Influence of light intensity at different temperatures on rate of respiration of douglas-fir seedlings

The rate of photorespiration of Douglas-fir seedlings was measured under different light intensities by: (1) extrapolating the curve for CO₂ uptake in relation to atmospheric CO₂ content to zero CO₂ content, and (2) measuring CO₂ evolution of the plants into a CO₂-free airstream. Different results, obtained from these techniques, were believed to be caused by a severe restriction of the photosynthetic activity when the latter was used. With the first method, CO₂ evolution was lower than the dark respiration rate at low light intensity. For all temperatures studied (6°, 20°, 28°) a further increase in light intensity raised the CO₂ evolution above dark respiration before it leveled off. The rate of CO₂ evolution was stimulated by increase in temperature at all light intensities. With the CO₂-free air method, CO₂ evolution in the light was less than dark respiration at all light intensities.     

Moisture content and CO2 exchange of lichens

Summary Net photosynthesis (10 klx light intensity, 150 E m^-2 s^-1 PAR) and dark respiration of the lichen Ramalina maciformis at different temperatures are measured in relation to thallus water content. Both first increase with increasing hydration. Dark respiration then remains constant with increased water content until thallus saturation. In contrast, a further increase in water content leads to a depression of net photosynthesis, as shown in previous studies, after a maximum of CO₂ uptake has been attained. However, the extent of this depression depends strongly on temperature. In saturated thalli (160% water content in relation to lichen dry weight) the depression amounts to about 15% and 63% of the maximum unsaturated rate at 5°C and 25°C thallus temperature, respectively. The moisture compensation-point of net photosynthesis is also decisively determined by temperature (for 0°C at 20% water content; for 25°C at 15%), and the water content that allows maximum rates of CO₂ uptake (for 0°C at 80%; for 25°C at less than 40% water content). An electrical analogue of CO₂ exchange in a lichen thallus is presented, and it is suggested that the experimental results may be interpreted in terms of temperature-dependent CO₂ diffusion resistances in imbibed lichen thalli.     

Einfluß der Temperatur auf die Lichtatmung der Blaualge Anacystis nidulans

Summary CO₂ exchange, ¹⁴CO₂ fixation and ¹⁴C-products of Anacystis nidulans (strain L 1402-1) were studied during the induction period at temperatures of +15°C and+35°C. At+15°C the stationary rates of CO₂ uptake and respiration were reached directly. At+35°C a maximum of CO₂ uptake could be observed at the beginning of the illumination period followed by a lower steady rate of photosynthesis. In the following dark period a CO₂ gush appeared at+35°C. The magnitude of the CO₂ outburst is relatively independent of the photosyntbetic period. The autoradiographic studies showed that the Calvin cycle is the main carboxylation pathway in Anucystis. At a temperature of +35°C serine was labelled after 20 sec of photosynthesis. At+15°C, on the other hand, a low radio-activity appeared in serine after 5 min of photosynthesis. The results show that photorespiration of Anacystis is stimulated by high temperatures.     

In situ CO2 gas-exchange in fruits of a tropical tree,Durio zibethinus Murray

We examined the in situ CO₂ gas-exchange of fruits of a tropical tree, Durio zibethinus Murray, growing in an experimental field station of the Universiti Pertanian Malaysia. Day and night dark respiration rates were exponentially related to air temperature. The temperature dependent dark respiration rate showed a clockwise loop as time progressed from morning to night, and the rate was higher in the daytime than at night. The gross photosynthetic rate was estimated by summing the rates of daytime dark respiration and net photosynthesis. Photosynthetic CO₂ refixation, which is defined as the ratio of gross photosynthetic rate to dark respiration rate in the daytime, ranged between 15 and 45%. The photosynthetic CO₂ refixation increased rapidly as the temperature increased in the lower range of air temperature T _c (T _c <28.5 °C), while it decreased gradually as the temperature increased in the higher range (T _c 28.5 °C). Light dependence of photosynthetic CO₂ refixation was approximated by a hyperbolic formula, where light saturation was achieved at 100 mol m^–2 s^–1 and the asymptotic CO₂ refixation was determined to be 37.4%. The estimated gross photosynthesis and dark respiration per day were 1.15 and 4.90 g CO₂ fruit^–1, respectively. Thus the CO₂ refixation reduced the respiration loss per day by 23%. The effect of fruit size on night respiration rate satisfied a power function, where the exponent was larger than unity.     

Limitation of photosynthesis by changes in temperature

The aim of this work was to examine the effect of abrupt changes in temperature in the range 5 to 30°C upon the rate of photosynthetic carbon assimilation in leaves of barley (Hordeum vulgare L.). Measurement of the CO₂-assimilation rate in relation to the intercellular partial pressure of CO₂ at different temperatures and O₂ concentrations and at saturating irradiance showed that as the temperature was decreased photosynthesis was saturated at progressively lower CO₂ partial pressures and that the transition between the CO₂-limited and ribulose-1,5-bisphosphate-regeneration-limited rate became more abrupt. Feeding of orthophosphate to leaves resulted in an increased rate of CO₂ assimilation at lower temperatures at around ambient or higher CO₂ partial pressures both in 20% O₂ and in 2% O₂ and it removed the abruptness in the transition between the CO₂-limited and ribulose-1,5-bisphosphate-regeneration-limited rates. Phosphate feeding tended to inhibit carbon assimilation at higher temperatures. The response of carbon assimilation to temperature was altered by feeding orthophosphate, by changing the concentrations of CO₂ or of O₂ or by leaving plants in the dark at 4°C for several hours. Similarly, the response of carbon assimilation to phosphate feeding or to changes in 2% O₂ was altered by leaving the plants in the dark at 4°C. The mechanism of limitation of photosynthesis by an abrupt lowering of temperature is discussed in the light of the results.Abbreviations A rate of CO₂ assimilation - P _i intercellular partial pressure of CO₂ - RuBP ribulose-1,5-bisphosphate     

Apparent Reassimilation of Respiratory Carbon Dioxide by Different Plant Species

Differences among species in respiration rates in CO₂-free air, in light and dark, were studied using the standard leaf chamber technique and the infrared carbon dioxide analyzer. Photosynthesis, transpiration and respiration were measured. In all species studied, rates of respiration were considerably higher in dark than in light. This effect was assumed to be due to reassimilation of the respiratory CO₂. A resistance analogy model was derived to account for the apparent differences in internal recycling of CO₂ among species; the differences were correlated with differences in maximum photosynthetic rates in normal air and optimal conditions (P₃₁₀) and with internal resistances to CO₂ diffusion (r_k). Species with high P₃₁₀ and low r_k appear to reassimilate all the endogenous CO₂, whereas other species with lower P₃₁₀ and higher r_k appear to reassimilate only a part of their respiratory CO₂. Experiments with the photosynthetic inhibitor, 3-(3,4-dichlorophcnyl)-l,l-dimethyl urea (DCMU), indicated that species with zero respiration in CO₂-free air and light release respiratory CO₂ when photosynthesis is inhibited. It is concluded that the CO₂ released in the presence of DCMU represents respiratory CO₂ which recycles to photosynthesis under normal conditions.     

Variation in leaf respiration in relation to growth and photosynthesis of Lolium

Six Lolium genotypes with contrasting apparent photorespiration and CO_a compensation concentration, [C0₂]_c, were compared for net photosynthesis, dark respiration, leaf starch accumulation, rate of leaf expansion and shoot regrowth. Plants were grown in day/night temperatures of 15/10 and 25/20 ^oC. There were significant (P < 0–05) differences between the genotypes in all these parameters. At 25/20 ^oC apparent photorespiration was correlated with [CO₂]_c. Correlation coefficients, pooled from both temperature regimes, revealed that genotypes with high rates of net photosynthesis accumulated more leaf starch during light periods than genotypes with slow photosynthesis, but rates of leaf expansion and dry matter increase were only correlated, negatively, with dark respiration. Apparent photorespiration was negatively correlated with dark respiration. These findings suggest that attributes related to photorespiration such as [CO₂]_c and O₂ uptake from CO₂-free air in the light are unlikely to be useful selection criteria for growth of C₃ grasses, that net photosynthesis was probably not limiting growth and that maintenance respiration may have been an important determinant of genotypic differences in growth rate. Selections for slow and fast rates of dark respiration of mature leaves were therefore made at 8 and at 25 ^oC from within two different populations of L. perenne, S.23. This characteristic showed repeatabilities (broad-sense heritability) of from 0–41 to o-66. Six independent comparisons of simulated swards of the slow- and fast-respiring selections were made under periodic cutting regimes, either in a growth room at 25 ^oC or in a glasshouse from August to May. Growth of all plots of slow-respiring genotypes was consistently more rapid than that of the fast-respiring, at 25 ^oC in the growth room, and during autumn and spring in the glasshouse. There was no difference in winter growth. The implications of these results for the use of gas exchange measurements as selection criteria in plant breeding programmes are discussed.     

Temperature and irradiance and the total daily photosynthetic production of the crown of a Pinus radiata tree

Summary Carbon dioxide exchange rates were recorded for different ages and positions of foliage and parts of the main stem of a 7-m tall Pinus radiata D. Don tree growing in a large, artificially lit, controlled-environment room. Irradiance levels were varied from dark to approximately full sunlight, and air temperatures from 10° to 35°C in 5°C steps. Leaf temperatures within the cuvettes used for CO₂ exchange measurements, however, were up to 5°C higher than the room air temperature set but this varied with position in the tree crown, the shaded lower crown being at approximately room temperature. A balance sheet was prepared to show the photosynthetic gains and respiratory losses of different parts of the crown over 24 h at each air temperature and at irradiances of 400, 270, and 135 W m^-2 during the 8-h photosynthetic period. The greatest daily photosynthetic gain was at 10° C, although this temperature is considered sub-optimal for growth. At temperatures greater than 25° C, even at the greatest irradiance level for 8 h, total respiration was greater than photosynthesis.     

A method for the study of CO2 exchanges in in vitro cultured Vitis rupestris plantlets

A CO₂ assay circuit adapted to in vitro culture was designed to investigate CO₂ exchanges in test tube-grown Vitis rupestris plantlets. The CO₂ concentration of the air in culture tubes was measured by injection of samples in the open circuit. It was observed under the culture conditions used that the CO₂ content stabilized during the light phase at 3 times the CO₂ compensation point.Measurements of dark respiration under closed circuit conditions at every two-hour interval during the night did not reveal any limiting by lack of the substrate under mixotrophic culture conditions. A mathematical model of the influence of ambient CO₂ concentration on net CO₂ uptake rates under closed circuit conditions was devised and used to compare net photosynthesis at different lighting levels. Measurement of CO₂ evolution into CO₂-free air under open circuit conditions revealed a post-illumination burst characteristic of photorespiration which increased with the temperature.     

Untersuchung der Beziehung zwischen extracellulärer Glykolsäure-Ausscheidung und der photosynthetischen CO2-Aufnahme bei der Blaualge Anacystis nidulans

Summary The formations of transients in CO₂ exchange in the blue-green alga Anacystic nidulans is dependent on the temperature used during the measurements. The algae were grown in a low light intensity (4000 lux) under normal air conditions and measured in the same low CO₂ concentration (0.03 vol. %) but under a higher light intensity (10 000 lux). At a temperature of +20°C the stationary rate of CO₂ uptake was reached directly. At a temperature of +35°C, on the other hand, a maximum of CO₂ uptake could be observed at the beginning of the light period followed by a steady rate of photosynthesis, which was higher than at +20°C. In the beginning of the dark period a CO₂ outburst appeared at 35°C.Only at a low temperature (+20°C) did we find a light induced glycollate excretion; after a maximum at 7 1/2 minutes illumination the release of glycollate ceases and the level decreases to a lower value. A similar time course exists during illumination in red light (621 nm, 1.5·10^-8 einsteins) and a temperature of +20°C. In blue light (432 nm, 1,5·10^-8 einsteins, +20°C) and in white light at a high temperature (+35°C) we could not find any light induced glycollate excretion. Our results are discussed in reference to the photorespiration. We explain the formation of transients in CO₂ uptake of Anacystis at a high temperature (+35°C) and in blue light (+20°C) on the basis of the influence of photorespiration.     

Prior illumination and the respiration of maize leaves in the dark

The course of respiration of attached maize (Zea mays L.) leaves was measured by infrared gas analysis of CO₂ efflux in the dark following illumination in atmospheres of 300 microliters of CO₂ per liter of air, CO₂-free air, and CO₂-free N₂ containing 400 microliters of O₂ per liter. CO₂ efflux from control leaves started 3 to 4 minutes after darkening, increased to a maximum after about 20 minutes, and returned to a steady minimum after 2 to 3 hours. Respiration was quantitatively related to prior illumination, independent of net CO₂ fixation in the light, and depressed by N₂. Light, but not air, was required to produce a substrate for respiration in the subsequent dark period; air was required for oxidation of the substrate to CO₂. The stimulation of respiration by prior illumination in maize leaves differs in its slower onset and greater duration from the postillumination burst of photorespiration.     

Investigation on photorespiration with a sensitive C-assay

A leaf disk assay for photorespiration has been developed based on the rate of release of recently fixed ¹⁴CO₂ in light in a rapid stream of CO₂-free air at 30° to 35°. In tobacco leaves (Havana Seed) photorespiration with this assay is 3 to 5 times greater than the ¹⁴CO₂ output in the dark. In maize, photorespiration is only 2% of that in tobacco.

The importance of open leaf stomata, rapid flow rates of CO₂-free air, elevated temperatures, and oxygen in the atmosphere in order to obtain release into the air of a larger portion of the ¹⁴CO₂ evolved within the tissue in the light was established in tobacco. Photorespiration, but not dark respiration, was inhibited by α-hydroxy-2-pyridinemethanesulfonic acid, an inhibitor of glycolate oxidase, and by 3-(4-chlorophenyl)-1,1-dimethylurea (CMU), an inhibitor of photosynthetic electron transport, under conditions which did not affect the stomata. These experiments show that the substrates of photorespiration and dark respiration differ and also provide additional support for the role of glycolate as a major substrate of photorespiration. It was also shown that at 35° the quantity of ¹⁴CO₂ released in the assay may represent only 33% of the gross ¹⁴CO₂ evolved in the light, the remainder being recycled within the tissue.

It was concluded that maize does not evolve appreciable quantities of CO₂ in the light and that this largely accounts for the greater efficiency of net photosynthesis exhibited by maize. Hence low rates of photorespiration may be expected to be correlated with a high rate of CO₂ uptake at the normal concentrations of CO₂ found in air and at higher light intensities.

     

Upper temperature limits for growth and diazotrophy in the thermophilic cyanobacterium HTF Chlorogloeopsis

The effect of temperature and oxygen on diazotrophic growth of the thermophilic cyanobacterium HTF (High Temperature Form) Chlorogloeopsis was investigated using cells grown in light-limited continuous culture at a dilution rate of 0.02 h^-1. Diazotrophy was more sensitive to elevated temperatures than growth with combined nitrogen. The maximum temperature for growth of cultures gassed with CO₂-enriched air was more than 55 °C but less than 60 °C with N₂ as the sole nitrogen source, but between 60°C and 65°C when nitrate was present in the medium. The effect of temperature on nitrogenase activity, photosynthesis and respiration in the dark was determined using cells grown at 55°C. Maximal rates of all three processes were observed at 55°C and rates at 60°C during shortterm incubations were not less than 75% of the maximum. However, nitrogenase activity at 60°C was unstable and decayed at a rate of 2.2 h^-1 under air and at 0.3 h^-1 under argon. Photosynthesis and respiration were more stable at 60°C than anoxic nitrogen fixation. The upper temperature limits for diazotrophic growth thus seem to be set by the stability of nitrogenase.Abbreviations chl chlorophyll a - DCMU N-(3,4-dichlorophenyl) N,N-dimethylurea - Taps N-tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid     

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C₃ photosynthesis and low apparent photorespiration and has been classified as a C₃/C₄ intermediate. Other species in the Laxa group are C₃ with normal photorespiration. Panicum prionitis is a C₄ species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C₃ and C₄ species, its photosynthetic response to irradiance and temperature was compared to the closely related C₃ species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m^-2 s^-1 at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m^-2 s^-1 and an optimum temperature near 40°C. P. milioides exhibited low CO₂ loss into CO₂-free air in the light and this loss was nearly insensitive to temperature. Loss of CO₂ in the light in the C₃ species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C₃ Panicum species.Abbreviations AP apparent photosynthesis - I CO₂ compensation point - gl leaf conductance; gm, mesophyll conductance - PPFD photosynthetic photon flux density - PR apparent photorespiration rate - RuBPC sibulose bisphosphate carboxylase     

The temperature dependance of photoinhibition in the tropical basidiomycete lichen Cora pavonia E. Fries

Summary The response of net photosynthesis (NP) and dark respiration to periods of high insolation exposure was examined in the tropical basidiomycete lichen Cora pavonia. Photoinhibition of NP proved quite dependant on temperature. Rates of light saturated NP were severely impaired immediately after pretreatment high light exposure at temperatures of 10, 20 and 40°C, while similar exposure at 30°C resulted in only minimal photoinhibition. Apparent quantum yield proved an even more sensitive indicator of photoinhibition, reduced in all temperature treatments, although inhibition was again greatest at low and high temperatures. Concurrent exposure to reduced O₂ tensions during high light exposure mitigated some of the deleterious effects of high light exposure at 10 and 20°C, suggesting an interaction of O₂ with the inactivation of photosynthetic function. This represents the first reported instance of light dependant chilling stress in lichens, and may be an important limitation on the distribution of this and other tropical lichen species. This narrow range of temperatures within which thalli of C. pavonia can withstand periods of high insolation exposure coincides with that faced by hydrated thalli during rare periods of high insolation exposure within the cloud/shroud zone on La Soufrière, and points to the necessity of considering periods of atypical or unusual climatic events when interpreting patterns of net photosynthetic response, both in tropical and in north temperate lichen species.     

RATES OF PHOTOSYNTHESIS AND RESPIRATION OF THE MOSS BRYUM SANDBERGII AS INFLUENCED BY LIGHT INTENSITY AND TEMPERATURE

Using differential respirometry and air enriched to 3% CO₂ (v/v), the rates of photosynthesis and dark respiration of the moss Bryum sandbergii were measured as influenced by temperature and light intensity. The optimal temperature for net (apparent) photosynthesis was between 24 to 30 C; however, the photosynthesis/respiration ratio was about 11 to 27 between 4 to 24 C and dropped to lower values at 34 C., which indicates a wide temperature tolerance for this moss in short-term experiments. The maximum temperature for photosynthesis was about 41 C and the minimum was below –5 C. At 20 C light saturation was approached at 6.2 mw cm^–2 (ca. 700 ft-c) but not completely reached at 12 mw cm^-2; the light compensation point was estimated to be 0.4 mw cm^-2 (ca. 40 ft-c). At 4 C light saturation and the compensation point were at lower levels and apparently solarization occurred at 12 mw cm^-2. Light intensity had little or no apparent effect on dark respiration. However, respiration increased with temperature over various ranges extending from –5 to 39 C with temperature quotients of about 2.5 to 1.2. The significance of these characteristics is discussed with respect to the ecological relationships of the species.     

Carbon exchange in biological soil crust communities under differential temperatures and soil water contents: implications for global change

Biological soil crusts (biocrusts) are an integral part of the soil system in arid regions worldwide, stabilizing soil surfaces, aiding vascular plant establishment, and are significant sources of ecosystem nitrogen and carbon. Hydration and temperature primarily control ecosystem CO₂ flux in these systems. Using constructed mesocosms for incubations under controlled laboratory conditions, we examined the effect of temperature (5–35 °C) and water content (WC, 20–100%) on CO₂ exchange in light (cyanobacterially dominated) and dark (cyanobacteria/lichen and moss dominated) biocrusts of the cool Colorado Plateau Desert in Utah and the hot Chihuahuan Desert in New Mexico. In light crusts from both Utah and New Mexico, net photosynthesis was highest at temperatures >30 °C. Net photosynthesis in light crusts from Utah was relatively insensitive to changes in soil moisture. In contrast, light crusts from New Mexico tended to exhibit higher rates of net photosynthesis at higher soil moisture. Dark crusts originating from both sites exhibited the greatest net photosynthesis at intermediate soil water content (40–60%). Declines in net photosynthesis were observed in dark crusts with crusts from Utah showing declines at temperatures >25 °C and those originating from New Mexico showing declines at temperatures >35 °C. Maximum net photosynthesis in all crust types from all locations were strongly influenced by offsets in the optimal temperature and water content for gross photosynthesis compared with dark respiration. Gross photosynthesis tended to be maximized at some intermediate value of temperature and water content and dark respiration tended to increase linearly. The results of this study suggest biocrusts are capable of CO₂ exchange under a wide range of conditions. However, significant changes in the magnitude of this exchange should be expected for the temperature and precipitation changes suggested by current climate models.     

Photosynthetic characteristics of the shade-adapted C4 grass Muhlenbergia sobolifera (Muhl.) Trin.: control of development of photorespiration by growth temperature

Muhlenbergia sobolifera (Muhl.) Trin., a C₄ grass, occurs in understory habitats in the northeastern United States. Plants of M. sobolifera were grown at 23 and 30°C at 150 and 700 μmol photons m⁻² s⁻¹. The photosynthetic CO₂ compensation point, maximum CO₂ assimilation, dark respiration and the absorbed quantum use efficiency (QUE) were measured at 23 and 30°C at 2 and 20% O₂. Photosynthetic CO₂ compensation points ranged from 4 to 14mm³ dm⁻³ CO₂ and showed limited O₂ sensitivity. The mean photosynthetic CO₂ compensation point of plants grown at 30°C (4·5 mm³ dm⁻³) was 57% lower and 80% less inhibited by O₂ than that of plants grown at 23°C. Photosynthesis was similarly affected by growth temperature, with 70% more O₂ inhibition in plants grown at 23°C; suppression over all treatments ranging from 2 to 11%. Unlike typical C₄ species, plants of M. sobolifera from both temperature regimes exhibited higher CO₂ assimilation rates when grown at low light. Growth temperature and light also affected QUE; plants grown at low light and 23°C had the highest value (0·068 mol CO₂/mol quanta). Measurement temperature and growth light regime significantly affected dark respiration; however, O² did not affect QUE or dark respiration under any growth or measurement conditions. The results indicate that M. sobolifera is adapted to low PPFD, and that complete suppression of photorespiration is dependent upon high growth temperature.     

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.     

Effect of carbon dioxide and temperature on photosynthetic CO2 uptake and photorespiratory CO2 evolution in sunflower leaves

Using an open gas-exchange system, apparent photosynthesis, true photosynthesis (TPS), photorespiration (PR) and dark respiration of sunflower (Helianthus annuus L.) leaves were determined at three temperatures and between 50 and 400 l/l external CO₂. The ratio of PR/TPS and the solubility ratio of O₂/CO₂ in the intercellular spaces both decreased with increasing CO₂. The rate of PR was not affected by the CO₂ concentration in the leaves and was independent of the solubility ratio of oxygen and CO₂ in the leaf cell. At photosynthesis-limiting concentrations of CO₂, the ratio of PR/TPS significantly increased from 18 to 30°C and the rate of PR increased from 4.3 mg CO₂ dm^-2 h^-1 at 18°C to 8.6 mg CO₂ dm^-2 h^-1 at 30°C. The specific activity of photorespired CO₂ was CO₂-dependent but temperature-independent, and the carbon traversing the glycolate pathway appeared to be derived both from recently fixed assimilate and from older reserve materials. It is concluded that PR as a percentage of TPS is affected by the concentrations of O₂ and CO₂ around the photosynthesizing cells, but the rate of PR may also be controlled by other factors.Abbreviations APS apparent photosynthesis (net CO₂ uptake) - PR photorespiration (CO₂ evolution in light) - RuBP ribulose-1,5-bisphosphate - TPS true photosynthesis (true CO₂ uptake)