Assimilation of xylem-transported 13C-labelled CO2 in leaves and branches of sycamore (Platanus occidentalis L.).

Previous reports have shown that CO(2) dissolved in xylem sap in tree stems can move upward in the transpiration stream. To determine the fate of this dissolved CO(2), the internal transport of respired CO(2) at high concentration from the bole of the tree was simulated by allowing detached young branches of sycamore (Platanus occidentalis L.) to transpire water enriched with a known quantity of (13)CO(2) in sunlight. Simultaneously, leaf net photosynthesis and CO(2) efflux from woody tissue were measured. Branch and leaf tissues were subsequently analysed for (13)C content to determine the quantity of transported (13)CO(2) label that was fixed. Treatment branches assimilated an average of 35% (SE=2.4) of the (13)CO(2) label taken up in the treatment water. The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles. Overall, the fixation of internally transported (13)CO(2) label by woody tissues averaged 6% of the assimilation of CO(2) from the atmosphere by the leaves. Woody tissue assimilation rates calculated from measurements of (13)C differed from rates calculated from measurements of CO(2) efflux in the lower branch but not in the upper branch. The results of this study showed unequivocally that CO(2) transported in xylem sap can be fixed in photosynthetic cells in the leaves and branches of sycamore trees and provided evidence that recycling of xylem-transported CO(2) may be an important means by which trees reduce the carbon cost of respiration.

CO2 efflux, CO2 concentration and photosynthetic refixation in stems of Eucalyptus globulus (Labill.).

In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physiological factors control stem respiration are unclear. An experiment was set up in a Eucalyptus globulus plantation in central Portugal with monoculture stands of 5-year-old and 10-year-old trees. CO(2) efflux from stems under shaded and unshaded conditions, as well as the concentration of CO(2) dissolved in sap [CO(2)(*)], stem temperature, and sap flow were measured with the objective of improving our understanding of the factors controlling CO(2) release from stems of E. globulus. CO(2) efflux was consistently higher in 5-year-old, compared with 10-year-old, stems, averaging 3.4 versus 1.3 mumol m(-2) s(-1), respectively. Temperature and [CO(2)(*)] both had important, and similar, influences on the rate of CO(2) efflux from the stems, but neither explained the difference in the magnitude of CO(2) efflux between trees of different age and size. No relationship was found between efflux and sap flow, and efflux was independent of tree volume, suggesting the presence of substantial barriers to the diffusion of CO(2) from the xylem to the atmosphere in this species. The rate of corticular photosynthesis was the same in trees of both ages and only reduced CO(2) efflux by 7%, probably due to the low irradiance at the stem surface below the canopy. The younger trees were growing at a much faster rate than the older trees. The difference between CO(2) efflux from the younger and older stems appears to have resulted from a difference in growth respiration rather than a difference in the rate of diffusion of xylem-transported CO(2).

CO2 fluxes and respiration of branch segments of sycamore (Platanus occidentalis L.) examined at different sap velocities, branch diameters, and temperatures.

Respiration of stems and branches of trees (R(S)) has typically been estimated by measuring radial CO(2) efflux from woody tissue (E(A)) and rates of efflux are often scaled temporally using a temperature relationship (Q(10)). High concentrations of CO(2) in xylem sap ([CO(2)*]) have been shown to affect E(A), and the transport of CO(2) in the xylem stream has been suggested as a mechanism to explain field observations of temperature-independent fluctuations in E(A). Sap velocity and temperature were manipulated in detached branch segments of sycamore (Platanus occidentalis L.) under controlled conditions to quantify these effects. Within individual branches of similar size, E(A) and [CO(2)*] were greater at low sap velocity, while the amount of respired CO(2) transported in sap (transport flux, F(T)) was greater at high sap velocity. E(A) was linearly correlated with [CO(2)*]. In branches of three diameter classes (1, 2, and 3 cm), volume-based E(A), F(T), and R(S) did not differ, but surface-area based CO(2) fluxes increased with diameter class. Regardless of diameter, E(A) accounted for only 30% of respired CO(2) at high sap velocity, while at low sap velocity, E(A) accounted for 71% of respired CO(2). E(A), F(T), and R(S) measured at 5, 20, and 35 degrees C at the same sap velocity showed a typical exponential response to temperature. However, at the lowest temperature, E(A) accounted for only 18% of the CO(2) released from respiring cells compared with 44% at the highest temperature, perhaps due to the effect of temperature on the solubility of CO(2) in water. These results directly demonstrate the transport of respired CO(2) in the xylem stream and may help to explain inconsistencies in stem and branch respiration measurements made in situ.

Measurement of stem respiration of sycamore (Platanus occidentalis L.) trees involves internal and external fluxes of CO2 and possible transport of CO2 from roots.

CO(2) released by respiring cells in tree stems can either diffuse to the atmosphere or dissolve in xylem sap. In this study, the internal and external fluxes of CO(2) released from respiring stems of five sycamore (Platanus occidentalis L.) trees were calculated. Mean rates of stem respiration were highest in mid-afternoon and lowest at night, and were positively correlated with air temperature. Over a 24 h period, on average 34% of the CO(2) released by respiring cells in the measured stem segment remained within the tree. CO(2) efflux to the atmosphere consisted of similar proportions of CO(2) derived from local respiring cells (55%) and CO(2) that had been transported in the xylem (45%), indicating that CO(2) efflux does not accurately estimate respiration. A portion of the efflux of transported CO(2) appeared to have originated in the root system. A modification of the method for calculating stem respiration based on internal and external fluxes of CO(2) was developed to separate efflux due to local respiration from efflux of transported CO(2).

Estimating stem respiration in trees by a mass balance approach that accounts for internal and external fluxes of CO2.

The respiration rate of a tree stem has commonly been estimated from measurements of CO2 efflux to the atmosphere. These estimates assume that all CO2 efflux originates from respiration of local tissues and that all CO2 produced by local tissues escapes to the atmosphere through the bark. However, dissolved CO2 can be transported in the xylem stream, and CO2 concentration ([CO2]) in xylem can be up to three orders of magnitude greater than that of the atmosphere, suggesting that measurements of CO2 efflux do not account for all CO2 produced by respiration. Here, we propose a new mass balance approach for estimating the respiration rate of tree stems that accounts for both external and internal fluxes of CO2. We demonstrate this approach using measurements of CO2 efflux, sap flux and internal [CO(2)] to calculate the rate of CO2 production of a segment of stem tissue in situ. At different times of the day, CO2 produced by respiration of stem tissues followed different flux pathways. During daylight hours when sap was flowing, a large proportion of respired CO2 was carried away in the xylem stream, whereas at night, most respiratory CO2 escaped to the atmosphere through the bark. Our calculations showed errors in efflux-based estimates of respiration of up to 76% compared with estimates that include both internal and external fluxes.

Microelectrode technique for in situ measurement of carbon dioxide concentrations in xylem sap of trees.

We developed a new microelectrode technique for measuring CO2 concentration ([CO2]) in xylem sap of trees. This technique enabled us to make rapid and continuous measurements of xylem sap [CO2] in situ. In this report, we discuss the methodology and establish the feasibility of the technique. We also describe calibration procedures, temperature sensitivity, field use and other characteristics of the microelectrodes. An example of data collected in the field is provided. Microelectrode calibration was accomplished at constant temperature in air of known [CO2]. When sampling temperature differed from calibration temperature, correction was necessary. We developed an equation to correct for temperatures between 15 and 35 degrees C when calibration was conducted at 25 degrees C. Equations based on Henry's Law were used to convert measured gas phase [CO2] (%) to concentration of all products of CO2 dissolved in sap (mmol l(-1)). We inserted microelectrodes into stems of three tree species to measure diurnal changes in [CO2] in the xylem sap. A diurnal pattern with depression during the day and elevation at night was observed. Mean daily [CO2] ranged from 1.6 to 10.3 mmol l(-1). Microelectrodes were suitable for making diurnal measurements for up to 7 days without recalibration. We also used the microelectrodes to measure [CO2] of soil in situ. Soil [CO2] ranged from 1 to 4% (gas phase), with little diurnal variation.

Is coronary angioplasty safe in the elderly?

OBJECTIVE: To evaluate the use of percutaneous transluminal coronary angioplasty (PTCA) and the immediate procedural outcomes in the elderly at a tertiary care centre. PATIENTS AND METHODS: Between January 1992 and December 1997, a total retrospective cohort study of 2322 consecutive patients aged 60 years or older underwent PTCA. Patients were categorized into three age groups: group A (60 to 69 years of age), which included 1294 patients; group B (70 to 79 years), which included 895 patients; and group C (80 years of age or older), which included 133 patients. PTCA was performed using the newest catheter technology as it became available. RESULTS: Men comprised 63% of the patients in groups A and B combined, and 44% of group C (P<0.001). Canadian Cardiovascular Society angina class IV was present in 45% of group C compared with 30% and 35% in groups A and B, respectively (P<0.001). The proportion of patients with diabetes mellitus and hypertension was similar among the three groups. Acute myocardial infarction before PTCA was twice as common at 4.5% (95% CI 3.7% to 5.3%) in group C, compared with 2.9% (95% CI 2.7% to 3.1%) and 2.2% (95% CI 2.0% to 2.3%) in groups A and B, respectively. The procedural success rate was similar at 93%, 92.7% and 91.7% in groups A, B and C, respectively. A total of five (0.2%) deaths and eight (0.34%) myocardial infarctions occurred in groups A and B combined, while none occurred in group C (not significant). More patients in groups A and B underwent emergency coronary artery bypass graft than in group C: group A - 22, 3.4% (95% CI 3.2% to 3.6%); group B - 16, 3.4% (95% CI 3.2% to 3.6%) and group C - one, 0.75% (95% CI 0.6% to 0.9%). CONCLUSIONS: In this retrospective series of patients, it was shown that PTCA may be performed in the very elderly with high procedural success and acceptable risk. Age alone should not be the criterion to limit the use of PTCA.

Aortic valve area discrepancy by Gorlin equation and Doppler echocardiography continuity equation: relationship to flow in patients with valvular aortic stenosis.

BACKGROUND: In vitro studies have shown a discrepancy between aortic valve area (AVA) measurements derived invasively by Gorlin equation (Gorlin AVA) and noninvasively by Doppler echocardiography (Doppler-echo) continuity equation (Doppler AVA) during low flow states. OBJECTIVE: To assess whether a flow-related discrepancy between Gorlin AVA and Doppler AVA occurs in the clinical setting in patients with isolated valvular aortic stenosis. PATIENTS AND METHODS: Seventy-five consecutive patients with isolated valvular aortic stenosis, who had AVA determined both invasively by Gorlin equation and noninvasively by Doppler-echo continuity equation, were retrospectively reviewed. RESULTS: Gorlin AVA and Doppler AVA correlated (r=0.68) over the narrow AVA range (Gorlin AVA 0.30 to 1.22 cm2); however, Doppler AVA was systematically larger than Gorlin AVA (0.80+/-0.21 versus 0.70+/-0.23 cm2, AVA difference = 0.10+/-0.17 cm2, P<0.0001). The AVA difference was inversely related to invasive cardiac index (r=-0.51) and was significantly greater at low flow states (cardiac index less than 2.5 L/min/m2) than at normal flow states (cardiac index 2.5 L/min/m2 or more) (0.16+/-0.15 versus -0.03+/-0.15 cm2, P<0.0001). Independent predictors of the AVA difference were the difference between Doppler-echo and invasive cardiac output (P<0.0001); the difference between Doppler-echo and invasive mean transvalvular pressure gradient (P=0.0002); and the average cardiac output (Doppler-echo plus invasive cardiac output/2, P=0.001) at the time of the hemodynamic assessments. The AVA difference was not related to average pressure gradient, average AVA or patient characteristics. CONCLUSIONS: A flow-related discrepancy between Gorlin AVA and Doppler AVA occurs in the clinical setting of patients with isolated valvular aortic stenosis. This discrepancy should be considered when assessing aortic stenosis severity during low flow states, where Gorlin AVA may be significantly smaller than Doppler AVA.

Influence of rate of change in stomatal conductance to fluctuating irradiance on estimates of daily water use by Pinus taeda leaves.

Pinus taeda L. stomata respond slowly to changes in irradiance. Because incident irradiance on a leaf varies constantly, the rate of change in stomatal conductance to fluctuating irradiance may have a large effect on plant water use. We estimated total daily water use of Pinus taeda foliage for 10 days with very different irradiance patterns, assuming that rates of stomatal opening and closing were similar. To determine how the most extreme imbalance in rates of stomatal opening and closing affects estimates of water use, we also estimated total daily water use assuming instantaneous stomatal opening and a realistic rate of stomatal closing. Total daily water use was calculated by summing estimates of transpiration based on irradiance and vapor pressure deficit measured every minute at locations atop and within a canopy. Estimates of total daily water use calculated on the basis of realistic rates of stomatal opening and closing were similar to estimates calculated assuming instantaneous stomatal change (mean difference between methods of calculation was less than 0.2%). Estimates of total daily water use assuming instantaneous stomatal opening and a realistic rate of closing differed from estimates of total daily water use based on similar rates of stomatal opening and closing. The discrepancy was greater within the canopy (mean difference 6%) than at the top of the canopy (mean difference 1%). Calculation of mean daily conductance from mean daily irradiance, without accounting for minute-by-minute variations in irradiance, resulted in overestimations of daily stomatal conductance (13% mean error) and the magnitude of the error was directly related to the variation in irradiance for that day. We conclude that, provided variation in irradiance is accounted for, rates of stomatal opening and closing have little effect on estimates of daily water use.

Changes in rates of photosynthesis and respiration during needle development of loblolly pine.

Net photosynthetic rates of developing foliage and one-year-old foliage of loblolly pine (Pinus taeda L.) were measured under field conditions. In the subsequent year, net photosynthesis and dark respiration rates of current-year and one-year-old foliage were measured under controlled environmental conditions. Loblolly pine foliage grows slowly, reaching its final size 3.5 to 4 months after bud burst. Positive rates of net photosynthesis were recorded when the foliage was 13 and 18% of final length, in the controlled-environment and field study, respectively. However, because of high rates of dark respiration during the initial growth period, a positive diurnal carbon balance did not occur until foliage was about a third of final length (40 days after bud burst). Two months after bud burst, when foliage was about 55% of final length, its photosynthetic capacity exceeded that of one-year-old foliage. The highest rates of net photosynthesis were achieved when foliage was more than 90% fully expanded.

Effect of elevated carbon dioxide concentration and root restriction on net photosynthesis, water relations and foliar carbohydrate status of loblolly pine seedlings.

To determine the effects of CO(2)-enriched air and root restriction on photosynthetic capacity, we measured net photosynthetic rates of 1-year-old loblolly pine seedlings grown in 0.6-, 3.8- or 18.9-liter pots in ambient (360 micro mol mol(-1)) or 2x ambient CO(2) (720 micro mol mol(-1)) concentration for 23 weeks. We also measured needle carbohydrate concentration and water relations to determine whether feedback inhibition or water stress was responsible for any decreases in net photosynthesis. Across all treatments, carbon dioxide enrichment increased net photosynthesis by approximately 60 to 70%. Net photosynthetic rates of seedlings in the smallest pots decreased over time with the reduction occurring first in the ambient CO(2) treatment and then in the 2x ambient CO(2) treatment. Needle starch concentrations of seedlings grown in the smallest pots were two to three times greater in the 2x ambient CO(2) treatment than in the ambient CO(2) treatment, but decreased net photosynthesis was not associated with increased starch or sugar concentrations. The reduction in net photosynthesis of seedlings in small pots was correlated with decreased needle water potentials, indicating that seedlings in the small pots had restricted root systems and were unable to supply sufficient water to the shoots. We conclude that the decrease in net photosynthesis of seedlings in small pots was not the result of CO(2) enrichment or an accumulation of carbohydrates causing feedback inhibition, but was caused by water stress.

Preliminary assessment of patients' opinions of queuing for coronary bypass graft surgery at one Canadian centre.

OBJECTIVES: To explore psychological and socioeconomic concerns of patients who queued for coronary artery bypass surgery and the effectiveness of support existing in one Canadian cardiovascular surgical center. DESIGN: Standardised questionnaire and structured interview. SETTING: Victoria General Hospital, Halifax, Nova Scotia. SUBJECTS: 100 consecutive patients awaiting non-emergency bypass surgery. RESULTS: Most patients (96%) found the explanation of findings at cardiac catheterisation and the justification given for surgery satisfactory. However, 84 patients complained that waiting for surgery was stressful and 64 registered at least moderate anxiety. Anger over delays was expressed by 16%, but only 4% thought that queuing according to medical need was unfair. Economic hardship, attributed to delayed surgery, was declared by 15 patients. This primarily affected those still working--namely, blue collar workers and younger age groups. Only 41% of patients were satisfied with existing institutional supports. Problems related mainly to poor communication. CONCLUSIONS: Considerable anxiety seems to be experienced by most patients awaiting bypass surgery. Better communication and education might alleviate some of this anxiety. Economic hardship affects certain patient subgroups more than others and may need to be weighed in the selection process. A more definitive examination of these issues is warranted.

Water use by Pinus radiata trees in a plantation.

We used the heat-pulse velocity technique to estimate transpirational water use of trees in an experimental 16-year-old Pinus radiata D. Don plantation in South Australia during a 4-month period from November 1993 to March 1994 (spring-summer). Fertilization and other silvicultural treatments during the first 8 years of the plantation produced trees ranging in diameter at a height of 1.3 m from 0.251 to 0.436 m, with leaf areas ranging from 83 to 337 m(2). Daily water use was greater for large trees than for small trees, but transpiration per unit leaf area was nearly identical. Daily transpiration was highly correlated with available soil water in the upper 1 m of soil and weakly correlated with irradiance and air temperature. For the stand (0.4 ha), estimated rates of transpiration ranged from 6.8 to 1.4 mm day(-1) in wet and dry soil conditions, respectively. Total water use by the plantation during the 4-month study period was 346 mm. Water transpired by the trees was about three times that extracted from the upper 1 m of soil. Large trees extracted water from the same soil volume as small trees and did not exhibit a greater potential to extract water from deeper soil when the upper horizons become dry.

Noninvasive testing in women presenting with chest pain: evidence for diagnostic uncertainty.

OBJECTIVE: To gain insight into the diagnostic utility of exercise stress testing in women. DESIGN: Observational prospective cohort study. SETTING: The Victoria General Hospital, Halifax, Nova Scotia. PARTICIPANTS: Consecutive women with chest pain referred by cardiologists or internists for exercise stress testing between May 30, 1992 and November 30, 1992 and followed prospectively to February 28, 1993. INTERVENTION: The proportion of patients subsequently referred for thallium scintigraphy and/or coronary angiography and their clinical profiles were determined. MAIN RESULTS: Of 183 patients studied, stress testing was positive in 48 (26.2%), negative in 48 (26.2%) and nondiagnostic in 87 (47.5%). Women with negative results were more likely to have had normal baseline electrocardiograms (ECGs) (P = 0.002) and least likely to have undergone prior angiography (P = 0.0003). Subsequent thallium scintigraphy and/or coronary angiography was undertaken in 33.3%, 18.8% and 27.6% with positive, negative and nondiagnostic index stress tests, respectively. None of chest pain, cardiac risk factors, previous cardiac investigations or baseline ECG discriminated 33 patients with negative or nondiagnostic stress results who had additional tests from 102 who did not. CONCLUSION: Exercise stress testing poorly screens women with chest pain for coronary artery disease (diagnostic in only 52.5%). Further study was undertaken in 27.6% with nondiagnostic tests and, surprisingly, in 18.8% with negative results. Why certain women with nondiagnostic, and so many with negative, stress tests were referred for further investigation was unclear. These results suggest diagnostic uncertainty when females presenting with chest pain are assessed.

Spontaneous pericardial tamponade during PTCA.

A case of acute hemorrhagic pericardial tamponade complicating a successful percutaneous transluminal coronary angioplasty (PTCA) is described, in the setting of rheumatoid arthritis (with no evidence of prior or concomitant pericarditis), large doses of intravenous heparin administration and a relatively high activated clotting time. There was no evidence of coronary artery rupture and there was no recent use of other anticoagulants or thrombolytic agents. Successful treatment comprised emergency pericardial drainage and intravenous protamine sulphate. The authors believe this to be the first reported case of acute hemorrhagic pericardial tamponade due solely to heparin administration. The possibility of acute hemorrhage of a rheumatoid nodule was considered but subsequent magnetic resonance imaging scan with contrast gadolinium was normal.

Responses of foliar gas exchange to long-term elevated CO(2) concentrations in mature loblolly pine trees.

Branches of field-grown mature loblolly pine (Pinus taeda L.) trees were exposed for 2 years (1992 and 1993) to ambient or elevated CO(2) concentrations (ambient + 165 micro mol mol(-1) or ambient + 330 micro mol mol(-1) CO(2)). Exposure to elevated CO(2) concentrations enhanced rates of net photosynthesis (P(n)) by 53-111% compared to P(n) of foliage exposed to ambient CO(2). At the same CO(2) measurement concentration, the ratio of intercellular to atmospheric CO(2) concentration (C(i)/C(a)) and stomatal conductance to water vapor did not differ among foliage grown in an ambient or enriched CO(2) concentration. Analysis of the relationship between P(n) and C(i) indicated no significant change in carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase during growth in elevated CO(2) concentrations. Based on estimates derived from P(n)/C(i) curves, there were no apparent treatment differences in dark respiration, CO(2) compensation point or P(n) at the mean C(i). In 1992, foliage in the three CO(2) treatments yielded similar estimates of CO(2)-saturated P(n) (P(max)), whereas in 1993, estimates of P(max) were higher for branches grown in elevated CO(2) than in ambient CO(2). We conclude that field-grown loblolly pine trees do not exhibit downward acclimation of leaf-level photosynthesis in their long-term response to elevated CO(2) concentrations.

Dynamic response of stomata to changing irradiance in loblolly pine (Pinus taeda L.).

Dynamic changes in stomatal conductance and the rate of photosynthesis were measured as periods of shading (decrease in irradiance from 800 to 200 micro mol m(-2) s(-1)) between 5 and 60 min were imposed on needles of Pinus taeda L. trees under laboratory conditions. Shading induced a 39% decrease in stomatal conductance but the rate of change was slow. Average time constants (+/- standard error) were shorter for the decrease in stomatal conductance when shading was imposed for 30 min (14.8 +/- 1.3 min) than for the increase in stomatal conductance when shading was removed (25.5 +/- 3.4 min). The time constants for increasing stomatal conductance when shading was removed were linearly related to the length of the previous dark period. The rate of photosynthesis fell immediately by 58% when shading was imposed and increased more rapidly than the change in stomatal conductance when shading was removed. The increase in photosynthesis during the induction phase after shading was removed was limited by both stomatal and biochemical effects. The long time constants for stomatal response contributed to the poor correlations between stomatal conductance and instantaneous measurements of irradiance from field data. However, the slow response of stomatal conductance to changes in irradiance had little effect on total daily transpiration, carbon gain and water-use efficiency.

Influence of climate and fertilization on net photosynthesis of mature slash pine.

Net photosynthesis was measured under field conditions in 23-year-old slash pine (Pinus elliottii Engelm. var. elliottii) trees to determine how it was affected by fertilization and climate. There was only a small decrease in rates of net photosynthesis from late summer through winter demonstrating that appreciable carbon gain occurs throughout the year in slash pine. Although fertilization substantially increased leaf area and aboveground biomass, it only slightly increased the rate of net photosynthesis. Simultaneous measurements of gas exchange in fertilized and unfertilized (control) plots allowed the detection of a small, but statistically significant difference in average net photosynthesis of 0.14 micro mol m(-2) s(-1). Irradiance, and to a lesser extent air temperature, were the environmental factors that exerted the most control on net photosynthesis. The highest rates of net photosynthesis occurred between air temperatures of 25 and 35 degrees C. Because air temperatures were within this range for 46% of all daylight hours during the year, air temperature was not often a significant limitation. Soil and atmospheric water deficits had less effect on photosynthesis than irradiance or air temperature. Although the depth to the water table changed during the year from 10 to 160 cm, predawn and midday xylem pressure potentials only changed slightly throughout the year. Predawn values ranged from -0.63 to -0.88 MPa in the control plot and from -0.51 to -0.87 MPa in the fertilized plot and were not correlated with water table depth. There was no correlation between xylem pressure potentials and net photosynthesis, presumably because water uptake was adequate. Although vapor pressure deficits reached 3.5 kPa during the summer, they had little effect on net photosynthesis. Over a vapor pressure deficit range from 1.0 to 3.0 kPa, net photosynthesis only decreased 21%. No differences in responses to these environmental factors could be attributed to fertilization.

Movement of respiratory CO(2) in stems of loblolly pine (Pinus taeda L.) seedlings.

Temperature-independent fluctuations in stem CO(2) efflux were measured in Pinus taeda L. seedlings. Stem CO(2) efflux was measured during high and low transpiration rates, high and low net photosynthesis rates, and normal and interrupted substrate supply conditions. Stem CO(2) efflux rates were an average of 6.7% lower during periods of high transpiration compared to periods of low transpiration. This difference in stem CO(2) efflux rates was not due to water stress. The most likely cause was movement of respiratory CO(2) in the transpiration stream. Interruption of substrate supply to the stem by phloem girdling reduced stem CO(2) efflux rates. Increasing net photosynthesis rates from low to high had no effect on stem CO(2) efflux, but decreasing net photosynthesis from high to low caused relatively small reductions in stem CO(2) efflux. These results indicate that diurnal changes in net photosynthesis rate may play a small role in temperature-independent afternoon depressions of stem CO(2) efflux. The transport of respiratory CO(2) by the transpiration stream compromises measurements of woody tissue respiration obtained by commonly accepted gas exchange techniques. This phenomenon could also affect measurement of leaf net photosynthesis and branch woody tissue respiration.