Low vapour pressure deficit reduces the beneficial effect of elevated CO2 on growth of N2-fixing alfalfa plants

Plant responses to elevated CO₂ can be modified by many environmental factors, but very little attention has been paid to the interaction between CO₂ and changes in vapour pressure deficit (VPD). Thirty-day-old alfalfa plants ( Medicago sativa L. cv. Aragón), which were inoculated with Sinorhizobium meliloti 102F78 strain, were grown for 1 month in controlled environment chambers at 25/15°C, 14 h photoperiod, and 600 µmol m⁻² s⁻¹ photosynthetic photon flux (PPF), using a factorial combination of CO₂ concentration (400 µmol mol⁻¹ or 700 µmol mol⁻¹) and vapour pressure deficit (0.48 kPa or 1.74 kPa, which corresponded to relative humidities of 85% and 45% at 25°C, respectively). Elevated CO₂ strongly stimulated plant growth under high VPD conditions, but this beneficial effect was not observed under low VPD. Under low VPD, elevated CO₂ also did not enhance plant photosynthesis, and plant water stress was greatest for plants grown at elevated CO₂ and low VPD. Moreover, plants grown under elevated CO₂ and low VPD had a lower leaf soluble protein and photosynthetic activity (photosynthetic rate and carboxylation efficiency) than plants grown under elevated CO₂ and high VPD. Elevated CO₂ significantly increased leaf adaxial and abaxial temperatures. Because the effects of elevated CO₂ were dependent on vapour pressure deficit, VPD needs to be controlled in experiments studying the effect of elevated CO₂ as well as considered in the extrapolations of results to a warmer, high-CO₂ world.     

The role of light and CO2 in optimising the conditions for shoot proliferation of Actinidia deliciosa in vitro

Proliferating cultures of Actinidia deliciosa A. Chev., C. F. Liang and A. R. Ferguson cv. Tomuri (♂) were grown under photosynthetic photon flux density (PPFD) rates ranging from 30 to 250 μmol m⁻² s⁻¹ in order to determine certain physiological parameters in vitro: CO₂ evolution, photosynthesis at three CO₂ atmospheric concentrations (330, 1450 and 4500 μl l⁻¹), fresh and dry matter accumulation and proliferation rate.
A proportional response in dry weight, dry/fresh weight ratios and PPFD was found. The proliferation rate increased up to 120 μmol m⁻² s⁻¹ but decreased at higher rates. At the highest PPFD, the CO² released from cultures and accumulated in the vessels reached 200 μl l⁻¹ of; at the lowest rate the CO₂ concentration reached 10500 μl l⁻¹ after 28 days of culture. The photosynthetic rate at 1450 and 4500 μl l⁻¹ of CO₂ was nearly 4 times higher than at the lowest concentration tested.     

Regulation of Rubisco activity in vivo

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is not able to achieve and maintain adequate CO₂ and Mg²⁺ activation under physiological conditions. Higher plants and green algae contain Rubisco activase, a soluble protein which not only facilitates Rubisco activation in situ but also regulates enzyme activity in response to irradiance and other factors. Regulation of Rubisco activity by modulation of activation state coordinates the rate of CO₂ fixation with the rate of substrate regeneration. This regulation may be required to ensure that the levels of photosynthetic metabolites in the chloroplast are optimal for photosynthesis under a variety of environrmental conditions. Some plant species also appear to regulate Rubisco activity by synthesizing 2-carboxyarabinitol 1-phosphate, an inhibitor of Rubisco in the dark. This inhibitor may function primarily as a regulator of metabolite binding in the dark rather than as a modulator of Rubisco activity in the light.     

Effect of light and gibberellic acid on photosynthesis during leaf senescence of alstroemeria cut flowers.

The functioning of the photosynthetic apparatus during leaf senescence was investigated in alstroemeria cut flowers by a combination of gas-exchange measurements and analysis of in vivo chlorophyll fluorescence. Chlorophyll loss in leaves of alstroemeria cut flowers is delayed by light and by a treatment of the cut flowers with gibberellic acid (GA₃). The maximal photosynthesis of the leaves was approximately 6 μmol CO₂ m⁻² s⁻¹ at I 350 μmol m⁻² s⁻¹ (PAR) which is relatively low for intact C₃ leaves. Qualitatively the gas-exchange rates followed the decline in chlorophyll content for the various treatments, i.e. light and GA₃-treatment delayed the decline in photosynthetic rates. However, when chlorophyll loss could not yet be observed in the leaves, photosynthetic rates were already strongly decreased. In vivo fluorescence measurements revealed that the decrease in CO₂ uptake is (partly) due to a decreased electron flow through photosystem II. Furthermore, analysis of the fluorescence data showed a high nonphotochemical quenching under all experimental conditions, indicating that the consumption of reducing power in the Calvin cycle is very low. The chlorophyll, remaining after 9 days incubation of leaves with GA₃ in the dark should be considered as a 'cosmetic' pigment without any function in the supply of assimilates to the flowers.     

Effect of CO2-enrichnient on seedling physiology and growth of two tropical tree species

Seedlings of two tree species from the Atlantic lowlands of Costa Rica, Ochroma la-gopus Swartz, a fast-growing pioneer species, and Pentaclethra macroloba (Willd.) Kuntze, a slower-growing climax species, were grown under enriched atmospheric CO₂ in controlled environment chambers. Carbon dioxide concentrations were maintained at 350 and 675 μl 1⁻¹ under photosynthetic photon flux densities of 500 μol m⁻² s⁻¹ and temperatures of 26°C day and 20°C night. Total biomass of both species increased significantly in the elevated CO₂ treatment; the increase in biomass was greatest for the pioneer species, O. lagopus . Both species had greater leaf areas and specific leaf weights with increased atmospheric CO₂. However, the ratio of non-pho-tosynthetic tissue to leaf area also increased in both species leading to decreased leaf area ratios. Plants of both species grown at 675 μl 1⁻¹ CO₂ had lower chlorophyll contents and photosynthesis on a leaf area basis than those grown at 350 μl 1⁻¹. Reductions in net photosynthesis occurred despite increased internal CO₂ concentrations in the CO₂-enriched treatment. Stomatal conductances of both species decreased with CO₂-enrichment resulting in significant increases in water use efficiency.     

The effect of different growth conditions on dark and light carbon assimilation in Littorella uniflora

The effect of long-term exposure to different inorganic carbon, nutrient and light regimes on CAM activity and photosynthetic performance in the submerged aquatic plant, Littorella uniflora (L.) Aschers was investigated. The potential CAM activity of Littorella was highly plastic and was reduced upon exposure to low light intensities (43 μmol m⁻² s⁻¹), high CO₂ concentrations (5.5 mM, pH 6.0) or low levels of inorganic nutrients, which caused a 25–80% decline in the potential maximum CAM activity relative to the activity in the control experiments (light: 450 μmol m⁻² s⁻¹; free CO₂: 1.5 mM). The CAM activity was regulated more by light than by CO₂, while nutrient levels only affected the activity to a minor extent. The minor effect of low nutrient regimes may be due to a general adaptation of isoetid species to low nutrient levels.
The photosynthetic capacity and CO₂ affinity was unaffected or increased by exposure to low CO₂, irrespective of nutrient levels. High CO₂, low nutrient and low light, however, reduced the capacity by 22–40% and the CO₂ affinity by 35-45%, relative to control.
The parallel effect of growth conditions on CAM activity and photosynthetic performance of Littorella suggest that light and dark carbon assimilation are interrelated and constitute an integrated part of the carbon assimilation physiology of the plant. The results are consistent with the hypothesis that CAM is a carbon-conserving mechanism in certain aquatic plants. The investment in the CAM enzyme system is beneficial to the plants during growth at high light and low CO₂ conditions.     

Photosynthesis and transpiration by young Larix kaempferi trees: C3 responses to light and temperature

The effects of photon flux density and temperature on net photosynthesis and transpiration rates of mature and immature leaves of three-year-old Japanese larch Larix kaempferi (Lamb.) Sarg. trees were determined with an infrared, differential open gas analysis system. Net photosynthetic response to increasing photon flux densities was similar for different foliage positions and stage of maturity. Light compensation was between 25 and 50 μmol m⁻² s⁻¹. Rates of photosynthesis increased rapidly at photon flux densities above the compensation level and became saturated between 800 and 1000 μmol m⁻² s⁻¹. Transpiration rates at constant temperature likewise increased with increasing photon flux density, and leveled off between 800 and 1000 μmol m⁻² s⁻¹. Photosynthetic response to temperature was determined in saturating light and was similar for all foliage positions; it increased steadily from low temperatures to an optimum range betweeen 15 and 21°C and then decreased rapidly above 21°C. Transpiration rate, however, increased continuously with rising temperature up to the experimental maximum. CO₂ compensation concentrations for mature foliage varied between 58 and 59 μl l⁻¹; however, foliage borne at the apex of the terminal leader compensated at 75 μl l⁻¹. None of these data support the claim that Japanese larch possesses C₄ photosynthetic characteristics.     

Changes in pigment levels, Rubisco and respiratory enzyme activity of Ficus benjamina during acclimation to low irradiance

The objective of the present study was to determine the influence of reduced irradiance on the activities of ribulose bisphosphate carboxylase-oxygenase (Rubisco) and respiratory enzymes. Rooted cuttings of the tropical epiphyte. Ficus benjamina L., were grown in a shaded environment that excluded approximately 50% of the natural photosynthetically active irradiance (890 μmol m⁻² s⁻¹) for 4 months. Established plants were transferred and grown for 10 months under a range of irradiance levels with daily average maxima varying from a full-sun environment to 20% full sun (100%−1735; 50%−890; 40%−695; and 20%−303 μmol m⁻²s⁻¹). Chlorophyll, carotenoid and soluble protein content increased in Ficus leaves as irradiance level decreased, while Rubisco increased on a fresh weight basis but decreased on a protein basis. Glycolytic enzymes, enolase and pyruvate kinase, showed higher activities in full-sun plants on a protein and fresh weight basis. However, the activity of two mitochondrial enzymes, aconitase and malate dehydrogenase, was not different under the various irradiance levels. When transferred to a very low irradiance environment (18 μmol m⁻² s⁻¹), mature leaves exhibited increased chlorophyll and carotenoid levels regardless of previous irradiance treatment. Exposure to very low irradiance resulted in a large increase in enolase and pyruvate kinase activities. Only plants grown under full sun conditions showed a decline in Rubisco activity following growth at very low irradiance. Together, these studies demonstrate the ability of mature leaves of Ficus to biochemically adjust photosynthetic and respiratory components over a wide range of irradiance.     

Inhibition of photosynthesis by heat stress: the activation state of Rubisco as a limiting factor in photosynthesis

Although the catalytic activity of Rubisco increases with temperature, the low affinity of the enzyme for CO₂ and its dual nature as an oxygenase limit the possible increase in net photosynthesis with temperature. For cotton, comparisons of measured rates of net photosynthesis with predicted rates that take into account limitations imposed by the kinetic properties of Rubisco indicate that direct inhibition of photosynthesis occurs at temperatures higher than about 30°C. Inhibition of photosynthesis by moderate heat stress (i.e. 30–42°C) is generally attributed to reduced rates of RuBP regeneration caused by disruption of electron transport activity, and specifically inactivation of the oxygen evolving enzymes of photosystem II. However, measurements of chlorophyll fluorescence and metabolite levels at air-levels of CO₂ indicate that electron transport activity is not limiting at temperatures that inhibit CO₂ fixation. Instead, recent evidence shows that inhibition of net photosynthesis correlates with a decrease in the activation state of Rubisco in both C₃ and C₄ plants and that this decrease in the amount of active Rubisco can fully account for the temperature response of net photosynthesis. Biochemically, the decrease in Rubisco activation can be attributed to: (1) more rapid de-activation of Rubisco caused by a faster rate of dead-end product formation; and (2) slower re-activation of Rubisco by activase. The net result is that as temperature increases activase becomes less effective in keeping Rubisco catalytically competent. In this opinionated review, we discuss how these processes limit photosynthetic performance under moderate heat stress.     

The temperature response of photosynthesis in tobacco with reduced amounts of Rubisco

The reasons for the decline in net CO₂ assimilation ( A ) above its thermal optimum are controversial. We tested the hypothesis that increasing the ratio of Rubisco activase to Rubisco catalytic site concentration would increase the activation state of Rubisco at high temperatures. We measured photosynthetic gas exchange, in vivo electron transport ( J ) and the activation state of Rubisco between 15 and 45 °C, at 38 and 76 Pa ambient CO₂, in wild-type (WT) and anti- rbc S tobacco. The Rubisco content of the anti- rbc S lines was 30% (S7-1) or 6% (S7-2) of WT, but activase levels were the same in the three genotypes. Anti- rbc S plants had lower A than WT at all temperatures, but had a similar thermal optimum for photosynthesis as WT at both CO₂ levels. In WT plants, Rubisco was fully activated at 32 °C, but the activation state declined to 64% at 42 °C. By contrast, the activation state of Rubisco was above 90% in the S7-1 line, between 15 and 42 °C. Both A and J declined about 20% from T _opt to the highest measurement temperatures in WT and the S7-1 line, but this was fully reversed after a 20 min recovery at 35 °C. At 76 Pa CO₂, predicted rates of RuBP regeneration-limited photosynthesis corresponded with measured A in WT tobacco at all temperatures, and in S7-1 tobacco above 40 °C. Our observations are consistent with the hypothesis that the high temperature decline in A in the WT is because of an RuBP regeneration limitation, rather than the capacity of Rubisco activase to maintain high Rubisco activation state.     

Effects of long-term elevated [CO2] from natural CO2 springs on Nardus stricta: photosynthesis, biochemistry, growth and phenology

Plants of Nardus stricta growing near a cold, naturally emitting CO₂ spring in Iceland were used to investigate the long-term (> 100 years) effects of elevated [CO₂] on photosynthesis, biochemistry, growth and phenology in a northern grassland ecosystem. Comparisons were made between plants growing in an atmosphere naturally enriched with CO₂ (≈ 790 μ mol mol^–1) near the CO₂ spring and plants of the same species growing in adjacent areas exposed to ambient CO₂ concentrations (≈360 μ mol mol^–1). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (≈25%), Rubisco content (≈26%), Rubisco activity (≈40%), Rubisco activation state (≈23%), chlorophyll content (≈33%) and leaf area index (≈22%) compared with plants growing away from the spring. The potential positive effects of elevated [CO₂] on grassland ecosystems in Iceland are likely to be reduced by strong down-regulation in the photosynthetic apparatus of the abundant N. stricta species.     

Response of net photosynthesis in bean (Phaseolus vulgaris) leaves to the elevation of the partial pressures of oxygen and carbon dioxide

Bean ( Phaseolus vulgaris L. cv. Golden Saxa) plants were grown under low artificial light or under natural daylight. The rate of net photosynthesis (P_N) was measured at: CO₂ partial pressure, p(CO₂), of 0.03, 0.09 or 0.15 kPa; O₂ partial pressure, p(O₂), of 2, 21 or 31 kPa and at light intensities of 350 or 1000 μmol m⁻² s⁻¹ (photosynthetically active radiation). In plants which had been grown under natural light, stimulation of P_N at 21 kPa p(O₂) was found only at elevated p(CO₂) and high light. It is proposed that this phenomenon is dependent on a high capacity of the photosynthetic apparatus to regenerate ribulose 1.5-bisphosphate.     

Stomatal limitation of photosynthesis and reduced growth of the halophyte, Plantago maritima L., at high salinity

Abstract. Plantago maritima L. was grown at three levels of salinity, 50, 200, 350 mol m⁻³ NaCl, and the effects on growth, ion content and photosynthetic capacity were studied. Shoot and root dry weight, leaf production and leaf length were all substantially reduced in plants grown at high salinity. Total leaf area of plants grown at 350 mol m⁻³ NaCl was only 20% of that in plants at low salinity. Both the Na⁺ and K⁺ content of leaves and roots increased with external salinity. There was no change in the Na⁺/K⁺ ratio of leaves or roots at different salinity levels. Despite the large reductions in growth and high accumulation of Na⁺ ions, leaf photosynthetic rate was only slightly reduced by salinity stress. The reduction in photosynthesis was not caused by reduced biochemical capacity as judged by photosynthetic response to intercellular CO₂ and by ribulose-1,5-bisphosphate carboxylase activity, but was due to reduced leaf conductance and low intercellular CO₂ concentration. The increased stomatal limitation of photosynthesis resulted in higher water-use efficiency of plants grown at high salinity.     

Photoinhibition of photosynthesis in Lemna gibba as induced by the interaction between light and temperature. I. Photosynthesis in vivo

The floating angiosperm Lemna gibba L. was exposed for 2 h to various combinations of photosynthetic photon flux densities and temperature. The extent of photoinhibition of photosynthesis was assayed by measuring the net CO₂ uptake before and after a photoinhibitory treatment, and the time course for photoinhibition was studied. It was found that the maximum quantum yield and the light-saturated rate of CO₂ uptake were affected by the interaction between light and temperature during the photoinhibitory treatment. At a constant photon flux density of 650 μmol m⁻² s⁻¹ the extent of photoinhibition increased with decreasing temperature showing that even a chilling-resistant plant like L. gibba is much more susceptible to photoinhibition at chilling temperatures. About 60% photoinhibition of the quantum yield for CO₂ uptake could be obtained either by a high photon flux density of 1 750 μmol m⁻² s⁻¹ and 25°C or by a moderate photon flux density of 650 μmol m⁻² s⁻¹ and 3°C. The time courses of recovery from 60% photoinhibition produced by either of these two treatments were similar, indicating that the nature of the photoinhibition was intrinsically similar. The extent of photoinhibition was related to the amount of light absorbed in excess to what could be handled by photosynthesis at that temperature. The vital importance of photosynthesis in alleviating photoinhibition is discussed.     

Spectacular abundance of ciliates in anoxic pond water: contribution of symbiont photosynthesis to host respiratory oxygen requirements

Abstract: Very large numbers (3466 ml⁻¹) of ciliated protozoa were found living beneath the oxic-anoxic boundary in a stratified freshwater pond. Most ciliates (96%) contained symbiotic algae ( Chlorella spp.). Peak abundance was in anoxic water with almost 1 mol free CO₂ m⁻³ and a midday irradiance of 6 μmol photon m⁻² s⁻¹. Photosynthetic rate measurements of metalimnetic water indicated a light compensation point of 1.7 μmol photon m⁻² s⁻¹ which represents 0.6% of sub-surface light. We calculate that photosynthetic evolution of O₂ by symbionts is sufficient to meet the demand of the host ciliates for 13 to 14 hours each day. Each 'photosynthetic ciliate' may therefore become an aerobic island surrounded by anoxic water.     

Regulation of the photosynthetic capacity of primary bean leaves by the red:far-red ratio and photosynthetic photon flux density of incident light

The main objective of the present work was to examine the effects of the red:far-red ratio (R:FR) prevailing during leaf development on the photosynthetic capacity of mature leaves. Plants of Phaseolus vulgaris L. cv. Balin de Albenga were grown from time of emergence in a controlled environment room, 25 ± 3°C, 12-h photoperiod, with different light treatments:a) high photosynthetic photon flux density (PPFD) = 800 μmol m⁻¹ s⁻¹+ high R:FR= 1.3;b) low PPFD= 300 μmol m⁻² s⁻¹+ high R:FR= 1.3; c) high PPFD=800 μmol m⁻² s⁻¹+ low R:FR= 0.7; d) low PPFD= 300 μmol m⁻²s⁻¹+ low R:FR=0.7. With an R:FR ratio of 1.3, a decrease in irradiance during leaf growth reduced photosynthesis when measured at moderate to high PPFD; but when measured at low PPFD, leaves expanded under low irradiance actually had photosynthesis rates higher than those of leaves grown in high irradiance. A low R:FR ratio during development reduced the photosynthetic capacity of the leaves. In leaves expanded under R:FR = 0.7 and high irradiance photosynthesis was reduced by 42 to 89%, depending on the PPFD at which measurements were made, whereas for leaves developed at R:FR = 0.7 and low irradiance photosynthesis decreased by 21 to 24%, compared to leaves under R:FR = 1.3 and similar irradiance. The reduced photosynthetic capacity under R:FR = 0.7 and high irradiance. In natural environments, leaves may experience low R:FR conditions temporarily during their development, and this may affect their future photosynthetic capacity in full sunlight.     

Maximum CO2-assimilation rates of vascular plants on an Alaskan arctic tundra slope

Light-saturated CO₂-assimilation rates of 19 vascular plant species were measured on a tundra slope in the foothills of the Brooks Range, Alaska. Maximum assimilation capacities on a leaf area basis ranged from 20.3 μmol m⁻² s⁻¹ for the forb, Bistorta plumosa , to 6.0 μmol m⁻² s⁻¹ for the evergreen, Empetrum hermaphroditicum . Graminoids, deciduous shrubs, and forbs fell within a similar range of maximum photosynthetic rates on a leaf area basis. Evergreens had the lowest rates. On a leaf weight basis, maximum assimilation rates were greatest for forbs, followed by deciduous shrubs, graminoids, and evergreens. Rates of evergreens were less than half those of all other growth forms. Cassiope tetragona had the lowest rates per unit leaf weight of any species tested; mean maximum rates of C. tetragona were only 14% of those of B. plumosa , the species with the highest rates. When the data were subjected to canonical analysis, only a partial correspondence was found between species growth form and photosynthetic characteristics.     

Photosynthetic response of Eragrostis tef to temperature

Photosynthetic characteristics of leaves of tef, Eragrostis tef (Zucc.) Trotter, plants, grown at 25/15°C (day/night), were measured at temperatures from 18 to 48°C. The highest carbon exchange rates (CER) occurred between 36 and 42°C. and averaged 27 μmol m⁻² s⁻¹. At lower or higher temperatures, CER was reduced, but the availability of CO₂ to the mesophyll, measured as internal CO₂ concentration, was highest when temperatures were above or below the optimum for CER. In addition, CER and stomatal conductance were not correlated, but residual conductance was highly correlated with CER (r = 0.98). In additional experiments, relative ¹³C composition for leaf tissue grown at 25, 35 and 45°C averaged -14.4 per mille, confirming that tef is a C₄ grass species. Dry matter accumulation was higher at 35 than at 25, and lowest at 45°C. Leaf CER rates increased hyperbolically with increased light when measured from 0 to 2000 μmol m⁻² s⁻¹ PPFD. The highest CER, 31.8 μ-mol m_-2 s⁻¹, occurred at 35°C and 2000 μmol m⁻² s⁻¹ PPFR. At high light, CER at 25 and 35°C were nearly equal because of higher stomatal conductance at 25°C. Residual conductance was, however, clearly highest at 35°C compared to 25 and 45°C treatments. Stomatal conductance and residual conductance were not correlated in either set of experiments, yet residual conductance was always highest when temperatures were between 35 and 42°C across experiments, suggesting that internal leaf photosynthetic potential was highest across that temperature range.     

The CO2concentrating mechanism in cyanobactiria and microalgae

Over the past 10 years it has become clear that cyanobacteria and microalgae possess mechanisms for actively acquiring inorganic carbon from the external medium and are able to use this to elevate the CO₂ concentration around the active site of the primary photosynthetic carboxylating enzyme, ribulose bisphosphate carboxylase-oxygenase (Rubisco). This results in a vastly enhanced photosynthetic affinity for inorganic carbon (C_i) and improved photosynthetic efficiency. The CO₂ concentrating mechanism is dependent on the existence of membrane bound C_i transport systems, and a microenvironment within the cell where the accumulated C_i can be used to elevate CO₂ at the site of Rubisco. Evidence presented in this review suggests that in cyanobacteria this is achieved by the packaging of Rubisco and carbonic anhydrase (CA) into discrete structures, which are termed carboxysomes. Analogous structures in microalgae, termed pyrenoids, may perform a similar function. The recovery and analysis of high-CO₂-requiring mutants has greatly advanced our understanding of the mechanisms and genes underlying these systems, especially in cyanobacteria, and this review places particular emphasis on the contribution made by molecular genetic approaches.