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)     

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.     

Moss functioning in different taiga ecosystems in interior Alaska

Summary Carbon dioxide exchange rates in excised 2-year-old shoot sections of five common moss species were measured by infrared gas analysis in mosses collected from different stands of mature vegetation near Fairbanks, Alaska. The maximum rates of net photosynthesis ranged from 2.65 mg CO₂ g^-1h^-1 in Polytrichum commune Hedw. to 0.25 in Spagnum nemoreum Scop. Intermediate values were found in Sphagnum subsecundum Nees., Hylocomium splendens (Hedw.) B.S.G., and Pleurozium schreberi (Brid.) Mitt. Dark respiration rates at 15°C ranged from 0.24 mg CO₂ g^-1h^-1 in S. subsecundum to 0.57 mg CO₂ g^-1h^-1 in H. splendens. The dark respiration rates were found to increase in periods of growth or restoration of tissue (i.e., after desiccation). There was a strong decrease in the rates of net photosynthesis during the winter and after long periods of desiccation.Due to increasing amounts of young, photosynthetically active tissue there was a gradual increase in the rates of net photosynthesis during the season to maximum values in late August. As an apparent result of constant respiration rates and increasing gross photosynthetic rates, the optimum temperature for photosynthesis at light saturation and field capacity increased during the season in all species except Polytrichum, with a corresponding drop in the compensation light intensities. Sphagnum subsecundum seemed to be the most light-dependent species.Leaf water content was found to be an important limiting factor for photosynthesis in the field. A comparison between sites showed that the maximum rates of net photosynthesis increased with increasing nutrient content in the soil but at the permafrostfree sites photosynthesis was inhibited by frequent moisture stress.     

Inhibition of photosynthesis by chilling in moderate light: a comparison of plants sensitive and insensitive to chilling

Photosynthetic activity, in leaf slices and isolated thylakoids, was examined at 25° C after preincubation of the slices at either 25° C or 4° C at a moderate photon flux density (PFD) of 450 mol·m^–2·s^–1, or at 4° C in the dark. The plants used wereSpinacia oleracea L.,Cucumis sativus L. andNerium oleander L. which was acclimated to growth at 20° C or 45° C. The plants were grown at a PFD of 550 mol·m^–2·s^–1. Photosynthesis, measured as CO₂-dependent O₂ evolution, was not inhibited in leaf slices from any plant after preincubation at 25° C at a moderate PFD or at 4° C in the dark. However, exposure to 4° C at a moderate PFD induced an inhibition of CO₂-dependent O₂ evolution within 1 h inC. sativus, a chilling-sensitive plant, and in 45° C-grownN. oleander. The inhibition in these plants after 5 h reached 80% and 40%, respectively, and was independent of the CO₂ concentration but was reduced at O₂ concentrations of less than 3%. Methyl-viologen-dependent O₂ exchange in leaf slices from these plants was not inhibited. There was no photoxidation of chlorophyll, in isolated thylakoids, or any inhibition of electron transport at photosystem (PS)II, PSI or through both photosystems which would account for the inhibition of photosynthesis. The conditions which inhibit photosynthesis in chilling-sensitive plants do not cause inhibition inS. oleracea, a chilling-insensitive plant, or in 20° C-grownN. oleander. The CO₂-dependent photosynthesis, measured at 5° C, was reduced to about 3% of that recorded at 25° C in chilling-sensitive plants but only to about 30% in the chilling-insensitive plants. Methyl-viologen-dependent O₂ exchange, measured at 5° C, was greater than 25% of the activity at 25° C in all the plants. The results indicate that the mechanism of the chilling-induced inhibition of photosynthesis does not involve damage to PSII. That inhibition of photosynthesis is observed only in the chilling-sensitive plants indicates it is related, in some way, to the disproportionate decrease in photosynthetic activity in these plants at chilling temperatures.Abbreviations Chl chlorophyll - DPIPH reduced form of 2,6-dichlorophenol-indophenol - DMQ 2,5-dimethyl-p-benzoquinone - MV methyl viologen - 20°-oleander Nerium oleander grown at 20° C - 45°-oleander N. oleander grown at 45° C - PFD photon flux density (photon fluence rate) - PSI and PSII photosystem I and II, respectively     

The recovery of photosynthesis in tomato subsequent to chilling exposure

The overall success of a plant in coping with low temperature sensitivity of photosynthesis is dependent not only on the maximum extent of inhibition suffered for a given time of low temperature exposure but also on the persistence of the inhibition after normal growth temperatures are restored. Thus the capacity of recovery and the speed with which a plant can recover from the effects of chilling exposure are important parameters in determining how devastating the chilling event will be on season-long growth and yields. We have studied the recovery of CO₂-saturated photosynthesis from the injury caused by exposing intact tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) or detached tomato leaves to a temperature of 1°C in the dark for varying periods of time. We found that net photosynthesis was fully recovered within 12 h after returning the plants to 25°C in the dark, even after chilling exposures as long as 45 h. This was true for intact plants as well as for detached leaves that were supplied with water. When chilling took place in the light (4°C, 1000 E · m^-2 · s^-1, PAR) inhibition of photosynthesis was more severe and appeared more quickly and the recovery was slower and incomplete. A 12 h chilling exposure in the light resulted in injury to net photosynthesis that was not fully recovered even after 50 h. Chilling damage to photosynthesis developing in the light was distinguished from chilling in the dark by the decreased photosynthetic quantum yield. Not only did high intensity illumination enhance chilling damage of photosynthesis but bright light subsequent to the chilling exposure also delayed the recovery of photosynthesis. At none of the three ambient CO₂ concentrations investigated (300, 1500 and 5000 1.1^-1) did the recovery of photosynthesis depend on stomatal conductance.     

Temperature-dependent feedback inhibition of photosynthesis in peanut

Arachis hypogaea L. is a tropical crop that is slow-growing at temperatures below 25°C. Unadapted CO₂-assimilation rate (A) showed insufficient variation between 15 and 30°C in the short term (hours) to explain this marked reduction in growth. However, at longer periods (12 d), A was depressed as were growth rate and leafproduction rate. To examine the possible relationship between growth, A and sink demand plants were transferred from 30°C, which is near the optimum for growth, to a suboptimal temperature (19°C). In the first 2 d of cooling, A decreased by 50–70%, the stomata stayed open, and the intercellular CO₂ concentration (c_i) rose, i.e. the decrease in A of the cooled plants was the result of non-stomatal factors. Changes in dark respiration did not account for the decline in A.Clear evidence was obtained of sink control of A by independently manipulating the temperature of different leaves on the plant. Cooling (to 19°C) most of the plant (the sink) led to a 70% decline in A of the remaining leaves at 30°C after 3 d, whereas the converse treatments (30°C sink, 19°C source) resulted in small changes (17%). In plants at 19°C which were exposed to low CO₂ concentration to prevent photosynthesis, A was not reduced when measured at normal CO₂ concentrations, indicating that carbohydrate accumulation was responsible for the decline in A. Dry-matter build-up at suboptimal temperature was also consistent with end-product inhibition of photosynthesis.Abbreviations and symbols A (mol·m^-2·s^-1) rate of net CO₂ assimilation - C_i (l·l^-1) substomatal CO₂ concentration - DW (g) dry weight - g (mol·m^-2·s^-1) stomatal conductance to diffusion of water vapour - PFD (mol·m^-2·s^-1) photon flux density     

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.     

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.     

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.     

Microhabitat,water relations,and photosynthesis of a desert fern,Notholaena parryi

Summary Interrelationships between morphology, microhabitat, water relations, and photosynthesis of a xeric fern, Notholaena parryi D.C. Eat. (Pteridaceae), were examined in the western Colorado desert. In its typical microhabitat rock outcroppings protected N. parryi from direct sunlight and moderated the diurnal variations in air temperature. For example, frond temperature at noon in late winter was 15.3° C, which was 7.3° C cooler than an energy budget simulation predicting frond temperature at an exposed site. The lowest soil water potential leading to daytime stomatal opening was about-1.5 MPa (-15 bars). Rainfall runoff that was channeled to the periphery of the rocks caused ^soil near the fern roots to rise above-1.5 MPa even after light rainfalls, and it remained above-1.5 MPa longer after rainfall than in non-rocky sites.The water potential gradient along the stipe necessary to support the observed rates of transpiration was about-10 MPa m^-1; such a large gradient reflected the small conducting area in the xylem. The water vapor conductance decreased as the frond temperature was raised, an effect that became proportionally greater as the soil dried out. The daytime water-use efficiency (mass CO₂ fixed/mass water transpired) was 0.0058 for a spring day. Individual fronds reached 90% of light saturation for photosynthesis at only 100 Einsteins m^-2 s^-1, a photosynthetically active radiation similar to that from the diffuse sunlight incident on the generally north-facing microhabitat of the fern. Below 50 E m^-2 s^-1 the quantum requirement was 13 Einsteins absorbed/mole CO₂ fixed. The ratio of chlorophyll to P700 was 552, indicating a fairly large photosynthetic unit that is characteristic of plants adapted to shaded habitats. The temperature optimum for net photosynthesis shifted from 13° C in midwinter (mean daily air temperature of 11° C) to 19° C in early fall (air temperature of 23° C).     

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.     

Temperature effects on the photosynthetic response of C3 plants to long-term CO2 enrichment

To assess the long-term effect of increased CO₂ and temperature on plants possessing the C₃ photosynthetic pathway, Chenopodium album plants were grown at one of three treatment conditions: (1) 23 °C mean day temperature and a mean ambient partial pressure of CO₂ equal to 350 bar; (2) 34 °C and 350 bar CO₂; and (3) 34 °C and 750 bar CO₂. No effect of the growth treatments was observed on the CO₂ reponse of photosynthesis, the temperature response of photosynthesis, the content of Ribulose-1,5-bisphosphate carboxylase (Rubisco), or the activity of whole chain electron transport when measurements were made under identical conditions. This indicated a lack of photosynthetic acclimation in C. album to the range of temperature and CO₂ used in the growth treatments. Plants from every treatment exhibited similar interactions between temperature and CO₂ on photosynthetic activity. At low CO₂ (< 300 bar), an increase in temperature from 25 to 35 °C was inhibitory for photosynthesis, while at elevated CO₂ (> 400 bar), the same increase in temperature enhanced photosynthesis by up to 40%. In turn, the stimulation of photosynthesis by CO₂ enrichment increased as temperature increased. Rubisco capacity was the primary limitation on photosynthetic activity at low CO₂ (195 bar). As a consequence, the temperature response of A was relatively flat, reflecting a low temperature response of Rubisco at CO₂ levels below its k_m for CO₂. At elevated CO₂ (750 bar), the temperature response of electron transport appeared to control the temperature dependency of photosynthesis above 18 °C. These results indicate that increasing CO₂ and temperature could substantially enhance the carbon gain potential in tropical and subtropical habitats, unless feedbacks at the whole plant or ecosystem level limit the long-term response of photosynthesis to an increase in CO₂ and temperature.Abbreviations A net CO₂ assimilation rate - C _a ambient partial pressure of CO₂ - C _i intercellular partial pressure of CO₂ - Rubisco Ribulose-1,5-bisphosphate carboxylase - VPD vapor pressure difference between leaf and air     

Effects of temperature and CO2 enrichment on kinetic properties of phospho-enol-pyruvate carboxylase in two ecotypes of Echinochloa crus-galli (L.) Beauv., a C4 weed grass species

Summary Two populations of Echinochloa crus-galli (Québec, Mississippi) were grown at the Duke University Phytotron under 2 thermoperiods (28°/22°C, 21°/15°C day/night) and 2 CO₂ regimes (350 and 675 l l^-1). Thermostability, energy of activation (E _a ),K _m (PEP), K _m (Mg⁺⁺), and specific activity of phospho-enol-pyruvate carboxylase (PEP_c) were analyzed in partially purified enzyme preparations of plants grown for 5 weeks. Thermostability of PEP_c from extracts (in vitro) and leaves (in situ) was significantly higher in Mississippi plants. In vitro denaturation was not appreciably modified by thermal acclimation but CO₂ enrichment elicited higher thermostability of PEP_c. In situ thermostability was significantly higher than that of in vitro assays and was higher in Mississippi plants acclimated at 28°/22°C and in plants of the two ecotypes grown at 675 l l^-1 CO₂. E _a (Q ₁₀ 30°/20°C) for PEP_c was significantly lower in Québec plants as compared to Mississippi and no acclimatory shifts were observed. Significantly higher K _m's (PEP) in 20°C assays were obtained for Mississippi as compared to Québec plants but values were similar at 30°C and 40°C assays. K _m (Mg⁺⁺) decreased at higher assay temperatures and were significantly lower for PEP_c of the Québec ecotype. No significant changes in K _m (Mg⁺⁺) values were associated with modifications in temperature on CO₂ regimes. PEP_c activity measured at 30°C was significantly higher for Québec plants when measured on a leaf fresh weight, leaf area or protein basis but not on a chlorophyll basis. Significantly higher PEP_c activity for both genotypes was observed for plants acclimated at 21°/15°C or grown at 675 l l^-1 CO₂. Net photosynthesis (Ps) and net assimilation rates (NAR) were higher in Québec plants and were enhanced by CO₂ enrichment. NAR was higher in plants acclimated at low temperature, while an opposite trend was observed for Ps. PEP_c activities were always in excess of the amounts required to support observed rates of CO₂ assimilation.     

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.     

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.     

Irradiance and temperature effects on photosynthesis of tussock tundra Sphagnum mosses from the foothills of the Philip Smith Mountains,Alaska

Summary Photosynthetic characteristics of three species of Sphagnum common in the foothills of the Brooks Range on the North Slope of Alaska were investigated. Generally, light-saturated rates of net photosynthesis decreased in the order S. squarrosum, S. angustifolium, and S. warnstorfii when plants were grown under common growth chamber conditions. For field-grown S. angustifolium, average light compensation point at 10°C was 37 mol m^-2s^-1 photosynthetic photon flux density (PPFD), and light saturation occurred between 250 and 500 mol m^-2 s^-1. At 20°C, compensation point increased to 127 mol m^-2s^-1 and the PPFD required for light saturation increased to approximately 500 mol m^-2s^-1, while maximum rates of CO₂ uptake increased only slightly. Light response curves of chamber-grown plants exhibited substantially lower compensation points and higher light-saturated rates of CO₂ assimilation than field-grown material, due perhaps to a higher percentage of green, photosynthetically competent tissue. All three species exhibited broad responses to temperature, with optima near 20°C, and maintained at least 75% of maximum assimilation between approx. 13° and 30°C. Rates at 5°C were approx. 50% of maximum. Studies of the microclimate of Sphagnum at the field research site suggest that CO₂ uptake should occur at near light-saturated rates during the day in open tussock tundra but that PPFD may often be limiting under Salix and Betula canopies in a water track drainage. Simulations using a simple model provided a seasonal estimate of 0.78 g dry weight (DW) of S. angustifolium produced from each initial g of photosynthetic tissue under willow canopies, assuming no water limitations. Although the simulation model suggests that production would be 66% higher in open tussock tundra, S. angustifolium is rarely found in this potentially more stressful habitat. To explain the relative abundance of Sphagnum in shaded water track areas as compared to open tussock tundra, we postulate that the vascular plant canopies provide protection from adverse effects of high temperatures, excess irradiance and reduced water availability. Under conditions of normal water availability, removal of the vascular plant cover did not affect the tissue water content of S. squarrosum, but resulted in a strong decrease in photosynthetic capacity, accompanied by chlorophyll bleaching. These results suggest that photoinhibition may limit production under certain conditions.     

Pseudocyphellaria dissimilis: a desiccation-sensitive,highly shade-adapted lichen from New Zealand

Summary Pseudocyphellaria dissimilis, a foliose, cyanobacterial lichen, is shown not to fit into the normal ecological concept of lichens. This species is both extremely shade-tolerant and also more intolerant to drying than aquatic lichens previously thought to be the most desiccation-sensitive of lichens. Samples of P. dissimilis from a humid rain-forest site in New Zealand were transported in a moist state to Germany. Photosynthesis response curves were generated. The effect of desiccation was measured by comparing CO₂ exchange before and after a standard 20-h drying routine. Lichen thalli could be equilibrated at 15° C to relative humidities (RH) from 5% to almost 100%. Photosynthesis was saturated at a photosynthetically active radiation (PAR) level of 20 mol m^-2 s^-1 (350 bar CO₂) and PAR compensation was a very low 1 mol m^-2 s^-1. Photosynthesis did not saturate until 1500 bar CO₂. Net photosynthesis was relatively unaffected by temperature between 10° C and 30° C with upper compensation at over 40° C. Temporary depression of photosynthesis occurred after a drying period of 20 h with equilibration at 45–65% relative humidity (RH). Sustained damage occurred at 15–25% RH and many samples died after equilibration at 5–16% RH. Microclimate studies of the lichen habitat below the evergreen, broadleaf forest canopy revealed consistently low PAR (normally below 10–20 mol m^-2 s^-1) and high humidities (over 80% RH even during the day time). The species shows many features of an extremely deep shade-adapted plant including low PAR saturation and compensation, low photosynthetic and respiratory rates and low dry weight per unit area.     

Photosynthetic recovery in the resurrection plant Selaginella lepidophylla after wetting

Summary Photosynthetic recovery (PR) in a southwest Texas, USA population of Selaginella lepidophylla (Hook and Grev.) (Selaginellaceae), a poikilohydric spikemoss, was examined in the laboratory. Infrared CO₂ gas analysis and ribulose 1,5-bisphosphate (RuBP) carboxylase activity measurements indicated that optimal temperature for PR was near 25°C in terms of: (1) rapidity of net CO₂ uptake after hydration (5.4 h), (2) maximum net photosynthetic rate at 2000 E·m^-2·s^-1 (2.44 mg CO₂·g(DWT)^-1·h^-1), and (3) maximum net CO₂ assimilation per 30 h hydration event (43.8 mg CO₂·g(DWT)^-1·30 h^-1). The PR was much slower at both 15° and 35° C, with lower photosynthetic rates and net carbon gains per hydration event. High respiratory costs were incurred at 45°C and no net photosynthesis was observed. Increases in RuBP carboxylase activity and chlorophyll content during 24 h hydration were also greatest near 25°C. Dry plants had 60% of the enzyme activity of fully recovered (24 h hydration) plants, indicating enzyme conservation. Actinomycin D and cycloheximide did not appear to inhibit PR, but chloramphenicol appeared to totally inhibit RuBP carboxylase activity increases over levels conserved in dry plants. Therefore, rapid PR in S. lepidophylla was achieved by both rapid increase in RuBP carboxylase activity, possibly via de novo synthesis, and conservation of the photosynthetic enzyme. Both mechanisms are essential to maximize assimilation in S. lepidophylla in an environment where hydrated periods are rare and of short duration.     

A circadian rhythm in the level of carbon dioxide compensation in Bryophyllum fedtschenkoi with zero values during the transient

Summary Detached shoots of Bryophyllum fedtschenkoi maintained in a closed system in the light exhibited an endogenous circadian rhythm in CO₂ compensation. The rhythm was sensitive to changes in light intensity and temperature. At 15° C it damped rapidly in light of 78 J m^-2 s^-1, but at 10° C a rhythm of considerable amplitude was evident at this same light intensity. During the transient (i.e. the temporary state of the rhythm before it acquired its steady state) low compensation values between 0 and 5 ppm CO₂ were achieved. When the plants were maintained at a higher light intensity prior to the measurements, the period of low compensation during the transient was extended, and zero values were obtained under some conditions.Studies of gas exchange at opposite phases of the rhythm revealed: (i) that the rate of uptake of ¹⁴CO₂ differed, both in light and darkness (the epidermis was removed during these observations to avoid interference from stomatal rhythms); (ii) photorespiration, estimated by extrapolation of the graph relating photosynthetic rate and CO₂ concentration, was highest during the peaks of the rhythm in CO₂ compensation; (iii) estimates of the capacity for photorespiration by the glycine-1-¹⁴C assay indicated highest values during the troughs of the rhythm. These findings are discussed in relation to the C₄-acid metabolism of this species. Low CO₂ compensation is probably due to the activity of phosphoenolpyruvate carboxylase and not to the absence of processes involving CO₂ evolution.     

Correlated photosynthetic responses and habitat factors of two successional tree species

Summary Ulmus alata and Diospyros virginiana are components of the shrubearly tree communities of old-field succession in several areas in the deciduous forests of eastern North America. In these habitats, the plants experience high insolation, high temperatures, and low soil moisture during the summer. They exhibit pronounced daily changes in water potential and usually develop more negative water potentials as the season progresses. The species light saturate at 1,150 E m^-2 sec^-1 with photosynthetic rates of 15 mg CO₂ dm^-2 h^-1 for U. alata and 17 mg CO₂ dm^-2 h^-1 for D. virginiana. The optimum temperatures for photosynthesis are 25°C. Ulmus alata maintains maximum photosynthesis to water potentials of-14 bars and recovers from-20 bars to 60% of maximum photosynthesis within 10 hrs after watering. When they are deprived of water, twigs of D. virginiana exhibit faster decline in photosynthesis and leaf conductance than twigs of U. alata. The two species have somewhat different response to the environmental of high insolation and low water supply. Unlike Ulmus, Diospyros virginiana has some adaptations which may explain the persistence of a few individuals in mature forests.