JXB Advance Access originally published online on February 5, 2007
Journal of Experimental Botany 2007 58(6):1313-1320; doi:10.1093/jxb/erl296
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RESEARCH PAPER |
Effects of carbon dioxide and oxygen on sapwood respiration in five temperate tree species
1Rowland Institute at Harvard, 100 Edwin H. Land Boulevard, Cambridge, MA 02142, USA
2Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA
* To whom correspondence should be addressed. E-mail: spicer{at}rowland.harvard.edu
Received 26 June 2006; Revised 8 November 2006 Accepted 4 December 2006
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Abstract |
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The gaseous environment surrounding parenchyma in woody tissue is low in O2 and high in CO2, but it is not known to what extent this affects respiration or might play a role in cell death during heartwood formation. Sapwood respiration was measured in two conifers and three angiosperms following equilibration to levels of O2 and CO2 common within stems, using both inner and outer sapwood to test for an effect of age. Across all species and tissue ages, lowering the O2 level from 10% to 5% (v/v) resulted in about a 25% decrease in respiration in the absence of CO2, but a non-significant decrease at 10% CO2. The inhibitory effect of 10% CO2 was smaller and only significant at 10% O2, where it reduced respiration by about 14%. Equilibration to a wider range of gas combinations in Pinus strobus L. showed the same effect: 10% CO2 inhibited respiration by about 15% at both 20% and 10% O2, but had no net effect at 5% O2. In an extreme treatment, 1% O2+20% CO2 increased respiration by over 30% relative to 1% O2 alone, suggesting a shift in metabolic response to high CO2 as O2 decreases. Although an increase in respiration would be detrimental under limiting O2, this extreme gas combination is unlikely to exist within most stems. Instead, moderate reductions in respiration under realistic O2 and CO2 levels suggest that within-stem gas composition does not severely limit respiration and is unlikely to cause the death of xylem parenchyma during heartwood formation.
Key words: Anoxia, carbon dioxide, heartwood, hypoxia, oxygen, parenchyma, respiration, sapwood, senescence, xylem
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Introduction |
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In bulky organs or tissue with high diffusive resistance (e.g. fruits, tubers, woody stems, seeds with thick seed coats), oxidative respiration may deplete O2 and raise CO2 such that the internal gas composition differs substantially from that of ambient air. The gaseous environment within woody stems is enriched in CO2 and depleted in O2 as a result of the net effects of respiration, exchange with the atmosphere via diffusion through bark, and exchange with the transpiration stream through the dissolution of gases. That high CO2 inhibits respiration in woody stems has been suggested, but never tested (Carrodus and Triffett, 1975; Eklund, 1990, 1993), and evidence for the ability of CO2 to accelerate or induce heartwood formation is mixed (Eklund and Klintborg, 2000). Similarly, the low O2 levels within stems have been postulated, but never demonstrated, to limit respiration in woody tissue and lead to parenchyma cell death during heartwood formation (Eklund and Klintborg, 2000; Sorz and Hietz, 2006).
Within-stem CO2 concentrations range from less than 1% to greater than 10% and show both temporal and spatial variation. Continuous measurements made with infrared gas analysers and CO2 electrodes suggest diurnal fluctuations of within-stem CO2, with a maximum (6–8%) at the end of the dark period and a minimum (3–4%) during times of peak sap flow (McGuire and Teskey, 2002; Teskey and McGuire, 2002). Seasonally, within-stem CO2 shows a bimodal peak of 6–10% both early and late in the growing season (i.e., peaks of 6–10% were observed in June and October with values around 4% in the intervening months), but remains low (
1%) throughout dormancy (Eklund, 1990, 1993; Pruyn et al., 2002b). Within-stem CO2 also varies spatially, and likely increases from the cambium toward the sapwood/heartwood boundary (Pruyn et al., 2002b).
High concentrations of CO2 inhibit respiration in isolated mitochondria (Gonzalez-Meler et al., 1996), but there is considerable disagreement over the effects of CO2 on respiration at higher scales. At concentrations plausible in vivo (but uncommon in most tissues, e.g. 
5%), CO2 directly inhibits several glycolytic enzymes as well as cytochrome c oxidase and succinate dehydrogenase (Miller and Evans, 1956; Palet et al., 1991; Gonzalez-Meler et al., 1996). Although the mechanism for this is not known, likely candidates include reversible protein carbamylation and the action of bicarbonate as a competitive inhibitor (Drake et al., 1999; Gonzalez-Meler and Taneva, 2005). The extremely high levels of CO2 (2–20%) that can build up in bulky organs like fruits and tubers typically inhibit respiration (Kerbel et al., 1988; Gunes et al., 2001), and the use of high CO2 in combination with low temperatures and/or low O2 is a common horticultural practice during fruit storage and transportation. There are, however, a few reports of high CO2 levels (10%) increasing respiration (Day et al., 1978; Perez-Trejo et al., 1981). Other factors that may contribute to disagreement in the literature include differential effects of CO2 on the cytochrome and alternative oxidase pathways (Palet et al., 1991; Reuveni et al., 1995), and the potential for ‘dark’ CO2 fixation (i.e. via PEP carboxylase) to lower the rate of apparent CO2 production artificially (Smart, 2004).
Studies on the effects of elevated atmospheric CO2 (e.g. up to 2000 ppm, or 0.2% v/v) on plant respiration suggest little to no direct inhibitory effect at the tissue level (Gonzalez-Meler and Taneva, 2005), noting that early reports of respiratory inhibition in leaves and whole shoots included serious measurement artefacts (Jahnke and Krewitt, 2002). Reports of reduced root respiration under high levels of CO2 within the soil (0.1–1.0% v/v; McDowell et al., 1999) have been similarly questioned (Burton and Pregitzer, 2002).
Oxygen levels within woody stems are at a minimum during the growing season (2–8% v/v) and closer to atmospheric levels (15–20%) throughout dormancy (Eklund, 1990, 1993; Pruyn et al., 2002b). Diurnal patterns of O2 are more variable than those of CO2, but the O2 concentration of xylem sap reaches a minimum in the absence of transpiration and a maximum during times of peak flow (del Hierro et al., 2002; Mancuso and Marras, 2003), suggesting transpiration serves as a source of O2 (but see Gansert, 2003). Oxygen is progressively depleted toward the sapwood/heartwood boundary, but may be lowest near the cambial zone during the early part of the growing season (Pruyn et al., 2002b; van Dongen et al., 2003; Spicer and Holbrook, 2005). The lowest O2 concentrations reported for woody tissue are about 3–5% and may occur in both outer and inner sapwood (Eklund, 1990; Mancuso and Marras, 2003; Spicer and Holbrook, 2005).
The effects of low O2 on respiration are relatively clear: respiration decreases with decreasing O2 concentration, and this defines a ‘hypoxic’ range (0.5–5% for many tissues, but note that ‘hypoxic’ simply implies ‘O2 deficient’ and is often used loosely). However, the O2 level at which inhibition is first seen and the rate at which respiration declines with decreasing O2 depend on the tissue type, its history, and the scale of measurement. Both cytochrome oxidase and alternative oxidase have very high affinities for O2 (Km for O2=0.14 and 1.7 µmol, respectively; Millar et al., 1994), suggesting that O2 levels must fall well below 0.1% before the reaction terminating mitochondrial electron transport becomes substrate-limited. Inhibition of respiration is seen at roughly 0.01–0.1% O2 in plant mitochondria (Rawsthorne and LaRue, 1986; Millar et al., 1994), 0.5–2.5% in protoplasts (Lammertyn et al., 2001), 10% in roots grown in air (Saglio et al., 1984), and at 10–20% in tuber slices (Geigenberger et al., 2000).
The observation that apparent sensitivity to low O2 increases with increasing metabolic rate and/or increasing tissue size is often attributed to diffusion limiting the rate of substrate supply (Armstrong et al., 1994; Bidel et al., 2000; Lammertyn et al., 2001). More recently, it has become clear that the inhibition of respiration at only mildly hypoxic O2 levels (e.g. 5–10%) is often the result of regulated, adaptive changes in metabolism that prevent the onset of anoxia (Geigenberger, 2003b). These include reduced rates of biosynthesis, shifts to more ATP-conservative biochemical pathways, a decrease in adenylate energy status, and a regulated inhibition of glycolysis (Geigenberger et al., 2000; Geigenberger, 2003a; van Dongen et al., 2003). The expectation is that these metabolic shifts prevent the severe drop (>90%) in efficiency of ATP production and dangerous byproducts associated with anaerobic/fermentative metabolism.
In the tissue of bulky organs where diffusion is limiting, low O2 will be accompanied by high CO2, as is the case in large woody stems. Little is known about the interacting effects of O2 and CO2 on respiration, as inhibition due to high CO2 has been shown both to increase (Kidd, 1916) and to decrease (Lammertyn et al., 2001) with decreasing O2 concentration. The primary goal of this study was to ask whether the low O2 and/or high CO2 levels common in woody stems could limit parenchyma respiration to such an extent that internal gas composition might lead to the death of parenchyma during the transition from sapwood to heartwood. Secondary objectives were (i) to measure rates of sapwood respiration at physiologically relevant gas compositions, (ii) to determine whether the effects of O2 and CO2 on respiration vary with species or tissue age, and (iii) to consider whether the air volume fraction of sapwood is a determinant of the effects of either O2 or CO2. Results of this work should improve our understanding of parenchyma cell physiology by considering how the local environment within stems affects metabolism.
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Materials and methods |
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Plant material
Fresh sapwood was sampled from ten trees each of two conifers [Tsuga canadensis (L.) Carr and Pinus strobus L.] and three angiosperms (Acer rubrum L., Fraxinus americana L., and Quercus rubra L.). Trees were randomly selected from populations within a 5 ha tract of natural forest in Harvard Forest, Petersham, MA, USA (42.5° N lat, 72° W long, 220 m elev). Sapwood cores were extracted with a 12 mm increment borer at 1.4 m above ground to variable depths, but deep enough to reach the darker coloured heartwood in all cases. Four increment cores were taken per tree and randomly assigned to one of four gas treatments (see below). Two trees per species were sampled on each of five dates from June to July 2003, and stem diameter, sapwood age, and sapwood depth were recorded (Table 1). The phloem and cambium were removed with a razor blade immediately following core extraction. Increment cores were then wrapped in moist paper towel and stored in a cooler (
5 °C) for transport to the laboratory.
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Based on the results from the experiment described above, a test of the effect of CO2 on sapwood respiration across a wider range of O2 concentrations was included. For this experiment, the outermost sapwood of Pinus strobus was sampled by extracting two 12 mm diameter cores from each of 40 trees (over-bark diameter 36.6±1.2 cm; mean ±SE) to a sapwood depth of 2–3 cm. Cores were collected in August 2003 and treated as described above.
Oxygen and carbon dioxide treatments
Fresh tissue was sampled from the outermost (youngest) and innermost (oldest) sapwood by removing two approximately 1 cm3-cylinders from each increment core and lightly shaving the edges with a razor blade to remove any microbial contamination. The outermost position was adjacent to, but not including, the cambial zone (i.e. the cambium and developing xylem were removed with a razor blade); the innermost position was adjacent to, but not including, any heartwood (as judged by the colour change, which is a reliable indicator of the presence of live cells; Spicer and Holbrook, 2005). The white-coloured ring that is transitional between sapwood and heartwood in conifers was also excluded, when present, from the innermost samples of Pinus strobus and Tsuga canadensis. Distance and ring number from the cambium were recorded for the outer and inner ends of each sample, and the mean of these two ends was used to define each sample's age and position (Table 2).
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Four gas treatments in a factorial design (two levels of O2 and two levels of CO2) were randomly assigned at the level of the increment core such that each tree served as an experimental block containing all four treatments. Prior to O2 uptake measurements, samples were equilibrated under certified gas mixtures of 5% O2+0% CO2, 10% O2+0% CO2, 5% O2+10% CO2, and 10% O2+10% CO2 (v/v; balance N2). Each sample was weighed, wrapped in a 2 cmx5 cm strip of moistened cheesecloth, and placed in an open 10 ml glass vial. The open vials were enclosed in gas-impermeable, disposable glove bags and flushed repeatedly with the treatment gas mixture.
Samples were allowed to equilibrate in sealed glove bags for a total of 36 h with additional flushes of the treatment gas at 12 h and 24 h, with a final flush at 36 h before sealing the vials. The first 24 h of equilibration were at 4 °C followed by 12 h at room temperature (20 °C). Prior tests had shown that 36 h was sufficient to equilibrate the samples fully so that the internal gas composition matched that of treatment gas, such that drawdown of O2 from the gas headspace was linear. Rough calculations using a worst-case scenario diffusion coefficient for woody tissue (i.e. Dm=1x10–10; Sorz and Hietz, 2006) also suggested that 36 h was sufficient for equilibration, assuming an initial concentration difference of about 10% (v/v) between sample and headspace. Equilibrating at a low temperature suppressed respiration and extended the length of time respiration rates remained stable after a return to 20 °C. Returning the samples to 20 °C for the final 12 h of incubation allowed respiration rates to stabilize in response to temperature before measurements began. Samples from two trees of each species were equilibrated in each glove bag per sample date. Two trees from the 26 June sampling in the 5% O2+10% CO2 treatment were removed from the experiment because of a punctured glove bag.
Following equilibration and the final flush of treatment gas, vials were sealed inside the glove bags with crimp-top lids each containing a butyl rubber septum. Sealed vials were then transferred to a water bath where they were incubated at 20 °C throughout the measurement period. Oxygen was measured in each vial every 4–6 h for 24–36 h by penetrating the septum with a needle-tipped, fibre-optic O2 probe (Ocean Optics Inc., USB2000 spectrometer), which was calibrated with humidified standards at 20 °C. The slope of the linear portion of the curve (typically the first 12–24 h) was taken as the rate of O2 consumption and converted to mol h–1 by calculating the total volume of gas in the vial. Increases in CO2 within the vials during this time did not affect measurements: gas samples extracted with a syringe from 15 vials at the end of the measurement period were injected into a flow-through IRGA and suggested that final CO2 concentrations were 3%; similarly, samples incubated in vials suspended above soda lime respired at the same rates as matched controls. Following measurements, samples were measured for fresh volume by volume displacement, dried to a constant weight at 105 °C (>48 h) and weighed. The proportion of fresh volume occupied by cell wall, water, and air was calculated assuming a density of cell wall material of 1.5 g cm–3.
Samples of outer sapwood from Pinus strobus were located adjacent to the cambium as described above and treated identically, with the following exceptions. To test the effect of CO2 over a wider range of O2, 30 trees (i.e. 30 pairs of cores) were each randomly assigned to one of three O2 treatment levels (5%, 10%, 20%), with one core from each pair assigned to either 10% or 0% CO2. As an additional ‘worst case’ treatment (i.e. the most severe gas composition likely to exist with in the stem), the remaining ten trees were assigned to a 1% O2 treatment, with one core from each pair assigned to either 20% or 0% CO2.
Statistical analyses
The effects on respiration of O2, CO2, species and radial position within the stem were tested in a repeated measures ANOVA that included tree as a blocking factor, species as a ‘between subject’ factor, and radial position as a ‘within subject’ factor (i.e. radial position was a spatially repeated factor within each tree). Means of significant O2 and CO2 treatments were compared with Bonferroni-adjusted P-values to minimize the experiment error rate. The effects of species and radial position on the proportion of sapwood volume occupied by cell wall, water, and air were also tested in a repeated measures ANOVA, using the mean of all four cores per tree for each radial position. Contrasts were then used to test for a difference in air content between inner and outer sapwood within each species (i.e. to make a priori comparisons among the significant radial positionxspecies terms). In Pinus strobus outer sapwood, the per cent inhibition by 10% CO2 (relative to 0% CO2) was tested at each level of O2 with paired t tests.
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Results |
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Oxygen level had a significant effect on sapwood respiration (P value <0.0001), but that effect depended on the level of CO2 (i.e. there was a significant O2xCO2 interaction, P value=0.02; Table 3). Lowering the O2 level from 10% to 5% resulted in a significant, roughly 27% decrease in respiration in the absence of CO2, but a non-significant decrease at 10% CO2 (Fig. 1). High CO2 (10%) had a smaller inhibitory effect, but only at 10% O2, where it reduced respiration by about 14%. The interaction between O2 and CO2 was the same for all five species and at both sapwood ages/radial positions (both P values >0.5; Table 3).
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Species differed in the effect of radial position on respiration (interaction P value <0.0001; Tables 3, 4). Pooled across all gas treatments, respiration in inner sapwood was reduced by about 40% and 20% relative to outer sapwood in Acer rubrum and Quercus rubra, respectively (contrast P values <0.0001). Fraxinus americana showed only a marginally significant reduction in respiration of about 10% (contrast P value=0.06). In Tsuga canadensis, respiration rates were the same for both sapwood positions, whereas Pinus strobus showed a small (15%) but significant increase in respiration in the inner sapwood (contrast P value=0.03).
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The inhibitory effect of low O2, but not high CO2, differed among species and radial position (interaction term P value=0.02; Table 3). In most cases there was a 20–25% decrease in respiration under 5% relative to 10% O2 (Fig. 2). One exception was in the inner sapwood of Fraxinus, where 5% O2 had no effect. There were also only small, non-significant decreases in respiration under 5% O2 in the inner sapwood of Acer and the outer sapwood of Quercus.
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The proportion of sapwood volume occupied by cell wall did not differ with radial position, but the extent to which the remaining volume was filled with either water or air differed across species and with radial position (Table 5). Both Tsuga and Pinus had significantly greater proportions of air in the inner sapwood (contrast P values <0.001 and 0.02, respectively; Fig. 3), while Acer, Fraxinus, and Quercus had similar proportions of air in inner and outer sapwood (contrast P values
0.15; Fig. 3).
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In the outer sapwood of Pinus, the effect of high CO2 changed across O2 levels. At both 20% and 10% O2, there was about a 15% inhibition of respiration by 10% CO2 (Fig. 4a), but no inhibition at 5% O2. In the most extreme treatment, 1% O2 and 20% CO2, there was actually an increase in respiration of over 30% (P value 0.02; Fig. 4a). Respiration decreased by about 50% when O2 was lowered from 20% to 5%, with a smaller reduction under 10% CO2 (45% inhibition, versus 54% without CO2).
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Discussion |
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Oxygen had a more pronounced inhibitory effect on sapwood respiration than CO2, and its effect was roughly the same across five species that differ widely in sapwood structure, age and physiology. In the absence of significant levels of CO2, reducing O2 concentration from 10% to 5% resulted in about a 27% decrease in respiration. This decrease was cut roughly in half in the presence of 10% CO2. These values represent a realistic drop in O2 that could occur spatially, or even diurnally within the stem, with 5% O2 being a reasonable estimate of ‘low’ O2 in sapwood (Spicer and Holbrook, 2005). The observed reduction in respiration was moderate in the absence of CO2, and at more realistic CO2 levels there was an even smaller decrease, suggesting that internal gas composition in woody stems does not place any serious limits on respiration.
The inhibitory effect of CO2 on respiration was surprisingly small, and strongly dependent on the level of O2. 10% CO2 reduced respiration across all species, but only by about 15% when measured at 10% O2 (and at 20% O2 in Pinus strobus), and this inhibition disappeared at 5% O2 (Figs 1, 4a). A similar decrease in inhibition by CO2 with decreasing levels of O2 was shown in pear fruit (Lammertyn et al., 2001). The large variation in percentage inhibition by 10% CO2 at 5% O2 reflects the pairing of treatments within each tree, and the fact that respiration was just as likely to increase as it was to decrease under high CO2 (note that this within-tree pairing is not reflected in the overall treatment means; Fig. 4b). This was not true at higher levels of O2, where 10% CO2 reduced respiration between paired samples in all but a few cases. This fact, coupled with the observed increase in respiration at 1% O2 and 20% CO2, suggests a metabolic shift in the response to high CO2 with decreasing O2.
Two alternative (although not mutually exclusive) hypotheses are proposed to explain the observed variation in sapwood response to high CO2. First, increases in respiration found under high CO2 in tubers and callus tissue have been attributed to increasing engagement of the alternative oxidase pathway, as the cytochrome oxidase pathway is increasingly inhibited (Day et al., 1978; Perez-Trejo et al., 1981; Palet et al., 1991). Although the functional significance of the alternative pathway is not well understood, there is evidence that these two pathways compete for electron flow, with the relative contribution of the alternative pathway increasing under conditions that ‘over-reduce’ the ubiquinone pool (e.g. inhibition of the cytochrome pathway). If CO2 in the form of bicarbonate acts as a competitive inhibitor of cytochrome oxidase (Gonzalez-Meler et al., 1996), then combined levels of very high CO2 and low O2 might cause a marked increase in the alternative pathway.
Alternatively, the combination of very high CO2 and low O2 could trigger a sharp rise in glycolysis (and apparent respiration rates) if anoxic conditions develop within the tissue and lead to fermentative metabolism. Anoxic conditions might develop more rapidly under very high CO2 due to its proposed action as a competitive inhibitor, effectively increasing the Km of cytochrome oxidase. Up-regulation of alcohol dehydrogenase and accumulation of ethanol can occur in response to O2 depletion in woody tissue (Kimmerer and Stringer, 1988; Joseph and Kelsey, 2004). The induction of enzymes associated with fermentative metabolism prior to the onset of anoxia (e.g. at 1–4% O2; van Dongen et al., 2003) may also explain variation in the response to high CO2 at 5% O2. Future work with inhibitors of the alternative and cytochrome pathways, oxygen isotopes (18O), and gene expression profiling under conditions of low O2 and high CO2 will all help to develop a mechanistic understanding of these responses.
The over 35% increase in respiration observed at 1% O2 and 20% CO2 is clearly not sustainable and would quickly deplete any remaining O2 in the tissue. Although CO2 concentrations as high as 12–15% have been reported within stems (McGuire and Teskey, 2002, 2004), these are maxima and likely transient values. Similarly, although within-stem O2 levels as low as 2% have been reported (Eklund, 2000; Mancuso and Marras, 2003), it is unlikely that this deficit would be matched by a corresponding increase in CO2 (i.e. 19%, assuming a respiratory quotient of 1) given the steep concentration gradient driving diffusion of CO2 out of the stem. In addition to diffusion through the bark, within-stem CO2 concentrations will be reduced by axial transport of CO2 in the transpiration stream (Teskey and McGuire, 2002), and possibly through non-photosynthetic carboxylation reactions within stem tissue itself (Höll and Meyer, 1977; Smart, 2004). This ‘dark-fixation’ of CO2 via PEP carboxylase is a tantalizing mechanism to avoid dangerously high levels of CO2, although the capacity to produce and store organic acids may be limited by the cell's ability to buffer pH. It is not known whether ‘dark fixation’ plays any role in the apparent low (<1) respiratory quotient for sapwood (i.e. sapwood is typically more depleted in O2 than it is enriched in CO2).
The fact that respiration was reduced by a drop from 20% to 10% O2 in Pinus outer sapwood suggests either that diffusion was limiting or that respiration was actively down-regulated, possibly as an adaptive response to minimize further O2 consumption. Most likely it is a combination of the two, although to what extent excised tissue has the ability to sense and respond to O2 levels is not known. The variation in response to high CO2 at a range of O2 levels underscores the fact that respiration is a highly regulated metabolic process, and may reflect regulatory shifts in response to decreasing O2 levels (e.g. to conserve ATP as O2 levels decrease; Geigenberger, 2003b).
The rate of gas diffusion through plant tissue is fundamental to any measure of respiration (see Armstrong et al., 1994; Bidel et al., 2000; Lammertyn et al., 2001, for interesting approaches), and it is not known to what extent O2 diffusion through woody tissue might limit parenchyma respiration. Although it is tempting to think that diffusion coefficients could be estimated by the relative proportions of cell wall, water, and air, our results suggest that the geometric configuration of these components is also important. If the air content of fresh sapwood were a strong determinant of rates of diffusion, we would expect to see variation in the sensitivity of respiration to low O2, such that higher air content (i.e. faster O2 supply via diffusion) resulted in reduced sensitivity. This was not found to be the case: inner and outer sapwood responded identically to reduced O2 in both Pinus and Tsuga despite a difference in air content of over 30%. To model the effects of gas composition on plant respiration in a manner relevant to whole plant physiology, it will be important to have good estimates of diffusion coefficients for a range of tissue types, including secondary xylem, in which there is also likely to be spatial variation (i.e. bulk density and volumetric air fraction both vary spatially, and diffusional barriers are formed by bordered pit aspiration in conifers and tylosis formation/gum excretion in angiosperms).
Although the response among species to O2 and CO2 was similar, there were large species differences in the effect of radial position/age on respiration (Tables 3, 4). Previous work suggests a decline in respiration with sapwood age (Pruyn et al., 2002a, b), but this was not found to be the case in Pinus or Tsuga, and, in general, the age difference between inner and outer sapwood was not predictive of respiratory differences. These observations suggest that spatial patterns of respiration within stems are not the result of tissue age per se, but instead reflect physical position (e.g. proximity to meristematic tissue, foliage, carbohydrate supply) and/or regulatory cues.
In summary, although the levels of O2 and CO2 found inside woody stems can reduce the rate of sapwood respiration, the effects are moderate and unlikely to be related to cell death in the transition from sapwood to heartwood. Despite the extremely high levels of CO2 found in stems, the effect of low O2 is dominant and may, in some cases, be mitigated by high CO2. We previously found that sapwood parenchyma can remain alive at 1% O2 for over 72 h, and quickly returned to pre-hypoxic respiration rates upon the re-introduction of 10% O2 (Spicer and Holbrook, 2005). Future work should focus on determining the spatial and temporal patterns of the most extreme gas conditions within stems (i.e. highest CO2 and lowest O2), as well as more accurate estimates of diffusion coefficients in woody tissue.
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Acknowledgements |
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We wish to thank the US Environmental Protection Agency for funding through a STAR Graduate Fellowship (R Spicer), as well as generous logistical and financial support from both the Arnold Arboretum and Harvard Forest. We also gratefully acknowledge Teresa Abbott (NSF REU Program at Harvard Forest) for assistance in the field and laboratory, and two anonymous reviewers for helpful comments on the original manuscript.
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