Loblolly Pine Seedlings Research Articles

Growth and photosynthesis of loblolly pine (Pinus taeda) after exposure to elevated CO(2) for 19 months in the field.

To detect seasonal and long-term differences in growth and photosynthesis of loblolly pine (Pinus taeda L.) exposed to elevated CO(2) under ambient conditions of precipitation, light, temperature and nutrient availability, seedlings were planted in soil representative of an early, abandoned agricultural field and maintained for 19 months in the field either in open-top chambers providing one of three atmospheric CO(2) partial pressures (ambient, ambient +15 Pa, and ambient +30 Pa) or in unchambered control plots. An early and positive response to elevated CO(2) substantially increased total plant biomass. Peak differences in relative biomass enhancement occurred after 11 months of CO(2) treatment when biomass of plants grown at +15 and +30 Pa CO(2) was 111 and 233% greater, respectively, than that of plants grown at ambient CO(2). After 19 months, there was no significant difference in biomass between +15 Pa CO(2)-treated plants and ambient CO(2)-treated plants, whereas biomass of +30 Pa CO(2)-treated plants was 111% greater than that of ambient CO(2)-treated plants. Enhanced rates of leaf-level photosynthesis were maintained in plants in the elevated CO(2) treatments throughout the 19-month exposure period despite reductions in both leaf N concentration and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity during the first 11 months of CO(2) exposure. Reductions in Rubisco activity indicated photosynthetic adjustment to elevated CO(2), but Rubisco-mediated control of photosynthesis was small. Seasonal shifts in sink strength affected photosynthetic rates, greatly magnifying the positive effects of elevated CO(2) on photosynthesis during periods of rapid plant growth. Greater carbon assimilation by the whole plant accelerated plant development and thereby stimulated new sinks for carbon through increased plant biomass, secondary branching and new leaf production. We conclude that elevated CO(2) will enhance photosynthesis and biomass accumulation in loblolly pine seedlings under high nutrient conditions; however, reductions over time in the relative biomass response of plants to elevated CO(2) complicate predictions of the eventual magnitude of carbon storage in this species under future CO(2) conditions.

Photosynthetic response of loblolly pine and sweetgum seedling stands to elevated carbon dioxide, water stress, and nitrogen level

Seedling stands of loblolly pine (Pinustaeda L.) and sweetgum (Liquidambarstyraciflua L.) were grown in monoculture or mixed stands for two growing cycles in controlled-environment chambers. Treatments consisted of ambient (408 ppm) and elevated (806 ppm) CO2, concentrations, water-stressed and well-watered conditions, and low (20 kg N/ha) and high (215 kg N/ha) nitrogen application rates. Photosynthesis rates were measured under ambient and elevated cuvette CO2 concentrations for both whole stands and individual seedlings from these stands. Significant interactions between CO2 and water suggested that elevated CO2 concentration compensated for low water availability in individually measured loblolly pine and in whole seedling stands regardless of stand type. Expressing photosynthesis on a soil area versus a leaf-mass basis influenced the photosynthetic rankings of the three stand types relative to one another. Net photosynthetic rates per unit leaf mass were 390 and 880% higher in individually measured seedlings than in whole monoculture stands for loblolly pine and sweetgum, respectively. Lower photosynthetic contributions from lower canopy leaves in whole seedling stands compared with the upper canopy leaves used in individual-seedling measurements were thought to be responsible for lower photosynthetic rates in seedling stands. These results suggest that photosynthetic response is influenced by canopy dynamics that are unaccounted for by individual-seedling measurements of photosynthesis. Differences in photosynthetic response between loblolly pine and sweetgum stands and individuals are thought to be largely due to species-specific differences in canopy light extinction characteristics.

Seasonal ectomycorrhizal fungal biomass development on loblolly pine ( Pinus taeda L.) seedlings

Seasonal ectomycorrhizal fungal biomass development on loblolly pine ( Pinus taeda L.) seedlings

Aluminum sensitivity of loblolly pine and slash pine seedlings grown in solution culture.

To probe variation in Al sensitivity of two co-occurring pine species, seedlings from six full-sib families of loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Engelm.) were grown in solution culture containing 4.4 mM (high-Al) or 0.01 mM (low-Al) AlCl(3) at pH 4 for 58 days. On average, both pine species had 41% less total dry weight in the high-Al treatment than in the low-Al treatment. Stem volume growth of slash pine was more sensitive to the high-Al treatment than that of loblolly pine. In both species, the high-Al treatment inhibited root dry weight more than shoot dry weight. Within-species variation in Al sensitivity among families was greater in loblolly pine (24 to 52% inhibition of seedling dry weight) than in slash pine (35 to 47% inhibition of seedling dry weight). Foliar Al concentration was positively correlated with Al sensitivity in slash pine but not in loblolly pine; however, in both species, the concentration of Al in roots was 20-fold greater than in foliage.

Seasonal ectomycorrhizal fungal biomass development on loblolly pine (Pinus taeda L.) seedlings

Ergosterol, a membrane sterol found in fungi but not in plants, was used to estimate live mycelial biomass in ectomycorrhizae. Loblolly pine (Pinus taeda L.) seeds were sown in April 1993 and grown with standard nursery culture practices. Correlations between total seedling ergosterol and visual assessment of mycorrhizal colonization were high during July and August but low as ectomycorrhizal development continued into the growing season. Percentages of mycelial dry weight over lateral roots decreased from 9% in July to 2.5% in November because seedling lateral root dry weight accumulated faster than mycelial dry weight. Total ergosterol per seedling increased from July through February. As lateral root dry weight ceased to increase during winter months, ectomycorrhizal mycelia became the major carbohydrate sink of pine seedlings. No distinctive seasonal pattern of soil ergosterol content was observed. The impact of ectomycorrhizal fungi on plant carbohydrate source-sink dynamics can be quantitatively estimated with ergosterol analysis but not with conventional visual determination.

Loblolly pine needle nutrient and soil enzyme activity as influenced by ozone and acid rain chemistry

Atmospheric air pollutants such as acid rain and O 3 are present throughout the Southeastern U.S.A. and are known to suppress tree growth and crop productivity. Precipitation chemistry has shown changes in the ratio of N and S compounds in recent years. A greenhouse study was conducted using continuous stirred reactor chambers to determine the effects of simulated acid rain (SAR) chemistry, O 3 and their interaction on loblolly pine seedling biomass, loblolly pine secondary needly nutrient content and soil enzyme activities. The seedlings were exposed to 0, 80, 160, 240 or 320 nl O 3 l −1 at SAR pH 4.3 with two ratios of N-to-S (7N-to-3S and 3N-to-7S) for 11 wk. The secondary needle and root biomass were significantly suppressed by increasing O 3 concentrations. A decreased content of P, Ca and Mg in secondary needles was also found with increasing O 3 concentration. Neither SAR chemistry (N-to-S ratios) or the interaction of SAR chemistry and O 3 had significant effects on loblolly pine seedling biomass. Acid phosphatase and arylsulfatase activities were dependent on SAR chemistry. The activity of soil acid phosphatase and arylsulfatase was inhibited by O 3 when S application through simulated acid rain was higher (7-to-3, S-to-N). The inhibitory effects appeared at 160 nl O 3 l −1 for acid phosphatase and 240 nl O 3 l −1 for arylsulfatase. Dehydrogenase activity was not affected by either O 3 or SAR chemistry.

Species mixture and soil-resource availability affect the root growth response of tree seedlings to elevated atmospheric CO21

The effects of CO2 enrichment on root proliferation of loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styracijlua L.) seedlings were studied under varied water and nitrogen (N) regimes and in competitive interaction. Seedlings of each species were grown from seed as monocultures or as 50:50 pine-sweetgum mixtures in 22-L pots filled with forest soil. Seedlings were exposed to either ambient (400 ppm) or CO2-enriched (ambient plus 400 ppm) air for 32 weeks in continuously stirred tank reactors. Detailed sampling of very fine roots (>0.5 mm diam.) showed a general increase (up to 2-fold) in root length density (RLD, cm · m-3 with elevated CO2; however, the effects of CO2 on RLD differed according to species, culture type, water, and N availability. In monoculture, low water with low N conditions produced the largest RLD responses to elevated CO2: 75% increase for sweetgum and 31% increase for pine. In mixed culture, by contrast, the largest RLD responses to CO2 were observed under high water, high N regimes: pine showed a 110% increase and sweetgum a 96% increase. The total RLD of the standing crop in mixture under elevated CO2, high water, and high N was 2.6 cm · m-3 compared with 1.6 cm · m-3 in ambient CO2, with sweetgum accounting for >75% of the total RLD in both cases. These findings suggest that resource-rich rather than resource-poor soil environments could be th.e circumstances under which belowground interference from sweetgum would intensify in pine-sweetgum mixtures with rising atmospheric CO2.

Growth and dry matter partitioning in loblolly and ponderosa pine seedlings in response to carbon and nitrogen availability

summaryWe grew loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings in a factorial experiment with two CO2 partial pressures (35 and 70 Pa) and two nitrogen treatments (1‐0 and 3–5 mM NH4+) for one growing season in a ‘common garden’ experiment designed to examine the extent that dry matter and nitrogen accumulation and partitioning are environmentally controlled. Ponderosa pine seedlings grown in 35 Pa CO2 and 3.5 mM NH4+ showed symptoms of nitrogen toxicity, characterized by greatly reduced growth, and moderately reduced total plant N. With the exception of this treatment combination, there were no significant differences between species in total plant dry matter or total plant nitrogen, suggesting that responses of growth to environmental conditions were stronger than heritable responses. There were however large differences in dry matter and N partitioning between the two species. Increases in leaf mass were largest in loblolly pine, whilst ponderosa pine tended to have higher root: shoot (R:S) ratios. R:S ratio of loblolly increased in response to C availability and decreased in response to N availability, whilst R: S ratio of ponderosa pine was much less responsive to resource availability. Total plant N varied with N supply, and N partitioning was related to plant growth and carbon partitioning. Carbon and N were interactive, such that an increase in the accumulation of either resource was always accompanied by an increase in the other. Over several seasons the different patterns of resource acquisition and biomass allocation that we observed in a uniform environment could potentially result in different growth rates at most resource levels. In the first season, contrary to our expectations, heritable differences in growth rate did not appear.

Effects of the amount and composition of the forest floor on emergence and early establishment of loblolly pine seedlings

Five forest floor weights (0, 10, 20, 30, and 40 Mg/ha), three forest floor compositions (pine, pine–hardwood, and hardwood), and two seed placements (forest floor and soil surface) were tested in a three-factorial, split-plot design with four incomplete, randomized blocks. The experiment was conducted in a nursery setting and used wooden frames to define 0.145-m2 subplots. Forest floor composition had no significant effect on emergence or establishment of loblolly pine (Pinustaeda L.) seedlings. Numbers of emerging and established seedlings displayed a negative exponential relationship with forest floor weight (fit indices of 0.62 and 0.62, respectively). Seed placement significantly affected the number of emerging seedlings (forest floor > soil surface) and their survival (forest floor < soil surface). However, seed placement had no overall significant effect on the number of established seedlings because effects on emergence and survival essentially canceled out. Germination of herbaceous seeds in the soil bank also displayed a negative exponential trend with forest floor weight (fit indices of 0.46 and 0.50 for the weight of grasses and forbs, respectively) and was not significantly affected by forest floor composition. Results indicate that forest floor composition is not a factor in the natural regeneration of the pine component of mixed pine–hardwood stands and suggest that when pine seed production is adequate, moderate amounts of forest floor material will improve pine seedling development because of suppression of herbaceous vegetation.

Water relations and growth of loblolly pine seedlings planted under a shelterwood and in a clear-cut.

We investigated the influence of shelterwood conditions on water relations and growth of loblolly pine (Pinus taeda L.) seedlings on two harsh sites in eastern Texas. Site I was harvested to provide four overstory density treatments (0, 2.3, 4.6 and 9.2 m(2) of residual basal area per ha). To quantify the effects of overstory competition, trenched and nontrenched subplots, each containing 25 one-year-old seedlings, were established within each overstory treatment plot, and predawn and midday water potentials (Psi(w)), seedling growth and survival were measured during the growing season. Leaf area and seedling biomass partitioning were measured at the end of the growing season. Site II was harvested to provide two overstory density treatments (0 and 6.9 m(2) ha(-1)) and planted with one-year-old loblolly pine seedlings. Seedling Psi(w), stomatal conductance (g(wv)), transpiration flux density (E), leaf area, height and survival were determined. On Site I, seedling Psi(w) increased with increasing overstory basal area, whereas trenching only substantially affected Psi(w) of seedlings in the 9.2 m(2) ha(-1) overstory treatment. Growth was not affected by overstory treatment or trenching. On Site II, Psi(w) and g(wv) were highest during the morning hours and lowest in the afternoon, whereas E peaked in the afternoon. Vapor pressure deficits and photosynthetic photon flux density were major factors in determining g(wv) differences between treatments. On individual days, the presence of an overstory increased Psi(w) and reduced both g(wv) and E. On Site II, leaf area was affected by overstory treatment throughout most of the study. We conclude that the presence of an overstory can have ameliorative effects on harsh sites at the western fringe of the loblolly pine natural range.

Red spruce and loblolly pine nutritional responses to acidic precipitation and ozone

A multidisciplinary research program called Response of Plants to Interacting Stresses (ROPIS) was initiated by the Electric Power Research Institute in 1986 to develop a general mechanistic theory of plant response to interacting air pollutants and other stresses. As part of the program, the individual and combined impacts of acidic precipitation and elevated O 3 on nutritional responses of red spruce ( Picea rubens Sarg.) and loblolly pine ( Pinus taeda L.) were evaluated. Red spruce saplings were exposed to charcoal filtered, non-filtered, 1·5 times ambient or twice ambient levels of O 3 in combination with rain pH treatments of 3·1, 4·1, or 5·1 for 4 years. Similarly, loblolly pine seedlings were exposed to subambient, ambient or twice ambient O 3 levels in combination with acidic precipitation treatments of pH 3·8 or 5·2 for 3 years. Cation leaching was accelerated in the pH 3·1 rain treatment in the red spruce study, driven by atmospheric inputs of H + and SO 4 2−. Subsequent decreases in soil pH, exchangeable pools of Ca 2+ and Mg 2+, and increases in exchangeable Al 3+ in the organic horizon were observed. Calcium and Mg 2+ fluxes in throughfall of red spruce were also enhanced at pH 3·1, and foliar concentrations of Mg 2+ were reduced. In contrast, soil pH and nutrient concentrations as well as foliar leaching in the loblolly pine study were not significantly affected by either the pH 3·8 or 5·2 rain treatments. Ozone exposure had no effect on throughfall or soil solution ionic flux for either species. Results indicate that ambient rainfall acidities are not likely to affect the nutritional status of loblolly pine. High elevation red spruce forests, however, could be impacted by acidic deposition via enhanced soil acidification, leading to Al 3+ mobilization and reduced availability of important base cations as well as increased foliar leaching.

Monoterpene emissions and cuticular lipids of loblolly and slash pines: potential bases for oviposition preference of the Nantucket pine tip moth

Monoterpene emissions from intact 5- to 29-month-old loblolly and slash pine seedlings contained α-pinene, camphene, β-pinene, myrcene, sabinene, β-phellandrene, and limonene. α-Pinene made up > 50% of the volatiles from both species. β-Pinene was significantly more abundant in slash (35.6%) than in loblolly pine (15.3%), while myrcene was significantly more abundant in loblolly (10.9%) than in slash pine (3.4%). Cuticular lipids represented 0.11 and 0.06% of the dry weight biomass of shoots from loblolly and slash pines, respectively. Species differences in cuticular lipid composition were primarily in relative proportions of a group of unidentified compounds that appear to be saturated and unsaturated diols and (or) hydroxy alcohols with chain lengths of about 18 carbons. 10-Nonacosanol made up 16.2 and 14.1% of the total lipids recovered from loblolly and slash pines, respectively. The Nantucket pine tip moth, Rhyacionia frustrana (Comstock), may use these chemical differences to distinguish the susceptible loblolly pines from the resistant slash pines. Key words: Pinus taeda, Pinus elliottii, monoterpene emissions, cuticular lipids, Rhyacionia frustrana.

Genetic variation in rooting ability of loblolly pine cuttings: effects of auxin and family on rooting by hypocotyl cuttings.

After about 20 days, hypocotyl cuttings from 20-day-old loblolly pine (Pinus taeda L.) seedlings rooted easily in the presence of the auxin indole-3-butyric acid (IBA), with roots forming directly from xylem parenchyma. In contrast, woody cuttings from 1-2-year-old hedged seedlings formed roots indirectly from callus tissue in 60-90 days, but IBA had little effect on rooting. Variation in rooting among hypocotyls from both half- and full-sib families was highly significant in response to IBA, and rooting did not occur within 20 days unless IBA was applied. Hypocotyls from poor rooting families tended to produce fewer roots per cutting than hypocotyls from good rooting families. Rooting by woody cuttings and hypocotyl cuttings from the same nine full-sib families was weakly correlated, raising the possibility that at least some common genetically controlled processes were affecting rooting by both types of cutting. The phytotropin N-1-naphthylphthalamic acid (NPA), supplied at 1 micro M with 10 micro M IBA, significantly inhibited rooting by hypocotyl cuttings from both good and poor rooting families, but there was no significant family x treatment interaction. Family variation in rooting ability may be a function of the frequency of occurrence of auxin-responsive cells in the hypocotyls.

Contribution of ectomycorrhiza to the potential nutrient‐absorbing surface of pine

summaryThe increase in potential nutrient‐absorbing surface area of loblolly pine (Pinus taeda L.) seedlings as a result of ectomycorrhizal development was quantified. In one experimental. In one experiment, loblolly pine seedlings were inoculated with Pisolithus tinctorius Pers. Coker (Pt) after having first been grown from seed for 8 wk under a low phosphorus (P) nutrient regime. Eight weeks following inoculation, seedlings were harvested and ail components of the nutrient absorbing system (including the fungal mycelium) were quantified. External mycelium accounted for only 5% of the potential absorbing‐system dry weight, but this mycelium comprised 75% of the potential absorbing system area and over 99% of the absorbing length. In a second experiment, the P uptake and contribution to potential absorbing surface of Pt was compared to Cenococcum geophilum Fr. (Cg). Seedlings inoculated with Pt differed from those inoculated with Cg in that they (i) absorbed almost twice the P. (ii) had a higher frequency of fine‐root branching, (iii) developed an extensive network of rhizomorphs, and (iv) had over 1.5 times the mycelial absorbing surface area and over 3 times the mycelial length of Cg. We conclude that for both Pt and Cg inoculated plants, external mycelium made by far the greatest contribution to the overall potential absorbing surface area of the pine seedling. Differences in total mycelial surface area and mycelial length may account for differences in P uptake between seedlings inoculated with Pt and those inoculated with Cg.

Phosphorus supply affects the photosynthetic capacity of loblolly pine grown in elevated carbon dioxide.

Effects of phosphorus supply and mycorrhizal status on the response of photosynthetic capacity to elevated CO(2) were investigated in loblolly pine (Pinus taeda L.) seedlings. Seedlings were grown in greenhouses maintained at either 35.5 or 71.0 Pa CO(2) in a full factorial experiment with or without mycorrhizal inoculum (Pisolithus tinctorius (Pers.) Coker & Couch) and with an adequate or a limiting supply of phosphorus. Assimilation versus internal CO(2) partial pressure (C(i)) curves were used to estimate maximum Rubisco activity (V(c,max)), electron transport mediated ribulose 1,5-bisphosphate regeneration capacity (J(max)), phosphate regeneration capacity (PiRC) and daytime respiration rates (R(d)). Nonmycorrhizal seedlings grown with limiting phosphorus had significantly reduced V(c,max) and PiRC compared to seedlings in other treatments. Elevated CO(2) increased photosynthetic capacity in nonmycorrhizal seedlings in the low phosphorus treatment by increasing PiRC, whereas it induced phosphorus limitation in mycorrhizal seedlings in the low phosphorus treatment and did not affect the photosynthetic capacity of seedlings in the high phosphorus treatment. Despite the variety of effects on photosynthetic capacity, seedlings in the elevated CO(2) treatments had higher net assimilation rates than seedlings in the ambient CO(2) treatments. We conclude that phosphorus supply affects photosynthetic capacity during long-term exposure to elevated CO(2) through effects on Rubisco activity and ribulose 1,5-bisphosphate regeneration rates.

Effects of nitrogen on the response of loblolly pine to drought: II. Biomass allocation and C : N balance.

Effects of drought on carbon and nitrogen dynamics and biomass allocation patterns of loblolly pine (Pinus taeda L.) seedlings grown under 4 different nitrogen levels were determined Nitrogen stress caused an increase in root-shoot ratio. increased Foliar and mot Starch concentration, decreased soluble sugar concentration, and decreased free amino acid concentration in well-watered seedlings. Drought cent-rally caused an increase in foliar hexoses and decreases in foliar find root starch and root hexoses. while not affecting foliar free amino acids. However, nitrogen stress arid drought interacted in affecting carbohydrate pools and seedling morphology. Foliar starch reserves in high-N seedling were rapidly metabolized under drought, white N-deficient seedling maintained similar starch concentrations throughout the drought stress cycle. After one drought cycle, the N-induced difference in root:shoot ratio was no longer significant. These results suggest that N-stressed seedlings partition a higher percentage of fixed carbon to storage, and that (his storage pool is less available For immediate Use than in seedlings grown under adequate N.

Evaluation of potential evaporation as a means to infer loblolly pine seedling physiological response to a given microclimate

Foresters understand that there is no single correct way to treat every forest stand to produce optimum results, and that poor treatment can produce adverse effects on most sites. An example of this problem concerns areas that produce poor environmental conditions for regeneration upon clearcutting. Valigura and Messina (Journal of Environmental Management, 1994) found that loblolly shelterwoods substantially influence the temperature, radiation humidity and windspeed of the seedling-level microclimate relative to those in a clearcut, but they could not predict the effects of these modified understory conditions on seedling performance. The primary question addressed in this study is: on harsh sites, what advantages/disadvantages does a partial overstory impose on seedlings growing underneath relative to seedlings grown in a clearcut? The objective was to evaluate the combination equation for potential evaporation as a relative means to infer the amount of stress imposed on loblolly pine seedlings in a shelterwood or clearcut environment, subsequently inferring the effect of a partial overstory on seedling performance. It was shown that although PE statistically explained some of the variance measured in seedling physiological variables, it was not possible to use PE estimates to infer seedling performance to microclimate, on these sites, during the relatively wet period of 1990/1991. This study qualitatively confirmed the ability of a shelterwood to favorably influence the seedling microclimate relative to a clearcut microclimate. However, there is still a need to explore and develop an inferential technique to evaluate the effect of different environments on seedling performance.

Cyproconazole for Control of Fusiform Rust on Loblolly Pine Seedlings

Abstract The efficacy of cyproconazole seed treatments and foliar sprays for control of fusiform rust on loblolly pine seedlings was evaluated in greenhouse and nursery trials. At the nursery, the percentage of untreated seedlings that were galled at lifting was high (54%). No seedlings receiving cyproconazole at the lowest dosage foliar spray (4 sprays at 123.25 g ai/ha/application ) without the seed treatment, and only 0.1% of seedlings receiving only the seed treatment (1.25 g ai/kg seed), were galled Laboratory inoculations indicated that seed treatments with cyproconazole at 1.25 and 2.5 g ai/kg seed were effective for at least 20 and 30 days, respectively, and that foliar sprays of 185.0 or 246.5 g ai/ha were effective for 14 but not 21 days. Eleven months after sowing, mycorrhizal development in the nursery was similar among seedlings receiving approximately equal dosages of either cyproconazole or triadimefon. Seedling growth was not significantly different between control seedlings and those receiving the various dosages of cyproconazole. South J. Appl. For. 18(3): 101-104.

Effects of leaf nutrient status on photosynthetic capacity in loblolly pine (Pinus taeda L.) seedlings grown in elevated atmospheric CO(2).

We measured needle photosynthesis of loblolly pine seedlings grown in a factorial experiment with two CO(2) partial pressures (35 and 65 Pa) and three nutrient treatments (7 mM NH(4)NO(3) + 1 mM PO(4); 7 mM NH(4)NO(3) + 0.2 mM PO(4); 1 mM NH(4)NO(3) + 1 mM PO(4)). The data were used to parameterize a physiologically based photosynthetic model that included limitations imposed by ribulose-1,5-bisphosphate carboxylase/oxygenase activity, electron transport capacity and inorganic phosphate availability. With nonlimiting nutrients, seedlings grown at 65 Pa CO(2) had significantly higher net photosynthesis and lower stomatal conductance than seedlings grown at 35 Pa CO(2). Nutrient limitations by either N or P significantly reduced photosynthetic capacity. When either N or P was limiting, there was no effect of growth CO(2) partial pressure on photosynthesis, but stomatal conductance was significantly lower for seedlings grown at 65 Pa CO(2). Modeled biochemical parameters suggest that, in all cases, photosynthesis was co-limited by carboxylation, electron transport and phosphate regeneration. Acclimation to growth in elevated CO(2) involved a reduction in leaf N content. In the low-N and low-P treatments, modeled parameters indicated that the biochemical processes of photosynthesis were down regulated to the point that there was no effect of increasing CO(2) partial pressure. The capacity to regenerate phosphate was reduced in both low nutrient treatments, but was only reduced by elevated CO(2) when seedlings were grown under low soil P conditions. Increased photosynthetic water use efficiency and nutrient use efficiency in response to CO(2) enrichment occurred in all three nutrient treatments and have important implications for whole-plant water and nutrient balance. These data support the contention that soil nutrient status in forest ecosystems will be a critical influence on tree seedling response to increasing atmospheric CO(2) partial pressures.

Movement of respiratory CO(2) in stems of loblolly pine (Pinus taeda L.) seedlings.

Temperature-independent fluctuations in stem CO(2) efflux were measured in Pinus taeda L. seedlings. Stem CO(2) efflux was measured during high and low transpiration rates, high and low net photosynthesis rates, and normal and interrupted substrate supply conditions. Stem CO(2) efflux rates were an average of 6.7% lower during periods of high transpiration compared to periods of low transpiration. This difference in stem CO(2) efflux rates was not due to water stress. The most likely cause was movement of respiratory CO(2) in the transpiration stream. Interruption of substrate supply to the stem by phloem girdling reduced stem CO(2) efflux rates. Increasing net photosynthesis rates from low to high had no effect on stem CO(2) efflux, but decreasing net photosynthesis from high to low caused relatively small reductions in stem CO(2) efflux. These results indicate that diurnal changes in net photosynthesis rate may play a small role in temperature-independent afternoon depressions of stem CO(2) efflux. The transport of respiratory CO(2) by the transpiration stream compromises measurements of woody tissue respiration obtained by commonly accepted gas exchange techniques. This phenomenon could also affect measurement of leaf net photosynthesis and branch woody tissue respiration.