Rates Of Gross Photosynthesis Research Articles

Physiological differences among cryptic species of the Mediterranean crustose coralline alga Lithophyllum stictiforme (Corallinales, Rhodophyta)

ABSTRACT The crustose coralline alga Lithophyllum stictiforme is an important reef builder, forming coralligenous concretions in the Mediterranean Sea. This algal species complex is composed of eight cryptic species (or clades). The objective of our study was to compare light-dependent physiological processes within and between the two most abundant cryptic species (C1 and C4) of L. stictiforme from the Bay of Marseille (France). Physiological rates of respiration, photosynthesis and calcification were measured using incubation chambers under various irradiance levels and in the dark during the summer period. We compared the physiology of the C1 and C4 cryptic species of L. stictiforme among three algal groups: C1 (the most common in the Marseille area) from 28 m depth, C4 from the same site and depth, and C1 from a deeper site at 45 m depth. We found both interspecific (between C1 and C4) and intraspecific (within C1) physiological differences. Photosynthetic parameters showed acclimation to light (or depth) with higher values of initial slope of photosynthetic light response curve (α) and lower values of saturating irradiance (Ek) and compensating irradiance (Ec) for C1 from 45 m than for C1 and C4 from 28 m depth. At the shallower depth, significant physiological differences were observed between C1 and C4 with a higher value of maximum rate of gross photosynthesis (Pg max) in C4, suggesting better ability to grow under high irradiance. Light and dark calcification rates differed only between C4 from 28 m and C1 from 45 m depth, being intermediate in C1 from 28 m depth. On a 24 h basis, diel net calcification rates were significantly higher in specimens from the shallower waters than from the deeper waters. Physiological differences suggest physiological plasticity within the regionally dominant cryptic species C1 and species-specific physiological traits between the cryptic species C1 and C4.

The enhancement of photosynthetic performance, water use efficiency and potato yield under elevated CO2 is cultivar dependent.

As a fourth major food crop, potato could fulfill the nutritional demand of the growing population. Understanding how potato plants respond to predicted increase in atmospheric CO2 at the physiological, biochemical and molecular level is therefore important to improve potato productivity. Thus, the main objectives of the present study are to investigate the effects of elevated CO2 on the photosynthetic performance, water use efficiency and tuber yield of various commercial potato cultivars combined with biochemical and molecular analyses. We grew five potato cultivars (AC Novachip, Atlantic, Kennebec, Russet Burbank and Shepody) at either ambient CO2 (400 μmol CO2 mol-1) or elevated (750 μmol CO2 mol-1) CO2. Compared to ambient CO2-grown counterparts, elevated CO2-grown Russet Burbank and Shepody exhibited a significant increase in tuber yield of 107% and 49% respectively, whereas AC Novachip, Atlantic and Kennebec exhibited a 16%, 6% and 44% increment respectively. These differences in CO2-enhancement of tuber yield across the cultivars were mainly associated with the differences in CO2-stimulation of rates of photosynthesis. For instance, elevated CO2 significantly stimulated the rates of gross photosynthesis for AC Novachip (30%), Russet Burbank (41%) and Shepody (28%) but had minimal effects for Atlantic and Kennebec when measured at growth light. Elevated CO2 significantly increased the total tuber number for Atlantic (40%) and Shepody (83%) but had insignificant effects for other cultivars. Average tuber size increased for AC Novachip (16%), Kennebec (30%) and Russet Burbank (80%), but decreased for Atlantic (25%) and Shepody (19%) under elevated versus ambient CO2 conditions. Although elevated CO2 minimally decreased stomatal conductance (6-22%) and transpiration rates (2-36%), instantaneous water use efficiency increased by up to 79% in all cultivars suggesting that enhanced water use efficiency was mainly associated with increased photosynthesis at elevated CO2. The effects of elevated CO2 on electron transport rates, non-photochemical quenching, excitation pressure, and leaf chlorophyll and protein content varied across the cultivars. We did not observe any significant differences in plant growth and morphology in elevated versus ambient CO2-grown plants. Taken all together, we conclude that the CO2-stimulation of photosynthetic performance, water use efficiency and tuber yield of potatoes is cultivar dependent.

Hydrologic connectivity drives extremes and high variability in vegetation productivity across Australian arid and semi-arid ecosystems

Vegetation growth drives many of the interactions between the land surface and atmosphere including the uptake of carbon through photosynthesis and loss of water through transpiration. In arid and semi-arid regions water is the dominant driver of vegetation growth. However, few studies consider the fact that water can move laterally across the landscape as runoff via streams and floodplains, termed hydrologic connectivity. Using multiple observations alongside models and a hydromorphology dataset for Australia, we examine how ecosystems with high hydrologic connectivity differ in their vegetation response to water availability, soil properties, and interannual variability and extremes in vegetation productivity. We find that the average interannual variability of vegetation productivity is 21–34% higher in ecosystems with high hydrologic connectivity, with skewed annual anomalies showing larger extremes in carbon uptake. This is driven by a higher average and more variable surface soil moisture and significantly higher soil available water capacity and soil depth. These spatially small ecosystems, covering 14% of the study region, contribute 15–22% (median = 17%) to regional-scale carbon uptake through higher rates of gross photosynthesis, especially evident during wet years, and 3–37% (median = 19%) to annual anomalies. Current global land surface models do not reproduce the observed spatial patterns of interannual variability in carbon uptake over regions where hydrologic connectivity is high as they lack the mechanism of connectivity of water between discrete land surface elements. This study highlights the significant role of riparian and floodplain vegetation on the interannual variability and extremes of the regional carbon cycle.

Heating rate modulates the metabolic response of the staghorn coral Acropora cervicornis (Lamarck, 1816)

Corals are relatively stenothermic organisms and highly sensitive to thermal stress. To understand how heating rate modulates the holobiont response to thermal challenge, we compared the effects of acute heat shock and cumulative thermal exposures on Acropora cervicornis from Southeast Florida (26° 06 N, 80° 05 W). In March of 2017, maximum dark-adapted yield of photosystem II (FV/FM), rates of dark respiration (R), and rates of gross photosynthesis (Pg) were measured at temperatures spanning 25–36 °C. Thermal performance curves of each response variable were constructed as a function of temperature and thermal dose (i.e., degree heating minutes). Acute exposure (i.e., instantaneously increasing temperature by between 2.5 and 10 °C) resulted in a more marked decline in physiology despite a lower thermal dose, compared to the cumulative exposure (2.5 °C h−1). Apparent deficits in coral holobiont metabolism were observed as a function of both temperature and thermal dose, but examination of holobiont metabolism in a dose-context revealed more pronounced differences between acute and cumulative exposures. Subtle physiological differences may be more easily identified and better cross-study comparisons of cnidarian thermal tolerance may be achieved by placing temperature exposures in degree heating times. In addition, the ratio of daily Pg:R fell below 1 when temperatures exceeded 32 °C, corroborating prior observations that Pg:R may be a means of identifying physiological deficits in advance of visual signs of dysbiosis, such as bleaching.

Response of Winter Wheat Seedlings Respiration to Long-Term Cold Exposure and Short-Term Daily Temperature Drops

The effects of a long-term (6-day) exposure to low positive temperature (4°С, LT treatment) and short-term (3 h per a day for 6 days) daily temperature drops to 4°С (DROP treatment) on leaf respiration in winter wheat (Triticum aestivum L., var. Moskovskaya 39) were studied under controlled conditions. In the case of the LT treatment, the respiration rate in the darkness (Rd) increased by 36%, while the DROP treatment only resulted in a tendency to such an increase. The influence of both DROP and LT treatments on the respiration rate in the light (Rl) and temperature sensitivity of Rd and Rl was insignificant. In contrast to the DROP treatment, the LT treatment increased the level of light-induced inhibition of plant respiration. In both LT- and DROP-treated plants, the rate of gross photosynthesis (Ag) measured at 12°С increased by 23 and 54%, respectively, as compared to the control plants. The DROP treatment did not affect the Rd/Ag and Rl/Ag ratios in plant leaves. The Rd/Ag value in LT-treated plants measured at 22°С was higher than in control plants, while the Rl/Ag value measured at 12°С was lower than in the control. The DROP treatment did not affect the sensitivity of respiration to salicylhydroxamic acid (SHAM), an inhibitor of the alternative respiratory pathway. The LT treatment increased the share of a SHAM-sensitive respiration in the total respiration of plant leaves by 25% on average, which indirectly indicates the activation of an alternative pathway of the electron transfer in the respiratory chain. Thus, a long-term continuous exposure of winter wheat seedlings to low positive temperatures provided a modifying effect on respiration manifested via changes in the respiratory rate, activity of the SHAM-sensitive respiration, and the ratio between respiration and gross photosynthesis. In the case of a short-term daily exposure to low temperature high respiration stability was observed.

An investigation on possible effect of leaching fractions physiological responses of hot pepper plants to irrigation water salinity

BackgroundThe modification effect of leaching fraction (LF) on the physiological responses of plants to irrigation water salinity (ECiw) remains unknown. Here, leaf gas exchange, photosynthetic light–response and CO2–response curves, and total carbon (C) and nitrogen (N) accumulation in hot pepper leaves were investigated under three ECiw levels (0.9, 4.7 and 7.0 dS m− 1) and two LFs treatments (0.17 and 0.29).ResultsLeaf stomatal conductance was more sensitive to ECiw than the net photosynthesis rate, leading to higher intrinsic water use efficiency (WUE) in higher ECiw, whereas the LF did not affect the intrinsic WUE. Carbon isotope discrimination was inhibited by ECiw, but was not affected by LF. ECiw reduced the carboxylation efficiency, photosynthetic capacity, photorespiration rate, apparent quantum yield of CO2 and irradiance–saturated rate of gross photosynthesis; however, LF did not influence any of these responses. Total C and N accumulation in plants leaves was markedly increased with either decreasing ECiw or increasing LF.ConclusionsThe present study shows that higher ECiw depressed leaf gas exchange, photosynthesis capacity and total C and N accumulation in leaves, but enhanced intrinsic WUE. Somewhat surprisingly, higher LF did not affect the intrinsic WUE but enhanced the total C and N accumulation in leaves.

Soil moisture dynamics under two rainfall frequency treatments drive early spring CO2 gas exchange of lichen-dominated biocrusts in central Spain.

BackgroundBiocrusts, communities dominated by mosses, lichens, cyanobacteria, and other microorganisms, largely affect the carbon cycle of drylands. As poikilohydric organisms, their activity time is often limited to short hydration events. The photosynthetic and respiratory response of biocrusts to hydration events is not only determined by the overall amount of available water, but also by the frequency and size of individual rainfall pulses.MethodsWe experimentally assessed the carbon exchange of a biocrust community dominated by the lichen Diploschistes diacapsis in central Spain. We compared the effect of two simulated precipitation patterns providing the same overall amount of water, but with different pulse sizes and frequency (high frequency: five mm/day vs. low frequency: 15 mm/3 days), on net/gross photosynthesis and dark respiration.ResultsRadiation and soil temperature, together with the watering treatment, affected the rates of net and gross photosynthesis, as well as dark respiration. On average, the low frequency treatment showed a 46% ± 3% (mean ± 1 SE) lower rate of net photosynthesis, a 13% ± 7% lower rate of dark respiration, and a 24% ± 8% lower rate of gross photosynthesis. However, on the days when samples of both treatments were watered, no differences between their carbon fluxes were observed. The carbon flux response of D. diacapsis was modulated by the environmental conditions and was particularly dependent on the antecedent soil moisture.DiscussionIn line with other studies, we found a synergetic effect of individual pulse size, frequency, environmental conditions, and antecedent moisture on the carbon exchange fluxes of biocrusts. However, most studies on this subject were conducted in summer and they obtained results different from ours, so we conclude that there is a need for long-term experiments of manipulated precipitation impacts on the carbon exchange of biocrusts. This will enable a more complete assessment of the impacts of climate change-induced alterations in precipitation patterns on biocrust communities.

Ocean acidification alters early successional coral reef communities and their rates of community metabolism.

Ocean acidification is expected to alter community composition on coral reefs, but its effects on reef community metabolism are poorly understood. Here we document how early successional benthic coral reef communities change in situ along gradients of carbon dioxide (CO2), and the consequences of these changes on rates of community photosynthesis, respiration, and light and dark calcification. Ninety standardised benthic communities were grown on PVC tiles deployed at two shallow-water volcanic CO2 seeps and two adjacent control sites in Papua New Guinea. Along the CO2 gradient, both the upward facing phototrophic and the downward facing cryptic communities changed in their composition. Under ambient CO2, both communities were dominated by calcifying algae, but with increasing CO2 they were gradually replaced by non-calcifying algae (predominantly green filamentous algae, cyanobacteria and macroalgae, which increased from ~30% to ~80% cover). Responses were weaker in the invertebrate communities, however ascidians and tube-forming polychaetes declined with increasing CO2. Differences in the carbonate chemistry explained a far greater amount of change in communities than differences between the two reefs and successional changes from five to 13 months, suggesting community successions are established early and are under strong chemical control. As pH declined from 8.0 to 7.8, rates of gross photosynthesis and dark respiration of the 13-month old reef communities (upper and cryptic surfaces combined) significantly increased by 10% and 20%, respectively, in response to altered community composition. As a consequence, net production remained constant. Light and dark calcification rates both gradually declined by 20%, and low or negative daily net calcification rates were observed at an aragonite saturation state of <2.3. The study demonstrates that ocean acidification as predicted for the end of this century will strongly alter reef communities, and will significantly change rates of community metabolism.

Physiological and Pathogenic Contributors to the Summer Decline of Roughstalk Bluegrass

Roughstalk bluegrass (Poa trivialis L.) is a cool‐season perennial turfgrass species that is more sensitive to heat stress than other cool‐season grasses. This sensitivity has been associated with increased rates of respiration compared with photosynthesis, or an inability to hydrolyze carbohydrates to simple sugars for metabolism during heat stress, but may also involve common turfgrass diseases, since roughstalk bluegrass has been shown to maintain quality from repeated applications of strobilurin fungicides in summer. Our objective was to differentiate between physiological and pathogenic contributors to roughstalk bluegrass decline and determine the effects of strobilurin fungicides on its growth and physiology. Roughstalk bluegrass was treated with azoxystrobin (Heritage 50 WDG or Heritage TL) at 610 g a.i. ha−1 or pyraclostrobin (Insignia 20 WG or Insignia SC) at 556 g a.i. ha−1 prior to summer heat stress in field plots in Manhattan, KS, (2011 and 2012) and Mead, NE (2012). Fungicides improved quality and percentage green cover over untreated roughstalk bluegrass in all studies, but fungicides rarely affected rooting or rates of gross photosynthesis. Disease sampling did not reveal any fungal pathogen consistently associated with summer turf decline. Our data suggest that nontarget physiological effects of strobilurin fungicides likely increased quality and cover over untreated roughstalk bluegrass during summer.

Photosynthetic Acclimation of Symbiodinium in hospite Depends on Vertical Position in the Tissue of the Scleractinian Coral Montastrea curta.

Coral photophysiology has been studied intensively from the colony scale down to the scale of single fluorescent pigment granules as light is one of the key determinants for coral health. We studied the photophysiology of the oral and aboral symbiont band of scleractinian coral Montastrea curta to investigate if different acclimation to light exist in hospite on a polyp scale. By combined use of electrochemical and fiber-optic microsensors for O2, scalar irradiance and variable chlorophyll fluorescence, we could characterize the physical and chemical microenvironment experienced by the symbionts and, for the first time, estimate effective quantum yields of PSII photochemistry and rates of electron transport at the position of the zooxanthellae corrected for the in-tissue gradient of scalar irradiance. The oral- and aboral Symbiodinium layers received ∼71% and ∼33% of surface scalar irradiance, respectively, and the two symbiont layers experience considerable differences in light exposure. Rates of gross photosynthesis did not differ markedly between the oral- and aboral layer and curves of PSII electron transport rates corrected for scalar irradiance in hospite, showed that the light use efficiency under sub-saturating light conditions were similar between the two layers. However, the aboral Symbiodinium band did not experience photosynthetic saturation, even at the highest investigated irradiance where the oral layer was clearly saturated. We thus found a different light acclimation response for the oral and aboral symbiont bands in hospite, and discuss whether such response could be shaped by spectral shifts caused by tissue gradients of scalar irradiance. Based on our experimental finding, combined with previous knowledge, we present a conceptual model on the photophysiology of Symbiodinium residing inside living coral tissue under natural gradients of light and chemical parameters.

Trophic dynamics of scleractinian corals: stable isotope evidence.

Reef-building corals form symbioses with dinoflagellates from the diverse genus Symbiodinium. This symbiotic association has developed adaptations to acquire and share nutrients, which are essential for its survival and growth in nutrient-poor tropical waters. The host is thus able to prey on a wide range of organic food sources (heterotrophic nutrition) whereas the symbionts acquire most of the inorganic nutrients (autotrophic nutrition). However, nutrient fluxes between the two partners remain unclear, especially concerning heterotrophically acquired carbon and nitrogen. We combined physiological measurements and pulse-chase isotopic labeling of heterotrophic carbon and nitrogen, as well as autotrophic carbon to track nutrient fluxes in two coral species, Stylophora pistillata and Turbinaria reniformis, in symbiosis with Symbiodinium clades A, and C,D respectively. We showed a rapid acquisition, exchange and a long-term retention of heterotrophic nutrients within the symbiosis, whereas autotrophic nutrients were rapidly used to meet immediate metabolic needs. In addition, there was a higher retention of heterotrophic nitrogen compared with carbon, in agreement with the idea that tropical corals are nitrogen-limited. Finally, a coupling between auto- and heterotrophy was observed in the species S. pistillata, with a higher acquisition and retention of heterotrophic nutrients under low irradiance to compensate for a 50% reduction in autotrophic nutrient acquisition and translocation. Conversely, T. reniformis conserved an equivalent heterotrophic nutrient acquisition at both light levels because this coral species did not significantly reduce its rates of gross photosynthesis and autotrophic carbon acquisition between the two irradiances. These experiments advance the current understanding of the nutrient exchanges between the two partners of a symbiotic association, providing evidence of the complexity of the host-symbiont relationship.

87. Effects of elevated ambient pressure on the rates of gross photosynthesis and dark respiration

87. Effects of elevated ambient pressure on the rates of gross photosynthesis and dark respiration

Photosynthate translocation increases in response to low seawater pH in a coral–dinoflagellate symbiosis

Abstract. This study has examined the effect of low seawater pH values (induced by an increased CO2 partial pressure) on the rates of photosynthesis, as well as on the carbon budget and carbon translocation in the scleractinian coral species Stylophora pistillata, using a new model based on 13C labelling of the photosynthetic products. Symbiont photosynthesis contributes to a large part of the carbon acquisition in tropical coral species, and it is thus important to know how environmental changes affect this carbon acquisition and allocation. For this purpose, nubbins of S. pistillata were maintained for six months at two pHTs (8.1 and 7.2, by bubbling seawater with CO2). The lowest pH value was used to tackle how seawater pH impacts the carbon budget of a scleractinian coral. Rates of photosynthesis and respiration of the symbiotic association and of isolated symbionts were assessed at each pH. The fate of 13C photosynthates was then followed in the symbionts and the coral host for 48 h. Nubbins maintained at pHT 7.2 presented a lower areal symbiont concentration, and lower areal rates of gross photosynthesis and carbon incorporation compared to nubbins maintained at pHT 8.1. The total carbon acquisition was thus lower under low pH. However, the total percentage of carbon translocated to the host as well as the amount of carbon translocated per symbiont cell were significantly higher under pHT 7.2 than under pHT 8.1 (70% at pHT 7.2 vs. 60% at pHT 8.1), such that the total amount of photosynthetic carbon received by the coral host was equivalent under both pHs (5.5 to 6.1 μg C cm−2 h−1). Although the carbon budget of the host was unchanged, symbionts acquired less carbon for their own needs (0.6 compared to 1.8 μg C cm−2 h−1), explaining the overall decrease in symbiont concentration at low pH. In the long term, such decrease in symbiont concentration might severely affect the carbon budget of the symbiotic association.

Bromus tectorum litter alters photosynthetic characteristics of biological soil crusts from a semiarid shrubland

Invasion by the exotic annual grass Bromus tectorum has increased the cover and connectivity of fine litter in the sagebrush steppes of western North America. This litter tends to cover biological soil crusts, which could affect their metabolism and growth. To investigate this possible phenomenon, biological soil crusts dominated by either the moss Bryum argenteum or the lichen Diploschistes muscorum were covered with B. tectorum litter (litter treatment) or left uncovered (control treatment) and exposed to natural field conditions. After periods of five and ten months, we removed the litter and compared the photosynthetic performance of biological soil crusts from the two treatments. Litter induced photosynthetic changes in our samples. In both B. argenteum and D. muscorum, biological soil crusts that had been covered with litter for ten months had lower rates of gross photosynthesis and lower chlorophyll content than control samples. Similarly in both biological soil crust types, litter reduced the rate of dark respiration. For D. muscorum, the reduction in dark respiration fully compensated for the decrease in gross photosynthesis, resulting in similar values of net photosynthesis in the two treatments. In contrast, for B. argenteum, net photosynthesis was four-times greater in the control than the litter treatment. Also under litter cover, D. muscorum showed three common adaptations to shade conditions: a decrease in the light compensation point, in the light intensity needed to achieve 95% of maximal net photosynthesis, and in the chlorophyll a/b ratio. None of these changes was apparent in B. argenteum. Overall, our results indicate that photosynthetic responses to the presence of litter varied among species of the crust biota and that the litter can reduce the photosynthetic capacity of biological soil crusts. These results help to explain field observations of decreases in biological soil crust cover and changes in biological soil crust composition with increases in litter cover, and suggest that the landscape-wide invasion by B. tectorum may have substantial effects on biological soil crust performance and therefore their capacity to function in semiarid shrublands.

Carbon turnover rates in the One Tree Island reef: A 40‐year perspective

During November–December 2009 community rates of gross photosynthesis (Pg), respiration (R) and net calcification (Gnet) were estimated from low‐tide slack water measurements of dissolved oxygen, dissolved inorganic carbon and total alkalinity at the historical station DK13 One Tree Island reef, Great Barrier Reef, Australia. Compared to measurements made during the 1960s–1970s at DK13 in the same season,Pg increased from 833 to 914 mmol O2·m−2·d−1 and Pg:R increased from 1.14 to 1.30, indicating that the reef has become more autotrophic. In contrast, Gnet decreased from 133 mmol C·m−2·d−1 to 74 ± 24 mmol C·m−2·d−1. This decrease stems primarily from the threefold increase in nighttime CaCO3 dissolution from −2.5 mmol·m−2·h−1 to −7.5 mmol·m−2·h−1. Comparison of the benthic community survey results from DK13 and its vicinity conducted during this study and in studies from the 1970s, 1980s and 1990s suggest that there have been no significant changes in the live coral coverage during the past 40 years. The reduced Gnet most likely reflects the almost threefold increase in dissolution rates, possibly resulting from increased bioerosion due to changes in the biota (e.g., sea cucumbers, boring organisms) and/or from greater chemical dissolution produced by changing abiotic conditions over the past 40 years associated with climate change, such as increased temperatures and ocean acidification. However, at this stage of research on One Tree Island the effects of these changes are not entirely understood.

Variation in photosynthesis and respiration in ­geographically distinct populations of two reef‑building coral species

Studies of the regulation and importance of physiological processes such as coral photo- synthesis and respiration on coral reefs require knowledge of spatio-temporal patterns of variability at different scales. Oxygen microelectrodes were used to measure photosynthesis and dark respira- tion of 2 corals, Pocillopora damicornis and Turbinaria reniformis, in the northern (Lizard Island) and central (Davies and Broadhurst Reefs) regions of the Great Barrier Reef (GBR) in winter and summer. Genetic characterisation of Symbiodinium revealed that P. damicornis hosted a single symbiont type (Symbiodinium C1) in both regions, whereas T. reniformis harboured 2 types, dependent on location. Colonies at Lizard Island harboured Symbiodinium D, whereas colonies at Davies Reef harboured Symbiodinium C2. Rates of gross photosynthesis were greater in the central than in the northern GBR in summer. A similar pattern was detected for dark respiration rates in T. reniformis. No seasonal change in either photosynthesis or dark respiration was evident in the northern GBR, possibly due to less annual variability in light conditions, and for T. reniformis, additionally the presence of Symbio- dinium D. These results highlight that environmental conditions coupled with regional-scale distrib- ution of Symbiodinium are likely to exert important influences on respiration and photosynthetic per- formance of reef-building corals.

Photosynthetic performance of the aquatic macrophyte Althenia orientalis to solar radiation along its vertical stems

We have studied the plasticity of the photosynthetic apparatus in the endangered aquatic macrophyte Althenia orientalis to the gradient of light availability within its meadow canopy. We determined diurnal change in situ irradiance, light quality, in vivo chlorophyll a fluorescence, ex situ oxygen evolution rates, respiration rate and pigment concentration. The levels of photosynthetic photon flux density (PFD) and ultraviolet radiation (UVR) and the red/far-red ratio decreased with depth within the canopies of A. orientalis. Apical leaves had a greater decrease of the maximal quantum yield (F(v)/F(m)) in the morning and a faster recovery rate in the afternoon than those in the basal ones. The relative electron transport rate (ETRr) was not saturated at any time of the day, even in the apical leaves that received the highest light. The maximum light-saturated rate of gross photosynthesis (GP(max)) took place in apical leaves around noon. The chlorophyll a/b ratio values were higher, and the chlorophyll/carotenoid ratio values lower, in apical leaves than basal ones. The highest concentrations in total carotenoids were reached in the apical leaves around noon. A. orientalis has a high capacity to acclimatize to the changes in the light environment, both in quality and quantity, presenting sun and shade leaves in the same stem through the vertical gradient in the canopy.

Differences in CO2 dynamics between successional mire plant communities during wet and dry summers

Questions: What impact do a wet and a dry growing season have on CO2 dynamics of mire plant communities along a primary succession gradient from the initiation stage to the bog stage? Location: Mires on a land uplift coast, Finland. Methods: We measured CO2 dynamics and vascular plant green area development on five mires that form a sequence of mire succession. TWINSPAN was used to define successional mire plant communities and regression analyses were used to explore the temporal variation in CO2 dynamics of the communities. Results: CO2 dynamics of successional plant communities reacted differently to a wet and a dry growing season. The net CO2 uptake rate of the earlier successional communities decreased in the dry growing season due to a decrease in photosynthesizing leaf area. Concurrently, CO2 uptake of the later successional communities moderately increased or did not change. Generally, the difference in net ecosystem exchange (NEE) between a dry and a wet year resulted from the altered rate of gross photosynthesis (PG) rather than ecosystem respiration (RE). Conclusions: Critical factors for the more stable carbon (C) gas dynamics in the later stages of mire succession were (1) higher autogenic control of the physical environment and (2) an increase in the number of factors regulating the PG rate. These factors may buffer mire ecosystems (in terms of the C sink function) from extreme and unfavourable variations in environmental conditions.

Photosynthesis in chlorolichens: the influence of the habitat light regime

The hypothesis that CO(2) gas exchange and chlorophyll a fluorescence (ChlaF) of lichens vary according to the light regimes of their original habitat, as observed in vascular plants, was tested by analysing the photosynthetic performance of 12 populations of seven dorsoventral, foliose lichens collected from open, south-exposed rocks to densely shaded forests. Light response curves were induced at optimum thallus water content and ChlaF emission curves at the species-specific photon flux at which the quantum yield of CO(2) assimilation is the highest and is saturating the photosynthetic process. Photosynthetic pigments were quantified in crude extracts. The results confirm that the maximum rate of gross photosynthesis is correlated with the chlorophyll content of lichens, which is influenced by light as well as by nitrogen availability. Like leaves, shade tolerant lichens emit more ChlaF than sun-loving ones, whereas the photosynthetic quantum conversion is higher in the latter.

Measuring rates of gross photosynthesis and net community production in cryoconite holes: a comparison of field methods

AbstractPhotosynthesis by microbes on the surfaces of glaciers and ice sheets has the potential to fix carbon, alter the albedo of ice surfaces via the production of organic matter and so enhance ice melt. It could also be important for supplying labile organic matter and nutrients to in situ and downstream ecosystems. This study compares in situ 24 hour incubation methods for measuring rates of gross photosynthesis, respiration and net community production (NCP) in cryoconite holes on three Svalbard valley glaciers. Rates of gross photosynthesis and respiration measured by the ΔCO2 method were closely balanced, resulting in rates of NCP close to the detection limit (mean of –1.3 μg C g−1 d–1) consistent with previous measurements in Arctic cryoconite holes. This suggests that organic matter within cryoconite holes may be derived largely from allochthonous sources. The molar ratio of ΔO2 to ΔCO2 in incubations gave mean respiratory and photosynthetic quotients of 0.80 ± 0.17 (1 × SD) and 1.24 ± 0.20 (1 × SD), respectively. The 14C method typically underestimated rates of gross photosynthesis (ΔCO2 method) by more than one order of magnitude and measured a rate closer to NCP.