Search

Root growth and respiration in elevated CO 2 (700 μmol mol -1 ) was studied in three tree species, Fraxinus excelsior L., Quercus petraea. L. and Pinus sylvestris L. grown in open-top chambers (OTCs) during a long-term exposure (20 months), during which root systems were allowed to develop without restriction imposed by pots. Root growth, measured as root length using root in-growth bags was increased significantly in trees exposed to elevated CO 2 , although the magnitude of the response differed considerably between species and with time of sampling, the greatest effect observed after 6 months in ash (ratio of elevated: ambient, e∶a; 3·40) and the smallest effect observed in oak (e∶a; 1·95). This was accompanied by changes in specific root length, with a significant decrease in all species after 6 months, suggesting that root diameter or root density were increased in elevated CO 2 . Increases in root length might have resulted from an acceleration in root cell expansion, since epidermal cell size was significantly increased in the zone of elongation in ash root tips (P<0·05). Contrasting effects of elevated CO 2 were observed for root carbohydrates, with significant increases in soluble sugars for all species (P<0·05), but both increases and decreases in starch content were observed, depending on species, and producing a significant interaction between species and CO 2 (P<0·001). Exposure to elevated CO 2 increased the total root d. wt for whole trees of all three species after 8 months of exposure, although the magnitude of this effect, in contrast to the root in-growth study, was greatest in Scots pine and smallest in ash. No significant effect of elevated CO 2 was observed on the root∶shoot ratio. Further detailed analysis of whole root systems after 20 months confirmed that species differences in root responses to elevated CO 2 were apparent, with increased coarse and fine root production in elevated CO 2 for Scots pine and ash respectively. Lateral root number was increased in elevated CO 2 for all species, as was mean root diameter. Root respiration rates were significantly reduced in elevated CO 2 for all three species. These results provide firm evidence that exposure of trees to future CO 2 concentrations will have large effects on root system development, growth, carbohydrate status and respiration. The magnitude and direction of such effects will differ, depending on species. The consequences of such responses for the three species studied are discussed.

Carbon-isotope ratios ([delta]13Cs) were measured for various bio-chemical fractions quantitatively extracted from naturally exposed and shaded leaves of the C3-Crassulacean acid metabolism (CAM) intermediate Clusia minor, sampled at dawn and dusk on days during the wet and dry seasons in Trinidad. As the activity of CAM increased in response to decreased availability of water and higher photon flux density, organic acids and soluble sugars were enriched in 13C by approximately 3.5 to 4%[per mille (thousand) sign] compared to plants sampled during the wet season. The induction of CAM was accompanied by a doubling in size of the reserve carbohydrate pools. Moreover, stoichiometric measurements indicated that degradation of both chloroplastic reserves and soluble sugars were necessary to supply phosphoenolpyruvate for the synthesis of organic acids at night. Results also suggest that two pools of soluble sugars exist in leaves of C. minor that perform CAM, one a vacuolar pool enriched in 13C and the second a transport pool depleted in 13C. Estimates of carbon-isotope discrimination expressed during CAM, derived from the trafficking among inorganic carbon, organic acids, and carbohydrate pools overnight, ranged from 0.9 to 3.1%[per mille (thousand) sign]. The [delta]13C of structural material did not change significantly between wet and dry seasons, indicating that most of the carbon used in growth was derived from C3 carboxylation.

On-line instantaneous carbon isotope discrimination was measured in conjunction with net uptake of CO 2 in leaves of exposed and shaded plants of the C 3 -CAM intermediate Clusia minor growing under natural conditions in Trinidad. At the end of the rainy season (late January-early February, 1992) C 3 photosynthesis predominated although exposed leaves recaptured a small proportion of respiratory CO 2 at night for the synthesis of malic acid. Citric acid was the major organic acid accumulated by exposed leaves at this time with a citric: malic acid ratio of 11:1. Values of instantaneous discrimination (Δ) in exposed leaves during the wet season rose from 17.1‰ shortly after dawn to 22.7‰ around mid-day just before stomata closed, suggesting that most CO 2 was fixed by Rubisco at this time. During the late afternoon, instantaneous Δ declined from 22.2‰ to 17‰, probably reflecting the limited contribution from PEPc activity and an increase in diffusional resistance to CO 2 in exposed leaves. Shaded leaves showed no CAM activity and CO 2 uptake proceeded throughout the day in the wet season. The decrease in instantaneous Δ from 27‰ in the morning to 19.2‰ in the late afternoon was therefore entirely due to diffusional limitation. Leaves sampled in the dry season (mid-March, 1992) had by now induced full CAM activity with both malic and citric acids accumulated overnight and stomata closed for 4-5 h over the middle of the day. Values of instantaneous Δ measured over the first 3 h after dawn (6.4-9.1‰) indicated that C 4 carboxylation dominated CO 2 uptake for most of the morning when rates of photosynthesis were maximal, implying that under natural conditions, the down regulation of PEPc in phase II occurs much more slowly than laboratory-based studies have suggested. The contribution from C 3 carboxylation to CO 2 uptake during phase II was most marked in leaves which accumulated lower quantities of organic acids overnight. In exposed leaves, measurements of instantaneous Δ during the late afternoon illustrated the transition from C 3 to C 4 carboxylation with stomata remaining open during the transition from dusk into the dark period. Uptake of CO 2 by shaded leaves during the late afternoon however appeared to be predominantly limited by decreased stomatal conductance. The short-term measurements of instantaneous Δ were subsequently integrated over 24 h in order to predict the leaf carbon isotope ratios (δ p ) and to compare this with the δ p measured for leaf organic material. Whilst there was close agreement between predicted and measured δ p for plants sampled in the wet season, during the dry season the predicted carbon isotope ratios were 5-9‰ higher than the measured isotope ratios. During the annual cycle of leaf growth most carbon was fixed via the C 3 pathway although CAM clearly plays an important role in maintaining photochemical integrity in the dry season.

A method of monitoring and collecting CO 2 samples in the field has been developed which has been used to study both temporal and spatial variations in canopy CO 2 isotopic signatures in two contrasting tropical forest formations in Trinidad. These have been related to vertical gradients in the carbon isotope ratio (δ 13 C) of organic material in conjunction with measurements of other environmental parameters. The δ 13 C of leaf material from two canopies showed a gradient with respect to height, more negative values being found low in the understorey. The deciduous secondary forest, (Simla) showed a difference of 4.6‰ and the semi-evergreen seasonal canopy (Aripo), 2.8‰. The range of δ 13 C values at Simla was 4‰ less negative than those at Aripo. In order to relate these measurements to the interaction between diffusion or carboxylation limitation, and source CO 2 effects, variations in environmental parameters through the canopy have been compared with changes in CO 2 partial pressure (P a ) and isotopic composition δ 13 C throughout the day during the dry season. Values of P a 20 m above the ground at Aripo varied from 380 vpm at dawn to 340 vpm at midday, at which time the partial pressure 15 cm above the ground was 375 vpm. The CO 2 partial pressure did not stabilise during the course of the day, and there was good correlation (r 2 =0.82) between δ a and P a , with more negative values of δ a occuring in the understorey. Diuraal changes of 2‰ were evident at all canopy positions. In the more open canopy at Simla, these gradients were similar, but less marked. Leaf-air vapour pressure deficit (VPD) showed no relationship with height, possibly as a result of minimal water flux from both the soil and the canopy due to low soil water content; VPD was 1.5 kPa higher at midday than dawn. A 3° C temperature gradient between the understorey and upper canopy was observed at Aripo but not in the more open Simla canopy. CO 2 partial pressure stabilised for only 4 h in the middle of the day, while other parameters showed no stable period. The proportion of floor respired CO 2 reassimilated at Aripo has been calculated as 26%, 19%, and 8% for the periods 0600-1000, 1000-1400, and 1400-1800 hours. In order to quantify source CO 2 effects, measurements of the environmental parameters and assimilation rate must be made at all canopy positions and throughout the day.

Short-term measurements of instantaneous carbon-isotope discrimination have been determined from mass-spectrometric analyses of CO2 collected online during gas exchange for the epiphytic bromeliad Tillandsia utriculata L. Using this technique, the isotopic signature of CO2 exchange for each phase of Crassulacean acid metabolism (CAM) has been characterised. During night-time fixation of CO2 (Phase I), discrimination (Δ) ranged from 4.4 to 6.6‰, equivalent to an effective carbon-isotope ratio (δ(13)C) of -12.3 to -14.5‰ versus Pee Dee Belemnite (PDB). These values reflected the gross photosynthetic balance between net CO2 uptake and refixation of respiratory CO2, characteristic of CAM in the Bromeliaceae. When Δ for the relative proportion of external (p a ) and internal (p i) CO2 is taken into account, calculated p i/p a decreased during the later part of the dark period from 0.68 to 0.48. Measurements of Δ during Phase II, early in the light period, showed the transition between C4 and C3 pathways, with carboxylation being increasingly dominated by ribulose bisphosphate carboxylase (Rubisco) as Δ increased from 10.5 to 21.2‰ During decarboxylation in the light period (Phase III), CO2 leaked out of the leaf and the inherent discrimination of Rubisco was expressed. The value of Δ calculated from on-line measurements (64.4‰) showed that the CO2 lost was considerably enriched in (13)C, and this was confirmed by direct analysis of the CO2 diffusing out into a CO2-free atmosphere (δ (13)C = + 51.6‰ versus PDB). Instantaneous discrimination was characteristic of the C3 pathway during Phase IV (late in the light period), but a reduction in Δ showed an increasing contribution from phosphoenolpyruvate carboxylase. The results from this non-invasive technique confirm the observations that "double carboxylation" involving both phosphoenolpyruvate carboxylase and Rubisco occurs during the transient phases of CAM (II and IV) in the light period.