adaptation strategies in wetland plants: links between ecology and physiology. proceedings of a...

13
Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of Humidity-Induced Convection and Thermal Transpiration Author(s): William Armstrong, Jean Armstrong and Peter M. Beckett Source: Folia Geobotanica & Phytotaxonomica, Vol. 31, No. 1, Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop (1996), pp. 25-36 Published by: Springer Stable URL: http://www.jstor.org/stable/4181413 . Accessed: 14/06/2014 07:36 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . Springer is collaborating with JSTOR to digitize, preserve and extend access to Folia Geobotanica &Phytotaxonomica. http://www.jstor.org This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AM All use subject to JSTOR Terms and Conditions

Upload: jean-armstrong-and-peter-m-beckett

Post on 17-Jan-2017

215 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of Humidity-InducedConvection and Thermal TranspirationAuthor(s): William Armstrong, Jean Armstrong and Peter M. BeckettSource: Folia Geobotanica & Phytotaxonomica, Vol. 31, No. 1, Adaptation Strategies in WetlandPlants: Links between Ecology and Physiology. Proceedings of a Workshop (1996), pp. 25-36Published by: SpringerStable URL: http://www.jstor.org/stable/4181413 .

Accessed: 14/06/2014 07:36

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

Springer is collaborating with JSTOR to digitize, preserve and extend access to Folia Geobotanica&Phytotaxonomica.

http://www.jstor.org

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 2: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

Folia Geobot. Phytotax. 31: 25-36, 1996

PRESSURISED AERATION IN WETLAND MACROPHYTES: SOME THEORETICAL ASPECTS OF HUMIDITY-INDUCED CONVECTION AND THERMAL TRANSPIRATION

William Armstrong'), Jean Armstrong') & Peter M. Beckett2)

1) Department of Applied Biology and 2) Department of Applied Mathematics, University of Hull, Hull, HU6 7RX, United Kingdom; tel. +44 1482 465527, fax +44 1482 465458

Keywords: Diffusion, Gas-transport, Methane, Oxygen, Pressure-flow

Abstract: The pressurised gas-flows, humidity-induced convection (HIC) and thermal transpiration (TT), which are important for aeration and for greenhouse gas emissions in some wetland macrophytes, are described and discussed. Results obtained from simple mathematical modelling of the processes are presented to illustrate some of their more relevant features. It is emphasised that both processes require the presence of a micro-porous partition having a significantly greater resistance to pressure flow than to diffusion. In particular it is shown that whilst the potential to pressurise by these processes is inversely related to the pore diameters of the partition, the maximum gas flows are generated where pore diameters range from 0.1 to 0.2 jm. Where partitions are a surface feature (e.g. emergent macrophytes) a dominant role for HIC is predicted; where partitions are an embedded feature (e.g. water-lilies) it is deduced that HIC will still play a significant role, but the contribution of 1T could be greater.

INTRODUCTION

The below-ground aeration of most wetland plants depends upon internal oxygen transport from the shoot system, much of which is by gas-phase diffusion, and always within roots most will be diffusive (ARMSTRONG 1979, ARMSTRONG et al. 1994). However, in wetland macrophytes such as Phragmites australis (CAy.) TRIN. ex STEUD., and the water-lilies, which inhabit the deeper margins of lakes and rivers, diffusion alone may not be sufficient for normal development and various types of pressurised (convective) gas-flow also operate (DACEY 1981, 1987, SCHRODER et al. 1986, MEVI-SCHUTZ& GROSSE 1988, ARMSTRONG & ARMSTRONG 1990a, 1991, ARMSTRONG et al. 1991, 1992, GROSSE et al. 1991, BRIx et al. 1992, SORRELL & BOON 1994, BENDIX et al. 1994, TORNBJERG et al. 1994).

Two types of pressure-flow, humidity-induced convection (HIC) and thermal-transpiration (-osmosis), are initiated by diffusion processes, and depend upon the occurrence of porous partitions or surfaces in which the pores offer a more significant resistance to pressure flow than to diffusion. In water-lilies the cell layer separating the palisade parenchyma from the lower aerenchymatous spongy mesophyll, has been identified as such a partition (SCHRODER et al. 1986); in emergent macrophytes such as Phragmites leaf sheath and stem stomata appear to fulfill the requirements. If a concentration gradient of gases can be produced across such a partition, the resulting diffusion across it can create a real pressure difference between the two sides; it is these pressure differentials which drive the convections. In both the emergent and floating-leaved species these convections are of a throughflow type: in Phragmites air

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 3: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

26 W. Armstrong et al.

enters the leaf sheaths and is then driven to the rhizome system and, modified by respiratory activity and diffusion from the sediments, is vented to the atmosphere via old and broken culms. In water-lilies flows occur from leaf to rhizome and then to the atmosphere through the same (MEVI-SCHUTZ & GROSSE 1988) or another leaf depending upon species. Flows can be in excess of 15-20 cm3 min I, and can considerably enhance the oxygen supply to the rhizome, and so increase diffusion into the roots and rhizosphere (ARMSTRONG & ARMSTRONG 1990a, ARMSTRONG et al. 1992). At the same time they promote the escape to the atmosphere of carbon dioxide and ethene, and increase methane loss from the sediments (DACEY & KLUG 1979, SORRELL & BOON 1994, WHITING & CHANTON 1993).

Thermal-transpiration and humidity-induced convections are as yet not widely appreciated or understood. This paper seeks to redress this by reference to two simple models. Results derived from simple mathematical modelling of the processes are used to illustrate and emphasize some of their more relevant features and help in understanding how the processes operate in plants.

Diffusion versus pressure flow across microporous partitions

Fundamental to the development of pressure differentials in humidity-induced and thermal-transpiration-induced convections is the effectively smaller resistance offered to diffusion than to pressure flow by the micro-porous partitions. This can be readily appreciated by considering the gas-flows which, in dry air, would be induced across Nuclepore membrane partitions under an applied pressure differential, AP, of 100 Pa. Since an applied pressure of 100 Pa would increase the density on the pressured side of the membrane, there will be two components to the flow across the membrane: (a) a diffusive one under the partial pressure differential (or fractional volume concentration difference, AP/Pa), where Pa is atmospheric pressure, and (b) a convective (pressure flow) component under the total pressure differential of 100 Pa. The relative values of the two components will depend upon the relative diffusive and pressure-flow resistances of the partition, which in turn depend upon pore diameter.

The results in Fig. 1 show how pressure flow, and combined diffusive and pressure flow through the partition can be expected to change in relation to pore diameter. It can be seen that pressure flow dominates at pore diameters 2 1 gm, but at ca. 0.5 gm the pressure flows and diffusive flows are approximately equal. Below 0.3 gm, the pressure flows become subordinate to diffusion and at pore diameters << 0.1 ,Im may effectively cease to operate at low pressures because pore diameter becomes less than the mean free-path length of the molecules, and the pressure flow resistance of the partition may be thought of as infinite. Although this qualification has been ignored in constructing Fig. 1, it can be seen that pressure flow at pore diameters < 0.1 ,Im is still relatively insignificant.

Humidity-induced convection

The pressure and flow generation by humidity-induced convection is driven by what has been termed humidity-induced diffusion. The flows can be generated using physical models and the process can be explained by reference to a simple model system (Fig. 2): a cylindrical chamber having a micro-porous partition at one end, at first (Figs. 2a,b,c) temporarily sealed from the atmosphere by a cover. Through the base of the chamber is a venting pipe closed by a tap. It is assumed that the pore diameters in the partition are very small and well within

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 4: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

Pressurised aeration in wetland macrophytes 27

10000 . ............... .................. - .................................................. ........ ............................................................................-...........

_...............................I..........I........................ ..........II .................. 1 ...................I......................I................I

.............................................................I............1 ...........I......................................I......... . .............

- ... . ... I.................................... . .. . ...I.. ... .. .... .. ... . . ..I.. ...

co 100 M C_ C._I,

& _ ' ;-i: - 100:.W.

. -E ., . ,................................ " ... -......

0.1 .. j, / 1 0 0 ........................... ...............

2.......... ............................. :::::............. U ~~~~~~~~~~~~~............................................... ................. ........

0.1 0.1 1

.. ... .. . .I. .. ................................ ... ................ ..... .. .. ... .... .... .. . . .. . .. .. . .

................ ya. -.. ................... ........................ ....................................I............................ . ...... .............................. . ..................... ...................................

Fi.1.Th relat............. effect. o

... ... ....................... ........

.. .... .. ... .... . .. .. .. .. .. .. .. .

plus ressue-flw,acoss e m icroporous mebrne daetr2

mm, 10 jim thick and 10% porous) under a pressure differential of 100 Pa.

the Knudsen diffusion regime (LEUNING 1983), e.g. << 0.1 jm. Such a partition, while allowing the diffusion of gases through it, will effectively offer infinite resistance to pressurised flow (see Fig. 1).

Details of the stages outlined in Fig. 2 are as follows:

(a) If the chamber first contains dry air, is surrounded by an atmosphere of dry air at atmospheric pressure, is at the same temperature as its surroundings, and the tap open (Fig. 2a), the concentrations of oxygen and nitrogen (plus the rare gases) within, Ci, and without, Ca, will be equal, i.e. Ci = Ca and their percentage volumes inside and out will be 100%; similarly the total pressure inside and out will be the same, i.e. Pi = Pa.

(b) If water could now be introduced into the chamber without immediately entering the vapour phase, and the tap instantaneously closed, some water will subsequently enter the vapour phase. If temperature control is such that Ti = Ta, then the concentrations of oxygen and

nitrogen will be unchanged, i.e. Ci = Ca. However, due to the additional presence of the water vapour, the internal pressure will rise so that Pi > Pa (Fig. 2b). Since the system is presently gas-tight, the eventual value of Pi will be Pa + Pw,, where P"v is the saturated water vapour pressure at that temperature, e.g. 2.337 kPa at 20 ?C.

(c) If the tap is now opened briefly to equalise the internal pressures, and then closed again, some gas will be vented and the new conditions will be C1 < Ca, and once more, P1 = Pa (Fig. 2c). The amount of gas vented will approximate to the water vapour volume that had accumulated. At 20 ?C the combined percentage volume of the oxygen and nitrogen and rare gases (Ci) will now be 97.7% and the volume of water vapour 2.3%. In the dry air outside, Ca remains equivalent to 100%.

(d) If the seal is now removed from the porous partition (Fig. 2d), then because Ca > Ci, the external gases will diffuse into the chamber. Similarly water vapour will diffuse out, but will be instantly replaced from the reservoir. The diffusive entry of 02 and N2 will once more cause an increase in Pi, and if partition pore size is such as to prevent a pressurised outflow,

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 5: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

28 W. Armstrong et al.

(a) C

pi ___ = Ca (b)

pi > Pa sealed cover

mmmmmmungmm m (c) C. <

C,

W~~~~~~~~~~p P,m-

(d) c; > but < Ca

pi Pa + PW > bt <P.+ P.

P, - P, =P, C; < C&

.__ a____.(e) > Pa Pi - P = A Pd

dry air - rmkro-porous boundary- - - ---u . l layer "" -"mmmmm

header space

aerreservoir

vent tube

- cowecve flow

Fig. 2. Stages in the development of pressurisations and humidity-induced convective flows in a simple model by the process of humidity-induced diffusion across a microporous membrane. C1 refers to the concentration of atmospheric gases (excluding water vapour) inside the chamber, and Ca the concentration in the outside atmosphere. It is assumed that the atmosphere above the membrane is dry and that isothermal conditions prevail throughout. It should be noted, however, that convections would be produced even if the inside temperature (Ti) was lower than that outside (Ta), and if RH (relative humidity) was 100% inside and out, provided that Ti > Ta. Pwv is the saturated water vapour pressure. For (a) - (e) see the text.

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 6: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

Pressuised aeraton in wetland macrophytes 29

then as Ci approaches Ca, so will Pi approach (but not reach) Pa + Pwv. At equilibrium Pi - Pa is termed the static pressure differential. If the atmosphere inside the chamber was totally saturated with water vapour, Pi - Pa would be equal to Pw,. However, since water vapour can diffuse through the partition, it can never reach complete saturation under the partition and, under isothermal conditions, this value of 2.337 kPa could only be approached and never fully realised.

(e) If the tap is now opened (Fig. 2e), there will be a rapid initial outflow of gas from the chamber and Pi will fall. However, provided that the water vapour in the chamber is maintained by evaporation (involving latent heat input), the concentration of 02 and N2, Ci, will never reach Ca, and gases will continue to diffuse in across the partition to the extent that Pi will remain greater than Pa, and the pressure differential Pi - Pa, termed the dynamic pressure, will continue indefinitely to drive gases through the venting pipe. The rate will be equal to the rate of inward diffusion across the porous partition, and will be a function of (1) the partition thickness, (2) partition porosity, (3) partition pore diameters, (4) the concentration difference across the partition which is in turn a function of the water vapour concentration maintained beneath the partition, and (5) the venting path resistance. If the partition is very thin and highly porous, and provided that a high water vapour concentration can be maintained at the lower surface of the partition, very high rates of flow can be realised.

Predictions of the static pressures, dynamic pressures and flows which can be generated in such a system can be made using a number of relatively simple equations (ARMSTRONG 1992).

Thermal transpiration

For thermal transpiration, the gas concentration differences necessary to induce diffusion across the partition, and ultimately generate the pressure differential for driving the convections, are created by developing a temperature differential across the partition. The equation usually presented to express the relationship between Pi, Pa and the temperature differential Ti - Ta is:

Pi = Pa (TilTa)112

(Eq. 12 in DACEY 1981), but it must be stressed that this will only hold for extremely small pore sizes (see later).

Again the process can be generated and described using a physical model (Fig. 3). In the case of thermal transpiration, there is no water reservoir and the apparatus and atmosphere must be moisture free to avoid any competing or additive effects from humidity-induced diffusion and convection. The stages in the development of pressurisation and flow are as follows:

(a) Again we start with the partition sealed, the tap open and at the ambient temperature, so that Ti = Ta, Ci = Ca and Pi = Pa (Fig. 3a).

(b) If the tap is now closed and the inside temperature raised to a new, and constant value, the pressure in the chamber will also rise because of the entrapment of the warmed gas. However, because the gas is entrapped the concentrations will not change and the situation will be Ti > Ta, Ci = Ca and Pi > Pa (Fig. 3b).

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 7: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

30 W. Armstrong et al.

TI= Ta

(a) q = ca Pi =

P(b) > Ta

inin rnp-i ( P>a sealed cover ;> Ta

C_ = Ca (c) I = Pa

rnrninrnrnr_ nrn- i C< Ca

---~---rn-r

T > Ta

(d) P C bu < P. Ti

> Ta FW>+ bUt < Pa (fi 1 TD (e) q < Ca

--rnrnrrPi - Pa= A Pd dry air > micro-porous

pat - fon

< -dry air

<-- tap

K vent tube

Fig. 3. Thermal transpiration: stages in the development of pressurisation and flows by thermal transpiration across a microporous membrane. It is assumed that dry conditions prevail within the chamber and in the outside atmosphere. For (a) - (e) see the text.

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 8: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

Pressurised aeration in wetland macrophytes 31

~150O ;N AT=10

e1000

AT= 5 ?

50 AT= 2.5 '>

AT= 1 --

. . ... .. .. .., . .. .... I

0.01 0.1 1 10

1000

100

10~~~~~

I I ? W | T w w gI s T E * * * 1s1 F A T 10.0

A T= 5.0 1 0 A T= 2.5

A T= 1.0

0.1 . . , . . , . .. 0.01 0.1 1 10

Membmne pore diameter (im)

Fig. 4. Thermal transpiration: predictions based on an empirical equation by TAKAISHI & SENSUI (1963) to show the relationships between microporous partition pore diameter and static pressure development for a range of temperature differentials across the partition, where AT= Ti - Ta.

(c) If the tap is now opened momentarily and then closed again (Fig. 3c), the warned gas will expand while it is open and some will be forced out so that once more the inside and outside pressures become equal. However, this loss of gas will result in a lowering of the internal gas concentrations. If sufficient heat is supplied to maintain the constancy of the inside temperature the new conditions will be: Ti > Ta, Ci < Ca and Pi = Pa.

(d) If the partition seal is now removed (Fig. 3d), the atmospheric gases, being at a higher concentration than in the chamber, will diffuse in. Since the partition offers less resistance to inward diffusion than to any pressurised backflow, then, if the partition pore diameters are << 0.1 jIm, a net inflow would occur until the collisions on the outside of the partition (where the gas density is greatest) balance those on the inside where molecular velocities are greatest. This balance would be achieved when Pi

1/2 =Pa( Ti/Ta) . In the case of

thermal transpiration, therefore, the concentrations do not equalise at the static pressure; only a halving of the original concentration deficit is achieved.

(e) If the tap on the venting tube is now opened (Fig. 3e), there will be a rapid initial outflow of gas from the chamber and Pi will fall. However, since this will again reduce the concentration Ci, the atmospheric gases will once more diffuse in rapidly across the partition to such an extent that, as with HIC, Pi will remain greater than Pa. Provided that the same temperature differential is maintained, the dynamic pressure differential Pi - Pa, (APd), will

continue indefinitely to drive gases through the venting pipe.

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 9: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

32 W. Armstrong et al.

There is little information even in the non-biological literature on the pressurisation or gas flow by thermal transpiration in model systems. However, the relationship between effective static pressure differential, pore size and temperature differential in thermal transpiration can be predicted using an empirical equation derived by TAKAISHI & SENSUI (1963)(see Fig. 4).

To predict the dynamic pressures and convective flows generated by thermal transpiration we can use similar equations to those derived for HIC (ARMSTRONG 1992), with the potential static pressures found from the TAKAISHI & SENSUI equation, or read from plots such as those in Fig. 4.

RESULTS AND DISCUSSION

Mathematical modelling of the systems shown in Figs. 2 and 3 provide the means for studying some aspects of the convective flow mechanisms which would be very difficult, if not impossible to accomplish, using plants.

HIC, pore size, and pressurisation

An important feature of humidity-induced convection is its ability to operate under isothermal conditions. However, the static pressures which may be realised in plants in the field vary considerably and often fall far short of the ideal value implied in the often cited

equation: Pi = Pa + Pwv. Some of

2500 14jTP! ....... .... l , ! ,flj ', ,' .- .. .,._ . ,..........., .

R , , ,

_,.i.N.,.s.,,. j.,_,., ... ..... ....i.. _._.. , .- .... ,, ,.... ., .'''.'j.-

X ..' . , j _ j'-' ';...... i, ? .-,._'_ _ *' '-'w. ' '----j....................'.*'.'.-....i.

co - - ..... i-rrr. j.... rXlj 0L

^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. .......... ... ..... gBjX-. --j. .. t... ... .....ii. ii

i r.-iiiii 'ii'3.ii ............ .... ... + i;ii.. .. iri- 3i

O 1 _ .......... . . 1..50.. . .... n

-::::I' -ii---\-fl........................ ..........

8

i i i~~~~~~~~~~~~~~ l.^. ;.~..iii.i.ii-.. \ iiii.i i ... i .

-i.ir ... i ..iiii...ii C

1 j. i i ssijii ;\ijg ...... ...... ......j _

..........:3:.....::::::3:i: i.>iti :::::,:i: ............3:::I::i:-... ...........: - :i;::::;:::::;3ii CL i -r;4i|-ii 4 ::::4 -i:::i:- i:::::: ,............... .... .............. _

loo j :::.::': ,,,::::::: ................:: : ir.,:::i: .............. . . .:::::ii ....... ......

..,,,4a+;,..,,, .....2!.;.4 . .....-+-.. A. ,, ;; , tt 3

502 , ' ''-.,jjj,,-,,...'20 ................. ..... ............. -;'*

.........................,, ,,.,,.

10 0 1 Os 1- o1 1? 1

1 IM ........ -5 . -4 4 - 3 2 1 4

Membrane pore diameter (jim)

Fig. 5. Humidity-induced diffusion: predicted relationship between static pressure differentials and membrane pore diameters where the evaporating water surface is (i) 4 mm and, (ii) 10 jm, below the microporous membrane. It has been assumed that there was no boundary layer, that the air above the membrane had an RH of zero, that the porosity of the membrane is 10%. Knudsen diffusion coefficients have been applied at pore diameters < 0.12 jim.

the likely reasons for this can be appreciated from the data presented in Fig. 5. Static pressure differentials are shown to vary considerably with pore size and with the distance between the micro-porous partition and the evaporating surface beneath. To achieve a static pressure differential of 2300 Pa with the water surface at 4 mm is unattainable since, as the results show, this would require pore sizes even smaller than the dimensions of the gas molecules themselves. Clearly then, a high evaporating potential within the chamber (or plant), as well as small pore size, are important for humidity-induced pressurisation. In Phragmites, the stomatal aperture of the leaf sheath stomata appears to lie in the range 0.15-0.2 ,um, the stomatal pore length is about 10-15 ,um and the

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 10: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

Pressurised aeration in wetland macrophytes 33

sub-stomatal cavity is about 10 ,um deep. If we make an approximate extrapolation from the data shown in Fig. 5 we might expect to obtain static pressure differentials up to 1600 Pa in a completely dry atmosphere. Pressures measured in the field are frequently of the order of 300-400 Pa, but can rise to as high as 1100 Pa in warm, dry (30% relative humidity) conditions. The pressures of 300-400 Pa are more characteristic of humidities of ca. 50% in overcast conditions. The 1600 Pa above would be halved if the external humidity were 50% but a boundary layer of 700 m would reduce the figure still further: with partition porosities of 1% (akin to Phragmites) the figure approximates to ca. 500 Pa (data not shown). This is very close to the observed values and it is quite conceivable that a somewhat greater leakiness of the slit-like shape of the pores in Phragmites would be sufficient to account for any difference.

HIC, thermal transpiration, pore size, partition porosity and convective flow

Pore diameter and partition porosities can be shown to have important consequences also for the convective flows themselves: some of the effects, embracing both humidity-induced convection as well as thermal transpiration, can be seen in Fig. 6. The results are again predictions based on membrane partitions of 20 mm diameter, and thickness 10 ,um and for HIC assume dry air above the membrane, a free water surface at 10 jm below, a AT of zero and a membrane porosity of either 1% or 10%. For thermal transpiration the same two

HICe =0.1 1000

E HIC e=0.01 C C 100

0

* 10

0

0.01 0.10 1.00

Pore diameter (pm)

Fig. 6. Humidity-induced convection (HIC) and thermal transpiration (Th-T): effects of pore diameters and partition fractional porosities (E) on the convective flows. The predictions are based on model s2ystems in which the microporous membrane had an area of 314 mm and a thickness of 10 gm. For HIC it was assumed that there was no boundary layer, that the atmosphere above the membrane had an RH of zero, and that there was a free-water surface 10 gm below the membrane.

porosities have been used and a AT of 1 ?C; isothermal conditions would fail to create any thermal transpiration. What is particularly interesting about the results is that whereas static pressures continue to increase with decreasing pore size (Figs. 4 and 5), flow rates are seen to peak at pore diameters of around 0.1-0.2 gm. This has been confirmed experimentally for HIC (ARMSTRONG & ARMSTRONG 1994) but so far as we are aware this is the first time that a relationship between pore size and flow has been presented for thermal transpiration. Other features of interest in Fig. 6 are that (a) porosity can have a very marked influence on the flows, (b) HIC falls less sharply at pore diameters > 0.2 gm than does thermal transpiration.

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 11: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

34 W. Armstrong et al.

APd A(RVP)

(Pa) (Pa s m)

15 . ..... 600-

1a0 400/ 8+11

1? 30 / -011

1i5~~~~~~~~~~~0 tl\+/11

0

0 0 OeP000 0 20 40 60 60 100

Venting path resistance (1010 Pa s in4)

Fig. 7. Showing the predicted effects of venting path resistance on dynamic pressure development and convective gas-flows in a model system developing 500 Pa potential static pressure by humidity-induced diffusion. Flow (0); dynamic pressure differential (0); venting path resistance (V). Similar effects could be expected in the case of thermal transpiration-induced flows.

Effects of venting path resistance on flows

Whether convection is by HIC or by thermal transpiration, any increase in venting path resistance should lead to a raising of the dynamic pressure. Such an effect is illustrated in Fig. 7 where it has been assumed that a model system with a membrane diameter 20 mm, pore diameters 0.046 Rtm, porosity 1% and thickness 10 R1m, is capable of delivering a potential static pressure differential of 500 Pa by humidity-induced diffusion. The figure shows how the convective flows and dynamic pressures in such a system would be expected to change with increasing venting path resistance. It can be seen that increased venting path resistance can have serious consequences for flow; at the same time, the dynamic pressure is raised, and at very high resistances is approaching the static pressure differential of 500 Pa. As might be expected, plants which exhibit the highest rates of convection have minimal resistance to venting into and through their rhizomes (ARMSTRONG et al. 1988, BRIX et al. 1992). If the number of inflow points (e.g. the living shoots of Phragmites) much exceeds the number of outflow points, the potentially realisable flows can be significantly reduced and this condition can be detected by the occurrence of relatively high dynamic pressures (ARMSTRONG & ARMSTRONG l990b). There is mounting evidence that die-back of Phragmites is associated with increased resistance to flows due to blocking of the internal airways by callus (ARMSTRONG & ARMSTRONG 1995, KOHL et al. 1996).

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 12: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

Pressurised aeration in wetland macrophytes 35

FINAL COMMENTS

In conclusion, we offer the following comments on the relative properties of humidity-induced pressurisation and flow and those of thermal transpiration, since it is possible for both to operate simultaneously within plants.

In the case of HIC, since water vapour will always to some extent occupy any header space (e.g. Fig. 2d and e), humidity-induced convection should operate even where Ti < Ta (ARMSTRONG et al. 1991, BRIx et al. 1992, ARMSTRONG & ARMSTRONG 1994, SORRELL & BOON 1994, BENDIX et al. 1994). Also, since water vapour preferentially displaces more of the other atmospheric gases the higher the temperature, i.e. lowering Ci, it follows that HIC should be enhanced where Ti > Ta. Even if the relative humidities both within and outside a plant were at 100%, if Ti > Ta then Ca will be > Ci and humidity-induced diffusion, pressurisation and flows will still occur. In contrast, thermal transpiration can only operate positively when Ti > Ta, and if Ti < Ta the direction of flows is reversed. Hence, the situation can exist when thermal transpiration and HIC are opposing forces. Fortunately, in the case of emergent species where the pores (stomata) are superficial the much greater potential for pressurisation by HIC (cf. Figs. 4, 5 and 6) should prevail.

In the floating leaved species there is some question as to which process might be the more important. If the porous partition between the palisade layer and aerenchymatous spongy mesophyll (SCHRODER et al. 1986) is the relevant microporous layer, the problem to be resolved is how a significant water vapour gradient might be created across it. We suggest it as conceivable, therefore, that latent heat loss will cool the atmosphere in the palisade layer to a greater extent than in the aerenchymatous mesophyll. This should have two consequences: (a) it should help to create the temperature differential necessary for thermal transpiration, and (b) the temperature gradient will result also in a water vapour concentration gradient. It is difficult at this stage to predict what the exact outcome of this will be. What seems certain, however, is that humidity-induced convection will be a significant part of any gas-flows in floating leaved macrophytes, although thermal transpiration could play the greater role.

REFERENCES

ARMSTRONG J. (1992): Pathways and mechanisms of aeration in Phragmites australis. Ph.D. thesis, University of Hull, Hull.

ARMSTRONG J. & ARMSTRONG W. (1990a): Light-enhanced convective through-flow increases oxygenation in rhizomes and rhizosphere of Phragmites australis (CAV.) TRIN. ex STEUD. New Phytol. 114: 121-128.

ARMSTRONG J. & ARMSTRONG W. (1990b): Pathways and mechanisms of oxygen transport in Phragmites australis. In: COOPER P. & FINDLATER B.C. (eds.), The use of constructed wetlands in water pollution control, Pergamon Press, Oxford, pp. 529-533.

ARMSTRONG J. & ARMSTRONG W. (1991): A convective gas through-flow in Phragmites australis. Aquatic Bot. 39: 75-88.

ARMSTRONG J. & ARMSTRONG W. (1994): A physical model involving Nuclepore membranes to investigate the mechanism of humidity-induced convection in Phragmites australis. Proc. Roy. Soc. Edinburgh, Ser B, 102: 529-540.

ARMSTRONG J. & ARMSTRONG W. (1995): Phytotoxins, callus development and impeded gas-flow: critical links in Phragmites die-back. In: VAN DER PUITEN W.H. (ed.), Reed News 4, Final Reports of EU Project EUREED - EV5V-CT920083, Netherlands Institute of Ecology, Heteren, pp. 76-114.

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions

Page 13: Adaptation Strategies in Wetland Plants: Links between Ecology and Physiology. Proceedings of a Workshop || Pressurised Aeration in Wetland Macrophytes: Some Theoretical Aspects of

36 W. Armstrong et al.

ARMSTRONG J., ARMSTRONG W. & BECKET17 P.M. (1988): Phragmites australis - A critical appraisal of the ventilating pressure concept and an analysis of resistance to pressurised gas-flow and gaseous diffusion in horizontal rhizomes. New Phytol. 110: 383-390.

ARMSTRONG J., ARMSTRONG W. & BECKET'rP.M. (1 992): Phragmites australis. Venturi- and humidity-induced convections enhance rhizome aeration and rhizosphere oxidation. New Phytol. 120: 197-207.

ARMSTRONG W. (I1979): Aeration in higher plants (review). Advances Bot. Res. 7: 225-332. ARMSTRONG W., ARMSTRONG J., BECKETT P.M. & JUSTIN S.H.F.W. (1991): Convective gas-flows in wetland

plant aeration. In: JACKSON M.B., DAVIES D.D. & LAMBERS H. (eds.), Plant life under oxygen stress, SPB Academic Publishing bv, The Hague, pp. 283-302.

ARMSTRONG W., BRANDLE R. & JACKSON M.B. (1994): Mechanisms of flood tolerance in plants. Acta Bot. Neerl. 43: 1-52.

BENDIX M., TORNBJERG T. & BRIX H. (1 994): Internal gas transport in Typha latifolia L. and Typha angustifolia L. l. Humidity-induced pressurization and convective throughflow. Aquatic Bot. 49: 75-90.

BRIx H., SORRELL B.K. & ORR P.T. (1992): Internal pressurization and convective gas flow in some emergent freshwater macrophytes. Limnol. & Oceanogr. 37: 1420-1433.

DACEY J.W.A. (1981): Pressurized ventilation in the yellow water lily. Ecology 62: 1137-1147. DACEY J.W.A. (I1987. Knudsen-transitional flow and gas pressurization in leaves of Nelumbo. PI. Physiol. 85:

199-203. DACEY J.W.H. & KLUG M.J. (1979): Methane efflux from lake sediments through water lilies. Science 203:

1253- 1255. GROSSE W., BUCHEL H.B. & TIEBEL H. (1991): Pressurized ventilation in wetland plants. Aquatic Bot. 39:

89-98. KOHL J.G.. HENZE R. & KUHL H. (1996): Evaluation of the ventilation efficiency of the rhizomes of natural

reed beds by convective through-flow of gases in Phragmites australis (CAV.) TRIN. ex STEUDEL. Aquatic Bot. (in press).

LEUNING R. (1983): Transport of gases into leaves. Pl. Cell Environm. 6: 181-194. MEVI-SCHUTZJ. & GROSSE W. (1988): A two-way gas transport system in Nelumbo nucifera. P1. Cell Environm.

11: 27-34. SCHRODER P., GROSSE W. & WOERMANN D. (1986): Localisation of thermo-osmotically active partitions in

young leaves of Nuphar lutea. J. Exp. Bot. 37: 1450-1461. SORRELL B.K. & BOON P.I. (1994): Convective gas-flow in Eleocharis sphacelata R.BR.: methane transport

and release from wetlands. Aquatic Bot. 47: 197-212. TAKAISHI T. & SENSUI Y. (1963): Thermal transpiration effect of hydrogen, rare gases and methane. Trans.

Faraday Soc. 59: 2503-25 14. TORNBJERG T., BENDIX M. & BRIX H. (1994): Internal gas transport in Typha latifolia L. and Typha angustifolia

L. II. Convective throughflows and ecological significance. Aquatic Bot. 49: 91-106. WHITING G.J. & CHANTON J.P. (1993): Primary production control of methane emission from wetlands. Nature

364: 794-795.

This content downloaded from 185.44.77.128 on Sat, 14 Jun 2014 07:36:49 AMAll use subject to JSTOR Terms and Conditions