The effect of aqueous transport of CO(2) in xylem sap on gas exchange in woody plants

The influence of CO(2) transported in the transpiration stream on measurements of leaf photosynthesis and stem respiration was investigated. Measurements were made on trees in a temperate forest in Scotland and in a tropical rain forest in Cameroon, and on shrubs in the Sahelian zone in Niger. A chamber was designed to measure the CO(2) partial pressure in the gas phase within the woody stems of trees. High CO(2) partial pressures were found, ranging from 3000 to 9200 Pa. Henry's Law was used to estimate the CO(2) concentration of xylem sap, assuming that it was in equilibrium with the measured gas phase partial pressures. The transport of CO(2) in the xylem sap was calculated by multiplying sap CO(2) concentration by transpiration rate. The magnitude of aqueous transport in the studied species ranged from 0.03 to 0.35 &mgr;mol CO(2) m(-2) s(-1), representing 0.5 to 7.1% of typical leaf photosynthetic rates. These values strongly depend on sap pH. To examine the influence of aqueous transport of CO(2) on stem gas exchange, we made simultaneous measurements of stem CO(2) efflux and sap flow on the same stem. After removing the effect of temperature, stem CO(2) efflux was positively related to sap flow. The apparent effect on measurements of stem respiration was up to 0.7 &mgr;mol m(-2) s(-1), representing ~12% of peak stem respiration rates.     

Seasonal variation in xylem pressure of walnut trees: root and stem pressures

Measurements of air and soil temperatures and xylem pressure were made on 17-year-old orchard trees and on 5-year-old potted trees of walnut (Juglans regia L.). Cooling chambers were used to determine the relationships between temperature and sugar concentration ([glucose] + [fructose] + [sucrose], GFS) and seasonal changes in xylem pressure development. Pressure transducers were attached to twigs of intact plants, root stumps and excised shoots while the potted trees were subjected to various temperature regimes in autumn, winter and spring. Osmolarity and GFS of the xylem sap (apoplast) were measured before and after cooling or warming treatments. In autumn and spring, xylem pressures of up to 160 kPa were closely correlated with soil temperature but were not correlated with GFS in xylem sap. High root pressures were associated with uptake of mineral nutrients from soil, especially nitrate. In autumn and spring, xylem pressures were detected in root stumps as well as in intact plants, but not in excised stems. In contrast, in winter, 83% of the xylem sap osmolarity in both excised stems and intact plants could be accounted for by GFS, and both GFS and osmolarity were inversely proportional to temperature. Plants kept at 1.5 degrees C developed positive xylem pressures up to 35 kPa, xylem sap osmolarities up to 260 mosmol l(-1) and GFS concentrations up to 70 g l(-1). Autumn and spring xylem pressures, which appeared to be of root origin, were about 55% of the theoretical pressures predicted by osmolarity of the xylem sap. In contrast, winter pressures appeared to be of stem origin and were only 7% of the theoretical pressures, perhaps because of a lower stem water content during winter.     

Drought and the diurnal patterns of stem CO2 efflux and xylem CO2 concentration in young oak (Quercus robur)

A young potted oak (Quercus robur L.) tree was subjected to drought by interrupting the water supply for 9 days. The tree was placed in a growth chamber in which daily patterns of temperature and radiation were constant. The effects of drought on the water and carbon status of the stem were examined by measuring stem sap flow rate, stem water potential, stem diameter variations, stem CO(2) efflux rate (F(CO2)) and xylem CO(2) concentration ([CO(2)*]). Before and after the drought treatment, diurnal fluctuations in F(CO2) and [CO(2)*] corresponded well with variations in stem temperature (T(st)). Daytime depressions in F(CO2) did not occur. During the drought treatment, F(CO2) still responded to stepwise changes in temperature, but diurnal fluctuations in F(CO2) were no longer correlated with diurnal fluctuations in T(st). From the moment daily growth rate of the stem became zero, diurnal fluctuations in F(CO2) became closely correlated with diameter variations, exhibiting clear daytime depressions. The depressions in F(CO2) were likely the result of a reduction in metabolic activity caused by the lowered daytime stem water status. Xylem [CO(2)*] showed clear daytime depressions in response to drought. When the tree was re-watered, F(CO2) and [CO(2)*] exhibited sharp increases, coinciding with an increase in stem diameter. After resumption of the water supply, daytime depressions in F(CO2) and [CO(2)*] disappeared and diurnal fluctuations in F(CO2) and [CO(2)*] corresponded again with variations in T(st).     

Transient thermal dissipation method for xylem sap flow measurement: implementation with a single probe

Comparisons of tree water relations between treatments, species and sites are facilitated by the use of simple and low-cost measurements of xylem sap flow rates. The transient thermal dissipation (TTD) method is a variant of the constant thermal dissipation (CTD) method of Granier. It has the advantages of limiting thermal interference and of saving electrical energy. Here, our concern was to test a new step towards simplicity and low cost: the applicability of the TTD method with a single probe, i.e., without a reference sensor, following a cycle of 10 min heating and 10 min cooling, and using the same thermal index and multi-species calibration previously assessed with a dual probe. First, the responses of the dual and single probes were compared in an artificial hydraulic column of sawdust in the laboratory over a complete range of flux densities, from 0.3 to 4.0 l dm?2 h?1. Second, diurnal kinetics were compared in a young tree with rapid changes in the sapwood reference temperature of up to 5 °C h?1 for 5 consecutive days. With a relatively stable reference temperature, laboratory results showed that a single probe yielded the same temperature signal and thermal index as a dual probe for the full range of sap flux densities. Within the tree, the cooled temperature of the heated probe, linearly interpolated, proved to be an accurate indicator of the change in the reference temperature over time. Logically, the temperature signals and estimates of sap flux density with the single probe did not differ from the dual-sensor measurements when the cooled temperature was interpolated. Additionally, the responses of the thermal index, yielded in the hydraulic experiment with the sawdust column, fell within the variability of the multi-species calibration. This result supports the previous assessment of a non-species-specific calibration for the TTD method with diffuse porous media. In conclusion, our results showed that the TTD method can be directly applied with a single probe. Limitations and possible future progress are pointed out. This measurement system is probably the simplest technique currently available to measure xylem sap flow.     

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.     

Organic and inorganic sulfur transport in the xylem sap and the sulfur budget of Picea abies trees

Temporal changes in inorganic and organic sulfur compounds (sulfate, glutathione, cysteine, methionine) were analyzed in xylem sap of 40-year-old Norway spruce (Picea abies (L.) Karst.) trees growing on acidic soils at a healthy and a declining stand in the Fichtelgebirge (North Bavaria, Germany). Studies were carried out (1) to quantify glutathione (GSH) transport in the xylem of spruce, (2) to study the significance of reduced sulfur versus sulfate (SO(4) (2-)) transport in the xylem, and (3) to compare total sulfur (S) transport in the xylem with the amount of foliar uptake of SO(2) in an air-polluted environment. Glutathione was the main reduced S compound in the xylem ranging in concentration from 0.5 to 5 &mgr;mol l(-1). Concentrations of inorganic SO(4) (2-) in the xylem sap were up to 50 times higher than those of GSH ranging from 60 to 230 &mgr;mol l(-1). During the growing season, concentrations of all S compounds in the xylem were highest in May (up to 246 &mgr;mol l(-1)) and decreased during summer and fall (up to 21 &mgr;mol l(-1)). On average, SO(4) (2-) concentrations in xylem sap were 30% higher at the declining site compared with the healthy site. Diurnal changes in organic S compounds were significant for GSH and cysteine with high concentrations during the night and low concentrations during the day. Diurnal changes in inorganic concentrations were not significant. Xylem sap concentrations of SO(4) (2-) and cysteine were twice as high and GSH concentrations were tenfold higher in surface roots than in branches. At both sites, transport of organic S was low (up to 3% of total S) compared to transport of SO(4) (2-). Annual transport of total S in the xylem (SO(4) (2-) was the main component) ranged from 60 to 197 mmol tree(-1) year(-1) at the healthy site and from 123 to 239 mmol tree(-1) year(-1) at the declining site. Although gaseous uptake of SO(2) was estimated to be similar at both sites (38 mmol tree(-1) year(-1); Horn et al. 1989), the ratio between annual gaseous uptake of SO(2) and transport of S in the xylem was 1:4 and 1:5 at the healthy and declining sites, respectively.     

Winter variation in xylem sap pH of walnut trees: involvement of plasma membrane H+-ATPase of vessel-associated cells

We studied seasonal variation in xylem sap pH of Juglans regia L. Our main objectives were to (1) test the effect of temperature on seasonal changes in xylem sap pH and (2) study the involvement of plasma membrane H+-ATPase of vessel-associated cells in the control of sap pH. For this purpose, orchard-grown trees were compared with trees grown in a heated (> or = 15 degrees C) greenhouse. During autumn, sap pH was not directly influenced by temperature. A seasonal change in H+-ATPase activity resulting from seasonal variation in the amount of protein was measured in orchard-grown trees, whereas no significant seasonal changes were recorded in greenhouse-grown trees. Our data suggest that H+-ATPase does not regulate xylem sap pH directly by donating protons to the xylem, but by facilitating secondary active H+/sugar transport, among other mechanisms.     

A model of heat transfer in sapwood and implications for sap flux density measurements using thermal dissipation probes

A variety of thermal approaches are used to estimate sap flux density in stems of woody plants. Models have proved valuable tools for interpreting the behavior of heat pulse, heat balance and heat field deformation techniques, but have seldom been used to describe heat transfer dynamics for the heat dissipation method. Therefore, to better understand the behavior of heat dissipation probes, a model was developed that takes into account the thermal properties of wood, the physical dimensions and thermal characteristics of the probes, and the conductive and convective heat transfer that occurs due to water flow in the sapwood. Probes were simulated as aluminum tubes 20 mm in length and 2 mm in diameter, whereas sapwood, heartwood and bark each had a density and water fraction that determined their thermal properties. Base simulations assumed a constant sap flux density with sapwood depth and no wounding or physical disruption of xylem beyond the 2 mm diameter hole drilled for probe installation. Simulations across a range of sap flux densities showed that the dimensionless quantity k [defined as (ΔT(m) -ΔT)/ΔT, where ΔT(m) is the temperature differential (ΔT) between the heated and unheated probe under zero-flow conditions] was dependent on the thermal conductivity of the sapwood. The relationship between sap flux density and k was also sensitive to radial gradients in sap flux density and to xylem disruption near the probe. Monte Carlo analysis in which 1000 simulations were conducted while simultaneously varying thermal conductivity and wound diameter revealed that sap flux density and k showed considerable departure from the original calibration equation used with this technique. The departure was greatest for variation in sap flux density typical of ring-porous species. Depending on the specific combination of thermal conductivity and wound diameter, use of the original calibration equation resulted in an 81% under- to 48% overestimation of sap flux density at modest flux rates. Future studies should verify these simulations and assess their utility in estimating sap flux density for this widely used technique.     

Effect of elevated CO2 on monoterpene emission of young Quercus ilex trees and its relation to structural and ecophysiological parameters

We investigated growth, leaf monoterpene emission, gas exchange, leaf structure and leaf chemical composition of 1-year-old Quercus ilex L. seedlings grown in ambient (350 microl l(-1)) and elevated (700 microl l(-1)) CO2 concentrations ([CO2]). Monoterpene emission and gas exchange were determined at constant temperature and irradiance (25 degrees C and 1000 micromol m(-2) s(-1) of photosynthetically active radiation) at an assay [CO2] of 350 or 700 microl l(-1). Measurements were made on intact shoots after the end of the growing season between mid-October and mid-February. On average, plants grown in elevated [CO2] had significantly increased foliage biomass (about 50%). Leaves in the elevated [CO2] treatment were significantly thicker and had significantly higher concentrations of cellulose and lignin and significantly lower concentrations of nitrogen and minerals than leaves in the ambient [CO2] treatment. Leaf dry matter density and leaf concentrations of starch, soluble sugars, lipids and hemi-cellulose were not significantly affected by growth in elevated [CO2]. Monoterpene emissions of seedlings were significantly increased by elevated [CO2] but were insensitive to short-term changes in assay [CO2]. On average, plants grown in elevated [CO2] had 1.8-fold higher monoterpene emissions irrespective of the assay [CO2]. Conversely, assay [CO2] rapidly affected photosynthetic rate, but there was no apparent long-term acclimation of photosynthesis to growth in elevated [CO2]. Regardless of growth [CO2], photosynthetic rates of all plants almost doubled when the assay [CO2] was switched from 350 to 700 microl l(-1). At the same assay [CO2], mean photosynthetic rates of seedlings in the two growth CO2 treatments were similar. The percentage of assimilated carbon lost as monoterpenes was not significantly altered by CO2 enrichment. Leaf emission rates were correlated with leaf thickness, leaf concentrations of cellulose, lignin and nitrogen, and total plant leaf area. In all plants, monoterpene emissions strongly declined during the winter independently of CO2 treatment. The results are discussed in the context of the acquisition and allocation of resources by Q. ilex seedlings and evaluated in terms of emission predictions.     

Xylem sap composition of beech (Fagus sylvatica L.) trees: seasonal changes in the axial distribution of sulfur compounds

During different phases of the annual growth cycle, xylem sap was collected from trunk segments of adult beech (Fagus sylvatica L.) trees by the water displacement technique. Irrespective of the height of the trunk, both sulfate and reduced sulfur compounds were detected in the xylem sap throughout the year. Sulfate was the predominant sulfur compound in all samples analyzed. Its concentration in the xylem sap varied between 10 and 350 micro mol l(-1), with highest concentrations in April, shortly before bud break. In contrast to other tree species, cysteine and not glutathione was the predominant thiol transported in the xylem sap of beech trees. The cysteine concentration ranged between 0.1 and 1 micro mol l(-1). As observed for sulfate, maximum cysteine concentrations were found in April. Apparently, both sulfate and cysteine transport contribute to the sulfur supply of the developing leaves. Seasonal changes in the axial distribution of cysteine and sulfate differed, indicating differences in the source-sink relations of these sulfur compounds. High, but uniform, xylem sap sulfate concentrations in April may originate from balanced sulfate uptake by the roots, whereas high cysteine concentrations in April, increasing with increasing height of the trunk, may originate in part from protein breakdown in the trunk. Reversal of the axial distribution of xylem sap cysteine in late summer-early fall to higher concentrations in the lower part of the trunk than in the upper part of the trunk suggests that the upper part of the trunk becomes a sink for cysteine as a result of the synthesis of storage proteins at this time of the year.     

Winter stem xylem pressure in walnut trees: effects of carbohydrates, cooling and freezing

Pressure transducers were attached to twigs of orchard trees and potted trees of walnut (Juglans regia L.) to measure winter stem xylem pressures. Experimental potted trees were partially defoliated in the late summer and early autumn to lower the amount of stored carbohydrates. Potted trees were placed in cooling chambers and subjected to various temperature regimes, including freeze-thaw cycles. Xylem pressures were inversely proportional to the previous 48-h air temperature, but positively correlated with the osmolarity of the xylem sap. Defoliated trees had significantly lower concentrations of stored carbohydrates and significantly lower xylem sap osmolarities than controls. Plants kept at 1.5 degrees C developed xylem pressures up to 40 kPa, just 7% of the theoretical osmotic pressure of the xylem sap. However, exposure to low, nonfreezing temperatures followed by freeze-thaw cycles resulted in pressures over 210 kPa, which was 39% of the theoretical osmotic pressure. A simple osmotic model could account for the modest positive winter pressures at low, nonfreezing temperatures, but not for the synergistic effects of freeze-thaw cycles.     

Temperature responses of growth and wood anatomy in European beech saplings grown in different carbon dioxide concentrations

Effects of temperature on growth and wood anatomy were studied in young European beech (Fagus sylvatica L.) grown in 7-l pots for 2.5 years in field-phytotron chambers supplied with an ambient (approximately 400 micromol mol-1) or elevated (approximately 700 micromol mol-1) carbon dioxide concentration ([CO2]). Temperatures in the chambers ranged in increments of 2 degrees C from -4 degrees C to +4 degrees C relative to the long-term mean monthly (day and night) air temperature in Berlin-Dahlem. Soil was not fertilized and soil water and air humidity were kept constant. Data were evaluated by regression analysis. At final harvest, stem diameter was significantly greater at increased temperature (Delta1 degrees C: 2.4%), stems were taller (Delta1 degrees C: 8.5%) and stem mass tree-1 (Delta1 degrees C: 10.9%) and leaf area tree-1(Delta1 degrees C: 6.5%) were greater. Allocation pattern was slightly influenced by temperature: leaf mass ratio and leaf area ratio decreased with increasing temperature (Delta1 degrees C: 2.3% and 2.2% respectively), whereas stem mass/total mass increased (Delta1 degrees C: 2.1%). Elevated [CO2] enhanced height growth by 8.8% and decreased coarse root mass/total mass by 10.3% and root/shoot ratio by 11.7%. Additional carbon was mainly invested in aboveground growth. At final harvest a synergistic interaction between elevated [CO2] and temperature yielded trees that were 3.2% taller at -4 degrees C and 12.7% taller at +4 degrees C than trees in ambient [CO2]. After 2.5 seasons, cross-sectional area of the oldest stem part was approximately 32% greater in the +4 degrees C treatment than in the -4 degrees C treatment, and in the last year approximately 67% more leaf area/unit tree ring area was produced in the highest temperature regime compared with the lowest. Elevated [CO2] decreased mean vessel area of the 120 largest vessels per mm2 by 5.8%, causing a decrease in water conducting capacity. There was a positive interaction between temperature and elevated [CO2] for relative vessel area, which was approximately 38% higher at +4 degrees C than at -4 degrees C in elevated [CO2] compared with ambient [CO2]. Overall, temperature had a greater effect on growth than [CO2], but elevated [CO2] caused quantitative changes in wood anatomy.     

Impact of variable [CO2] and temperature on water transport structure-function relationships in Eucalyptus

Nearly 30 years ago, Whitehead and Jarvis and Whitehead et al. postulated an elegant mechanistic explanation for the observed relationship between tree hydraulic structure and function, hypothesizing that structural adjustments promote physiological homeostasis. To date, this framework has been nearly completely overlooked with regard to varying atmospheric carbon dioxide ([CO(2)]). Here, we evaluated Whitehead's hypothesis of leaf water potential (Ψ(l)) homeostasis in faster-growing (Eucalyptus saligna) and slower-growing (Eucalyptus sideroxylon) tree saplings grown under three [CO(2)] (pre-industrial, current and future) and two temperature (ambient and ambient + 4°C) treatments. We tested for relationships between physiological (stomatal conductance and Ψ(l)) and structural (leaf and sapwood areas (A(l), A(s)), height (h), xylem conductivity (k(s))) plant variables as a function of the [CO(2)] and temperature treatments to assess whether structural variables adjusted to maintain physiological homeostasis. Structural components (A(l), A(s), h) generally increased with [CO(2)] or temperature, while g(s) was negatively correlated with [CO(2)]. Contrary to Whitehead's hypothesis, Ψ(l) did not exhibit homeostasis in either species; elevated temperatures were associated with more negative Ψ(l) in faster-growing E. saligna, and less negative Ψ(l) in slower-growing E. sideroxylon. Moreover, individual structural variables were generally uncorrelated with Ψ(l). However, across both species, the integrated hydraulic property of leaf specific hydraulic conductance (K(l)) was positively correlated with an independent calculation of K(l) determined exclusively from leaf physiological variables. These results suggest that physiological homeostasis may not apply to saplings exposed to global change drivers including [CO(2)] and temperature. Nevertheless, Whitehead et al.'s formulation identified K(l) as a sensitive measure of plant structural-physiological co-variation across species.     

Calibration of sap flow estimated by the compensation heat pulse method in olive, plum and orange trees: relationships with xylem anatomy

The compensation heat pulse method is widely used to estimate sap flow in conducting organs of woody plants. Being an invasive technique, calibration is crucial to derive correction factors for accurately estimating the sap flow value from the measured heat pulse velocity. We compared the results of excision and perfusion calibration experiments made with mature olive (Olea europaea L. 'Manzanilla de Sevilla'), plum (Prunus domestica L. 'Songal') and orange (Citrus sinensis (L.) Osbeck. 'Cadenero') trees. The calibration experiments were designed according to current knowledge on the application of the technique and the analysis of measured heat pulse velocities. Data on xylem characteristics were obtained from the experimental trees and related to the results of the calibration experiments. The most accurate sap flow values were obtained by assuming a wound width of 2.0 mm for olive and 2.4 mm for plum and orange. Although the three possible methods of integrating the sap velocity profiles produced similar results for all three species, the best results were obtained by calculating sap flow as the weighted sum of the product of sap velocity and the associated sapwood area across the four sensors of the heat-pulse-velocity probes. Anatomical observations showed that the xylem of the studied species can be considered thermally homogeneous. Vessel lumen diameter in orange trees was about twice that in the olive and plum, but vessel density was less than half. Total vessel lumen area per transverse section of xylem tissue was greater in plum than in the other species. These and other anatomical and hydraulic differences may account for the different calibration results obtained for each species.     

Sap flux in pure aspen and mixed aspen-birch forests exposed to elevated concentrations of carbon dioxide and ozone

Elevated concentrations of atmospheric carbon dioxide ([CO2]) and tropospheric ozone ([O3]) have the potential to affect tree physiology and structure and hence forest water use, which has implications for climate feedbacks. We investigated how a 40% increase above ambient values in [CO2] and [O3], alone and in combination, affect tree water use of pure aspen and mixed aspen-birch forests in the free air CO2-O3 enrichment experiment near Rhinelander, Wisconsin (Aspen FACE). Measurements of sap flux and canopy leaf area index (L) were made during two growing seasons, when steady-state L had been reached after more than 6 years of exposure to elevated [CO2] and [O3]. Maximum stand-level sap flux was not significantly affected by elevated [O3], but was increased by 18% by elevated [CO2] averaged across years, communities and O(3) regimes. Treatment effects were similar in pure aspen and mixed aspen-birch communities. Increased tree water use in response to elevated [CO2] was related to positive CO2 treatment effects on tree size and L (+40%). Tree water use was not reduced by elevated [O3] despite strong negative O3 treatment effects on tree size and L (-22%). Elevated [O3] predisposed pure aspen stands to drought-induced sap flux reductions, whereas increased tree water use in response to elevated [CO2] did not result in lower soil water content in the upper soil or decreasing sap flux relative to control values during dry periods. Maintenance of soil water content in the upper soil in the elevated [CO2] treatment was at least partly a function of enhanced soil water-holding capacity, probably a result of increased organic matter content from increased litter inputs. Our findings that larger trees growing in elevated [CO2] used more water and that tree size, but not maximal water use, was negatively affected by elevated [O3] suggest that the long-term cumulative effects on stand structure may be more important than the expected primary stomatal closure responses to elevated [CO2] and [O3] in determining stand-level water use under possible future atmospheric conditions.     

Calibration of thermal dissipation sap flow probes for ring- and diffuse-porous trees

Thermal dissipation probes (the Granier method) are routinely used in forest ecology and water balance studies to estimate whole-tree transpiration. This method utilizes an empirically derived equation to measure sap flux density, which has been reported as independent of wood characteristics. However, errors in calculated sap flux density may occur when large gradients in sap velocity occur along the sensor length or when sensors are inserted into non-conducting wood. These may be conditions routinely associated with ring-porous species, yet there are few cases in which the original calibration has been validated for ring-porous species. We report results from laboratory calibration measurements conducted on excised stems of four ring-porous species and two diffuse-porous species. Our calibration results for ring-porous species were considerably different compared with the original calibration equation. Calibration equation coefficients obtained in this study differed by as much as two to almost three orders of magnitude when compared with the original equation of Granier. Coefficients also differed between ring-porous species across all pressure gradient conditions considered; however, no differences between calibration slopes were observed for data collected within the range of expected in situ pressure gradients. In addition, dye perfusions showed that in three of the four ring-porous species considered, active sapwood was limited to the outermost growth ring. In contrast, our calibration results for diffuse-porous species showed generally good agreement with the empirically derived Granier calibration, and dye perfusions showed that active sapwood was associated with many annual growth rings. Our results suggest that the original calibration of Granier is not universally applicable to all species and xylem types and that previous estimates of absolute rates of water use for ring-porous species obtained using the original calibration coefficients may be associated with substantial error.     

Spatial and seasonal variation in amino compounds in the xylem sap of a mistletoe (Viscum album) and its hosts (Populus spp. and Abies alba)

In a field study, the composition and concentrations of amino compounds in the xylem sap of the mistletoe, Viscum album L., and in the xylem sap of two host species, an evergreen conifer (Abies alba Mill.) and a deciduous broad-leaved tree (Populus x euramericana), were analyzed. The xylem sap of both hosts and mistletoe contained large, but similar amounts of total organic nitrogen in low molecular weight amino compounds (TONLW). Nevertheless, individual amino compounds accumulated in the xylem sap of mistletoe relative to the host xylem sap, indicating selective uptake. In the xylem sap of Populus, major amino compounds (asparagine (Asn) and glutamine (Gln)) and the bulk parameters, TONLW and proteinogenic amino acids, showed significant seasonal variation. In Abies and in mistletoe on either host, variation of amino compounds in xylem sap was largely explained by inter-annual differences, not by seasonal variation. In both hosts, TONLW in the xylem sap was dominated by Gln. There was a steady decrease in relative abundance of Gln from the host xylem sap to the mistletoe xylem sap and to the stems and leaves of mistletoe. Simultaneously, the abundance of arginine (Arg) increased. Arginine was the predominant amino compound in the stems and leaves of mistletoe, occurring at concentrations previously observed only in leaves of trees exposed to excess nitrogen. We conclude that Gln (2 mol N mol(-1)) delivered by the host xylem sap is converted, in mistletoe, to Arg (4 mol N mol(-1)) and that the organic carbon liberated from Gln contributes significantly to the parasite's heterotrophic carbon gain. Statistical analyses of the data support this conclusion. Accumulation of Arg in mistletoe is an indication of excess N supply as a result of the uptake of amino compounds from the host xylem sap and a lack of phloem uploading.     

Tree stem diameter variations and transpiration in Scots pine: an analysis using a dynamic sap flow model

A dynamic model for simulating water flow in a Scots pine (Pinus sylvestris L.) tree was developed. The model is based on the cohesion theory and the assumption that fluctuating water tension driven by transpiration, together with the elasticity of wood tissue, causes variations in the diameter of a tree stem and branches. The change in xylem diameter can be linked to water tension in accordance with Hookea s law. The model was tested against field measurements of the diurnal xylem diameter change at different heights in a 37-year-old Scots pine at Hyyti?l?, southern Finland (61 degrees 51' N, 24 degrees 17' E, 181 m a.s.l.). Shoot transpiration and soil water potential were input data for the model. The biomechanical and hydraulic properties of wood and fine root hydraulic conductance were estimated from simulated and measured stem diameter changes during the course of 1 day. The estimated parameters attained values similar to literature values. The ratios of estimated parameters to literature values ranged from 0.5 to 0.9. The model predictions (stem diameters at several heights) were in close agreement with the measurements for a period of 6 days. The time lag between changes in transpiration rate and in sap flow rate at the base of the tree was about half an hour. The analysis showed that 40% of the resistance between the soil and the top of the tree was located in the rhizosphere. Modeling the water tension gradient and consequent woody diameter changes offer a convenient means of studying the link between wood hydraulic conductivity and control of transpiration.     

Drought tolerance,xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of young plants of four temperate deciduous angiosperms

Patterns of water relations, xylem sap abscisic acid (ABA) concentration ([ABA]) and stomatal aperture were compared in drought-sensitive black walnut (Juglans nigra L.) and black willow (Salix nigra Marsh.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.). Strong correlations among reduction in predawn water potential, increase in xylem sap [ABA] and stomatal closure were observed in all species. Stomatal response was more highly correlated with xylem [ABA] than with ABA flux. Xylem sap pH and ion concentrations appeared not to play a major role in the stomatal response of these species. Stomata were more sensitive to relative changes in [ABA] in drought-sensitive black walnut and black willow than in sugar maple and white oak. In the early stages of drought, increased [ABA] in the xylem sap of black walnut and black willow was probably of root origin and provided a signal to the shoot of the water status of the roots. In sugar maple and white oak, leaf water potential declined with the onset of stomatal closure, so that stomatal closure also may have occurred in response to the change in leaf water potential.     

寒温带兴安落叶松林土壤CO气体通量变化特征研究

潜在温室气体CO气体通量相关研究很少,多集中于热带、亚热带地区,大兴安岭兴安落叶松林作为我国北方寒温带最大的天然林,研究该森林土壤CO变化特征具有典型性与创新性.于2020年6—9月采用加拿大LGR-N2O∕CO分析仪持续测定大兴安岭兴安落叶松林CO气体通量及土壤温湿度,分析大兴安岭兴安落叶松林土壤CO气体通量的变化特...