Accumulation of galloyl derivatives in a green alga, Spirogyra varians, in response to cold stress

The green alga Spirogyra varians accumulated antioxidative compounds in response to cold stress. When the algae were transferred from 20°C to 4°C, the amount of phenolic contents and flavonoids in the cell increased 17 times and 30 times, respectively, in 2 months. At this time, the radical scavenging activity of the methanolic extract of S. varians was 238 times higher than that of initial culture. To identify the responsible antioxidants, the methanolic extract was obtained from the algae grown at 4°C. HPLC analysis of the extract showed six compounds newly produced or increased over time. Four of the compounds were successfully purified, and the structures were identified using ¹H NMR spectroscopy. The compounds were galloyl derivatives—methyl gallate, 1-O-Galloyl-β-d-glucose, 1,2,3,6-tetra-O-Galloyl-β-d-glucose and 1,2,3,4,6-penta-O-Galloyl-β-d-glucose which are intermediates of the shikimate pathway.     

Responses of a new isolated <Emphasis Type="Italic">Cyanobacterium aponinum</Emphasis> strain to temperature,pH, CO<Subscript>2</Subscript> and light quality

A new strain of cyanobacteria was isolated from seawater samples collected near Jimo hot springs, Qingdao, China, and was identified as Cyanobacterium aponinum by 16S rDNA analysis. This study examined the effects of temperature, pH, light quality and high CO₂ concentration on the growth of the cyanobacteria. Results showed that the strain exhibited a higher growth rate (about 168.4 mg L^?1 day^?1) at 35 °C than other temperatures (surviving at up to 50 °C) and a wide growth tolerance to acidic stress (pH 3.0 to 4.0) resulting from either H₂SO₄ or HNO₃. The four light qualities, ranked by greatest to least biomass effect, were as follows: LED white light (LW) > LED red light (LR) > fluorescent white light (FW) > LED blue light (LB), achieving a higher lighting effect at a LW light intensity (60 μmol photons m^?2 s^?1) lower than other light qualities, which implied less energy consumption therewith. This strain demonstrates excellent CO₂ tolerance at least 10% CO₂ with the highest productivity in biomass (about 337.8 mg L^?1 day^?1) measured at 1% CO₂ level. Results indicate that this strain is a promising candidate for use in biofixation of CO₂ from flue gases emitted by thermoelectric plants.     

Light-dependent damage to photosynthesis in olive leaves during chilling and high temperature stress

Abstract The leaves of olive are long lived and likely to experience both chilling and high temperature stress during their life. Changes in photosynthetic CO₂ assimilation resulting from chilling and high temperature stress, in both dim and high light, are investigated. The quantum yield (φ) of photosynthesis at limiting light levels was reduced following chilling (at 5°C for 12 h), in dim light by approximately 10%, and in high light by 75%; the difference being attributed to photoinhibition. Similar reductions were observed in the light-saturated rate of CO₂ uptake (A_max). Decrease in A_max correlated with a halving of the leaf internal CO₂ concentration (c_i), suggesting an increased limitation by stomata following photoinhibition. Leaves were apparently more susceptible to photoinhibitory damage if the whole plant, rather than the leaf alone, was chilled. On return to 26 °C, I he photosynthetic capacity recovered to pre-stress levels within a few hours if leaves had been chilled in high light for 8 h or less, but did not fully recover from longer periods of chilling when loss of chlorophyll occurred. Leaves which were recovering from chilling in high light showed far more damage on being chilled a second time in high light. Three hours in high light at 38 °C reduced φ by 80%, but φ recovered within 4h of return to 26 °C. Although leaves of Olive are apparently less susceptible to photoinhibitory damage during chilling stress than the short-lived leaves of chilling-sensitive annual? crops, the results nevertheless show that photoinhibition during temperature stress is potentially a major factor influencing the photosynthetic productivity of Olive in the field.     

Photosynthetic response of Chlamydomonas reinhardtii to short-term storage on ice in darkness. Dependence of the response on growth conditions

The effect of storage of the unicellular green alga Chlamydomonas reinhardtii (strain 137+) in the pelleted state in darkness on ice (0.2–0.5°C) (further simply “SPDI-treatment”) on its photosynthetic and respiratory activities was studied. To this end, the steady-state rates of O₂ exchange in darkness (dark respiration) and under saturating light (apparent photosynthesis) as well as the induction periods (IP) of apparent photosynthesis were measured at 25°C in the SPDI-untreated and SPDI-treated for the period from ～0.5 to ～30 h algal cells. In contrast to expectations, the SPDI-treatment consistently affected the rate and IP of photosynthesis depending on the physiological state of C. reinhardtii. Dark respiration was affected by the SPDI-treatment as well. However, in absolute values the respiratory changes were much less than the photosynthetic ones, and they were insufficiently reproducible. The SPDI-treatment affected photosynthesis most significantly in high-CO₂-grown cells (cells grown at 5% CO₂ in white light). The rate of photosynthesis in these cells declined quasi-exponentially as a function of time during the SPDI-treatment with a t _1/2 ～1.5 h and finally became by about 60% lower than that before the SPDI-treatment. This decline of photosynthesis was accompanied by continuous and essential increase in the photosynthetic IP. The SPDI-induced photosynthetic changes in high-CO₂-grown cells resulted from the firm disfunction of the photosynthetic apparatus. After switch from growth at 5% CO₂ in white light to growth at ～0.03% CO₂ (air) in white, blue, or red light, the alga gradually transited to a physiological state, in which the negative effects of the SPDI-treatment on the rate and IP of photosynthesis became weak and absent, respectively. Remarkably, this transition was faster in blue (≤5 h) than in white and red light (>10 h). Similar changes in the response of the alga to the SPDI-treatment occurred when high-CO₂-grown cells (5% CO₂, white light, 26°C) were incubated in darkness (air, 24–26°C) for 20–25 h. The results of study were analyzed in the light of literature data relating to the effects of CO₂ concentration, darkness, and light quality on carbohydrates in plant organisms. The analysis led to suggestion that there is connection between the negative effect of the SPDI-treatment on C. reinhardtii and nonstructural carbohydrates presented in the alga: the more carbohydrates contain the alga, the more extensive inactivation of the photosynthetic apparatus occurs in it during its storage in the dense (pelleted) state in darkness on ice.     

Arbuscular mycorrhizae improves low temperature stress in maize via alterations in host water status and photosynthesis

The effect of arbuscular mycorrhizal (AM) fungus, Glomus etunicatum, on growth, water status, chlorophyll concentration and photosynthesis in maize (Zea mays L.) plants was investigated in pot culture under low temperature stress. The maize plants were placed in a sand and soil mixture at 25°C for 7 weeks, and then subjected to 5°C, 15°C and 25°C for 1 week. Low temperature stress decreased AM root colonization. AM symbiosis stimulated plant growth and had higher root dry weight at all temperature treatments. Mycorrhizal plants had better water status than corresponding non-mycorrhizal plants, and significant differences were found in water conservation (WC) and water use efficiency (WUE) regardless of temperature treatments. AM colonization increased the concentrations of chlorophyll a, chlorophyll b and chlorophyll a + b. The maximal fluorescence (Fm), maximum quantum efficiency of PSII primary photochemistry (Fv/Fm) and potential photochemical efficiency (Fv/Fo) were higher, but primary fluorescence (Fo) was lower in AM plants compared with non-AM plants. AM inoculation notably increased net photosynthetic rate (Pn) and transpiration rate (E) of maize plants. Mycorrhizal plants had higher stomatal conductance (g_s) than non-mycorrhizal plants with significant difference only at 5°C. Intercellular CO₂ concentration (Ci) was lower in mycorrhizal than that in non-mycorrhizal plants, especially under low temperature stress. The results indicated that AM symbiosis protect maize plants against low temperature stress through improving the water status and photosynthetic capacity.     

Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

Effects of heat stress on photosynthetic electron transport in a marine cyanobacterium <Emphasis Type="Italic">Arthrospira</Emphasis> sp.

Arthrospira (Spirulina) is widely used as human health food and animal feed. In cultures grown outdoors in open ponds, Arthrospira cells are subjected to various environmental stresses, such as high temperature. A better understanding of the effects of high temperature on photosynthesis may help optimize the productivity of Arthrospira cultures. In this study, the effects of heat stress on photosynthetic rate, chlorophyll a fluorescence transients, and photosystem (PS) II, PSI activities in a marine cyanobacterium Arthrospira sp. were examined. Arthrospira cells grown at 25 °C were treated for 30 min at 25 (control), 30, 34, 37, or 40 °C in the dark. Heat stress (30–37 °C) enhanced net photosynthetic O₂ evolution rate. Heat stress caused over-reduction PSII acceptor side, damage of donor side of PSII, decrease in the energetic connectivity of PSII units, and decrease in the performance of PSII. When the temperature changed from 25 to 37 °C, PSII activity decreased, while PSI activity increased, the enhancement of photosynthetic O₂ evolution was synchronized with the increase in PSI activity. When temperature was further increased to 40 °C, it induced a decrease in photosynthetic O₂ evolution rate and a more severe decrease in PSII activity, but an increase in PSI activity. These results suggest that PSI activity was the decisive factor determining the change of photosynthetic O₂ evolution when Arthrospira was exposed to a temperature from 25 to 37 °C, but then, PSII activity became the decisive factor adjusting the change of photosynthetic O₂ evolution when the temperature was increased to 40 °C.     

The effects of phenotypic plasticity on photosynthetic performance in winter rye, winter wheat and Brassica napus

The contributions of phenotypic plasticity to photosynthetic performance in winter (cv Musketeer, cv Norstar) and spring (cv SR4A, cv Katepwa) rye (Secale cereale) and wheat (Triticum aestivum) cultivars grown at either 20°C [non‐acclimated (NA)] or 5°C [cold acclimated (CA)] were assessed. The 22–40% increase in light‐saturated rates of CO₂ assimilation in CA vs NA winter cereals were accounted for by phenotypic plasticity as indicated by the dwarf phenotype and increased specific leaf weight. However, phenotypic plasticity could not account for (1) the differential temperature sensitivity of CO₂ assimilation and photosynthetic electron transport, (2) the increased efficiency and light‐saturated rates of photosynthetic electron transport or (3) the decreased light sensitivity of excitation pressure and non‐photochemical quenching between NA and NA winter cultivars. Cold acclimation decreased photosynthetic performance of spring relative to winter cultivars. However, the differences in photosynthetic performances between CA winter and spring cultivars were dependent upon the basis on which photosynthetic performance was expressed. Overexpression of BNCBF17 in Brassica napus generally decreased the low temperature sensitivity (Q₁₀) of CO₂ assimilation and photosynthetic electron transport even though the latter had not been exposed to low temperature. Photosynthetic performance in wild type compared to the BNCBF17‐overexpressing transgenic B. napus indicated that CBFs/DREBs regulate not only freezing tolerance but also govern plant architecture, leaf anatomy and photosynthetic performance. The apparent positive and negative effects of cold acclimation on photosynthetic performance are discussed in terms of the apparent costs and benefits of phenotypic plasticity, winter survival and reproductive fitness.     

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 temperature acclimation of photosynthetic responses to CO2 in Zea mays and its relationship to the activities of photosynthetic enzymes and the CO2-concentrating mechanism of C4 photosynthesis

Abstract Associations between photosynthetic responses to CO₂ at rate-saturating light and photosynthetic enzyme activities were compared for leaves of maize grown under constant air temperatures of 19, 25 and 31°C. Key photosynthetic enzymes analysed were ribulose bisphosphatc (RuBP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, NADP-malic enzyme and pyruvate, P_i dikinasc. Rates of CO₂-saturated photosynthesis were similar in leaves developed at 19°C and 25°C but were decreased significantly by growth at 31°C. In contrast, carboxylation efficiency differed significantly between all three temperature regimes. Carboxylation efficiency was greatest in leaves developed at 19°C and decreased with increasing temperature during growth. The changes of carboxylation efficiency were highly correlated with changes in the activity of pyruvate, P_i dikinase (r= 0.95), but not with other photosynthetic enzyme activities. The activities of these latter enzymes, including that of RuBP carboxylase, were relatively insensitive to temperature during growth. The sensitivity of quantum yield to O₂ concentration was lower in leaves grown at 19°C than in leaves grown at 31°C. These observations support the novel hypothesis that variation in the capacity for CO₂ delivery to the bundle sheath by the C₄ cycle, relative to the capacity for net assimilation by the C₂ cycle, can be a principal determinant of C₄ photosynthetic responses to CO₂.     

Protection Against Oxidative Stress Caused by Intermittent Cold Exposure by Combined Supplementation with Vitamin E and C in the Aging Rat Hypothalamus

This study investigated the effects of combined supplementation with vitamin E and C against oxidative stress (OS) caused by intermittent cold exposure (ICE) in the hypothalamus (HY) of aging male Wistar rats [adult (3-months), middle-aged (18-months) and old (24-months)]. Each age was divided into sub-groups: control (CON), cold-exposed at 10 °C (C10), cold-exposed at 5 °C (C5), supplemented control (CON+S) and supplemented cold-exposed at either 5 °C (C5+S) or 10 °C (C10+S). The supplement was a daily dose of 400 mg vitamin C and 50 IU of vitamin E/kg body weight. Cold exposure lasted 2 h/day for 4 weeks. All age groups exposed to cold showed increase in body mass and feeding efficiency. Feeding efficiency in the supplemented old group showed a statistically significant increase in the cold (p < 0.001). Age-related increases in levels of hydrogen peroxide (H₂O₂), protein carbonyl (PrC), advanced oxidation protein products and thiobarbituric acid reactive substances (TBARS) were further increased by cold in the HY. Cold reduced thiol(P-SH) levels and increased superoxide dismutase (SOD) and, catalase (CAT) activities as well as Hsp72 levels. However, supplementation lowered H₂O₂, PrC and TBARS with decreases in Hsp72 levels and in SOD and CAT activities. These changes were concomitant with elevations in P-SH, vitamin E and C levels. The results show that the OS caused by ICE in the HY and its subsequent protection following supplementation is related to the intensity of ICE as well as age of the animal. Immunohistochemical studies are underway to examine the findings on ICE-induced oxidative injury in the HY, and the prospects for vitamin E and C supplementation in the senescent.     

Physiological response to heat stress of tomato ‘Micro-Tom’ plants expressing high and low levels of mitochondrial sHSP23.6 protein

Tomato ‘Micro-Tom’ plants were transformed for high or low expression of the mitochondrial small “heat shock” protein (HSP) (MT-sHSP23.6) to evaluate their response to high temperature. The plants were raised for 59 days under a controlled temperature, photoperiod and photon flow density and then subjected to heat stress for 24 h at 37 °C, followed by a recovery period under normal conditions (21 ± 2 °C). The cycle was repeated. The chlorophyll a fluorescence intensity was measured, and the parameters of the JIP-test were calculated. The gas exchange was also evaluated. The JIP-test showed significantly different responses of the genotypes to heat stress. The parameters of photosystem I activity and the net assimilation of CO₂ increased during the first stress cycle in genotypes with a high expression of MT-sHSP23.6 and in non-transformed plants; however, the net assimilation of CO₂ decreased in genotypes with a low expression of MT-sHSP23.6. The data suggest that MT-sHSP23.6 participates in the heat tolerance mechanism, considering that the suppression of this protein resulted in greater physiological damage during heat stress.     

Prolonging the hydration and active metabolism from light periods into nights substantially enhances lichen growth

This study investigates how hydration during light and dark periods influences growth in two epiphytic old forest lichens, the green algal Lobaria pulmonaria and the cyanobacterial L. scrobiculata. The lichens were cultivated in growth chambers for 14 days (200 μmol m^?1 s^?2; 12 h photoperiod) at four temperature regimes (25/20 °C, 21/16 °C, 13/8 °C, and 6/1 °C; day/night temperatures) and two hydration regimes (12 h day-time hydration; 12 h day-time + 12 h night-time hydration). Growth was highly dynamic, showing that short-term growth experiments in growth cabinets have a high, but largely unexplored potential in functional lichen studies. The highest measured growth rates were not far from the maximal dry matter gain estimated from published net photosynthetic CO₂ uptake data. For the entire data set, photobiont type, temperature, hydration regime and specific thallus mass accounted for 46.6 % of the variation in relative growth rate (RGR). Both species showed substantially higher relative growth rates based on both biomass (RGR) and thallus area (RT_AGR) when they were hydrated day and night compared to hydration in light only. Chronic photoinhibition was substantial in thalli hydrated only during the day time and kept at the highest and lowest temperature regimes, resulting in exponential increases in RGR with increasing maximal PSII efficiency (F _v/F _m) in both species. However, the depression in F _v/F _m was stronger for the cyanolichen than for the cephalolichen at extreme temperatures. The growth-stimulating effect of night-time hydration suggests that nocturnal metabolic activity improves recovery of photoinhibition and/or enhances the conversion rate of photosynthates into thallus extension.     

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

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

Effects of elevated CO2 and temperature on the growth,elemental composition,and cell size of two marine diatoms: potential implications of global climate change

Two pennate diatoms, Amphora coffeaeformis and Nitzschia ovalis, were used to evaluate potential responses to the future CO₂ and temperature increases with respect to cell-specific growth rate, elemental composition, size, population growth rate, and carrying capacity. Diatoms were subjected to four different treatments over a 2 week period (approximately 4 generations): a control (28°C and present-day CO₂, ~400 ppm), high CO₂ (28°C with high CO₂, ~750 ppm), high temperature (31°C and present-day CO₂, ~400 ppm), and greenhouse-effect treatment (31°C with high CO₂, ~750 ppm). The results indicated that both the cell-specific growth rates and the carrying capacity of A. coffeaeformis decreased at the higher temperature treatment, whereas N. ovalis did not differ among all treatments. No significant difference was found in either species’ elemental cell composition, but higher C:N and C:P ratios were observed for A. coffeaeformis and N. ovalis, respectively, in high CO₂ and greenhouse-effect treatments. Smaller cell sizes were observed for both species under the greenhouse-effect treatment, a phenomenon that could alter benthic food webs in the future.     

The effect of temperature on photosynthetic induction under fluctuating light in Chrysanthemum morifolium

The photosynthetic response was investigated on Chrysanthemum morifolium under dynamic light conditions in the 20–35 °C temperature range to evaluate the effect of climatic variables on photosynthetic induction. The plant material was grown under uniform, controlled conditions and its gas exchange was analyzed. The gas exchange measurements were used to investigate the rate of induction, momentary induction state, and the opening of stomata. At the varying temperature ranges and under dynamic light conditions, C. morifolium reached a quasi-steady-state induction equilibrium (IS_eq(PAR,T)) within 14–45 min. For the same level of photosynthetically active radiation (PAR), the equilibrated level of steady-state induction increased as the temperature increased. It was highest approximately at 30 °C. The induction state was equilibrated at a lower level as the temperature increased to 35 °C. The interaction effect of PAR and temperature on induction state was not significant. The rate of photosynthetic induction and the time required at which the induction reached its 90 % value (t ₉₀) was influenced by PAR significantly. The light history of a leaf had a significant effect on t ₉₀, indicating that the time to reach a steady-state induction is different depending on the light environment and the period at which the leaf was exposed to light. The velocity of the photosynthetic induction was not affected by the temperature. It was associated with stomatal conductance of the leaf prior to the onset of light (g _Sini).     

The effects of development at sub-optimal growth temperatures on photosynthetic capacity and susceptibility to chilling-dependent photoinhibition in Zea mays

When plants of Zea mays L. cv. LG11 that have been grown at optimal temperatures are transferred to chilling temperatures (0–12°C) photoinhibition of photosynthetic CO₂ assimilation can occur. This study examines how growth at sub-optimal temperatures alters both photosynthetic capacity and resistance to chilling-dependent photoinhibition. Plants of Z. mays cv. LG11 were grown in controlled environments at 14, 17, 20 and 25°C. As a measure of the capacity for photosynthesis under light limiting conditions, the maximum quantum yields of CO₂ assimilation (φ^a.c) and O₂ evolution (φ^a.o) were determined for the laminae of the second leaves at photon fluxes of 50–150 μmol m^-2s^-1. To determine photosynthetic capacity at photon fluxes approaching light saturation, rates of CO₂ uptake (A₁₅₀₀) and O₂ evolution (A₁₅₀₀) were determined in a photon flux of 1500 μmol m^-2s^-1. In leaves developed at 14°C, φ and φ were 26 and 43%, respectively, of the values for leaves grown at 25°C. Leaves grown at 17°C showed intermediate reductions in φ and φ, whilst leaves developed at 20°C showed no significant differences from those grown at 25°C. Similar patterns of decrease were observed for A₁₅₀₀ and A_1500.0 with decreasing growth temperature. Leaves developed at 25°C showed higher rates of CO₂ assimilation at all light levels and measurement temperatures in comparison to leaves developed at 17 and 14°C. A greater reduction in A₁₅₀₀ relative to A_1500.0 with decreasing growth temperature was attributed to increased stomatal limitation. Exposure of leaves to 800–1000 μmol m^-2 s^-1 when plant temperature was depressed to ca 6.5°C produced a photoinhibition of photosynthetic CO₂ assimilation in all leaves. However, in leaves developed at 17°C the decrease in A₁₅₀₀ following this chilling treatment was only 25% compared to 90% in leaves developed at 25°C. Recovery following chilling was completed earlier in leaves developed at 17°C. The results suggest that growth at sub-optimal temperatures induces increased tolerance to exposure to high light at chilling temperatures. This is offset by the large loss in photosynthetic capacity imposed by leaf development at sub-optimal temperatures.     

Combined effects of CO2 enrichment and elevated growth temperatures on metabolites in soybean leaflets: evidence for dynamic changes of TCA cycle intermediates

Soybean (Glycine max [Merr.] L.) was grown in indoor chambers with ambient (38 Pa) and elevated (70 Pa) CO₂ and day/night temperature treatments of 28/20, 32/24 and 36/28 °C. We hypothesized that CO₂ enrichment would mitigate the deleterious effects of elevated growth temperatures on metabolites in soybean leaflets. Net CO₂ assimilation rates increased incrementally with growth temperature and were enhanced up to 24 % on average by CO₂ enrichment. Stomatal conductance about doubled from the lowest to highest temperature but this was partially reversed by CO₂ enrichment. Metabolites were measured thrice daily and 19 and 28 of 43 total leaf metabolites were altered by the 32/24 and 36/28 °C temperature treatments, respectively, in both CO₂ treatments. Polyols, raffinose and GABA increased and 23 nonstructural carbohydrates, organic acids and amino acids decreased when the temperature was increased from 28 to 36 °C under ambient CO₂. Citrate, aconitate and 2-oxoglutarate decreased over 90 % in the 36/28 °C compared to the 28/20 °C temperature treatment. Temperature-dependent changes of sugars, organic acids and all but three amino acids were almost completely eliminated by CO₂ enrichment. The above findings suggested that specific TCA cycle intermediates were highly depleted by heat stress under ambient CO₂. Mitigating effects of CO₂ enrichment on soybean leaflet metabolites were attributed to altered rates of photosynthesis, photorespiration, dark respiration, the anaplerotic pathway and to possible changes of gene expression.     

Photosynthetic refixing of CO2 is responsible for the apparent disparity between mitochondrial respiration in the light and in the dark

The net photosynthetic rate (P _N), the sample room CO₂ concentration (CO₂S) and the intercellular CO₂ concentration (C _i) in response to PAR, of C₃ (wheat and bean) and C₄ (maize and three-colored amaranth) plants were measured. Results showed that photorespiration (R _p) of wheat and bean could not occur at 2 % O₂. At 2 % O₂ and 0 μmol mol^?1 CO₂, P _N can be used to estimate the rate of mitochondrial respiration in the light (R _d). The R _d decreased with increasing PAR, and ranged between 3.20 and 2.09 μmol CO₂ m^?2 s^?1 in wheat. The trend was similar for bean (between 2.95 and 1.70 μmol CO₂ m^?2 s^?1), maize (between 2.27 and 0.62 μmol CO₂ m^?2 s^?1) and three-colored amaranth (between 1.37 and 0.49 μmol CO₂ m^?2 s^?1). The widely observed phenomenon of R _d being lower than R _n can be attributed to refixation, rather than light inhibition. For all plants tested, CO₂ recovery rates increased with increasing light intensity from 32 to 55 % (wheat), 29 to 59 % (bean), 54 to 87 % (maize) and 72 to 90 % (three-colored amaranth) at 50 and 2,000 μmol m^?2 s^?1, respectively.