CO2 and O2 transport in the aerenchyma of Cyperus papyrus L.

~i',! ' " :. ! •i ¸i ~.~ •~.~ ;i~i:

uatic Aqbot.a ny

ELSEVIER

Aquatic Botany 52 (1995) 93-106

CO 2

and 0

transport in the aerenchyma of Cyperus papyrus L.

2

Meirong Li 1, Michael B. Jones* Department of Botany, Trinity College, Universityof Dublin, Dublin 2, Ireland Accepted 13 April 1995

Abstract Cyperus papyrus L. (papyrus) is an emergent wetland species with C4 photosynthesis. Culms of papyrus possess numerous large intercellular air cavities and functional 'Kranz' chlorenchyma which are involved in CO2 recycling in the culm. In darkness, the CO2 concentration in the culms increased to 74 times that of the ambient air. In the light, the culms greatly reduce the intercellular CO2 concentrations by internal CO2 recycling via photosynthesis. Results suggest that 35-57% of the CO2 respired by the culm pith and rhizomes may be refixed by culm photosynthesis. The dynamics of 02 transport in the intercellular spaces of the culms and the rhizomes were also studied. Both illumination and prolonged darkness had significant effects on the 02 concentrations in the culm and rhizomes. While the water surrounding the rhizomes remained strongly hypoxic, the 02 concentration in the submerged rhizomes was 15.1% during the day and 10.3% at night. The diffusive fluxes of CO2 and 02 within the papyrus plant during the day and night were calculated. Results suggest that rapid CO2 exchange occurs between the ambient air, internal atmosphere and the culm photosynthetic tissue. Also, there is a high 02 flux, particularly at night, which is generated in the intercellular air spaces between the culm and the rhizome. Keywords: C4 photosynthesis; Carbon dioxide; Oxygen; Gas transport; Cyperuspapyrus

1. Introduction Emergent wetland plants contain extensive aerenchyma which allows long distance gasphase transport (Armstrong, 1979, 1989). In these plants, CO2 produced in respiration by submerged roots and rhizomes moves upwards to the aerial shoots, resulting in either an accumulation of CO2 in the dark or an enhancement of CO2 supply for photosynthesis in 1 Present address: Department of Botany, University of Toronto, Toronto M5S 3B2, Canada. * Corresponding author: e-mail [email protected]; Fax. + 353-1-6081147. 0304-3770/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDIO304-3770(95)OO484-X

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M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

daytime (Brix, 1990a; Constable et al., 1992). Also, it has long been recognized that besides the apical gas-phase CO2 transport, a requirement exists for effective downward gas-phase oxygen transport from the aerial shoots to the root system (Conway, 1940; Van Raalte, 1940; Armstrong, 1979; Raskin and Kende, 1985; Dacey, 1987; Brix, 1988, 1989). As a consequence, CO2 and 02 must move in opposite directions through the intercellular space system in the plant (Brix, 1988). When gas flow resistances in Nelumbo leaves (Dacey, 1987) and Phragmites culms (Armstrong et al., 1988) were measured they were found to be sufficiently small to allow for substantial long-distance CO2 or 02 transport by convection or diffusion. The refixation of respiratory CO2 from the roots and rhizomes by the above-ground photosynthetic tissue has been identified as a significant component of the carbon balance of several wetland species (Wetzel and Grace, 1983). In the most extreme cases, some plants which are without stomata obtain all of their CO2 from the root environment (Keeley et al., 1984). In another example, Vapaavuori and Pelkonen (1985) found that inorganic carbon uptake through the roots o f Salix may account for approximately 30% of total biomass production. Recently, this work has been extended and Vuorinen et al. ( 1989, 1992) have reported on the effect of light exposure of the shoots on inorganic carbon uptake through the root system of Salix. They found that although the fixation of inorganic carbon in Salix roots occurred in both light and darkness, the incorporation of 14C after 24 h darkness is about half of that incorporated in the light, indicating a light enhancement of carbon uptake into the shoots. Field experiments with rice and water hyacinth under nutrient-rich conditions have also demonstrated substantial increases in plant growth due to CO2 enrichment in the root environment (Wetzel and Grace, 1983). None of these examples investigated to date are of C4 plants which have the Ca pathway of photosynthesis. In C4 plants the initial reaction in the CO2 fixation pathway is with the 3-carbon compound phosphoenolpyruvate (PEP) which is mediated by the enzyme PEP carboxylase. PEP carboxylase has a higher affinity for CO2 than ribulose bisphosphate carboxylase (Rubisco) which is responsible for the initial CO2 fixation step in the (socalled) C3 plants. The photosynthetic attributes of Cyperuspapyrus. (papyrus) have been investigated by Jones and Milburn (1978) and Jones (1988) and this plant has been shown to have C4 characteristics. It is also a plant with very high levels of productivity, which have been shown to be in excess of 50 t ha- 1 year- l (Muthuri et al., 1989). One aim of our work was to quantify the carbon balance of papyrus-dominated wetlands and an important component of this is the recycling of respiration derived CO2 in the plant. The objective of the work reported here was to determine the amount of CO2 refixation and 02 movement within papyrus rhizomes and culms. Papyrus is a particularly convenient plant to use for this type of experimental work because it has large culms, up to 3 m long, from which it is relatively easy to sample intercellular gases. The plant has large intercellular air spaces in its culms and there is a large proportion of biomass underground in the form of rhizomes and roots (Jones and Muthuri, 1985; Jones, 1988; Li, 1992). Measurement of CO2 and 02 concentrations in the culm of C. papyrus was used to test the hypotheses that (1) internal CO2 accumulation from respiration and refixation in photosynthesis is significant and lightdependent and (2) aerobic conditions in the roots and rhizomes of C. papyrus are maintained by 02 transport from the atmosphere through the aerenchyma of the culms.

M. Li, M.B. Jones~AquaticBotany52 (1995) 93-106

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2. Materials and methods

2.1. Plant material and growth conditions Plants of Cyperuspapyrus were grown from seed collected in Kenya and maintained in the Trinity College Botanic Gardens, Dublin. Plant material for this experiment was grown in a water tank (0.7 m 2 water surface, 70 cm deep) placed in a growth room for 7 months before the start of measurements. The average length of the culms at the start of measurements was 100 cm. The level of water in the tank was maintained approximately 20-25 cm above the substrate in this artificial 'swamp'. The substrate in which the rhizomes were located consisted of sandy soil and plant detritus, about 40 cm deep. Lighting conditions in the growth room ranged from 200 to 1000/zmol m - 2 s - ~photosynthetic photon flux density (PPFD) from water surface to the top of the canopy, respectively. The temperature in the room was regulated by a thermostatically controlled air-conditioner through which fresh air entered and the exhaust was removed by an outlet fan.

2.2. Anatomy of the gaseous pathways Light microscopy was used for examining transverse freehand sections of culms and rhizomes of C. papyrus. Sections, 30-50/zm thick, were cut using razor blades and stained with iodine in KI to determine the presence and distribution of starch in chlorenchymatous cells of the culms. For more detailed structural observations, leaf tissue approximately 1 mm thick was fixed in 3% glutaraldehyde, post-fixed in buffered 2% OsO4 and finally embedded in Emix resin (medium hardness). Sections, 1.0-2.0/zm thick, were cut using an LKB ultratome (Type 8801A/8802A, LKB Producter AB, Stockholm, Sweden) with glass knives and then stained with toluidine blue, and examined using a light microscope (Leitz Laborlux K, Ernst Leitz Wetzlar GMBH, Germany). Scanning electron microscopy (SEM) was employed for examining in more detail the arrangement of intercellular air spaces. For this, fresh material was cut from the plant using razor blades and immediately dropped into liquid nitrogen for cryo-fixation. Fixed material was then transferred into a vacuum (5 kPa) and maintained overnight before coating with gold. Coated specimens were examined at 20 kV in a scanning electron microscope (Hitachi S-520, Hitachi Co., Japan).

2.3. Dynamic C02 exchange measurements In order to make continuous measurements of CO2 production in the intercellular air system of intact culms, a continuous air flow was passed directly through the intercellular air channels. The air entered through a syringe needle 20 cm above the rhizome and exited through another needle 10 cm below the umbel. It therefore passed through about 70 cm of culm. Flow rates were between 0.2 and 25 cm 3 s - I. Photosynthetic rates of intact culms were determined by placing a chamber, consisting of 25 cm of perspex tubing 2.5 cm in diameter, around the culm prior to umbel unfolding and sealing the ends around the culm with blue-tac and Vaseline. Air of known CO2 concentration was passed into the chamber through a port near the base and exited through

96

M. Li, M.B. Jones~Aquatic Botany 52 (1995) 93-106

a port near the top. CO 2 exchange was measured with an infrared CO2 analyser (IRGA, Type 225 MK-3, Analytical Development Co. Ltd., Hoddesdon, UK) which was calibrated using a standard gas mixture containing 300/xmol mol- 1 CO2 balanced by nitrogen with a tolerance of _ 5% (CryoService Ltd., Worcester, UK). Ambient CO2 concentration was measured with a portable IRGA (Type LCA-2, Analytical Development Co. Ltd.). 2.4. Intercellular C02 and 02 concentrations

Intercellular gas samples were also collected using gas-tight syringes (1 ml with a resolution of 0.03 ml) (Brix, 1988). After sampling, the syringe needle was immediately sealed with either liquid paraffin or by embedding in a rubber stopper until measurement. The systems used for the measurement of CO2 and 02 concentrations of gases sampled from the intercellular air-spaces were based on methods described by Atkins and Pate (1977), Clegg et al. (1978) and Walker (1988). This method is based on a linear relationship between CO2/O2 concentration and analyser/recorder response, when gas samples are injected into a flowing carrier gas (N2) that passes though an IRGA or over a Clark-type O2-electrode (Model LD-2, Hansatech Ltd., Kings Lynn, UK). CO2 concentrations up to 3% ( v / v ) and 02 concentrations up to 21% ( v / v ) can be measured by these methods. The IRGA was calibrated using a range of known CO2 concentrations between zero and 1000 /.tmol mol- 1, generated using a CO2 gas diluter (Analytical Development Co. Ltd.). For CO2 concentrations higher than 1000/zmol mol-l, measurements were carried out using a method described by Atkins and Pate (1977) and Clegg et al. (1978). For measurement of 02 concentrations in water, samples were collected at different water depths with a 2 ml pipette and a Clark-type O2-electrode (Model LD-2) was used for measuring 02 concentration (Walker, 1988, 1989; Armstrong and Armstrong, 1991). The 02 electrode was calibrated against diluted atmospheric 02 concentration (21%, v/v) according to Walker (1988). 2.5. Calculations

In the present study, the convection of 0 2 and CO2 in the intercellular air channels is ignored as measurements were assumed to be made under isothermal conditions. As the intercellular air spaces in the culm of C. papyrus are interconnected, it is considered to be a single tube surrounded by chlorenchyma and epidermis. The porous nature of the culms and the rhizomes of C. papyrus can be similarly simplified because the diameters of the pores are large enough (over l 0 / z m ) to allow free gaseous diffusion along the gradient (Armstrong, 1979; Armstrong et al., 1988). The fluxes of a gas (Fi, tool m - 2 S - 1) through this system were calculated using the expression Fi=DiACJL

(1)

where Di (m 2 s -1) is the diffusion coefficient of gas i (i.e. CO2 or 02) at 20°C, Dco2 = 1.51 × 10 -5 m 2 S- l and Do2 = 1.95 × 10 -5 m 2 s - l (Nobel, 1983). Cil and Ci2 are the concentrations at X~ and X2, A C~ is the concentration difference between Cgl and Ciz, and L is the path distance (XI-X2) along which the gas diffuses. Therefore, the rate of diffusion of a gas is directly proportional to the difference in concentration of the gas along

M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

97

the path, and is inversely proportional to the length of the path. The rate of the diffusion process, therefore, is assumed to be dependent on the dimensions and structure of the culm, and the gas concentration gradients along the culm. The volume fractions of CO2 or 02 measured during experiments using the IRGA or 02 electrodes were converted to molar concentrations using M---(c/22.4)(273/T)(P/ 101.3) X 10-3, where M is molar concentration (mol m-3), c is volume fraction (ppmv), T is the absolute temperature (K) and P is the pressure (kPa) of the gas.

2.6. Data analysis Experimental data were logged into a graphics package, Cricket Graphics v. 1.3.1. (Cricket Software for Apple Macintosh, USA) and linear regressions were calculated using the same software. Significantly different means were separated by analysis of variance (ANOVA) using the statistical packages, Microsoft Exel v. 4.0 (Microsoft Corp.) and Data Desk v. 4.0 (Data Description, Inc., Ithaca, NY, USA).

3. Results and discussion

3.1. Anatomy of the gas pathways In the pith matrix of papyrus culms each lacuna is surrounded by a single layer of nonphotosynthetic parenchymatous cells (Fig. la). Within the central region of the aerenchyma are the randomly arranged vascular bundles and this region is surrounded by chlorenchymatous photosynthetic cells. There are obvious inter-cavity connections between the lacunae (Fig. ld). The diameters of the lacunae vary (Figs. la and lb) but for any individual its diameter does not significantly change from the base to the top (Fig. 1b). These air cavities are continuous along the culm without tortuosity (Fig. lb), but they close at the junction with the rhizome (Figs. 2a and 2b), and umbel (Fig. 2c). In these transitional junctions (average thickness 15 mm) there are parenchymatous cells with starch grains and between them are a limited number of intercellular air spaces (Fig. 2d). It is likely that, at these junctions, an interface with a relatively high resistance to gas transport exists and it may function as a significant barrier to gaseous diffusion between the rhizome, culms and umbels. A feature ofC4 plants is a ring of chloroplast-containing primary carbon assimilation (PCA) cells in the mesophyll surrounding the vascular bundles which contain photosynthetic carbon reduction (PCR) cells (Li and Jones, 1994; Dengler et al., 1994). In papyrus culms these C4 'Kranz' anatomical units are located in the peripheral chlorenchyma (Fig. 3a ). The interveinal distances (IVD) are similar to those of C. papyrus leaves and bracteoles with the exception of the basal parts (Li and Jones, 1994). The thickness of the peripheral chlorenchyma (240 +__20/xm) varies little along the culm and there are no direct air channels across it (Fig. 3a). Papyrus culms have a high rate of metabolic activity, similar to the scale leaves and bracteoles, with CO2 respired at 1.83-3.65 p.mol g-~ dry weight s-~ (M. Li, unpublished data, 1992). The stomatal complex on the culm epidermis consists of two guard cells and two subsidiary cells (Fig. 3b). No obvious anti-stomatal cavity is formed by the cuticle extension, but

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M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

Fig. 1. Aerenchymatouscharacteristicsofa Cyperuspapyrusculm. (a) Transverseappearanceof fractured whole calm showing a structure with a large proportion of intercellular air channels. Note that vascular bundles are predominant in the peripheral areas, whereas air cavities are predominant in the pith (magnification× 12). (b) Longitudinalfractureof culm showingcontinuousair cavities withouttortuosity (magnification× 36). (c) Transverse fracture of the culm showing large air channels surrounding a vascularbundle (magnification× 100). ( d ) Detailed portion of an intercellular air channel in longitudinal section showing a single layer of cells and intercavity air spaces (magnification× 360). cavities within the stomatal pores and substomatal cavities are formed (Fig. 3b). The stomata of the culms are functional, as indicated by the observation of open stomata (Fig. 3c), and the fact that opened and closed stomata were observed when nail varnish was applied to the surface to make epidermal impressions (M. Li, unpublished data, 1992). The stomata are distributed throughout the epidermis of the culm, except where the culm is covered by scale leaves. In fully developed culms, stomatal frequency increases with the distance from the culm base and reaches the highest value at about two-thirds of the distance from the base to the top (Li, 1992). The scale leaves covering the base of the culm also contain porous tissue but their intercellular air spaces are not continuous with the culm as they are separated by the culm epidermis (Fig. 3d). 3.2. C02 recycling in culms In the light, the internal CO2 concentration in the culms is reduced markedly (Figs. 4 and 5a). Measurements using attached culms showed that they had a net CO2 assimilation from the atmosphere of 16.7 + 4.3/zmol m - 2 s - i at a PPFD of 200/xmol m - 2 s - ~ (M. Li,

M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

99

Fig. 2. The characteristics of air spaces in different regions of Cyperus papyrus. (a) Three-dimensionalimage showing air cavities in the calm region near the rhizome (magnification× 42). (b) Longitudinalfractureshowing the termination of air cavities in the culm-rhizome transition (magnification× 36). (c) Longitudinal fracture showing the termination of air cavities in the culm-umbei transition zone. Note the tortuosity of the vascular bundle systems and constricted air diffusion pathways (magnificationX18). (d) Transverse fracture of the rhizome showing reduced intercellular air spaces ( magnification× 660). unpublished data, 1992). The total assimilation rate of the culms is the sum of this uptake rate and the refixation of respired CO2 from within the culms. When CO2-free air was passed into and through attached culms the internal CO2 concentration in the culms decreased as the rate of airflow passing through the intercellular air cavities increased (Fig. 4). This method of measurement requires a continuous airflow, but the average internal CO2 level in the absence of air flow can be estimated by extrapolating the plot of reciprocal of CO2 concentration against the flow rates back to zero flow. According to this, the CO2 concentration in the culm reached 700/xmol m o l - 1 when the plant was illuminated with a PPFD of 200/xmol m -2 s - 1 and 1600 ~mol m o l - 1 when the plant was in darkness. This indicates that approximately 57% of CO2 accumulated in the culm from respiration was refixed in the light by photosynthesis. Replicated measurements using culms from other papyrus plants showed that 35% and 53% of internal CO2 was refixed by culm photosynthesis. In papyrus it is likely that the outer parts of the chlorenchyma of the culms are responsible for atmospheric CO2 uptake and their inner parts for the internal CO2 utilisation. Direct sampling of intercellular CO2 also showed that CO2 accumulation occurs in the parenchyma tissue of the culm in situ (Fig. 5a). The CO2 gradients in culm intercellular air spaces (Fig. 5a) indicate that respired CO2 produced by the rhizomes is an important source

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M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

Fig. 3. Anatomical characteristics of the epidermal region of C. papyrus culm. (a) Transverse section of the peripheral area of a culm showing 'Kranz' anatomy in the chlorenchymaand intercellular air-spaces between vascular bundles. Note that air cavities are in immediate proximityto PCA cells in peripheral areas of the culm ( magnification× 90 ). (b) Fracture of a stomatal complex showing guard cells, subsidiary cells and substomatal cavity (magnification× 1500). (c) Epidermalsurfaceof culm withpartiallyopen stomata (magnification× 1500). (d) Transverse fracture of the culm and the surrounding scale leaf (top fight) showing the poorly developed epidermal region of the culm and the lack of continuous air cavities in the scale leaf (magnification× 30). of the CO2 in the culm. In darkness, this CO2 accumulates within the intercellular spaces and at the base o f the culm is about 74 times that of the atmospheric CO2 concentration. Fig. 6a shows how CO2 accumulates in the culms during darkness. These results are consistent with those reported by Constable et al. (1992). These authors found that the CO2 concentrations in leaf aerenchyma of Typha latifolia L. were as much as 18 times atmospheric levels at dawn but declined to near atmospheric levels at midday. 3.3. 02 aeration 0 2 concentrations in the intercellular air spaces of papyrus culms show less of a gradient up the culm in daytime than at night time (Fig. 5b). At a given culm position, the intercellular 02 level was higher during the day than at night, particularly nearer the base. The higher intercellular 02 concentrations during the daytime is probably due to more rapid diffusion inwards through open stomatal pores and the photosynthetic production of oxygen as the internal CO2 production is refixed by culm photosynthesis. While the intercellular CO2 concentration increased during night time the 02 concentrations decreased significantly ( P < 0 . 0 0 1 ) (Fig. 6b). At the base of the culms, which are

M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

101

2.1

1.8

1.5" 0

Y

1.2' a

// ,,~

0.9'

/

/

//

/ /

//

•

Light Dark

0.6 20

40

60

80

1O0

120

140

Airflow, (era 3 m i n "1 )

Fig. 4. Lightand air flowrate dependentchangeof CO2concentrationin the intercellularair-spaceof a C.papyrus culm. The regressions were fitted to the relationshipbetweenairflowand the reciprocalCO,.concentration(mmol mol-, ) with (open symbols) and without (solid symbols) illumination (200/zmol m-2 s- ~PPFD). Ambient CO2 concentration was 478 /.~mol mol-l and ambient air temperature 20°C. The linear regressions for the relationships are: (1) for light, y=l.4222+0.012873x (r2=0.984); (2) for dark, y=0.61704+0.010725x ( r2= 0.994). directly connected to rhizomes, 0 2 concentrations were always lower compared with upper sections, indicating that 02 consumption is predominantly in the rhizomes. The decline in 02 concentration in the culm during the night is probably due to closure of stomatal pores in the culm epidermis which increased the resistance to 02 diffusion inwards. Consequently, as 02 was consumed in respiration it was not rapidly replenished by an inward flux of 02 from the atmosphere surrounding the culms. The oxygen concentration in the water of the papyrus 'swamp' significantly (P < 0.01 ) decreased with depth. Conditions were hypoxic (low 02) rather than anoxic (no 02), with the 02 concentrations ranging from 55 nmol ml - ~at a depth of 1 cm to about 40 nmol m l at 15 cm. However, concentrations as low as 17.5 nmol ml-~ were recorded from water collected close to the roots. Because of the hypoxic state of the water surrounding the roots and rhizomes it is likely that the 02 required for their respiration is supplied via intercellular air-spaces connected to aerial parts of the plant (Armstrong, 1979; Higuchi, 1982; Higuchi et al., 1984). As a result, although the roots and rhizomes of papyrus are in a low 02 environment they do not generally grow under 02 deprived conditions. Our results therefore support the theory that the flux of 02 through the culms of C. papyrus is used mainly to sustain the aerobic metabolism of roots and rhizomes, as proposed by Brix ( 1989, 1990b) for Phragmites.

3.4. Fluxes of C02 and 02 transport in papyrus culms In the culm about 20 cm above the rhizome, the C O 2 concentration rises to a maximum of 2.6% and the 02 level falls to a minimum of 13.6% at night (Fig. 5). Our results have shown that prolonged darkness had a highly significant (P < 0.001) effect on the internal 02 and CO2 concentrations in the culms of Cyperus papyrus (Fig. 6). Fig. 7 shows the

M. Li, M.B. Jones~Aquatic Botany 52 (1995) 93-106

102

3.0"

A 2.5"

•I~

2.0"

1.5" O 1.0"

0.5'

0.0 22

B

20

20

37.5

55

72.5

90

Distance from rhizome (cm)

Fig. 5. Intercellular CO2 and 02 concentrations along the culms of C. papyrus ( average length 100 cm ) maintained in a water tank, with light-on (open) or light-off (filled) for 8 h. Samples were taken from the intercellular air space of the culm. The zero position is about 1 cm below the water surface which is equivalent to about 20 cm from the rhizome. Bars represent one standard error (n = 3).

resistance pathway of CO2 diffusion within the plant, in the form of an Ohm's law analogue, and the typical concentrations of CO2 in the light and dark. In this example, the CO2 concentrations in the intercellular air spaces of C. papyrus culms and rhizomes are as follows. (a) CO2 concentrations in the rhizomes were calculated from the concentration gradients in the culm because the intercellular air-spaces in the rhizomes and roots are too small to be sampled directly by syringe. Using a two-order polynomial correlation for the CO2 concentration data presented in Fig. 5 (a), the curves were fitted with r 2 values of 0.907 and 0.936 respectively, and the CO2 concentration in the rhizomes was estimated to be 4.5% in both the day and night. (b) CO2 concentrations in the culms, which are assumed to be 1 m in length, were estimated based on Fig. 4 and are 0.07% during the day and 0.16% at night. (c) Intercellular CO2 concentrations in the chlorenchyma during daytime (C~) was assumed to be 0.012%, a typical Ci value for PCA cells in C4 plants.

M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

30

103

A

2.5'

2.0

•~

15

e. 1.0

05

0.0 20

16

~'~ 14

O

0

0.5

1.5

3

5.5

Time in darkness (h)

Fig. 6. Intercellular CO2 and 02 concentrations in the culms of C. papyrus in prolonged darkness with air temperature 20.5 + 2.6°C and water temperature 18.3 + 1.3°C. Bars represent one standard error (n = 12).

(d) No samples were taken from roots because their intercellular air-spaces are too small to be sampled by syringe. During the day, CO2 flux between rhizome and culm is lower than that between the culm and PCA intercellular spaces, reflecting the declining CO2 concentration in the culm. This pattern was reversed at night because photosynthesis stopped in the dark (Table 1). Brix (1990b) has reported a CO2 emission of 1.06 × 10 - 5 mol m - 2 s - ~in the culm of Phragmites australis. In contrast, Table 1 shows that fluxes in papyrus culms towards the atmosphere are - 1.20× 10 -3 mol m -2 s -1 (day) and 3.27× 10 -4 mol m -2 s -~ (night), suggesting that C4 photosynthesis in the culm plays an important role in CO2 movement. Because culms can have positive CO2 uptake during the day, both internal and external CO2 can be fixed via photosynthesis. Therefore, there are two CO2 fluxes towards the PCA cells during the day: one is from outside the plant and the other is via the intercellular air spaces. Further research is needed to estimate mesophyll, epidermal, cuticular, stomatal and boundary layer resistance to CO2 transport, as these were not determined in this study. The volume fluxes 02 transported downwards towards the roots are also illustrated in Fig. 7 and tabulated in Table 1. The 02 concentrations in the intercellular air spaces of C. papyrus culms were estimated as follows.

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M. Li, M.B. Jones/Aquatic Botany 52 (1995) 93-106

Table 1 The estimation of CO2 and 02 diffusive transport in Cyperus papyrus under isothermal conditions (20°C), based on the data in Fig. 7 ( 101.3 kPa, PPFD = 1500 p.mol m-2 s - t ) Interface

L

A [CO21 ( m o l m -3)

Fcoz. (molm-2s -t)

a [O21 ( m o l m -3)

F~ (molm-2s -l)

1.5 cm 120 g,m 120/xm

1.84 2.41 X 10 -3 9.59 X 10 -3

1.85X 10 .3 3.04 X 10 -3 - 1.20X 10 -3

2.45 0 0

- 3 . 1 9 × 10 -3 0 0

1.5 cm 240/zm

1.81 5.20 × 10 -2

1.82× 10 -3 3.27 × 10 -4

3.29 1.16

- 4 . 2 7 × 10 -3 - 1.51 × 10 -3

Day time Rhizome--culm Culm-PCA cell PCA cell-atmosphere

Night time Rhizome-culm Calm-atmosphere

A [CO2], CO 2 concentration gradient; Fco_,, CO2 flux; A[ O2], 02 concentration gradient; Fo2, 02 flux; L, interface

thickness or the path distance along which the gas diffuses; PCA cells are primary carbon assimilation cells in the culm peripheral. Negative flux values indicate basipetal transport and the positive flux values indicate acropetal transport. For further details see text.

(a) In the rhizome the 02 concentrations were 15.1% during the day and 10.3% at night. These values were estimated using the 02 concentration data presented in Fig. 5b, using the same method used for estimating COz in the rhizome, and the curves for correlation between 02 concentration and the distance from the rhizome were fitted with r2 values of 0.907 and 0.936 respectively. (b) The 02 concentrations in the culm were 21% during the day and 18.2% at night (Fig. 5b). The Oz fluxes between the culm and the rhizome were high during both day and night in order to provide oxygen for the root system (Table 1). Oxygen fluxes required for root respiration have been extensively investigated by Armstrong (1979), Gaynard and ArmDay

Io2_ - - Waterlevel

OO2

001 Aerobic

0.16% 18.2%

[co2] = 4.5% [02] =15.1%

4.5% 10.3%

[CO2]= ? % [o21= ?%

?% ?%

Rw oots

Anoxic

[CO2]= 0.07% [02] = 21%

(daytime)

izome Hypoxic [02] = 0.028.0.104%

Night

]

IDetritassedimen~J

Fig. 7. Resistance diagram for the model describing the internal accumulation and recycling of CO2 and 02 in C. papyrus. Rb, boundary layer resistance; Re, epidermal cuticular resistance; Rcu, resistance along the culm pith; Rm, mesophyll resistance; Rrm, resistance in the rhizome; Rrt, resistance to the roots; Rs, stomatal resistance; Ru, umbel resistance; Rw, resistance in rhizosphere.

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strong (1987) and Brix (1990b). It appears that the requirements of mitochondria and intact cells or tissues for 02 are only met by maintaining relatively high 02 concentrations (Bonner, 1973; Saglio et al., 1984; Atwell et al., 1985; Brix, 1988; Armstrong, 1989; Drew, 1991). Although the 02 flux towards to the roots was not determined in this study, it is proposed that the high 02 fluxes towards the rhizome may not only allow the rhizome to respire aerobically, but also allow the root system to have an efficient 02 supply regardless of the fact that they are submerged in hypoxic conditions. The dynamics o f CO2 and 02 transport in the intercellular air-spaces described here suggest that: (1) the balance between photosynthesis and respiration processes have an effect on the 02 and CO2 exchange in the culm intercellular air-spaces; (2) respiration processes are mainly aerobic in the culm, as well as in the rhizomes and roots, through a passive 02 transport mechanism; ( 3 ) intense CO2 exchange occurs between ambient air and culm pith, owing to the high conductance of the epidermis and the C4 photosynthesis in the periphery o f the culms; (4) a high O2 flux is generated during both day and night in the intercellular air spaces between the culm and rhizome. It is hypothesised that the CO2 produced by rhizome respiration diffuses into the intercellular space systems of the culm along continuous canals but further diffusion into the air is prevented by the chlorenchyma which is suberised, lignified and peripherally located in the culm. The vertical diffusive path to the umbel is stopped by a barrier which is located in the terminal end of the culm (Fig. 2c). As a consequence, CO2 can be either fixed through photosynthesis during the day or accumulate in the intercellular space system at night and reach a concentration very much higher than that of the ambient air.

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