Can the progressive increase of C4 bundle sheath leakiness at low PFD be explained by incomplete suppression of photorespiration?

The ability to concentrate CO₂ around Rubisco allows C₄ crops to suppress photorespiration. However, as phosphoenolpyruvate regeneration requires ATP, the energetic efficiency of the C₄ pathway at low photosynthetic flux densities (PFD) becomes a balancing act between primary fixation and concentration of CO₂ in mesophyll (M) cells, and CO₂ reduction in bundle sheath (BS) cells. At low PFD, retro‐diffusion of CO₂ from BS cells, relative to the rate of bicarbonate fixation in M cells (termed leakiness φ), is known to increase. This paper investigates whether this increase in ? could be explained by incomplete inhibition of photorespiration. The PFD response of φ was measured at various O₂ partial pressures in young Zea mays plants grown at 250 (LL) and 750 µmol m^?2 s^?1 PFD (HL). φ increased at low PFD and was positively correlated with O₂ partial pressure. Low PFD during growth caused BS conductance and interveinal distance to be lower in the LL plants, compared to the HL plants, which correlated with lower φ. Model analysis showed that incomplete inhibition of photorespiration, especially in the HL plants, and an increase in the relative contribution of mitochondrial respiration at low PFD could explain the observed increases in φ.     

Effect of CO2 concentration on the PIC/POC ratio in the coccolithophore Emiliania huxleyi grown under light-limiting conditions and different daylengths

We compared the effect of CO₂ concentration ([CO₂], ranging from ∼5 to ∼34 μmol l⁻¹) at four different photon flux densities (PFD=15, 30, 80 and 150 μmol m⁻² s⁻¹) and two light/dark (L/D) cycles (16/8 and 24/0 h) on the coccolithophore Emiliania huxleyi. With increasing [CO₂], a decrease in the particulate inorganic carbon to particulate organic carbon (PIC/POC) ratio was observed at all light intensities and L/D cycles tested. The individual response in cellular PIC and POC to [CO₂] depended strongly on the PFD. POC production increased with rising [CO₂], irrespective of the light intensity, and PIC production decreased with increasing [CO₂] at a PFD of 150 μmol m⁻² s⁻¹, whereas below this light level it was unaffected by [CO₂]. Cell growth rate decreased with decreasing PFD, but was largely independent of ambient [CO₂]. The diurnal variation in PIC and POC content, monitored over a 38-h period (16/8 h L/D, PFD=150 μmol m⁻² s⁻¹), exceeded the difference in carbon content between cells grown at high (∼29 μmol l⁻¹) and low (∼4 μmol l⁻¹) [CO₂]. However, consistent with the results described above, cellular POC content was higher and PIC content lower at high [CO₂], compared to the values at low [CO₂], and the offset was observed throughout the day. It is suggested that the observed sensitivity of POC production for ambient [CO₂] may be of importance in regulating species-specific primary production and species composition.     

The input and fate of new C in two forest soils under elevated CO2

The aim of this study was to estimate (i) the influence of different soil types on the net input of new C into soils under CO₂ enrichment and (ii) the stability and fate of these new C inputs in soils. We exposed young beech–spruce model ecosystems on an acidic loam and calcareous sand for 4 years to elevated CO₂. The added CO₂ was depleted in ¹³C, allowing to trace new C inputs in the plant–soil system. We measured CO₂‐derived new C in soil C pools fractionated into particle sizes and monitored respiration as well as leaching of this new C during incubation for 1 year. Soil type played a crucial role in the partitioning of C. The net input of new C into soils under elevated CO₂ was about 75% greater in the acidic loam than in the calcareous sand, despite a 100% and a 45% greater above‐ and below‐ground biomass on the calcareous sand. This was most likely caused by a higher turnover of C in the calcareous sand as indicated by 30% higher losses of new C from the calcareous sand than from the acidic loam during incubation. Therefore, soil properties determining stabilization of soil C were apparently more important for the accumulation of C in soils than tree productivity. Soil fractionation revealed that about 60% of the CO₂‐derived new soil C was incorporated into sand fractions. Low natural ¹³C abundance and wide C/N ratios show that sand fractions comprise little decomposed organic matter. Consistently, incubation indicated that new soil C was preferentially respired as CO₂. During the first month, evolved CO₂ consisted to 40–55% of new C, whereas the fraction of new C in bulk soil C was 15–23% only. Leaching of DOC accounted for 8–23% of the total losses of new soil C. The overall effects of CO₂ enrichment on soil C were small in both soils, although tree growth increased significantly on the calcareous sand. Our results suggest that the potential of soils for C sequestration is limited, because only a small fraction of new C inputs into soils will become long‐term soil C.     

The influence of plants grown under elevated CO2 and N fertilization on soil nitrogen dynamics

We investigated the effects of spring barley growth on nitrogen (N) transformations and rhizosphere microbial processes in a controlled system under elevated carbon dioxide (CO₂) at two levels of N fertilization (applied with ¹⁵N labelling). After 25 d, elevated CO₂ (twice ambient) increased plant growth (dry weight, DW) by 141% at low‐N fertilization and by 60% at high‐N fertilization, but its positive effect on the root‐to‐shoot ratio was only significant at low‐N input. As a result of this plant response, elevated CO₂ caused a greater soil CO₂ efflux, rhizosphere soil DW, and soil microbial biomass under N‐limiting conditions than under high N availability. Elevated CO₂ also caused a significant (P < 0.001) increase in the N recovered by the plant from both the labelled (N_f) and unlabelled (N_s + N_uf) N pools. The dynamics of N in the system as affected by elevated CO₂ were driven principally by mineralization–immobilization turnover, with little loss by denitrification. Under N‐limiting conditions, there is evidence to suggest enhanced nutrient release from soil organic matter (SOM) pools—a process which could be defined as priming. The results of our experiment did not indicate a direct plant‐mediated effect of elevated CO₂ on nitrous oxide (N₂O) fluxes or denitrification activity.     

Plant, soil microbial and soil inorganic nitrogen responses to elevated CO2: a study in microcosms of Holcus lanatus

The impact of elevated atmospheric CO₂ concentrations on the nitrogen cycle was evaluated in a 2-month experiment in monospecific grassland microcosms (Holcus lanatus L.) grown on reconstituted grassland soil. The responses of the N pools in the plants, soil, and soil microbes were studied. The impact of high CO₂ on key stages of the N cycle, especially nitrification and denitrification processes, were also measured. Our study showed a strong plant response to high CO₂: total biomass increased by 76% (P < 0.001) and root length density increased by 77% (P = 0.010). However, total plant N was not significantly modified by high CO₂, because the percent N in the plant decreased by 40% (P < 0.001). We observed a large decrease in soil NO₃^– concentration under elevated CO₂ (–50%; P = 0.002). Soil ammonium concentrations were much less affected by CO₂ enrichment, and only in resin bags (–8%, P = 0.019). Soil nitrifying enzyme activity (NEA) had a tendency to increase (+17%; P = 0.061) and denitrifying enzyme activity (DEA) decreased (-12%; P = 0.013). We found evidence of increased microbial N sink (microbial N increased by 17%, P = 0.004). This and other studies suggest that rising CO₂ often reduces soil nitrate concentrations, which may lead to decreased nitrate leaching. Elevated CO₂ led to environmental conditions that were less favourable for denitrification in our study.     

Net soil carbon input under ambient and elevated CO2 concentrations: isotopic evidence after 4 years

Elevation of atmospheric CO₂ concentration is predicted to increase net primary production, which could lead to additional C sequestration in terrestrial ecosystems. Soil C input was determined under ambient and Free Atmospheric Carbon dioxide Enrichment (FACE) conditions for Lolium perenne L. and Trifolium repens L. grown for four years in a sandy‐loam soil. The ¹³C content of the soil organic matter C had been increased by 5‰ compared to the native soil by prior cropping to corn (Zea mays) for > 20 years. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above‐ground biomass produced by L. perenne during the 4‐year period was strongly dependent on the amount of N fertilizer applied but did not respond to increased CO₂. In contrast, the total accumulated above‐ground biomass of T. repens doubled under elevated CO₂ but remained independent of N fertilizer rate. The C:N ratio of above‐ground biomass for both species increased under elevated CO₂ whereas only the C:N ratio of L. perenne roots increased under elevated CO₂. Root biomass of L. perenne doubled under elevated CO₂ and again under high N fertilization. Total soil C was unaffected by CO₂ treatment but dependent on species. After 4 years and for both crops, the fraction of new C (F‐value) under ambient conditions was higher (P= 0.076) than under FACE conditions: 0.43 vs. 0.38. Soil under L. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO₂ had no effect on total soil organic matter content for both systems. The net amount of C sequestered in 4 years was unaffected by the CO₂ concentration (overall average of 8.5 g C kg^?1 soil). There was a significant species effect and more new C was sequestered under highly fertilized L. perenne. The amount of new C sequestered in the soil was primarily dependent on plant species and the response of root biomass to CO₂ and N fertilization. Therefore, in this FACE study net soil C sequestration was largely depended on how the species responded to N rather than to elevated CO₂.     

Elevated CO2 and limited nitrogen nutrition can restrict excitation energy dissipation in photosystem II of Japanese white birch (Betula platyphylla var. japonica) leaves

Elevated atmospheric CO₂ concentration [CO₂] and different levels of nitrogen (N) nutrition can influence the amount of excess excitation energy in photosystem (PS) II and related photosynthetic properties. The interactive effect of two [CO₂] levels (ambient: 360 µM M⁻¹ and elevated: 720 µM M⁻¹) and two N levels (high: 700 mg N plant⁻¹ and low: 100 mg N plant⁻¹) on these properties was examined in seedlings of Japanese white birch (Betula platyphylla var. japonica) using simultaneous measurements of gas exchange and chlorophyll fluorescence. Photosynthetic acclimation to elevated [CO₂], as indicated by a decline in carboxylation efficiency (CE), was observed in plants grown at elevated [CO₂] especially under low N. Elevated [CO₂] resulted in a decrease in area-based leaf N content (N_area) irrespective of N treatment. The adverse effect of elevated [CO₂] and low N on CE may have been exacerbated by a greater accumulation of leaf sugar and starch contents in these plants leading to a lower electron transport rate (ETR). While these plants also showed higher non-photochemical quenching (Nq_P) that could offset the reduction in energy dissipation through ETR to some extent, they still have a higher risk of photoinhibition from excessive excitation energy in PSII as indicated by a decrease in photochemical quenching (q_P). However, chronic photoinhibition was not observed in plant grown at elevated [CO₂] and low N because they showed no difference in F_v/F_m (the maximum photochemical efficiency of PSII) from those grown at ambient [CO₂] and low N after an overnight dark adaptation. High levels of Nq_P in plants grown at elevated [CO₂] and low N reflect a near saturation of thermal energy dissipation. This impaired capacity of photoprotection would render these plants more vulnerable to photoinhibition in the event of additional environmental stresses such as drought, low or high temperature.     

Short-term decomposition of litter produced by plants grown in ambient and elevated atmospheric CO2 concentrations

The effects of elevated atmospheric CO₂ (ambient + 340 μmol mol^–1) on above-ground litter decomposition were investigated over a 6-week period using a field-based mesocosm system. Soil respiratory activity in mesocosms incubated in ambient and elevated atmospheric CO₂ concentrations were not significantly different (t-test, P > 0.05) indicating that there were no direct effects of elevated atmospheric CO₂ on litter decomposition. A study of the indirect effects of CO₂ on soil respiration showed that soil mesocosms to which naturally senescent plant litter had been added (0.5% w/w) from the C₃ sedge Scirpus olneyi grown in elevated atmospheric CO₂ was reduced by an average of 17% throughout the study when compared to soil mesocosms to which litter from Scirpus olneyi grown in ambient conditions had been added. In contrast, similar experiments using senescent material from the C₄ grass Spartina patens showed no difference in soil respiration rates between mesocosms to which litter from plants grown in elevated or ambient CO₂ conditions had been added. Analysis of the C:N ratio and lignin content of the senescent material showed that, while the C:N ratio and lignin content of the Spartina patens litter did not vary with atmospheric CO₂ conditions, the C:N ratio (but not the lignin content) of the litter from Scirpus olneyi was significantly greater (t-test;P < 0.05) when derived from plants grown under elevated CO₂ (105:1 compared to 86:1 for litter derived from Scirpus olneyi grown under ambient conditions). The results suggest that the increased C:N ratio of the litter from the C₃ plant Scirpus olneyi grown under elevated CO₂ led to the lower rates of biodegradation observed as reduced soil respiration in the mesocosms. Further long-term experiments are now required to determine the effects of elevated CO₂ on C partitioning in terrestrial ecosystems.     

Carbon partitioning to mobile and structural fractions in poplar wood under elevated CO2 (EUROFACE) and N fertilization

To determine whether globally increasing atmospheric carbon dioxide (CO₂) concentrations can affect carbon partitioning between nonstructural and structural carbon pools in agroforestry plantations, Populus nigra was grown in ambient air (about 370 μmol mol^?1 CO₂) and in air with elevated CO₂ concentrations (about 550 μmol mol^?1 CO₂) using free‐air CO₂ enrichment (FACE) technology. FACE was maintained for 5 years. After three growing seasons, the plantation was coppiced and one half of each experimental plot was fertilized with nitrogen. Carbon concentrations and stocks were measured in secondary sprouts in seasons of active growth and dormancy during 2 years after coppicing. Although FACE, N fertilization and season had significant tissue‐specific effects on carbon partitioning to the fractions of structural carbon, soluble sugars and starch as well as to residual soluble carbon, the overall magnitude of these shifts was small. The major effect of FACE and N fertilization was on cell wall biomass production, resulting in about 30% increased above ground stocks of both mobile and immobile carbon pools compared with fertilized trees under ambient CO₂. Relative C partitioning between mobile and immobile C pools was not significantly affected by FACE or N fertilization. These data demonstrate high metabolic flexibility of P. nigra to maintain C‐homeostasis under changing environmental conditions and illustrate that nonstructural carbon compounds can be utilized more rapidly for structural growth under elevated atmospheric [CO₂] in fertilized agroforestry systems. Thus, structural biomass production on abandoned agricultural land may contribute to achieving the goals of the Kyoto protocol.     

Effects of elevated atmospheric CO2 in agro-ecosystems on soil carbon storage

Increasing global atmospheric CO₂ concentration has led to concerns regarding its potential effects on the terrestrial environment. Attempts to balance the atmospheric carbon (C) budget have met with a large shortfall in C accounting (≈1.4 × 10¹⁵ g C y^–1) and this has led to the hypothesis that C is being stored in the soil of terrestrial ecosystems. This study examined the effects of CO₂ enrichment on soil C storage in C3 soybean (Glycine max L.) Merr. and C4 grain sorghum (Sorghum bicolor L.) Moench. agro-ecosystems established on a Blanton loamy sand (loamy siliceous, thermic, Grossarenic Paleudults). The study was a split-plot design replicated three times with two crop species (soybean and grain sorghum) as the main plots and two CO₂ concentration (ambient and twice ambient) as subplots using open top field chambers. Carbon isotopic techniques using δ¹³C were used to track the input of new C into the soil system. At the end of two years, shifts in δ¹³C content of soil organic matter carbon were observed to a depth of 30 cm. Calculated new C in soil organic matter with grain sorghum was greater for elevated CO₂ vs. ambient CO₂ (162 and 29 g m^–2, respectively), but with soybean the new C in soil organic matter was less for elevated CO₂ vs. ambient CO₂ (120 and 291 g m^–2, respectively). A significant increase in mineral associated organic C was observed in 1993 which may result in increased soil C storage over the long-term, however, little change in total soil organic C was observed under either plant species. These data indicate that elevated atmospheric CO₂ resulted in changes in soil C dynamics in agro-ecosystems that are crop species dependent.     

Relative enhancement of photosynthesis and growth at elevated CO2 is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling

The survivorship of dipterocarp seedlings in the deeply shaded understorey of South‐east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO₂ upon photosynthesis and growth under sunflecks, seedlings of Shorealeprosula were grown in controlled environment conditions at ambient or elevated CO₂. Equal total daily photon flux density (PFD) (～7·7 mol m^?2 d^?1) was supplied as either uniform irradiance (～170 µmol m^?2 s^?1) or shade/fleck sequences (～30 µmol m^?2 s^?1/～525 µmol m^?2 s^?1). Photosynthesis and growth were enhanced by elevated CO₂ treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO₂ but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO₂ were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post‐irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO₂ under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.     

Changes in soil carbon and nitrogen properties and microbial communities in relation to growth of Pinus radiata and Nothofagus fusca trees after 6 years at ambient and elevated atmospheric CO2

Increasing atmospheric CO₂ concentration can influence the growth and chemical composition of many plant species, and thereby affect soil organic matter pools and nutrient fluxes. Here, we examine the effects of ambient (initially 362 μL L^?1) and elevated (654 μL L^?1) CO₂ in open‐top chambers on the growth after 6 years of two temperate evergreen forest species: an exotic, Pinus radiata D. Don, and a native, Nothofagus fusca (Hook. F.) Oerst. (red beech). We also examine associated effects on selected carbon (C) and nitrogen (N) properties in litter and mineral soil, and on microbial properties in rhizosphere and hyphosphere soil. The soil was a weakly developed sand that had a low initial C concentration of about 1.0 g kg^?1 at both 0–100 and 100–300 mm depths; in the N. fusca system, it was initially overlaid with about 50 mm of forest floor litter (predominantly FH material) taken from a Nothofagus forest. A slow‐release fertilizer was added during the early stages of plant growth; subsequent foliage analyses indicated that N was not limiting. After 6 years, stem diameters, foliage N concentrations and C/N ratios of both species were indistinguishable (P>0.10) in the two CO₂ treatments. Although total C contents in mineral soil at 0–100 mm depth had increased significantly (P<0.001) after 6 years growth of P. radiata, averaging 80±0.20 g m^?2 yr^?1, they were not significantly influenced by elevated CO₂. However, CO₂‐C production in litter, and CO₂‐C production, microbial C, and microbial C/N ratios in mineral soil (0–100 mm depth) under P. radiata were significantly higher under elevated than ambient CO₂. CO₂‐C production, microbial C, and numbers of bacteria (but not fungi) were also significantly higher under elevated CO₂ in hyphosphere soil, but not in rhizosphere soil. Under N. fusca, some incorporation of the overlaid litter into the mineral soil had probably occurred; except for CO₂‐C production and microbial C in hyphosphere soil, none of the biochemical properties or microbial counts increased significantly under elevated CO₂. Net mineral‐N production, and generally the potential utilization of different substrates by microbial communities, were not significantly influenced by elevated CO₂ under either tree species. Physiological profiles of the microbial communities did, however, differ significantly between rhizosphere and hyphosphere samples and between samples under P. radiata and N. fusca. Overall, results support the concept that a major effect on soil properties after prolonged exposure of trees to elevated CO₂ is an increase in the amounts, and mineralization rate, of labile organic components.     

Widespread foliage δ15N depletion under elevated CO2: inferences for the nitrogen cycle

Leaf ¹⁵N signature is a powerful tool that can provide an integrated assessment of the nitrogen (N) cycle and whether it is influenced by rising atmospheric CO₂ concentration. We tested the hypothesis that elevated CO₂ significantly changes foliage δ¹⁵N in a wide range of plant species and ecosystem types. This objective was achieved by determining the δ¹⁵N of foliage of 27 field‐grown plant species from six free‐air CO₂ enrichment (FACE) experiments representing desert, temperate forest, Mediterranean‐type, grassland prairie, and agricultural ecosystems. We found that within species, the δ¹⁵N of foliage produced under elevated CO₂ was significantly lower (P<0.038) compared with that of foliage grown under ambient conditions. Further analysis of foliage δ¹⁵N by life form and growth habit revealed that the CO₂ effect was consistent across all functional groups tested. The examination of two chaparral shrubs grown for 6 years under a wide range of CO₂ concentrations (25–75 Pa) also showed a significant and negative correlation between growth CO₂ and leaf δ¹⁵N. In a select number of species, we measured bulk soil δ¹⁵N at a depth of 10 cm, and found that the observed depletion of foliage δ¹⁵N in response to elevated CO₂ was unrelated to changes in the soil δ¹⁵N. While the data suggest a strong influence of elevated CO₂ on the N cycle in diverse ecosystems, the exact site(s) at which elevated CO₂ alters fractionating processes of the N cycle remains unclear. We cannot rule out the fact that the pattern of foliage δ¹⁵N responses to elevated CO₂ reported here resulted from a general drop in δ¹⁵N of the source N, caused by soil‐driven processes. There is a stronger possibility, however, that the general depletion of foliage δ¹⁵N under high CO₂ may have resulted from changes in the fractionating processes within the plant/mycorrhizal system.     

Effect of CO2, CO2 and Diamox on photosynthesis and photorespiration in Chlamydomonas reinhardtii (green alga) and Anacystis nidulans (Cyanobacterium, blue-green alga)

Photorespiration by Chlamydomonas reinhardtii and Anacystis nidulans was measured as the oxygen inhibition of CO₂ uptake and the CO₂ compensation points. Net photosynthesis was oxygen dependent in Chlamydomonas grown in 5% CO₂, but CO₂ insensitive in cultures bubbled with air. Anacystis, even when cultured in 5% CO₂, exhibited an CO₂ insensitive net photosynthesis. The CO₂ compensation point of Chlamydomonas grown in cultures bubbled with air and Anacystis grown in 5% CO₂ enriched air, were reached shortly after the measurement was begun and the values were very low, less than 10 μl CO₂ 1^?1; while Chlamydomonas grown in 5% CO₂ enriched air for 4 days showed a high, but temporary CO₂ compensation point (60 μl CO₂ 1^?1). After a two hour adaptation in low CO₂, a stable, low CO₂ compensation point was reached. It seems that photorespiration can only be detected by the methods used in this study when the algae are cultured in high CO₂, but a mechanism exists which blocks photorespiration when the green algae are adapted to low CO₂ concentrations. When Chlamydomonas was treated with Diamox, an inhibitor of carbonic anhydrase, after cultivation in low CO₂ (air), the cells behaved as if they had been grown in high CO₂. They showed an oxygen sensitive net photosynthesis and a high CO₂ compensation point. This indicates that carbonic anhydrase plays an important role in the regulation of a measurable photorespiration in Chlamydomonas. The results are discussed in relation to previous observations of photorespiration measured by enzyme assay, metabolic products and gas exchange properties.     

Seed germination and seedling physiology of Larix kaempferi and Pinus densiflora in seedbeds with charcoal and elevated CO2

We investigated the effect of ectomycorrhizal colonization, charcoal and CO₂ levels on the germination of seeds of Larix kaempferi and Pinus densiflora, and also their subsequent physiological activity and growth. The seeds were sown in brown forest soil or brown forest soil mixed with charcoal, at ambient CO₂ (360 μmol mol⁻¹) or elevated CO₂ (720 μmol mol⁻¹), with or without ectomycorrhiza. The proportions of both conifer seeds that germinated in forest soil mixed with charcoal were significantly greater than for seeds sown in forest soil grown at each CO₂ level (P < 0.05; t-test). However, the ectomycorrhizal colonization rate of each species grown in brown forest soil mixed with charcoal was significantly lower than in forest soil at each CO₂ treatment [CO₂] (P < 0.01; t-test). The phosphorus concentrations in needles of each seedling colonized with ectomycorrhiza and grown in forest soil were greater than in nonectomycorrhizal seedlings at each CO₂ level, especially for L. kaempferi seedlings (P < 0.05; t-test), but the concentrations in seedlings grown in brown forest soil mixed with charcoal were not increased at any CO₂ level. Moreover, the maximum net photosynthetic rate of each seedling for light and CO₂ saturation (P _max) increased when the seedlings were grown with ectomycorrhiza at 720 μmol mol⁻¹ [CO₂]. Ectomycorrhizal colonization led to an increase in the stem diameter of each species grown in each soil treatment at each CO₂ level. However, charcoal slowed the initial growth of both species of seedling, constraining ectomycorrhizal development. These results indicate that charcoal strongly assists seed germination and physiological activity.     

Changes in the condition of photosynthetic apparatus of a diatom alga <Emphasis Type="Italic">Thallassiosira weisflogii</Emphasis> during photoadaptation and photodamage

Two populations of a diatom alga Thallassiosira weisflogii were grown at photon flux densities (PFD) of 0.8 and 8 μmol/(m² s). For both diatom populations, the recovery of chlorophyll fluorescence parameters (F ₀, F _m, F _v/F _m, and NPQ) was monitored after nondestructive irradiation by visible light at PFD of 40 μmol/(m² s) and after high-intensity irradiation by visible light (1000–4000 μmol/(m² s)). The exposure of diatoms to PFD of 40 μmol/(m² s)—higher than PFD used for algal growth but still nondamaging to photosynthetic apparatus—induced nonphotochemical quenching (NPQ), which was stronger in algae grown at higher PFD (8 μmol/(m² s)) than in algae grown at low light. After irradiation with high-intensity light, the recovery of chlorophyll fluorescence parameters was more pronounced in algae grown at elevated PFD level. During short-term irradiation of diatoms with high-intensity visible light (1000 μmol/(m² s)), a stronger NPQ was observed in the culture adapted to high irradiance. After the treatment of algae with dithiothreitol (an inhibitor of carotenoid deepoxidase in the diadinoxanthin cycle) or NH₄Cl (an agent abolishing the proton gradient at thylakoid membranes), a short exposure of algae to PFD of 40 μmol/(m² s) induced hardly any nonphotochemical quenching. The results indicate the dominant contribution of xanthophyll cycle carotenoids to energy-dependent quenching.     

Photosynthetic Responses for Vitis vinifera Plants Grown at Different Photon Flux Densities Under Field Conditions

In grapevine (Vitis vinifera L.) leaf chlorophyll (Chl) a and Chl b and carotenoid contents were higher in plants grown at low photon flux densities (PFD) than in those grown at medium and high PFD. The highest Chl a variable to maximum fluorescence ratio F_v/F_m was observed in plants grown at medium PFD while the minimum fluorescence F₀ was highest in those at high PFD. In isolated thylakoids, both high and low PFD caused marked inhibition of whole chain and photosystem 2 (PS2) activities. The artificial exogenous electron donor diphenyl carbazide significantly restored the loss of PS2 activity in low PFD leaves.     

Effects of growth-light quantity,growth-light quality and CO2 concentration on Rubisco deactivation during low PFD or darkness

The effects of CO₂ concentration and the effects of growth-light conditions on Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) deactivation were examined for Spinacea oleracea (spinach). Rubisco deactivation kinetics and the degree that Rubisco activation limited the rise in photosynthesis following an increase in photon flux density (PFD) were determined from gas-exchange time courses. There were no significant differences in the apparent relaxation time for Rubisco deactivation among leaves exposed to high or low CO₂ (50 or 1000 mol mol^-1) and low PFD (170 mol m^-2 s^-1) or darkness. However, when PFD was increased to 1700 mol m^-2 s^-1 following a period of low PFD or darkness, leaves exposed to low CO₂ × low PFD showed a lower contribution to the photosynthetic induction process by the activation of Rubisco than leaves exposed to the other treatments. For the growth-light experiments, spinach was grown under high PFD × high red:far-red ratio (R:FR), low PFD × high R:FR, or low PFD × low R:FR light environments. Leaves that matured under the low PFD × low R:FR treatment showed a lower percent change in photosynthesis due to Rubisco activation than leaves exposed to the other growth-light treatments. However, there were no significant differences among the growth-light treatments in the maximum contribution of Rubisco activation to the induction response or in the apparent relaxation time for Rubisco deactivation during shade events.     

土壤氮水交互对马尾松和杉木COS和CO2通量的影响

羰基硫(COS)和CO_2化学结构相似,且植物对COS和CO_2具有共吸收特性,因此可利用COS作为示踪物来估算生态系统总初级生产力,而不同植物吸收COS和CO_2对环境因子变化的响应差异较大。以南亚热带典型树种马尾松(Pinus massoniana)和杉木(Cunninghamia lanceolata)为研究对象,设置2个氮水平及3个土壤水分梯度处理。采取顶空套袋法采集气体样品,用预浓缩—气质联用仪分析样品COS浓度,同时测量植物光合参数。结果表明:马尾松和杉木吸收COS,吸收速率均值分别为39.58—127.27 pmol m~(-2) s~(-1)和0.81—66.92 pmol m~(-2) s~(-1)。整体而言,施氮可促进植物吸收COS,但除施氮对马尾松COS通量有显著影响外(P0.05),施氮、土壤水分和两者交互作用对马尾松和杉木的COS和CO_2通量及其比值均无显著性影响。施氮情况下,高土壤水分处理促进马尾松COS吸收而低土壤水分处理促进杉木COS吸收。中等土壤水分和高土壤水分条件下马尾松和杉木COS通量与气孔导度呈正相关关系。线性拟合结果表明,植物COS通量(F_(COS))与CO_2通量(F_(CO_2))呈极显著正相关(P0.01),马尾松和杉木F_(COS)/F_(CO_2)值分别为1.48×10~(-6)和1.01×10~(-6)。中等土壤水分条件均可提高马尾松F_(COS)/F_(CO_2)比值,而低土壤水分条件下施氮增加杉木F_(COS)/F_(CO_2)比值,高土壤水分条件下施氮降低杉木F_(COS)/F_(CO_2)比值。低土壤水分和高土壤水分使马尾松蒸汽压亏缺增大,促使气孔导度减小从而降低净光合速率。低土壤水分和高土壤水分下施氮导致杉木气孔导度增加从而增强净光合速率。研究结果不仅对进一步了解区域氮沉降和降水对树木COS通量及F_(COS)/F_(CO_2)的影响有重要意义,而且可为模型估算总初级生产力提供区域性数据支持。