Predawn Xylem Water Potential Research Articles

Simulations of canopy net photosynthesis and transpiration in Quercus ilex L. under the influence of seasonal drought

A mechanistically based C 3 leaf photosynthesis model combined with an empirical stomatal model and a canopy model of light interception and microclimate was used to simulate Quercus ilex canopy net photosynthesis and transpiration at l'Avic watershed (NE Spain). The model takes into account the sun-shade leaf differentiation of photosynthetic characteristics as affected by depth within the canopy. Based on field studies, simulations were carried out for two locations within the watershed along a gradient in elevation, microclimate and forest structure. Effective predictions of diurnal and seasonal courses of stomatal conductance of sun and shade leaves for different days during the year were obtained by changing a single model variable termed g F. The value of g F determined from least squares of observed vs. simulated time courses was linearly related to pre-dawn xylem water potential over critical ranges of the response curve. Response to g F in the model may to a great extent be thought of as the integrated expression of canopy response to root system generated signals or control mechanisms. For development of predictive capability, g F is extremely useful because it allows seasonal assessments of water use and carbon dioxide uptake with differing patterns in water availability. Based on simulated responses on representative clear, overcast and variable days throughout the year, only small differences in annual totals for net photosynthesis and transpiration were found between the two sites, despite large differences in soil drying. Annual estimates of canopy water loss were in close agreement with independent estimates of evapotranspiration using the hydrological input/output method.

Physiological responses to water stress and waterlogging in Nothofagus species.

Gas exchange and water relations were investigated in Nothofagus solandri var. cliffortioides (Hook. f.) Poole (mountain beech) and Nothofagus menziesii (Hook. f.) Oerst (silver beech) seedlings in response to water stress and waterlogging. At soil matric potentials (Psi(soil)) above -0.005 MPa, N. solandri had significantly higher photosynthetic rates (A), and stomatal and residual conductances (g(sw) and g(rc)), and lower predawn xylem water potentials (Psi(predawn)) than N. menziesii. The relative tolerance of plants to water stress was defined in terms of critical soil matric potential (Psi(cri)) and lethal xylem water potential (Psi(lethal)). The estimated values of Psi(cri) and Psi(lethal) were -1.2 and -7 MPa, respectively, for N. solandri, and -0.7 and -4 MPa, respectively, for N. menziesii. Photosynthesis was sustained to a xylem water potential (Psi(xylem)) of -7 MPa in N. solandri compared with -4 MPa in N. menziesii. Following rewatering, both A and Psi(xylem) recovered quickly in N. solandri, whereas the two variables recovered more slowly in N. menziesii. During the development of water stress, nonstomatal inhibition significantly affected A in both N. solandri and N. menziesii. Nothofagus menziesii was more susceptible to inhibition of A by waterlogging than N. solandri. However, the tolerance of N. solandri to severe waterlogging was also limited as a result of a failure to form adventitious roots, suggesting a lack of adaptation to these conditions. The differences in tolerance to water stress and waterlogging between the two species are consistent with the distribution patterns of N. solandri and N. menziesii in New Zealand.

Gas exchange of 20-year-old black spruce families displaying a genotype × environment interaction in growth rate

Gas exchange and xylem water potential were measured on 20-year-old black spruce (Piceamariana (Mill.) B.S.P) trees from four full-sib families from a 7 × 7 diallel. These families constitute a 2 parent × 2 parent factorial breeding structure. One female parent (59) produced families that have displayed high productivity on three plantation sites, whereas the other female parent (63) produced families that had high growth rates on two of the sites but not on the third, least productive, site. In 1991 (a hot, dry year) measurements were obtained from trees at two sites; in 1992 (a cool, wet year) measurements were obtained from only one site. Gas exchange was measured on detached branches cut from the upper third of the crown; measurements were initiated within 2 min from cutting. In 1991, daytime xylem water potential, predawn xylem water potential, net photosynthesis (Pn), and needle conductance (gwv) were all lower in trees growing on the least productive site (2) than in trees growing on the most productive site (3), indicating that trees on site 2 were under greater water stress than those on site 3. Progeny of female 59 displayed higher Pn than progeny of female 63 over both years of contrasting rainfall and on both the wetter and drier sites during the first year. However, progeny of the two female parents did not differ with respect to gwv in either year nor on either site during the first year. The genetic variation in Pn/gwv, observed over sites and years, is positively related to growth rate only on the dry site. Thus, the genotype × environment interaction in growth appears to be due to relatively stable genetic differences in Pn. We hypothesize that water stress on the dry site reduces Pn below a threshold value at which point genetic variation in Pn impacts on productivity and results in genetic variation in growth rate. Furthermore, we hypothesize that on the wet site carbon gain per unit leaf area is not as limiting to growth rate; thus, the observed genetic differences in Pn do not influence genetic rankings in growth rate.

Physiology, morphology, and ozone uptake of leaves of black cherry seedlings, saplings, and canopy trees

Patterns of ozone uptake were related to physiological, morphological, and phenological characteristics of different-sized black cherry trees ( Prunus serotina Ehrh.) at a site in central Pennsylvania. Calculated ozone uptake differed among open-grown seedlings, forest gap saplings, and canopy trees and between leaves in the upper and lower crown of saplings and canopy trees. On an instantaneous basis, seedling leaves had the greatest ozone uptake rates of all tree size classes due to greater stomatal conductance and higher concentrations of ozone in their local environment. A pattern of higher stomatal conductance of seedlings was consistent with higher incident photosynthetically-active radiation, stomatal density, and predawn xylem water potentials for seedlings relative to larger trees. However, seedlings displayed an indeterminate pattern of shoot growth, with the majority of their leaves produced after shoot growth had ceased for canopy and sapling trees. Full leaf expansion occurred by mid-June for sapling and canopy trees. Because many of their leaves were exposed to ozone for only part of the growing season, seedlings had a lower relative exposure over the course of the growing season, and subsequently lower cumulative uptake, of ozone than canopy trees and a level of uptake similar to upper canopy leaves of saplings. Visible injury symptoms were not always correlated with patterns in ozone uptake. Visible symptoms were more apparent on seedling leaves in concurrence with their high instantaneous uptake rates. However, visible injury was more prevalent on leaves in the lower versus upper crown of canopy trees and saplings, even though lower crown leaves had less ozone uptake. Lower crown leaves may be more sensitive to ozone per unit uptake than upper crown leaves because of their morphology. In addition, the lower net carbon uptake of lower crown leaves may limit repair and anti-oxidant defense processes.

Vertical Mulching of Trees: Effects on Roots and Water Status

We examined the effect of vertical mulching on root growth and xylem water potential of large sugar maples (Acersaccharum) growing in pastures. Root mass in the 35-40 cm depth of undisturbed soil was about 50% that in the 5-10 cm depth, and root mass at the edge of the canopy (7.5 m from the trunk) was about 40% that found at 1 /4 the distance to the edge of the canopy. After three growing seasons, the abundance of roots in perlite-filled vertical mulch channels (0.6 kg/m3) was significantly (P<0.0001) less than in undisturbed soil 1-2 m away from mulch channels (4.4 kg/m3) and in undisturbed soil immediately surrounding mulch channels (3.5 kg/m3). Predawn and midday xylem water potentials, measured in late summer and used as indices of tree health, did not differ between mulched and non-mulched trees. Our results indicated that perlite-filled vertical mulch channels did not affect tree health, and, rather than being utilized as preferred pathways of growth, were avoided by tree roots.

Effects of plant size and water relations on gas exchange and growth of the desert shrub Larrea tridentata.

Larrea tridentata is a xerophytic evergreen shrub, dominant in the arid regions of the southwestern United States. We examined relationships between gasexchange characteristics, plant and soil water relations, and growth responses of large versus small shrubs of L. tridentata over the course of a summer growing season in the Chihuahuan Desert of southern New Mexico, USA. The soil wetting front did not reach 0.6 m, and soils at depths of 0.6 and 0.9 m remained dry throughout the summer, suggesting that L. tridentata extracts water largely from soil near the surface. Surface soil layers (<0.3 m) were drier under large plants, but predawn xylem water potentials were similar for both plant sizes suggesting some access to deeper soil moisture reserves by large plants. Stem elongation rates were about 40% less in large, reproductively active shrubs than in small, reproductively inactive shrubs. Maximal net photosynthetic rates (Pmax) occurred in early summer (21.3 μ mol m-2 s-1), when pre-dawn xylem water potential (XWP) reached ca. -1 MPa. Although both shrub sizes exhibited similar responses to environmental factors, small shrubs recovered faster from short-term drought, when pre-dawn XWP reached about -4.5 MPa and Pmax decreased to only ca. 20% of unstressed levels. Gas exchange measurements yielded a strong relationship between stomatal conductance and photosynthesis, and the relationship between leaf-to-air vapor pressure deficit and stomatal conductance was found to be influenced by pre-dawn XWP. Our results indicate that stomatal responses to water stress and vapor pressure deficit are important in determining rates of carbon gain and water loss in L. tridentata.

Morphological and water-stress characteristics of three Douglas-fir stocktypes in relation to seedling performance under different soil moisture conditions

This one-time greenhouse study examined the phenology, morphology, frost hardiness and response to moisture stress of three Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stocktypes from the same seed lot. The types were mini-plugTM transplants (MPT), 1+1 bareroot transplants (1+1), and 2+0 bareroot seedlings (2+0). In late summer and fall before lifting, 2+0 seedling set bud before 1+1 seedlings, 1+1 seedlings before MPT seedlings. The 2+0 seedlings appeared slowest to acquire frost hardiness in fall and seemed to deharden most rapidly in spring. Although the 2+0 seedlings were taller than the MPT stocktype, MPT and 2+0 seedlings were relatively similar in other morphological respects, but 1+1 seedlings were much larger. All stocktypes were potted on January 20, 1989, placed in a greenhouse, and subjected to 39%, 18%, 16%, or 6% soil water content (% dry weight) until the end of the experiment in mid-July 1990. The largest decrease in pre-dawn xylem water potential occurred with 16% and 6% soil water content; pre-dawn xylem water potential averaged over the three stocktypes generally declined 219% from low to high soil moisture stress. The 1+1 seedlings used more water than the other two stocktypes, and at maximum soil moisture stress, plant moisture stress increased in the order: MPT <2+0<1+1. During the 6-month greenhouse experiment, the larger 1+1 stocktype showed the most absolute growth, but the smaller stocktypes grew more on a relative scale. Growth of the stocktypes appeared to be related to differences in morphology and water-use patterns as the seedlings competed for available water within each pot. The results show that MPT seedlings, a new stocktype, performed as well as the more traditionally used 2+0 and 1+1 seedlings and that stocktype selection is important in reforestation efforts.

Physiological Responses of Loblolly Pine (Pinus taeda L.) Seedlings to Drought Stress: Osmotic Adjustment and Tissue Elasticity

Summary The physiological responses of osmotic adjustment and turgor maintenance may be critical for seedling survival under drought stress. The objectives were to determine: 1) if osmotic adjustment in shoots and changes in shoot tissue elasticity could be induced by cyclical gradual development of water deficits in seedlings, 2) if osmotic adjustment occurred in roots, and 3) to characterize important solutes contributing to osmotic adjustment. Cyclical drought stress treatments were applied to 5- to 6-month-old Pinus taeda L. seedlings from two half-sib families in a controlled-environment chamber by withholding irrigation until predawn xylem potentials reached desired levels. Predawn xylem water potentials did not fall below -0.6MPa in the irrigated, nonstressed seedlings, while in the stressed treatment, seedlings were irrigated to saturation only when predawn xylem potentials reached -1.5 to - 2.0 MPa. Pressure-volume curve analysis and solute analyses after five or more stress cycles confirmed significant osmotic adjustment. Osmotic adjustment through proline and monosaccharide (fructose, glucose) accumulation in fascicles was observed. Shoot tissue elasticity was reduced by drought stress. Osmotic adjustment was also demonstrated in both suberized and nonsuberized root portions of 16-month-old seedlings after only one drought cycle. No differences between the two families in cell wall elasticity capacities were observed. Pressure-volume curve analyses showed no significant differences in osmotic adjustment between the two families.

Transplanted red oak seedlings mediate transplant shock by reducing leaf surface area and altering carbon allocation

One-year-old red oak seedlings (Quercusrubra L.) from three open-pollinated families were produced in 1 m tall containers during 1989. In spring 1990, the seedlings were either transplanted (which included pruning the main root to a 15-cm length) or not. Transplanted seedlings either received a 5-s basal dip in 20 mM indole-3-butyric acid or did not. The seedlings were placed in a greenhouse and harvested at the beginning of the first lag phase, at the beginning of elongation of the second growth flush, and 70 days after the beginning of the experiment. Root-pruning removed 42 to 50% of whole-plant dry weight. Transplanted seedlings had significantly reduced leaf surface area and began a second growth flush later than untransplanted seedlings. There were no treatment differences in CO2 assimilation rate on a per unit leaf surface area. Predawn xylem water potential in transplanted seedlings was lower than in untransplanted seedlings. Transplanted seedlings used less root and more shoot reserves to develop the first flush than untransplanted seedlings. Treating root-pruned seedlings with 20 mM indole-3-butyric acid did not significantly increase growth potential compared with untreated transplanted seedlings. For transplanted red oak, transplant shock seems to be mediated through reduced leaf surface area, which reduces whole-plant water use.

Water Relations in Cowpea and Pearl Millet Under Soil Water Deficits. III. Extent of Predawn Equilibrium in Leaf Water Potential

Pearl millet [Penniseturn americanum (L.) Leeke] consistently develops lower predawn leaf water potentials than cowpea [Vigna unguiculata (L.) Walp.] when plants are subjected to progressive soil drying, even when they are grown as intercrops in the same pot. Lack of equilibrium in water potential during the predawn, within the plants and between plants and soil, was studied as a possible explanation for this difference. Experiments were conducted in a glasshouse in pots containing an artificial rooting medium which had a high water-holding capacity and was easy to separate from roots. Leaf and root predawn xylem water potentials were measured with a pressure chamber. In cowpea, leaf water potential (ΨL) values during the dry treatment were similar to values of root water potential (ΨR), indicating an approximate equilibrium within the plants. In millet, measurements were made on plants grown in both large and small pots. With large pots, ΨL and ΨR values in millet were similar, but with small pots, predawn ΨL was lower than ΨR. When the surfaces of these small pots were covered with evaporation barriers, however, no significant differences developed between ΨL and ΨR at predawn during soil drying, indicating an approximate equilibrium within millet. During the early stages of the dry treatment, leaf surface conductances measured at predawn indicated that significant water flux occurred at night from the leaves of both cowpea and millet. Leaf surface conductances declined to negligible levels as the dry treatment progressed, however, and night-time fluxes of water from leaves cannot explain the significant difference in predawn ΨL that developed between cowpea and millet. In order to determine whether the lower predawn ΨL in millet was due to a resistance to water uptake at the root surface, an osmoticum (- 0.5 MPa mannitol solution) was applied to pots of cowpea and millet intercrops after substantial differences in predawn ΨL had developed, and ΨL was measured. Measurements taken 9 h later indicated that predawn ΨL had recovered in millet, and the values of ΨL in millet and cowpea after mannitol treatment were similar to the osmotic potential of the mannitol solution. These results suggest that the lower predawn ΨL in millet than in cowpea under drought is due to the development of a substantial soil resistance to water movement to the root surfaces of millet.

Deep tritiated water uptake and predawn xylem water potentials as indicators of vertical rooting extent in a Quercus–Carya forest

Transpiration uptake from a tritiated water source (3H2O) associated with a sandstone bed at 3 m depth was examined in a second-growth Quercus–Carya forest. The effect of utilization of deep water on predawn xylem water potentials (Pstem) was also investigated. The 3H2O saturated the soil contacting the sandstone bed. As surface soils dried during the growing season, 3H2O uptake increased. Although all trees growing over the 3H2O reserve were capable of utilizing some of this water, Q. alba trees from all canopy levels generally exhibited higher foliage tritium (3H) activities than C. glabra (Mill.) Sweet. Higher (less negative) Pstem values were associated with trees having higher foliage 3H activities. The Q. alba tree with the highest foliage 3H activity, 1.2 × 106 Bq•L−1, exhibited the highest Pstem value, −0.875 MPa; the Q. alba tree with the lowest 3H activity also had the lowest Pstem value, 4.5 × 104 Bq•L−1 and −1.75 MPa, respectively. Pstem and the logarithm of 3H activity were highly correlated (r2 = 0.89), suggesting that predawn moisture status was at least partially controlled by the ability of trees to utilize the deep water reserve at or near the fractured sandstone bed.

Transplanting shock in Corsican pine and Cedar of Atlas seedlings: Internal water deficits, growth and root regeneration

The studies were carried out in 3-year-old seedlings of Cedrus atlantica Manetti and Pinus nigra Arn. ssp. laricio Poiret var. Corsicana. Water relations, root regeneration, bud break and shoot growth were studied in the transplanted seedlings either regularly irrigated or pre-conditioned by drought before planting. Transplanted seedlings remained water-stressed for a minimum of 190 days. Root regeneration did not occur below a predawn xylem water potential of −1.7 MPa for cedar and −1.5 MPa for pine. Drought-pre conditioned seedlings did regenerate at still lower xylem water potentials, −2.8 MPa for cedar and −1.6 MPa for pine. Bud break was greatly reduced below predawn xylem water potential of −1.5 MPa in cedar and −1.1 MPa in pine. First-season terminal-shoot growth was low for both species.

Water relations characteristics of competing singleleaf pinyon seedlings and sagebrush nurse plants

Encroachment of singleleaf pinyon ( Pinus monophylla) into adjacent low sagebrush ( Artemisia arbuscula) and basin big sagebrush ( Artemisia tridentata ssp. tridentata) communities may be enhanced by the efficient use of limited water resources by tree seedlings. Seedlings and sagebrush nurse plants were monitored over two growing seasons to determine water-use patterns. Predawn xylem water potential of low sagebrush declined rapidly, reaching −3.5 to −5.5 MPa by late summer. Big sagebrush values dropped to −2.0 to −3.0 MPa during summer drought. The drop in sagebrush xylem water potential was related to the decline in soil water potential ( r=0.68 and 0.82). The change in pinyon predawn xylem water potential was moderate, declining to values of −1.5 to −2.5 MPa. An apparent diurnal threshold xylem water potential (−2.3 to −3.0 MPa) that results in stomatal closure enables pinyon seedlings to maintain a seasonally stable xylem water potential. Water use by pinyon seedlings declined by 50% from May to August in association with a reduction in stomatal conductance. Despite reduced stomatal conductance, sagebrush water use continued to increase during summer and reached levels up to five times greater (per unit leaf area) than associated pinyon. Pinyon seedlings appear to have greater drought avoidance than sagebrush nurse plants.

High Root-zone Temperatures, Mycorrhizal Fungi, Water Relations, and Root Hydraulic Conductivity of Container-grown Woody Plants

Abstract High root-zone temperatures can stress container-grown plants and ultimately reduce nursery productivity in the southern United States. Water relations of glasshouse-grown Berberis thunbergii DC ‘Atropurpurea’ Buxus microphylla Seibold and Zucc japonica and Pittosporum tobira, (Thunb.) Ait. ‘Wheeler’ were studied under high-temperature root-stress conditions using container-grown plants that were either colonized with vesicular arbuscular mycorrhizal fungi (VAM) or noncolonized. Predawn xylem water potential in stems (ψstem) increased initially (more positive) in response to high root-zone temperatures (40° to 45°C), and then decreased over a 5-day period. Stomatal conductance (gs) and evapotranspiration (ET) were reduced incrementally over time in response to high root-zone temperatures. Root damage occurred, as indicated by reductions in root quality and gs at 35° and 40° for B. thunbergii and P. tobira, and at 40° and 45° for the more high-temperature-resistant B. microphylla. Colonization increased gs and ET of B. microphylla at ambient (25°) and high temperatures (45°) and increased ET of B. thunbergii at 25°. Colonized plants had lower (more negative) ψshoot with initial exposure to increased root-zone temperatures; however, throughout the remainder of the study period there was little reduction in plant stress with the mycorrhizal isolates used. Root hydraulic conductivity (Lp) increased markedly in B. thunbergii compared to B. microphylla at 40° and 45°, indicating less high-temperature resistance in B. thunbergii roots. Mycorrhizal colonization did not moderate hydraulic conductivity at high root-zone temperatures of 40° and 45°. Of the two species, mycorrhizal B. thunbergii had lower Lp at 25° and B. microphylla had lower Lp at 35°.

Competitive Effects of Various Grasses and Forbs on Ponderosa Pine Seedlings

Abstract Competition between ponderosa pine seedlings and various grasses and forbs was studied on a site in northern Arizona burned in 1982 by a wildfire. Two-year-old pine seedlings were planted in 3.05 x 3.05 m plots in April 1983, followed by the sowing of grass and forb seeds on the same plots in July 1983 after summer rains had begun. Predawn xylem water potential of the pine seedlings was measured biweekly throughout the 1983 and 1984 growing seasons. Extractable soil nitrogen was measured at the beginning and end of both growing seasons. After the wildfire, nitrate and ammonium levels were significantly higher in the burned area than in an adjacent unburned area. Nitrate and ammonium concentrations in various grass treatment plots were significantly (P &amp;lt; 0.05) different at both the beginning and end of the growing season. Pine seedling xylem water potential differed among treatments, with potentials being lowest on plots sown with Agropyron desertorum. The differences in seedling xylem water potential and available soil nitrogen were reflected in differences in pine seedling growth. The most effective competitors were Agropyron desertorum and Dactylis glomerata. Results indicated that competition occurred for both moisture and available nitrogen. For. Sci. 33(2):356-366.

Water Relations of Oak Species On and Adjacent to a Maryland Serpentine Soil

The predominant oaks of the Maryland Piedmont (Quercus alba and Q velutina) do not occur on serpentine soils. On serpentine sites they are replaced by Q. stellata and Q marilandica. Soil and plant water relations were measured to evaluate water as a limiting factor to distribution of oaks on serpentine soil. May through August soil water potentials of serpentine soils were not lower than adjacent nonserpentine soils. Dawn xylem water potentials of oaks on serpentine soils were similar to nonserpentine oaks during early and midsummer. By late summer, water potentials of the oaks on serpentine sites were higher. Dawn xylem water potentials and soil water potentials were significantly correlated for all species. A significant correlation between xylem water potential and stomatal conductance was found for the oaks on the serpentine soils. No such correlation was found for oaks on nonserpentine soil, but a xylem water potential threshold for stomatal closure was observed. Despite differences in their responses to moisture availability, water does not appear to be the environmental factor responsible for the distributional pattern of these species. INTRODUCTION The occurrence of serpentine soils with their atypical vegetations is a worldwide phenomenon (Walker, 1954). Serpentine soils vary widely in their mineral composition and hence in the edaphic problems encountered by the associated vegetation. Many serpentine soils are high in Mg and have a low Ca:Mg ratio. Toxic levels of Ni, Cr and Co may be present. In addition, one or more mineral nutrients (N, P, K, Mo) may be limiting to plant growth (Proctor and Woodeil, 1975). The physical properties of serpentine soils may also be adverse to plant growth (Proctor and Woodell, 1975). Frequently the soils are severely eroded, shallow and stony. These conditions often result in low soil water availability, which has been hypothesized (Monteferrante, 1973; Miller, 1977) to be an important influence on the vegetation. Whittaker (1954) observed that along a moisture continuum, plant communities of serpentine soils are more xeric than the vegetation of other soils. Few studies have examined the availability of water to plants on serpentine soils. Whittaker (1960) measured soil water in northern California and southern Oregon and found no significant differences between serpentine and nonserpentine soils. Miller (1977) measured soil water parameters (capillary water, permanent wilting percentage and available water) and found no intrinsic differences between a serpentine soil of southeastern Pennsylvania and an adjacent nonserpentine site. However, when he measured the available water in June 1964, there was less available water in the serpentine soil. He concluded that soil moisture and nutrients may have a synergistic effect in limiting the vegetation on serpentine soils. Monteferrante (1973) suggested that, in addition to soil minerals, the shallowness of the soils with their lowered water storage capacity was responsible for the patterns of vegetation. S. N. Turitzen (pers. comm.) studied the diurnal water relations of populations of Smilax rotundifo1ia on and off serpentine soil in southeastern Pennsylvania. He found similar predawn xylem water potentials in both populations, but lower midday xylem water potentials in the serpentine populations. He suggested that differences in exposure accounted for the differences in diurnal water relations. The purpose of this study was to evaluate water relations of oak species found on and off serpentine soils in order to infer if plant moisture stress may be a limiting factor for the oaks on serpentine soils.

Estimation of twig xylem water potential in young Douglas-fir trees

Methodology, based on water flow theory, is described which can be used to estimate twig xylem water potential for 10–15 m tall Douglas-fir (Pseudotsugamenziesii) (Mirb.) Franco) trees. Using pressure chamber measurements, values of needle xylem water pressure potential were found to be similar to twig xylem water pressure potential. For root zone soil water potential (ψs) &gt; −0.4 MPa, measured predawn total twig xylem water potential (ψTtx) of these Douglas-fir trees was significantly less than ψs. A transpiration rate (E) dependent single soil to twig liquid resistance (Rst) accounted for the difference between predawn ψTtx and ψs. For ψs &gt; −0.4 MPa, during the daytime when E was high, Rst could be described by a logarithmic function of ψs. The effect of capacitance on twig xylem water potential (ψtx) was small. A model incorporating the transpiration dependent Rst accurately estimated the course of ψtx on a clear day when ψs was −0.04 MPa.

Water Stress of Tallgrass Prairie Plants in Central Oklahoma

The predawn xylem water potentials of Andropogon gerardi, Schizachyrium scoparti, Panicum oligosanthes, Sporobolus asper, Ambrosia psilostachya, Psoralea tenuiflora and Sohwuun eleagnifolium were determined by the pressure equilibration chamber method during the 1980 growing season in a Central Oklahoma tallgrass prairie. Water potentials declined rapidly after June indicating high levels of water stress in all species. The decline in plant water potential for Schizachyrium scoparium, and to a lesser extent, Panicum oligosanthes, was much greater than that for ,the other 5 species. Andropogon gerardi apparently avoids dehydration by having a deep root system, whereas Schizachyrium scoparium survives in spite of a high degree of dehydration. Aboveground live biomasses declined sharply at about the same time plant water potential values decreased sharply. Results indicate plant water potential data are useful for interpreting range plant growth responses and predicting adaptability of species to harsh growing conditions.

Environmental control of leaf water conductance in conifers

Diurnal measurements of leaf conductance, xylem water potential, temperature, humidity, and radiation were taken on six sites throughout Oregon. Xylem water potential was hypothesized to influence leaf conductance in two ways. First, predawn xylem water potential controlled the early-morning maximum leaf conductance of Pseudotsugamenziesii (Mirb.) Franco. Second, irrespective of predawn water potential, a threshold diurnal water potential was found. Whenever water potential of Pseudotsugamenziesii dropped below −20 bars (−20 × 105 Pa) during the day, a significant decrease in leaf conductance resulted. If the water potential threshold was not reached during the day, leaf conductance stayed at the morning maximum or decreased slightly in response to decreasing humidity.