Assimilate partitioning affects 13C fractionation of recently assimilated carbon in maize

Coupling ¹³C natural abundance and ¹⁴C pulse labelling enabled us to investigate the dependence of ¹³C fractionation on assimilate partitioning between shoots, roots, exudates, and CO₂ respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling of maize shoots in a ¹⁴CO₂ atmosphere, ¹⁴C was traced to determine the amounts of recently assimilated C in the four pools and the δ¹³C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered ¹⁴C in NS and DNS treatments) led to a 0.3‰ ¹³C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered ¹⁴C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings support the hypothesis that ¹³C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ¹³C of CO₂ evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced (NS treatment), the respired CO₂ became 0.7‰ ¹³C depleted compared to roots. Increasing amounts of recently assimilated C in the CO₂ from NS via DNS to DW treatments resulted in a 1.6‰ δ¹³C increase of root respired CO₂ from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool led to a δ¹³C increase. In DW and DNS plants there was no ¹³C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ ¹³C enrichment in exudates compared to roots. We conclude that ¹³C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools. Section Editor: H. Lambers     

Magnesium uptake kinetics in loblolly pine seedlings

Recent studies have suggested that the growth of loblolly pine (Pinus taeda L.) has declined in the southern United States and it has been hypothesized that foliar Mg deficiency may play an important role in the perceived decline. Quantitative nutrient uptake models such as the Barber-Cushman model have been used successfully to investigate nutrient uptake by crop species under a variety of field and experimental conditions and may provide one approach to evaluating this question. However, in order to use this approach it is necessary to develop, for the plant species and nutrient of interest, values for maximal nutrient influx rate at high solution concentrations (I_max), the solution concentration where net influx is 0.5 I_max (K_m), and the nutrient concentration below which influx ceases (C_min). As a first step in evaluating the potential of such an approach, two sets of experiments using established solution nutrient depletion techniques were used to define these values for loblolly pine seedlings 180, 240, 365, and 425 days in age. Observed I_max values for Mg range from 7.90E-8 to 1.29E-7 mol cm^–2 s^–1 with younger seedlings having higher values. Values of K_m for all seedling ages were quite similar ranging from 8.69 to 8.58E-3 mol cm^–3. Most importantly, the results of both experiments indicate that during a growth flush, seedlings will withdraw Mg from solution until the concentration is essentially zero (C_min=0). During non-flush periods uptake rates appear to be greatly reduced. Therefore, efforts to model Mg uptake will need to take these differences as well as seedling age influences into consideration.     

Turnover of recently assimilated carbon in arctic bryophytes

Carbon (C) allocation and turnover in arctic bryophytes is largely unknown, but their response to climatic change has potentially significant impacts on arctic ecosystem C budgets. Using a combination of pulse-chase experiments and a newly developed model of C turnover in bryophytes, we show significant differences in C turnover between two contrasting arctic moss species (Polytrichum piliferum and Sphagnum fuscum). ¹³C abundance in moss tissues (measured up to 1 year) and respired CO₂ (traced over 5 days) were used to parameterise the bryophyte C model with four pools representing labile and structural C in photosynthetic and stem tissue. The model was optimised using an Ensemble Kalman Filter to ensure a focus on estimating the confidence intervals (CI) on model parameters and outputs. The ratio of aboveground NPP:GPP in Polytrichum piliferum was 23% (CI 9–35%), with an average turnover time of 1.7 days (CI 1.1–2.5 days). The aboveground NPP:GPP ratio in Sphagnum fuscum was 43% (CI 19–65%) with an average turnover time of 3.1 days (CI 1.6–6.1 days). These results are the first to show differences in C partitioning between arctic bryophyte species in situ and highlight the importance of modelling C dynamics of this group separately from vascular plants for a realistic representation of vegetation in arctic C models.     

Dynamic and allocation of recently assimilated carbon in Korean pine (Pinus koraiensis) and birch (Betula platyphylla) in a temperate forest

Biogeochemistry - Understanding how assimilated carbon (C) is allocated in forest ecosystems is crucial for process-based models and accurate prediction of global C cycling. We tracked the fate of...     

Winter nitrogen fertilization of loblolly pine seedlings

Loblolly pine (Pinus taeda L.) seedlings were nitrogen fertilized during winter in a bare root forest tree nursery located in the coastal plain of the southeastern United States. Total application rates were 0, 50, 100, and 200 kg/N/ha applied in split applications 4 weeks apart in January and February. Seedlings were lifted and outplanted in March, 4 weeks after the second fertilization and measured at 3 and 6 months after outplanting. No seedling morphological differences were encountered at the time of lifting and outplanting although seedling shoot nitrogen content was 28% greater in the highest fertilization treatment compared to the check. Shoot nitrogen concentrations fell after outplanting regardless of treatment, decreasing from an average of 1.51% across all treatments at the time of planting to 0.64% at 6 months after planting. When measured at 6 months after outplanting, seedling dry weight and height growth after planting was shown to increase by 12% and 24%, respectively, for the high nitrogen treatment. This and other studies across a variety of sites have found positive post-outplanting seedling growth response after nutrient loading in the nursery.     

Soil respiration under elevated CO2 and its partitioning into recently assimilated and older carbon sources

Plant and Soil - Efflux of soil CO2 (soil respiration) plays a crucial role in the global carbon cycle and may be strongly altered by global change. In this study, we measured soil respiration in...     

The utilization of recently assimilated carbon in graminaceous plants

Isotopic carbon and infra-red gas analysis techniques were used to measure the following growth attributes in maize, sorghum, winter wheat and perennial ryegrass: the rate of entry of carbon into each main shoot leaf; the rate of translocation of leaf assimilate to meristems; the fraction of leaf and total shoot assimilate respired in one diurnal period; and the distribution of residual assimilate to new leaf, stem, axillary shoots and root. The two tropical plants possessed higher leaf assimilation rates and larger leaves than the temperate species, but their efficiency of translocation was only marginally superior in the experimental conditions. In all species, c. 25% of the assimilate generated in the 8·4h photoperiod was respired in in the same diurnal period. Maize and sorghum partitioned a greater proportion of their total shoot assimilate to new leaf tissue at the main shoot apex and to root than wheat and ryegrass. On the other hand, wheat and ryegrass exported up to 30% of their assimilate to axillary shoots; in sorghum, little assimilate was translocated to axillary shoots, while in maize this activity was completely absent. Plant habit, as exemplified by the contrast between the annual, single-axis maize plant and the perennial, multi-tillering ryegrass plant, appears to be a reflexion of the pattern of assimilate distribution to areas of potential growth. With the exception of superior leaf assimilation rates in maize and sorghum, the four species showed no marked differences in respect of the production, transport and respiratory utilization of assimilates.     

Nutrient uptake by intact and disturbed roots of loblolly pine seedlings

Most measurements of nutrient uptake use either hydroponic systems or soil-grown roots that have been disturbed by excavation. The first objective of this study was to test how root excavation affects nitrate uptake. Rates of NO₃^? uptake by mycorrhizal loblolly pine (Pinus taeda L.) seedlings were measured in intact sand-filled columns, hydroponics, and disturbed sand-filled columns. Total nitrate uptake in intact sand-filled columns was higher than in disturbed columns, indicating that disturbance lowers uptake. Transferring plants from the sand-filled columns to hydroponics had little effect on NO₃^? uptake beyond delaying uptake for an hour. The second objective of this study was to determine whether NH₄⁺, Ca²⁺, Mg²⁺ and K⁺ uptake could be studied using sand-filled columns, since previous studies had tested this method only for nitrate uptake. Uptake rates of NH₄⁺ and K⁺ were positive, while Ca²⁺ and Mg²⁺ uptake rates were negative in intact sand-filled columns, indicating that net efflux may occur even without physical disturbance to the root system. The sand-filled column approach has some limitations, but holds promise for conducting nutrient uptake studies with minimal disturbance to the root system.     

The role of harvest residue in rotation cycle carbon balance in loblolly pine plantations. Respiration partitioning approach

Timber harvests remove a significant portion of ecosystem carbon. While some of the wood products moved off‐site may last past the harvest cycle of the particular forest crop, the effect of the episodic disturbances on long‐term on‐site carbon sequestration is unclear. The current study presents a 25 year carbon budget estimate for a typical commercial loblolly pine plantation in North Carolina, USA, spanning the entire rotation cycle. We use a chronosequence approach, based on 5 years of data from two adjacent loblolly pine plantations. We found that while the ecosystem is very productive (GEP up to 2900 g m^?2 yr^?1, NEE at maturity about 900 g C m^?2 yr^?1), the production of detritus does not offset the loss of soil C through heterotrophic respiration (R_H) on an annual basis. The input of dead roots at harvest may offset the losses, but there remain significant uncertainties about both the size and decomposition dynamics of this pool. The pulse of detritus produced at harvest resulted in a more than 60% increase in R_H. Contrary to expectations, the peak of R_H in relation to soil respiration (SR) did not occur immediately after the harvest disturbance, but in years 3 and 4, suggesting that a pool of roots may have remained alive for the first few years. On the other hand, the pulse of aboveground R_H from coarse woody debris lasted only 2 years. The postharvest increase in R_H was offset by a decrease in autotrophic respiration such that the total ecosystem respiration changed little. The observed flux rates show that even though the soil C pool may not necessarily decrease in the long‐term, old soil C is definitely an active component in the site C cycle, contributing about 25–30% of the R_H over the rotation cycle.     

Ethylene production by loblolly pine seedlings associated with water stress

Effects of water stress on production of ethylene and its precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), by loblolly pine ( Pinus taeda L.) seedlings from a Texas drought-hardy and a Virginia Coastal Plain source were investigated. Ethylene production rates in needles from the Virgnia source increased slightly with initial stress (-1.3 MPa), declined until water potential reached -1.6 MPa and then increased sharply at -2.5 MPa. The ethylene production rates in needles from the Texas also increased slightly with initial stress, then decreased with decreasing water potential. Ethylene production by root tissue was two to three times higher than needle tissue and decreased with decreasing water potential. ACC concentrations in needles of both seed sources decreased as water potential began decreasing. Below -1.4 MPa, ACC levels started increasing (Texas source) or remained constant until -2.8 MPa (Virginia source) at which time its level increased three-fold. Mean ACC levels in root tissue [122 nmol (g dry weight)⁻¹] were slightly higher than the mean levels in the needle tissue [92 nmol (g dry weight) ⁻¹]; roots apparently were more efficient in converting it to ethylene since ethylene production was two to three times higher than needle tissue. The modulation of ethylene synthesis by ACC synthase and ethyleneforming enzyme appeared to be influenced by stress level, organ and seed source.     

Diurnal changes in allocation of newly fixed carbon in exporting sugar beet leaves

Storage of newly fixed carbon as starch and sucrose follows a regular daily pattern in exporting sugar beet leaves under constant day length and level of illumination. Up to the final two hours of the light period, when starch storage declines, a nearly constant proportion of newly fixed carbon was allocated to carbohydrate storage, principally starch. Sucrose is stored only early in the light period, when there is little accumulation of starch. Pulse labeling with ¹⁴CO₂ revealed that considerable starch synthesis was taking place at this time. Starch made the previous day was not mobilized during this period but breakdown of newly synthesized starch may occur when carbon flow into sucrose synthesis increases early in the day. At the end of the day, starch storage declined from the constant level observed during most of the day, but no diversion of label into export of specific alternative compounds could be detected. Lowered storage of starch persisted when the 14-hour light period was lengthened. Changed allocation of recently fixed carbon to sucrose and starch at the beginning and end of the light period was not the result of outright inactivation of pathways but of regulation of carbon flow.     

Interactive effects of soil nitrogen and atmospheric carbon dioxide on root/rhizosphere carbon dioxide efflux from loblolly and ponderosa pine seedlings

We measured CO₂ efflux from intact root/rhizosphere systems of 155 day old loblolly (Pinus taeda L.) and ponderosa (Pinus ponderosa Dougl. ex Laws.) pine seedlings in order to study the effects of elevated atmospheric CO₂ on the below-ground carbon balance of coniferous tree seedlings. Seedlings were grown in sterilized sand culture, watered daily with either 1, 3.5 or 7 mt M NH ₄ ⁺ , and maintained in an atmosphere of either 35 or 70 Pa CO₂. Carbon dioxide efflux (mol CO₂ plant^–1 s^–1) from the root/rhizosphere system of both species significantly increased when seedlings were grown in elevated CO₂, primarily due to large increases in root mass. Specific CO₂ efflux (mol CO₂ g root^–1 s^–1) responded to CO₂ only under conditions of adequate soil nitrogen availability (3.5 mt M). Under these conditions, CO₂ efflux rates from loblolly pine increased 70% from 0.0089 to 0.0151 mol g^–1 s^–1 with elevated CO₂ while ponderosa pine responded with a 59% decrease, from 0.0187 to 0.0077 mol g^–1 s^–1. Although below ground CO₂ efflux from seedlings grown in either sub-optimal (1 mt M) or supra-optimal (7 mt M) nitrogen availability did not respond to CO₂, there was a significant nitrogen treatment effect. Seedlings grown in supra-optimal soil nitrogen had significantly increased specific CO₂ efflux rates, and significantly lower total biomass compared to either of the other two nitrogen treatments. These results indicate that carbon losses from the root/rhizosphere systems are responsive to environmental resource availability, that the magnitude and direction of these responses are species dependent, and may lead to significantly different effects on whole plant carbon balance of these two forest tree species.     

The influence of root assimilated inorganic carbon on nitrogen acquisition/assimilation and carbon partitioning

Understanding of the influences of root-zone CO2 concentration on nitrogen (N) metabolism is limited. The influences of root-zone CO2 concentration on growth, N uptake, N metabolism and the partitioning of root assimilated 14C were determined in tomato (Lycopersicon esculentum). Root, but not leaf, nitrate reductase activity was increased in plants supplied with increased root-zone CO2. Root phosphoenolpyruvate carboxylase activity was lower with NO3(-)- than with NH4(+)-nutrition, and in the latter, was also suppressed by increased root-zone CO2. Increased growth rate in NO3(-)-fed plants with elevated root-zone CO2 concentrations was associated with transfer of root-derived organic acids to the shoot and conversion to carbohydrates. With NH4(+)-fed plants, growth and total N were not altered by elevated root-zone CO2 concentrations, although 14C partitioning to amino acid synthesis was increased. Effects of root-zone CO2 concentration on N uptake and metabolism over longer periods (> 1 d) were probably limited by feedback inhibition. Root-derived organic acids contributed to the carbon budget of the leaves through decarboxylation of the organic acids and photosynthetic refixation of released CO2.     

Influence of CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation in longleaf pine seedlings

One-year old, nursery-grown longleaf pine (Pinus palustris Mill.) seedlings were grown in 45-L pots containing a coarse sandy medium and were exposed to two concentrations of atmospheric CO₂ (365 or 720 mol^-1) and two levels of nitrogen (N) fertility (40 or 400 kg N ha^-1 yr^-1) within open top chambers for 20 months. At harvest, needles, stems, coarse roots, and fine roots were separated and weighed. Subsamples of each tissue were frozen in liquid N, lyophilized at -50°C, and ground to pass a 0.2 mm sieve. Tissue samples were analyzed for carbon (C), N, nonpolar extractives (fats, waxes, and oils = FWO), nonstructural carbohydrates (total sugars and starch), and structural carbohydrates (cellulose, lignin, and tannins). Increased dry weights of each tissue were observed under elevated CO₂ and with high N; however, main effects of CO₂ were significant only on belowground tissues. The high N fertility tended to result in increased partitioning of biomass aboveground, resulting in significantly lower root to shoot ratios. Elevated CO₂ did not affect biomass allocation among tissues. Both atmospheric CO₂ and N fertility tended to affect concentration of C compounds in belowground, more than aboveground, tissues. Elevated CO₂ resulted in lower concentrations of starch, cellulose, and lignin, but increased concentrations of FWO in root tissues. High N fertility increased the concentration of starch, cellulose, and tannins, but resulted in lower concentrations of lignin and FWO in roots. Differences between CO₂ concentrations tended to occur only with high N fertility. Atmospheric CO₂ did not affect allocation patterns for any compound; however the high N treatment tended to result in a lower percentage of sugars, cellulose, and lignin belowground.Joseph W. Jones     

Diurnal changes in the chilling sensitivity of seedlings

Seedlings of tomato (Lycopersicon esculentum, Mill.) varied diurnally in their sensitivity to chilling temperatures. If chilled near the end of the dark period when they were most sensitive, the time taken to kill half of the seedlings was approximately 3 days, whereas in samples taken 4 hours after the onset of dark, a period of 6 days of chilling was required. Sensitivity dropped rapidly after the onset of the light period. This rhythm was exogenously controlled by the diurnal changes in light, rather than in the temperature. The susceptibility of predawn seedlings could be reduced by exposure to light, by water stress, or by abscisic acid applied to the leaves. However, the subsequent changes in sensitivity to chilling did not correlate with stomatal aperture. Six other chilling-sensitive species showed similar diurnal changes in their chilling sensitivity.     

Productivity differences among loblolly pine genotypes are independent of individual-tree biomass partitioning and growth efficiency

Genetic differences in individual-tree biomass partitioning, growth efficiency, and stem relative growth rate (RGR) could confer intraspecific productivity differences and might strongly influence forest ecosystem carbon storage. We examined the relationship between genotype productivity (stem volume), whole-tree biomass partitioning, growth efficiency (stem wood production per unit leaf area), and stem RGR among nine different loblolly pine (Pinus taeda L.) genotypes from three different genetic groups of contrasting inherent genetic homogeneity: three open-pollinated (half-sib) families, three mass-control pollinated (full-sib) families, and three clonal varieties. We hypothesized that genotype productivity would be positively associated with increased partitioning to stem wood relative to other plant parts, higher stem RGR, and enhanced growth efficiency. After 3 years under plantation conditions, genotypes showed significant differences in stem volume, percent stem wood, percent branch wood, and partitioning to fine roots, yet no differences in stem RGR or growth efficiency. Furthermore, genotypic differences in stem volume were independent of genotypic differences in biomass partitioning, and overall, we found no evidence to support the hypothesized relationships. Even so, the observed variation in biomass partitioning has implications for forest C sequestration as genotypes which partition more biomass to long-lived biomass pools such as stems, may sequester more C. Moreover, the lack of a genetic relationship between stem volume and belowground partitioning suggests that highly productive genotypes may be planted without compromising belowground C storage.     

Mycorrhiza development of loblolly pine seedlings in relation to soil reaction and the supply of nitrate

Summary A study was made of the relative influence of nitrate and soil pH on mycorrhiza development in seedlings of loblolly pine (Pinus taeda L.). The investigation was conducted in the greenhouse, using as the growth medium topsoil collected from a young pine plantation.Lime-induced Fe-deficiency occurred at pH 7.5, resulting in chlorotic seedlings with few mycorrhizas. Chlorosis was corrected, and normal mycorrhiza development restored, by adding Fe-EDTA without altering soil pH.Application of 18 1/2, 37, and 74 pounds of N per acre as NaNO₃ reduced mycorrhiza development at age 21 weeks, but had no effect at age 45 weeks. At 21 weeks, the degree of infection varied inversely as the percentage total N in seedling roots. The effect of NaNO₃ was due to the nitrate ion, since Na₂CO₃ did not reduce mycorrhiza development even though it raised soil pH.Alkalinityper se did not affect mycorrhiza formation in loblolly pine seedlings, but only indirectly through its influence on host nutrition. Normal mycorrhiza development was possible at pH values of 7.2 and 7.5, provided Fe deficiency was corrected, and soil nitrate level was kept low. The results may be interpreted in terms of the carbohydrate — nitrogen balance in the root tissues.     

Modeling magnesium,phosphorus and potassium uptake by loblolly pine seedlings using a Barber-Cushman approach

The Barber-Cushman mechanistic nutrient uptake model, which has been utilized extensively to describe and predict nutrient uptake by crop plants, was evaluated for its ability to predict K, Mg, and P uptake by loblolly pine (Pinus taeda L.) seedlings. Sensitivity analyses were also used to investigate the impact of changes in soil nutrient supply, root morphological, and root uptake kinetics parameters on simulated nutrient uptake. Established experimental techniques were utilized to define the 11 parameters needed to model uptake by 1-0 seedlings of K, Mg, and P from a modified A horizon soil (Lilly series). Model predictions of K and P uptake over a 180-d growth period were underestimated by 6 and 11%, respectively. Estimates of Mg uptake were underestimated by 62%. While the level of agreement between predicted and observed K and P values was quite acceptable, analysis of parameter values and results of sensitivity analyses both indicated that the model underestimation of Mg uptake was the result of applying an I_max value developed under relatively low Mg concentration to a situation in which the functional I_max would be much higher due to the dominance of passive versus active uptake. Overall results of sensitivity analyses indicate that under the circumstances investigated, I_max, was the primary variable controlling plant uptake of K, Mg, and P. The dominance of this term over others was due to the relatively high C_li values for all three nutrients. Reducing (-50%) or increasing (+ 100%) other soil supply, root morphological, and remaining root uptake kinetics values did not substantially alter model estimates of nutrient uptake.