AOBPreview originally published online on October 2, 2002
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Annals of Botany 90: 593-598, 2002
© 2002 Annals of Botany Company
Initial Net CO2 Uptake Responses and Root Growth for a CAM Community Placed in a Closed Environment
1 Biosphere 2 Center, Columbia University, Box 689, Oracle, AZ 85623, USA
* For correspondence at: Department of Organismic Biology, Ecology, and Evolution, University of California, Los Angeles, CA 90095-1606, USA. Fax +1 310 825 9433, e-mail psnobel{at}biology.ucla.edu
Received: 22 May 2002; Returned for revision: 25 June 2002; Accepted: 16 July 2002 Published electronically: 2 October 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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To help understand carbon balance between shoots and developing roots, 41 bare-root crassulacean acid metabolism (CAM) plants native to the Sonoran Desert were studied in a glass-panelled sealable room at day/night air temperatures of 25/15 °C. Net CO2 uptake by the community of Agave schottii, Carnegia gigantea, Cylindropuntia versicolor, Ferocactus wislizenii and Opuntia engelmannii occurred 3 weeks after watering. At 4 weeks, the net CO2 uptake rate measured for south-east-facing younger parts of the shoots averaged 1·94 µmol m–2 s–1 at night, considerably higher than the community-level nocturnal net CO2 uptake averaged over the total shoot surface, primarily reflecting the influences of surface orientation on radiation interception (predicted net CO2 uptake is twice as high for south-east-facing surfaces compared with all compass directions). Estimated growth plus maintenance respiration of the roots averaged 0·10 µmol m–2 s–1 over the 13-week period, when the community had a net carbon gain from the atmosphere of 4 mol C while the structural C incorporated into the roots was 23 mol. Thus, these five CAM species diverted all net C uptake over the 13-week period plus some existing shoot C to newly developing roots. Only after sufficient roots develop to support shoot water and nutrient requirements will the plant community have net above-ground biomass gains.
Key words: Agave schottii, Carnegiea gigantea, CO2 uptake, crassulacean acid metabolism, Cylindropuntia versicolor, Ferocactus wislizenii, Opuntia engelmannii, plant community, respiration, roots.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Considerable information is available regarding net CO2 gain by above-ground portions of plants and net CO2 loss by below-ground portions under specific environmental conditions (Fitter and Hay, 2002; Taiz and Zeiger, 2002; Waisel et al., 2002), yet much fewer data exist for entire plants and there are almost no data for plant communities. Such data are crucial for understanding the carbon balance of plants in the field and for predicting responses of community-level CO2 exchanges to environmental conditions. Measuring community-level net CO2 exchanges of both roots and shoots is difficult in the field, and few resources are available for such measurements under controlled, yet relatively natural conditions (Nelson et al., 1993; Langhans and Tibbitts, 1997).
Measurements in sealed rooms with glass walls that allow the entry of solar irradiation pose special challenges (Wheeler et al., 2001). For example, when the air temperature changes from 15 °C at night to 25 °C during the daytime, the ideal gas law indicates that the pressure in a constant-volume room will increase by 3·5 % (Nobel, 1999); the force on a 1 m x 2 m glass panel in the walls of such a room would then increase by 720 kg if there were no provisions for air volume increases. Means of accommodating the air pressure changes, necessary for sealed rooms with rigid walls, are available at the Biosphere 2 Center in Oracle, Arizona, USA (Nelson et al., 1993), where the present experiment was performed to examine net CO2 uptake by shoots and to determine root growth for five succulent species expected to exhibit crassulacean acid metabolism (CAM). Because temperature influences essentially all processes in plants (Nobel, 1999), a room with controlled temperatures would simplify carbon-balance interpretations, as was the case in the present study. Day and night temperatures in the room can also be increased by a specific amount, as in the present experiments.
CAM species, such as the agave and the four species of cacti considered here, take up CO2 predominantly at night (Nobel, 1988, 1999; Taiz and Zeiger, 2002), so atmospheric CO2 concentrations surrounding a community of such species are expected to decrease at night and to increase during the daytime. Moreover, multi-year-old CAM plants can successfully be transplanted to new locations without concern about maintaining fine roots or even any roots at all (Hewitt, 1997; P. S. Nobel and E. G. Bobich, pers. obs.), so such plants can act as a useful model for investigating the inter-relations between root and shoot carbon balances of perennials during an establishment period. Moreover, there are unique features of carbon allocation for species that reproduce vegetatively (Magda et al., 1993; Poorter and Nagel, 2000), such as during establishment for CAM species that rely on vegetative reproduction by offshoots or stem segments (Holthe and Szarek, 1985). Although a community consisting entirely of CAM species is not realistic, the five species chosen occur sympatrically in the Sonoran Desert of Arizona (Benson, 1982; Gentry, 1982), where CAM species are often the dominant vegetation (Drennan and Nobel, 1997). Thus, the data obtained for carbon balances may be useful for examining the CAM component of natural plant communities in various arid, semi-arid and even tropical regions, where most CAM species actually occur (as epiphytes and hemiepiphytes; Benzing, 1990; Lüttge, 1997).
The availability of a sealable room at the Biosphere 2 Center allowed the testing of three specific hypotheses: (1) because bare-root CAM plants develop new roots and take up water within a matter of days after planting and watering (Nobel and Sanderson, 1984; Nobel, 1988), net CO2 uptake by the plant community should be evident within a few weeks after planting and watering; (2) the respiratory rates for root growth and root maintenance, which have been determined for various agaves and cacti (Palta and Nobel, 1989a, b; Nobel et al., 1992), should be relatively low compared with the net CO2 uptake rate by the shoots of such CAM plants under moist conditions; and (3) roots represent only a small portion of the biomass of CAM plants, but the initial carbon requirements for developing new roots should require a large proportion of the initial net CO2 uptake by the shoots over a 3-month period.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Plant material
Eight or nine bare-root specimens of five CAM species of similar size and shoot surface area and with minimal amounts of existing roots (Table 1) were planted on 18–19 Dec. 2001 in a sealable room at Columbia University’s Biosphere 2 Center near Oracle, Arizona, USA (32°35'N, 110°51'W, 1165 m elevation). (The room is called the ‘Test Module’ because it was designed and originally used to evaluate the influence of plants and a single human on atmospheric concentrations of CO2 and O2 when sealed from the external atmosphere.) Agave schottii Engelmann var. schottii with approx. 75 unfolded living leaves was obtained from the Biosphere 2 Center campus, the columnar cactus Carnegiea gigantea (Engelmann) Britton & Rose was purchased from Bach’s Greenhouse Cactus Nursery (Tucson, Arizona), Cylindropuntia versicolor (Engelmann ex J.M. Coulter) F.M. Knuth with one trunk and five or six main branches was obtained from the campus, the barrel cactus Ferocactus wislizenii (Engelmann) Britton & Rose was purchased from Keller Nursery and Landscaping (Tucson, Arizona), and the platyopuntia Opuntia engelmannii Salm-Dyck ex Engelmann with approx. nine cladodes was obtained from the campus. Mean plant height ranged from 29 to 74 cm, and mean shoot surface area ranged from 0·37 to 0·55 m2 (Table 1).
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The 41 plants were randomly placed at approximately equal distances of about 0·8 m (to simulate typical interplant spacing and ground cover in the field) in a loamy sand obtained from the Biosphere 2 Center campus. The soil was 40 cm deep in two planters that averaged 5·5 m in length and 2·4 m in width with a 1·2 m walkway in between to facilitate measurements of net CO2 exchange. The total area of the leaves plus stems was 18·2 m2 (Table 1) for the 26·4 m2 of soil surface area. Based on the areas of polygons circumscribing each plant, the calculated ground cover was 0·27, which is the same as the mean measured ground cover for perennials in the north-western Sonoran Desert (Drennan and Nobel, 1997). Each week, beginning on 20 Dec. 2001, plants received water equivalent to a depth of 30 mm for the first 2 weeks, 10 mm for the next 2 weeks, and then 5 mm weekly thereafter for 13 weeks, or 125 mm total, similar to the winter rainfall in the local Sonoran Desert (Venable and Pake, 1999; Bowers, 2000). At 13 weeks, plants were excavated and fresh and dry masses of the newly developed roots were determined.
Experimental conditions
The Test Module is a sealable room with glass walls to which is attached a bellows (‘lung’) exposed to atmospheric pressure to accommodate the changes in air volume that occur with changes in room air temperature. The CO2 concentration in the room was determined at 15-min intervals using a Ventrostat 2001V CO2 sensor [Telaire (formerly Engelhard Sensor Technologies), Goleta, CA, USA] to determine the CO2 exchange by the five-species plant community (because of the temperature sensitivity of the sensor, reliable readings could be obtained only when constant air temperatures prevailed in the room for a few hours, such as at midday or for various nights). The rate of change of room CO2 concentration multiplied by the volume of air in the room plus its bellows (286 m3 at 25 °C for a moderately filled bellows) yields the amount of CO2 exchanged per unit time. The room was opened to the external environment for a few hours on a weekly basis to maintain an internal CO2 concentration similar to current atmospheric CO2 concentrations, i.e. to prevent major increases or decreases in the room CO2 concentration (the reported weekly CO2 concentration changes took these adjustments into consideration). Room CO2 concentration is expressed in parts per million by volume (p.p.m.), which equals µl l–1 and Pa MPa–1 by the ideal gas law (Nobel, 1999).
Room temperature was routinely maintained at 15·0 ± 0·7 °C for 12 h nights and 25·0 ± 0·8 °C for 12 h days. Temperature changes from the two set points generally required about 45 min, except when the atmospheric air temperatures differed substantially from the design temperatures, when a few hours were required. Measurements of CO2 concentration in the room and hence community-level net CO2 exchange rates were also made at elevated day/night air temperatures of 32·2 ± 1·0 °C22·1 ± 1·1 °C maintained for 2 weeks (the eighth and ninth weeks). The total daily photosynthetic photon flux (PPF, wavelengths of 400–700 nm) on a horizontal surface at the tops of the plants inside the room during the 3-month experimental period averaged 18 mol m–2 d–1, as measured at 15-min intervals using a LI-190S quantum sensor (LI-COR, Lincoln, NB, USA); this value was 38 % less than the total daily PPF measured just outside the test module with the same type of sensor and 55 % less than the average value of 41 mol m–2 d–1 predicted for clear days from the winter solstice to the spring equinox (essentially the experimental period) at that latitude (Nobel, 1988).
Shoot net CO2 uptake
Net CO2 uptake rates were also measured directly on shoots of individual plants during the day and night on 16–17 Jan. 2002 using a LI-COR LI-6200 portable photosynthesis system. The cuvette was fitted to the leaves of the agave or the stems of the cacti (after locally removing spines in the latter cases) by replacing the lid of the cuvette with an acrylic plate having a rectangular extension with an opening of 8 x 8 mm. The margin of the opening was covered with a foam-rubber gasket to form an air-tight seal with the shoot surface. Surfaces were chosen that faced approximately south-east on the younger (upper) portions of the plants approximately two-thirds of the way from the base to the apex for each of the five species. The total daily PPF on a horizontal surface for the 4 d preceding the nocturnal CO2 measurements, which affects the amount of glucan available in the chlorenchyma cells and hence the net CO2 uptake capacity of a CAM plant (Nobel, 1988; Black et al., 1996), averaged 12 mol m–2 d–1 at plant height, which was 39 % lower than just outside the room and 54 % lower than predicted for clear days at that time of the year (Nobel, 1988).
Root growth
The entire root systems for all 41 plants in the sealed room were carefully excavated on 25–26 Mar. 2002. New root growth was readily distinguished as unbranched white roots originating from the stem base for A. schottii (plus a few white extensions of the relatively numerous reddish-brown roots that existed initially) and whitish, branched roots for the four species of cacti, nearly all newly developed roots coming from the base of the stems (only a few old existing roots were present initially for the cacti, especially F. wislizenii). The fresh masses of the new roots were obtained immediately after excavating; dry masses were determined after drying the new roots to constant mass for 4 d at 80 °C in a forced-draft oven.
Data are presented as means ± s.e. (n = number of measurements).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Daily CO2 measurements during the first 2 weeks after planting indicated that the CO2 concentration in the sealed room was increasing steadily. The weekly increase during the first week was 67 p.p.m. CO2 (Fig. 1) for the 286 m3 air volume of the room plus bellows, which represents 0·78 mol CO2 based on the ideal gas law at 25 °C (Nobel, 1999). For the week, and using shoot and root areas of 18·2 and 26·4 m2, respectively, the 67 p.p.m. increase corresponds to an average rate of CO2 release per unit soil area of 0·049 µmol m–2 s–1 or per unit shoot area of 0·071 µmol m–2 s–1 (henceforth, shoot area basis will be used for calculations). During the fourth week, the room CO2 concentration decreased by 47 p.p.m., so the net CO2 uptake per unit shoot area was then positive and averaged 0·050 µmol m–2 s–1 over the day and night.
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After 4 weeks in the sealed room, net CO2 uptake was measured directly on south-east-facing upper portions of the shoots of all five species to ascertain whether each species could then have net carbon gain. The rates for all five species were similar (Table 2); the average net CO2 uptake rate at night was 1·94 ± 0·16 µmol m–2 s–1, and the average daytime net CO2 loss rate was 0·64 ± 0·13 µmol m–2 s–1. The total daily net CO2 uptake for the south-east-facing surfaces of the five species averaged 56 ± 4 mmol m–2 d–1 (Table 2). For comparison, the community net CO2 uptake rate at night averaged over the total shoot surface area of the five species was determined during the fifth week in the sealed room, as the night-time temperatures then remained constant at 15·1 ± 0·4 °C for nearly the entire 12-h period. The nightly decrease in the room CO2 concentration for 6 d averaged 41 ± 4 p.p.m., which corresponds to a mean nightly net CO2 uptake rate per unit total shoot area (i.e. on a community-level basis) of 0·61 µmol m–2 s–1.
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When the weekly community net CO2 uptake rate had achieved a steady value at 4–7 weeks after planting (a weekly decrease in CO2 concentration of 52 ± 3 p.p.m.; Fig. 1), the day/night air temperatures were increased to 32/22 °C for 2 weeks. This caused the room CO2 con centration to increase by an average of 66 p.p.m. weekly (Fig. 1). One week after returning to the usual day/night air temperatures of 25/15 °C, the weekly CO2 concentration decrease actually exceeded previous values, and the weekly decrease averaged 94 p.p.m. for the next 3 weeks (Fig. 1). Averaged over the 13-week experimental period, the weekly change of CO2 concentration in the room was a 26 p.p.m. decrease.
After 13 weeks in the sealable room, all 41 plants were excavated so that growth of new roots could be ascertained for each plant (Table 3). Most new roots for all five species originated from the bases of the stems. A few, old, reddish-brown roots of Agave schottii developed new whitish extensions, and a few new roots developed from existing large roots of the cacti, especially Carnegia gigantea and Opuntia engelmannii. Mean new root production per plant varied considerably among the species, from fresh masses of less than 2 g for A. schottii to 78 g for C. gigantea (Table 3). Dry mass production per plant also varied, from 0·6 g for A. schottii to 25 g for O. engelmannii; the total dry mass gain for new roots of all plants of the five species was 598 g (Table 3). The relatively high dry mass : fresh mass ratio, averaging 0·36 ± 0·03 for the five species, reflects the woodiness and dead cortex observed for many of the roots. New shoot growth at 13 weeks among the four cacti was evident only for Cylindropuntia versicolor, which produced a few new stem segments that averaged 0·1 g dry mass per plant, a dry mass that was negligible compared with the total dry mass gain for the roots of all five species. Also, two new leaves unfolded from the central spike of leaves on two of the nine plants of A. schottii.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The five-species CAM community placed in a sealed room with day/night air temperatures of 25/15 °C had a net CO2 release for the first 2 weeks after planting and also when air temperatures were raised by 8 °C for 2 weeks, but otherwise had a net weekly uptake of carbon over the 3-month experimental period. Four weeks after planting and watering the five species, the instantaneous rate of net CO2 uptake at night measured for upper portions of shoot surfaces facing approximately south-east averaged 1·94 µmol m–2 s–1. On the other hand, measurements of community-level net CO2 uptake under essentially the same conditions during the next week yielded a net CO2 uptake rate of 0·61 µmol m–2 s–1 at night when expressed based on the total area of all shoot surfaces. Most of the discrepancy between net CO2 uptake for south-east-facing surfaces vs. community-level observations resulted from the orientation of the selected photosynthetic surfaces and the non-linear response between net CO2 uptake and total daily PPF (Geller and Nobel, 1987; Nobel, 1988).
When the net rate of CO2 uptake was measured directly on shoots 1 month after the winter solstice at 33° N (essentially the conditions that applied here), Nobel (1988) found the total daily PPF on clear days was 5 mol m–2 d–1 for surfaces facing north, 11 mol m–2 d–1 for surfaces facing north-east or north-west, 17 mol m–2 d–1 for surfaces facing east or west, 28 mol m–2 d–1 for surfaces facing south-east or south-west and 33 mol m–2 d–1 for surfaces facing south. Taking into consideration the reduction of PPF by structural supports in the room and by transmission of the glass as well as the effect of clouds, the PPF available to the shoot surfaces was 54 % less than these values. For Agave deserti, the barrel cactus Ferocactus acanthodes, and the platyopuntia Opuntia ficus-indica, species that are morphologically similar and taxonomically closely related to three of the five species considered here, no net CO2 uptake occurred when the total daily PPF was less than 3 mol m–2 d–1 (Nobel, 1988), as was the case for shoot surfaces facing north in the present experiment. Based on the non-linear response of nocturnal net CO2 uptake to total daily PPF for these three species, the surfaces facing south-east are predicted to have a nocturnal net CO2 uptake rate that is double the average for all directions. Thus, the average nocturnal rate of net CO2 uptake per unit shoot area is predicted to be about 0·97 µmol m–2 s–1, considerably above the measured community-level net CO2 uptake rate averaged over the shoot surface area (0·61 µmol m–2 s–1).
Net CO2 uptake measurements were made on younger portions of the shoots which, in agaves and cacti, tend to have higher net CO2 uptake rates than older parts of the shoots (Nobel, 1988). Also, release of respiratory CO2 from the soil would reduce the apparent community-level nocturnal net CO2 uptake rate averaged over the shoots from the actual mean shoot net CO2 uptake rate. Over the 3-month experimental period, a positive net CO2 uptake occurred, but its rate per unit shoot area was only 0·03 µmol m–2 s–1 when averaged over day and night-time. In this regard, the daytime loss measured directly on the shoots after 4 weeks averaged 0·64 µmol m–2 s–1. Such a daytime loss rate is relatively little influenced by PPF (Nobel, 1988) and so probably closely reflects a community-level daytime net CO2 loss rate for the shoots. Moreover, this rate is similar to that observed for the night-time net CO2 uptake on a community-level basis at 5 weeks (0·61 µmol m–2 s–1), helping to account for the relative small average net weekly CO2 uptake for the community, if the flux density of CO2 emanating from the soil is relatively small. Such a CO2 flux density from the soil represents growth and maintenance respiration for the roots as well as respiratory CO2 coming from other organisms in the soil.
The temperature responses for shoot net CO2 uptake have been determined for an agave and four species of cacti from arid or semi-arid regions whose morphology and taxonomy are similar to those of the five species used here, viz. A. deserti, F. acanthodes, O. ficus-indica (Nobel, 1988), Cylindropuntia acanthocarpa (Nobel and Bobich, 2002) and the columnar cactus Stenocereus queretaroensis (Nobel and Pimienta-Barrios, 1995). Optimal net CO2 uptake occurs at mean night-time air temperatures of 14–15 °C for these species. Compared with the optimum, nocturnal net CO2 uptake rates average 62 ± 6 % as much at mean night-time temperatures of 22 °C. By itself, a 38 % decrease in a mean nocturnal CO2 uptake rate of 0·61 µmol m–2 s–1 would lead to a 110 p.p.m. increase in CO2 concentration during 1 week, which is similar to the difference between the 52 p.p.m. weekly decrease that occurred for 4–7 weeks at 25/15 °C vs. the 66 p.p.m. weekly increase that occurred for a subsequent 2 weeks at 32/22 °C. Thus, the responses of nocturnal net CO2 uptake to elevated temperatures for the five CAM species acting as a community are consistent with the individual responses of five morphologically and taxonomically similar species studied previously.
Although the shoot surface area was similar for the five species placed in the sealed room, the fresh and dry masses of newly produced roots differed considerably. Agave schottii, which had more existing old roots at the time of planting than did the cacti, had by far the lowest mass of new roots. Because it exhibited a net CO2 uptake capacity similar to that for the four cactus species, its existing old roots were apparently able to take up water from the soil, consistent with root hydraulic conductivity measurements for A. deserti (Nobel and Sanderson, 1984). Specifically, rehydration of existing roots of A. deserti leading to appreciable water uptake can occur within 2 d of rewetting droughted plants (North and Nobel, 1998). On the other hand, most water uptake from the soil for the four species of cacti apparently depended on the newly produced roots.
Roots require carbohydrates both for growth respiration and for maintenance respiration as well as for carbon structurally incorporated into the roots (Amthor, 1989; Buwalda, 1993; Gregory et al., 1996; Lambers et al., 2002). The total root dry mass increase over the 13 weeks for the five species in the sealed room was 598 g. For A. deserti, F. acanthodes and O. ficus-indica, the carbon incorporated into root dry mass averaged 38 mol kg–1 (Nobel et al., 1992), so 23 mol C would be required in the present case, if the same conversion rate applies. At 20 °C (the average of day and night-time temperatures in the sealed room), growth respiration on a dry mass basis for young roots is 7·0 mol CO2 kg–1 for A. deserti, 9·4 mol CO2 kg–1 for F. acanthodes and 8·7 mol CO2 kg–1 for O. ficus-indica (Nobel et al., 1992), leading to an average of 8·4 mol CO2 kg–1; 598 g of roots would thus require 5·0 mol CO2 for growth respiration. When averaged over the 3-month experimental period and the total shoot surface area, structural carbon costs amount to 0·15 µmol CO2 m–2 s–1 and growth respiration amounts to 0·033 µmol CO2 m–2 s–1. At 20 °C, the maintenance respiration for young roots is 3 µmol kg–1 s–1 for A. deserti (Palta and Nobel, 1989a), 4 µmol kg–1 s–1 for F. acanthodes and 6 µmol kg–1 s–1 for O. ficus-indica (Palta and Nobel, 1989b), or an average of 4·3 µmol kg–1 s–1. Assuming for convenience of calculation that the root dry mass increased linearly with time over the experimental period and using this mean respiration rate for young roots, the maintenance respiration rate in the room would average 1·3 µmol CO2 s–1; expressed on the basis of shoot area, this corresponds to 0·071 µmol CO2 m–2 s–1. Hence, the growth plus maintenance respiration could be 0·033 + 0·071 or 0·10 µmol CO2 m–2 s–1.
Consistent with the first hypothesis, net CO2 uptake was evident on a community-level basis 3 weeks after watering the plants, and was confirmed by direct measurements on the shoots of all five species after 4 weeks. Consistent with the second hypothesis, CO2 fluxes for root growth plus maintenance respiration were small relative to the nocturnal net CO2 uptake rate for the shoots; in particular, root respiration averaged 0·10 µmol CO2 m–2 s–1, whereas the net CO2 uptake rate at night after 5 weeks was about 0·61 µmol CO2 m–2 s–1 at day/night air temperatures of 25/15 °C. A large fraction of the carbon taken up over the 3-month experimental period was hypothesized to be diverted to the roots. Based on the mean weekly decreases in room CO2 concentration of 26 p.p.m., the net carbon gain by the entire plant community over 3 months was 4·1 mol. Yet roots required an estimated 23 mol C for structural costs, in addition to the 16 mol C recycled by respiration. The shoots therefore had a net C loss of 23 minus 4, or 19 mol C over the 3-month period. Thus, the requirements of roots dominated carbon partitioning over the 3-month experimental period after placing bare-root specimens of the five CAM species in the soil. Assuming that the dry mass was about 14 % of the fresh mass, as is typical for shoots of CAM species under drought conditions (Nobel, 1988), a 19 mol shoot to root diversion of carbon corresponds to 1·0 % of the carbon initially in the shoots. Only after the plants develop sufficient roots to satisfy their water and nutrient needs, which on a dry mass basis generally means a root biomass equal to 6–10 % of the shoot biomass for mature agaves and cacti (Nobel, 1988), will the community begin to have net above-ground biomass productivity. A sealable room allows for the study of such root/shoot carbon balances for a CAM community and, by extension, can allow better understanding of the carbon costs for root development of other plant communities.
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ACKNOWLEDGEMENTS |
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This experiment was encouraged by Dr C. Barry Osmond and was financially supported by a grant from the Packard Foundation to the Biosphere 2 Center.
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LITERATURE CITED |
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