The objective of this study was to evaluate the effect of different CO2 concentrations on carbohydrate, chlorophyll contents and net photosynthetic productivity in selected clones of Eucalyptus (Eucalyptus camaldulensis) .Plants were exposed to double the atmospheric CO2 concentrations over a period of six months. The study revealed CO2-induced increases and differences in the response of different clones of E. camaldulensis. The results showed that elevated CO2 had a significant influence on all the biochemical parameters. A significant increase was observed in the biomass of the plants, though only shoot development was enhanced significantly as a result of elevated CO2. Root biomass was not affected by elevated CO2. Similarly, internal CO2 levels and stomatal conductance significantly varied suggesting that Ci might be an important determinant of photosynthetic acclimation in this species. It is inferred that among the four productive clones released for commercial use by IFGTB, clone was found to be the most resilient to elevated CO2 levels suggesting its planting over long periods of time.
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Keywords: Eucalyptus, biochemical parameters, biomass production, net photosynthetic productivity, OTC
Increase in atmospheric CO2 concentrations and the associated rise in temperature and precipitation patterns will have profound effects on terrestrial plant growth and productivity in the near future. According to the Intergovernmental Panel on Climate Change (IPCC, 2007), the preindustrial levels of carbon in the atmosphere rose from 285 μmol l-1 (600 gigatonnes (Gt)) to the current level of 384 μmol l-1 (800 Gt) and the predicted rise in the atmospheric CO2 would approach 1000 Gt by the year 2050. Such an abnormal rise in the levels of atmospheric CO2 would result in direct and indirect global climate changes. The increase in CO2 concentrations as well as other greenhouse gases, due to anthropogenic intensification, will result in an increase in global average temperatures which would further result in drastic shifts in the annual precipitation (Reddy and Gnanam, 2000; Chaplot, 2007). The alarming and unprecedented rise in the atmospheric concentration of greenhouse gases under global climate change warrants an urgent need to understand the synergistic and holistic mechanisms associated with plant growth and productivity.
Climate change affects plant growth and development primarily due to changes in photosynthetic carbon assimilation patterns. Physiological processes are the critical intermediaries through which heredity and environment interact to regulate plant growth. Tree species show high genetic variation in size, crown form, longevity, growth rate, cold hardiness, and tolerance to environmental stresses. Trees are subjected to multiple abiotic and biotic stresses that affect growth by influencing physiological processes. Environmental stresses set in motion a series of physiological disturbances that adversely affect growth. Indepth knowledge of the physiology of woody perennials provides deeper insights into the complexity and control of plant growth which supports useful application of this information in efficient measurements of these varied responses (Warrier, 2010).
The acclimatory responses of plants to the rapidly changing environment and understanding the potential impacts of multiple interacting factors (water availability, temperature, soil nutrition and ozone) have become a subject of debate over the past two decades. Conflicting reports on plant responses to elevated CO2, and several such differential photosynthetic responses, could be attributed to differences in experimental methods, plant species used for the experiments, age of the plant as well as duration of the treatment (Sage, 2002: Davey et al., 2006).
Effects of elevated CO2 on C3 photosynthetic rates have been the subject of many CO2 enrichment studies and have been reported in numerous papers. Most of these studies show that photosynthetic rate is increased following initial exposure to elevated CO2 (hours to days). Increases in photosynthetic rate are brought about by increased availability of CO2 at the chloroplasts and reductions in photorespiration resulting from an increased ratio of CO2 to O2 (Pearcy et al. 1987). Short-term exposure of C3 plants to elevated atmospheric CO2 concentrations often stimulates photosynthesis (Gifford, 1992), producing major gains in biomass as a result of the improved competitiveness of CO2 over O2 as a substrate for the main C3 photosynthetic enzyme, ribulose-l,5-bisphosphate carboxylase- oxygenase (Rubisco) (Bowes, 1993). Plants grown in elevated CO2 can show a degree of photosynthetic acclimation (Besford et al, 1990), i.e. an increase or more commonly a decrease in photosynthetic perform-ance as compared with plants grown in low (ambient) concentrations of CO2, when measured under the same conditions, due to intrinsic changes in the photosynthetic machinery (Gunderson and Wullschleger, 1994).
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However, many studies report that high photosynthetic rates are not maintained over long time periods and substantial reductions in photosynthesis (down-regulation) may occur within days to weeks after initial exposure to elevated CO2 (Long et al. 1993, Sims et al. 1998). Therefore, short-term measurements of photosynthetic rate may overestimate the potential for carbon assimilation of plants subjected to long-term exposure to elevated CO2 (Oechel and Strain 1985). Progress has been made in determining the biochemical and molecular mechanisms by which photosynthesis is down-regulated in response to elevated CO2. Photosynthetic down-regulation is characterized at the biochemical and leaf levels by reduced chlorophyll content, reduced Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) content and activity, limitations in RuBP and Pi regeneration, higher leaf mass/leaf area ratios and decreased leaf nitrogen concentration on a leaf mass basis (Sage 1994, Tissue et al. 1995).
Global climate change is predicted to change the growth conditions of forest trees. Short term experiments with Pinus pondrosa, Quercus coccinea, Pinus radiata and Populus deltoids have shown a definite increase in photosynthesis rate up to 40-80% under 600 ppm levels of CO2 (Couteaux et al, 1992). Devakumar et al. (1998) studied the effect of elevated CO2 concentration on growth and photosynthesis in two clones of Hevea brasillensisi. They reported higher biomass accumulation, leaf area and better growth when compared to ambient air grown plants. Thus adaptive variations are being reported in perennials with altered climatic conditions.
Rising atmospheric CO2 will directly affect forest plantation productivity by its impact on photosynthetic carbon fixation. While mature trees may not retain more carbon under elevated CO2 (Korner et al., 2005), fixation rates in young trees or seedlings grown in elevated CO2 have been shown to increase by up to 50% (Davey et al., 2006). However, there is expected to be considerable between and within species variation in responses to elevated CO2. Seedlings of the sub-tropical species E. grandis have been observed to grow at approximately four times the rate of seedlings grown under atmospheric CO2 levels (Conroy et al., 1992). Conversely, no response was observed in the arid zone species E. occidentalis (Southerton, 2007). It appears that fast growing coppice systems could be considerably more productive in elevated CO2, and could contribute to slowing the rate of rise in atmospheric CO2.
In India, studies on Open top Chambers (OTC) to understand the synergistic and holistic mechanisms associated with plant growth and productivity in relation to global elevated CO2 concentration was taken up as early as 1995. However, the major species which have been focused on for their response to elevated CO2 are only food crops which include rice and brassica (Upreti et al., 2000), castor bean (Ricinus communis L.) and blackgram (Vigna mungo), Greengram (Srivastava et al. 2001), sorghum and sunflower (Vanaja et al. 2006). Tree species have received very scant attention in this regard.
The plantation forests of India accounts for 17% of the global plantations, and is the second largest in the world after China. India is also the largest planter of Eucalypts in the world. Thus, this species demands attention in terms of productivity under varied climatic conditions. Our paper aims (1) to describe the effects of elevated CO2 on photosynthetic gas exchange behaviour of selected clones of Eucalyptus camaldulensis treated under open top chamber conditions, and (2) to relate the observed differences in photosynthetic CO2 uptake to underlying biochemical characteristics and assimilatory functions.
Materials and Methods
Materials: First generation provenance trials were established in ten different locations and about 100 clones of E. camaldulensis were selected, based on individual tree superiority for height, diameter at breast height and straightness of stem through index selection method. The clonal trials were established in three different locations, viz., Coimbatore (Tamil Nadu), Sathyavedu (Andhra Pradesh) and Kulathupuzha (Kerala). Thirty three clones across all the three trials were compared with 10 commercial clones and seed origin plants of Eucalyptus camaldulensis (3 entries) and E. tereticornis (2 entries) to prove clonal superiority. Top four clones namely IFGTB EC1, IFGTB EC2, IFGTB EC3 and IFGTB EC4 which showed consistent performance in all the three trials over the control were selected and released for commercial use. These four clones were selected for the present study (Tripati, 2011).
Methods: The selected four clones were grown inside the open top chambers (OTCs) 3 m diameter and 10 m height) lined with transparent PVC sheets (0.125 mm thickness). Pure CO2 gas was used for the enrichment. Rubber pipes with small holes throughout were circulated inside the OTC, which acted as the elevated CO2 environment and the same was connected to the gas cylinders containing pure CO2 gas. The flow of the CO2 was adjusted with a flow meter to get the exact concentration of CO2 (600 ±50 ïƒ¬ï€ mol mol-1). Similarly OTCs were used as control where the clones were grown under ambient CO2 (360 ïƒ¬ï€ mol mol-1). The clones were also grown in open field with ambient CO2 (360 ïƒ¬ï€ mol mol-1). The experiments were laid in a Complete Randomized Design. The period of CO2 enrichment was 180 days. A software facility called Supervisory Control and Data Acquisition (SCADA) was used to continuously control, record and display the actual and desired CO2 level, relative humidity and temperature in each OTC by feedback control loop passing through Programmable Logical Controllers (PLC) (Buvaneswaran et al., 2010).
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Measurements of photosynthesis and related parameters: The net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci) and transpiration rate (E) were measured with a portable photosynthesis system LI-6200 (LICOR, Inc, Lincoln, NE, USA). The measurements were taken between 9.30 am and 11.30 am under cloud free conditions. Three observations each from ten ramets per clone were recorded for all the physiological parameters. Water Use Efficiency (WUE) was also estimated from the clones. Intrinsic water use efficiency was estimated as the ratio of net photosynthetic rate to stomatal conductance (Pn/gs) whereas instantaneous water use efficiency was estimated as the ratio of net photosynthetic rate to transpiration (Pn/E). Intrinsic carboxylation efficiency was derived as the ratio of net photosynthetic rate to intercellular CO2 concentration (Pn/Ci). Intrinsic mesophyll efficiency was estimated as the ratio of intercellular CO2 concentration to stomatal conductance (Ci/gs).The leaves and stem portion were separated after the recording of the plant height and total number of leaves. All the plant parts were dried at 80oC for determining the dry mass. The fresh and dry masses of the plant samples were recorded.
Biochemical analysis: To determine chlorophyll content fully expanded leaf from top was collected at random from four ramets per clone and after cleaning the leaves were cut into small pieces, the pigments extracted in 80 per cent acetone, and measured colorimetrically with an UV-VIS spectrophotometer (Labtronics, India) at 645, 654 and 663 nm. Chlorophyll (a,b and total) contents in fresh mass basis were calculated using the method of Yoshida et. al. (1976). For soluble protein estimation fresh leaves were ground in a pre-chilled pestle and mortar with 1:2 (m/v) 50 mM phosphate buffer, pH 7.0. Homogenate was centrifuged at 40oC for 20 min. at 15000 g. This extract was used for estimating soluble protein following the procedure of Lowry et al. (1951). Total carbohydrates were extracted into solution by acidifying with 1.5 N HCl, and estimated colorimetrically using anthrone method (Hedge and Hofreiter, 1962). Free reducing sugars were estimated by method of Miller (1972). The data were analyzed by analysis of variance (ANOVA). Prior to statistical analyses, variables were checked for normality and transformed wherever necessary.
Results and Discussion
Growth: Exposure to elevated CO2 (600 ± 50 ïƒ¬ï€ mol mol-1) in open-top chambers increased the growth of Eucalyptus clones. Plant height and shoot (Stem and leaf) biomass increased in elevated CO2 grown plants significantly. Among the clones EC 1 attained maximum height (105.8 cm) followed by EC 4 (88.6 cm), EC 2 (73 cm) and EC 3 (63.9 cm) under elevated CO2 at the end of six months and it was 35 percent over the control (Fig. 1). The rate of growth and branching increased in some tree species exposed to elevated CO2 (Curtis and Wang, 1998). .
A significant increase in the leaf number was observed in the clones under elevated CO2 (Table 1). The number of leaves increased from 8.4 to 25.2 over the six months period, the maximum increase observed in clone EC 3. The shoot biomass increased significantly when the plants were grown under high concentration of CO2 (Table 1). Though there was an overall increase in the plant biomass (a total of the dry weights of the root, shoot and leaves), the root biomass did not significantly vary suggesting that CO2 enhanced the carbohydrate assimilation resulting in increased height and shoot biomass, while roots were not affected by elevated CO2 levels. This result supports the observations of Sharma and Sengupta (1990), which showed that the extra carbon fixed by the plants due to CO2 enrichment translocated towards the growing axis. In our experiment, high CO2 stimulated increased number of leaves per plant and allocation of more biomass to stem and leaves. According to Poorter et al. (1979) this pronounced increase in biomass is due to changes in leaf chemical composition, mainly due to the accumulation of total nonstructural saccharides. As leaf number increases, leaf area index (leaf area/land area) may also increase, resulting in higher carbon assimilation on an ecosystem level. Jach and Ceulemans (1999) found evidence for these responses in Pinus sylvestris seedlings grown at elevated CO2. In our finding also, we confirmed that the biomass in all the genotypes increased significantly when the species was subjected to elevated CO2 environment (Fig 2). Ceulemans et al. (1996) reported Poplar clones exhibited different and significant positive responses to elevated atmospheric CO2 resulting in increased investment in branch and leaf biomass. However, this is contrary to a recent report by Reddy et al., 2010 who has stated increased root volume in Gmelina, a tropical tree species, under elevated CO2.
IFGTB EC1 showed a steady and maximum increase in dry biomass which is in agreement with other results (Dev Kumar et al., 1998; Uprety et al., 2000; Vanaja et al., 2006). There was a 50 per cent increase in dry biomass with a 2-fold increase in CO2 level. Higher biomass production under OTC, with ambient CO2 was also observed in EC 3 and EC 4, which could be attributed to the marginal increase in the temperature in the chamber. Elevated CO2 thus stimulated total dry biomass accumulation in all the Eucalyptus clones. Steady increase of dry matter is a common physiological response to high CO2 concentration (Atkinson et al., 1997).
Photosynthetic parameters: There was substantial variation between clones in the extent and nature of alteration in photosynthetic characteristics. Net leaf photosynthetic rate of plants grown and measured at the elevated CO2 concentration was significantly decreased (P < 0.05) by about 25 to 60 per cent for Eucalyptus clones compared with that of plants grown and measured at ambient CO2 (Figure 3). The photosynthetic rates differed significantly between the clones (P < 0.05), with significant interaction between CO2 concentration and clones over a period of six months. Figure 3 demonstrates the A/Ci curve relation in Eucalyptus clones as a result of elevated CO2 levels. These parameters are commonly used when monitoring stress sensitive photosynthetic characteristics. The changes in PN under elevated CO2 are often associated with altered ribulose-1,5-biphosphate carboxylase/oxygenase content (Stitt, 1986).
Fig 3 demonstrates the changes observed in Eucalypus clones with the Pn-Ci curve under open field, ambient CO2 in OTC and OTC with elevated Co2 levels. The control clones have been depicted as empty circles, the clones in OTC under ambient CO2 as partially shaded, and the clones in elevated CO2 levels as fully shaded. All the four clones showed a reduction in the levels of Pn when subjected to elevated CO2, while Ci showed variations. Clones EC 2 and EC 3 were able to maintain a higher concentration of CO2 within tissues, while EC 1 and EC 4 had concentrations lower than those of the control. It was observed that there was a corresponding change in the stomatal conductance values also, association being depicted as significant negative correlation (r=-0.81**). Decreased PN during growth could be interpreted in terms of high CO2 induced transient inactivation of photosynthesis as a stress response (Lichtenthaler, 1996). The PN decreased under OTC (without elevated CO2) also in all the clones (Table 2). The stomatal conductance followed similar pattern as Pn. Stomatal conductance is of utmost importance when photosynthesis is concerned. Stomates play a pivotal role in controlling the balance between assimilation and transpiration (Beadle et al., 1981). According to Harley et al. (1992) stomatal conductance (gs) decreases in elevated CO2. The reduction in Pn under elevated CO2 occurred may be due to lower stomatal conductance, which also declined under elevated CO2 levels in EC 1 and EC 4 while it was higher and 100 per cent more in EC 2.
The role of stomata in determining the water use efficiency is well understood (Leverenz et al., 1999; Li, 2000). The genotypes that can maintain higher water use efficiency will have an efficient stomatal regulatory capacity (Maroco et al., 1997). Instantaneous WUE is estimated as the ratio of net photosynthesis rate to transpiration (Petite et al., 2000). Higher the value, better the efficiency of the plant to divert water for photosynthesis than transpiration. Measurement of WUE might be a useful trait for selecting genotypes with improved drought adaptation and biomass productivity under different environmental conditions (Li, 2000).In our study (Fig 4a), it was observed that though clones EC 2 and EC 3 had the highest WUE under control conditions, they showed poor WUE under elevated CO2 levels. The other two clones namely, EC 1 and EC 4 were able to demonstrate better WUE over the control under high CO2 levels, EC1 showing almost 200 per cent increase. Zhang and Marshall (1994) reported genotypic differences in long-term measures of instantaneous WUE among the native populations of Larix occidentalis. Though relatively higher WUE was noticed in Salix viminalis (Lindroth et al., 1996), water availability was identified as the critical factor in short rotation willow forestry. Tuomela (1997), studying the physiological and morphological responses of Eucalyptus microtheca provenances suggested that the efficient control of water loss was indicated by high instantaneous WUE. This suggests that EC 1 and EC 4 could be considered as efficient clones, especially under elevated levels of CO2 with reference to water utilization. The ratio of net photosynthesis rate to intercellular CO2 concentration is termed as intrinsic carboxylation efficiency (Hamerlynck et al., 2000). Higher the ratio, better the efficiency for carboxylation. In the present study, clones IFGTB EC 1 and EC 4 recorded the highest CE under elevated CO2 levels over the control (Fig 4b). This ratio varied between 0.001 and 0.030 µmol m-2 s-1 (µl l-1)-1. . The ratio of intercellular CO2 concentration (Ci) to stomatal conductance (gs) represents the intrinsic mesophyll efficiency (Sheshshayee et al. 1996). At a given stomatal conductance, lower Ci indicated better mesophyll efficiency and better draw down rate of the substrate CO2. It has been reported that drought tolerant cultivars of Morus alba exhibited greater mesophyll efficiency than the drought sensitive genotypes (Ramanjulu et al. 1998). In the present study, none of the clones showed an increase in the mesophyll efficiency under elevated CO2 conditions (Fig 4c).
Fig 4a. WUE as influenced by elevated Co2 in Eucalyptus clones
Many researchers have studied the physiological adaptations of eucalypts. Srivastava (1993) reported that Eucalyptus enhanced water holding capacity in the soil. There was more soil moisture under Eucalyptus than a nearby open area even after three consecutive drought years. Osorio and Pereira (1993) studied the effect of drought on productivity and WUE in E. globulus clones and reported that WUE was significantly increased by water deficit. Abbasi and Vinithan (1997) have established that Eucalyptus hybrid plantations do not deplete soil moisture. Kumar (1984) has refuted the allegation that Eucalyptus has a high transpiration rate. According to him, Eucalyptus has a low transpiration rate and it controls stomatal openings according to water availability without serious reduction in biomass production. Eucalyptus has the inherent capacity for luxury consumption of water when moisture is abundantly available. The high rate of transpiration reported in certain physiological studies on Eucalyptus is thus an adaptability mechanism operative under adequate soil moisture only (Srivastava et al. 2003).
Fig 4b. Intrinsic carboxylation efficiency as influenced by elevated CO2 in Eucalyptus clones
Fig 4c. Intrinsic mesophyll efficiency as influenced by elevated CO2 in Eucalyptus clones
Table 1. Variation in Plant height, dry weight and number of leaves in Eucalyptus clones at the end of six months as influenced by CO2
Plant Height (cm)
No. of leaves
78.8 ± 15.4
105.8 ± 17.6
86.8 ± 12.5
12.62 ± 5.02
16.06 ± 5.05
15.29 ± 4.12
23 ± 3.3
33 ± 10.4
24.2 ± 5.5
49.1 ± 4.9
64.2 ± 9.4
63.9 ± 8.0
8.95 ± 2.40
7.09 ± 2.00
7.44 ± 2.16
17.2 ± 4.7
21.4 ± 3.6
19.6 ± 2.6
65.8 ± 4.9
70.6 ± 10.9
73.0 ± 14.2
6.99 ± 0.78
11.22 ± 2.06
10.78 ± 1.58
29.6 ± 4.8
22.2 ± 6.4
20.6 ± 4.5
41.6 ± 7.4
80.2 ± 24.4
88.6 ± 12.6
5.59 ± 0.98
15.58 ± 19.92
14.29 ± 3.82
19.2 ± 5.7
35.6 ± 2.7
19.4 ± 6.1
T = 6.52
C x T =5.33
C x T =3.28
C x T=1.69
OPEN = Ambient condition, OTC =Open top chamber with ambient CO2, OTC+CO2 = Open top chamber with elevated CO2,
T = Treatment, C = Clone, Values significant at p<0.05 level
Table 2. Variation in photosynthetic parameters in Eucalyptus clones at the end of six months as influenced by CO2
OPEN = Ambient condition, OTC =Open top chamber with ambient CO2, OTC+CO2 = Open top chamber with elevated CO2,
T = Treatment, C = Clone, Values significant at p<0.05
Table 3. Variation in photosynthetic pigments in Eucalyptus clones at the end of six months as influenced by CO2
Chlorophyll a:b ratio
Biochemical parameters: Elevated CO2 positively influenced the accumulation of photosynthetic pigments in all the clones with an increase in chlorophyll a, b and total chlorophyll over the control. This suggests an increase in efficiency of radiant energy capture through a shift in carbon allocation. There was a significant variation in the chlorophyll a: b ratio also both amongst the clones and as a result of elevated CO2. There was substantial variation between the clones in the extent and nature of alteration in photosynthetic characteristics. This is demonstrated in Fig. 5 (PN and Chl a/b). It was observed that though there was a reduction in the photosynthetic rate, the trend observed was the same in all the four clones. Another significant observation was that the reductions in photosynthetic rate due to elevated CO2 did not significantly impact the carbon gains being made, as plants exposed to elevated CO2 had approximately twice the biomass of plants grown at ambient CO2. This implies that leaves grown at high CO2 can capture the photons for photosynthesis similar to ambient CO2 conditions and may be able to overcome this physiological stress with time.
Fig 5. Chlorophyll a/b ratio plotted against the net photosynthetic rate, Pn
The accumulation of soluble protein in the oat leaves decreased under elevated CO2 in all the clones except EC1 where significant increase was recorded under elevated CO2 levels. Similarly, under OTC at ambient CO2 all the clones except EC4 showed a significant decrease in the soluble protein content of the leaves. Several reports have shown a decline in soluble proteins of leaves grown in elevated CO2 (Campbell et al., 1988; Stitt, 1991; Akin et al., 1995). Leaf total carbohydrate content and the reducing sugar levels in the clones decreased significantly on exposure to elevated CO2 levels in the clones EC 2 and Ec 3. Clones EC 4 and EC 1 showed significant difference in total soluble sugar content between the CO2 treatments (Figure 6b). Two-way ANOVA showed a significant (P < 0.05) interaction between clones and elevated CO2 for total carbohydrate content and free reducing sugar levels.
Fig 6a. Soluble proteins as influenced by elevated CO2 in Eucalyptus clones
Fig 6b. Total carbohydrates and reducing sugars as influenced by elevated CO2 in Eucalyptus clones
In trees, elevated CO2 can increase total leaf area (Koch et al. 1986), leaf weight (Brown and Higginbotham 1986, Norby and O'Neill 1989), leaf weight to area ratio (Conroy et al. 1986, Berryman et al. 1993, Pettersson et al. 1993), and branching frequency (Sionit et al. 1985, Samuelson and Seiler 1993). Root biomass, root length, root branching and lateral root production are also reported to increase in response to elevated CO2 (Rogers et al. 1994, Day et al. 1996, Janssens et al. 1998). Because elevated CO2 enhances photosynthetic rates in tropical and sub-tropical trees, it should also lead to increased carbohydrate and biomass production in these species. At a tropical forest research site in Panama, twice-ambient CO2 concentrations enhanced foliar sugar concentrations by up to 30 percent (Wurth et al., 1998), while doubling the foliar concentrations of starch (Lovelock et al., 1998) in a number of tree species. In the eight-month study of Roden et al. (1999), Eucalyptus pauciflora seedlings growing at 700 ppm CO2 displayed seasonal rates of net photosynthesis that were approximately 30 percent greater than those exhibited by their ambiently grown counterparts. In another eight-month study, Palanisamy (1999) reported that well-watered Eucalyptus cladocalyx seedlings exposed to 800 ppm CO2 exhibited photosynthetic rates that were 120 percent higher than those observed in control plants growing at 380 ppm CO2. Moreover, after a one-month period of water stress, photosynthetic rates of CO2-enriched seedlings were still 12 percent greater than rates displayed by ambiently grown water-stressed seedlings.
Because elevated CO2 enhances photosynthetic rates in eucalyptus species, this phenomenon should lead to increased biomass production in these rapidly growing trees. And so it does. In the eight-month experiment of Gleadow et al. (1998), for example, Eucalyptus cladocalyx seedlings growing at 800 ppm CO2 displayed 134 and 98 percent more biomass than seedlings growing at 400 ppm CO2 at low and high soil nitrogen concentrations, respectively. Similarly, Eucalyptus pauciflora seedlings growing at twice ambient CO2 concentrations for eight months produced 53 percent more biomass than control seedlings (Roden et al., 1999). After the first six weeks of the study, the plantlets grown in air of elevated CO2 concentration exhibited an average net photosynthetic rate across all media treatments that was 26% greater than that displayed by plantlets grown in air of 400 ppm CO2. This phenomenon led to a 23% increase in CO2-enriched plantlet total dry weight across all media treatments. In addition, after the final four weeks of growth in air maintained at 400 ppm CO2, the plantlets that were previously exposed to air of 1200 ppm CO2 displayed survival percentages that were 13% greater than those of plantlets previously grown in ambient air. As the air's CO2 content continues to rise, Eucalyptus plantlets - and perhaps recently germinated seedlings - will probably display enhanced rates of photosynthesis and biomass production. Thus, young Eucalyptus trees will likely sequester ever more carbon within their woody tissues as time progresses (Kirdmanee et al., 1995).
As suggested by Gleadow et al. (1998), with increasing atmospheric CO2 concentration, this Eucalyptus species will probably experience photosynthetic down regulation without significantly affecting the growth stimulation brought about by elevated CO2. This phenomenon would thus lead to larger Eucalyptus trees with better-developed root systems. In addition, increasing levels of CO2 would allow batter allocation of the nitrogen allocation for mobilization into leaf defence components so that this species can maintain a stable degree of protection as the CO2 content of the air rises ever higher.
More research is underway to understand the clonal response to increasing CO2 concentration and the consequent molecular changes occurring towards adaptability to changing CO2 levels.