Why the free floating macrophyte Stratiotes aloides mainly grows in highly CO2-supersaturated waters

We examined how the freely floating macrophyte, Stratiotes aloides L., sampled from a CO₂-supersaturated pond, changes leaf morphology, photosynthesis and inorganic carbon acquisition during its different submerged and emerged life stages in order to evaluate whether S. aloides requires consistently supersaturated CO₂ conditions to grow and complete its life cycle. Submerged rosettes formed from over-wintering turions had typical traits of submerged plants with high specific leaf area and low chlorophyll a concentrations. Emergent leaf parts of mature, floating specimens had typical terrestrial traits with stomata, low specific leaf area and high chlorophyll a content, while offsets formed vegetatively and basal, submerged parts of mature plants showed traits in between. All submerged leaf types exhibited some ability to use HCO₃⁻ but only rosettes formed from turions had efficient HCO₃⁻ use. Rosettes also had the highest CO₂ affinity and maximum CO₂-saturated photosynthesis in water. Half-saturation constants for CO₂ (21–74 μM CO₂) were for all submerged leaf parts 5–140 times lower than the concentrations of free CO₂ in the pond (350–2800 μM CO₂). Emergent leaves were less efficient in water but had significantly higher photosynthesis than submerged, mature leaf parts in air, and rates of photosynthesis of emergent leaves in air were three to five times higher than rates of CO₂-saturated photosynthesis of the three submerged leaf types in water. Underwater photosynthetic rates estimated at CO₂ concentrations corresponding to air equilibrium were not sufficiently high to support any noticeable growth except for rosettes, in which bicarbonate utilization combined with high CO₂ affinity resulted in photosynthetic rates corresponding to almost 34% of maximum rates at high free CO₂. We conclude that S. aloides requires consistently high CO₂-supersaturation to support high growth and to complete its life cycle, and we infer that this requirement explains why S. aloides mainly grows in ponds, ditches and reed zones that are characterized by strong CO₂-supersaturation.     

Does long-term cultivation of saplings under elevated CO2 concentration influence their photosynthetic response to temperature?

Background and Aims Plants growing under elevated atmospheric CO₂ concentrations often have reduced stomatal conductance and subsequently increased leaf temperature. This study therefore tested the hypothesis that under long-term elevated CO₂ the temperature optima of photosynthetic processes will shift towards higher temperatures and the thermostability of the photosynthetic apparatus will increase.Methods The hypothesis was tested for saplings of broadleaved Fagus sylvatica and coniferous Picea abies exposed for 4–5 years to either ambient (AC; 385 µmol mol⁻¹) or elevated (EC; 700 µmol mol⁻¹) CO₂ concentrations. Temperature response curves of photosynthetic processes were determined by gas-exchange and chlorophyll fluorescence techniques.Key Results Initial assumptions of reduced light-saturated stomatal conductance and increased leaf temperatures for EC plants were confirmed. Temperature response curves revealed stimulation of light-saturated rates of CO₂ assimilation (A_max) and a decline in photorespiration (R_L) as a result of EC within a wide temperature range. However, these effects were negligible or reduced at low and high temperatures. Higher temperature optima (T_opt) of A_max, Rubisco carboxylation rates (V_Cmax) and R_L were found for EC saplings compared with AC saplings. However, the shifts in T_opt of A_max were instantaneous, and disappeared when measured at identical CO₂ concentrations. Higher values of T_opt at elevated CO₂ were attributed particularly to reduced photorespiration and prevailing limitation of photosynthesis by ribulose-1,5-bisphosphate (RuBP) regeneration. Temperature response curves of fluorescence parameters suggested a negligible effect of EC on enhancement of thermostability of photosystem II photochemistry.Conclusions Elevated CO₂ instantaneously increases temperature optima of A_max due to reduced photorespiration and limitation of photosynthesis by RuBP regeneration. However, this increase disappears when plants are exposed to identical CO₂ concentrations. In addition, increased heat-stress tolerance of primary photochemistry in plants grown at elevated CO₂ is unlikely. The hypothesis that long-term cultivation at elevated CO₂ leads to acclimation of photosynthesis to higher temperatures is therefore rejected. Nevertheless, incorporating acclimation mechanisms into models simulating carbon flux between the atmosphere and vegetation is necessary.     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.     

Carbon dioxide diffusion across stomata and mesophyll and photo-biochemical processes as affected by growth CO2 and phosphorus nutrition in cotton

Nutrients such as phosphorus may exert a major control over plant response to rising atmospheric carbon dioxide concentration (CO₂), which is projected to double by the end of the 21st century. Elevated CO₂ may overcome the diffusional limitations to photosynthesis posed by stomata and mesophyll and alter the photo-biochemical limitations resulting from phosphorus deficiency. To evaluate these ideas, cotton (Gossypium hirsutum) was grown in controlled environment growth chambers with three levels of phosphate (Pi) supply (0.2, 0.05 and 0.01 mM) and two levels of CO₂ concentration (ambient 400 and elevated 800 μmol mol⁻¹) under optimum temperature and irrigation. Phosphate deficiency drastically inhibited photosynthetic characteristics and decreased cotton growth for both CO₂ treatments. Under Pi stress, an apparent limitation to the photosynthetic potential was evident by CO₂ diffusion through stomata and mesophyll, impairment of photosystem functioning and inhibition of biochemical process including the carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxyganase and the rate of ribulose-1,5-bisphosphate regeneration. The diffusional limitation posed by mesophyll was up to 58% greater than the limitation due to stomatal conductance (g_s) under Pi stress. As expected, elevated CO₂ reduced these diffusional limitations to photosynthesis across Pi levels; however, it failed to reduce the photo-biochemical limitations to photosynthesis in phosphorus deficient plants. Acclimation/down regulation of photosynthetic capacity was evident under elevated CO₂ across Pi treatments. Despite a decrease in phosphorus, nitrogen and chlorophyll concentrations in leaf tissue and reduced stomatal conductance at elevated CO₂, the rate of photosynthesis per unit leaf area when measured at the growth CO₂ concentration tended to be higher for all except the lowest Pi treatment. Nevertheless, plant biomass increased at elevated CO₂ across Pi nutrition with taller plants, increased leaf number and larger leaf area.     

C4 photosynthetic carbon metabolism in the leaves of aromatic tropical grasses — I. Leaf anatomy,CO2 compensation point and CO2 assimilation

A few species of Cymbopogon and Vetiveria are potentially important tropical grasses producing essential oils. In the present study, we report on the leaf anatomy and photosynthetic carbon assimilation in five species of Cymbopogon and Vetiveria zizanioides. Kranz-type leaf anatomy with a centrifugal distribution of chloroplasts and exclusive localization of starch in the bundle sheath cells were common among the test plants. Besides the Kranz leaf anatomy, these grasses displayed other typical C₄ characteristics including a low (0–5 µl/l) CO₂ compensation point, lack of light saturation of CO₂ uptake at high photon flux densities, high temperature (35°C) optimum of net photosynthesis, high rates of net photosynthesis (55–67 mg CO₂ dm^-2 leaf area h^-1), little or no response of net photosynthesis to atmospheric levels of O₂ and high leaf ¹³C/¹²C ratios. The biochemical studies with ¹⁴CO₂ indicated that the leaves of the above plant species synthesize predominantly malate during short term (5 s) photosynthesis. In pulse-chase experiments it was shown that the synthesis of 3-phosphoglycerate proceeds at the expense of malate, the major first formed product of photosynthesis in these plant species.     

Rapid isolation of mesophyll cells from leaves of soybean for photosynthetic studies

Mesophyll cells were rapidly isolated from soybean (Glycine max [L.]) leaves using a combined Macerase enzyme-stirring technique. About 50% to 70% of the leaf cells on a chlorophyll basis from 3 grams of leaves could be isolated in 15 minutes. The cells obtained by this method were capable of high rates of photosynthesis even after storage in the dark for periods of up to 9 hours. The CO₂-saturated rate of photosynthesis increased from 5 μm CO₂/mg Chl·hour at 5 C to 170 μm CO₂/mg Chl·hour at 40 C. At atmospheric CO₂ concentration, the rate varied from 5 to 55 μm CO₂/mg Chl·hour over this temperature range. The reduced temperature response of photosynthesis at low CO₂ concentration was due to an increased Km(CO₂) of the cells with increasing temperature. The products of photosynthesis in the isolated cells were similar to the products of leaf photosynthesis.     

Effects of CO2 concentration on growth of filamentous algae and Littorella uniflora in a Danish softwater lake

An experiment was conducted from May to November in Lake Hampen, Denmark, to study the effect of higher CO₂ concentration on the biomass of filamentous algae. Three enclosures (1.5 m diameter) were enriched with free CO₂ to ∼10 times atmospheric equilibrium (∼170 μM) and three enclosures were kept at atmospheric equilibrium (∼17 μM). The isoetid Littorella uniflora dominated the vegetation in the enclosures. Low concentrations of nitrate and phosphate in the water were observed, especially in the summer months. During the summer, a high biomass of filamentous algae (dominated by Zygnema sp.) developed in both types of enclosures (18–58 g dry wt. m⁻² in July and August), but the biomass of algae was significantly higher (1.9–38 times) in the CO₂ enriched enclosures than in enclosures with low CO₂ concentration. L. uniflora biomass, especially leaf biomass, also showed a significant positive response to increased CO₂ concentration (75.0 ± 10.4 and 133.3 ± 42.5 g dry wt. m⁻² at low and high CO₂ concentrations, respectively) even though the massive filamentous algal growth decreased the light intensity. Both filamentous algae (in August) and L. uniflora showed lower tissue concentrations of N and P at high CO₂ concentration.     

Responses of Gmelina arborea, a tropical deciduous tree species, to elevated atmospheric CO2: Growth, biomass productivity and carbon sequestration efficacy

The photosynthetic response of trees to rising CO₂ concentrations largely depends on source-sink relations, in addition to differences in responsiveness by species, genotype, and functional group. Previous studies on elevated CO₂ responses in trees have either doubled the gas concentration (>700 μmol mol⁻¹) or used single large addition of CO₂ (500-600 μmol mol⁻¹). In this study, Gmelina arborea, a fast growing tropical deciduous tree species, was selected to determine the photosynthetic efficiency, growth response and overall source-sink relations under near elevated atmospheric CO₂ concentration (460 μmol mol⁻¹). Net photosynthetic rate of Gmelina was ∼30% higher in plants grown in elevated CO₂ compared with ambient CO₂-grown plants. The elevated CO₂ concentration also had significant effect on photochemical and biochemical capacities evidenced by changes in F_V/F_M, ABS/CSm, ET₀/CSm and RuBPcase activity. The study also revealed that elevated CO₂ conditions significantly increased absolute growth rate, above ground biomass and carbon sequestration potential in Gmelina which sequestered ∼2100 g tree⁻¹ carbon after 120 days of treatment when compared to ambient CO₂-grown plants. Our data indicate that young Gmelina could accumulate significant biomass and escape acclimatory down-regulation of photosynthesis due to high source-sink capacity even with an increase of 100 μmol mol⁻¹ CO₂.     

Cotton bracts are adapted to a microenvironment of concentrated CO2 produced by rapid fruit respiration

Background and Aims

Elucidation of the mechanisms by which plants adapt to elevated CO₂ is needed; however, most studies of the mechanisms investigated the response of plants adapted to current atmospheric CO₂. The rapid respiration rate of cotton (Gossypium hirsutum) fruits (bolls) produces a concentrated CO₂ microenvironment around the bolls and bracts. It has been observed that the intercellular CO₂ concentration of a whole fruit (bract and boll) ranges from 500 to 1300 µmol mol⁻¹ depending on the irradiance, even in ambient air. Arguably, this CO₂ microenvironment has existed for at least 1·1 million years since the appearance of tetraploid cotton. Therefore, it was hypothesized that the mechanisms by which cotton bracts have adapted to elevated CO₂ will indicate how plants will adapt to future increased atmospheric CO₂ concentration. Specifically, it is hypothesized that with elevated CO₂ the capacity to regenerate ribulose-1,5-bisphosphate (RuBP) will increase relative to RuBP carboxylation.

Methods

To test this hypothesis, the morphological and physiological traits of bracts and leaves of cotton were measured, including stomatal density, gas exchange and protein contents.

Key results

Compared with leaves, bracts showed significantly lower stomatal conductance which resulted in a significantly higher water use efficiency. Both gas exchange and protein content showed a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio (J_max/V_cmax) in bracts than in leaves.

Conclusions

These results agree with the theoretical prediction that adaptation of photosynthesis to elevated CO₂ requires increased RuBP regeneration. Cotton bracts are readily available material for studying adaption to elevated CO₂.     

Regulation of photosynthetic rates of submerged rooted macrophytes

Summary Fourteen temperate, submerged macrophytes were cultivated in the laboratory at high DIC levels (3.3–3.8 mM), 10.4–14.4 mol photons (PAR) m^-2 d^-1 and 15°C. Photosynthesis at photosaturation ranged between 0.59 and 17.98 mg O₂ g^-1 DW h^-1 at ambient pH (8.3) and were markedly higher between 1.76 and 47.11 mg O₂ g^-1 DW h^-1 at pH 6.5 under elevated CO₂ concentrations. Photosynthetic rates were significantly related to both the relative surface area and the chlorophyll content of the leaves. Consequently, the photosynthetic rate was much less variable among the species when expressed per surface area and chlorophyll content instead of dry mass. The chlorophyll content was probably a main predictor of photosynthesis of submerged leaves because of the direct relationship of chlorophyll to the light harvesting capacity and/or a coupling to the capacity for photosynthetic electron transport and carboxylation. A comparison with terrestrial leaves characterized the submerged leaves by their low chlorophyll concentrations and low photosynthetic rates per surface area due to the thin leaves. Photosynthetic rates per chlorophyll content in submerged leaves at CO₂ saturation, however, were at the same level as photosynthesis in terrestrial leaves measured at ambient CO₂ when appropriate corrections were made for differences in incubation temperature.     

Plasticity in carbon acquisition of the heterophyllous Luronium natans: An endangered freshwater species in Europe

Luronium natans (L.) Raf. (Floating Water-plantain) is an endangered amphibious freshwater species endemic to Europe. We examined the plasticity in carbon acquisition and photosynthesis in L. natans to assess if lack of plasticity could contribute to explain the low competitive ability of the species. The plasticity of photosynthesis in submerged leaves towards inorganic carbon availability was examined and the photosynthesis of submerged, floating and aerial leaves was contrasted. L. natans was shown to be plastic in inorganic carbon uptake, as it was able to effectively acclimate to changed concentrations of free-CO₂. The photosynthetic apparatus was down-regulated in plants grown at high CO₂. Chlorophyll concentration, Rubisco activity and maximum photosynthesis were significantly lower in submerged leaves of plants grown at high CO₂ (200 μM free-CO₂) compared to plants grown at low CO₂ (18 μM free-CO₂). Furthermore, bicarbonate utilization was down-regulated in response to high CO₂. Carbon acquisition of submerged, floating and aerial leaves of L. natans differed significantly. The aerial leaves were superior in photosynthesising in air and, surprisingly, the floating leaves had the highest rates of photosynthesis in water. The study did not support the hypothesis that the low competitive ability of L. natans is caused by inefficient photosynthesis or a lack of plasticity in photosynthesis. However, the somewhat low photosynthetic performance of the submerged leaves may be a contributing factor.     

Chlorophyll fluorescence parameters,CO2 photosynthetic rate and regeneration capacity as a result of complete submergence and subsequent re-emergence in rice (Oryza sativa L.)

Regeneration capacity of submerged rice (Oryza sativa) seedlings in terms of CO₂ photosynthetic rate, chlorophyll a fluorescence and chlorophyll and carbohydrate content were investigated in three Indica rice cultivars namely FR 13A, Kalaputia and IR 42 that differed in submergence tolerance. Twenty-one day old plants were completely submerged under water for 8 days. Subsequently, plants were kept under normal conditions with 5–10 cm of stagnant water above soil surface for a further period of 15 days. After complete submergence, all genotypes showed inhibition of CO₂ photosynthetic rate. Submergence treatment resulted in a significant reduction of Rubisco activity. Maximal photochemical efficiency (F_v/F_m) of PS II and area above the fluorescence curve between F_o and F_m decreased more under submergence especially in susceptible cultivar IR 42. When re-aerated, the plants recovered to various degrees. The carbohydrate content of plants was found to be significantly and positively associated with submergence tolerance and regeneration growth. The tolerant cultivar (FR 13A) could survive submergence apparently because it possessed 1.9–2.0 and 3.2–3.7-fold more non-structural carbohydrate content before and after submergence compared to the susceptible cultivar (IR 42) and it had a better capability to restore its photosynthetic capacity during post-submergence periods.     

In vitro enzyme activities and products of 14CO2 assimilation in flag leaf and ear parts of wheat (Triticum aestivum L.)

Activities of key enzymes of Calvin cycle and C₄ metabolism, rate of ¹⁴CO₂ fixation in light and dark and the initial products of photosynthetic ¹⁴CO₂ fixation were determined in flag leaf and different ear parts of wheat viz. pericarp, awn and glumes. Compared to the activities of RuBP carboxylase and other Calvin cycle enzymes viz. NADP-glyceraldehyde-3-phosphate dehydrogenase, NAD-glyceraldehyde-3-phosphate dehydrogenase and ribulose-5-phosphate kinase, the levels of PEP carboxylase and other enzymes of C₄ metabolism viz. NADP-malate dehydrogenase, NAD-malate dehydrogenase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase genase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase, were generally greater in ear parts than in the flag leaf. In contrast to CO₂ fixation in light, the various ear parts incorporated CO₂ in darkness at much higher rates than flag leaf. In short term assimilation of ¹⁴CO₂ by illuminated ear parts, most of the ¹⁴C was in malate with less in 3-phosphoglyceric acid, whereas flag leaves incorporated most into 3-phosphoglyceric acid. It seems likely that ear parts have the capability of assimilating CO₂ by the C₄ pathway of photosynthesis and utilise PEP carboxylase for recapturing the respired CO₂.     

Soybean leaf hydraulic conductance does not acclimate to growth at elevated [CO2] or temperature in growth chambers or in the field

Background and Aims

Leaf hydraulic properties are strongly linked with transpiration and photosynthesis in many species. However, it is not known if gas exchange and hydraulics will have co-ordinated responses to climate change. The objective of this study was to investigate the responses of leaf hydraulic conductance (K_leaf) in Glycine max (soybean) to growth at elevated [CO₂] and increased temperature compared with the responses of leaf gas exchange and leaf water status.

Methods

Two controlled-environment growth chamber experiments were conducted with soybean to measure K_leaf, stomatal conductance (g_s) and photosynthesis (A) during growth at elevated [CO₂] and temperature relative to ambient levels. These results were validated with field experiments on soybean grown under free-air elevated [CO₂] (FACE) and canopy warming.

Key results

In chamber studies, K_leaf did not acclimate to growth at elevated [CO₂], even though stomatal conductance decreased and photosynthesis increased. Growth at elevated temperature also did not affect K_leaf, although g_s and A showed significant but inconsistent decreases. The lack of response of K_leaf to growth at increased [CO₂] and temperature in chamber-grown plants was confirmed with field-grown soybean at a FACE facility.

Conclusions

Leaf hydraulic and leaf gas exchange responses to these two climate change factors were not strongly linked in soybean, although g_s responded to [CO₂] and increased temperature as previously reported. This differential behaviour could lead to an imbalance between hydraulic supply and transpiration demand under extreme environmental conditions likely to become more common as global climate continues to change.     

To Float or Not to Float: How Interactions between Light and Dissolved Inorganic Carbon Species Determine the Buoyancy of Stratiotes aloides

Structural diversity formed by dense, floating Stratiotes aloides stands, generates hotspots of biodiversity of flora and fauna in wetlands. However, only part of the populations become emergent and provide this important facilitation. Since it has been hypothesised that its buoyancy depends on the rates of underwater photosynthesis, we investigated the role of dissolved CO₂ availability and PAR on photosynthesis, biomass production and buoyancy in a controlled greenhouse experiment. Photosynthesis and growth were strongly influenced by both PAR and CO₂ availability. At low PAR, plants formed less biomass and produced no emergent leaves, even when CO₂ was abundant. At low CO₂ levels, S. aloides switched to HCO₃ ^- use, resulting in a lower photosynthetic O₂ production, decreased emergent leaf formation and increased CaCO₃ precipitation on its leaves, all of which impaired buoyancy. At high PAR, low CO₂ availability resulted in slower colonisation of the water layer, whereas CO₂ availability did not influence PAR-limited plants. Our study shows that site conditions, rather than the sole abundance of potentially facilitating species, may strongly determine whether or not they form the structure necessary to act as a facilitator for biodiversity in aquatic environments.     

Photosynthetic characteristics of mesophyll eells isolated from sunflower (helianthus annuus L.) leaves

Mesophyll cells were isolated from sunflower leaves by an enzymic procedure. The cell suspensions possessed high photosynthesis rates. The products of cell photosynthesis were similar to the products of leaf disc photosynthesis. The relatively high radioactivity incorporated into malate after ¹⁴CO₂ feeding suggests that PEP carboxylase might participate in CO₂ fixation. Sunflower leaf extracts possessed a PEP carboxylase activity slightly higher than that of other C₃ species. Inhibition of PEP carboxylase by maleate decreased cell photosynthesis by only 15% and the first products of cell photosynthesis were phosphorylated compounds. It is concluded that the high photosynthesis rates displayed by sunflower are not due to a parallel C₄ pathway of photosynthesis but are rather dependent, at least in part, on the activity, or the amount, of RuBP carboxylase.Abbreviations PVP polyvinylpyrrolidone - PDS potassium dextran sulfate - DTT dithiothreitol - PEG polyethyleneglycol - RuBP ribulose 1,5-bisphosphate - PEP phosphoenolpyruvate - Mes 2-(N-morpholino) ethanesulfonic acid - Hepes N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid     

Environmental factors regulating the radial oxygen loss from roots of Myriophyllum spicatum and Potamogeton crispus

Myriophyllum spicatum and Potamogeton crispus are common species of shallow eutrophic lakes in north-eastern Germany, where a slow recovery of the submersed aquatic vegetation was observed. Thus, the characterisation of the root oxygen release (ROL) as well as its implication for geochemical processes in the sediment are of particular interest. A combination of microelectrode measurements, methylene blue agar and a titanium(III) redox buffer was used to investigate the influence of the oxygen content in the water column on ROL, diel ROL dynamics as well as the impact of sediment milieu. Oxygen gradients around the roots revealed a maximum oxygen diffusion zone of up to 250 μm. During a sequence with a light/dark cycle as well as alternating aeration of the water column, maximum ROL with up to 35% oxygen saturation at the root surface occurred under light/O₂-saturated conditions. A decrease to about 30% was observed under dark/O₂-saturated conditions, no ROL was detected at dark/O₂-depleted conditions and only a weak ROL with 5–10% oxygen saturation at the root surface was measured under light but O₂-depleted water column. These results indicate, that during darkness, ROL is supplied by oxygen from the water column and even during illumination and active photosynthesis production, ROL is modified by the oxygen content in the water column. Visualisation of ROL patterns revealed an enhanced ROL for plants which were grown in sulfidic littoral sediment in comparison to plants grown in pure quartz sand. For both plant species grown in sulfidic littoral sediment, a ROL rate of 3–4 μmol O₂ h⁻¹ plant⁻¹ was determined with the Ti(III) redox buffer. For plants grown in pure quartz sand, the ROL rate decreased to 1–2 μmol O₂ h⁻¹ plant⁻¹. Hence, aside from the oxygen content in the water column, the redox conditions and microbial oxygen demand in the sediment has to be considered as a further major determinant of ROL.     

Gas exchange and photosynthetic performance of the tropical tree <Emphasis Type="Italic">Acacia nigrescens</Emphasis> when grown in different CO<Subscript>2</Subscript> concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO₂ concentrations—from sub-ambient to super-ambient—have been studied. Light-saturated rates of net photosynthesis (A _sat) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 μL L⁻¹) to current ambient (386 μL L⁻¹) CO₂ growth conditions but did not increase any further as [CO₂] became super-ambient (597 μL L⁻¹). Examination of photosynthetic CO₂ response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A _sat seen was a consequence of growth [CO₂] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO₂ but could be explained by current models of photosynthesis.     

Acclimation of Photosynthesis to Elevated CO(2) in Five C(3) Species

The effect of long-term (weeks to months) CO₂ enhancement on (a) the gas-exchange characteristics, (b) the content and activation state of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) leaf nitrogen, chlorophyll, and dry weight per area were studied in five C₃ species (Chenopodium album, Phaseolus vulgaris, Solanum tuberosum, Solanum melongena, and Brassica oleracea) grown at CO₂ partial pressures of 300 or 900 to 1000 microbars. Long-term exposure to elevated CO₂ affected the CO₂ response of photosynthesis in one of three ways: (a) the initial slope of the CO₂ response was unaffected, but the photosynthetic rate at high CO₂ increased (S. tuberosum); (b) the initial slope decreased but the CO₂-saturated rate of photosynthesis was little affected (C. album, P. vulgaris); (c) both the initial slope and the CO₂-saturated rate of photosynthesis decreased (B. oleracea, S. melongena). In all five species, growth at high CO₂ increased the extent to which photosynthesis was stimulated following a decrease in the partial pressure of O₂ or an increase in measurement CO₂ above 600 microbars. This stimulation indicates that a limitation on photosynthesis by the capacity to regenerate orthophosphate was reduced or absent after acclimation to high CO₂. Leaf nitrogen per area either increased (S. tuberosum, S. melongena) or was little changed by CO₂ enhancement. The content of rubisco was lower in only two of the five species, yet its activation state was 19% to 48% lower in all five species following long-term exposure to high CO₂. These results indicate that during growth in CO₂-enriched air, leaf rubisco content remains in excess of that required to support the observed photosynthetic rates.