Water-stressed Seedlings Research Articles

Ozone and drought stress: Interactive effects on gas exchange in Aleppo pine ( Pinus halepensis Mill.)

Two-year-old Aleppo pine seedlings were exposed over twenty months in open-top chambers to either above-ambient ozone (O 3) concentrations or charcoal-filtered air. Subsequently, water was applied following plant requirements in the well watered treatment (WW), while it was withheld for 11 days in the water-stressed seedlings (WS). Then, the seedlings of both water treatments were irrigated to field capacity. Four seedlings per water treatment were used within each chamber. The daily variation in gas exchange rates of Aleppo pine seedlings from the different treatments were studied during and after water stress. Ozone exposure and a moderate water shortage induced similar reductions in the gas exchange rates. An antagonistic response modulated by the severity of water stress followed the combined exposure to both O 3 and water stress; a partial recovery from water stress was observed two days after rewatering to field capacity, while the seedlings that were not exposed to O 3 showed a faster recovery.

Ferricyanide restores nitrate reductase activity in leaf extracts of water stressed wheat seedlings

Summary Nitrate reductase (EC 1.6.6.1) activity declined rapidly in leaves of wheat seedlings following polyethylene glycol-induced water stress. The decrease in NADH-nitrate reductase was accompanied by a concomitant loss of FMNH 2 − and reduced benzyl viologen-linked nitrate reductase activities. Incubation of dialysed leaf extracts with potassium ferricyanide restored the enzyme activity almost to the same level as that in non-stressed plants. The decline in nitrate reductase, at least during the initial stages of water stress, hence seems to be primarily due to redox-mediated inactivation rather than diminished synthesis of the enzyme.

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.

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.

Photosynthesis and growth of Erythrina variegata as affected by water stress and triacontanol

Erythrina variegata Lam. seedlings were grown under water stress (Ψ = -3.2 MPa) and subsequently sprayed with triacontanol (Tria). Water stress significantly reduced shoot growth rate, while roots continued to grow. Content of chlorophyll (Chl) a decreased more than that of Chl b. Water stress also reduced photosynthetic activity of chloroplasts as measured by Chl fluorescence induction. Stress effect was identified at the oxidation site of photosystem (PS) 2 prior to the hydroxylamine donating site and perhaps close to or after the diphenylcarbazide donating site. The loss of O2 evolving thylakoid polypeptides (33, 23, 17 kDa) and the large (55 kDa) and small (15 kDa) subunits of ribulose-1,5-bisphosphate carboxylase (RuBPC) were found in water stressed seedlings. The reduction in RuBPC activity was accompanied by reduction of CO2 fixation and stomatal conductance. All photosynthetic parameters were improved by Tria.

Nucleus and nucleolus in pea root meristems are affected by variations in moisture content

ABSTRACT This paper examines the morphological and biochemical variations occurring in the nucleus when considerable changes in moisture content take place in the tissue. The tissue used for this investigation was the root meristem of Pisum sativum L. at three different developmental stages: during seed maturation, during seed germination and during a vegetative stage. These different developmental stages were representative of three different degrees of moisture content. Seed maturation represented the case of tissue undergoing desiccation whereas seed germination represented the case of tissue undergoing hydration. PEG-treated seedlings represented the case of a vegetative tissue subjected to water stress. The data demonstrated that when moisture content in root meristems was low, the nucleus underwent a reduction in diameter and a change in chromatin arrangement. The nucleolus shrank considerably in diameter and became less visible. These events were simultaneous to qualitative and quantitative variations in the nuclear protein pattern. The relationship between morphological and biochemical events is unknown. However, when the moisture content in the tissue was low, the cell cycle in meristematic cells was arrested in G1 and G2 phases in the case of seed maturation. In water-stressed seedlings the cell cycle was arrested in late S or early Gl phase. The differences observed between natural desiccation and artificially induced water stress could be due to the different rate of water loss from tissues. This paper reports for the first time the presence of Dehydrins in nuclei of higher plants. These proteins undergo quantitative variations related to changes in the moisture content of the tissue. The hypothesis of a possible role of Dehydrins in protecting DNA is discussed.

Growth, water relations and solute accumulation in osmotically stressed seedlings of the tropical tree Colophospermum mopane.

Root and hypocotyl elongation, water status and solute accumulation were studied in osmotically stressed seedlings of the tropical tree, Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard, which grows in hot arid areas of southern and central Africa. Seeds were imbibed for 24 h and then subjected to a polyethylene-glycol-generated osmotic stress of -0.03 (control), -0.2, -0.8, -1.6 or -2.0 MPa for 60 h. Seedlings subjected to moderate water stress (-0.2 MPa) had higher root growth rates (2.41 +/- 0.24 mm h(-1)), greater final root lengths (111 +/- 3.8 mm) and longer cells immediately behind the root elongation zone than control seedlings (1.70 +/- 0.15 mm h(-1) and 93 +/- 3.9 mm, respectively). Root lengths of seedlings in the -0.8 and -1.6 MPa treatments were similar to those of control seedlings, whereas the -2.0 MPa seedlings had significantly shorter roots. Both root and hypocotyl tissues exhibited considerable osmotic adjustment to the external water potential treatments. Seedlings in the -0.03, -0.2, and -0.8 MPa treatments had similar cell turgor pressures (0.69 +/- 0.10, 0.68 +/- 0.07 and 0.57 +/- 0.04 MPa, respectively), whereas the -2.0 MPa treatment lowered cell turgor pressure to 0.17 +/- 0.04 MPa. Root vacuolar osmotic pressures were generally similar to sap osmotic pressures, indicating that the increased root elongation observed in moderately water-stressed seedlings was not caused by increased turgor pressure difference. Neutral-fraction solute concentrations, including the osmoticum pinitol, increased approximately two-fold in root sap in response to a low external water potential. In hypocotyl sap of seedlings in the -2.0 MPa treatment, pinitol more than doubled, sucrose increased from about 2 to 75 mol m(-3) but glucose and fructose remained unchanged and, as a result, total sugars increased only slightly. The benefits of rapid early root elongation and osmoticum accumulation under conditions of water stress are discussed in relation to seedling establishment.

The Effect of Elevated [CO2] on Growth and Photosynthesis of Two Eucalyptus Species Exposed to High Temperatures and Water Deficits.

Two species of eucalyptus (Eucalyptus macrorhyncha and Eucalyptus rossii) were grown for 8 weeks in either ambient (350 [mu]L L-1) or elevated (700 [mu]L L-1) CO2 concentrations, either well watered or without water additions, and subjected to a daily, 3-h high-temperature (45[deg]C, maximum) and high-light (1250 [mu]mol photons m-2 s-1, maximum) stress period. Water-stressed seedlings of E. macrorhyncha had higher leaf water potentials when grown in elevated [CO2]. Growth analysis indicated that increased [CO2] may allow eucalyptus species to perform better during conditions of low soil moisture. A down-regulation of photosynthetic capacity was observed for seedlings grown in elevated [CO2] when well watered but not when water stressed. Well-watered seedlings grown in elevated [CO2] had lower quantum efficiencies as measured by chlorophyll fluorescence (the ratio of variable to maximal chlorophyll fluorescence [Fv/Fm]) than seedlings grown in ambient [CO2] during the high-temperature stress period. However, no significant differences in Fv/Fm were observed between CO2 treatments when water was withheld. The reductions in dark-adapted Fv/Fm for plants grown in elevated [CO2] were not well correlated with increased xanthophyll cycle photoprotection. However, reductions in the Fv/Fm were correlated with increased levels of nonstructural carbohydrates. The reduction in quantum efficiencies for plants grown in elevated [CO2] is discussed in the context of feedback inhibition of electron transport associated with starch accumulation and variation in sink strength.

The effect of humidity on photosynthesis and water relations of white spruce seedlings during the early establishment phase

Newly transplanted, 3-year-old bare-root white spruce (Piceaglauca (Moench) Voss) seedlings were grown in three levels of absolute humidity difference (AHD), under water-stressed and well-watered conditions in growth chambers. Stomatal conductance to CO2 as well as net assimilation were more than twice as high in the low AHD treatment (6.8 μg H2O•cm−3) as in the high AHD treatment (21.0 μg H2O•cm−3). Transpiration rates were uniform among humidity treatments, but in the low AHD treatment water use efficiency was more than double that of the high AHD treatment. Water use efficiency was greatest in the low AHD conditions, in the water-stressed seedlings, and during the time immediately after planting. There were no differences in the number of new roots produced among humidity treatments, despite the different levels of photosynthesis. In the stress treatment, bud flush of seedlings in the high AHD treatment occurred 3 days later than in the low AHD treatment. A field study on the effects of varying aspen (Populustremuloides Michx.) canopies on humidity levels showed that AHD under partial and full canopies was lower than that of the clearcut. Results suggest that silvicultural treatments that promote higher humidity levels on planting sites should improve white spruce seedling photosynthesis immediately after planting.

Characterization of cDNAs encoding Triticum durum dehydrins and their expression patterns in cultivars that differ in drought tolerance

From a cDNA library prepared from roots of Triticum durum water-stressed seedlings, we have characterized four dehydrin clones. Two clones, pTd27e and pTd16, code for proteins with classical features of dehydrins, i.e. the consensus motif KIKEKLPG that is present beyond the tract of serine residues and at the carboxy terminus. The encoded proteins, Tddhn15 and Tddhn16, show similarities with Triticum aestivum sequences. Two clones, pTd25a and pTd38, code for a dehydrin which lacks the stretch of serine residues and shows sequence similarity to T. aestivum Cor proteins. To correlate T. durum drought tolerance with dehydrin gene expression, we have for four cultivars compared the accumulation of dehydrin transcripts in roots and shoots of seedlings in response to a water-stress, and under application of exogenous ABA. A water-stress time course showed that accumulation of the dehydrin transcripts is delayed in the drought-tolerant cultivars. Also, the level of accumulated transcripts appeared to be greater in the drought-tolerant cultivars than in the drought-sensitive cultivar. A similar result was observed after application of exogenous ABA.

Stomatal resistance and the ratio of intercellular to ambient carbon dioxide in container-grown yellow-poplar seedlings exposed to chronic ozone fumigation and water stress

One-year old container-grown seedlings of yellow-poplar ( Liriodendron tulipifera L.) were exposed to 0.15 ppm ozone (O 3) fumigation and water stress (two l-week drying cycles) in continuously stirred tank reactors prior to measuring stomatal resistance ( R s), intercellular carbon dioxide [CO 2 ( C i)], and the ratio of C i and ambient CO 2 ( C a) after 20 and 36 days. Water stress increased R s by more than 250% and reduced C i and the C i C a ratio by just over 20%. There were no significant effects of 0.15 ppm O 3 on C i and the C i C a ratio but R s was significantly increased by the interaction of water stress and O 3 fumigation. For water-stressed seedlings fumigated with charcoal-filtered air, C i and the C i C a ratio were significantly lower than those of the other treatments. The data show that interactions involving low levels of ozone and short periods of water stress may have a substantial influence on stomatal physiology.

Effect of plant moisture stress and application surface on uptake and translocation of triclopyr with organosilicone surfactant in red maple seedlings

This study investigated the effect of plant moisture stress and surface of application on the absorption, translocation, and "rainfastness" (short-term ability to retain herbicide) of organosilicone-adjuvated (adjuvant added) [14C]triclopyr amine on greenhouse-grown, 4-month-old red maple (Acerrubrum L.) seedlings. Xylem water potentials were −1.6 and −0.9 MPa and leaf conductances were 0.07 and 0.13 cm s−1 for the stressed and control seedlings, respectively. At 2 h, rainfastness was 13% less for stressed seedlings. Uptake increased with time, and by 72 h no effect of moisture stress treatment was apparent. Abaxial absorption into living leaf tissue was 57% greater at 72 h than was adaxial absorption, but application surface did not significantly affect translocation. Plant moisture stress did, however, reduce translocation of herbicide into the shoots and roots. Organosilicone surfactants may enhance long-term triclopyr uptake in water-stressed seedlings, but do not appear to facilitate translocation in stressed seedlings. Plant moisture status, therefore, should continue to be a concern when scheduling herbicide applications.

Steady-state nutrition of Pinus resinosa seedlings: response to nutrient loading, irrigation and hardening regimes.

To test effects of fertilization on late-season growth and nutrient content of container-grown red pine (Pinus resinosa Ait.), seedlings were subjected to a factorial combination of two fertilization regimes (exponentially increasing concentrations providing 25 (1N) and 75 (3N) mg N seedling(-1), respectively) and two irrigation regimes (well-watered and water-stressed) followed by drought-hardening or nonhardening. The fertilization treatments gave rise to steady-state nutrition in the seedlings during the prehardening phase. The 3N treatment increased macronutrient uptake significantly more than dry matter production, particularly in the roots, giving rise to what has been called luxury consumption of nutrients, or nutrient loading. Nutrient loading was higher in well-watered seedlings than in water-stressed seedlings. Among well-watered seedlings, relative growth rate of nutrient-loaded seedlings was more sensitive to drought during the hardening phase than that of nonloaded seedlings. However, when watering was resumed at the end of the hardening treatment, the relative growth rate of the nutrient-loaded seedlings (421%) exceeded that of the nonloaded seedlings (213%). Nitrogen accumulation was also higher in nutrient-loaded seedlings than in nonloaded seedlings during the post-hardening phase.

Blue-Oak Regeneration and Seedling Water Relations in Four Sites within a California Oak Savanna

Water relations of transplanted and naturally occurring blue-oak seedlings (Quercus douglasii Hook. & Arn.) were studied to examine the effects of site differences with regard to the rates of progressing drought on the current lack of blue-oak establishment in California oak savannas. Seedling water relations were compared from May to September 1987 at four sites, differing in terms of canopy structure, grazing history, and grass density. During the dry season, water status of transplanted seedlings was more favorable at a grazed open site where grass density was minimal but soil and atmospheric drought were most pronounced. In contrast, transplanted seedlings experienced the highest rates of water deficit and senesced earliest at a grazed canopy site that had the highest density of grasses. Naturally occurring seedlings were present only in the grazed canopy and ungrazed canopy sites, and their water relations were similar at both sites. Naturally occurring seedlings were 3-4 yr old, and thus their roots might have already extended to deeper soil layers where competition from grass roots was low. We conclude that blue-oak seedlings resist drought by osmotic adjustment, particularly when seedling water stress progresses slowly because of lack of severe belowground competition from grasses. The process of osmotic adjustment was perhaps more critical for seedling survival than for seedling growth. Seedling stomatal conductances responded primarily to seedling water status rather than to soil or atmospheric drought. It seemed that the density of grasses was more important in explaining patterns of seedling water stress than was either microclimate or the change in dominance from perennial to annual grasses during the past century.

Population differences in water-use efficiency of well-watered and water-stressed western larch seedlings

Gas exchange, water-use efficiency (WUE), and carbon-isotope discrimination (Δ) were compared among 14 populations of western larch (Larixoccidentalis Nutt.), a deciduous conifer. Seedlings grown from seed collected across the natural distribution of the species were maintained under water-stressed and well-watered conditions. No differences in gas exchange or instantaneous water-use efficiency (WUEi) were detected among populations. However, population differences in Δ, a second measure of water-use efficiency, were observed. WUE measured as the rate of dry matter accumulation per cumulative water transpired (WUET) was different among populations at the P = 0.07 level. The three measures of WUE were intercorrelated and all were higher among water-stressed seedlings than among well-watered seedlings. Overall means were 4.30 ± 0.08 (± SE) and 3.61 ± 0.07 g dry matter accumulation per kg water transpired (WUET), 2.80 ± 0.09 and 2.02 ± 0.06 μmol CO2 uptake per μmol H2O transpired (WUEi), and Δ was 18.24 ± 0.08‰ and 20.26 ± 0.10‰ under the water-stressed and well-watered treatments, respectively. Population differences in the two integrated measurements, WUET and Δ, were significantly correlated with the altitude at which seed was collected. No interaction between genotype and watering treatment was observed, which means that despite pronounced shifts in WUE between watering treatments, population rankings remained unchanged. These results provide evidence of genotypic differences in long-term measures of WUE among native populations of western larch.

Growth and physiological response of water-stressed yellow-poplar seedlings exposed to chronic ozone fumigation and ethylenediurea

One-year-old containerized seedlings of yellow-poplar ( Liriodendron tulipifera L.) were exposed to ethylenediurea (EDU), ozone (O 3) fumigation (0, 0.07, and 0.15 ppm), and drought (one or two 1-week drying cycles) from late April until early June 1990, at the Forestry Sciences Laboratory, Delaware, Ohio, USA. Measurements of net photosynthesis ( P n), dark respiration ( R d), stomatal conductance ( G s), and transpiration ( T s) were made on seedlings from each treatment on 15 May and 31 May. Treatment effects on seedling growth were determined by measuring height increase, shoot and root biomass, and leaf area. Water stress significantly reduced P n, G s, and T s (by more than 70%), whereas EDU and O 3 treatments had no appreciable effect on these parameters. There was no indication that ethylene evolution was influenced by any of the stress factors imposed in this study. There were no significant interactions between these stress factors and the physiological parameters measured. Growth measurements showed that height increase was reduced by 22% in water-stressed seedlings, whereas new shoot biomass, root biomass and leaf area declined by 10%, 20%, and 12%, respectively. New shoot biomass, root biomass, and leaf area were reduced by the EDU treatment. Root/shoot ratio was not significantly altered by the drought treatment employed. Overall, there were no significant effects of O 3 fumigation on growth. The interaction of water stress and O 3 fumigation at 0.07 ppm significantly reduced root biomass and the root/shoot ratio. The EDU and 0.15 ppm O 3 together reduced height growth more than either factor alone.

Effect of plant moisture stress on absorption and translocation of triclopyr in oak seedlings

Ways to maximize herbicide efficacy are of great concern to most forest managers. Many factors influence the efficacy of herbicides, including the physiological status of the plant and its effect on the translocation of herbicides. This study investigated how plant moisture status affected herbicide absorption and translocation. The ester formulation of [14C]triclopyr was applied to the foliage of 8-month-old, container-grown water oak (Quercusnigra L.) and southern red oak (Quercusfalcata Michx.) seedlings. At the time of treatment, seedlings were in soil with average moisture contents of 5 and 35% by weight. Corresponding seedling xylem water potentials averaged −1.8 and −0.3 MPa, respectively. Triclopyr absorption was not significantly affected by the soil moisture treatment. However, translocation to stems and roots was 62 and 48% lower, respectively, in water-stressed seedlings. Water oak absorbed 23% less than southern red oak when averaged over the soil moisture treatments. Scheduling herbicide applications when moisture is plentiful could improve triclopyr efficacy.

Regulating Competition on Conifer Plantations with Prescribed Cattle Grazing

Abstract On conifer plantations, competitive understory vegetation often retards growth and establishment of tree seedlings. Livestock grazing exemplifies a method of controlling the understory vegetation and increasing the availability of site resources to tree seedlings. We hypothesized that prescribed cattle grazing ameliorates water stress of young tree seedlings by reducing root growth of competing understory species. On a Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and ponderosa pine (Pinus ponderosa Dougl.) plantation in southwest Oregon planted in 1986, seedling water stress was evaluated with the pressure chamber technique and supplemented with gravimetric soil water determinations in 1986-1989. Root growth of orchardgrass (Dactylis glomerata L.), the major understory competing species, was quantified in 1988 and 1989 with the root periscope/mini-rhizotron technique. Seedling water stress levels during spring and summer were similar in a cattle-grazed vs. ungrazed area in 1986 through 1988, but in summer 1989, water stress was reduced significantly in the grazed area. Soil water content was higher in the grazed area in 1989, especially at the 10-20 cm soil depth. End of season (July) orchardgrass root growth was reduced 18% and 15% with grazing in 1988 and 1989, respectively. We conclude that repeated cattle grazing of orchardgrass reduced transpirational surface area and root growth sufficiently to increase soil water availability to seedlings. Thus, prescribed cattle grazing on conifer plantations can enhance seedling physiological status by acting as a regulator of above- and belowground competition. For. Sci. 39(3):405-418.

EFFECTS OF WATER DEFICIT ON CIRCADIAN PATTERNS OF ROOT WATER FLUX IN HONEY LOCUST

The volume flux of water (Jv) through roots of 2-month-old honey locust seedlings (Gleditsia triacanthos var. inermis) was estimated by using applied hydrostatic pressure. Shoots were severed 8 cm above the root collar, and root systems were mounted in the lid of a pressure chamber with the cut stump protruding through a gasket. The chamber, filled with Hoagland solution, was pressurized to 0.34 MPa, and exudate was delivered from each root system to an electronic balance via an automated system. Jv was expressed on a fibrous-root dry weight basis and plotted over time for water-stressed seedlings and well-watered controls. A highly defined endogenous circadian rhythm in Jv was observed, regardless of treatment, with maximal and minimal fluxes occurring at mid-day and midnight, respectively; the rhythm continued for up to 90 h. Seedlings subjected to water stress exhibited an approximately 4-fold reduction in Jv at comparable points in the cycle. The severity of stress influenced both the amplitude and trajectory of the rhythm. Daily peaks of Jv decreased gradually over a 3-day period in both moderately-stressed and well-watered plants. In contrast, peak Jv from roots of severely-stressed plants increased by approximately 28% per day over the same time course. The electrical conductivity of the efflux also exhibited a highly defined rhythm which was phase-shifted from that of the Jv rhythm by approximately 12 h.

Interactive Role of Elevated CO2, Nutrient Limitations, and Water Stress in the Growth Responses of Red Spruce Seedlings

Abstract Red spruce (Picea rubens Sarg.) seedlings were grown from seed for 5 mo in ambient (362 ppm) or elevated (711 ppm) CO2 to determine the potential effect of an increase in global CO2 concentration on seedling growth and establishment. CO2 exposure treatments were crossed with two levels of soil fertility and water stress treatments to determine if seedling dry weight, size, and fixed growth responses to elevated CO2 depended on nutrient and water supply. Seedling dry weight and size responses to elevated CO2 at 5 mo did not depend on nutrient and water supply. Seedlings grown in both soil fertility treatments and water stress treatments responded similarly to CO2 treatment. Water stress and CO2 treatments did have an interactive influence on the fixed growth potential of the terminal leader. Leaf weight, leaf area, and height of the terminal leader of water-stressed seedlings were greater in seedlings exposed to elevated CO2 during budset than seedlings exposed to ambient CO2. Total new fixed growth (lateral plus terminal) and total terminal fixed growth (leaf plus stem) were greater in seedlings that formed shoot primordia in elevated CO2 than in ambient CO2. Red spruce seedlings grown in elevated CO2 for 5 mo had greater stem diameter, height, branching density, leaf weight, root weight, stem weight, total weight, and mean relative growth rate (RGR) from 3 to 5 mo than seedlings grown in ambient CO2. Red spruce seedling responses to elevated CO2 suggest that seedling establishment in natural environments may be enhanced when ambient CO2 concentrations rise even if water and nutrient availabilities are limited. FOR. SCI. 39(2):348-358.