Aquatic Botany, 23 (1986) 329--339 329 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
G A S E X C H A N G E O F T Y P H A O R I E N T A L I S P R E S L . C O M M U N I T I E S I N A R T I F I C I A L P O N D S
PHILIP J.M. SALE and PHILIP T. ORR
CSIRO Centre for Irrigation Research, Griffith, N.S. W. 2680 (Australia)
(Accepted for publication 4 September 1985)
ABSTRACT
Sale, P.J.M. and Orr, P.T., 1986. Gas exchange of Typha orientalis Presl. communit ies in artificial ponds. Aquat. Bot., 23 : 329--339.
Large field chambers were used for measuring the net carbon dioxide exchange of vigorously growing communit ies of Typha orientalis Presl. Uptake was greateJt at the begin- ning of summer, when that of a communi ty with leaf area index 6.8 reached almost 6 g CO 2 m -2 (water surface area) h -~. In a second communi ty with LAI 4.8 maximum uptake was 3.8 g CO~ m -2 h -~. In both communit ies there was a marked decline in uptake over the middle part of each day: uptake also declined quite rapidly as the communities aged. Maximum dry weight increases of 30 and 24 g m -2 24 h TM for the 2 communities, respec- tively, were calculated from day-time net assimilation and night-time respiration. Mea- surements of maximum water loss from the first communi ty showed a water-use effi- ciency of 7.86, which declined as the summer progressed.
The photosynthet ic performance of the Typha communit ies was similar to that of some well-irrigated C3 crop plants grown in a similar climate.
INTR ODUCTION
Typha s p e c i e s a re w i d e l y d i s t r i b u t e d in b o t h n a t u r a l a n d a r t i f i c i a l w e t l a n d a reas . D u r i n g s u m m e r m o n t h s , w h e n g r o w i n g c o n d i t i o n s a r e f a v o u r a b l e a n d w a t e r a b u n d a n t , t h e y a r e we l l a d a p t e d , f o r m i n g d e n s e c o m m u n i t i e s w h i c h o f t e n have h igh r a t e s o f d r y m a t t e r p r o d u c t i o n ( W e s t l a k e , 1 9 6 3 ; D y k y j o v a ,
1 9 7 1 ) . I n t h e M u r r u m b i d g e e I r r i g a t i o n A r e a s o f N . S . W . , T. orientalis Pres l . , o n e o f
t w o spec i e s n a t i v e t o A u s t r a l i a a n d a d o m i n a n t c o m p o n e n t o f n a t u r a l f r e s h - w a t e r e c o s y s t e m s , has c o n s i d e r a b l e a n d i n c r e a s i n g n u i s a n c e v a l u e in b l o c k i n g d r a i n a g e a n d i r r i g a t i o n c h a n n e l s , a n d as a w e e d in r i c e p a d d i e s . O n t h e o t h e r h a n d , i t has p o t e n t i a l u se in b i o l o g i c a l w a s t e - w a t e r f i l t e r s ( C a r y e t a l . , 1 9 8 2 ) .
T h e h igh p r o d u c t i v i t y o f Typha s p e c i e s s u g g e s t s t h a t c a r b o n a s s i m i l a t i o n m a y b e t h r o u g h t h e e f f i c i e n t C4 p a t h w a y , b u t M c N a u g h t o n a n d F u l l e m
( 1 9 7 0 ) d e m o n s t r a t e d t h a t o n l y t h e n o r m a l C3 p a t h w a y is u s e d in T. latifolia L. P h o t o s y n t h e s i s in l eaves o r s h o o t s o f Typha p l a n t s has b e e n s t u d i e d b y
several workers (e.g. Gartner, 1976), but f rom an ecological viewpoint, it is the performance of the whole plant communi ty which is most impor tan t in understanding and predicting the interaction of a species with its environ- ment. This paper describes photosynthe t ic measurements, made on 2 stands of T. orientalis grown in artificial ponds, in relation to radiation and tem- perature, to provide basic information which will help predict its growth in different habitats.
METHODS
Plant communit ies
In 1979, a series of plastic-lined ponds, each 1.1 m deep, 3 m long and 3.6 m wide, was built and planted with young rhizomes of Typha orientalis at different densities. A 10-cm layer of gravel on the bo t tom was covered with 50 cm of channel sediment. When the plants were established, the water level was maintained at a depth of 30 cm above the sediment and fertiliser was added each year to maintain vigorous growth. The measurements described here were made during 1981--82 on 2 ponds when the plants had grown undisturbed for 2 seasons.
Gas exchange measurements
Carbon dioxide exchange of the plant communit ies was measured using the Teflon-covered field assimilation chambers of base area 2.8 m 2 described previously (Sale, 1974). They were f i t ted with an air-tight skirt and sup- por ted with the base of the skirt below and the top of the chamber 2 m above the water level. To reduce artificial shading of enclosed plants to a minimum, the chambers were or ienta ted with equipment at the south end, and a guard area of approximate ly 1 m to the nor th ; the guard area on either side was unlimited. Carbon assimilation was measured as the rate of injection of pure carbon dioxide which kept the air circulated around the enclosed plants at ambient carbon dioxide concentrat ion. An infrared gas analyser with chart recorder ou tpu t was used as a null-balance controller. Dark respiration was measured short ly after dusk from the slope of the recorder trace after flushing the chamber with heated or cooled ambient air. The temperature of the circulating air was thermostatically control led and its relation to leaf tempera ture was periodically determined using an infrared the rmomete r (Everest Interscience model 110). Aspirated psychrometers with diode-based sensors and thermocouples measured air-dry-bulb tem- peratures and wet-bulb depression. Total incident radiation on a horizontal surface was measured using thermopi le solarimeters with integrators, and the radiation incident on the plants within the chambers was estimated as 85% of this value. The day-time evapotranspiration of the plant--water system in one of the chambers was measured cont inuously by collecting the condensate from the air-cooling system.
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The field chamber measured the net carbon dioxide changes in the en- closed air, which resulted from exchange both with plants and across the water surface. Changes within the water, resulting f rom exchange with the sediment, submerged plant parts and atmosphere, were assessed by using the Gran titration technique (Denny et al., 1983). Continuous measurements of pH (using Beckman 960A meters) and pond water temperature were made at depths of 5 and 20 cm. When the plants were finally harvested, by cutt ing below the water surface, chamber measurements were continued for 3 days to assess carbon dioxide flux across the air--water interface. All measured parameters were recorded at 15-min intervals on a data-logger.
Plan t parameters
Gas exchange measurements were made for three 2-week periods on 2 communities (designated A and B), which had a similar number of uni- formly distributed shoots. While the plants in Communi ty A were large and vigorous, most producing flowering spikes, those in Communi ty B were smaller and the majori ty did not flower. During the growing season, leaf area and dry weight parameters for enclosed shoots were estimated by sampling similar shoots outside the chambers. After the final measurement period, all shoots from within each chamber were cut at water level and green leaf areas and dry weights of the separated plant parts were determined. The leaf area indices (LAIs) given below are for both surfaces of all green leaves measured from where the laminae diverged from the main shoot. Finally, the ponds were drained, the surface litter collected and an area of 1 m: ex- cavated to collect the roots and rhizomes, which were washed free from sediment, oven~lried and weighed.
RESULTS
Carbon dioxide uptake
The pH of the water body tended to increase slightly by day and decrease at night, e.g. from 6.6 or 6.7 at 09.00 h to 6.8 or 6.9 at 16.00 h, measured in early November. Free dissolved carbon dioxide in the water profile, cal- culated from measured pH and alkalinity, decreased through the day by about 0.05--0.20 g CO2 m -2 (water surface) h -1. When the plants were cut, there was a fluctuating net efflux of carbon dioxide from the water, which was too small to assess accurately using the chamber system. For normal chamber measurements, it was estimated that carbon dioxide fluxes, other than between emergent plant parts and the atmosphere, were only 1--5% of the total flux, and no a t tempt was made to correct for them in the mea- surements described below. Almost no direct sunlight penetrated through the canopy to the water surface, and the water temperature varied at most by only 5---6°C between dawn and dusk.
332
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T I M E OF D A Y (h)
Fig. 1. Net carbon d ioxide uptake of Typha orientalLs ( o C o m m u n i t y A; e, C o m m u n i t y B) and total irradiance (solid line) on (a) 22 November 1981, (b) 3 December 1981 and (c) 9 February 1982. The condensate col lected f rom the chamber over C o m m u n i t y A is also shown (4). Effec t ive leaf area indices of Communi t i e s A and B, respectively, were (a) 6.8 and 4.8, (b) 6.4 and 4.5 and (c) 1.07 and 1.43. The sharp radia t ion peaks occurred when light was ref lected o f f approaching cloud.
All gas exchange rates given here are described as rates per m 2 of plant communi ty , i.e. they are related to the surface area of the water in which the plants were growing. Figure 1 shows typical daily rates of carbon dioxide uptake for the 2 communities on 3 occasions. The greatest measured uptake,
333
almost 6 g CO2 m -2 h-~ for Communi ty A, occurred at the end of November at the time of maximum leaf development (LAI 6.8). Maximum uptake in Communi ty B, with LAI 4.8, was about 3.8 g CO2 m -2 h -l Thereafter , both carbon dioxide uptake and effective leaf area indices declined, the leaves on flowering shoots senescing most rapidly, until by early February the uptake in Communi ty A was only slightly greater than in Communi ty B, although the latter had the greatest green leaf area at this stage (Table I). A marked feature of carbon dioxide exchange in both communit ies was a significant reduct ion in uptake between 10.00 and 14.00 h with peaks at about 09.00 and 15.00 h. This was particularly evident on cloudless days (Fig. lb ) , but was less pronounced when scattered cloud resulted in more diffuse radiation (e.g. Fig. la) . The mid , l ay reduct ion was still evident, bu t no t so marked, as the leaf canopy aged (Fig. lc ) .
The reduction in uptake at the time of peak radiation is clearly seen in Fig. 2, which also distinguishes uptake before and after mid-day.
TABLE I
Green leaf Lrea indices (both surfaces), stem numbers and dry weights (g) per m 2 o f c o m p o n e n t parts of the 2 Typha communities harvested on 25 February 1982
Typha Shoot LA1 Leaf Leaf Dead Inflores- Root Rhizome Su~ace T o t ~ community N o . m lamina bMe leaf cence litter weight
] O f these , 1 4 . 7 i n C o m m u n i t y A and 5 .4 i n C o m m u n i t y B were f l o w e r i n g s h o o t s .
S ~ A
O O o
7 0 I ! l I i 200 ~00 600 800 I~
Tofat radiation (W m -2)
Fig. 2. Ne t c a r b o n d iox ide u p t a k e o n 2 D e c e m b e r 1981 fo r C o m m u n i t y A ( o p e n c i rc les ) and C o m m u n i t y B (sol id c i rc les) p l o t t e d aga ins t t o t a l i n c o m i n g r a d i a t i o n . Sol id l ine up- take u p to m i d - d a y ; b r o k e n l ine u p t a k e a l t e r m i d - day . C u r v e s are f i t t ed p a r a b o l a s w i t h r 2 = 0 .90 o r be t t e r . F o r C o m m u n i t y B, a f t e r n o o n u p t a k e is s i gn i f i c a n t l y less t h a n m o r n - ing u p t a k e (P <: 0 .01) .
334
18 27-~) Nov
1"6
"~ 1"~ ~ 1 ~ - 1 7 Dec
1'2 ~E
cm •• & AA • 27-30 Nov. ._6 0"8 "
i 7+,
0"2 A J "
I I I I I I I
O'Oo 5 10 15 20 25 30, Temperafure (°C}
Fig. 3. Dark respiration rates for Typha orientalis plotted against air temperature for Community A (open symbols) and Community B (solid symbols). Circles are used for the period 27--30 November 1981; triangles are used for the period 14--17 December 1981.
Effects o f temperature on dark respiration
In early summer, there was a linear relationship between dark respiration and temperature over about 15--35°C (Fig. 3). Two sets of measurements separated by 18 days showed that a marked reduct ion in respiration oc- curred over this period. Dark respiration was not propor t ional to live above- water biomass, which on 20 December 1982 was est imated to be 1840 and 950 g (dry weight) m -2 in Communit ies A and B, respectively, of which 47 and 28%, respectively, was flowering tissue. By 9 February 1982, dry weights of live shoots had fallen to 1310 g m -2 (Communi ty A) and 510 g m -2 (Communi ty B), and communi ty dark respiration measured at 20°C was 0.45 and 0.20 g CO2 m -2 h -1 in A and B, respectively.
Effects o f temperature on net photosynthesis
The response of net canopy photosynthesis to temperature was deter- mined at the end of November 1981. The values shown in Fig. 4 are for light-saturated canopies, avoiding the period of mid-day depression, and were calculated f rom the slopes of the IRGA traces. Leaf temperatures in-
335
6
3 I I . Uptake B
I . . . . I ' - - I I I I I 10 15 20 25 30 35 /~0
Temperature (°F-)
Fig. 4. The e f f e c t o f air t emperature on carbon d i o x i d e uptake and dark respirat ion for 2 Typha c o m m u n i t i e s b e t w e e n 27 and 29 N o v e m b e r 1 9 8 1 . Bars are s ignif icant d i f f erences at P < 0 .05 .
side the chamber were found to be the same as, or about I°C higher than, air temperature. Dark respiration rates, taken from Fig. 3, show the relative magnitude of respiration and net photosynthesis.
Water loss
For the chamber on Communi ty A, collection of condensate from the air- cooler showed that an initial period was required each day for the measure- ment system to equilibrate, but thereafter, evapotranspiration was fairly constant during daylight hours. Figure l a shows slight reductions in step with fluctuations in radiation and carbon dioxide uptake, indicating a rapid response by the measuring system. A similar response was found following artificial shading. However, f luctuations sometimes occurred which could not be related to radiation (Fig. lb) , The system did not measure water loss at night. Water loss was apparently related to green leaf area. The maximum loss of about 700 g water m -2 communi ty h -1 occurred at the end of Novem- ber when LAI was 6.8 (Fig. la) , but had declined markedly by early Febru- ary when LAI was 1.07 (Fig. lc) , although radiation was still very high.
Dry weight production
The net seasonal dry weight product ion was not determined, but Table I gives a summary of the dry weights of the component parts of the two plant communities at harvest, when the seed heads were beginning to dehisce and the leaves were senescing rapidly, particularly on the larger flowering shoots which predominated in Communi ty A. Biomass above the sediment bore no relation to that below it at this time, and while the total plant dry weight in Community A was 42% greater than in Communi ty B, the roots in B were 32% heavier than those in A.
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DISCUSSION
In these experiments, measurements of carbon dioxide uptake were made on Typha oriental~ communit ies under conditions which would be expected to lead to high rates of uptake; the plants had a high LAI with upright leaves, well-developed storage organs to which assimilate could be translocated, abundant water, high incident radiation and favourable growth temperatures. The maximum communi ty assimilation rate measured was about 6 g CO2 m -~ h-~, which is fairly high for plants having the C3 photosynthet ic path- way, and almost double the maximum uptake achieved by a well developed communi ty of the floating macrophyte Eichhornia crassipes (Mart). Solms under similar condit ions (Sale et al., 1985). However, well-irrigated C3 vege- table crops, where the canopy architecture was very different to that of Typha, have also been found to achieve assimilation rates of 5--6 g CO2 m -2 (ground area) h -1 (Sale, 1977), whi.le the C4 Zea mays L. achieved over 9 g m-: h -~. Typha does not, therefore, appear as exceptional in its photo- synthetic ability as has sometimes been suggested (e.g. McNaughton and Fullem, 1970).
While for many plants an increase in effective LAI above 4 or 5 confers little extra photosynthet ic advantage because of increasing self-shading (e.g. Sale, 1975), this limit did not apply to Typha, since assimilation at LAI 6.8 was considerably greater than at LAI 4.8. This was probably a conse- quence of the almost vertical orientation of Typha leaves, which enables light to penetrate to the lower levels of the canopy with a more efficient use of incident photosynthet ic radiation (e.g. Loomis et al., 1967). Although a LAI of 6.8 represents a vigorous and well-developed communi ty , some ex- ceptionally high LAI values have occasionally been reported for Typha species, e.g. 18 in T. angustifolia, by Dykyjova (1971), and such communit ies may therefore have higher assimilation rates. Gustafson (1976) suggests, from a simulation model, that maximum net photosynthet ic uptake will occur in T. latifolia at a LAI of about 12, representing 40 shoots per m 2, the uptake then being about 17% greater than at LAI 7. In very dense stands, there may be net respiratory loss from heavily shaded leaves, but some of the respired carbon dioxide may be re-assimilated.
Previous workers, using single leaves of T. latifolia, have reported either that maximum photosynthesis occurs at mid-day (Gartner, 1976; Gustafson, 1976) or, in contrast, that there is a mid-day reduction similar to that found here for T. orientalis communit ies (Salageanu and Tipa, 1967). There is no obvious explanation for this mid-day reduction. It is unlikely to be a func- tion of light interception, since it was both marked and consistent in can- opies of different LAIs; also light penetrat ion and therefore efficiency of utilization in a Typha communi ty should be greatest when the sun is directly overhead (Ondok, 1978). Although stomatal resistance was not measured in the present experiments, the absence of a concurrent mid-day reduct ion in transpiration (Fig. 1) suggests that there was no stomatal closure, a conclu-
337
sion supported by previous work with T. angustifolia (Sale and Wetzel, 1983). Since photosynthe t ic uptake returned almost to the original high rate towards the end of the afternoon, it is unlikely that mid-day depres- sion was caused by an accumulation of assimilates in the leaves, which was suggested as a possible cause of the af ternoon depression in uptake in Eich- hornia crassipes (Sale et al., 1985).
On some occasions, uptake at a given radiation level in the morning, when radiation was increasing, was slightly greater than at the same level in the afternoon, when radiation was decreasing, e.g. for Communi ty B (Fig. 2), where the difference was significant at P < 0.01. However, this response in Typha was never as marked as in some other species (Sale, 1977; Sale et al., 1985).
Net photosynthesis of the communities was not very sensitive to air tem- perature (Fig. 4). McNaughton (1973) found a greater temperature response with single leaves of T. latifolia, and also a higher opt imum temperature (25--30°C) for an ecotype from a warm climate compared with one f rom a cool climate. The op t imum of 15--25°C found here for T. orientalis com- munities (Fig. 4) seems fairly low for an area with high summer tempera- tures, but may reflect adaptation to the cooler temperatures at the begin- ning of spring when leaf development is rapid.
In the 11 days between 22 November and 3 December, the mean rates of net photosynthesis in the 2 Typha communit ies declined by 16--20%, while the effective LAIs declined only slightly, which suggests a marked reduct ion in photosynthet ic efficiency as the leaves aged. McNaughton's (1973) single-leaf data suggest a similar reduction for T. latifolia over the first 30 days or so from full expansion.
Respiration rates of the communities also fell markedly over this t ime (Fig. 3), possibly as a consequence of re-distribution of dry matter into the rapidly developing inflorescences. Respiration was highly temperature- sensitive, and night temperatures determined the percentage of the dayt ime carbon dioxide uptake which was lost each night. The average loss was about 20% of uptake, which was similar to tha t found for some summer row-crops (Sale, 1975), and although respiration of Typha per unit of dry mat ter was less than for row-crops, the dry weight per unit area was greater.
The results of the present experiments are difficult to compare with those of previous workers, who often measured assimilation using single leaves of T. latifolia. However, the maximum uptake of 45--71 mg CO2 g-' dry weight 24 h- ' reported by McNaughton (1974) for single whole shoots of T. latifolia under artificial light is very similar to the 48 and 55 mg CO2 g-1 (above-water shoot dry weight) 24 h-1, calculated as maximum uptake for T. orientalis Communities A and B, respectively, from the data in Figs. 1 and 3. The maximum net uptake related to water surface area was 46 and 37 g CO2 m -2 24 h -1 for A and B, respectively, equivalent to about 30 and 24 g dry matter m- ~ for the 24 h period. This is in the middle of the range of 12--48 g dry matter m -2 24 h -1 which Westlake (1975) suggests may be
338
expected from C3 freshwater macrophytes, and compares with a value of 19 g m -2 24 h -z derived from the results of Gustafson (1976) for T. latifolia. The conversion efficiency for dry weight production of T. orientalis over 24 h, with the high incident levels of photosynthetically active radiation at the end of November, was calculated to be about 2.6--3.2%, which was com- parable to that of summer row-crops (Sale, 1977).
Vapour pressure deficit within the chamber was calculated to be about half that of ambient vapour pressure deficit, and although this is unlikely to have affected photosynthesis of unstressed plants (Rawson et al., 1977), evapotranspiration was probably lower than that outside. Assuming that no water was lost from the heavily shaded water surface, the maximum transpiration rate of about 700 g m -2 (communi ty water surface) h -1
represented 1.15 g water g-I leaf dry weight h- ~, which is 25--50% of single leaf rates measured in mid-summer for T. latifolia (McNaughton, 1973; Kroli- kowska, 1978). Over the period 22 November--3 December, water loss was constant bu t communi ty photosynthesis declined, with a consequent reduction in water-use efficiency (rag CO2 uptake per g water transpired) from 7.86 to 6.42. McNaughton (1973) found a similar decline from 10.61 to 6.92 as single leaves of T. latifolia aged from 12 to 30 days. Since Typha is normally abundant ly supplied with water in its natural habitat, it might be expected to have a low water-use efficiency compared with terrestrial plants, but the value found here is very similar to that o f 7.33 determined for a crop of tomatoes (Lycopersicon esculentum L.) of LAI 3, grown in nutrient solution in a closed greenhouse on the same site over a similar summer period (P.J.M. Sale, C.P. Meyer and G.I. Moss, unpublished data, 1984).
ACKNOWLEDGEMENTS
The authors are indebted to G. Shell for assistance with the data logging system and D. Erskine for much technical assistance and data analysis.
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