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.     

Spatial activity and expression of plasma membrane H+-ATPase in stem xylem of walnut during dormancy and growth resumption

Plasma membrane H+-ATPase (PM H+-ATPase) plays a key role in nutrient transport, stress responses and growth. To evaluate proton motive force differences between apical and basal parts of acrotonic 1-year-old shoots of walnut (Juglans regia L. cv 'Franquette') trees, spatial and seasonal changes in PM H+-ATPase were studied in mature xylem tissues. During both the dormancy and growth resumption periods, and in both the apical and basal parts of the stem, PM H+-ATPase activity showed positive correlations with the amount of immunodetectable protein. In spring, at the time of growth resumption, higher activities and immunoreactivities of PM H+-ATPase were found in the apical part of the stem than in the basal part of the stem. In spring, the decrease in xylem sugar concentration reflected the high sugar uptake rate. Our data suggest that PM H+-ATPase plays a major role in the uptake of carbohydrates from xylem vessels during growth resumption. These results are discussed in the context of the acrotonic tendency of walnut shoots.     

Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch

Two theories have been proposed to explain how high positive pressures are developed in sugar maple stems when temperatures fluctuate around freezing. The Milburn-O'Malley theory proposes that pressure development is purely physical and does not require living cells or sucrose. The osmotic theory invokes the involvement of living cells and sucrose to generate an osmotic pressure difference between fibers and vessels, which are assumed to be separated by an osmotic barrier. We analyzed wood of Acer saccharum Marsh., Juglans cinerea L. and Betula papyrifera Marsh. (all generate positive pressures) examining three critical components of the osmotic model: pits in cell walls, selectivity of the osmotic barrier and stability of air bubbles under positive xylem pressure. We examined the distribution and type of pits directly by light and scanning electron microscopy (SEM), and indirectly by perfusion of branch segments with fluorescent dyes with molecular masses similar to sucrose. The latter approach allowed us to use osmotic surrogates for sucrose that could be tracked by epifluorescence. Infusion experiments were used to assess the compartmentalization of sucrose and to determine the behavior of gas bubbles as predicted by Fick's and Henry's laws. The SEM images of sugar maple revealed a lack of pitting between fibers and vessels but connections between fiber-tracheids and vessels were present. Fluorescein-perfusion experiments demonstrated that large molecules do not diffuse into libriform fibers but are confined within the domain of vessels, parenchyma and fiber-tracheids. Results of the infusion experiments were in agreement with those of the fluorescein perfusions and further indicated the necessity of a compartmentalized osmolyte to drive stem pressure, as well as the inability of air bubbles to maintain such pressure because of instability. These results support the osmotic model and demonstrate that the secondary cell wall is an effective osmotic barrier for molecules larger than 300 g mol(-1).     

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.     

Winter embolism,mechanisms of xylem hydraulic conductivity recovery and springtime growth patterns in walnut and peach trees

Xylem vessels of Prunus persica Batsch (peach) and Juglans regia L. (walnut) are vulnerable to frost-induced embolism. In peach, xylem embolism increased progressively over the winter, reaching a maximum of 85% loss of hydraulic conductivity (PLC) in early March. Over winter, PLC in walnut approached 100%, but the degree of xylem embolism varied during the winter, reflecting the ability of walnut to generate positive xylem pressures in winter and spring. In contrast, positive xylem pressures were not observed in peach. Controlled freeze-thaw experiments showed that frost alone is insufficient to increase embolism in peach; evaporative conditions during thawing are also required. However, when both species were protected from frost, PLC was zero. At bud break, there was complete recovery from embolism in walnut, whereas PLC remained high in peach. Three mechanisms responsible for the restoration of branch hydraulic conductivity were identified in walnut: the development of stem pressure, the development of root pressure and the formation of a new ring of functional xylem, whereas only one mechanism was observed in peach (new functional ring). The climatic conditions necessary for the manifestation of these mechanisms were investigated.     

Water content and bark thickness of Norway spruce (Picea abies) stems: phloem water capacitance and xylem sap flow

To determine the relationship between phloem transport and changes in phloem water content, we measured temporal and spatial variations in water content and sucrose, glucose and fructose concentrations in phloem samples and phloem exudates of 70- and 30-year-old Norway spruce trees (Picea abies (L.) Karst.). Large temporal and spatial variations in phloem water content (1.4-2.6 mg mg(dw)(-1)) and phloem total sugar concentration (31-70 mg g(dw)(-1)) paralleled each other (r(2) = 0.83, P < 0.0001 for the temporal profile and r(2) = 0.96, P < 0.008 for the spatial profile), indicating that phloem water content depends on the total amount of sugar to be transferred. Changes in phloem water content were unrelated to changes in bark thickness. Maximum changes in phloem water content calculated from dendrometer readings were only 8-11% of the maximum measured changes in phloem water content, indicating that reversible changes in bark thickness did not reflect changes in internal water relations. We also studied the relationship between xylem sap velocity and changes in bark thickness in 70-year-old trees during summer 1999 and winter 1999-2000. Sap flow occurred sporadically throughout the winter, but there was no relationship between bark shrinkage or swelling and sap velocity. In winter, mean daily xylem sap velocity was significantly correlated with mean daily vapor pressure deficit and air temperature (P < 0.0001, in both cases). Changes in bark thickness corresponded with both short- and long-term changes in relative humidity, in both winter and summer. Under controlled conditions at > 0 degrees C, changes in relative humidity alone caused changes in thickness of boiled bark samples. Because living bark of Norway spruce trees contains large areas with crushed and dead sieve cell zones-up to 24% of the bark is air-filled space-we suggest that this space can compensate for volume changes in living phloem cells independently of total tissue water content. We conclude that changes in bark thickness are not indicative of changes in either phloem water capacitance or xylem sap flow.     

Effects of simulated thaw on xylem cavitation,residual embolism,spring dieback and shoot growth in yellow birch

Yellow birch seedlings (Betula alleghaniensis Britt.) that had lost more than 90% of their stem hydraulic conductivity during ambient winter temperatures were exposed to 0 and 20 days of a simulated winter thaw followed by a 48-h freezing treatment at 0, -5, -10, -20 and -30 degrees C. After measuring freezing injury to shoots and roots, the seedlings were placed in a greenhouse where recovery of xylem conductivity and new growth were measured. Shoot xylem cavitation was measured as percent loss of hydraulic conductivity. Shoot freezing injury was assessed by electrolyte leakage (EL) and root freezing injury was assessed by EL and triphenyl tetrazolium chloride reduction. Seedlings pretreated with thaw had higher stem water contents and suffered more freezing damage to roots and shoots (at -20 and -30 degrees C, respectively) than unthawed seedlings. After 3 weeks in a greenhouse, seedlings from the 0, -5 and -10 degrees C freezing treatments showed complete recovery of xylem conductivity, with substantially increased stem water contents. Poor recovery of hydraulic conductivity was observed only in seedlings that were subjected to freezing treatments at -20 and -30 degrees C, regardless of thaw treatment. Of these embolized seedlings, however, only those not previously thawed showed recovery of hydraulic conductivity or regained stem water content after 9 weeks in the greenhouse. Shoot dieback, bud burst and length of new shoots were significantly related to the extent of stem xylem cavitation and freezing injury. We conclude that (1) the simulated winter thaw predisposed yellow birch seedlings to freezing damage in shoots and roots by dehardening tissues and increasing their water content; (2) root freezing damage in turn affected the seedlings' ability to refill embolized stem xylem, resulting in considerable residual xylem embolism after spring refilling; (3) further recovery of stem xylem conductivity was attributable to growth of new vessels; (4) and the permanent residual embolism, together with root and shoot freezing injury, caused increased dieback, bud mortality and reduced growth of new shoots.     

Photosynthetic activity in winter needles of the evergreen tree Taxus cuspidata at low temperatures

Photosystems harvest light energy, yet this energy cannot be efficiently employed for CO(2) assimilation at the below-freezing temperatures to which plants are typically exposed during winter in the temperate and boreal zones. To elucidate the mechanisms whereby this energy is dissipated, I evaluated performance of photosystems in winter needles of the evergreen tree Taxus cuspidata Sieb. et Zucc. Chloroplasts were localized adjacent to plasma membranes in needle cells in summer, whereas they congregated together in the centers of the cells during winter. When winter needles were acclimated to a temperature of 20 degrees C, their chloroplasts gradually dispersed to the edges of the cells, as in the summer. Acclimation-dependent relocalization coincided with changes in CO(2) uptake. Examination of photosystem II fluorescence kinetics in winter needles indicated that the quinone electron acceptor (Q(A)) reduction rate exceeded the Q(A) oxidation rate at low temperatures. The majority of Q(A) remained reduced even when winter needles were subjected to a temperature of -5 degrees C at low irradiance.     

Formation and seasonal occurrence of xylem embolism in Alnus cordata

We investigated the vulnerability of xylem to embolism and the seasonal occurrence of xylem embolism in Italian alder (Alnus cordata Loisel.) by acoustic and hydraulic methods. Wood anatomy was also studied. More than eighty percent of the vessels were less than 50 mm long and no vessels were longer than 120 mm. Mean vessel diameter was 48 micro m. Ultrasound acoustic emissions from root and branch segments dehydrating in air followed a similar pattern: in both tissues, emission peaks were recorded when the relative water content of the xylem was around 0.2. In branches dehydrating in air, xylem embolism increased linearly as water potential decreased. In trees in the field, more than 80 percent of hydraulic conductivity was lost in the tree crowns during winter. Recovery from winter embolism occurred mostly before bud burst. In summer, xylem embolism was low (< 30%) and acoustic emissions from roots, stem and branches of trees in the field were also low.     

A semi-physiological model of cold hardening and dehardening in walnut stem

It has been hypothesized that the increase in temperature in this century could lead to an increase in frost damage to plant tissues. Several models have been proposed to describe the development of cold hardiness, but never taking into account extreme climatic and/or physiological events. Our results on walnut tree (Juglans regia L.) show that cold hardiness was best correlated with average daily temperatures minimal temperatures over the last 15 days before sampling (T(min 15 days)), indicating that the freezing tolerance depended on the tree's climatic history. Moreover, this study also shows that the accumulation of sucrose and the water content (WC) decrease are an essential step towards cold hardiness. Thus, a simple linear model based on climatic (T(min 15 days)) and physiological (soluble sugars, WC) explanatory variables was developed to predict the cold hardiness level in walnut stem at any time during the leafless period. Each of the three input variables can be assigned a specific role contributing to the simulated function, cold hardiness. The extent and robustness of this relation was assessed on extreme physiological events on walnut trees bearing three main branches. On each tree, one branch was defoliated to limit the local carbohydrate and transpiration, one was girdled to increase local carbohydrate and prevent carbohydrate export and the third one was kept untreated as control. As expected, these treatments impacted both local carbon reserves and WC in the stems born by each main branch in comparison with the control on the same tree. The impact of these treatments on stem's freezing tolerance, as evaluated by an electrolyte leakage method (LT??), confirmed the direct impact of soluble sugar and WC on cold hardiness over a wide range of carbohydrate and WC. This is discussed in relation to the branch autonomy theory for carbon but also for water during summer growth and winter periods. The present study demonstrates the importance of physiological parameters in the prediction of cold hardiness and proposes a way to model cold hardiness with extreme climatic and/or physiological events.     

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.     

Drought tolerance,xylem sap abscisic acid and stomatal conductance during soil drying: a comparison of canopy trees of three temperate deciduous angiosperms

Patterns of water relations, xylem sap abscisic acid concentration ([ABA]) and stomatal aperture were characterized and compared in drought-sensitive black walnut (Juglans nigra L.), less drought-sensitive sugar maple (Acer saccharum Marsh.) and drought-tolerant white oak (Quercus alba L.) trees co-occurring in a second-growth forest in Missouri, USA. There were strong correlations among reduction in predawn leaf water potential, increased xylem sap [ABA] and stomatal closure in all species. Stomatal conductance was more closely correlated with xylem sap ABA concentration than with ABA flux or xylem sap pH and cation concentrations. In isohydric black walnut, increased concentrations of ABA in the xylem sap appeared to be primarily of root origin, causing stomatal closure in response to soil drying. In anisohydric sugar maple and white oak, however, there were reductions in midday leaf water potential associated with stomatal closure, making it uncertain whether drought-induced xylem sap ABA was of leaf or root origin. The role of root-originated xylem sap ABA in these species as a signal to the shoot of the water status of the roots is, therefore, less certain.     

Seasonal fluctuations of low‐molecular‐weight sugars,starch and nitrogen in sapwood of Pinus sylvestris L.

The content of water‐soluble substances and starch in the living tree stem at the time of felling influences wood durability during further utilization. The aim of the study was to describe the annual and seasonal fluctuations in the contents of stored carbohydrates and nitrogenous compounds. The contents of soluble sugars, starch, and nitrogen were measured in the outer sapwood (0–15 mm from cambium) of 65‐yr‐old Scots pine {Pinus sylvestris L.) trees during an annual cycle. To study the influence of growth conditions, samples were taken from different stands in Sweden, in one stand from a control and a fertilized block. The effect of the age of the trees on the content of stored substances was also considered by comparing young (10–15 yrs old) and older trees (40–65 yrs old). Determination of the carbohydrates was carried out using enzymatic analysis. The outer pine sapwood contained a higher content of low‐molecular weight sugars during autumn and winter than during spring and summer. Starch content rose at the beginning of the growth period and decreased in autumn. The content of soluble sugars increased towards the cambium on all sampling occasions. Fructose and glucose were dominant sugars in all the stands studied. Seasonal changes of sucrose were different from those of glucose and fructose, in that the sucrose content was already decreasing in February. The variations in the nitrogen content of the sapwood were far smaller than those of the soluble sugars. No significant differences were found between the stands with regard to glucose, fructose and the sum of all three sugars. The fertilized block showed low content of sucrose and nitrogen. It is concluded that seasonal fluctuations are more important with regard to the sugar content than the effect of growth conditions. Data indicate a lower content of soluble sugars and nitrogen in the outermost sapwood of young trees as compared with older trees.     

Freezing pattern and frost killing temperature of apple (Malus domestica) wood under controlled conditions and in nature

The freezing pattern and frost killing temperatures of apple (Malus domestica Borkh.) xylem were determined by differential thermal analysis and infrared differential thermal analysis (IDTA). Results from detached or attached twigs in controlled freezing experiments and during natural field freezing of trees were compared. Non-lethal freezing of apoplastic water in apple xylem as monitored during natural winter frosts in the field occurred at -1.9?±?0.4 °C and did not change seasonally. The pattern of whole tree freezing was variable and specific to the environmental conditions. On detached twigs high-temperature freezing exotherms (HTEs) occurred 2.8 K below the temperature observed under natural frosts in the field with a seasonal mean of -4.7?±?0.5 °C. Microporous apple xylem showed freezing without a specific pattern within a few seconds in IDTA images during HTEs, which is in contrast to macroporous xylem where a 2D freezing pattern mirrors anatomical structures. The pith tissue always remained unfrozen. Increasing twig length increased ice nucleation temperature; for increased twig diameter the effect was not significant. In attached twigs frozen in field portable freezing chambers, HTEs were recorded at a similar mean temperature (-4.6?±?1.0 °C) to those for detached twigs. Upon lethal intracellular freezing of apple xylem parenchyma cells (XPCs) low-temperature freezing exotherms (LTEs) can be recorded. Low-temperature freezing exotherms determined on detached twigs varied significantly between a winter minimum of -36.9 °C and a summer maximum -12.7 °C. Within the temperature range wherein LTEs were recorded by IDTA in summer (-12.7?±?0.5 to -20.3?±?1.1 °C) various tiny clearly separated discontinuous freezing events could be detected similar to that in other species with contrasting XPC anatomy. These freezing events appeared to be initially located in the primary and only later in the secondary xylem. During the LTE no freezing events in the bark and central pith tissue were recorded. Attached twigs were exposed to various freezing temperatures at which LTEs occur. Even if 60% of XPCs were frost-damaged twigs were able to recuperate and showed full re-growth indicating a high regeneration capacity even after severe frost damage to XPCs.     

Stem girdling manipulates leaf sugar concentrations and anthocyanin expression in sugar maple trees during autumn

To better understand the effects of sugar accumulation on red color development of foliage during autumn, we compared carbohydrate concentration, anthocyanin expression and xylem pressure potential of foliage on girdled versus non-girled (control) branches of 12 mature, open-grown sugar maple (Acer saccharum Marsh.) trees. Half of the study trees were known to exhibit mostly yellow foliar coloration and half historically displayed red coloration. Leaves from both girdled and control branches were harvested at peak color expression (i.e., little or no chlorophyll present). Disruption of phloem export by girdling increased foliar sucrose, glucose and fructose concentrations regardless of historical tree color patterns. Branch girdling also increased foliar anthocyanin expression from 50.4 to 66.7% in historically red trees and from 11.7 to 54.2% in historically yellow trees, the latter representing about a fivefold increase compared with control branches. Correlation analyses indicated a strong and consistent relationship between foliar red coloration and sugar concentrations, particularly glucose and fructose, in both girdled and control branches. Measures of xylem pressure potentials confirmed that girdling was a phloem-specific treatment and had no effect on water transport to distal leaves. Results indicate that stem girdling increased foliar sugar concentrations and enhanced anthocyanin expression during autumn in sugar maple foliage. Native environmental stresses (e.g., low autumn temperatures) that reduce phloem transport may promote similar physiological outcomes.     

Water storage capacitance and xylem tension in isolated branches of temperate and tropical trees

Trees of tropical semi-deciduous forests range from "drought-avoiding" stem-succulent species with low-density wood (< 0.5 g cm(-3)), which maintain high stem water potentials (psi(STEM) > -0.7 MPa) throughout the year, to "drought-tolerant" deciduous hardwood species (wood density > 0.75 g cm(-3)), which dehydrate strongly during seasonal drought (psi(STEM) < -6 MPa). In stem-succulent and other drought-avoiding species, xylem vessels are surrounded by extensive parenchyma providing intracellular water storage, whereas in deciduous species stem water storage is mainly extracellular. Thirteen tropical and two temperate tree species, representing different functional types, were studied. The contribution of stem water storage to these species' water use during water stress was determined by time-series analysis of dehydration and rehydration of excised leaf-bearing branches of these trees. During dehydration, stem water potential slowly declined 1-2 MPa in drought-avoiding species, but in deciduous species it rapidly fell 4-5 MPa, suggesting that water storage capacitance was related to xylem anatomy. After immersion of dehydrated, leafless branches in water, the decline in xylem tension and rate of water uptake during rehydration were linearly related, as predicted by application of Ohm's law to water flux. The decline of xylem tension during rehydration was biphasic, with a phase of rapid water uptake into extracellular spaces being followed by a prolonged phase of slow water uptake into living cells. The rate of water uptake during rehydration and the minima of leaf water potential observed in the field during the dry season were highly correlated with water storage capacitance, indicating that wood anatomy is a major determinant of drought adaptation.     

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.     

Field measurements of root respiration indicate little to no seasonal temperature acclimation for sugar maple and red pine

Increasing global temperatures could potentially cause large increases in root respiration and associated soil CO2 efflux. However, if root respiration acclimates to higher temperatures, increases in soil CO2 efflux from this source would be much less. Throughout the snow-free season, we measured fine root respiration in the field at ambient soil temperature in a sugar maple (Acer saccharum Marsh.) forest and a red pine (Pinus resinosa Ait.) plantation in Michigan. The objectives were to determine effects of soil temperature, soil water availability and experimental N additions on root respiration rates, and to test for temperature acclimation in response to seasonal changes in soil temperature. Soil temperature and soil water availability were important predictors of root respiration and together explained 76% of the variation in root respiration rates in the red pine plantation and 71% of the variation in the sugar maple forest. Root N concentration explained an additional 6% of the variation in the sugar maple trees. Experimental N additions did not affect root respiration rates at either site. From April to November, root respiration rates measured in the field increased exponentially with increasing soil temperature. For sugar maple, long-term Q10 values calculated from the field data were slightly, but not significantly, less than short-term Q10 values determined for instantaneous temperature series conducted in the laboratory (2.4 versus 2.62.7). For red pine, long-term and short-term Q10 values were similar (3.0 versus 3.0). Sugar maple root respiration rates at constant reference temperatures of 6, 18 and 24 degrees C were measured in the laboratory at various times during the year when field soil temperatures varied from 0.4 to 16.8 degrees C. No relationship existed between ambient soil temperature just before sampling and root respiration rates at 6 and 18 degrees C (P = 0.37 and 0.86, respectively), and only a very weak relationship was found between ambient soil temperature and root respiration at 24 degrees C (P = 0.08, slope = 0.09). We conclude that root respiration in these species undergoes little, if any, acclimation to seasonal changes in soil temperature.