Báo cáo khoa học: "Whole shoot hydraulic resistance in Quercus species measured with a new high-pressure flowmeter"

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Nội dung Text: Báo cáo khoa học: "Whole shoot hydraulic resistance in Quercus species measured with a new high-pressure flowmeter"

Original article Whole shoot hydraulic resistance in Quercus species measured with a new high-pressure flowmeter A Granier 3 B Sinclair MT P Lu Tyree 1 US Department of Agriculture, Forest Service, Northeastern Forest Experiment Station, PO Box 968, Burlington, Vermont, 05402 USA; 2 INRA, Laboratoire de Physiologie Intégrée de l’Arbre Fruitier, 63039 Clermont-Ferrand, 3 INRA, Laboratoire d’Écophysiologie Forestière, 54280 Champenoux, France (Received 16 December 1992; accepted 21 April 1993) Summary — Whole shoot resistance to water flow was measured in 4 species of oak, Quercus ro- bur L, Q petraea Matt Liebl, Q pubescens Willd, and Q rubra L. Shoots were 1.1 to 1.5 m long with 16-19 mm basal wood diameter and were 4-8 yr old. Whole shoot resistances accounted for 20- 40% of the total resistance to water flow from soils to leaves based on comparisons with literature values. Leaf blade resistances accounted for 80-90% of total shoot resistances measured in this study. Stem resistances to water flow were≈ twice as large in Q robur than in the other species comparable which had stem resistances. Differences in shoot resistance between Q robur versus Q petrae are discussed in terms of the differential response of these species to drought in mixed stands. Quercus/ hydraulic resistance I water stress Résumé — Mesure de la résistance au transfert de l’eau chez différentes espèces de chênes moyen d’un nouveau fluxmètre haute pression. La résistance au transfert de l’eau de au branches a été mesurée chez 4 espèces de chênes : Quercus robur L, Q petraea Matt Liebl, Q pu- bescens Willd et Q rubra L. Les branches avaient une longueur comprise entre 1,1 et 1,5 m, pour un diamètre de 16 à 19 mm à leur base, et étaient âgées de 4 à 8 ans. La comparaison des mesures avec des données de la littérature a montré que la résistance au transfert de l’eau dans les branches était de l’ordre de 20 à 40% de la résistance hydraulique totale, calculée entre le sol et les feuilles. La résistance au transfert dans les feuilles représentait de 80 à 90% de la résistance totale de la branche. Les résistances dans les parties ligneuses étaient environ deux fois plus élevées chez Q robur que chez les autres espèces, celle-ci montrant des valeurs comparables. Les diffé- rences de résistance hydraulique entre Q robur et Q petraea sont discutées en termes de diffé- rences de réponse à la sécheresse de ces espèces dans les peuplements mélangés. Quercus / résistance hydraulique totale / résistance au transfert de l’eau
INTRODUCTION In thisstudy, we have used a new high- Some mid-European oak species are flowmeter to make rapid compari- pressure more sensitive to drought than others. Pre- sons of the hydraulic architecture of shoots liminary observations have shown that in of 4 oak species (Q robur, Q petraea, mixed stands of Quercus robur and Q pe- Q pubescens, and Q rubra). traea only the former species was in de- cline following the exceptional drought that occurred in France in 1976 (Becker and MATERIALS AND METHODS Lévy, 1982). Another related species, Q pubescens, is mostly found in Southern Plant material Europe where severe drought develops every summer. So taxa of subgenus Le- pidobalanus section robur (Krussmann, Branches of Quercus robur, Q petraea, Q pu- 1978), which includes all the above spe- bescens, and Q rubra were collected from cies, exhibit very different responses to Champenoux, France (16 km east of Nancy) from the same trees as those used in the study water stress. Since 1976, a number of of Cochard et al (1992). Branches = 2 m long studies have been undertaken to deter- and 25 mm in diameter at the base were cut mine the mechanisms of this difference in with pole pruners from the south side of mature drought resistance but no striking differ- trees in a sunny location. Within 5 min the ences have yet been found except for dif- branches were transported back to the labora- ferences in vulnerability to cavitation, Q tory where the base of the branch was placed under water and recut 0.3 m from the base to robur being more sensitive to drought- = remove some of the air bubbles sucked into the induced xylem dysfunction by cavitation stem during the initial cut. than Q petraea which is as vulnerable as Prior to connecting shoots to the high- Q pubescens (Cochard et al, 1992). pressure flowmeter described below, all cut sur- Differences in hydraulic architecture of faces were shaved with a razor blade to remove trees may contribute to their adaptation to blockage of cut vessels by cell-wall fragments formed by the initial cuts. drought (Zimmermann 1983; Tyree and Ewers 1991).The hydraulic resistance of the xylem of trees will determine, in part, The high-pressure flowmeter the degree of water stress in leaves as measured by xylem pressure potential, ψ A reduced ψ (more negative) can The flowmeter shown in figure 1 permitted the . xp xp perfusion of water into the base of a branched cause reduced cell expansion, wall synthe- system while measuring the flow rate F (kg s ). -1 sis, protein synthesis, stomatal conduc- The main body of the system was constructed tance and photosynthesis and an in- from glass tubing, tygon tubing, stopcocks, and creased xylem dysfunction by cavitation plastic T-junctions. Water was held in a flexible events. According to the soil-plant- plastic bag inside a pressure reservoir (R). Wa- atmosphere-continuum model of water ter contained in the reservoir was distilled water filtered through an 0.1 μm filter. The water was flow in trees, the ψ of leaves will be de- xp placed under pressure by compressed air, con- termined by the soil water potential, ψ , soil trolled with a pressure regulator (PR) using gas the hydraulic resistances of the root and from a compressed-air tank (not shown). The shoot (R and R respectively) and the r , s water was directed through a capillary tube (CT, evaporative flux density from leaves, E, 0.7 mm diameter and 0.12 m long) and then according to the following equation. onto the shoot. The rate of flow, F, across the
CT is proportional to the pressure drop across ent values by changing the air pressure in R and the tube; this pressure drop was recorded with a the rate of flow (kg s into the con- ) -1 measuring 2-arm water manometer system made from tainer of water on the balance. Calibration curves thick-walled glass capillary tubes of 1.5 mm in- were linear with a maximum deviation from the ternal diameter. The water level in the right arm best fit straight line of 1.5% full scale. The differ- of the manometer (MR) was always at the same ence in water levels, Δh, was rarely 0 at F 0, = level as the water in the reservoir (R). The same because of differences in surface tension of wa- air pressure used to pressurize water in the ter in MR and ML. The height difference at F = 0 reservoir (R) was transmitted to the top of the was measured and subtracted from all readings right and left manometer columns via lengths of (usually a correction of 1-3 mm). The problem tygon tubing. This prevented the water in the of a non-zero Δh could have been eliminated o right arm of the manometer (MR) from rising by replacing the manometer columns with a dif- above the level of water in the reservoir when ferential pressure gauge like that used in a low- the water was under pressure. The level of wa- pressure flow meter described by Tyree (1983). ter in the left arm of the manometer (ML) de- However, that would have eliminated the main pended on the rate and direction of flow across advantages of the present high-pressure flow CT. Usually, flow was from right to left across meter, ie, that it was inexpensive and could be CT (fig 1) and this made the level in ML below used without a power source under field condi- that in MR. To facilitate more accurate measure- tions. ment of the height difference, Δh, between MR and ML, a water level (WL) was used to transfer the level of water from MR to ML. The WL con- Measurement of shoot resistances sisted of a length of tygon tubing partly filled with water. The position of the tubing was ad- justed so that the level of water in WL coincided Shoot resistances were measured by connect- with that in MR; the Δh could be measured at ing the flowmeter to a shoot and perfusing water the place shown in figure 1. Three-way stop- at 0.2 MPa pressure for 2 or 3 h. Initially, flow cocks (S and S were used to fill the flowmeter 1 ) 2 rate was high but declined gradually. The initially and reservoir with water and S was used to re- 3 high flow rate was attributed to negative leaf wa- lease air pressure from the system. ter potentials, ψ However, after 2 or 3 h the leaf The flowmeter was calibrated by directing leaf air spaces were visibly infiltrated with water flow of water across a length of stem segment and water dripped from the stomata of some via water-filled tubing to a container of water on leaves and F became stable. Shoot resistance a balance. Flow rate, F, was adjusted to differ- was computed from:
where P was the applied water pressure, and A was the total leaf area of the shoots measured with a delta-T leaf area meter (Delta-T Devices Ltd, Cambridge, UK) at the end of the experi- ment. Normalization of R by multiplying P/F by s A was justified because preliminary experiments revealed that large shoots (with large A) had smaller value of P/F than small shoots; see Yang and Tyree (1993) for a discussion of how P/F depends on branch size in Acer saccharum. Resistances of the components of a shoot measured by making resistance measure- were ments after removal of each component. For ex- ample, the resistance of the whole shoot was measured before and after removal of leaf blades. Leaf-blade resistance was calculated from Subsequently, all petioles were removed, then current-year shoots, then all 1-yr-old shoots, all etc. Measurements of the branch resistance be- fore and after each removal were used to calcu- late resistances of each component by differ- ence. All values were normalized by multiplying P/F by A. RESULTS Shoot resistances of oak were measured on shoots 1.1-1.5 m long with leaf areas of 1.1 to 2.1 m and basal diameters of 2 16-19 mm. The shoots ranged in age from 4-8 yr. Resistances of removed compo- nents are shown in figure 2A. Leaf blade 20-fold that of any resistances were > other component (eg, petioles, current- year shoots, 1-yr-old shoots etc). The leaf blade resistance of Q pubescens (2.42 ± 0.12 x 10 MPa s m kg was significant- 2 -1 4 ) ly higher (P 0.05) than that of the other = species which were not significantly differ- ent from each other (1.82 ± 0.12, 1.89 ± 0.16, 2.04 ± 0.07 x 10 for Q petraea, Q 4 robur, and Q rubra, respectively). Petioles of Q robur were too small to remove sep-
arately, but the petiole resistances of all and Alexander, unpublished data). The other species were significantly less than leaf-blade resistance includes vascular and nonvascular pathways from the base that from the current-year shoots. Petioles of the leaves to mesophyll airspaces, but were removed by breaking them off from we are of the opinion that the main resis- the current-year shoots. They broke near tance to water flow is probably in the non- where the abscission zone would have vascular part of the path (Tyree and formed in fall, but part of the vascular in- Cheung, 1977). sertion zone would have remained behind in the current-year shoots. Our methods Leaf-blade resistances are relevant to a did not permit us to estimate the junction better understanding of stomatal physio- constrictions (if any were present) between logy because they allow us to estimate the petioles and current-year shoots. gradient in water potential between minor There was a general trend of declining veins and stomata, ie, leaf-blade resistanc- stem component resistance to water flow es can be used to predict localized stoma- with increasing age of the stem. tal desiccation. Leaf blade resistances were very high when considered in terms In figure 2B the data are replotted to of the water potential drop that would oc- show the shoot resistance remaining after cur in them during normal transpiration. removal of each component labelled on Quercus leaves have evaporative flux den- the x-axis. "W" refers to the whole-shoot sities of 6 x 10 kg s m at midday (Bré- -1 -2 -5 resistance (with leaves present). The resis- da and Granier, unpublished data). Ac- tance for "LB" refers to the resistance re- cordingly the drop in ψ from the base of maining after removal of leaf blades (peti- the blade to mesophyll air spaces must be oles and all stems were still present). "P" E•R blade 0.87-1.45 MPa for Q pubes- leaf = refers to the resistance of the shoots after cens and Q petraea, respectively, with the removal of the petioles (all stems were still other 2 species within the above range. present). The other notations on the x-axis The resistances measured in this paper have analogous meanings. The percent- are probably about the same as or less age of the whole shoot resistance remain- than the resistance encountered by water ing after removal of the leaf blades was 8.7 during normal transpiration. The resistance ± 0.3, 11.4 ± 1.6, 13.5 ± 1.5, and 18.5 ± to water flow in Quercus leaf blades could 1.8 for Q pubescens, Q rubra, Q petraea, be higher during normal transpiration if and Q robur, respectively. Thus, the leaf- most water evaporation occurs near the blade resistances were 80-90% of the stomata in accordance with the evidence whole-shoot resistance. in support of peristomatal evaporation in substomatal cavities (Tyree and Yianoulis, 1980; Yianoulis and Tyree, 1984). The DISCUSSION large resistance to water flow in leaves would cause a large reduction in the water potential of the guard cells of stomata and The leaf-blade resistances of Quercus could account for the partial closure of (1.87 to 2.4 x 10 MPa m s kg are 2-4 2 -1 4 ) stomata around midday observed in many times more than that which is found in Quercus species (Tenhunen et al, 1985; other species where the values range from Epron et al, 1992). 0.5 to 1 x 10 MPa m s kg for Fagus 2 -1 4 and Cheung, 1977), Ju- of this study was One of the objectives grandifolia (Tyree glans regia (Tyree et al, 1993) and for Acer if we could find further physiological to see evidence for Q robur being more in decline saccharum and Populus deltoides (Tyree
after drought episodes than Q petraea. Q predict that 30 or 50% of the total resis- tance to water flow is contained in the robur is more vulnerable to cavitation than Q petraea, the former reaching 50% loss above-ground portion of trees with perhaps 80% of the shoot resistance contained in of the conductivity in petioles and current the leaf blades. The remainder of the whole year stems at ψ -2.7 MPa whereas the = xp latter did not reach 50% loss of conductivi- tree resistance to water flow is accounted ty until ψ -3.3 MPa (Cochard et al, for by roots and soil near the roots. = xp 1992). Evaporative flux densities, E, are Studies have shown that R increases tree about the same for Q robur and Q petraea, by 400-500% as predawn ψ fall from 0 to s but the shoot resistances to water flow are -2 MPa (Bréda et al, 1993; Simonin et al, 1.5- to 2-fold higher in Q robur than in Q 1993) but that embolisms in small branch- petraea (fig 3B). This difference in shoot es and petioles can account for only a 20 resistance will tend to make stem ψ more s 30% increase in resistance of small or negative in Q robur than in Q petraea. branches. It therefore seems unlikely that These differences in shoot resistance and cavitation and differences in shoot resi- in vulnerability to cavitation could make Q tance can account for all the observed robur cavitate earlier in a drought cycle changes in the hydraulics of whole trees than Q petraea. However, it is difficult to during drought. How whole-tree resistances say if the observed differences in shoot re- to water flow changes during drought, may sistances of relatively small shoots in this be important for a better understanding of study will have a dominating affect on field adaptation to drought. performance of the 2 species without fur- However, differences in stem resistanc- ther knowledge of root and bole resistanc- could account for differences in growth es es of the 2 species. rate under mild drought. Higher stem resis- The shoot resistances we have meas- tances will cause lower stem ψ and thus s ured are only a small fraction of the sum of lower stem cell turgor pressures in meri- the resistances in the soil, root, shoot and stematic zones. This in turn could cause leaf of whole trees of Quercus. Whole tree slower growth rates in Q robur versus Q resistances, R have been estimated for , tree petraea (Cosgrove, 1986). More studies Q robur and Q petraea based on meas- will be necessary to determine the effect of ures of predawn water potential (as an es- differences in shoot resistance on differ- timate of ψ and the relatioship between ) soil ences in performance of tree species dur- ψ and stem water flow under well- leaf ing drought. watered conditions. These R values are tree in the range of 5 to 10 x 10 MPa s m kg- 4 2 1 and do not vary much with tree size (Cer- REFERENCES mak et al, 1980; Bréda et al, 1993; Simo- nin et al, 1993). Accordingly, the shoot re- Becker M, Lévy G (1982) Le dépérissement du sistance of this study accounts for about chêne en forêt du Troncais. Les causes éco- 20-40% of the resistance of the entire logiques. Ann Sci For 36, 439-444 soil-plant hydraulic pathway. In a study on Bréda N, Cochard H, Dreyer E, Granier A leafless shoots of Acer saccharum,≈ 50% (1993) Water transfer in a mature oak stand the total resistance to water flow in shoots (Quercus petraea): seasonal evolution and 0.12 m in diameter at the base is con- effects of a severe drought. Can J For Res tained in branches < 0.02 m basal diame- 23, 1130-1143 ter (Yang and Tyree, 1993). If the same Cermak J, Huzulak J, Penka M (1980) Water po- pattern holds in Quercus, then we might tential and sap flow rate in adult trees with
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