Abstracts

Cochard, H., Martin, R., et al. 2000, 'Temperature Effects on Hydraulic Conductance and Water Relations of Quercus robur L.', Journal of Experimental Botany, vol. 51, no. 348, pp. 1255-1259.

The effects of temperature on root and shoot hydraulic conductances (g_shoot) and (g_root) were investigated for Quercus robur L. saplilngs. In a first experiment, conductances were measured with a High Pressure Flow Meter on excised shoots and detopped root systems. The g_root and g_shoot increased considerably with temperature from 0-50°C. Between 15°C and 35°C, g_shoot and g_root varied with water viscosity. In a second experiment, the impact of temperature-induced changes in groot on sapling transpiration (E) and leaf water potential (Ψ_leaf)was assessed. Intact plants were places in a growth cabinet with constant air and variable soil temeratures. E increase linearly with soil temperature but Ψ_leaf remained constant. As a consequence, a linear relationship was found between E and g_plant. The results illustrate the significance of g_plant for the stomatal control of transpiration and the significance of temperature for tree water transport.

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Domec, J.-C., Meinzer, F.C., et al. 2006, 'Transpiration-induced Axial and Radial Tension Gradients in Trunks of Douglas-fir Trees', Tree Physiology, vol. 26, pp. 275-284.

We determined the axial and radial xylem tension gradients in trunks of young Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Axial specific conductivity (k_s-a) and sap flux density (J_s) were measured at four consecutive depths within the sapwood at a stem height of 1 m. By definition, at a given position in the bole, J_s is a function not only of k_s-a but also of the driving force for water movement. The J_s:k_s-a ratio was therefore used to estimate axial tension gradients and the radial gradients at a stem height of 1 m were calculated from the differences in axial tension gradients at each depth. Tracheid lumen diameter and tracheid length were used to predict differences in k_s-a and its divergence from the theoretical k_s-a determined by the Hagen Poisseuille equation. The ratio of k_s-a (determined in the laboratory) to J_s (measured in the field) varied with depth in the sapwood, resulting in non-uniform axial and radial tension gradients from inner to outer sapwood. Transpiration-induced axial tension gradients were in the range of 0.006-0.01 MPa m^-1 excluding the gravitational tension gradient. At a stem height of 1 m, radial tension gradients were in the range of 0.15-0.25 MPa m^-1 and were lower in the middle sapwood than in the inner or outer sapwood. Axial tension gradients were 44-50% higher in the outer sapwood than in the inner sapwood. At a stem height of 1 m, radial J_s, calculated on the basis of radial tension gradients and measured radial specific conductivity (k_s-r), was about two orders of magnitude smaller than axial J_s. Our findings indicate that large radial tension gradients occur in the sapwood and clarify the role played by xylem k_s-a and k_s-r in determining in situ partitioning of J_s in the axial and radial directions.

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Trifilò, P., Raimondo, F., et al. 2004, 'Drought Resistance of Ailanthus altissima: Root Hydraulics and Water Relations', Tree Physiology, vol. 24, pp. 107-114.

Drought resistance of Ailanthus altissima (Mill.) Swingle is a major factor underlying the impressively wide expansion of this species in Europe and North America. We studied the specific mechanism used by A. altissima to withstand drought by subjecting potted seedlings to four irrigation regimes. At the end of the 13 week treatment period, soil water potential was -0.05 MPa for well-watered control seedlings (W) and -0.04, -0.08 and -1.7 MPa for drought-stressed seedlings (S) in irrigation regimes S1, S2 and S3, respectively. Root and shoot biomass production did not differ significantly among the four groups. A progressively marked stomatal closure was observed in drought-stressed seedlings, leading to homeostasis of leaf water potential, which was maintained well above the turgor loss point. Root and shoot hydraulics were measured with a high-pressure flow meater. When scaled by leaf surface area, shoot hydraulic conductance decreased by about 20 % in S1 and S2 seedlings and by about 70% in S3 seedlings, with respect to the well-watered control value. Similar differences were observed when root hydraulic conductance was scaled by root surface area, suggesting that roots had become less permeable to water. Anatomical observations of root cross sections revealed that S3 seedlings had shrunken cortical cells and a multilayer endodermal-like tissue that probably impaired soil-to-root stele water transport. We conclude that A. altissima seedlings are able to withstand drought by employing a highly effective water-saving mechanism that involves reduced water loss by leaves and reduced root hydraulic conductance. This water-saving mechanism helps explain how A. altissima successfully competes with native vegetation.

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Tsuda, M. and Tyree, M.T. 1997, 'Whole-plant Hydraulic Resistance and Vulnerability Segmentation in Acer saccharinum', Tree Physiology, vol. 17, pp. 351 - 357.

Hydraulic properties were studied in Acer sccharinum L., a riparian species that also grows well on a dry soil when transplanted. Hydraulic resistances were measured by two independent techniques; a new high-pressure flowmeter (HPFM) method and a conventional evaporative flux (EF) method. Vulnerability to cavitation was also investigated on petiols, stems and roots using a hydraulic conductivity technique.

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Tsuda, M. and Tyree, M.T. 2000, 'Plant Hydraulic Conductance Measured by the High Pressure Flow Meter in Crop Plants', Journal of Experimental Botany, vol. 51, no. 345, pp. 823-828.

A new high pressure flow meter (HPFM) method for measuring plant hydraulic conductances (K) was investigated to examine whether its results are comparable to those from a conventional evaporative flux (EF) method in crops. Hydraulic conductance (K) was measured by the two methods under quasi-steady-state conditions in six crops grown in pots: soybean (Glycine max L. Merr. cv. Tsurunoko daizu), sunflower(Helianthus annuus L. cv. Russian mammoth), kidney bean (Phaseolus vulgaris L. cv. Tsurunashi morocco), tomato (Lycopersicon esculentum Mill. cv. Sekai- ichi), green pepper (Capsicum annuum L. cv. shishitou), and eggplant (Solanum melongena L.cv. Siguro chunaganasu). There was a 1:1 agreement between K values measured by the two methods for K values of whole plant, root and stem, and leaf under quasi-steady-state conditions, Leaf water potential (Ψ_leaf) and evaporative flux density (E) in sunflower was curvilinear, indication whole plant K estimated by the EF method increased with increase of E. Predicted Ψ_leaf (=E divided by whole plant K measured by the HPFM method) agreed with measured Ψ_leaf Diurnal changes were also found in K measured by the HPFM confiming that K changed in reponse to temperature and E. The HPFM revealed that variable conductance was located in all organs: roots, stems, petioles, and leaves. These observations indicated that the HPFM is valid for crops as well as for trees (as previously established by Tsuda and Tyree) and has advantages over the EF method because of the speed and ease of the HPFM method.

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Tyerman, S.D., Tilbrook, J., et al. 2004, 'Direct Measurement of Hydraulic Properties in Developing Berries of Vitis vinifera L. cv Shiraz and Chardonnay', Australian Journal of Grape and Wine Research, vol. 10, pp. 170-181.

Berries of Vitis vinifera L. cv Shiraz can undergo weight loss during later stages of ripening. Existing published views on how weight loss occurs are based on changes in capacity of the vascular system to import water during development (McCarthy and Coombe, Australian Journal of Grape and Wine Research, 5, 17-21, 1999). One important element of these views is the proposed cessation of water flow through the xylem after veraison. We have now measured the water flow into berries of Shiraz and Chardonnay as they develop using the pressure probe and the high pressure flow meter (HPFM). The pressure probe connected to the pedicel of individual berries provided measurements of single berry hydraulic conductance. By systematic excision of tissue segments of the berry and pedicel we determined where in the pathway hydraulic conductance changed during development. The HPFM was used on whole bunches showing that berries (including pedicels) represent parallel high hydraulic resistances and that the hydraulic resistance of the bunch axis was rather small. The hydraulic conductance per berry could be determined from excision experiments. There was close agreement between the pressure probe and HPFM measurements. Both showed a ten-fold reduction in hydraulic conductance of whole berries from veraison to full ripeness. Shiraz had hydraulic conductances that were 2- to 5-fold higher than those for Chardonnay. Shiraz maintained a higher hydraulic conductance past 90 days after flowering than Chardonnay. The decrease in hydraulic conductance occurred in both the distal and proximal parts of the berry for both varieties. The pressure probe also provided measurements of the xylem pressure that nontranspiring berries could develop. These pressures were -0.2 to -0.1 MPa until veraison and increased to zero when the juice osmotic potential reached about -3 MPa in Chardonnay and -4 MPa in Shiraz. The results suggest values of the reflection coefficient of the osmotic barrier around the xylem vessels of about 0.1-0.2 at veraison decreasing to 0 at harvest. It is suggested that in addition to changes in xylem anatomy, aquaporins in berry membranes may play a role in regulating hydraulic conductance. Water movement from the berry back to the parent vine via the xylem (backflow) may be an important component of berry weight loss in Shiraz, particularly if the phloem ceases functioning at high osmoticpotentials near maximum weight. Backflow could account for a weight loss of 43 mg per day in Shiraz berries for a relatively small gradient of 0.1 MPa.

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Tyree, M.T. 1997, 'The Cohesion-Tension Theory of Sap Ascent: Current Controversies', Journal of Experimental Botany, vol. 48, no. 315, pp. 1753-1765.

In recent years, the Cohesion-Tension (C-T) theory of sap ascent in plants has come under question because of work published by professor Ulrich Zimmermann and colleagues at the University of Wüzburg, Germany. The purpose of this review is to (1) state the essential and testable elements of the C-T theory, (2) summarize the negative evidence for the C-T theory, and (3) review critically the positive evidence for the C-T theory and the evidence that the Scholander Hammel pressure bomb measures xylem pressure potential (P_x) correctly, because much of the evidence for the C-T theory depends on pressure bomb data. Much of the current evidence negates the contlusions drawn by Zimmermann fro studies using the xylem pressure probe (XPP), but it is not yet clear in every instance why the XPP results disagree with those of other methods for estimating xylem pressure. There is no reason to reject the XPP as a useful new tool for studying xylem tensions in the range of 0 to -0.6MPa. Additional research is needed to test the C-T theory with both the XPP and traditional methods.

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Tyree, M.T., Nardini, A., et al. 2001, Hydraulic Architecture of Whole Plants and Single Leaves, L'Abre The Tree, Montreal, Isabelle Quentin. 2000 pp. 215-221.

This paper presents a brief account of hydraulic architecture models of whole plants and of single leaves. The model for whole plants, T-Plant, is intended to investigate the consequences of reducing the complex resistance network of large plants to single resistance values. It is argued that the single resistance values depend on the method used to measure them, i.e., the evaporative flux method versus the high pressure flowmeter method, and the resistance can depend on the time of day in the evaporative flux method.

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Tyree, M.T., Patiño, S., et al. 1995, 'Dynamic Measurements of Root Hydraulic Conductance using a High-pressure Flowmeter in the Laboratory and Field', Journal of Experimental Botany, vol. 46, no. 282, pp. 83-94.

A new high-pressure flowmeter (HPFM) is described which is capable of rapid water-flow measurements. The HPFM permits dynamic determination of hydraulic conductance of roots, K_r, and can be used in the laboratory or field. The base of a root is connected to the PRFM and water is perfused into the root system opposite to the normal direction of flow during transpiration. The perfusion pressure is changed at a constant rate of 3-7kPa s^-1 while measuring the flow into the root every 2-4s . The slope of the plot of flow versus applied pressure is K_r.

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Tyree, M.T., Velez, V., et al. 1998, 'Growth Dynamics of Root and Shoot Hydraulic Conductance in Seedlings of Five Neotropical Tree Species: Scaling to show Possible Adaption to Differing Light Regimes.', Oecologia, vol. 114, pp. 293-298.

The dynamics of growth (shoot and root dry weights, surface areas, hydraulic conductances, and root length) were measured in seedlings of five neotropical tree species aged 4-16 months. The species studied included two light demanding pioneers (Miconia argentea and Apeiba membranacea) and three shade-tolerant young- or old forest species (Pouteria reticulata, Gustavia superba and Trchillia tuberculata). Growth analysis revealed that shoot and root dry weights and hydraulic conductances on leaf area all increased exponentially with time. Alternative methods of scaling measured parameters to reveal differences that might explain adaptions to microsites are discussed. Scaling root conductance to root surface area or root length revealed a few species differences but nothing that correlated with adaption to light regimes. Scaling of root surface area or root length revealed that pioneers produced significantly more root area and length per gram dry weight investment than shade tolerant species. Scaling of root and shoot hydraulic conductances to leaf area and scaling of root conductance to root dry weight and shoot condectance to shoot dry weight also revealed that pioneers were significanly more conductive to water than shade tolerant species. The advantages of scaling hydraulic parameters to leaf surface area are discussed in terms of the Ohm's law analogue of water flow in plants.

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Wei, C., Tyree, M.T., et al. 1999, 'Direct Measurement of Xylem Pressure in Leaves of Intact Maize Plants. A Test of the Cohesion-tension Theory taking Hydraulic Architecture into Consideration', Plant Physiology, vol. 121, pp. 1191-1205.

The water relations of maize (Zea mays L. cv Helix) were documented in terms of hydraulic architecture and xylem pressure. A high-pressure flowmeter was used to characterize the hydraulic resistances of the root, stalk and leaves. Xylem pressure measurements were made with a Scholander-Hammel pressure bomb and with a cell pressure probe. Evaporation rates were measured by gas exchange and by gravimetric measurements. Xylem pressure was altered by changing gas pressure applied to the soil mass (using a root pressure bomb). Xylem pressure measured by the cell pressure probe and by the pressure bomb agreed over the entire measured range of 0 to -0.7 MPa. Experiments were consistent with the cohesion-tension theory. Xylem pressure changed rapidly and reversibly with changes in light intensity and root-bomb pressure. Increasing the root-bomb pressure increased evaporation rate slightly when xylem pressure was negative and increased water flow rate through the shoots dramatically when xylem pressure was positive and guttation was observed. The hydraulic architecture model could predict all observed changes in water flow rate and xylem. We measured the cavitation threshold for oil- and water-filed pressure probes and provide some suggestions for improvement.

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Wikberg, J. and Ögren, E. 2004, 'Interrelationships between water use and growth traits in biomass-producing willows', Trees - Structure and Function, vol. 18, no. 1, pp. 70-76.

Water use, drought response and growth were examined under controlled conditions in four interbreeding willow species from different geographical origins (two clones of Salix viminalis L., one clone of S. viminalis x S. schwerenii E. Wolf and one clone of S. purpurea L.). The levels of soil water depletion that plants could sustain without wilting varied markedly between the clones. The level of drought resistance expressed this way was positively related to resistance to xylem cavitation, negatively related to the maximum stomatal conductance, and positively related to early stomatal closure. The rate of stomatal closure, however, was negatively related to the resistance to xylem cavitation. Prior to drought, there were no significant differences between leaf-specific hydraulic conductances of the clones when whole plants were considered. However, there were differences if the roots and shoots were considered separately. Drought resistance was negatively related to maximum growth yields. This is probably because resources were diverted away from leaf production to the production of denser wood (wood density was positively related to cavitation resistance), and, for one clone, to the growth of a larger root system. In addition, because the level of drought resistance was negatively related to the maximum stomatal conductance, growth may have been adversely affected as a result of reduced photosynthesis. Given its high water extraction ability, one of the clones started to wilt sooner than expected, although only lateral shoots were affected. This appeared to indicate a strategy of sacrificing expendable shoots.

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