Carbon dioxide exchange characteristics of C4 Hawaiian Euphorbia species native to diverse habitats

Summary The characteristics of the photosynthetic apparatus of 11 Hawaiian Euphorbia species, all of which possess C₄ photosynthesis but range from arid habitat, drought-deciduous shrubs to mesic or wet forest evergreen trees and shrubs, were investigated under uniform greenhouse conditions. Nine species exhibited CO₂ response curves typical of C₄ plants, but differed markedly in photosynthetic capacity. Light-saturated CO₂ uptake rates ranged from 48 to 52 mol m^-2 s^-1 in arid habitat species to 18 to 20 mol m^-2 s^-1 in mesic and wet forest species. Two possessed unusual CO₂ response curves in which photosynthesis was not saturated above intercellular CO₂ pressures [p(CO₂)] of 10 to 15 Pa, as typically occurs in C₄ plants.Both leaf (g₁) and mesophyll (g_m) conductances to CO₂ varied widely between species. At an atmospheric p(CO₂) of 32 Pa, g₁ regulated intercellular p(CO₂) at 12–15 Pa in most species, which supported nearly maximum CO₂ uptake rates, but did not result in excessive transpiration. Intercellular p(CO₂) was higher in the two species with unusual CO₂ response curves. This was especially apparent in E. remyi, which is native to a bog habitat. The regulation of g₁ and intercellular p(CO₂) yielded high photosynthetic water use efficiencies (P/E) in the species with typical CO₂ response curves, whereas P/E was much lower in E. remyi.Photosynthetic capacity was closely related to leaf nitrogen content, whereas correlations with leaf morphological characteristics and leaf cell surface area were not significant. Thus, differences in photosynthetic capacity may be determined primarily by investment in the biochemical components of the photosynthetic apparatus rather than by differences in diffusion limitations. The lower photosynthetic capacities in the wet habitat species may reflect the lower light availability. However, other factors, such as reduced nutrient availability, may also be important.     

Simultaneous gas exchange and fluorescence measurements indicate differences in the response of sunflower,bean and maize to water stress

Gas exchange and fluorescence measurements of attached leaves of water stressed bean, sunflower and maize plants were carried out at two light intensities (250 mol quanta m^-2s^-1 and 850 mol quanta m^-2s^-1). Besides the restriction of transpiration and CO₂ uptake, the dissipation of excess light energy was clearly reflected in the light and dark reactions of photosynthesis under stress conditions. Bean and maize plants preferentially use non-photochemical quenching for light energy dissipation. In sunflower plants, excess light energy gave rise to photochemical quenching. Autoradiography of leaves after photosynthesis in ¹⁴CO₂ demonstrated the occurrence of leaf patchiness in sunflower and maize but not in bean. The contribution of CO₂ recycling within the leaves to energy dissipation was investigated by studies in 2.5% oxygen to suppress photorespiration. The participation of different energy dissipating mechanisms to quanta comsumption on agriculturally relevant species is discussed.Abbreviations F_o minimal fluorescence - F_m maximal fluorescence - F_p peak fluorescence - g leaf conductance - P_N net CO₂ uptake - q_N coefficient of non-photochemical quenching - q_P coefficient of photochemical quenching     

Leaf photosynthetic characteristics of seedlings of actinorhizal Alnus spp. and Elaeagnus spp.

Single leaf photosynthetic characteristics of Alnus glutinosa, A. incana, A. rubra, Elaeagnus angustifolia, and E. umbellata seedlings conditioned to ambient sunlight in a glasshouse were assessed. Light saturation occurred between 930 and 1400 mol m^-2s^-1 PAR for all species. Maximum rates of net photosynthesis (Pn) measured at 25°C ranged from 12.8 to 17.3 mol CO₂m^-2s^-1 and rates of dark respiration ranged from 0.74 to 0.95 mol CO₂m^-2s^-1. These values of leaf photosynthetic variables are typical of early to midsuccessional species. The rate of Pn measured at optimal temperature (20°C) and 530mol m^-2s^-1 PAR was significantly (p<0.01) correlated with leaf nitrogen concentration (r=0.69) and negatively correlated with the mean area of a leaf (r=–0.64). We suggest that the high leaf nitrogen concentration and rate of Pn observed for Elaeagnus umbellata and to a lesser degree for E. angustifolia are genetic adaptations related to their crown architecture.Abbreviations Pn net photosynthesis     

The analysis of photosynthesis in air and water of Ascophyllum nodosum (L.) Le Jol.

Summary The photosynthetic characteristics for the intertidal macroalga Ascophyllum nodosum were examined in air and water. Under ambient conditions of temperature (10° C) inorganic carbon concentrations (15.63 mmol CO₂ m^-3 or 2.0 mol TIC m^-3) and light (500 mol photons m^-2 s^-1) photosynthesis was slightly greater by the exposed alga than by the submerged alga. In both environments photosynthesis was light saturated at 200 mol photons m^-2 s^-1. The relationship between CO₂ concentration and photosynthesis in air could be accurately analysed using Michaelis-Menten kinetics, although the range of concentrations used were not saturating. In contrast the application of the Lineweaver-Burk and Woolf plots to aquatic photosynthesis was not suitable as the experimental data was similar to the Blackman type curves and not rectangular hyperbolae. This was reflected by the applicability of the Hill-Whittingham equation to describe the photosynthesis curves. The effect of unstirred layers and other limiting factors is discussed in relation to the kinetic parameters, V _max and K _m.     

The specific radioactivity of glycolic acid in relation to the specific activity of carbon dioxide evolved in light in photosynthesizing sunflower leaves

Attached leaves of sunflower (Helianthus annuus L.) were exposed to ¹⁴CO₂ during steady-state photosynthesis for 2 to 30 min in 345 l/l CO₂ and 21% O₂ at 29° C and a light intensity of 1300 E m^-2s^-1. Glycolic acid was extracted with water and diethyl ether, and was determined in the aqueous residue by high-pressure liquid column chromatography. The relative specific radioactivity of the glycolic acid synthesized during photosynthesis reached about 100% after 30 min of photosynthesis and was almost equal to that of the CO₂ evolved during photorespiration, their ratio at all times being nearly one. These results provide strong in-vivo evidence that the glycolic acid is the substrate for CO₂ evolved by sunflower leaves in light.     

Carbon dioxide exchange of C3 and C4 tree species in the understory of a Hawaiian forest

Summary Field measurements of photosynthetic CO₂ exchange were made on saplings of a C₄ tree species, Euphorbia forbesii, and a C₃ tree species, Claoxylon sandwicense, in a shaded mesic forest on Oahu, Hawaii. Both species had light responses typical of those generally found in shade plants. Light saturated photosynthetic rates were 7.15 and 4.09 mol m² s¹ and light compensation points were 6.3 and 1.7 mol m² s¹ in E. forbesii and C. sandwicense, respectively. E. forbesii maintained a higher mesophyll conductance and a higher water use efficiency than C. sandwicense as is typically found in comparisons of C₄ and C₃ plants. Under natural light regimes, both species maintained positive CO₂ uptake rates over essentially the entire day because of low respiration rates and light compensation points. However, photosynthesis during sunflecks accounted for a large fraction of the daily carbon gain. The results show that the carbon-gaining capacity of E. forbesii is comparable to that of a C₃ species in a moderately cool, shaded forest environment. There appears to be no particular advantage or disadvantage associated with the C₄ photosynthetic pathway of E. forbesii in this environment.     

Seasonal changes in CO2 and H2O gas exchange of young European beech (Fagus sylvatica L.)

Summary The CO₂ and H₂O gas exchange of young beech trees (Fagus sylvatica L.) were measured over a growing season. Of particular interest was the adaptation of gas exchange to the low level of photon flux density in the understorey of the old beech. The recorded diurnal courses were subdivided into several classes of irradiance. The most frequent class was from only 30–40 E * m^-2 * s^-1. Even at the highest irradiance values, no light saturation in assimilation occurred. The light compensation point lies below 3 E * m^-2 * s^-1, because net dark respiration values are very low. Calculated from the initial slope of the light response curves a mean value of 0.02 mol CO₂ * mol photons^-1 shows a very efficient use of light be the young trees. At the optimal phase of assimilation, the relationship between the daily sum of irradiance and net photosynthesis is highly significantly correlated. Under the local climatic situation, the stomatal opening primarily depends on irradiance. In response to a change in irradiance, stomatal opening also changes rapidly. Therefore, there is only a loose relationship between transpiration rate and vapour pressure saturation deficit. Towards autumn, the transpiration coefficient (E/A-ratio, estimated under light saturation) increases strongly because net photosynthesis decreases simultaneously.     

Analysis of gas exchange in seedlings of Acer saccharum: integration of field and laboratory studies

In the field, photosynthesis of Acer saccharum seedlings was rarely light saturated, even though light saturation occurs at about 100 mol quanta m^-2 s^-1 photosynthetic photon flux density (PPFD). PPFD during more than 75% of the daylight period was 50 mol m^-2 s^-1 or less. At these low PPFD's there is a marked interaction of PPFD with the initial slope (CE) of the CO₂ response. At PPFD-saturation CE was 0.018 mol m^-2 s^-1/(l/l). The apparent quantum efficiency (incident PPFD) at saturating CO₂ was 0.05–0.08 mol/mol. and PPFD-saturated CO₂ exchange was 6–8 mol m^-2 s^-1. The ratio of internal CO₂ concentration to external (C _i /C _a ) was 0.7 to 0.8 except during sunflecks when it decreased to 0.5. The decrease in C _i /C _a during sunflecks was the result of the slow response of stomates to increased PPFD compared to the response of net photosynthesis. An empirical model, which included the above parameters was used to simulate the measured CO₂ exchange rate for portions of two days. Parameter values for the model were determined in experiments separate from the daily time courses being sumulated. Analysis of the field data, partly through the use of simulations, indicate that the elimination of sunflecks would reduce net carbon gain by 5–10%.List of symbols A measured photosynthetic rate under any set of conditions (mol m^-2 s^-1) - A _m (atm) measured photosynthetic rate at saturating PPFD, 350 l/l CO₂ and 21% (v/v) O₂ (mol m^-2 s^-1) - C constant in equation of Smith (1937, 1938) - C _a CO₂ concentration in the air (l/l) - C _i CO₂ concentration in the intercellular air space (l/l) - C _i ^/* C _i corrected for CO₂ compensation point, i.e., C _i -I ^*, (l/l) - CE initial slope of the CO₂ response of photosynthesis (mol m^-2 s^-1/(l/l)) - CEM CE at PPFD saturation - E transpiration rate (mmol m^-2 s^-1) - F predicted photosynthetic rate (mol m^-2 s^-1) - G leaf conductance to H₂O (mol m^-2 s^-1) - I photosynthetic photon flux density (mol m^-2 s^-1) - N number of data points - P _m predicted photosynthetic rate at saturating CO₂ and given PPFD (mol m^-2 s^-1) - P _ml predicted photosynthetic rate at saturating CO₂ and PPFD (mol m^-2 s^-1) - R _d residual respiratory rate (mol m^-2 s^-1) - T _a air temperature (°C) - T _l leaf temperature (°C) - V reaction velocity in equation of Smith (1937, 1938) - V _max saturated reaction velocity in equation of Smith (1937, 1938) - VPA vapor pressure of water in the air (mbar/bar) - VPD vapor pressure difference between leaf and air (mbar/bar) - X substrate concentration in equation of Smith (1937, 1938) - initial slope of the PPFD response of photosynthesis at saturating CO₂ (mol CO₂/mol quanta) - (atm) initial slope of the PPFD response of photosynthesis at 340 l/l CO₂ and 21% (v/v) O₂ (mol CO₂/mol quanta) - I ^* CO₂ compensation point after correction for residual respiration (l/l) - PPFD compensation point (mol m^-2 s^-1)     

Responses of C4 grasses to atmospheric CO2 enrichment

Summary The growth and photosynethetic responses to atmospheric CO₂ enrichment of 4 species of C₄ grasses grown at two levels of irradiance were studied. We sought to determine whether CO₂ enrichment would yield proportionally greater growth enhancement in the C₄ grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 l 1^-1 CO₂ and 1,000 or 150 mol m^-2 s^-1 photosynthetic photon flux density (PPFD). An increase in CO₂ concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO₂. Plants grown in CO₂-enriched atmosphere had lower photosynthetic capacity relative to the low CO₂ grown plants when exposed to lower CO₂ concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO₂ compensation point for photosynthesis.     

The effect of different night conditions on the CO2 fixation in a lichen Xanthoria parietina

CO₂ fixation was studied in a lichen, Xanthoria parietina, kept in continuous light, and with cyclic changes in light intensity, dark period or temperature. The diurnal and seasonal courses of CO₂ exchange were followed. The rate of net photosynthesis was observed to fall from morning to evening, and this decline was more pronounced in winter than in summer. The maximal net photosynthetic rate, 223 ng CO₂g^-1dws^-1, occured in winter and the minimum, 94 ng CO₂g^-1dws^-1, late in spring. The light compensation point in summer was four times as high as in winter. In continuous light (180 or 90 mol photons m^-2s^-1, 15°C) net photosynthesis decreased noticeably during one week, falling below the level maintained in a 12 h light: 12 h dark cycle. Photosynthetic activity did not decrease, however, in lichens held in continuous light (90 mol photons m^-2s^-1) with cyclic changes of temperature (12 h 20 °C: 12 h 5 °C). Active photosynthesis was also maintained in light of cyclically changing intensity (12 h: 12 h, 15 °C) when night-time light was at least 75% lower than illumination by day. A dark period of 4 hours in a 24-h light:dark cycle was sufficient to keep CO₂ fixation at the control level. It seems that plants need an unproductive period during the day to survive and this can be induced by fluctuations in light and/or temperature.     

Light-emitting diodes as a light source for photosynthesis research

Light-emitting diodes (LED) can provide large fluxes of red photons and so could be used to make lightweight, efficient lighting systems for photosynthetic research. We compared photosynthesis, stomatal conductance and isoprene emission (a sensitive indicator of ATP status) from leaves of kudzu (Pueraria lobata (Willd) Ohwi.) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. Stomatal conductance was measured to determine if red LED light could sufficiently open stomata. The LEDs produced an even field of red light (peak emission 656±5 nm) over the range of 0–1500 mol m^-2 s^-1. Under ambient CO₂ the photosynthetic response to red light deviated slightly from the response measured in white light and stomatal conductance followed a similar pattern. Isoprene emission also increased with light similar to photosynthesis in white light and red light. The response of photosynthesis to CO₂ was similar under the LED and xenon arc lamps at equal photosynthetic irradiance of 1000 mol m^-2 s^-1. There was no statistical difference between the white light and red light measurements in high CO₂. Some leaves exhibited feedback inhibition of photosynthesis which was equally evident under irradiation of either lamp type. Photosynthesis research including electron transport, carbon metabolism and trace gas emission studies should benefit greatly from the increased reliability, repeatability and portability of a photosynthesis lamp based on light-emitting diodes.     

Productivity and temperature biology of two snowbed bryophytes

Summary Chlorophyll distribution within the carpets, CO₂ gas exchange under controlled conditions, and heat resistance of the snowbed bryophyte Anthelia juratzkana (Limpr.) Trev. were investigated. Also the gas-exchange parameters of the co-occurring Polytrichum sexangulare Floercke were assessed. Only the uppermost 4 mm layer of Anthelia carpets contains sufficient pigments for photosynthesis. At light saturation and optimal temperatures (6–11°C) the maximum rates of CO₂ uptake are 0.7 mg CO₂ g^-1dw h^-1 in Anthelia and 1.5 mg CO₂ g^-1dw h^-1 in Polytrichum. Gas exchange reaches light saturation at about 300 E m^-2s^-1 in both species. At +2°C the light compensation point is reached at ca. 10E m^-2s^-1 and increases significantly with increasing temperature. The lower temperature compensation point is reached at-4°C in Anthelia and does not drop much below-5°C in Polytrichum. Anthelia cannot sustain net photosynthesis beyond 30°C and Polytrichum not beyond 32°C. Nine month storage under dark, cold and wet conditions does not affect the photosynthetic capability of Anthelia. As a response, however, the net photosynthesis rate is depressed due to an increase of the respiration rates. Polytrichum sexangulare did not tolerate the storage so well. The heat resistance limit of Anthelia is low (39°C). There is evidence that the distribution of the two bryophytes within snowbed communities is determined by their capability to make use of low light intensities and their low temperature demand for optimal photosynthetic rates. Being resistant to long lasting cold, wet, and dark conditions, Anthelia is particularly adapted to grow in the border zone along permanent snowpatches. Polytrichum is more productive and is therefore capable of competing successfully at sites which are less extreme and therefore accessible for higher plants.     

Effects of environmental conditions on isoprene emission from live oak

Live-oak plants (Quercus virginiana Mill.) were subjected to various levels of CO₂, water stress or photosynthetic photon flux density to test the hypothesis that isoprene biosynthesis occurred only under conditions of restricted CO₂ availability. Isoprene emission increases as the ambient CO₂ concentration decreased, independent of the amount of time that plants had photosynthesized at ambient CO₂ levels. When plants were water-stressed over a 4-d period photosynthesis and leaf conductance decreased 98 and 94%, respectively, while isoprene emissions remained constant. Significant isoprene emissions occurred when plants were saturated with CO₂, i.e., below the light compensation level for net photosynthesis (100 mol m^-2 s^-1). Isoprene emission rates increased with photosynthetic photon flux density and at 25 and 50 mol m^-2 s^-1 were 7 and 18 times greater than emissions in the dark. These data indicate that isoprene is a normal plant metabolite and not — as has been suggested — formed exclusively in response to restricted CO₂ or various stresses.Abbreviation PPFD photosynthetic photon flux density     

Leaf and canopy photosynthetic CO2 uptake of a stand of Echinochloa polystachya on the Central Amazon floodplain

The C₄ grass Echinochloa polystachya, which forms dense and extensive monotypic stands on the Varzea floodplains of the Amazon region, provides the most productive natural higher plant communities known. The seasonal cycle of growth of this plant is closely linked to the annual rise and fall of water level over the floodplain surface. Diurnal cycles of leaf photosynthesis and transpiration were measured at monthly intervals, in parallel with measurements of leaf area index, canopy light interception and biomass. By artificial manipulation of the light flux incident on leaves in the field light-response curves of photosynthesis at the top and near to the base of the canopy were generated. Fitted light-response curves of CO₂ uptake were combined with information of leaf area index, incident light and light penetration of the canopy to estimate canopy rates of photosynthesis. Throughout the period in which the floodplains were submerged photosynthetic rates of CO₂ uptake (A) for the emergent leaves were high with a mean of c. 30 mol m^-2 s^-1 at mid-day and occasional values of 40 mol m^-2 s^-1. During the brief dry phase, when the floodplain surface is uncovered, there was a significant depression of A, with mid-day mean values of c. 17 mol m^-2 s^-1. This corresponded with a c. 50% decrease in stomatal conductance, and a c. 35% depression in the ratio of the leaf inter-cellular to external CO₂ concentration (c _i/c _a). During the dry phase, a midday depression of rates of CO₂ assimilation was observed. The lowest leaf area index (F) was c. 2 in November–December, when the flood plain was dry, and again in May, when the rising floodwaters were submerging leaves faster than they were replaced. The maximum F of c. 5 was in August when the floodwaters were receding rapidly. Canopy light interception efficiency varied from 0.90 to 0.98. Calculated rates of canopy photosynthesis exceeded 18 mol C m^-2 mo^-1 throughout the period of flooding, with a peak of 37 mol C m^-2 mo^-1 in August, but declined to 13 mol C m^-2 mo^-1 in November during the dry phase. Estimated uptake of carbon by the canopy from the atmosphere, over 12 months, was 3.57 kg C m^-2. This was insufficient to account for the 3.99 kg C m^-2 of net primary production, measured simultaneously by destructive harvesting. It is postulated that this discrepancy might be accounted for by internal diffusion of CO₂ from the CO₂-rich waters and sediments via the roots and stems to the sites of assimilation in the leaves.     

Crassulacean acid metabolism in the shade. Studies on an epiphytic fern,Pyrrosia longifolia,and other rainforest species from Australia

Summary Crassulacean acid metabolism (CAM) was studied in a tropical epiphytic fern, Pyrrosia longifolia, from a fully sun-exposed and from a very shaded site in Northern Queensland, Australia. Measurements of instantaneous net CO₂ exchange showed carbon gain via CO₂ dark fixation with some net CO₂ uptake also occuring during late afternoon, in both sun and shade fronds. Maximum rates of net CO₂ uptake and the nocturnal increase in titratable acidity were lower in shade than in sun fronds. ¹³C values of sun and shade fronds were not significantly different, and ranged between-14 and-15 suggesting that, in the long term, carbon gain was mainly via CO₂ dark fixation. Sun fronds had a higher light compensation point of photosynthesis than shade fronds but the same quantum yield. Yet there was no acclimation of photosynthetic O₂ evolution, (measured at 5% CO₂) in sun and shade fronds and photosynthesis saturated at between 200 and 400 mol quanta m^-2 s^-1. Use of higher light intensities for photosynthesis of sun fronds was probably precluded by low nutrient availability. Total nitrogen was less than 1% of dry weight in fully expanded sun and shade fronds. Exposure of shade fronds to full sunlight for 6 h led to a 60% decline in the quantum yield of photosynthesis and to a decline in variable fluorescence measured at room temperature. Photoinhibition by high light was also observed in Hoya nicholsoniae, a rainforest climber growing in deep shade. This species also exhibited CAM as demonstrated by nocturnal net CO₂ uptake, nocturnal acidification and a ¹³C value of-14. Photosynthetic O₂ evolution in this species was saturated at 2.5% of full sunlight. Two species of Dendrobium (Orchidaceae) from sun-exposed sites, one species exhibiting CAM and the other one exhibiting net CO₂ uptake exclusively during daytime via conventional C₃ photosynthesis, showed similar light response curves and the same quantum yield for photosynthetic O₂ evolution.     

CO2 environment,microclimate and photosynthetic characteristics of the moss Hylocomium splendens in a subarctic habitat

Summary In order to document the natural CO₂ environment of the moss Hylocomium splendens, and ascertain whether or not the moss was adapted to this, and its interactions with other microenvironmental factors, two studies were carried out. Firstly, the seasonal variations of CO₂ concentration, photosynthetically active radiation (PAR), tissue water content and temperature were measured in the natural microenvironment of H. splendens in a subarctic forest during the summer period (July–September). Secondly, the photosynthetic responses of the species to controlled CO₂ concentrations, PAR, temperature, and hydration were measured in the laboratory. CO₂ concentrations around the upper parts of the plant, when PAR was above the compensation point (30 mol m^–2 s^–1), were mostly between 400 and 450 ppm. They occasionally increased up to 1143 ppm for short periods. PAR flux densities below saturating light levels for photosynthesis (100 mol m^–2 s^–1), occurred during 65% (July), 76% (August) and 96% (September) of the hours of the summer period. The temperature optimum of photosynthesis was 20° C: this temperature coincided with PAR above the compensation point during 5%, 6% and 0% of the time in July, August and September, respectively. Optimal hydration of tissues was infrequent. Hence PAR, temperature and water limit CO₂ uptake for most of the growing season. Our data suggest that the higher than normal ambient CO₂ concentration in the immediate environment of the plant counteracts some of the limitations in PAR supply that it experiences in its habitat. This species already experiences concentrations of atmospheric CO₂ predicted to occur over the next 50 years.     

Predicting CO2 gain and photosynthetic light acclimation from fluorescence yield and quenching in cyano-lichens

Modulated chlorophylla fluorescence is useful for eco-physiological studies of lichens as it is sensitive, non-invasive and specific to the photobiont. We assessed the validity of using fluorescence yield to predict CO₂ gain in cyano-lichens, by simultaneous measurements of CO₂ gas exchange and chlorophylla fluorescence in five species withNostoc-photobionts. For comparison, O₂ evolution and fluorescence were measured in isolated cells ofNostoc, derived fromPeltigera canina (Nostoc PC). At irradiances up to the growth light level, predictions from fluorescence yield underestimated true photosynthesis, to various extents depending on species. This reflected the combined effect of a state transition in darkness, which was not fully relaxed until the growth light level was reached, and a phycobilin contribution to the minimum fluorescence yield (F_o). Above the growth light level, the model progressively overestimated assimilation, reflecting increased electron flow to oxygen under excess irradiance. In cyanobacteria, this flow maintains photosystem II centres open even up to photoinhibitory light levels without contributing to CO₂ fixation. Despite this we show that gross CO₂ gain may be predicted from fluorescence yield also in cyanolichens when the analysis is made near the acclimated growth light level. This level can be obtained even when measurements are performed in the field, since it coincides with a minimum in non-photochemical fluorescence quenching (NPQ). However, the absolute relation between fluorescence yield and gross CO₂ gain varies between species. It may therefore be necessary to standardise the fluorescence prediction for each species with CO₂ gas exchange.Abbreviations CCM CO₂-Concentrating mechanism - Chl chlorophyll - C_i inorganic carbon - 0 convexity (curvature of the light response curve) - ETR electron transport rate - F_o minimum fluorescence yield - F_m maximal fluorescence yield - F_s fluorescence yield at steady-state photosynthesis - F_v variable fluorescence yield - F_v/F_m dark ratio of variable to maximal fluorescence yield after dark adaptation - F_vF_mmax ratio of variable to maximal fluorescence yield in the absence of quenching - _CO2 maximum quantum yield of CO₂ assimilation - _PS quantum yield of photosystem II photochemistry - GP gross photosynthesis - I irradiance (mol quanta·m^–2·s^–1) - NPQ non photochemical fluorescence quenching - qp photochemical fluorescence quenching     

Influence of CO2, light and watering on growth of Nostoc flagelliforme mats

The terrestrial blue-green alga (cyanobacterium), Nostoc flagelliforme, was cultured in air at variouslevels of CO₂, light and watering to see theireffects on its growth. The alga showed the highestrelative growth rate at the conditions of highCO₂ (1500 ppm), high light regime (219–414mol m^-2s^-1) and twice daily watering,but the lowest rate at the conditions of low light(58–114 mol m^-2s^-1) and daily twicewatering. Increased watering had little effect ongrowth rate at 350 ppm CO₂, but increased byabout 70% at 1500ppm CO₂ under high lightconditions. It was concluded that enriched CO₂could enhance the growth of N. flagelliformewhen sufficient light and water was supplied.     

Calculation of leaf photosynthetic parameters from light-response curves for ecophysiological applications

Measurement of the light response of photosynthetic CO₂ uptake is often used as an implement in ecophysiological studies. A method is described to calculate photosynthetic parameters, such as the maximum rate of whole electron transport and dissimilative respiration in the light, from the light response of CO₂ uptake. Examples of the light-response curves of flag leaves and ears of wheat (Triticum aestivum cv. ARKAS) are shown.Abbreviations and symbols A net photosynthesis rate - D ¹ rate of dissimilative respiration occurring in the light - f loss factor - I incident PPFD - I effective absorbed PPFD - J rate of whole electron transport - J _m maximum rate of whole electron transport - p ^c intercellular CO₂ partial pressure - PPFD photosynthetic photon flux density - q effectivity factor for the use of light (electrons/quanta) - absorption coefficient - I ^* CO₂ compensation point in the absence of dissimilative respiration (bar) - II conversion factor for calculation of CO₂ uptake from the rate of whole electron transport - convexity factor Gas-exchange rates relate to the projective area and are given in mol·m^-2·s^-1. Electron-transport rates are given in mol electrons·m^-2·s^-1; PPFD is given in mol quanta·m^-2·s^-1.     

Moisture content and CO2 exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance

Summary Thalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 E m^-2 s^-1 light intensity (PAR) were obtained during drying of the thalli. At ambient CO₂ concentrations from 200 to 1,000 ppm, CO₂ uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO₂ concentration, and it was fully absent at 1,600 ppm ambient CO₂. Under these conditions of CO₂ saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO₂ uptake of the water saturated lichen at lower (e.g. natural) ambient CO₂ must be due exclusively to increased resistance to CO₂ diffusion from the external air to the sites of carboxylation.