Photosynthetic Stimulation Research Articles

Stimulated photosynthesis of regrowth after fire in coastal scrub vegetation: increased water or nutrient availability?

Fire-prone landscapes experience frequent fires, disrupting above-ground biomass and altering below-ground soil nutrient availability. Augmentation of leaf nutrients or leaf water balance can both reduce limitations to photosynthesis and facilitate post-fire recovery in plants. These modes of fire responses are often studied separately and hence are rarely compared. We hypothesized that under severe burning, woody plants of a coastal scrub ecosystem would have higher rates of photosynthesis (Anet) than in unburned areas due to a transient release from leaf nutrient and water limitations, facilitating biomass recovery post-burn. To compare these fire recovery mechanisms in regrowing plants, we measured leaf gas exchange, leaf and soil N and P concentrations, and plant stomatal limitations in Australian native coastal scrub species across a burn sequence of sites at 1 year after severe fire, 7 years following a light controlled fire, and decades after any fire at North Head, Sydney, Australia. Recent burning stimulated increases in Anet by 20% over unburned trees and across three tree species. These species showed increases in total leaf N and P as a result of burning of 28% and 50% for these macronutrients, respectively, across the three species. The boost in leaf nutrients and stimulated leaf biochemical capacity for photosynthesis, alongside species-specific stomatal conductance (gs) increases, together contributed to increased photosynthetic rates after burning compared with the long-unburned area. Photosynthetic stimulation after burning occurred due to increases in nutrient concentrations in leaves, particularly N, as well as stomatal opening for some species. The findings suggest that changes in species photosynthesis and growth with increased future fire intensity or frequency may be facilitated by changes in leaf physiology after burning. On this basis, species dominance during regrowth depends on nutrient and water availability during post-fire recovery.

Estimating the CO2 Fertilization Effect on Extratropical Forest Productivity From Flux‐Tower Observations

AbstractThe land sink of anthropogenic carbon emissions, a crucial component of mitigating climate change, is primarily attributed to the CO2 fertilization effect on global gross primary productivity (GPP). However, direct observational evidence of this effect remains scarce, hampered by challenges in disentangling the CO2 fertilization effect from other long‐term confounding drivers, particularly climatic changes. Here, we introduce a novel statistical approach to separate the CO2 fertilization effect on photosynthetic carbon uptake using eddy covariance (EC) records across 38 extratropical forest sites. We find the median stimulation rate of GPP to be 3.2 ± 0.9 gC m−2 yr−1 ppm−1 (or 16.4 ± 4.2% per 100 ppm) under increasing atmospheric CO2 across these sites, respectively. To validate the robustness of our findings, we test our statistical method using factorial simulations of an ensemble of process‐based land surface models. We address additional factors, including nitrogen deposition and land management, that may impact plant productivity, potentially confounding the attribution to the CO2 fertilization effect. Assuming these site‐specific effects offset to some extent across sites as random factors, the estimated median value still reflects the strength of the CO2 fertilization effect. However, disentanglement of these long‐term effects, often inseparable by timescale, requires further causal research. Our study provides direct evidence that the photosynthetic stimulation is maintained under long‐term CO2 fertilization across multiple EC sites. Such observation‐based quantification is key to constraining the long‐standing uncertainties in the land carbon cycle under rising CO2 concentrations.

Links across ecological scales: Plant biomass responses to elevated CO2.

The degree to which elevated CO2 concentrations (e[CO2 ]) increase the amount of carbon (C) assimilated by vegetation plays a key role in climate change. However, due to the short-term nature of CO2 enrichment experiments and the lack of reconciliation between different ecological scales, the effect of e[CO2 ] on plant biomass stocks remains a major uncertainty in future climate projections. Here, we review the effect of e[CO2 ] on plant biomass across multiple levels of ecological organization, scaling from physiological responses to changes in population-, community-, ecosystem-, and global-scale dynamics. We find that evidence for a sustained biomass response to e[CO2 ] varies across ecological scales, leading to diverging conclusions about the responses of individuals, populations, communities, and ecosystems. While the distinct focus of every scale reveals new mechanisms driving biomass accumulation under e[CO2 ], none of them provides a full picture of all relevant processes. For example, while physiological evidence suggests a possible long-term basis for increased biomass accumulation under e[CO2 ] through sustained photosynthetic stimulation, population-scale evidence indicates that a possible e[CO2 ]-induced increase in mortality rates might potentially outweigh the effect of increases in plant growth rates on biomass levels. Evidence at the global scale may indicate that e[CO2 ] has contributed to increased biomass cover over recent decades, but due to the difficulty to disentangle the effect of e[CO2 ] from a variety of climatic and land-use-related drivers of plant biomass stocks, it remains unclear whether nutrient limitations or other ecological mechanisms operating at finer scales will dampen the e[CO2 ] effect over time. By exploring these discrepancies, we identify key research gaps in our understanding of the effect of e[CO2 ] on plant biomass and highlight the need to integrate knowledge across scales of ecological organization so that large-scale modeling can represent the finer-scale mechanisms needed to constrain our understanding of future terrestrial C storage.

Carbon sink strength of nodules but not other organs modulates photosynthesis of faba bean (Vicia faba) grown under elevated [CO2 ] and different water supply.

Photosynthetic stimulation by elevated [CO2 ] (e[CO2 ]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N2 -fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO2 ]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO2 ] and drought on carbon sink strength of nodules and other organs. To compare carbon sink strength, faba bean was grown under ambient (400ppm) or elevated (700ppm) atmospheric [CO2 ] and subjected to well-watered or drought treatments, and then exposed to 13 C pulse-labelling using custom-built chambers to track the fate of new photosynthates. Drought decreased 13 C uptake and nodule sink strength, and this effect was even greater under e[CO2 ], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N2 fixation, and increased accumulation of new photosynthates (13 C/sugars) in leaves, which in turn can feed back on photosynthesis. Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO2 ], and legumes may only be able to achieve greater C gain if nodule activity is maintained.

Temporal and stoichiometric patterns of algal stimulation of litter‐associated heterotrophic microbial activity

Abstract Periphyton communities associated with submerged plant detritus contain interacting autotrophic and heterotrophic microbes, and are sites of extracellular enzymatic activity. The strength and nature of these interactions might be expected to change over time as microbial communities develop on plant litter. Microbial interactions and enzymatic activity can be altered by nutrient availability, suggesting that litter stoichiometry could also affect these phenomena. We grew wetland plants under ambient and nutrient‐enriched conditions to generate plant litter of differing nutrient content. In two experiments, we investigated: (1) the influence of algal photosynthesis on fungal and bacterial production and the activities of four extracellular enzymes throughout a 54‐day period of microbial colonisation and growth; and (2) the influence of litter stoichiometry on these relationships. Ambient and nutrient‐enriched standing‐dead plant litter was collected and then submerged in wetland pools to allow for natural microbial colonisation and growth. Litter samples were periodically retrieved and transported to the laboratory for experiments manipulating photosynthesis using the photosystem II inhibitor DCMU (which effectively prevents algal photosynthetic activity). Algal (14C‐bicarbonate), bacterial (3H‐leucine), and fungal (14C‐acetate) production, and β‐glucosidase, β‐xylosidase, leucine aminopeptidase, and phosphatase activities (MUF‐ or AMC‐labelled fluorogenic substrates) were measured under conditions of active and inhibited algal photosynthesis. Photosynthesis stimulated overall fungal and bacterial production in both experiments, although the strength of stimulation varied amongst sampling dates. Phosphatase activity was stimulated by photosynthesis during the first, but not the second, experiment. No other enzymatic responses to short‐term photosynthesis manipulations were observed. Microbial communities on high‐nutrient litter occasionally showed increased extracellular enzyme activity, fungal growth rates, and bacterial production compared to communities on non‐enriched litter, but algal and fungal production were not affected. Litter stoichiometry had no effects on fungal, bacterial, or enzymatic responses to photosynthesis, but the mean enzyme vector analysis angle (a measure of P‐ versus N‐acquiring enzyme activity) was positively correlated to litter N:P, suggesting that elevated litter N:P led to an increase in the relative activity of P‐acquiring enzymes. These results supported the hypothesis that algal photosynthesis strongly influences heterotrophic microbial activity on macrophyte leaf litter, especially that of fungi, throughout microbial community development. However, the strength of this photosynthetic stimulation does not generally depend on small differences in litter nutrient content. Stimulation of microbial heterotrophs by algal photosynthesis could drive diurnal shifts in periphyton community and aquatic ecosystem function, as well as linking green (photoautotroph‐based) and brown (detrital‐based) food webs.

Photosynthetic stimulation of saplings by the interaction of CO2 and water stress

Climate change necessitates research into interactions between elevated carbon dioxide (CO2) concentrations and drought on plant photosynthetic physiology. This study describes the physiological properties of Platycladus orientalis (Chinese thuja) and Quercus variabilis (Chinese cork oak) saplings cultivated through orthogonal treatments of four CO2 concentrations combined with five soil volumetric water contents (SWC). It highlights the differences between the interactive effects from the treatments. Water stress had little effect on photosynthetic traits until the soil volumetric water contents exceeded 70–80 or 100%. Similar variations in carbon-13 isotope abundance (δ13C) of water soluble compounds (δ13CWSC) extracted from leaves of two species have been observed. Whether soil volumetric water contents exceeded or fell below the water threshold values (70–80% of field capacity for P. orientalis and 100% of field capacity for Q. variabilis), instantaneous water use efficiency decreased. Elevated carbon dioxide could increase iWUE and enhance drought tolerance, depending on stimulating net photosynthetic rates and declining stomatal conductance and transpiration rates. Augmenting either drought, excess water, or ambient carbon dioxide could alleviate the physiological inhibition caused by the stresses described above.

Photosynthetic enhancement by elevated CO₂ depends on seasonal temperatures for warmed and non-warmed Eucalyptus globulus trees.

Arguments based on the biochemistry of photosynthesis predict a positive interaction between elevated atmospheric [CO2] and temperature on photosynthesis as well as growth. In contrast, few long-term studies on trees find greater stimulation of photosynthesis in response to elevated [CO2] at warmer compared with cooler temperatures. To test for CO2 × temperature interactions on leaf photosynthesis and whole-plant growth, we planted Eucalyptus globulus Labill. in climate-controlled chambers in the field at the Hawkesbury Forest Experiment research site, and investigated how photosynthetic enhancement changed across a range of seasonal temperatures. Trees were grown in a complete two-way factorial design with two CO2 concentrations (ambient and ambient + 240 ppm) and two temperatures (ambient and ambient + 3 °C) for 15 months until they reached ∼10 m height, after which they were harvested for biomass. There was significant enhancement of photosynthesis and growth with elevated [CO2], with the photosynthetic stimulation varying with season, but there was no significant effect of warming. Photosynthetic enhancement was higher in summer (+46% at 28 °C) than in winter (+14% at 20 °C). Photosynthetic enhancement as a function of leaf temperature was consistent with theoretical expectations, but was strongly mediated by the intercellular [CO2]/ambient [CO2] (Ci/Ca) ratio across seasons. Total tree biomass after 15 months was 66% larger in elevated CO2 (P = 0.017) with no significant warming effect detected. The fraction of biomass in coarse roots was reduced in warmed trees compared with ambient temperature controls, but there was no evidence of changed biomass allocation patterns in elevated CO2. We conclude that there are strong and consistent elevated CO2 effects on photosynthesis and biomass of E. globulus. It is crucial to consider stomatal conductance under a range of conditions to appraise the interactive effect of [CO2] and temperature on photosynthetic enhancement and subsequent implications for tree growth and forest productivity in future climates.

Biochemical and molecular characteristics of leaf photosynthesis and relative seed yield of two contrasting rice cultivars in response to elevated [CO₂

Understanding the basis for intraspecific yield variability may be important in elucidating biological mechanisms that are associated with superior yield performance in response to projected increases in carbon dioxide concentration, [CO₂]. Using a free-air CO₂ enrichment (FACE) facility, two rice lines, S63 and W14, which differed consistently in their enhancement of seed yield when grown at elevated [CO₂] in multiple field trials, were examined. To determine if the different cultivar responses were linked to changes in photosynthetic characteristics at elevated [CO₂], spatial and temporal changes in photosynthetic stimulation and the occurrence of down-regulation, or acclimation, in relation to panicle sink development were quantified for the uppermost canopy leaves. Changes in photosynthetic capacity were determined by quantifying changes in the sink:source ratio, leaf nitrogen (N) content, the concentration and mRNA expression of the large Rubisco subunit, and changes in V c,max, the maximum ribulose bisphosphate (RuBP)-saturated rate of carboxylation. For the W14 cultivar, significant reductions in photosynthesis at the elevated, relative to ambient [CO₂], signalling photosynthetic acclimation, were observed following panicle initiation. The observance of photosynthetic acclimation was consistent with significant reductions in N, Rubisco content and expression, and V c,max. In contrast, for the cultivar S63, elevated [CO₂] resulted in increased spikelet number and grain weight, increased sink:source ratios, and continued stimulation of photosynthesis up to grain maturity. Overall, these data suggest that the greater response of the S63 line to elevated [CO₂] may be associated with enhanced carbon sinks relative to sources, and the ability to maintain photosynthetic capacity during grain development.

Biochemical acclimation, stomatal limitation and precipitation patterns underlie decreases in photosynthetic stimulation of soybean (Glycine max) at elevated [CO2] and temperatures under fully open air field conditions

The net effect of elevated [CO2] and temperature on photosynthetic acclimation and plant productivity is poorly resolved. We assessed the effects of canopy warming and fully open air [CO2] enrichment on (1) the acclimation of two biochemical parameters that frequently limit photosynthesis (A), the maximum carboxylation capacity of Rubisco (Vc,max) and the maximum potential linear electron flux through photosystem II (Jmax), (2) the associated responses of leaf structural and chemical properties related to A, as well as (3) the stomatal limitation (l) imposed on A, for soybean over two growing seasons in a conventionally managed agricultural field in Illinois, USA. Acclimation to elevated [CO2] was consistent over two growing seasons with respect to Vc,max and Jmax. However, elevated temperature significantly decreased Jmax contributing to lower photosynthetic stimulation by elevated CO2. Large seasonal differences in precipitation altered soil moisture availability modulating the complex effects of elevated temperature and CO2 on biochemical and structural properties related to A. Elevated temperature also reduced the benefit of elevated [CO2] by eliminating decreases in stomatal limitation at elevated [CO2]. These results highlight the critical importance of considering multiple environmental factors (i.e. temperature, moisture, [CO2]) when trying to predict plant productivity in the context of climate change.

Photosynthetic traits and growth of Quercus mongolica var. crispula sprouts attacked by powdery mildew under free-air CO2 enrichment

To predict the performance of coppice forests with Japanese oak (Quercus mongolica var. crispula) in future changing environment, we studied the growth, photosynthesis, and powdery mildew (Erysiphe alphitoides) infection of sprouts of Japanese oak under free-air CO2 enrichment. Elevated CO2 reduced powdery mildew infection in both leaves of the shoot emerged in spring (1st flush) and the lammas and proleptic shoots (2nd flush) of sprouts. We observed significant increase in the net photosynthetic rate at growth CO2 concentration (i.e., 370 and 500 μmol mol−1 for ambient and elevated CO2 treatments, respectively) in both 1st and 2nd flush leaves of sprouts grown under elevated CO2. On the other hand, no significant increase in net photosynthetic rate under elevated CO2 was found before cutting. The photosynthetic activity of 2nd flush leaves in the sprouts under ambient condition was greatly reduced by severe infection to powdery mildew. Growth of sprouts was enhanced in the elevated CO2 condition. We conclude the growth enhancement in Japanese oak sprouts under elevated CO2 in the present study was achieved not only by physiological response (i.e., photosynthetic stimulation) but also by disease interaction.

Cassava about‐FACE: Greater than expected yield stimulation of cassava (Manihot esculenta) by future CO2 levels

AbstractGlobally, cassava is the second most important root crop after potatoes and the fifth most important crop overall in terms of human caloric intake. In addition to its growing global importance for feed, fuel, and starch, cassava has long been vital to food security in Sub‐Saharan Africa. Climate change is expected to have its most severe impact on crops in food insecure regions, yet little is known about how cassava productivity will respond to climate change. The most important driver of climate change is globally increasing atmospheric CO2 concentration ([CO2]). However, the potential for cassava to enhance food security in an elevated [CO2] world is uncertain as greenhouse and open top chamber (OTC) study reports are ambiguous. Studies have yielded misleading results in the past regarding the effect of elevated [CO2] on crop productivity, particularly in cases where pots restricted sink growth. To resolve these conflicting results, we compare the response of cassava to growth at ambient (ca. 385 ppm) and elevated [CO2] (585 ppm) under field conditions and fully open air [CO2] elevation. After three and half months of growth at elevated [CO2], above ground biomass was 30% greater and cassava root tuber dry mass increased over 100% (fresh weight increased 89%). High photosynthetic rates and photosynthetic stimulation by elevated [CO2], larger canopies, and a large sink capacity all contributed to cassava's growth and yield stimulation. Cassava exhibited photosynthetic acclimation via decreased Rubisco capacity early in the season prior to root tuber initiation when sink capacity was smaller. Importantly, and in contrast to a greenhouse study, we found no evidence of increased leaf N or total cyanide concentration in elevated [CO2]. All of our results are consistent with theoretical expectations; however, the magnitude of the yield increase reported here surpasses all other C3 crops and thus exceeds expectations.

Changes in leaf nutrient traits and photosynthesis of four tree species: effects of elevated [CO2], N fertilization and canopy positions

Aims Leaf traits of trees exposed to elevated [CO2] in association with other environmental factors are poorly understood in tropical and subtropical regions. Our goal was to investigate the impacts of elevated [CO2] and N fertilization on leaf traits in southern China. Methods Four tree species, Schima superba Gardn. et Champ. (S. superba), Ormosia pinnata (Lour.) Merr (O. pinnata), Castanopsis hystrix AC. DC. (C. hystrix) and Acmena acuminatissima (Blume) Merr. et Perry (A. acuminatissima) were studied in a factorial combination of atmospheric [CO2] (ambient at ;390 lmol mol 1 and elevated [CO2 ]a t;700 lmol mol 1 ) and N fertilization (ambient and ambient + 100 kg N ha 1 year 1 ) in open-top chambers in southern China for 5 years. Leaf mass per unit leaf area (LMA), leaf nutrient concentration and photosynthesis (Asat) were measured. Important Findings Results indicated that leaf traits and photosynthesis were affected differently by elevated [CO2] and N fertilization among species. Elevated [CO2] decreased LMA in all species, while N fertilization did not affect LMA. Leaf mass-based N concentration (NM) was significantly greater in O. pinnata and C. hystrix grown in elevated [CO2] but was lower in S. superba. Leaf mass-based P concentration (PM) was significantly greater in C. hystrix and A. acuminatissima exposed to elevated [CO2] but was lower in S. superba. N fertilization significantly increased PM in O. pinnata but decreased PM in S. superba. Photosynthetic stimulation in O. pinnata, C. hystrix and A. acuminatissima was sustained after 5 years of CO2 fumigation. N fertilization did not modify the effects of elevated [CO2] on photosynthesis. Leaf traits (NM ,N A ,P M ,P A) and light-saturated photosynthesis were decreased from the upper to lower canopy. Canopy position did not alter the responses of leaf traits and photosynthesis to elevated [CO2]. Results suggest that photosynthetic stimulation by elevated [CO2] in native species in subtropical regions may be sustained in the long term.

Effect of the soil yeast Cryptococcus laurentii on the photosynthetic water and nutrient-use efficiency and respiratory carbon costs of a Mediterranean sclerophyll, Agathosma betulina (Berg.) Pillans

Nothing is known about the effect of soil yeasts on the photosynthetic resource-use and carbon dynamics of plants. Here, we determined the effect of a plant growth-promoting isolate of Cryptococcus laurentii on the photosynthetic water and nutrient-use efficiencies, as well as the carbon economy of a Mediterranean sclerophyll, Agathosma betulina, grown under axenic conditions. The data showed that the higher photosynthetic water-use efficiency in yeast-inoculated plants was a consequence of higher maximum rates of CO2 assimilation, which was not related to foliar N and P content. We propose that photosynthetic stimulation in yeast-inoculated plants was a result of the increased demand for photosynthates of the yeast-root symbiosis.

Arbuscular mycorrhizae affect the N and C economy of nodulated Phaseolus vulgaris ( L.) during NH 4+ nutrition

In the symbiosis between nodulated legume roots and arbuscular mycorrhizal (AM) fungi, the C and N economy can be influenced by the source of N-supply from either AM-derived NH 4 + uptake or nodule-derived biological nitrogen fixation (BNF). This relationship was investigated in terms of NH 4 + supply and BNF by the two symbionts. Nodulated Phaseolus vulgaris seedlings with and without AM, were hydroponically grown with either 0 N or 1 mM NH 4 + supply. Plants were harvested at 30 days after emergence and measurements were taken for biomass, N 2 fixation, photosynthesis, CO 2 and O 2 root respiration, calculated C and N economy. AM roots had higher NH 4 + uptake and this was associated with the suppression of BNF and nodule growth. The higher NH 4 + uptake in AM roots occurred with lower root maintenance respiration, compared to when N was derived from BNF. There was also an increase in the below-ground sink strength of NH 4 + fed AM roots compared to NH 4 + fed non-AM roots, as evidenced by the increases in root CO 2 and O 2 respiration and photosynthetic stimulation. These results indicate that although the AM root had higher total below-ground respiratory costs during NH 4 + nutrition, there were lower respiratory C costs associated with N derived from AM symbionts in comparison to N from BNF.

Assessment of soil nitrogen and phosphorous availability under elevated CO2 and N-fertilization in a short rotation poplar plantation

Photosynthetic stimulation by elevated [CO2] is largely regulated by nitrogen and phosphorus availability in the soil. During a 6 year Free Air CO2 Enrichment (FACE) experiment with poplar trees in two short rotations, inorganic forms of soil nitrogen, extractable phosphorus, microbial and total nitrogen were assessed. Moreover, in situ and potential nitrogen mineralization, as well as enzymatic activities, were determined as measures of nutrient cycling. The aim of this study was to evaluate the effects of elevated [CO2] and fertilization on: (1) N mineralization and immobilization processes; (2) soil nutrient availability; and (3) soil enzyme activity, as an indication of microbial and plant nutrient acquisition activity. Independent of any treatment, total soil N increased by 23% in the plantation after 6 years due to afforestation. Nitrification was the main process influencing inorganic N availability in soil, while ammonification being null or even negative. Ammonium was mostly affected by microbial immobilization and positively related to total N and microbial biomass N. Elevated [CO2] negatively influenced nitrification under unfertilised treatment by 44% and consequently nitrate availability by 30% on average. Microbial N immobilization was stimulated by [CO2] enrichment and probably enhanced the transformation of large amounts of N into organic forms less accessible to plants. The significant enhancement of enzyme activities under elevated [CO2] reflected an increase in nutrient acquisition activity in the soil, as well as an increase of fungal population. Nitrogen fertilization did not influence N availability and cycling, but acted as a negative feed-back on phosphorus availability under elevated CO2.

Quantitative evaluation of the counterbalance between photosynthetic stimulation and depression caused by low partial pressure of O 2 and CO 2 in alpine atmospheres

To evaluate the effects of low atmospheric pressure on leaf photosynthesis, we compared the photosynthesis of identical leaves of Fagus crenata at lowland (0 m a.s.l.) and at highland (2360 m a.s.l.). At the high altitude, the atmospheric pressure and partial pressure of CO 2 at intercellular air spaces in the leaf ( C i360) decreased to 77% and 78% of those at the low altitude, respectively. On the other hand, the efficiency of photosynthetic CO 2-utilization was apparently higher at the high altitude because of a mitigation of the O 2-inhibition of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) under low ambient partial pressure of O 2. This stimulation of photosynthesis partly compensated a photosynthetic depression due to the low C i360, and the net assimilation rate ( A n360) at the high altitude retained 94% of the value at the low altitude. A theoretical model indicated that the stimulation of photosynthesis at high altitudes depend on internal conductance ( g i) and/or on Rubisco content. The model demonstrated that low atmospheric pressure at high altitudes caused severe restrictions of photosynthesis when leaves had a small g i and/or a large amount of Rubisco, whichever are repeatedly reported in alpine plants.

Photosynthetic stimulation under long‐term CO2 enrichment and fertilization is sustained across a closed Populus canopy profile (EUROFACE)

The long-term response of leaf photosynthesis to rising CO2 concentrations [CO2] depends on biochemical and morphological feedbacks. Additionally, responses to elevated [CO2] might depend on the nutrient availability and the light environment, affecting the net carbon uptake of a forest stand. After 6 yr of exposure to free-air CO2 enrichment (EUROFACE) during two rotation cycles (with fertilization during the second cycle), profiles of light, leaf characteristics and photosynthetic parameters were measured in the closed canopy of a poplar (Populus) short-rotation coppice. Net photosynthetic rate (A(growth)) was 49% higher in poplars grown in elevated [CO2], independently of the canopy position. Jmax significantly increased (15%), whereas leaf carboxylation capacity (Vcmax), leaf nitrogen (N(a)) and chlorophyll (Chl(a)) were unaffected in elevated [CO2]. Leaf mass per unit area (LMA) increased in the upper canopy. Fertilization created more leaves in the top of the crown. These results suggest that the photosynthetic stimulation by elevated [CO2] in a closed-canopy poplar coppice might be sustained in the long term. The absence of any down-regulation, given a sufficient sink capacity and nutrient availability, provides more carbon for growth and storage in this bioenergy plantation.

Periphytic Photosynthetic Stimulation of Extracellular Enzyme Activity in Aquatic Microbial Communities Associated with Decaying Typha Litter

We examined the effect of light on extracellular enzyme activities of periphytic/endogenous microbial assemblages associated with decomposing litter of an emergent macrophyte Typha angustifolia within a small inland wetland in southeastern Michigan. Standing-dead Typha leaf litter was collected, placed into floating wire mesh litter baskets, and submerged in a wetland pool. Enzyme saturation assays were conducted on three occasions following litter submergence (days 9, 28, and 44) to generate saturation curves for the individual enzymes tested and to examine potential differences in enzyme saturation kinetics during microbial colonization and development. Experimental light manipulations were conducted on two occasions during microbial development (days 10 and 29). Short-term (30 min) light exposure significantly increased extracellular beta-glucosidase activity of litter-associated microbial communities. Activities of beta-xylosidase and leucine-aminopeptidase were not stimulated, and stimulation of phosphatase activity was variable. The exact mechanism for increased enzyme activity remains unknown, but it may have been increased pH arising from periphytic algal photosynthesis. These results suggest that extracellular enzyme activity in microbial communities colonizing natural organic substrata may be influenced by light/photosynthesis, as has previously been demonstrated for periphyton communities grown on artificial, inert substrata. Thus, light/photosynthetic mediated stimulation of extracellular enzyme activities may be a common occurrence in microbial communities associated with natural decaying plant litter in wetlands and might engender diurnal patterns in other microbial decay processes (e.g., production, organic matter decomposition, and mineralization).

Photosynthetic responses to elevated CO 2 and O 3 in Quercus ilex leaves at a natural CO 2 spring

Photosynthetic stimulation and stomatal conductance (Gs) depression in Quercus ilex leaves at a CO 2 spring suggested no down-regulation. The insensitivity of Gs to a CO 2 increase (from ambient 1500 to 2000 μmol mol −1) suggested stomatal acclimation. Both responses are likely adaptations to the special environment of CO 2 springs. At the CO 2-enriched site, not at the control site, photosynthesis decreased 9% in leaves exposed to 2× ambient O 3 concentrations in branch enclosures, compared to controls in charcoal-filtered air. The stomatal density reduction at high CO 2 was one-third lower than the concomitant Gs reduction, so that the O 3 uptake per single stoma was lower than at ambient CO 2. No significant variation in monoterpene emission was measured. Higher trichome and mesophyll density were recorded at the CO 2-enriched site, accounting for lower O 3 sensitivity. A long-term exposure to H 2S, reflected by higher foliar S-content, and CO 2 might depress the antioxidant capacity of leaves close to the vent and increase their O 3 sensitivity.

Constraints on photosynthesis of C3 and C4 crop plants by trichloroacetic acid, an atmogenically generated pollutant

This study examined the physiological and biochemical bases of the inhibition of photosynthesis by trichloroacetic acid (TCA) in Phaseolus vulgaris L. and Zea mays L., representing dicotyledonous and mono-cotyledonous plants with C3 and C4 photosynthetic pathways respectively. TCA is a phytotoxic compound transformed in the atmosphere from chloro-organic substances (C2-CHCs) present in solvents and cleaning agents used in the metal and textile industry, as well as in combustion products released from coal-fired power stations and biomass burning. Photosynthetic gas exchange and fast chlorophyll fluorescence kinetics were measured in 3-week-old plants exposed for 3 days to TCA concentrations ranging from 0.05–3.20g TCA-Na m− 2. Photosynthetic inhibition by TCA was observed in both species, this being greater at the highest level of TCA supplied in Z. mays (85% decrease) than in P. vulgaris (37% decrease), though at low levels of TCA supplied some photosynthetic stimulation was also observed in P. vulgaris. Analysis of Rubisco activity revealed that TCA decreased the activation state of the enzyme in both species, especially in Z. mays, a possible consequence also of a measured decrease in stomatal conductance. However, the decline in net CO2 assimilation rate at saturating CO2 levels (Jmax) was also indicative of a decrease in RuBP regeneration capacity. This ostensible mesophyll limitation was supported by chlorophyll fluorescence measurements, which indicated a disruption of electron transport in photosystem II (PS II) reaction centres requisite for effective RuBP regeneration. It is concluded that increased emission of C2-CHCs by industry and veld-fire catastrophes pose a threat particular to grain production by monocotyledonous crops especially in southern Africa which is experiencing greater food demands from a rapidly expanding population.