Armstrong (1990a) found that considerable rates of humidity-induced convection can be sustained with pore
M. Bendix et al. /Aquatic Botany 49 (1994) 75-89 87
diameters of 1/zm and pore lengths of 10/zm. The pathways through the diverse layers of cells between the lacunae and the ambient air with varying sizes of in- tercellular spaces are much longer than 10/zm in the two Typha species. Stomata might also possess sufficiently high resistance to Poisseuille outflow to secure some pressurization providing that the pathways through the stomata are deep and tor- tuous. Several investigators have discussed the location of the temperature and humidity gradients in the plant tissue. If a layer with pores in the Knudsen range is situated under the palisade parenchyma inside the leaf (Schr6der et al., 1986), the humidity and temperature gradients across the porous layer will be less steep than if the stomata are the location for resistance (Armstrong and Armstrong, 1991 ). This will influence the magnitude of the humidity-induced diffusion and thermal transpiration and might be a possible reason for the fact that the effi- ciency of pressurization in the plants tested seldomly is greater than 20%.
Light influenced the pressurization efficiency of T. latifolia, indicating that light- induced stomatal movements affect internal gas transport. Opening of stomata might enhance the humidity-induced diffusion for P. australis, because when sto- mata are closed, projecting waxes cover the openings and reduce the humidity gradient across the pores by introducing additional boundary layers (Armstrong and Armstrong, 1990a, 1991 ). A similar effect has been observed for the sedge E. sphacelata (Brix et al., 1992, unpublished results). In T. angustifolia the pres- surization efficiency was generally two-fold higher in the dark than in the light, indicating that light has a negative effect on pressurization. This may be caused by light-induced opening of the stomata decreasing the resistance to outward Poisseuille flow. Further studies are needed to clarify if the observed effects of light on pressurization and convective gas flow were caused by stomata move- ments or by other light-induced processes.
Acknowledgements
The authors wish to thank Brian Sorrell for reviewing the manuscript and B. Lorenzen and A. Sloth for technical assistance. This study was financed by the Danish Natural Science Research Council, project No. 11-0074-1: 'Ecophysiol- ogy of aquatic macrophytes'.
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