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PubMed1998

Effects of temperature and tissue nitrogen on dormant season stem and branch maintenance respiration in a young loblolly pine (Pinus taeda) plantation.

We measured dormant season (November through February) maintenance respiration rates (R(m)) in stems and branches of 9-year-old loblolly pine (Pinus taeda L.) growing in plots under conditions of controlled nutrient and water supply in an effort to determine the relationships between R(m) and tissue size (surface area, sapwood volume, sapwood dry weight), tissue nitrogen content and temperature. Dormant season R(m) per unit size (i.e., surface area, &mgr;mol m(-2) s(-1); sapwood volume, &mgr;mol m(-3) s(-1); or sapwood dry weight, nmol g(-1) s(-1)) varied with tissue size, but was constant with respect to tissue nitrogen content (&mgr;mol mol(-1) N s(-1)). Cambium temperature accounted for 61 and 77% of the variation in stem and branch respiration, respectively. The basal respiration rate (respiration at 0 degrees C) increased with tissue nitrogen content, however, the Q(10) did not. Improved nutrition more than doubled stem basal respiration rate and increased branch basal respiration by 38%. Exponential equations were developed to model stem and branch respiration as a function of cambium temperature and tissue nitrogen content. We conclude that failure to account for tissue nitrogen effects on respiration rates will result in serious errors when estimating annual maintenance costs.

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PubMed1998

Net ecosystem productivity, net primary productivity and ecosystem carbon sequestration in a Pinus radiata plantation subject to soil water deficit.

Tree carbon (C) uptake (net primary productivity excluding fine root turnover, NPP') in a New Zealand Pinus radiata D. Don plantation (42 degrees 52' S, 172 degrees 45' E) growing in a region subject to summer soil water deficit was investigated jointly with canopy assimilation (A(c)) and ecosystem-atmosphere C exchange rate (net ecosystem productivity, NEP). Net primary productivity was derived from biweekly stem diameter growth measurements using allometric relations, established after selective tree harvesting, and a litterfall model. Estimates of A(c) and NEP were used to drive a biochemically based and environmentally constrained model validated by seasonal eddy covariance measurements. Over three years with variable rainfall, NPP' varied between 8.8 and 10.6 Mg C ha(-1) year(-1), whereas A(c) and NEP were 16.9 to 18.4 Mg C ha(-1) year(-1) and 5.0-7.2 Mg C ha(-1) year(-1), respectively. At the end of the growing season, C was mostly allocated to wood, with nearly half (47%) to stems and 27% to coarse roots. On an annual basis, the ratio of NEP to stand stem volume growth rate was 0.24 +/- 0.02 Mg C m(-3). The conservative nature of this ratio suggests that annual NEP can be estimated from forest yield tables. On a biweekly basis, NPP' repeatedly lagged A(c), suggesting the occurrence of intermediate C storage. Seasonal NPP'/A(c) thus varied between nearly zero and one. On an annual basis, however, NPP'/A(c) was 0.54 +/- 0.03, indicating a conservative allocation of C to autotrophic respiration. In the water-limited environment, variation in C sequestration rate was largely accounted for by a parameter integrative for changes in soil water content. The combination of mensurational data with canopy and ecosystem C fluxes yielded an estimate of heterotrophic respiration (NPP' - NEP) approximately 30% of NPP' and approximately 50% of NEP. The estimation of fine-root turnover rate is discussed.

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PubMed1998

Variability of the photosynthetic characteristics of mature needles within the crown of a 25-year-old Pinus pinaster.

Photosynthetic characteristics of 1- and 2-year-old needles were determined in excised shoots of maritime pine (Pinus pinaster Ait.) with an open gas exchange system. We used the nonlinear least mean squares method to derive values for quantum yield of electron transport (alpha), maximum carboxylation velocity (V(cmax)), and maximum electron transport rate (J(max)), from photosynthetic response curves to light and CO(2). Crown height had no significant effect on any of the parameters; however, V(cmax) and J(max), as well as alpha were 43, 26 and 35% higher, respectively, in 1-year-old needles than in 2-year-old needles. The main effect of irradiance on needles was a small decline in leaf concentrations of nitrogen and phosphorus from the top to the bottom of the canopy. Only J(max) demonstrated a linear relationship with both nitrogen content (R(2) = 0.42) and irradiance at the shoot level. Because needle age accounted for most of the variability in photosynthesis, we incorporated needle age into the photosynthesis model of Farquhar et al. (1980). The modified model underestimated the daily assimilation rate of 1-year-old needles in the field, especially when assimilation rates were high.

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PubMed1998

Canopy conductance of Pinus taeda, Liquidambar styraciflua and Quercus phellos under varying atmospheric and soil water conditions.

Sap flow, and atmospheric and soil water data were collected in closed-top chambers under conditions of high soil water potential for saplings of Liquidambar styraciflua L., Quercus phellos L. and Pinus taeda L., three co-occurring species in the southeastern USA. Responses of canopy stomatal conductance (g(t)) to water stress induced by high atmospheric water vapor demand or transpiration rate were evaluated at two temporal scales. On a diurnal scale, the ratio of canopy stomatal conductance to maximum conductance (g(t)/g(t,max)) was related to vapor pressure deficit (D), and transpiration rate per unit leaf area (E(l)). High D or E(l) caused large reductions in g(t)/g(t,max) in L. styraciflua and P. taeda. The response of g(t)/g(t,max) to E(l) was light dependent in L. styraciflua, with higher g(t)/g(t,max) on sunny days than on cloudy days. In both L. styraciflua and Q. phellos, g(t)/g(t,max) decreased linearly with increasing D (indicative of a feed-forward mechanism of stomatal control), whereas g(t)/g(t,max) of P. taeda declined linearly with increasing E(l) (indicative of a feedback mechanism of stomatal control). Longer-term responses to depletion of soil water were observed as reductions in mean midday g(t)/g(t,max), but the reductions did not differ significantly between species. Thus, species that employ contrasting methods of stomatal control may show similar responses to soil water depletion in the long term.

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PubMed1998

The response of Pinus sylvestris to drought: stomatal control of transpiration and hydraulic conductance.

We investigated the impact of drought on the physiology of 41-year-old Scots pine (Pinus sylvestris L.) in central Scotland. Measurements were made of the seasonal course of transpiration, canopy stomatal conductance, needle water potential, xylem water content, soil-to-needle hydraulic resistance, and growth. Comparison was made between drought-treated plots and those receiving average precipitation. In response to drought, transpiration rate declined once volumetric water content (VWC) over the top 20 cm of soil reached a threshold value of 12%. Thereafter, transpiration was a near linear function of soil water content. As the soil water deficit developed, the hydraulic resistance between soil and needles increased by a factor of three as predawn needle water potential declined from -0.54 to -0.71 MPa. A small but significant increase in xylem embolism was detected in 1-year-old shoots. Stomatal control of transpiration prevented needle water potential from declining below -1.5 MPa. Basal area, and shoot and needle growth were significantly reduced in the drought treatment. In the year following the drought, canopy stomatal conductance and soil-to-needle hydraulic resistance recovered. Current-year needle extension recovered, but a significant reduction in basal area increment was evident one year after the drought. The results suggest that, in response to soil water deficit, mature Scots pine closes its stomata sufficiently to prevent the development of substantial xylem embolism. Reduced growth in the year after a severe soil water deficit is most likely to be the result of reduced assimilation in the year of the drought, rather than to any residual embolism carried over from one year to the next.

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PubMedDate not supplied

Response of effluent-irrigated Eucalyptus grandis and Pinus radiata to salinity and vapor pressure deficits.

Effects of high vapor pressure deficit (VPD) and soil salinity on growth and physiology of Pinus radiata D. Don and Eucalyptus grandis Hill ex Maiden were studied in a five-year-old plantation irrigated with salt-enhanced effluent (2.2 dS m(-1)) or freshwater (0.2 dS m(-1)) for 14 weeks during spring and summer. Salt was then rapidly leached by over-irrigation with low-salinity effluent. Soil water and salinity, tree water stress, sap flux, substrate carbon conversion efficiency, foliage and stem growth, and foliar cations and chloride were monitored throughout the study. An average of 9 and 1 Mg ha(-1) of salt with an average hydraulic load of 660 and 780 mm was applied to the salt and control plots, respectively. Maximum soil salinity in the root zone was 5.8 and 6.8 dS m(-1) in the eucalypt and pine plots, respectively. Predawn water potential was more than twice as sensitive to increasing salinity in E. grandis as in P. radiata. The salt treatment reduced rates of leaf and stem growth of the eucalypts by 60 to 70% but had no effect on leaf and stem growth of the pines. In the eucalypts, salinity decreased mean leaf area by 26% and increased specific leaf area by 12% compared with control values, indicating less biomass per unit leaf area in the salt treatment. Salinity had no effects on these two parameters in pine. The salt treatment significantly increased mean foliar concentrations of Na and Cl in both species, and of K in the pines. Foliar Na concentration was 6-10 times higher in the eucalypts than in the pines. Lowered water potential and increased Na concentration in the eucalypts in response to salinity resulted in about a 50% reduction in the efficiency of conversion of carbon into biomass; however, three weeks after leaching the salt, there was no significant difference in efficiency of conversion of carbon into biomass between the treatments. Salinity had no effect on water use by eucalypts, but caused a nonsignificant decrease (7%) in water use by pines. As evaporative demand increased, crop factor (transpiration divided by pan evaporation) declined by up to 50 and 60% in the pines and eucalypts, respectively. We conclude that stomatal response to high VPD, not soil salinity, accounts for most of the reduction in summertime water use.

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PubMedDate not supplied

Transpiration rates and canopy conductance of Pinus radiata growing with different pasture understories in agroforestry systems.

We measured tree transpiration and canopy conductance in Pinus radiata D. Don at two low rainfall sites of differing soil fertility in Canterbury, New Zealand. At the more fertile Lincoln site, we also assessed the effects of two common pasture grasses on tree transpiration and canopy conductance. At the less fertile Eyrewell Forest site, the effect of no understory, and the effects of irrigation in combination with mixtures of grass or legume species were determined. Tree xylem sap flux (F(d)') was measured by the heat pulse method. Total canopy conductance to diffusion of water vapor (G(t)) was calculated by inverting a simplified Penman-Monteith model. The different treatment effects were modeled by the simple decaying exponential relationship G(t) = G(tmax)e((-bD)), where D = air saturation deficit. At the Lincoln site, trees with an understory of cocksfoot had lower F(d)' and G(tmax) than trees with an understory of ryegrass, although the sensitivity of G(t) to increasing D (i.e., the value of b) did not differ between treatments. At the Eyrewell site, irrigation only increased F(d)' in the absence of an understory, whereas the presence of understory vegetation, or lack of irrigation, or both, significantly reduced G(tmax) and increased b. We conclude that the selection of understory species is critical in designing successful agroforestry systems for low rainfall areas.

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PubMed1999

Interrelationships among light, photosynthesis and nitrogen in the crown of mature Pinus contorta ssp. latifolia.

Scaling leaf-level measurements to estimate carbon gain of entire leaf crowns or canopies requires an understanding of the distribution of photosynthetic capacity and corresponding light microenvironments within a crown. We have compared changes in the photosynthetic light response and nitrogen (N) content (per unit leaf area) of Pinus contorta Dougl. ssp. latifolia Engelm. (lodgepole pine) leaves in relation to their age and light microenvironment. The vertical gradient in integrated daily photosynthetic photon flux density (PPFD) from the upper to the lower crown of lodgepole pine was similar in magnitude to the horizontal gradient in daily PPFD along shoots from young to old leaves. The relationship between light-saturated net photosynthesis (A(max)) and daily PPFD was significant for both young and old leaves. However, old leaves had a lower A(max) than young leaves in a similar daily irradiance regime. For leaves of all ages from throughout the crown, A(max) was linearly related to the estimated daily net carbon gain that leaves could achieve in their natural PPFD environment (estimated A(day)) (r(2) = 0.84, P < 0.001, n = 39), indicating that estimated A(day) may be dominated by carbon fixed when leaves are light-saturated and operating at A(max). Comparison of the PPFD required to achieve A(max) and the PPFD available to the leaves showed that all of the measured leaves (n = 39), regardless of their position in the crown or age, were in light environments that could light-saturate photosynthesis for a similar proportion of the day. For all data pooled, foliar N was weakly correlated with daily PPFD. Analyzing each leaf age class separately showed that foliar N was significantly related to daily PPFD, A(max), and estimated A(day) for the youngest leaves but not for middle-aged or old leaves. Therefore, the general theory that foliar N is allocated within a crown according to total daily light availability was supported only for young (1-4 years old) leaves in this study.

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PubMed1999

Stand-level allometry in Pinus taeda as affected by irrigation and fertilization.

Changing environmental conditions have the potential to alter allometric relationships between plant parts, possibly leading to ecosystem-level feedbacks. We quantified allometric shifts in field-grown loblolly pine (Pinus taeda L.) in response to altered resource availability based on data from multiple harvests to correct for size-related changes in biomass partitioning. A replicated factorial arrangement of irrigation and fertilization treatments was applied for 4 years to an 8-year-old loblolly pine plantation on a well-drained, low fertility site in North Carolina. Destructive and nondestructive growth measurements were used to develop treatment-specific regressions to estimate stand-level biomass for ephemeral and perennial plant parts, both above- and belowground. Stand-level allometric analysis indicated that irrigation increased biomass partitioning to fine roots and decreased partitioning to foliage, relative to other plant parts. Fertilization increased partitioning to perennial tissues (coarse roots, taproots, and branches) and decreased partitioning to ephemeral tissues (foliage and fine roots). Changes in allometry were small (< 6 %) but statistically significant, indicating that biomass partitioning in loblolly pine changes with altered resource availability, but is probably under strong ontogenetic control.

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PubMed1999

Abscisic acid in needles of Pinus cembra in relation to ozone exposure.

Ozone pollution was analyzed in arolla pine (Pinus cembra L.) forests growing over two mountain ranges located in southern France by using specific ozone-sensitive tobacco plants as bio-indicators and a physico-chemical analyzer. Concentrations of abscisic acid (ABA) were determined in needles of healthy and declining trees in a massif with a declining forest and in a massif with a healthier forest. In addition, ABA was quantified in needles of trees exposed to either charcoal-filtered air or unfiltered air supplemented with ozone in open-top chambers located at the Col du Donon. The concentration of ABA in needles of injured trees increased when the trees were exposed to ozone either under field conditions or in open-top chambers; however, the difference in ABA concentration between control and ozone-exposed needles was less in the open-top chambers, where ozone was the sole variable, than in the field. The results are discussed in the context of the effects of ozone on plant water relations and hormone-mediated cell defense.

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PubMed1999

Responses of Scots pine (Pinus sylvestris) seedlings grown in different nutrient regimes to changing root zone temperature in spring.

We examined effects of nutrient availability and changing root zone temperature (RZT) on growth, gas exchange and plasma membrane H(+)-ATPase (PM-ATPase) activity of roots of 1-year-old Scots pine (Pinus sylvestris L.) seedlings during spring flushing. The 6-week growth-chamber experiment was carried out in hydroponic cultures that supplied the seedlings with low (0.5 mM N) or high (3 mM N) nutrient concentration and two rates of increase in RZT were simulated: slow warming (SW-treatment) and fast warming (FW-treatment). Air temperature, humidity, and light conditions were similar in all treatments. Growth of roots and shoots was retarded at low RZT, and fresh mass increment of roots was closely correlated with RZT sum. High nutrient availability increased nitrogen concentrations of needles and stems, but only at RZTs >13 degrees C. Low RZT and low availability of nutrients suppressed gas exchange of the seedlings. Real PM-ATPase activity was highly dependent on RZT. At high RZTs, real PM-ATPase activity was affected by nutrient availability but this effect was related to root growth. We conclude that, under conditions of high nutrient availability, Scots pine seedlings can compensate for the suppressive effects of long-term exposure to low RZT by rapidly accelerating growth, gas exchange and root metabolism, but only when RZT has increased above a threshold value, which was 13 degrees C in this study.

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PubMed1999

Evidence that hydraulic conductance limits photosynthesis in old Pinus ponderosa trees.

We tested the hypotheses that hydraulic conductance is lower in old (about 250 years old and 30 m tall) compared to young (about 40 years old and 10 m tall) Pinus ponderosa Dougl. ex Laws. trees and that lower hydraulic conductance of old trees limits their photosynthesis. Hydraulic conductance at the end of summer 1995, calculated from leaf water potential and leaf gas exchange measurements on one-year-old needles, was 44% lower in old trees compared to young trees growing in a mixed age-class stand on the east slope of the Oregon Cascades. Whole-tree sapflow per unit leaf area averaged 53% lower in old trees compared to young trees and mean hydraulic conductance calculated from sapflow and water potential data was 63% lower in old trees than in young trees. For the entire summer, stomatal conductance (g(s)) and assimilation (A) declined more steeply with air saturation deficit (D) in old trees than in young trees. For both old and young trees, mean g(s) and A were approximately 32 and 21% lower, respectively, at typical midday D values (2.5-3.0 kPa). We hypothesized that if hydraulic conductance limits g(s) and A, then increasing or decreasing the leaf specific conductance of a branch will result in proportional changes in the responses of g(s) and A with D. Removal of 50% of the foliage from a set of experimental branches on old trees caused g(s) and A to decline less steeply with D in early summer, but values were not significantly different from control values in late summer. Cutting transverse notches in branches on young trees had no effect on the responses of g(s) and A with D. Leaf nitrogen content and photosynthetic capacity were similar suggesting that differences in g(s) and A between old and young trees were not caused by differences in photosynthetic capacity.

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PubMed1999

Nutrient uptake by intact mycorrhizal Pinus sylvestris seedlings: a diagnostic tool to detect copper toxicity.

We developed a nondestructive method for detecting early toxic effects of inflethal copper (Cu) concentrations on ectomycorrhizal and non-mycorrhizal (NM) Scots pine (Pinus sylvestris L.) seedlings. The fungal symbionts examined were Paxillus involutus (Fr.) Fr., Suillus luteus (Fr.) S.F. Gray and Thelephora terrestris (Ehrh.) Fr. The accumulation of Cu in needles and fungal development (ergosterol) in roots and infstrate were assessed. Inorganic phosphate (P(i)) and ammonium (NH(4) (+)) uptake capacities were determined in a semi-hydroponic cultivation system on intact P-limited plants that were exposed for 3 weeks to 0.32 (control), 8 or 16 &mgr;moles Cu(2+). Short-term effects of a 1-hour exposure to 32 &mgr;moles Cu(2+) on nutrient uptake rates were also determined. None of the Cu(2+) treatments affected plant growth or root ergosterol concentrations. The active fungal biomass in infstrate invaded by S. luteus was reduced by 50% in the 16 &mgr;M Cu(2+) treatment compared with the control treatment; however, colonization by S. luteus prevented an increased accumulation of Cu in the needles. In contrast, the 16 &mgr;M Cu(2+) treatment caused a 2.2-fold increase in needle Cu concentration in NM plants. Ergosterol concentrations in the infstrate colonized by P. involutus and T. terrestris were not affected by 16 &mgr;molar Cu(2+). Although P. involutus and T. terrestris were less sensitive to Cu(2+) than S. luteus, T. terrestris did not prevent the accumulation of Cu in needles of its host plant in the 16 &mgr;molar Cu(2+) treatment. Mycorrhizal plants consistently had higher P(i) and NH(4) (+) uptake capacities than NM plants. In the control treatment, specific P(i) uptake rates were almost 10, 4 and 3 times higher in plants associated with P. involutus, S. luteus and T. terrestris, respectively, than in NM plants, and specific NH(4) (+) uptake rates were about 2, 2 and 5 times higher, respectively, than those of NM seedlings. Compared with the corresponding control plants, a 3-week exposure to 8 &mgr;M Cu(2+) had no effect on the nutrient uptake potential of plants. In contrast, the 16 &mgr;M Cu(2+) treatment significantly reduced P(i) uptake capacity of all plants and decreased NH(4) (+) uptake capacity of seedlings colonized by S. luteus or T. terrestris. The 32 &mgr;M Cu(2+) 1-h shock treatment reduced specific NH(4) (+) and P(i) uptake rates of roots colonized by S. luteus to 39 and 77%, respectively, of the original rates. The Cu(2+) 1-h shock treatment reduced the NH(4) (+) uptake rate of NM plants by 51%.

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PubMed1999

Photosynthetic responses of loblolly pine (Pinus taeda) needles to experimental reduction in sink demand.

Sink strength in loblolly pine (Pinus taeda L.) was experimentally manipulated on two sun-exposed branches on each of two neighboring trees by excising the emerging terminal cohort (second flush of 1996) during a period of rapid needle expansion. In addition, export of photosynthate was restricted on one of these branches from each tree by removal of bark and phloem just below the second flush of 1995. Treatment-induced changes in needle biochemistry were measured in 3-month-old (first flush of 1996) and 1-year-old (final flush of 1995) needles collected 1, 5 and 8 days after treatment. In 3-month-old needles, sugar concentration increased by 24% one day after leader excision, and increased by 86% on Day 8 after leader excision and girdling. Starch concentration increased by 33% in 3-month-old needles on Day 1 after leader excision, and by 400% in 1-year-old needles on Day 8 after leader excision and girdling. Physiological changes in 3-month-old and 1-year-old needles were measured by open-flow gas exchange and chlorophyll fluorescence on Day 8 after leader excision and girdling. Light- and CO(2)-saturated net photosynthesis decreased following treatment in both 3-month-old and 1-year-old needles (23 and 17%, respectively). Maximum rate of carboxylation (V(cmax)) decreased by 25% in 3-month-old needles and by 31% in 1-year-old needles in response to leader excision and girdling. The combined treatment resulted in a 38% decrease in maximum rate of electron transport (J(max)) in 3-month-old needles and a 37% decrease in J(max) in 1-year-old needles. Before leader excision and girdling, 2% oxygen in air stimulated photosynthesis by 17 to 19%, but this stimulation was only 3 to 4% at 9 days after treatment. These physiological responses indicate that experimentally lowered sink strength resulted in rapid feedback inhibition of leaf-level photosynthetic capacity in loblolly pine.

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PubMed1999

Effects of elevated atmospheric CO(2) on phenology, growth and crown structure of Scots pine (Pinus sylvestris) seedlings after two years of exposure in the field.

Three-year-old Scots pine (Pinus sylvestris L.) seedlings were grown for two years in the ground in open-top chambers supplied with either an ambient or elevated (ambient + 400 &mgr;mol mol(-1)) CO(2) concentration. Phenological observations and measurements of height and stem diameter growth, absolute and relative growth rates, starch and soluble carbohydrate concentrations of the needles, and crown structure and needle properties were made at frequent intervals throughout the two growing seasons. Elevated CO(2) significantly advanced the date of bud burst in both years. The increase in total needle area in response to elevated CO(2) was accounted for by longer shoots and an increase in individual needle area in the first year, and by an increase in the number and length of shoots in the second year. Stem diameter and tree height were enhanced more by the elevated CO(2) treatment in the first year than in the second, indicating a decreased effect of CO(2) on growth over time. This was confirmed by a study of absolute and relative growth rates of leader shoots. During the first growing season of CO(2) enrichment, mean weekly relative growth rates over the growing season (RGR(m)) were significantly enhanced. During the second year, RGR(m) in ambient CO(2) closely matched that in elevated CO(2).

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PubMed1999

Simulated nitrogen cycling response to elevated CO(2) in Pinus taeda and mixed deciduous forests.

Interactions between elevated CO(2) and N cycling were explored with a nutrient cycling model (NuCM, Johnson et al. 1993, 1995) for a Pinus taeda L. site at Duke University, North Carolina, and a mixed deciduous site at Walker Branch, Tennessee. The simulations tested whether N limitation would prevent growth increases in response to elevated CO(2), and whether growth responses to CO(2) in N-limited systems could be facilitated by increasing the biomass/N ratio (reducing N concentration) or increasing litter N mineralization, or both. Nitrogen limitation precluded additional growth when target growth rates and litterfall were increased (simulating potential response to elevated CO(2)) at the Duke University site. At the Walker Branch site, increasing target growth and litterfall caused a 7% increase in growth. Reducing foliar N concentrations reduced growth because of N limitation created by reduced litter quality (C:N ratio), reduced decomposition and increased N accumulation on the forest floor. These effects were most pronounced at the Duke University site, because the forest floor N turnover rate was lower than at the Walker Branch site. Reducing wood N concentration allowed prolonged increases in growth because of greater biomass/N; however, N uptake was reduced, allowing greater N immobilization on the forest floor and in soil. Increased N mineralization caused increased growth at the Duke University site, but not at the Walker Branch site. These simulations pose the counterintuitive hypothesis that increased biogeochemical cycling of N (as a result of increased litterfall N) causes reduced growth in an N-limited system because of increased accumulations of N on the forest floor and in soil. Translocation of N from senescing leaves before litterfall mitigates this response by allowing the trees to retain a greater proportion of N taken up rather than recycle it back to the forest floor and soil where it can be immobilized. Eliminating N translocation at Walker Branch changed the direction as well as the magnitude of the responses in three of the four scenarios simulated. Because the NuCM model does not currently allow translocation in coniferous species, the effects of translocation on N cycling in the Duke University simulations are not known.

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PubMed1999

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.

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PubMed1999

Root induction in Pinus ayacahuite by co-culture with Agrobacterium tumefaciens strains.

Transformation of in-vitro-derived shoots of Pinus ayacahuite Ehrenb. was achieved by co-culture with an oncogenic strain (A281 x 200) of Agrobacterium tumefaciens. During co-culture rooting also occurred; however, this rooting was not induced by genetic transformation of host cells, because a "disarmed" strain of A. tumefaciens (EHA101) also induced rooting. Furthermore, direct contact between shoots and bacterial cells was not required. Rooting occurred in agar-solidified medium and in a soilless substrate (9:1 vermiculite:peat mix). We conclude that A. tumefaciens strains induced rooting in P. ayacahuite through a change in the rhizosphere, probably by producing some root-inducing compound(s), and not through transformation of host cells.

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