Respiratory CO2 Fluxes in Photosynthesizing Leaves of C3 Species Varying in Rates of Starch Synthesis1

The rates of CO₂ fixation and respiratory CO₂ fluxes in six C₃ species, namely Solanum tuberosum, Nicotiana tabacum, Arabidopsis thaliana, Hordeum vulgare, Triticum aestivum, and Secale cereale, were determined under steady-state photosynthesis. The plants may be divided into two groups: (a) cereals with a low rate of starch synthesis (7–5% of true photosynthesis); (b) plants with a high rate of starch synthesis (45–35% of true photosynthesis). In the light, primary and stored photosynthates are consumed as substrates for both respiratory and photorespiratory pathways. In leaves of cereals, the total rate of respiratory and photorespiratory decarboxylations of stored photosynthates was higher in the light than in the dark, while, in starch-synthesizing species, stored photosynthates were consumed at a higher rate in the dark. Under normal environmental conditions, respiratory decarboxylation of stored photosynthates was suppressed by light in all species studied. The total rate of respiration as the sum of decarboxylation of stored and primary photosynthates was not affected by light in cereals, but suppressed in starch-accumulating plants. This suppression was not compensated for by the additional supply of respiratory substrates from primary photosynthates in the light.     

The Effects of Cold Acclimation of Winter Wheat Plants on Changes in CO<Subscript>2</Subscript> Exchange and Phenolic Compound Formation

We studied CO₂ exchange and phenolic compound production in various organs of unhardened and hardened winter wheat (Triticum aestivum L.) plants. The rates of CO₂ assimilation at saturating illumination (photosynthesis) and CO₂ evolution in darkness (respiration) declined substantially at the autumnal decrease of ambient temperature. However, because of a higher cold resistance of photosynthesis, the ratio of photosynthesis to respiration rates increased 1.5-fold. These gas exchange changes were accompanied by the accumulation of total soluble phenolics in leaves and a polymeric phenolic compound lignin in roots. We did not observe any changes in the production of either soluble or polymeric (lignin) phenolics in crowns.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 3, 2005, pp. 366–371.Original Russian Text Copyright © 2005 by Zagoskina, Olenichenko, Klimov, Astakhova, Zhivukhina, Trunova.     

Carbon balance in a monospecific stand of an annual herb <Emphasis Type="Italic">Chenopodium album</Emphasis> at an elevated CO<Subscript>2</Subscript> concentration

Elevated CO₂ enhances carbon uptake of a plant stand, but the magnitude of the increase varies among growth stages. We studied the relative contribution of structural and physiological factors to the CO₂ effect on the carbon balance during stand development. Stands of an annual herb Chenopodium album were established in open-top chambers at ambient and elevated CO₂ concentrations (370 and 700 μmol mol⁻¹). Plant biomass growth, canopy structural traits (leaf area, leaf nitrogen distribution, and light gradient in the canopy), and physiological characteristics (leaf photosynthesis and respiration of organs) were studied through the growing season. CO₂ exchange of the stand was estimated with a canopy photosynthesis model. Rates of light-saturated photosynthesis and dark respiration of leaves as related with nitrogen content per unit leaf area and time-dependent reduction in specific respiration rates of stems and roots were incorporated into the model. Daily canopy carbon balance, calculated as an integration of leaf photosynthesis minus stem and root respiration, well explained biomass growth determined by harvests (r ² = 0.98). The increase of canopy photosynthesis with elevated CO₂ was 80% at an early stage and decreased to 55% at flowering. Sensitivity analyses suggested that an alteration in leaf photosynthetic traits enhanced canopy photosynthesis by 40–60% throughout the experiment period, whereas altered canopy structure contributed to the increase at the early stage only. Thus, both physiological and structural factors are involved in the increase of carbon balance and growth rate of C. album stands at elevated CO₂. However, their contributions were not constant, but changed with stand development.     

The effects of oligomycin on content of adenylates in mesophyll protoplasts,chloroplasts and mitochondria from Pb<Superscript>2+</Superscript> treated pea and barley leaves

The effects of oligomycin on photosynthesis and respiration in relation to ATP production in chloroplasts and mitochondria were investigated in protoplasts isolated from the detached pea (Pisum sativum L cv. Iłowiecki.) and barley (Hordeum vulgare L. cv. Gunilla) leaves treated 5 mM Pb(NO₃)₂. The oligomycin (OM), an inhibitor of oxidative phosphorylation at 0.1 μM concentration caused the inhibition of photosynthesis rate in the protoplasts from both the control and the Pb-treated pea leaves. The respiration rate and ATP/ADP ratio in the protoplasts and the activity of ATPase in mitochondria, were also diminished in the control protoplasts. These effects were not observed in the protoplasts and mitochondria isolated from the Pb-treated leaves. Oligomycin, an inhibitor of photophosphorylation at 10 μM concentration decreased ATPase activity in chloroplasts from both the control and the Pb- treated leaves. Using the method of rapid fractionation of barley protoplasts it was shown that the ATP/ADP ratio in the mitochondria from Pb-treated leaves was largely suppressed (from 1.8 to 0.4) by OM under nonphotorespiratory conditions (high CO₂), whereas under photorespiratory conditions (low CO₂) this ratio was high (5.3) and under OM decreased less (to 3.1). Our results indicate that oligomycin, in organelle isolated from Pb-treated leaves, had no inhibitory effect on the mitochondrial ATPase, whereas it inhibited chloroplasts ATPase. We suggest that Pb ions affected the catalytic cycle and/or conformational changes of ATPase in pea chloroplasts differently than in mitochondria. The differences in Pb responses may reflect fine mechanisms for the regulation of ATP production in the plant cells under stress conditions.     

CO<Subscript>2</Subscript> sequestration in plants: lesson from divergent strategies

Most organisms inhabiting earth feed directly or indirectly on the products synthesized by the reaction of photosynthesis, which at the current atmospheric CO₂ levels operates only at two thirds of its peak efficiency. Restricting the photorespiratory loss of carbon and thereby improving the efficiency of photosynthesis is seen by many as a good option to enhance productivity of food crops. Research during last half a century has shown that several plant species developed CO₂-concentrating mechanism (CCM) to restrict photorespiration under lower concentration of available CO₂. CCMs are now known to be operative in several terrestrial and aquatic plants, ranging from most advanced higher plants to algae, cyanobacteria and diatoms. Plants with C₄ pathway of photosynthesis (where four-carbon compound is the first product of photosynthesis) or crassulacean acid metabolism (CAM) may consistently operate CCM. Some plants however can undergo a shift in photosynthetic metabolism only with change in environmental variables. More recently, a shift in plant photosynthetic metabolism is reported at high altitude where improved efficiency of CO₂ uptake is related to the recapture of photorespiratory loss of carbon. Of the divergent CO₂ assimilation strategies operative in different oraganisms, the capacity to recapture photorespiratory CO₂ could be an important approach to develop plants with efficient photosynthetic capacity.     

Effects of salinity on chlorophyll fluorescence and CO<Subscript>2</Subscript> fixation in C<Subscript>4</Subscript> estuarine grasses

The effects of salinity (sea water at 0 ‰ versus 30 ‰) on gross rates of O₂ evolution (J _O2) and net rates of CO₂ uptake (P _N) were measured in the halotolerant estuarine C₄ grasses Spartina patens, S. alterniflora, S. densiflora, and Distichlis spicata in controlled growth environments. Under high irradiance, salinity had no significant effect on the intercellular to ambient CO₂ concentration ratio (C _i/C _a). However, during photosynthesis under limiting irradiance, the maximum quantum efficiency of CO₂ fixation decreased under salinity across species, suggesting there is increased leakage of the CO₂ delivered to the bundle sheath cells by the C₄ pump. Growth under salinity did not affect the maximum intrinsic efficiency of photosystem 2, PS2 (F_V/F_M) in these species, suggesting salinity had no effect on photosynthesis by inactivation of PS2 reaction centers. Under saline conditions and high irradiance, P _N was reduced by 75 % in Spartina patens and S. alterniflora, whereas salinity had no effect on P _N in S. densiflora or D. spicata. This inhibition of P _N in S. patens and S. alterniflora was not due to an effect on stomatal conductance since the ratio of C _i/C _a did not decrease under saline conditions. In growth with and without salt, P _N was saturated at ∼500 μmol(quantum) m⁻² s⁻¹ while J _O2 continued to increase up to full sunlight, indicating that carbon assimilation was not tightly coupled to photochemistry in these halophytic species. This increase in alternative electron flow under high irradiance might be an inherent function in these halophytes for dissipating excess energy.     

Genotypic variation in drought stress response and subsequent recovery of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Three wheat (Triticum aestivum L.) genotypes, Sadovo, Katya and Prelom, with different tolerance to drought were comparatively evaluated in terms of leaf respiratory responses to progressing dehydration and consecutive rewatering. Under drought stress, the respiration of all varieties gradually decreased, as the drought-tolerant Katya showed the most pronounced decline at earlier stages of dehydration. When water stress intensified, this genotype gave relatively stable respiration rates compared with the drought-sensitive varieties. Additionally, dehydrated Katya leaves displayed lower stomatal conductance and higher photosynthesis values, which resulted in greater water use efficiency during the dehydration period. Combination of drought stress and short-term changes in leaf temperature also induced genotype-specific response that differed from the response to drought only. Over the whole temperature range, the leaves of Katya exposed to dehydration for 14 days, showed higher respiration rates compared to the drought-sensitive varieties. The sensitive varieties maintained higher respiration rates under control conditions and mild dehydration, and very low rates under severe drought. In Katya, respiration and photosynthesis were fully restored from the stress within the first day of rewatering. The drought-sensitive genotypes displayed a considerably slower recovering capacity. The results are discussed in terms of possible physiological mechanisms underlying plant tolerance to drought.     

Interaction between photorespiration and respiration in transgenic potato plants with antisense reduction in glycine decarboxylase

Potato (Solanum tuberosum L. cv. Désirée) plants with an antisense reduction in the P-protein of the glycine decarboxylase complex (GDC) were used to study the interaction between respiration and photorespiration. Mitochondria isolated from transgenic plants had a decreased capacity for glycine oxidation and glycine accumulated in the leaves. Malate consumption increased in leaves of GDC deficient plants and the capacity for malate and NADH oxidation increased in isolated mitochondria. A lower level of alternative oxidase protein and decreased partitioning of electrons to the alternative pathway was found in these plants. The adenylate status was altered in protoplasts from transgenic plants, most notably the chloroplastic ATP/ADP ratio increased. The lower capacity for photorespiration in leaves of GDC deficient plants was compensated for by increased respiratory decarboxylations in the light. This is interpreted as a decreased light suppression of the tricarboxylic acid cycle in GDC deficient plants in comparison to wild-type plants. The results support the view that respiratory decarboxylations in the light are restricted at the level of the pyruvate dehydrogenase complex and/or isocitrate dehydrogenase and that this effect is likely to be mediated by mitochondrial photorespiratory products.     

CO<Subscript>2</Subscript> exchange and structural organization of chloroplasts under hypothermia in potato plants transformed with a gene for yeast invertase

Growth, CO₂ exchange, and the ultrastructure of chloroplasts were investigated in the leaves of potato plants (Solanum tuberosum L., cv. Désirée) of wild type and transformed with a gene for yeast invertase under the control of patatin class I B33 promoter (for apoplastic enzyme) grown in vitro on the Murashige and Skoog medium supplemented with 2% sucrose. At a temperature of 22°C optimal for growth, the transformed plants differed from the plants of wild type in retarded growth and a lower rate of photosynthesis as calculated per plant. On a leaf dry weight basis, photosynthesis of transformed plants was higher than in control plants. Under hypothermia (5°C), dark respiration and especially photosynthesis of transformed plants turned out to be more intense than in control material. After a prolonged exposure to low temperature (6 days at 5°C), in the plants of both genotypes, the ultrastructure of chloroplasts changed. Absolute areas of sections of chloroplasts and starch grains rose, and the area of plastoglobules decreased; in transformed plants, these changes were more pronounced. By some ultrastructural characteristics: a reduction in the cold of relative total area of sections of starch grains and plastoglobules (in percents of the chloroplast section area) and in the number of granal thylakoids (per a chloroplast section area), transformed plants turned out to be more cold resistant than wild-type plants. The obtained results are discussed in connection with changes in source-sink relations in transformed potato plants. These changes modify the balance between photosynthesis and retarded efflux of assimilates, causing an increase in the intracellular level of sugars and a rise in the tolerance to chilling.     

Response of total night-time respiration to differences in total daily photosynthesis for leaves in a Quercus rubra L. canopy: implications for modelling canopy CO2 exchange

Measurements of photosynthesis and respiration were made on leaves in summer in a Quercus rubra L. canopy at approximately hourly intervals throughout 5 days and nights. Leaves were selected in the upper canopy in fully sunlit conditions (upper) and in the lower canopy (lower). In addition, leaves in the upper canopy were shaded (upper shaded) to decrease photosynthesis rates. The data were used to test the hypothesis that total night‐time respiration is dependent on total photosynthesis during the previous day and that the response is mediated through changes in storage in carbohydrate pools. Measurements were made on clear sunny days with similar solar irradiance and air temperature, except for the last day when temperature, especially at night, was lower than that for the previous days. Maximum rates of photosynthesis in the upper leaves (18.7 μmol m^?2 s^?1) were approximately four times higher than those in the lower leaves (4.3 μmol m^?2 s^?1) and maximum photosynthesis rates in the upper shaded leaves (8.0 μmol m^?2 s^?1) were about half those in the upper leaves. There was a strong linear relationship between total night‐time respiration and total photosynthesis during the previous day when rates of respiration were normalized to a fixed temperature of 20°C, removing the effects of temperature from this relationship. Measurements of specific leaf area, nitrogen and chlorophyll concentration and calculations of the maximum rate of carboxylation activity, V_cmax, were not significantly different between upper and upper shaded leaves 5 days after the shading treatment was started. There were small, but significant decreases in the rate of apparent maximum electron transport at saturating irradiance, J_max (P>0.05), and light use efficiency, ? (P<0.05), for upper shaded leaves compared with those for upper leaves. This suggests that the duration of shading in the experiment was sufficient to initiate changes in the electron transport, but not the carboxylation processes of photosynthesis. Support for the hypothesis was provided from analysis of soluble sugar and starch concentrations in leaves. Respiration rates in the upper shaded leaves were lower than those expected from a relationship between respiration and soluble sugar concentration for fully exposed upper and lower leaves. However, there was no similar difference in starch concentrations. This suggests that shading for the duration of several days did not affect sugar concentrations but reduced starch concentrations in leaves, leading to lower rates of respiration at night. A model was used to quantify the significance of the findings on estimated canopy CO₂ exchange for the full growing season. Introducing respiration as a function of total photosynthesis on the previous day resulted in a decrease in growing season night‐time respiration by 23% compared with the value when respiration was held constant. This highlights the need for a process‐based approach linking respiration to photosynthesis when modelling long‐term carbon exchange in forest ecosystems.     

Correlation between photorespiration,CO<Subscript>2</Subscript>-assimilation and spatiotemporal dynamics of photosynthesis in leaves of the C<Subscript>3</Subscript>-photosynthesis/crassulacean acid metabolism-intermediate species <Emphasis Type="Italic">Clusia minor</Emphasis> L. (Clusiaceae)

The tree Clusia minor L. (Clusiaceae) operates with different modes of photosynthesis in response to different combinations of environmental parameters. Here plants were subjected to experimental conditions eliciting performance of C₃-photosynthesis and crassulacean acid metabolism (CAM), respectively. A combination of instruments was used to determine CO₂ and water vapour gas exchange, relative quantum use efficiency of photosynthesis (Φ_PSII) and for the first time in such studies also photorespiration simultaneously with the other parameters. In the C₃-mode photorespiration was constant during the light period, where oxygenase activity of ribulose-bis-phosphate carboxylase/oxygenase (RubisCO) was ranging between 32.1 and 35.7% of total RubisCO activity. In the CAM-mode photorespiration depended on the CAM phases. In phase II in the morning was 15.6%. In phase IV in the afternoon initially it was 37.9% and then declined to 17.6% of total RubisCO activity towards the evening. Anatomically leaves of C. minor are differentiated in palisade and spongy parenchyma with an internal air space of 9.3% of the total volume and therefore could be structurally homobaric. However, heterogeneity of Φ_PSII under both non-photorespiratory and photorespiratory conditions in the C₃- and CAM-mode indicated that lateral diffusion of CO₂ and O₂ were subject to limitations showing that leaves are functionally heterobaric.     

Advanced radiogasometric method for the determination of the rates of photorespiratory and respiratory decarboxylations of primary and stored photosynthates under steady-state photosynthesis

An advanced radiogasometric method for the study of plant leaf CO₂ exchange is presented. The method enables determination of the rates of CO₂ fixation, photorespiration and respiration in the light under steady‐state photosynthesis and discrimination between primary and stored photosynthates as substrates of photorespiratory and respiratory decarboxylations. The method is based on the analysis of the time curves of ¹⁴CO₂ evolution from labeled primary and stored photosynthates in leaves previously exposed to ¹⁴CO₂. The molar rates of different decarboxylation reactions are calculated from the initial slopes of the curves taking into account the specific radioactivity of CO₂ fed to leaves and/or evolved from leaves. To estimate the contribution of primary and stored photosynthates, the measurements of ¹⁴CO₂ evolution are performed after feeding plant leaves for different periods with ¹⁴CO₂. Photorespiration and respiration are distinguished on the basis of data obtained from measurements of ¹⁴CO₂ evolution under normal (210 ml l⁻¹) and low (15 ml l⁻¹) concentrations of oxygen. A principally new method for the determination of the rate of intracellular refixation of respiratory CO₂ has been developed. The method is based on the measurements of ¹⁴CO₂ evolution from leaves into the medium of very high concentrations (30 ml l⁻¹) of ¹²CO₂, where the probability of refixation of ¹⁴CO₂ evolved inside the cell is close to zero. The results obtained were comparable with the data derived from parallel refixation measurements by means of gasometric methods. As an example of application, the data on CO₂ exchange in leaves of two contrasting groups of C₃‐species, differing in the ability of starch accumulation, are presented.     

Light-enhanced dark respiration in leaves and mesophyll protoplasts of pea in relation to photorespiration,respiration and some metabolites content

The respiration rate of leaves and mesophyll protoplasts of pea (Pisum sativum L.), from plants which were previously kept in darkness for 24 h was doubled following a period of photosynthesis at ambient level of O₂ (21 %), whereas the low level of O₂ (1 % and 4 % for leaves and protoplasts, respectively) reduced this light-enhanced dark respiration (LEDR) to the rate as noted before the illumination. Similarly to respiration rate, the oxygen at used concentrations had no effect on the ATP/ADP ratio in the dark-treated leaves. However, the ATP/ADP ratio in leaves photosynthesizing at 21 % O₂ was higher (up to 40 %, dependence on CO₂ concentration in the range 40–1600 1 dm⁻³) than in those photosynthesizing at 1 % O₂ or darkened at air (21 % O₂). Also, at 1 % O₂ the accumulation of malate was suppressed (by about 40 %), to a value noted for leaves darkened at 21 % O₂. The dark-treatment of leaves reduced the ability of isolated mitochondria to oxidize glycine (by about twofold) and succinate, but not malate. Mitochondria from both the light- and dark-treated leaves did not differ in qualitative composition of free amino acids, however, there were significant quantitative differences especially with respect to aspartate, alanine, glutamate and major intermediates of the photorespiratory pathway (glycine, serine). Our results suggest that accumulation of photorespiratory and respiratory metabolites in pea leaves during photosynthesis at 1 % O₂ is reduced, hence the suppression of postillumination respiration rate.     

The effect of water stress on photosynthetic carbon metabolism in four species grown under field conditions

Abstract. The effect of gradually-developing water-stress has been studied in Lupinus albus L., Helianthus annuus L., Vitis vinifera cv. Rosaki and Eucalyptus globulus Labill. Water was withheld and diurnal rhythms were investigated 4–8d later, when the predawn water deficit was more negative than in watered plants, and the stomata closed almost completely early during the photoperiod. The contribution of ‘stomatal’ and ‘non-stomatal’ components to the decrease of photosynthetic rate was investigated by (1) comparing the changes of the rate of photosynthesis in air with the changes of stomatal conductance and (2) measuring photosynthetic capacity in saturating irradiance and 15% CO₂. Three species (lupin, eucalyptus and sunflower) showed larger changes of stomatal conductance than photosynthesis in air, and showed little or no decrease of photosynthetic capacity in saturating CO₂. Photosynthesis in air also recovered fully overnight after watering the plants in the evening. In grapevines, stomatal conductance and photosynthesis in air changed in parallel, there was a marked decrease of photosynthetic capacity, and photosynthesis and stomatal conductance did not recover overnight after watering water-stressed plants. Relative water content remained above 90% in grapevine. We conclude that non-stomatal components do not play a significant role in lupins, sunflower or eucalyptus, but could in grapevine. The effect of water-stress on partitioning of photosynthate was investigated by measuring the amounts of sucrose and starch in leaves during a diurnal rhythm, and by measuring the partitioning of ¹⁴C-carbon dioxide between sucrose and starch. In all four species, starch was depleted in water-stressed leaves but sucrose was maintained at amounts similar to, or higher than, those in watered plants. Partitioning into sucrose was increased in lupins and eucalyptus, and remained unchanged in grapevine and sunflower. It is concluded that water-stressed leaves in all four species maintain high levels of soluble sugars in their leaves, despite having lower rates of field photosynthesis, decreased rates of export, and low amounts of starch in their leaves.     

Pioneering research on C<Subscript>4</Subscript> leaf anatomical,physiological, and agronomic characteristics of tropical monocot and dicot plant species: Implications for crop water relations and productivity in comparison to C<Subscript>3</Subscript> cropping systems

The review is done to summarise the history of the discoveries of the many anatomical, agronomical, and physiological aspects of C₄ photosynthesis (where the first chemical products of CO₂ fixation in illuminated leaves are four-carbon dicarboxylic acids) and to document correctly the scientists at the University of Arizona and the University of California, Davis, who made these early discoveries. The findings were milestones in plant science that occurred shortly after the biochemical pathway of C₃ photosynthesis in green algae (where the first chemical product is a three-carbon compound) was elucidated at the University of California, Berkeley, and earned a Nobel Prize in chemistry. These remarkable achievements were the result of ground-breaking pioneering research efforts carried out by many agronomists, plant physiologists and biochemists in several laboratories, particularly in the USA. Numerous reviews and books written in the past four decades on the history of C₄ photosynthesis have focused on the biochemical aspects and give an unbalanced history of the multidisciplinary/multinstitutional nature of the achievements made by agronomists, who published much of their work in Crop Science. Most notable among the characteristics of the C₄ species that differentiated them from the C₃ ones are: (I) high optimum temperature and high irradiance saturation for maximum leaf photosynthetic rates; (II) apparent lack of CO₂ release in a rapid stream of CO₂-free air in illuminated leaves in varying temperatures and high irradiances; (III) a very low CO₂ compensation point; (IV) lower mesophyll resistances to CO₂ diffusion coupled with higher stomatal resistances, and, hence, higher instantaneous leaf water use efficiency; (V) the existence of the so-called “Kranz leaf anatomy” and the higher internal exposed mesophyll surface area per cell volume; and (VI) the ability to recycle respiratory CO₂ by illuminated leaves.     

Metabolism of <Superscript>14</Superscript>C-glycine as a substrate for photorespiration of the leaf at different developmental stages of ephemeroides

The metabolism of ¹⁴C-glycine (a substrate for photorespiration) was studied in the light and in darkness under natural CO₂ concentration (0.03%) in the leaves of ephemeroides Scilla sibirica Haw. and Ficaria verna Huds. at different developmental stages. Using one and the same sample, potential photosynthesis (at 1% CO₂), true photosynthesis (at 0.03% CO₂), and leaf respiratory capacity were measured by the radiometric and manometric methods, respectively. All measurements were performed at 15°C, an average temperature during ephemer growth. It was found that, in the white zone of the Scilla leaf, the rate of CO₂ evolution resulting from metabolization of exogenous ¹⁴C-glycine was similar in the light and in darkness. In the green zone of the Scilla leaf and in the green leaf of Ficaria, both ¹⁴C-glycine absorption and ¹⁴CO₂ evolution were lower in the light as compared with darkness, which is explained by CO₂ reassimilation. In all treatments of both plant species, a specific inhibitor of glycine decarboxylase complex (GDC), aminoacetonitrile (5 mM) suppressed CO₂ evolution by 20–40%. It was concluded that in ephemeroides mitochondrial GDC, responsible for CO₂ evolution in photorespiration, is formed at the earliest stage of leaf development. This indicates that photorespiration can occur simultaneously with the development of the leaf photosynthetic activity. On the basis of the assumption that carbon losses in the form of CO₂ evolved during photorespiration comprise 25% of true photosynthesis, it was calculated that, in ephemer leaves, the highest rates of photorespiration and photosynthesis were attained during flowering when the leaf area was the largest and the rate of dark respiration was reduced by 1.5–2.0 times. The highest rates of dark respiration were observed in the beginning of growth. In senescing leaves by the end of the plant vegetation, potential photosynthesis and true photosynthesis were reduced, whereas dark respiration remained essentially unchanged. It is concluded that the high rates of potential and true photosynthesis are characteristic of ephemeroides when they complete their short developmental program in early spring (at 15°C); theoretically, photorespiration also occurs at a high rate during this period, when this process provides for a defense against the threat of photoinhibition at low temperature and high insolation.     

Antisense inhibition of sorbitol synthesis leads to up-regulation of starch synthesis without altering CO<Subscript>2</Subscript> assimilation in apple leaves

Sorbitol is a primary end-product of photosynthesis in apple (Malus domestica Borkh.) and many other tree fruit species of the Rosaceae family. Sorbitol synthesis shares a common hexose phosphate pool with sucrose synthesis in the cytosol. In this study, Greensleeves apple was transformed with a cDNA encoding aldose 6-phosphate reductase (A6PR, EC 1.1.1.200) in the antisense orientation. Antisense expression of A6PR decreased A6PR activity in mature leaves to approximately 15–30% of the untransformed control. The antisense plants had lower concentrations of sorbitol but higher concentrations of sucrose and starch in mature leaves at both dusk and predawn. ¹⁴CO₂ pulse-chase labeling at ambient CO₂ demonstrated that partitioning of the newly fixed carbon to starch was significantly increased, whereas that to sucrose remained unchanged in the antisense lines with decreased sorbitol synthesis. Total activities of ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), sucrose-phosphate synthase (EC 2.4.1.14), and ADP-glucose pyrophosphorylase (EC 2.7.7.27) were not significantly altered in the antisense lines, whereas both stromal and cytosolic fructose-1,6-bisphosphatase (EC 3.1.3.11) activities were higher in the antisense lines with 15% of the control A6PR activity. Concentrations of glucose 6-phosphate and fructose 6-phosphate (F6P) were higher in the antisense plants than in the control, but the 3-phosphoglycerate concentration was lower in the antisense plants with 15% of the control A6PR activity. Fructose 2, 6-bisphosphate concentration increased in the antisense plants, but not to the extent expected from the increase in F6P, comparing sucrose-synthesizing species. There was no significant difference in CO₂ assimilation in response to photon flux density or intercellular CO₂ concentration. We concluded that cytosolic FBPase activity in vivo was down-regulated and starch synthesis was up-regulated in response to decreased sorbitol synthesis. As a result, CO₂ assimilation in source leaves was sustained at both ambient CO₂ and saturating CO₂.     

C<Subscript>3</Subscript> and C<Subscript>4</Subscript> photosynthetic pathways and life form types for native species from agro-forestry region,Northeastern China

Of the total 570 species, 194 species in 116 genera and 52 families were found with C₃ photosynthesis, 24 species in 17 genera and 6 families with C₄ photosynthesis, and 2 species in 1 genera and 1 family with CAM photosynthesis. 90 % of the total species can be found in Changbai Mountain flora, more a half (69 %) in North China flora, and about 1/3 in Mongolian flora and Xinan flora, respectively. The occurrence of C₄ species was not as common as that in adjacent grasslands and deserts, but relatively more than in the adjacent forests. Of the total 24 C₄ species, 63 % C₄ species (15 of 24) was found in Gramineae. Nine life form types can be found, reflecting the moist climate in the region, especially the occurrence of epiphyte and liana forms. Relatively more geophyte life form plants suggested the winter in the region was much colder than in grasslands. These indicated that both ecological studies and land management decisions must take into account plant photosynthetic pathway and life form patterns, for both of them are closely related to climatic changes and land use.