Xylem Pressure Potential Research Articles

Effects of Wet Acid Deposition and Soil Fertility on Seedling Growth, Nutrition, and Water Relations

Responses of juvenile white oak (Quercus albaL.) and loblolly pine (Pinus taedaL.) to simulated acid precipitation (pH 4.8, 4.2, and 3.6), as influenced by soil fertility, were examined. Height and stem diameter growth of white oak decreased with increasing rainfall acidity, although the height response was confined to the high fertility treatment, and root mass was reduced by pH 3.6 rains as well. Height and diameter growth also declined with increasing acidity in loblolly pine, and shoot and root mass and root length and specific length were reduced in low fertility by pH 3.6 rains. Direct foliar injury increased with increasing acidity in loblolly pine, but pH 3.6 rainfall stimulated mycorrhization of this species. Foliar Ca, Mg, and Cu were reduced in both species by pH 3.6 and pH 4.2 rains as was S by the former. Conversely, the Fe concentration was elevated by pH 3.6 and pH 4.2 rainfall in loblolly pine as was Zn by the former. Foliar Al increased with increasing acidity in both white oak and loblolly pine, a response partially attenuated by high soil fertility. Xylem pressure potential of loblolly pine that had received pH 3.6 rainfall declined below that of other treatments during a simulated drought episode.

Leaf and root control of stomatal closure during drying in soybean

The stomatal conductance of an illuminated 2.5 cm2 area of an intact soybean leaflet was the same whether the rest of the shoot was in light or darkness. This was true throughout soil drying cycles. Water potential of tissue immediately outside the illuminated area consistently decreased about 0.3 MPa upon illumination of the shoot. This erroneously suggested that stomatal conductance during soil drying did not respond to diurnal reductions in leaf water potential, but was controlled by root or soil water status. Tests showed that the water potential of tissue in the illuminated area did not change in the steady‐state upon illumination of the rest of the shoot. Water potentials of shaded sections of leaves were not different from predawn water potentials, and were higher than leaf xylem pressure potentials as determined with a pressure chamber. These steep local gradients of leaf water potential suggest that there is minimal interchange of water among xylem elements leading from roots to different sections of leaves. The relationship between stomatal conductance and leaf water potential was the same whether leaf water potential was reduced by soil drying, application of polyethylene glycol (PEG) to the root system, lowering root temperature, or leaf excision. In the root cooling experiment, there was no soil drying, and with leaf excision, there was no root drying. The similarity of stomatal responses to leaf water potential in all cases strongly suggests control of conductance by a signal produced by local leaf water potential rather than root or soil water status in these experiments.

Refilling of embolized vessels in young stems of laurel. Do We need a new paradigm?

Recovery of hydraulic conductivity after the induction of embolisms was studied in woody stems of laurel (Laurus nobilis). Previous experiments confirming the recovery of hydraulic conductivity when xylem pressure potential was less than -1 MPa were repeated, and new experiments were done to investigate the changes in solute composition in xylem vessels during refilling. Xylem sap collected by perfusion of excised stem segments showed elevated levels of several ions during refilling. Stem segments were frozen in liquid N2 to view refilling vessels using cryoscanning electron microscopy. Vessels could be found in all three states of presumed refilling: (a) mostly water with a little air, (b) mostly air with a little water, or (c) water droplets extruding from vessel pits adjacent to living cells. Radiographic probe microanalysis of refilling vessels revealed nondetectable levels of dissolved solutes. Results are discussed in terms of proposed mechanisms of refilling in vessels while surrounding vessels were at a xylem pressure potential of less than -1 MPa. We have concluded that none of the existing paradigms explains the results.

Growth, water relations, and survival of drought-exposed seedlings from six maternal families of honey mesquite (Prosopis glandulosa): responses to CO(2) enrichment.

Low water availability is a leading contributor to mortality of woody seedlings on grasslands, including those of the invasive shrub Prosopis. Increasing atmospheric CO(2) concentration could favor some genotypes of this species over others if there exists intraspecific variation in the responsiveness of survivorship to CO(2). To investigate such variation, we studied effects of CO(2) enrichment on seedling survival in response to uniform rates of soil water depletion in six maternal families of honey mesquite (P. glandulosa Torr. var. glandulosa). Three families each from the arid and mesic extremes of the species' distribution in the southwestern United States were studied in environmentally controlled glasshouses. Relative water content at turgor loss and osmotic potential were not affected by CO(2) treatment. Increased atmospheric CO(2) concentration, however, increased growth, leaf production and area, and midday xylem pressure potential, and apparently reduced transpiration per unit leaf area of seedlings as soil dried. Consequently, CO(2) enrichment about doubled the fraction of seedlings that survived soil water depletion. Maternal families of honey mesquite differed in percentage survival of drought and in several other characteristics, but differences were of similar or of smaller magnitude compared with differences between CO(2) treatments. There was no evidence for genetic variation in the responsiveness of survivorship to CO(2). By increasing seedling survival of drought, increasing atmospheric CO(2) concentration could increase the abundance of honey mesquite where establishment is limited by water availability. Genetic types with superior ability to survive drought today, however, apparently will maintain that advantage in the future.

Longleaf Pine Photosynthetic Response to Soil Resource Availability and Elevated Atmospheric Carbon Dioxide

AbstractGas exchange responses during a drought cycle were studied in longleaf pine (Pinus palustris Mill.) seedlings after prolonged exposure to varying levels of atmospheric CO2 (≈365 or ≈730 µmol CO2 mol−1), soil N (40 or 400 kg N ha−1 yr−1), and water (“adequate” and “stressed”). Elevated atmospheric CO2 concentration increased photosynthesis, tended to decrease stomatal conductance, and increased water‐use efficiency (WUE). Although soil resource availability influenced gas exchange measurements, it generally did not affect the magnitude or direction of the response to CO2 concentration. However, significant interactions among treatment variables were observed for plant xylem pressure potential. In seedlings grown with high N, a positive growth response to elevated atmospheric CO2 increased whole‐plant water use resulting in more severe plant water stress, despite increased leaf‐level WUE; however, under low N conditions the lack of a growth response to elevated CO2 reduced whole‐plant water use, decreased water stress severity, and increased WUE. Photosynthetic response to CO2 was greatest in the high N treatment at the beginning of the drought cycle, but diminished as water stress increased; however, plants grown with low N showed greater photosynthetic responses to CO2 later in the drought cycle. Therefore, plant gas exchange rates interact with growth response in determining the severity of water stress under drought and, thus, the ability of elevated atmospheric CO2 to ameliorate the effects of drought and allow plants to maintain increased rates of photosynthesis may be influenced by the availability of other resources, such as N and water.

COMPARATIVE PHYSIOLOGICAL ECOLOGY OF LUPINES COLONIZING EARLY SUCCESSIONAL HABITATS ON MOUNT ST. HELENS

Lupinus lepidus, a prostrate, evergreen perennial and Lupinus latifolius, a robust, deciduous perennial were prominent survivors and among the first colonizers of early successional habitats following the 1980 eruption of Mount St. Helens. In this study, interspecific and age-specific variation in the water relations and photosynthetic properties of these lupines were assessed to determine their physiological capacity to colonize these volcanic habitats. Comparative water-relations studies showed that seasonal and diurnal trends in plant water balance were similar between species and age classes. Stomatal conductance and xylem pressure potential of seedlings and adults were closely coupled with soil water potential and leaf-to-air vapor-pressure gradients. Relatively high osmotic potentials were linked with high leaf water contents and cell-wall elasticity. Collectively, these water-relations properties are indicative of mesophytic, herbaceous species with leaf tissues sensitive to soil and atmospheric water deficits. The photosynthetic capacity of L. lepidus was higher over a wider range of environmental conditions than that of L. latifolius. L. lepidus had a higher light saturation point and was able to acclimate photosynthetically to higher temperatures, relative to L. latifolius. The photosynthetic activity of L. lepidus seedlings was significantly greater at higher light and temperature regimes compared to seedlings of L. latifolius. Differing photosynthetic responses to temperature and light help explain the contrasting demographic patterns of these lupines and the relative dominance of L. lepidus in early successional habitats on Mount St. Helens. The physiological capacity to respond to extreme temperatures and periodic water deficits also appears to be an important determinant of the natural distribution patterns of these two species in the Pacific Northwest: L. lepidus occupies windy, exposed alpine habitats, whereas L. latifolius is common within subalpine meadows and coniferous treeline communities.

Drought Stress Increases Densities but Not Populations of Two-spotted Spider Mite on Buddleia davidii `Pink Delight'

Two experiments were conducted to determine the effect of drought stress on the susceptibility of Buddleia davidii Franch. `Pink Delight' to the two-spotted spider mite (Tetranychus urticae Koch). In the first experiment, drought stress was imposed by withholding water until predawn xylem pressure potential fell below -1 MPa. Shoot growth was 75% less in drought-stressed than in nonstressed plants. Mite population densities were not affected, but noninfested leaf area was 14% higher, and degree of mite damage was lower, in nonstressed plants. Evidently, the greater amount of new growth in nonstressed plants leads to lower spider mite densities by diluting populations. In a second experiment, nonstressed B. davidii `Pink Delight' plants were watered every 1 to 2 days and drought-stressed plants were watered every 3 days. Spider mite populations were monitored by sampling newly expanded and mature foliage. Mite populations on mature foliage were not affected by stress, but stressed plants grew less and had larger spider mite populations on their newly expanded foliage than did nonstressed plants.

Tissue chemistry and carbon allocation in seedlings of Pinus palustris subjected to elevated atmospheric CO(2) and water stress.

Longleaf pine (Pinus palustris Mill.) seedlings were grown in 45-l pots and exposed to ambient or elevated (365 or 730 &mgr;mol CO(2) mol(-1)) CO(2) concentration in open-top chambers for 20 months. Two water-stress treatments (target values of -0.5 or -1.5 MPa xylem pressure potential) were imposed 19 weeks after initiation of the study. At harvest, tissues (needles, stems, taproots, coarse roots, and fine roots) were analyzed for carbon (C), nitrogen (N), nonpolar extractives (fats, waxes, and oils), nonstructural carbohydrates (sugars and starch), structural components (cellulose and lignin), and tannins. The greatest dry weights and lowest N concentrations occurred in tissues of plants grown at elevated CO(2) or with adequate water. Although allocation of C fractions among tissues was generally unaffected by treatments, concentrations of the analyzed compounds were influenced by treatments in needles and taproots, but not in stems and lateral roots. Needles and taproots of plants exposed to elevated CO(2) had increased concentrations of nonstructural carbohydrates. Among plant tissues, elevated CO(2) caused reductions in structural C concentrations and foliar concentrations of fats, waxes and oils.

Thinning, fertilization, and crown position interact to control physiological responses of loblolly pine.

To examine physiological responses to thinning, fertilization, and crown position, we measured net photosynthesis (P(n)), transpiration (E), vapor pressure difference (VPD), stomatal conductance (g(s)), and xylem pressure potential (Psi(1)) between 0930 and 1130 h under ambient conditions in the upper and lower crowns of a 13-year-old loblolly pine (Pinus taeda L.) plantation six years (1994) after the treatments were applied. Photosynthetic photon flux density (PPFD) and air temperature (T(a)) within the canopy were also recorded. Needle P(n) of thinned trees was significantly enhanced by 22-54% in the lower crown, because canopy PPFD increased by 28-52%. Lower crown foliage of thinned plots also had higher E and g(s) than foliage of unthinned plots, but thinning had no effect on needle Psi(1) and predawn xylem pressure potential (0430-0530 h; Psi(pd)). Tree water status did not limit P(n), E and g(s) during the late-morning measurements. Fertilization significantly decreased within-canopy PPFD and T(a). Needle Psi(1) was increased in fertilized stands, whereas P(n), E and g(s) were not significantly altered. Upper crown foliage had significantly greater PPFD, P(n), VPD, g(s), E, and more negative Psi(1) than lower crown foliage. In both crown positions, needle P(n) was closely related to g(s), PPFD and T(a) (R(2) = 0.77 for the upper crown and 0.82 for the lower crown). We conclude that (1) silvicultural manipulation causes microclimate changes within the crowns of large trees, and (2) needle physiology adjusts to the within-crown environmental conditions.

The canopy water relations of old-growth Douglas-fir trees

We investigated whole tree water relations in 56–65 m tall, old-growth Pseudotsuga menziesii trees within the Wind River Canopy Crane site, Carson, Washington, USA. We measured at predawn and solar noon the vertical gradients in xylem pressure potential using a pressure chamber. On an Abies amabilis sapling located in the understory at the base of one of the study trees, predawn and solar noon xylem pressure potentials were also measured. Xylem pressure potential data were measured from late June through early September 1996 on foliage sampled from 1 to 64.5 m. Over this height gradient, predawn water potentials ranged from –0.23 to –1.10 MPa. Solar noon values showed an even greater range (from –0.44 to –2.51 MPa). At predawn, the water potential gradient approached the theoretical hydrostatic gradient (–0.0105 vs –0.010 MPa m–1). The gradient at solar noon was steeper (–0.0331 MPa m–1). Instantaneous stomatal conductances were not greatly different between young, sapling-sized and old-growth trees [0.094±0.033 (SD) vs 0.086±0.045 cm s–1, respectively]. Stomata of both size classes of trees appeared very sensitive to increasing vapor pressure deficits. A comparison of stable carbon isotope values from the old-growth and sapling-sized trees indicated lower stomatal conductances in the old-growth. This study provides sound documentation regarding the utility of the cohesion theory in the interpretation of water potential gradients. This study also emphasizes inherent differences between sapling-sized and tall, old-growth trees.

Anatomical and Morphological Alterations in Longleaf Pine Needles Resulting from Growth in Elevated CO2: Interactions with Soil Resource Availability

Studies of anatomical changes in longleaf pine (Pinus palustris Mill.) needles for plants exposed to elevated atmospheric CO2 may provide insight into the potential influences of global CO2 increases on plant productivity. Longleaf pine seedlings were grown in open‐top field chambers supplied with either ambient (∼365 μmol mol−1) or elevated (∼720 μmol mol−1) atmospheric CO2 for 20 mo. Two levels of soil nitrogen (40 and 400 g ha−1 yr−1) and two soil moisture regimes (−0.5 or −1.5 MPa predawn xylem pressure potential) were used in combination with CO2 treatments. Needle tissue was collected 12 and 20 mo after treatment initiation and subjected to light and scanning electron microscopy. There was no effect of elevated CO2 on stomatal distribution or the proportion of internal leaf area allocated to a given tissue type at either sampling date. Although the relationships between vascular, transfusion, mesophyll, and epidermal tissue cross‐sectional areas to total leaf cross‐sectional areas appear nonplastic,...

Effect of Biostimulants on Drought Tolerance of Ornamental Annuals

This study determined the effect of the granular humate Earthgreen (Menefee Mining Corp., Dallas) and the biostimulant ROOTS2™ (LISA Products Corp., New Haven, Conn.) on drought tolerance of the ornamental annuals Impatiens wallerana `Accent White' and Salvia splendens `Red Hot Sally'. Earthgreen was incorporated into the soil and ROOTS2™ applied as a root drench at transplanting and 30 days after transplanting, Transplants were established in greenhouse beds under irrigated conditions for 8 weeks, then subjected to a five week drought. Data indicates rates of 43.9 and 87.9 g·m–2 of Earthgreen or 300 ml of 1% or 2% ROOTS2™ per plant had no significant effect on Salvia shoot dry weight or chlorophyll content of Salvia or Impatiens at the end of the 8-week establishment period. However, Salvia flower dry weight was significantly increased with the higher rate of Earthgreen. During the 5-week drought, Earthgreen or ROOTS2™ did not significantly affect xylem pressure potential of Salvia or relative water content of Impatiens. After the 5-week drought no significant effect of either product on shoot dry weight of Salvia or Impatiens or flower dry weight of Salvia was observed.

Effects of nitrogen and water limitation and elevated atmospheric CO2 on ectomycorrhiza of longleaf pine

summaryLongleaf pine (Pinus palustris Mill.) seedlings were exposed to two concentrations of atmospheric CO2 (365 or 720 μmol mol−1) and two levels of N (0.02 or 0.20 mg N g−1 soil yr−1) within open‐top chambers for 20 months. Seedlings were adequately watered for 19 wk to ensure seedling establishment, after which two water‐stress treatments (target values −0·5 or −1·5 MPa xylem pressure potential) were implemented. Fine‐root samples were collected in July and November 1993, and in March and November 1994. Ectomycorrhizal and non‐mycorrhizal short roots per unit length of fine root were quantified. The percentage of ectomycorrhizal short roots and numbers of ectomycorrhizas per unit root length were higher for seedlings grown with elevated CO2, low N and adequate water. Interactions among main treatment variables demonstrated higher percentages of ectomycorrhizal short roots, fine root length per seedling, and total numbers of ectomycorrhizas per seedling for plants grown with high CO2 (compared with ambient) or adequate water (compared with water stress) only under high N conditions. Increased fine‐root length and ectomycorrhizal colonization under elevated CO2 resulted in higher (almost double) numbers of ectomycorrhizas per seedling at each sampling.

Influence of atmospheric CO2 enrichment, soil N, and water stress on needle surface wax formation in Pinus palustris (Pinaceae)

Interactive effects of increasing atmospheric CO2 with resource limitations on production of surface wax in plants have not been studied. Pinus palustris seedlings were grown for 1 yr at two levels of soil N (40 or 400 kg N_ha-1_yr-1) and water stress (-0.5 or -1.5 MPa xylem pressure potential) in open-top field chambers under two levels of CO2 (365 or 720 mumol/mol). Needle surface wax content was determined at 8 mo (fall) and 12 mo (spring) and epicuticular wax morphology was examined using scanning electron microscopy (SEM) at 12 mo. Wax content expressed on both a leaf area and dry mass basis was increased due to main effects of low N and water stress. No main effects of CO2 were observed; however, a CO2 x N interaction at 12 mo indicated that under low soil N the elevated CO2 treatment had less wax (surface area or dry mass basis) compared to its ambient counterpart. Morphologically, low N needle surfaces appeared rougher compared to those of high N needles due to more extensive wax ridges. Although the main effect of water treatment on wax density was not reflected by changes in wax morphology, the CO2 x N interaction was paralleled by alterations in wax appearance. Decreases in density and less prominent epicuticular wax ridges resulting from growth under elevated CO2 and limiting N suggest that dynamics of plant/atmosphere and plant/pathogen interactions may be altered.

Carbon isotopic composition, gas exchange, and growth of three populations of ponderosa pine differing in drought tolerance.

Effects of water supply on gas exchange, carbon isotopic composition, and relative growth rate were compared among seedlings from three populations of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) grown in a controlled environment chamber. The three populations were chosen to represent high, moderate and low drought tolerance. There was no indication that drought tolerance was related to high water-use efficiency. Populations differed (P < 0.05) in relative growth rate (RGR), but did not differ (P > 0.10) in gas exchange variables or carbon isotope ratio (delta(13)C). Well-watered seedlings had significantly higher RGR, xylem pressure potential (Psi(xpp)), net photosynthesis (A), stomatal conductance to water vapor (g), and lower delta(13)C and instantaneous water-use efficiency than water-stressed seedlings. With decreasing Psi(xpp), A decreased linearly, whereas g decreased exponentially. Seedlings of the highly drought-tolerant population were more sensitive to water availability than seedlings from the other populations; they used water quickly when water was available, but closed their stomata in response to water stress. We conclude that, in ponderosa pine, the drought avoidance mechanism is more important for survival and growth in arid and semiarid environments than the efficient use of water.

Water vapour fluxes and their impact under elevated CO2 in a C4‐tallgrass prairie

We measured leaf‐level stomatal conductance, xylem pressure potential, and stomate number and size as well as whole plant sap flow and canopy‐level water vapour fluxes in a C4‐tallgrass prairie in Kansas exposed to ambient and elevated CO2. Stomatal conductance was reduced by as much as 50% under elevated CO2 compared to ambient. In addition, there was a reduction in stomate number of the C4 grass, Andropogon gerardii Vitman, and the C3 dicot herb, Salvia pitcheri Torr., under elevated CO2 compared to ambient. The result was an improved water status for plants exposed to elevated CO2 which was reflected by a less negative xylem pressure potential compared to plants exposed to ambient CO2. Sap flow rates were 20 to 30% lower for plants exposed to elevated CO2 than for those exposed to ambient CO2. At the canopy level, evapotranspiration was reduced by 22% under elevated CO2. The reduced water use by the plant canopy under elevated CO2 extended the photosynthetically‐active period when water became limiting in the ecosystem. The result was an increased above‐ and belowground biomass production in years when water stress was frequent.

Effects of atmospheric CO(2) on longleaf pine: productivity and allocation as influenced by nitrogen and water.

Longleaf pine (Pinus palustris Mill.) seedlings were exposed to two concentrations of atmospheric CO(2) (365 or 720 micro mol mol(-1)) in combination with two N treatments (40 or 400 kg N ha(-1) year(-1)) and two irrigation treatments (target values of -0.5 or -1.5 MPa xylem pressure potential) in open-top chambers from March 1993 through November 1994. Irrigation treatments were imposed after seedling establishment (i.e., 19 weeks after planting). Seedlings were harvested at 4, 8, 12, and 20 months. Elevated CO(2) increased biomass production only in the high-N treatment, and the relative growth enhancement was greater for the root system than for the shoot system. In water-stressed trees, elevated CO(2) increased root biomass only at the final harvest. Root:shoot ratios were usually increased by both the elevated CO(2) and low-N treatments. In the elevated CO(2) treatment, water-stressed trees had a higher root:shoot ratio than well-watered trees as a result of a drought-induced increase in the proportion of plant biomass in roots. Well-watered seedlings consistently grew larger than water-stressed seedlings only in the high-N treatment. We conclude that available soil N was the controlling resource for the growth response to elevated CO(2) in this study. Although some growth enhancement was observed in water-stressed trees in the elevated CO(2) treatment, this response was contingent on available soil N.

Effects of Acidic Precipitation and Ectomycorrhizal Inoculation on Growth, Mineral Nutrition, and Xylem Water Potential of Juvenile Loblolly Pine and White Oak

Individual and interactive effects of simulated acidic rainfall and mycorrhizal inoculation on growth and nutrient and water relations of loblolly pine (Pinus taedaL.) and white oak (Quercus albaL.) grown in a loam soil were examined. Seedlings of each species inoculated with basidiospores of the ectomycorrhizal fungus Pisolithus tinctorius(Pers.) Coker and Couch, a known my-cobiont of both loblolly pine and white oak, and uninoculated control seedlings received two simulated rains per week of either pH 3.6, 4.2, or 4.8 for 26 weeks. Higher acidity rainfall reduced the growth but increased mycorrhizal colonization of loblolly pine, while both loblolly pine and white oak exposed to these rains exhibited greater foliar injury. Inoculation with P. tinctoriusincreased growth and reduced foliar injury of both species. Foliar concentrations of P, S, and Cu in loblolly pine and white oak, Ca in loblolly pine, and Fe and Zn in white oak decreased with increasing rain acidity while the Al concentration of both species increased. Higher rainfall acidity also reduced soil pH and Ca and Mg concentrations while increasing soil AI. Foliage of inoculated seedlings of both species had higher N and P concentrations and lower Al concentrations than control seedlings. Following the final rain applications, a drought cycle was simulated by withholding irrigation for two weeks during which seedling xylem pressure potential and soil water potential were measured. One day after cessation of irrigation, xylem pressure potential of loblolly pine that had received pH 3.6 rains was lower than that of other treatments. Thereafter, xylem pressure potential and soil water potential of the inoculated treatment decreased below those of the control treatment in both species. These results suggest that acid deposition is detrimental to juvenile loblolly pine and white oak, but the magnitude of this effect is less than the positive response to ectomycorrhizal inoculation.

Elevated atmospheric CO2 differentially affects needle chloroplast ultrastructure and phloem anatomy in Pinus palustris: interactions with soil resource availability

ABSTRACTThe response of forest species to increasing atmospheric CO2, particularly under resource limitations, will require study in order to predict probable changes which may occur at the plant, community and ecosystem levels. Longleaf pine (Pinus palustris Mill.) seedlings were grown for 20 months at two levels of CO2 (365 and 720 μol mol1) in two levels of soil nitrogen (4 and 40 g m−2), and with two levels of soil moisture (–0·5 and –1·5 MPa xylem pressure potential). Leaf tissue was collected in the spring (12 months exposure) and autumn (20 months exposure) and examined using transmission electron microscopy (TEM) and light microscopy. During early spring, elevated CO2 magnified effects of N and water treatment on starch accumulation and in some cases contributed to altered organization of mesophyll chloroplasts. Disruption of chloroplast integrity was pronounced under elevated CO2, low N and water stress. In autumn, needles contained little starch; however, chloroplasts grown under high CO2 exhibited stress symptoms including increased plastoglobuli and shorter grana. A trend for reduced needle phloem cross‐sectional area resulting from fewer sieve cells was also observed under elevated CO2. These results suggest that, in nature, longleaf pine seedlings may not benefit from a doubling of CO2, especially when soil resources are limiting.

Xylem cavitation and loss of hydraulic conductance in western hemlock following planting.

Following planting, western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings experience water stress and declining xylem pressure potential (Psi(x)). Low Psi(x) can result in xylem cavitation and embolism formation, causing a decline in hydraulic conductance. This study focused on the relationship between Psi(x), xylem cavitation and transpiration (E) of newly planted seedlings. Leaf specific hydraulic conductance (k(AB)) declined from 0.56 to 0.09 mmol m(-2) s(-1) MPa(-1) over a 9-day period. Stomatal conductance (g(s)) declined from 143.5 to 39.15 mmol m(-2) s(-1) over the same period without an associated change in environmental conditions. A vulnerability profile indicated a 30% loss in hydraulic conductivity when seedlings experienced a Psi(x) between -2.5 and -3.0 MPa. A Psi(x) of -4.0 MPa led to a complete loss of conductivity. We conclude that following planting, western hemlock seedlings often experience Psi(x) values that are low enough to cause xylem cavitation and a decline in k(AB).