Effect of soil nitrogen supply on carbon assimilation by tree stems

• ? Nitrogen (N) is one of the most important resources for plants, generally enhancing leaf photosynthesis because a large part of it is allocated to Rubisco and thylakoïds. This is well known in leaves where photosynthesis (i.e. gas exchange, Rubisco activity, chlorophyll content) is positively correlated to leaf N content.
• ? In order to test this hypothesis in stems, N concentration, CO₂ exchange and also Rubisco and PEP carboxylase activities were measured in summer on current-year stems of young European beeches (Fagus sylvatica L.) growing on soils of different N content.
• ? The CO₂ refixation rate of stems increased from 58.5% to 74.3% when stem N concentration increased from 5.7 to 10.1 mg g^?1 DW. A hyperbolic relationship was obtained between stem gross photosynthesis and N concentration, with an x-intercept of 0.3 mmol N g^?1 DW. Stem PEP carboxylase activity was higher in stems than in leaves and increased with stem N concentration whereas Rubisco activity did not change between treatments in both tissues.
• ? In spite of a low nitrogen investment in stem photosynthesis (low PNUE), these results suggest that (1) stems invest more N in CO₂ refixation when more N is widely available, (2) stem photosynthesis is able to operate at low N concentration and (3) stem PEP carboxylase is involved in stem carbon refixation, but also simultaneously supplies carbon skeletons for N assimilation.

     

Modeling effects of hydrological changes on the carbon and nitrogen balance of oak in floodplains

We extended the applicability of the ecosystem model BIOME-BGC to floodplain ecosystems to study effects of hydrological changes on Quercus robur L. stands. The extended model assesses floodplain peculiarities, i.e., seasonal flooding and water infiltration from the groundwater table. Our interest was the tradeoff between (a). maintaining regional applicability with respect to available model input information, (b). incorporating the necessary mechanistic detail and (c). keeping the computational effort at an acceptable level. An evaluation based on observed transpiration, timber volume, soil carbon and soil nitrogen content showed that the extended model produced unbiased results. We also investigated the impact of hydrological changes on our oak stands as a result of the completion of an artificial canal network in 1971, which has stopped regular springtime flooding. A comparison of the 11 years before versus the 11 years after 1971 demonstrated that the hydrological changes affected mainly the annual variation across years in leaf area index (LAI) and soil carbon and nitrogen sequestration, leading to stagnation of carbon and nitrogen stocks, but to an increase in the variance across years. However, carbon sequestration to timber was unaffected and exhibited no significant change in cross-year variation. Finally, we investigated how drawdown of the water table, a general problem in the region, affects modeled ecosystem behavior. We found a further amplification of cross-year LAI fluctuations, but the variance in soil carbon and nitrogen stocks decreased. Volume increment was unaffected, suggesting a stabilization of the ecosystem two decades after implementation of water management measures.     

Interactive effects of nitrogen and water availabilities on gas exchange and whole-plant carbon allocation in poplar

Cuttings of balsam spire hybrid poplar (Populus trichocarpa var. Hastata Henry x Populus balsamifera var. Michauxii (Dode) Farwell) were grown in sand culture and irrigated every 2 (W) or 10 (w) days with a solution containing either 3.0 (N) or 0.5 (n) mol nitrogen m(-3) for 90 days. Trees in the WN (control) and wn treatments had stable leaf nitrogen concentrations averaging 19.4 and 8.4 mg g(-1), respectively, over the course of the experiment. Trees in the Wn and wN treatments had a similar leaf nitrogen concentration, which increased from 12.0 to 15.8 mg g(-1) during the experiment. By the final harvest, mean stomatal conductances of trees in the wN and wn treatments were less than those of trees in the Wn and WN treatments (1.8 versus 4.6 mm s(-1)). Compared to the WN treatment, biomass at the final harvest was reduced by 61, 72 and 75% in the Wn, wN and wn treatments, respectively. At the final harvest, WN trees had a mean total leaf area of 4750 +/- 380 cm(2) tree(-1) and carried 164 +/- 8 leaves tree(-1) with a specific leaf area of 181 +/- 16 cm(2) g(-1), whereas Wn trees had a smaller mean total leaf area (1310 +/- 30 cm(2) tree(-1)), because of the production of fewer leaves (41 +/- 6) with a smaller specific leaf area (154 +/- 2 cm(2) g(-1)). A greater proportion of biomass was allocated to roots in Wn trees than in WN trees, but component nitrogen concentrations adjusted such that there was no Wn treatment effect on nitrogen allocation. Compared with WN trees, rates of photosynthesis and respiration per unit weight of tissue of Wn trees decreased by 28 and 31%, respectively, but the rate of photosynthesis per unit leaf nitrogen remained unaltered. The wN and Wn trees had similar leaf nitrogen concentrations; however, compared with the Wn treatment, the wN treatment decreased mean total leaf area (750 +/- 50 cm(2) tree(-1)), number of leaves per tree (29 +/- 2) and specific leaf area (140 +/- 6 cm(2) g(-1)), but increased the allocation of biomass and nitrogen to roots. Net photosynthetic rate per unit leaf nitrogen was 45% lower in the wN treatment than in the other treatments. Rates of net photosynthesis and respiration per unit weight of tissue were 48 and 33% less, respectively, in wN trees than in Wn trees.     

Variations in carbon and nitrogen ratios and their effects on seed germination in Cupressus sempervirens populations

This study investigated the variations in the carbon (C), nitrogen (N), and C/N ratios in the seeds of the Mediterranean Cypress (Cupressus sempervirens L.), a typical forest tree of the Mediterranean Region. The data were gathered from populations distributed across Turkey. We also evaluated the relationship between these variations and germination parameters such as the germination percentage (GP) and mean germination time (MGT). All of the evaluated characteristics demonstrated statistically significant variations among the populations. The GPs were generally low and varied between 14% in Datça and 51% in the Köprülü Kanyon. The MGTs ranged from 14.9 in the Köprülü Kanyon to 18.9 in Marmaris. In addition, the C ratios varied between 46% in Marmaris and 52% in the Köprülü Kanyon, while the N ratios ranged from 1.1% in Beycik and Göksu to 1.8% in the Köprülü Kanyon. Furthermore, we observed a positive correlation between the GP and the C and N contents (R² = 0.51 and R² = 0.49, respectively) but found a negative correlation between the MGT and the C and N contents (R² = 0.56 and R² = 0.32, respectively). Moreover, with regard to the combined C and N seed ratios of the regression models, R² = 0.95 for the GP and R² = 0.87 for the MGT, and the populations with higher quantities of C and N in their seeds had shorter MGTs and higher GPs.     

Long-term effects of fire frequency on carbon storage and productivity of boreal forests: a modeling study

Climate change is predicted to shorten the fire interval in boreal forests. Many studies have recorded positive effects of fire on forest growth over a few decades, but few have modeled the long-term effects of the loss of carbon and nitrogen to the atmosphere. We used a process-based, dynamic, forest ecosystem model, which couples the carbon, nitrogen and water cycles, to simulate the effects of fire frequency on coniferous forests in the climate of Prince Albert, Saskatchewan. The model was calibrated to simulate observed forest properties. The model predicted rapid short-term recovery of net primary productivity (NPP) after fire, but in the long term, supported the hypotheses that (1) current NPP and carbon content of boreal forests are lower than they would be without periodic fire, and (2) any increase in fire frequency in the future will tend to lower NPP and carbon storage. Lower long-term NPP and carbon storage were attributable to (1) loss of carbon on combustion, equal to about 20% of NPP over a 100-200 year fire cycle, (2) loss of nitrogen by volatilization in fire, equal to about 3-4 kg N ha(-1) year(-1) over a 100-200 year fire cycle, and (3) the fact that the normal fire cycle is much shorter than the time taken for the forest (especially the soil) to reach an equilibrium carbon and nitrogen content. It was estimated that a shift in fire frequency from 200 to 100 years over 1000 Mha of boreal forest would release an average of about 0.1 Gt C year(-1) over many centuries.     

Carbon and nitrogen assimilation in red oaks (Quercus rubra L.) subject to defoliation and nitrogen stress

To examine how rates of net photosynthesis and N uptake of red oak seedlings respond to defoliation under contrasting conditions of N availability, nitrogen-deficient plants were grown in sand culture and subjected to partial defoliation and increased N availability under low light conditions. Both photosynthesis and N uptake rates were measured regularly before and after the treatments. Defoliation resulted in elevated rates of net photosynthesis in both low-N and high-N trees, but the high-N trees were able to maintain the high photosynthetic rates for a longer period of time. Nitrogen availability did not affect the photosynthetic rate of the undefoliated plants. Nitrogen uptake was not affected by the defoliation treatment, but was increased by increasing N availability in both the defoliated and undefoliated plants. Nitrogen uptake rates increased less than would be expected on the basis of N availability alone, but the uptake rates were apparently not limited by carbon supply in the short term. Suboptimal concentrations of N in plant tissues resulted in a strong sink for N even in the absence of refoliation.     

Effects of carbon and nitrogen amendment on soil carbon and nitrogen mineralization in volcanic immature soil in southern Kyushu,Japan

Soil N mineralization is affected by microbial biomass and respiration, which are limited by available C and N. To examine the relationship between C and N for soil microbial dynamics and N dynamics, we conducted long-term laboratory incubation (150 days) after C and N amendment and measured changes in C and N mineralization, microbial biomass C, and dissolved C and N throughout the incubation period. The study soil was volcanic immature soil from the southern part of Japan, which contains lower C and N compared with other Japanese forest soils. Despite this, the area is covered by well-developed natural and plantation forests. Carbon amendment resulted in an increase in both microbial biomass and respiration, and net N mineralization decreased, probably due to increasing microbial immobilization. In contrast, N amendment resulted in a decrease in microbial respiration and an increase in net N mineralization, possibly due to decreased immobilization by microbes. Amendment of both C and N simultaneously did not affect microbial biomass and respiration, although net N mineralization was slightly increased. The results suggested that inhibitory effect on microbial respiration by N amendment should be reduced if carbon availability is higher. Thus, soil available C may limit microbial biomass and respiration in this volcanic immature soil. Even in immature soil where C and N substrate is low, soil C, such as plant root exudates and materials from above- and belowground dead organisms, might help to maintain microbial activity and N mineralization in this study site.     

Effect of nitrogen deposition reduction on biodiversity and carbon sequestration

Global warming and loss of biodiversity are among the most prominent environmental issues of our time. Large sums are spent to reduce their causes, the emission of CO₂ and nitrogen compounds. However, the results of such measures are potentially conflicting, as the reduction of nitrogen deposition may hamper carbon sequestration and thus increase global warming. Moreover, it is uncertain whether a lower nitrogen deposition will lead to a higher biodiversity. We applied a dynamic soil model, a vegetation dynamic model and a biodiversity regression model to investigate the effect of nitrogen deposition reduction on the carbon sequestration and plant species diversity. The soil and vegetation models simulate the carbon sequestration as a result of nitrogen deposition and they provide the biodiversity model with information on the soil conditions groundwater table, pH and nitrogen availability. The plant diversity index resulting from the biodiversity model is based on the occurrence of ‘red list’ species for the tree soil conditions. Based on the model runs we forecast that a gradual decrease in nitrogen deposition from 40 to 10 kg N ha⁻¹ y⁻¹ in the next 25 years will cause a drop in the net carbon sequestration of forest in The Netherlands to 27% of the present amount, while biodiversity remains constant in forest, but may increase in heathland and grassland.     

Actual and potential transpiration and carbon assimilation in an irrigated poplar plantation

We examined the tradeoffs between stand-level water use and carbon uptake that result when biomass production of trees in plantations is maximized by removing nutrient and water limitations. A Populus trichocarpa Torr. x P. deltoides Bartr. & Marsh. plantation was irrigated and received frequent additions of nutrients to optimize biomass production. Sap flux density was measured continuously over four of the six growing-season months, supplemented with periodic measurements of leaf gas exchange and water potential. Measurements of tree diameter and height were used to estimate leaf area and biomass production based on allometric relationships. Sap flux was converted to canopy conductance and analyzed with an empirical model to isolate the effects of water limitation. Actual and soil-water-unlimited potential CO(2) uptakes were estimated with a canopy conductance constrained carbon assimilation (4C-A) scheme, which couples actual or potential canopy conductance with vertical gradients of light distribution, leaf-level conductance, maximum Rubisco capacity and maximum electron transport. Net primary production (NPP) was about 43% of gross primary production (GPP); when estimated for individual trees, this ratio was independent of tree size. Based on the NPP/GPP ratio, we found that current irrigation reduced growth by about 18% compared with growth with no water limitation. To achieve maximum growth, however, would require 70% more water for transpiration, and would reduce water-use efficiency by 27%, from 1.57 to 1.15 g stem wood C kg(-1) water. Given the economic and social values of water, plantation managers appear to have optimized water use.     

蒙古口蘑的碳氮源营养研究

为了给蒙古口蘑人工驯化栽培提供依据,开展了蒙古口蘑的碳、氮源营养研究。结果表明,蒙古口蘑的最佳碳源为蔗糖,淀粉、麦芽糖、葡萄糖、甘露醇也较好;牛肉膏为蒙古口蘑的最佳氮源,营养肉汤、酵母膏、蛋白胨、硝酸铵为蒙古口蘑的较好氮源。用蔗糖20g和牛肉膏2g加入琼脂20g、水1 000ml配制成培养基,培养10d后蒙古口蘑菌丝体可长满9cm直径的培养皿。     

Vertical profiles reveal impact of ozone and temperature on carbon assimilation of Betula pendula and Populus tremula

Rising temperature and tropospheric ozone (O(3)) concentrations are likely to affect carbon assimilation processes and thus the carbon sink strength of trees. In this study, we investigated the joint action of elevated ozone and temperature on silver birch (Betula pendula) and European aspen (Populus tremula) saplings in field conditions by combining free-air ozone exposure (1.2?×?ambient) and infrared heaters (ambient +1.2 °C). At leaf level measurements, elevated ozone decreased leaf net photosynthesis (P(n)), while the response to elevated temperature was dependent on leaf position within the foliage. This indicates that leaf position has to be taken into account when leaf level data are collected and applied. The ozone effect on P(n) was partly compensated for at elevated temperature, showing an interactive effect of the treatments. In addition, the ratio of photosynthesis to stomatal conductance (P(n)/g(s) ratio) was decreased by ozone, which suggests decreasing water use efficiency. At the plant level, the increasing leaf area at elevated temperature resulted in a considerable increase in photosynthesis and growth in both species.     

Harvest residue management and fertilisation effects on soil carbon and nitrogen in a 15-year-old Pinus radiata plantation forest

Growing interest in the use of planted forests for bioenergy production could lead to an increase in the quantities of harvest residues extracted. We analysed the change in C and N stocks in the forest floor (LFH horizon) and C and N concentrations in the mineral soil (to a depth of 0.3 m) between pre-harvest and mid-rotation (stand age 15 years) measurements at a trial site situated in a Pinus radiata plantation forest in the central North Island, New Zealand. The impacts of three harvest residue management treatments: residue plus forest floor removal (FF), residue removal (whole-tree harvesting; WT), and residue retention (stem-only harvesting; SO) were investigated with and without the mean annual application of 190 kg N ha⁻¹ year⁻¹ of urea-N fertiliser (plus minor additions of P, B and Mg). Stocks of C and N in the forest floor were significantly decreased under FF and WT treatments whereas C stocks and mass of the forest floor were significantly increased under the SO treatment over the 15-year period. Averaged across all harvesting treatments, fertilisation prevented the significant declines in mass and C and N stocks of the forest floor which occurred in unfertilised plots. The C:N ratio of the top 0.1 m of mineral soil was significantly increased under the FF treatment corresponding to a significant reduction in N concentration over the period. However, averaged across all harvesting treatments, fertilisation prevented the significant increase in C:N ratio of the top 0.1 m of mineral soil and significantly decreased the C:N ratio of the 0-0.3 m depth range. Results indicate that residue extraction for bioenergy production is likely to reduce C and N stocks in the forest floor through to mid-rotation and possibly beyond unless fertiliser is applied. Forest floors should be retained to avoid adverse impacts on topsoil fertility (i.e., increased C:N ratio). Based on the rate of recovery of the forest floor under the FF treatment, stocks of C and N in the forest floor were projected to reach pre-harvest levels at stand age 18-20. While adverse effects of residue extraction may be mitigated by the application of urea-N fertiliser, it should be noted that, in this experiment, fertiliser was applied at a high rate. Assessment of the sustainability of harvest residue extraction over multiple rotations will require long-term monitoring.     

凋落物输入对根系分泌物碳输入及土壤氮的影响

以马尾松人工林及林下灌木短柱茶为研究对象,采用根系分泌物原位收集法,分析凋落物处理[对照(保持原状凋落物不变,CK)、去除凋落物(LR)、添加凋落物(LA)]对两树种根系分泌物碳输入速率、根际土壤氮含量和氮转化相关酶活性的影响,并分析其相关关系。结果表明:凋落物输入量变化对马尾松和短柱茶根系分泌物碳输入速率无显著影响。同一凋落物处理下,马尾松和短柱茶根系分泌物碳输入速率无显著差异。凋落物输入量变化不显著影响根际土壤氮含量及马尾松根际土壤脲酶、羟胺还原酶、硝酸还原酶和亚硝酸还原酶活性,短柱茶根际土壤硝酸还原酶活性表现为去除凋落物处理显著大于对照,其余氮转化相关酶活性未表现出显著差异。可能是凋落物处理时间较短,凋落物未完全分解或者是林分年龄较大有关。相关关系结果表明,单位根长根系分泌物碳输入速率与根际土壤全氮(TN)含量呈显著负相关关系,单位根表面积根系分泌物碳输入速率与根际土壤硝态氮含量(NO-3-N)含量呈显著负相关关系。表明根系分泌物碳输入与根际土壤氮素相互影响。     

绿竹林碳,氮动态研究

主要讨论了闽南绿竹（Dendrocalamopsisoldhami）林C、N元素的含量、库存量、年动态及C／N。结果表明：（1）绿竹林各组分C、N元素含量有一定差异,含量范围分别为：C38．74％～44．77％、No．557％～1．012％;C／N在38．281～77．792之间。（2）绿竹林C、N元素的现存库存量分别为6760．0369／m2和112．3979／m2,其中地上部分别为5891．5739／m2和94．101g／m2;地下部分别为868．463g／m2和18．296g／m2。（3）1996年7月至1997年6月一年中绿竹叶的C、N元素含量基本上是在生长期的春末夏初含量高,在冬季含量低：C／N在24．707～38．281之间,在生长期的春季比例低,在冬季比例高。（4）绿竹细根的分解过程中C、N元素含量处于释放或累积的相间波动之中,而C／N在细根分解的前几个月下降,而后处于上升和下降的波动之中。     

Defoliation and nitrogen effects on photosynthesis and growth of Eucalyptus globulus

Plant responses to defoliation are complex. We established a field experiment in a nine-month-old Eucalyptus globulus Labill. plantation to examine the effects of pattern (upper crown versus lower crown removal), frequency (single, double or triple defoliation within a 12-month period) and severity (25 versus 38% of leaf area removed) of defoliation and the effect of soil nitrogen (N) on photosynthetic processes and stem growth. The photosynthetic responses observed following defoliation could be attributed to changes in source:sink ratios. Light-saturated CO(2) uptake (A(max)) increased with increasing severity and frequency of defoliation irrespective of defoliation pattern. Seedlings defoliated in autumn did not exhibit increases in A(max) until the following spring, whereas there was no such delay in photosynthetic responses associated with spring defoliation. Application of N before defoliation allowed trees to compensate for the effect of defoliation on stem diameter growth, which could not be explained simply in terms of increases in A(max). The observed increases in stem diameter increment following N fertilization of defoliated trees suggested increases in leaf area development, and there were changes in the leaf area:leaf dry mass ratio that may have increased light absorption by the crown. Nitrogen fertilization also increased partitioning of dry mass to branches at the expense of main stems, suggesting that N supply was important in rebuilding crowns following a defoliation event.     

Atmospheric carbon dioxide, irrigation, and fertilization effects on phenolic and nitrogen concentrations in loblolly pine (Pinus taeda) needles

Concentrations of total soluble phenolics, catechin, proanthocyanidins (PA), lignin and nitrogen (N) were measured in loblolly pine (Pinus taeda L.) needles exposed to either ambient CO(2) concentration ([CO(2)]), ambient plus 175 or ambient plus 350 micromol CO(2) mol(-1) in branch chambers for 2 years. The CO(2) treatments were superimposed on a 2 x 2 factorial combination of irrigation and fertilization treatments. In addition, we compared the effects of branch chambers and open-top chambers on needle chemistry. Proanthocyanidin and N concentrations were measured in needles from branch chambers and from trees in open-top chambers exposed concurrently for two years to either ambient [CO(2)] or ambient plus 200 micromol CO(2) mol(-1) in combination with a fertilization treatment. In the branch chambers, concentrations of total soluble phenolics in needles generally increased with needle age. Concentrations of total soluble phenolics, catechin and PA in needle extracts increased about 11% in response to the elevated [CO(2)] treatments. There were no significant treatment effects on foliar lignin concentrations. Nitrogen concentrations were about 10% lower in needles from the elevated [CO(2)] treatments than in needles from the ambient [CO(2)] treatments. Soluble phenolic and PA concentrations were higher in the control and irrigated soil treatments in about half of the comparisons; otherwise, differences were not statistically significant. Needle N concentrations increased 23% in response to fertilization. Treatment effects on PA and N concentrations were similar between branch and open-top chambers, although in this part of the study N concentrations were not significantly affected by the CO(2) treatments in either the branch or open-top chambers. We conclude that elevated [CO(2)] and low N availability affected foliar chemical composition, which could in turn affect plant-pathogen interactions, decomposition rates and mineral nutrient cycling.     

The impact of nitrogen deposition on carbon sequestration by European forests and heathlands

In this study, we present estimated ranges in carbon (C) sequestration per kg nitrogen (N) addition in above-ground biomass and in soil organic matter for forests and heathlands, based on: (i) empirical relations between spatial patterns of carbon uptake and influencing environmental factors including nitrogen deposition (forests only), (ii) ¹⁵N field experiments, (iii) long-term low-dose N fertilizer experiments and (iv) results from ecosystem models. The results of the various studies are in close agreement and show that above-ground accumulation of carbon in forests is generally within the range 15–40 kg C/kg N. For heathlands, a range of 5–15 kg C/kg N has been observed based on low-dose N fertilizer experiments. The uncertainty in C sequestration per kg N addition in soils is larger than for above-ground biomass and varies on average between 5 and 35 kg C/kg N for both forests and heathlands. All together these data indicate a total carbon sequestration range of 5–75 kg C/kg N deposition for forest and heathlands, with a most common range of 20–40 kg C/kg N. Results cannot be extrapolated to systems with very high N inputs, nor to other ecosystems, such as peatlands, where the impact of N is much more variable, and may range from C sequestration to C losses.     

氮沉降对森林土壤碳动态过程的影响

本文综述了国内外关于森林土壤碳动态过程对氮沉降响应机制的研究进展,概述了大气氮沉降对土壤碳释放及其影响因子的作用机制,从土壤生物学特性、凋落物动态、土壤碳释放等方面揭示大气氮沉降对土壤碳平衡过程影响机制和机理,探讨了森林土壤碳动态过程对氮沉降响应的不确定性因素,并指出未来该领域研究重点。     

Effect of nitrogen and carbon sources on lignin and cellulose degradation by Armillaria ostoyae

Lignin has been hypothesized to be the primary mechanism of resistance to fungal pathogens in plant tissue. Degradation of lignin and cellulose by Armillaria ostoyae cultured for six weeks in Melin-Norkrans medium containing various nitrogen and carbon sources was measured radiometrically. No consistent pattern of lignin or cellulose degradation was found, regardless of A. ostoyae isolate, nitrogen source and concentration, or carbon concentration. More lignin was degraded as the concentration of glucose and fructose increased but not when the concentration of sucrose increased.     

Effect of photon flux density on carbon assimilation and chlorophyll a fluorescence of cold-stored white spruce and lodgepole pine seedlings

White spruce (Picea glauca (Moench.) Voss) and lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) seedlings previously held in dark, frozen storage (-2 degrees C) for 2.5 or 6 months, and nursery-grown white spruce seedlings lifted in summer were exposed to photon flux densities (PFDs) similar to those that might be encountered at planting. Photosynthetic gas exchange and chlorophyll a (chl a) fluorescence were examined in cold-stored and summer-lifted seedlings before and after a 9 h-exposure to artificial illumination of high PFD (2000 micro mol m(-2) s(-1)) or low PFD (ca. 500 micro mol m(-2) s(-1)), and during exposure to 400 micro mol m(-2) s(-1) for 4-9 days. In the 2.5-month-stored and summer-lifted seedlings, the high-PFD treatment caused a small decrease in carbon fixation and a large decrease in the ratio of variable to maximum fluorescence (F(v)/F(m)) relative to the effect of the low-PFD treatment. In contrast, in the 6-month-stored seedlings the high-PFD treatment caused a significant decrease in rate of light-saturated carbon fixation but little decrease in F(v)/F(m) relative to the effect of the low-PFD treatment, indicating that the mechanisms for maintaining integrity of the photochemical apparatus had changed during the storage interval.