FREEZING STRESS AND OSMOTIC DEHYDRATION IN FUCUS DISTICHUS (PHAEOPHYTA): EVIDENCE FOR PHYSIOLOGICAL SIMILARITY1

The effects of osmotic dehydration and freezing on photosynthesis were studied in the brown alga Fucus distichus L. The data indicated that F. distichus exhibits similar physiological responses to both osmotic dehydration and freezing stress and that these responses resemble those in the literature for the effect of desiccation in air. Both stresses inhibited light-limited (P_subsat) and light-saturated (P_max) photosynthesis measured immediately after plants were reimmersed in seawater. The degree of initial inhibition and subsequent recovery of photosynthesis were proportional to the severity of the dehydration or freezing treatment. P_subsat and P_max recovered completely from osmotic dehydration for 3 h in 200% and 3 hr at – 10°C, but recovery was only partial following 3 h in 300%o or 3 h at – 15°C. In most cases, recovery was complete within 2 h following dehydration, with little further recovery occurring between 2 and 24 h posttreatment. No time-dependent recovery occurred following severe freezing. Observations using the vital stain fluorescein diacetate suggested that the lack of complete recovery might be due to severe damage or death of a proportion of cells in the thallus. There were no clear effects of either osmotic dehydration or freezing on dark respiration (R_d), although R_d was stimulated in all emersed treatments (frozen plants and 5° C controls) immediately following reimmersion. Measurement of chlorophyll fluorescence induction kinetics indicated that both osmotic dehydration and freezing reduced the ratio of variable to maximum florescence (F_v/F_m), indicating a decrease in the quantum efficiency of photosystem I. Based on these data, we suggest that there are common cellular and physiological components involved in the response of fucoid algae to a range of water stresses. This hypothesis was supported by experiments that showed that osmoacclimation in hyperosmotic seawater (51%o)for 2 weeks increased the ability of F. distichus to recover from freezing at – 15° C. During acclimation, mannitol content increased under hyperosmotic conditions and decreased under hypoosmotic conditions. Changes in plasma membrane integrity, determined by fresh weight: dry weight ratio, and amino acid release following freezing indicated an increasing gradient of freezing tolerance from low to high salinity. However, none of these physiological changes fully explained the marked increase in the freezing tolerance of photosynthesis observed in plants acclimated under hyperosmotic conditions.     

Long- and short-term responses of leaf carbohydrate levels and photosynthesis to decreased sink demand in soybean

Axillary buds and the apical portion of shoots of soybean [Glycine max (L.) Merr. cultivar Turchina] plants were trimmed to investigate long-term regulation of photosynthesis by sink demand at ambient CO₂ and 22 °C. Also, in intact and trimmed shoots, the CO₂ level was increased to 660 μmol mol^?1 and temperature was lowered to 5°C to examine the superimposed short-term responses of photosynthesis to low sink demand. Under growth conditions, trimming the shoots increased leaf photosynthesis and the levels of sucrose, glucose-6-phosphate (G6P) and 3-phosphoglycerate (PGA), as well as the G6P/fructose-6-phosphate (F6P) and sucrose/starch ratios, while it decreased the level of starch and the triose-phosphate (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP)/PGA ratio. Photosynthesis enhancement was accompanied by increased chlorophyll contents and ribulose-l,5-bisphosphate carboxylase oxygenase (Rubisco) activity. Sink removal consistently increased photosynthesis measured under a variety of conditions (growth CO₂ or a short-term change to 660 μmol mol_-1 CO₂; growth temperature or a short-term change to 5 °C), except when low temperature was combined with ambient CO₂; the increase in photosynthesis was higher under short-term elevated CO₂ than at ambient CO₂. In contrast with its effect at ambient CO₂, shoot trimming increased the levels of TP and ribulose-1,5-bisphosphate (RuBP) and the TP/PGA ratio under high-CO₂ conditions.     

Photosynthetic Acclimation to Temperature in the Desert Shrub, Larrea divaricata: I. Carbon Dioxide Exchange Characteristics of Intact Leaves

Larrea divaricata, a desert evergreen shrub, has a remarkable ability to adjust its photosynthetic temperature response characteristics to changing temperature conditions. In its native habitat on the floor of Death Valley, California, plants of this C₃ species when provided with adequate water are able to maintain a relatively high and constant photosynthetic activity throughout the year even though the mean daily maximum temperature varies by nearly 30 C from winter to summer. The temperature dependence of light-saturated net photosynthesis varies in concert with these seasonal temperature changes whereas the photosynthetic rate at the respective optimum temperatures shows little change.

Experiments on plants of the same age, grown at day/night temperatures of 20/15, 35/25, and 45/33 C with the same conditions of day length and other environmental factors, showed a similar photosynthetic acclimation response as observed in nature. An analysis was made of a number of factors that potentially can contribute to the observed changes in the temperature dependence of net CO₂ uptake at normal CO₂ and O₂ levels. These included stomatal conductance, respiration, O₂ inhibition of photosynthesis, and nonstomatal limitations of CO₂ diffusive transport. None of these factors, separately or taken together, can account for the observed acclimation responses. Measurements under high saturating CO₂ concentrations provide additional evidence that the observed adaptive responses are primarily the result of changes in intrinsic characteristics of the photosynthetic machinery at the cellular or subcellular levels. Two apparently separate effects of the growth temperature regime can be distinguished: one involves an increased capacity for photosynthesis at low, rate-limiting temperatures with decreased growth temperature, and the other an increased thermal stability of key components of the photosynthetic apparatus with increased growth temperature.

     

Photosynthetic response of Tempranillo grapevine to climate change scenarios

Photosynthesis in C₃ plants is CO₂ limited and therefore any increase in Rubisco carboxylation substrate may increase net CO₂ fixation, unless plants experience acclimation or other limitations. These aspects are largely unexplored in grapevine. Photosynthesis analysis was used to assess the stomatal, mesophyll, photochemical and biochemical contributions to the decreasing photosynthesis observed in Tempranillo grapevines (Vitis vinifera) from veraison to ripeness, modulated by CO₂, temperature and water availability. Photosynthesis and photosystem II photochemistry decreased from veraison to ripeness. The elevated CO₂ and temperature increased photosynthesis, but transiently, in both well irrigated (WI) and water‐stressed plants. Photosynthetic rates were maxima 1 week after the start of elevated CO₂ and temperature treatments, but differences with treatments of ambient conditions disappeared with time. There were not marked changes in leaf water status, leaf chlorophyll or leaf protein that could limit photosynthesis at ripeness. Leaf total soluble sugars remained at ripeness as high as 2 weeks after the start of treatments. On the other hand, and as expected, CO₂ diffusional limitations impaired photosynthesis in grapevine plants grown under water scarcity, stomatal and mesophyll conductances to CO₂ decreased and in turn low chloroplastic CO₂ concentrations limited photosynthetic CO₂ fixation. In summary, photochemistry and photosynthesis from veraison to ripeness in Tempranillo grapevine were dominated by a developmental‐related decreasing trend that was only transiently influenced by elevated CO₂ concentrations.     

Co-overproducing Rubisco and Rubisco activase enhances photosynthesis in the optimal temperature range in rice

Rubisco limits C₃ photosynthesis under some conditions and is therefore a potential target for improving photosynthetic efficiency. The overproduction of Rubisco is often accompanied by a decline in Rubisco activation, and the protein ratio of Rubisco activase (RCA) to Rubisco (RCA/Rubisco) greatly decreases in Rubisco-overproducing plants (RBCS-ox). Here, we produced transgenic rice (Oryza sativa) plants co-overproducing both Rubisco and RCA (RBCS-RCA-ox). Rubisco content in RBCS-RCA-ox plants increased by 23%–44%, and RCA/Rubisco levels were similar or higher than those of wild-type plants. However, although the activation state of Rubisco in RBCS-RCA-ox plants was enhanced, the rates of CO₂ assimilation at 25°C in RBCS-RCA-ox plants did not differ from that of wild-type plants. Alternatively, at a moderately high temperature (optimal range of 32°C–36°C), the rates of CO₂ assimilation in RBCS-ox and RBCS-RCA-ox plants were higher than in wild-type plants under conditions equal to or lower than current atmospheric CO₂ levels. The activation state of Rubisco in RBCS-RCA-ox remained higher than that of RBCS-ox plants, and activated Rubisco content in RCA overproducing, RBCS-ox, RBCS-RCA-ox, and wild-type plants was highly correlated with the initial slope of CO₂ assimilation against intercellular CO₂ pressures (A:Ci) at 36°C. Thus, a simultaneous increase in Rubisco and RCA contents leads to enhanced photosynthesis within the optimal temperature range.

A simultaneous increase in Rubisco and RCA contents in transgenic rice leads to an enhancement of photosynthesis at moderately high temperatures within the optimal temperature range.     

Photosynthesis in cold acclimated leaves of plants with various degrees of freezing tolerance

The objective of this study was to compare the photosynthetic changes during cold acclimation in various plant types able to acquire different degrees of freezing tolerance. Four herbaceous and six woody plants were hardened under natural or artificial conditions and – after determination of their frost resistance (LT₅₀) – the net photosynthetic rate at an ambient CO₂ of 33 Pa (P_n33), the dependencies of P_n to light and to CO₂ and the room temperature chlorophyll a fluorescence were recorded under optimal conditions. Herbaceous plants acquired freezing tolerances to temperatures between ?10 and ?15°C when hardened at temperatures around 0°C. Most leaves fully developed prior to frost hardening exhibited typical symptoms of senescence after frost hardening. In non-senescing leaves P_n33 was reduced by 15 to 50% mainly due to a reduced stomatal conductance. After hardening at temperatures around ?10°C Brassica survived down to ?24°C, but P_n33 was almost abolished as a result of disturbances in the chloroplasts. After transferring the plants to 20/15°C P_n33 recovered completely within a few days. Woody plants hardened at temperatures around 0°C tolerated – 15 to ?36°C: P_n33 was reduced by 25 to 60% and hardly recovered at 20/15°C. Hardening at ?10°C induced a tolerance of ?32 to n33 was almost totally blocked, but at 20/15°C it returned to the values of the plants hardened at 0°C within a few days. In woody plants disturbances were invariably localized in the chloroplasts. Thus, conifers, and especially Pinus cembra, can survive much lower temperatures than herbaceous plants and, at the same level of freezing tolerance, exhibit appreciably less restriction in relative P_n33.     

Whole plant respiration and photosynthesis of wheat under increased CO2 concentration and temperature: long-term vs. short-term distinctions for modelling

Short- and long-term effects of elevated CO₂ concentration and temperature on whole plant respiratory relationships are examined for wheat grown at four constant temperatures and at two CO₂ concentrations. Whole plant CO₂ exchange was measured on a 24 h basis and measurement conditions varied both to observe short-term effects and to determine the growth respiration coefficient (r_g), dry weight maintenance coefficient (r_m), basal (i.e. dark acclimated) respiration coefficient (r_g), and 24 h respiration:photosynthesis ratio (R:P). There was no response of r_g to short-term variation in CO2 concentration. For plants with adequate N supply, r_g was unaffected by the growth-CO₂ despite a 10% reduction in the plant's N concentration (%N). However, r_m was decreased 13%, and r_b was decreased 20% by growth in elevated CO₂ concentration relative to ambient. Nevertheless, R:P was not affected by growth in elevated CO₂. Whole plant respiration responded to short-term variation of ± 5 °C around the growth temperature with low sensitivity (Q₁₀= 1.8 at 15 °C, 1.3 at 30 °C). The shape of the response of whole plant respiration to growth temperature was different from that of the short term response, being a slanted S-shape declining between 25 and 30 °C. While r_m, increased, r_g decreased when growth temperature increased between 15 and 20 °C. Above 20 °C r_m became temperature insensitive while r_g increased with growth temperature. Despite these complex component responses, R:P increased only from 0.40 to 0.43 between 15° and 30 °C growth temperatures. Giving the plants a step increase in temperature caused a transient increase in R:P which recovered to the pre-transient value in 3 days. It is concluded that use of a constant R:P with respect to average temperature and CO₂ concentration may be a more simple and accurate way to model the responses of wheat crop respiration to ‘climate change’ than the more complex and mechanistically dubious functional analysis into growth and maintenance components.     

Interactions of elevated carbon dioxide and temperature with aphid feeding on transgenic oilseed rape: Are Bacillus thuringiensis (Bt) plants more susceptible to nontarget herbivores in future climate?

Climate change factors such as elevated carbon dioxide (CO₂) and temperature typically affect carbon (C) and nitrogen (N) dynamics of crop plants and the performance of insect herbivores. Insect‐resistant transgenic plants invest some nutrients to the production of specific toxic proteins [i.e. endotoxins from Bacillus thuringiensis (Bt)], which could alter the C–N balance of these plants, especially under changed abiotic conditions. Aphids are nonsusceptible to Lepidoptera‐targeted Bt Cry1Ac toxin and they typically show response to abiotic conditions, and here we sought to discover whether they might perform differently on compositionally changed Bt oilseed rape. Bt oilseed rape had increased N content in the leaves coupled with reduced total C compared with its nontransgenic counterpart, but in general the C : N responses of both plant types to elevated CO₂ and temperature were similar. Elevated CO₂ decreased N content and increased C : N ratio of both plant types. Elevated temperature increased C and N contents, total chlorophyll and carotenoid concentrations under ambient CO₂, but decreased these under elevated CO₂. In addition, soluble sugars were increased and starch decreased by elevated temperature under ambient but not under elevated CO₂, whereas photosynthesis was decreased in plants grown under elevated temperature in both CO₂ levels. Myzus persicae, a generalist aphid species, responded directly to elevated temperature with reduced developmental time and decreased adult and progeny weights, whereas the development of the Brassica specialist Brevicoryne brassicae was less affected. Feeding by M. persicae resulted in an increase in the N content of oilseed rape leaves under ambient CO₂, indicating the potential of herbivore feeding itself to cause allocation changes. The aphids performed equally well on both plant types despite the differences between C–N ratios of Bt and non‐Bt oilseed rape, revealing the absence of plant composition‐related effects on these pests under elevated CO₂, elevated temperature or combined elevated CO₂ and temperature conditions.     

Prechilling of Xanthium strumarium L. Reduces Net Photosynthesis and, Independently, Stomatal Conductance, While Sensitizing the Stomata to CO(2)

Greenhouse-grown plants of Xanthium strumarium L. were exposed in a growth cabinet to 10 C during days and 5 C during nights for periods of up to 120 hours. Subsequently, CO₂ exchange, transpiration, and leaf temperature were measured on attached leaves and in leaf sections at 25 or 30 C, 19 C dew point of the air, 61 milliwatts per square centimeter irradiance, and CO₂ concentrations between 0 and 1000 microliters per liter ambient air. Net photosynthesis and stomatal conductance decreased and dark respiration increased with increasing duration of prechilling. The reduction in net photosynthesis was not a consequence of decreased stomatal conductance because the intercellular CO₂ concentration in prechilled leaves was equal to or greater than that in greenhouse-grown controls. The intercellular CO₂ concentration at which one-half maximum net photosynthesis occurred remained the same in prechilled leaves and controls (175 to 190 microliters per liter). Stomata of the control plants responded to changes in the CO₂ concentration of the air only slightly. Prechilling for 24 hours or more sensitized stomata to CO₂; they responded to changes in CO₂ concentration in the range from 100 to 1000 microliters per liter.     

Temperature dependence of photosynthesis in cotton

Cotton plants (Gossypium hirsutum L., var. Deltapine Smooth Leaf) were grown under controlled environmental conditions over a range of day/night temperatures from 20/15 to 40/35 C. Their photosynthetic characteristics were then measured over a comparable temperature range. Net photosynthesis tended stongly to be greatest, and intracellular resistance to CO₂ transport to be lowest, when the measurement temperature corresponded to the daytime growth temperature, suggesting pronounced acclimation of the plants to the growth temperature. The preferred growth temperature was close to the 25/20 C regime, since net photosynthesis of these plants, regardless of measurement temperature, was higher and intracellular resistance lower, than in plants from any other regime.     

Acclimation of photosynthesis to elevated CO2 in onion (Allium cepa) grown at a range of temperatures

Onion (Allium cepa) was grown in the field within temperature gradient tunnels (providing about ‐2.5°C to +2.5°C from outside temperatures) maintained at either 374 or 532 μmol mol^?1 CO₂. Plant leaf area was determined non‐destructively at 7 day intervals until the time of bulbing in 12 combinations of temperature and CO₂ concentration. Gas exchange was measured in each plot at the time of bulbing, and the carbohydrate content of the leaf (source) and bulb (sink) was determined. Maximum rate of leaf area expansion increased with mean temperature. Leaf area duration and maximum rate of leaf area expansion were not significantly affected by CO₂. The light‐saturated rates of leaf photosynthesis (A_sat) were greater in plants grown at normal than at elevated CO₂ concentrations at the same measurement CO₂ concentration. Acclimation of photosynthesis decreased with an increase in growth temperature, and with an increase in leaf nitrogen content at elevated CO₂. The ratio of intercellular to atmospheric CO₂ (C₁/C₃ ratio) was 7.4% less for plants grown at elevated compared with normal CO₂. A_sat in plants grown at elevated CO₂ was less than in plants grown at normal CO₂ when compared at the same C₁. Hence, acclimation of photosynthesis was due both to stomatal acclimation and to limitations to biochemical CO₂ fixation. Carbohydrate content of the onion bulbs was greater at elevated than at normal CO₂. In contrast, carbohydrate content was less at elevated compared with normal CO₂ in the leaf sections in which CO₂ exchange was measured at the same developmental stage. Therefore, acclimation of photosynthesis in fully expanded onion leaves was detected despite the absence of localised carbohydrate accumulation in these field‐grown crops.     

Role of Photorespiration and Cyclic Electron Transport in C<Subscript>4</Subscript> Photosynthesis Evolution in the C<Subscript>3</Subscript>–C<Subscript>4</Subscript> Intermediate Species <Emphasis Type="Italic">Sedobassia sedoides</Emphasis>

Plants from two Sedobassia sedoides (Pall.) Aschers populations (Makan and Valitovo) (Chenopodiaceae) with C₂ photosynthesis (precursor of C₄ photosynthesis in phylogenesis) and photorespiratory CO₂-concentrating mechanism were studied. Genetic polymorphism and isotope discrimination (δ¹³С) levels of the plants were determined under natural conditions, and their morpho-physiological parameters such as fresh and dry biomass of the above ground parts of plants, functioning of photosystem I (PSI) and photosystem II (PSII), intensity of net photosynthesis (A), transpiration (E), photorespiration and water use efficiency (WUE) of plants were calculated under control and salinine conditions (0 and 200 mM NaCl). Results of the population-genetic analysis showed that the Makan population is polymorphic (plastic) and the Valitovo population is monomorphic (narrowly specialized). There were no significant differences between the populations based on δ¹³С values or growth parameters, PSII, A, E and WUE under control conditions. Under saline conditions, dry biomass accumulation decreased in the Makan population by 15% and by more than 2- fold in the Valitovo population. Population differences were revealed in terms of photorespiration intensity and P700 oxidation kinetics under control and saline conditions. Under control conditions, Makan plants were characterized by a higher photorespiration intensity, which decreased by 2-fold under saline conditions to the photorespiration level of Valitovo plants. Cyclic electron transport activity was minimal in the control Makan plants, and it increased by almost 2-fold under saline conditions to the level of that in Valitovo plants under control and saline conditions. Under control conditions, photosynthesis in Makan plants can be specified as the proto-Kranz type (transitional type from C₃ to C₂) and that in Valitovo plants can be specified as the C₂ type (C₄ photosynthesis with photorespiratory CO₂-concentrating mechanism), based on their photorespiration level and cyclic electron transport activity. Under saline conditions, Makan plants exhibited features of C₂ photosynthesis. Intraspecific functional differences of photosynthesis were revealed in different populations of intermediate C₃–C₄ plant species S. sedoides which reflect the initial stages of formation of a photorespiratory CO₂-concentrating mechanism during C₄ photosynthesis evolution, accompanied by decrease in salt tolerance.     

Light acclimation, CO2 response and long-term capacity of underwater photosynthesis in three terrestrial plant species

To characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO₂, and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea, Rumex crispus (both submergence-tolerant) and Arrhenatherum elatius (submergence-intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non-submerged (control) conditions, only R. crispus displayed shade acclimation. Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO₂ affinity of the elongated leaves. In control plants of the grass species P. arundinacea and A. elatius, CO₂ affinities were higher than for R. crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration. In contrast, underwater photosynthesis of the submergence-intolerant species A. elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay. To describe photosynthesis versus irradiance curves, four models were evaluated. The hyperbolic tangent produced the best goodness-of-fit, whereas the rectangular hyperbola (Michaelis-Menten model) gave relatively poor results.     

Short-term growth of meadow fescue with atmospheric CO2 enrichment decreases freezing tolerance, modifies photosynthetic apparatus performance and changes the expression of some genes during cold acclimation

The objective of this study was to assess the effect of an elevated atmospheric CO₂ molar ratio on freezing tolerance, photosynthetic apparatus performance and expression of CBF6, Cor14b and LOS2 in meadow fescue (Festuca pratensis Huds.). It was shown that cold acclimation under a CO₂ molar ratio of 800 μmol mol(air)^?1 decreased the freezing tolerance of meadow fescue when compared to the ambient CO₂ level. This effect was not related either to changes observed in PSII redox state or to photosynthetic acclimation to cold, which was in fact more effective at an elevated CO₂ level. The decrease in freezing tolerance was linked to changes in the expression of CBF6 and LOS2 genes, whereas the protective effect on photosynthetic apparatus was connected with the activation of a non-photochemical mechanism of photoprotection as well as upregulation of FpCOR14b expression.     

The influence of increasing growth temperature and CO2 concentration on the ratio of respiration to photosynthesis in soybean seedlings

Using controlled environmental growth chambers, whole plants of soybean, cv. ‘Clark’, were examined during early development (7–20 days after sowing) at both ambient (≈ 350 μL L^–1) and elevated (≈ 700 μL L^–1) carbon dioxide and a range of air temperatures (20, 25, 30, and 35 °C) to determine if future climatic change (temperature or CO₂ concentration) could alter the ratio of carbon lost by dark respiration to that gained via photosynthesis. Although whole-plant respiration increased with short-term increases in the measurement temperature, respiration acclimated to increasing growth temperature. Respiration, on a dry weight basis, was either unchanged or lower for the elevated CO₂ grown plants, relative to ambient CO₂ concentration, over the range of growth temperatures. Levels of both starch and sucrose increased with elevated CO₂ concentration, but no interaction between CO₂ and growth temperature was observed. Relative growth rate increased with elevated CO₂ concentration up to a growth temperature of 35 °C. The ratio of respiration to photosynthesis rate over a 24-h period during early development was not altered over the growth temperatures (20–35 °C) and was consistently less at the elevated relative to the ambient CO₂ concentration. The current experiment does not support the proposition that global increases in carbon dioxide and temperature will increase the ratio of respiration to photosynthesis; rather, the data suggest that some plant species may continue to act as a sink for carbon even if carbon dioxide and temperature increase simultaneously.     

Leaf senescence in response to elevated atmospheric CO<Subscript>2</Subscript> concentration and low nitrogen supply

This review reports the physiological and metabolic changes in plants during development under elevated atmospheric carbon dioxide concentration and/or limited-nitrogen supply in order to establish their effects on leaf senescence induction. Elevated CO₂ concentration and nitrogen supply modify gene expression, protein content and composition, various aspects of photosynthesis, sugar metabolism, nitrogen metabolism, and redox state in plants. Elevated CO₂ usually causes sugar accumulation and decreased nitrogen content in plant leaves, leading to imbalanced C/N ratio in mature leaves, which is one of the main factors behind premature senescence in leaves. Elevated CO₂ and low nitrogen decrease activities of some antioxidant enzymes and thus increase H₂O₂ production. These changes lead to oxidative stress that results in the degradation of photosynthetic pigments and eventually induce senescence. However, this accelerated leaf senescence under conditions of elevated CO₂ and limited nitrogen can mobilize nutrients to growing organs and thus ensure their functionality.     

Long- and short-term effects of decreased sink demand on carbohydrate levels and photosynthesis in wheat leaves

Wheat (Triticum aestivum L.) ears were removed to investigate long-term regulation of photosynthesis by sink demand at ambient CO₂ and 22 °C. The CO₂ level was also increased to 660 μmol mol^?1 and temperature was lowered to 5 °C to examine short-term responses of photosynthesis to low sink demand. Sink removal inhibited photosynthesis and increased leaf levels of glucose, fructose and ribulose-1, 5-bisphosphate (RuBP), and the glucose-6-phosphate (G6P)/fructose-6-phosphate (F6P) and RuBP/3-phosphoglycerate (PGA) ratios under growth conditions, but had no effect on the activity and activation state of ribulose-1, 5-bisphosphate carboxylase oxygenase (Rubisco) either under growth or short-term conditions, suggesting an inhibition of photosynthesis by decreased in vivo catalysis of Rubisco. Photosynthesis increased similarly in eared and earless shoots after a rise in CO₂ concentration, and the ratio of triose-phosphates (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP) to PGA was similar or higher for removed than intact ears, suggesting that feedback inhibition of photosynthesis was not caused by a limitation of ATP synthesis in chloroplasts. Under short-term conditions (660 μmol mol^?1 CO₂, 5 °C), TP and RuBP levels and the TP/PGA and TP/RuBP ratios were increased by sink removal, indicating an additional limitation of photosynthesis by the rate of RuBP regeneration.     

Photosynthesis and Transpiration as a Function of Gaseous Diffusive Resistances in Orange Leaves

The CO₂ and H₂O vapour exchange of single attached orange, Citrus sinensis (L.), leaves was measured under laboratory conditions using infrared gas analysis. Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm^-1) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (ri less than 10 s cm^-1) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H₂O vapour loss than over CO₂ assimilation. At higher leaf resistance (ri greater than 10 s cm^-1) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO₂ diffusive resistances (in a -O₂ gas stream) emphasised the presence of a significant mesophyll resistance; i.e., an additional and rate limiting resistance to CO₂ assimilation over and above that encountered by H₂O vapour escaping from the leaf.     

Oxygen Inhibition of Photosynthesis: I. Temperature Dependence and Relation to O(2)/CO(2) Solubility Ratio

The magnitude of the percentage inhibition of photosynthesis by atmospheric levels of O₂ in the C₃ species Solanum tuberosum L., Medicago sativa L., Phaseolus vulgaris L., Glycine max L., and Triticum aestivum L. increases in a similar manner with an increase in the apparent solubility ratio of O₂/CO₂ in the leaf over a range of solubility ratios from 25 to 45. The solubility ratio is based on calculated levels of O₂ and CO₂ in the intercellular spaces of leaves as derived from whole leaf measurements of photosynthesis and transpiration. The solubility ratio of O₂/CO₂ can be increased by increased leaf temperature under constant atmospheric levels of O₂ and CO₂ (since O₂ is relatively more soluble than CO₂ with increasing temperature); by increasing the relative levels of O₂/CO₂ in the atmosphere at a given leaf temperature, or by increased stomatal resistance. If the solubility ratio of O₂/CO₂ is kept constant, as leaf temperature is increased, by varying the levels of O₂ or CO₂ in the atmosphere, then the percentage inhibition of photosynthesis by O₂ is similar. The decreased solubility of CO₂ relative to O₂ (decreased CO₂/O₂ ratio) may be partly responsible for the increased percentage inhibition of photosynthesis by O₂ under atmospheric conditions with increasing temperature.     

Evidence of enhanced freezing damage in treeline plants during six years of CO2 enrichment and soil warming

Climate change and elevated atmospheric CO₂ levels could increase the vulnerability of plants to freezing. We analyzed tissue damage resulting from naturally occurring freezing events in plants from a long–term in situ CO₂ enrichment (+ 200 ppm, 2001–2009) and soil warming (+ 4°C since 2007) experiment at treeline in the Swiss Alps (Stillberg, Davos). Summer freezing events caused damage in several abundant subalpine and alpine plant species in four out of six years between 2005 and 2010. Most freezing damage occurred when temperatures dropped below –1.5°C two to three weeks after snow melt. The tree Larix decidua and the dwarf shrubs Vaccinium myrtillus and Empetrum hermaphroditum showed more freezing damage under experimentally elevated CO₂ and/or temperatures than under control conditions. Soil warming induced a 50% die‐back of E. hermaphroditum during a single freezing event due to melting of the protective snow cover. Although we could not identify a clear mechanism, we relate greater freezing susceptibility to a combination of advanced plant phenology in spring and changes in plant physiology. The climate record since 1975 at the treeline site indicated a summer warming by 0.58°C/decade and a 3.5 days/decade earlier snow melt, but no significant decrease in freezing events during the vegetation period. Therefore, in a warmer climate with higher CO₂ levels but constant likelihood of extreme weather events, subalpine and alpine plants may be more susceptible to freezing events, which may partially offset expected enhanced growth with global change. Hence, freezing damage should be considered when predicting changes in growth of alpine plants or changes in community composition under future atmospheric and climate conditions.