Photosynthetic carbon metabolism in potato leaves under changes in light intensity

Photosynthetic assimilation of ¹⁴CO₂ was examined in leaves of potato (Solanum tuberosum L.) plants that were grown under direct sunlight and then transferred to 50% irradiance for various periods. The rate of ¹⁴CO₂ assimilation correlated with light intensity: the photosynthetic rate reduced by 52% after 5-day shading and by 70% after 30-min shading. In all shaded and shade-adapted plants, the sucrose/hexose ratio decreased by a factor of 3.5–4.1; furthermore, the radioactivity of glycolate cycle metabolites and the serine/glycine ratio were lowered. In plants shaded for 5 days or 30 min, the radioactivity of aspartate and malate was higher than at continuous high irradiance, especially in plants shaded for 30 min, whereas a sudden illumination of the shaded plants reduced the radioactivity of these substances. We suppose that low irradiance averted the reentry of glycolate path carbon into the Calvin cycle and redirected this carbon source for the production of four-carbon acids that acidified the apoplast. This acidification activated the apoplastic invertase, which enhanced sucrose hydrolysis and hindered the sucrose export from the leaf. Hydrolysis of sucrose promoted the increase in osmolarity of the apoplastic solution, this increase being stronger at close distances to the stomatal pores where water is intensely evaporated. The increase in osmolarity of extracellular medium led to closing of stomata and the suppression of photosynthesis.     

Effects of sodium nitroprusside,the nitric oxide donor,on photosynthesis and ultrastructure of common flax leaf blades

Solutions of sodium nitroprusside, a nitric oxide donor, were introduced at various concentrations into common flax (Linum usitatissimum L.) shoots with the transpirational water flow. Sodium nitroprusside and nitrate were found to exert similar effects on incorporation of ¹⁴C into photosynthetic products, leaf cell ultrastructure, and the export of assimilates from leaves. The results suggest that export of assimilates from leaves might be regulated by the products of incomplete nitrate reduction and that regulation may involve the NO-signaling system.     

Characteristics of photosynthesis in maize leaves (С<Subscript>4</Subscript> plants) upon changes in the level of illuminance and nitrate nutrition

We studied assimilation of ¹⁴СО₂ and distribution of ¹⁴С among the products of 3-min-long photosynthesis of maize (Zea mays L.) leaves. The day before the experiment, half of the plants were fertilized with Ca(NO₃)₂ (1 g/L of water) at a rate of 6 L/m². Five days before the experiment, some plants were shaded for adaptation (illuminance was reduced by 50%). On the day of the experiment (before the application of ¹⁴СО₂), several shaded plants were exposed to direct sunlight for 3 min, and some plants grown at full light (light plants) were shaded for 3 min (illuminance of 50%). Unfertilized plants adapted for 5 days to shading showed photosynthesis of 75.9% of control level (full light). If light plants were transferred to shading for 3 min, their photosynthesis decreased to 42.1%. In plants shaded for 5 days and then transferred to full light, photosynthesis in 3 min was 96.3% of control level. At full light, fertilization with nitrate boosted photosynthesis to 132.6% as compared with control material, but photosynthesis decreased to 43.5 and 65.4% of control level in plants shaded for 5 days and those shaded for 3 min, respectively. At the same time, the plants shaded for 5 days and then exposed for 3 min to full light restored photosynthesis to almost control level (95.5%). Analysis of ¹⁴С distribution among the products of 3-min-long photosynthesis showed that, the same as in C₃ plants, a decrease in illuminance (especially a sudden one) in maize reduced the ratio between labeled sucrose and hexoses and elevates incorporation of ¹⁴С into malate, which indicated that its consumption in bundle sheath cells was suppressed. A decrease in the ratio between labeled sucrose and hexoses became more pronounced under the influence of nitrates with this effect also occurring in transport products of photosynthesis (20 cm below ¹⁴С-providing leaf area). In plants fertilized with nitrates, radioactivity of sucrose (% of radioactivity of soluble compounds) decreased in all the types of illumination. When illuminance was suddenly reduced for 3 min, incorporation of ¹⁴С into sucrose was 21.5 against 51.2% in light plants, and radioactivity of aspartate and malate sharply rose to 13.7 and 26.1% (against 2.1 and 8.9% in control material). Incorporation of ¹⁴С into compounds of glycolate pathway was low (less than 2.5%), but it was somewhat greater in nitrate plants. We concluded that the same mechanism of interaction between stomatal apparatus of leaf epidermis, invertase of mesophyll apoplast, and photosynthetic metabolism of carbon with electron flux via electron transport chain in chloroplasts of bundle sheath cells, which governs the rate of photosynthesis and assimilate export from the leaf but is triggered by the extent of consumption in the bundle sheath cells of C₄ acids produced in the mesophyll operates in C₄ plants (the same as in C₃ plants).     

Postphotosynthetic Utilization of Labeled Assimilates in Fiber Flax

The distribution of ¹⁴C in various tissues of fiber flax was assayed 1, 17, and 21 days after 30-min assimilation of ¹⁴CO₂ by the whole rapidly growing plant. Polymeric photosynthetic products were largely hydrolyzed in the ¹⁴C-donor part of the shoot, and the hydrolysates were transported upward. The content of ¹⁴C in pigments and lipids of the donor leaves (that absorbed ¹⁴CO₂) was significantly higher than that in the ¹⁴C-acceptor leaves. Additional nitrogen nutrition decreased the labeled sucrose-to-hexose ratio and inhibited transport of the assimilates from both ¹⁴C-donor and acceptor leaves. ¹⁴C transported to the shoot tip was largely used for the synthesis of poorly soluble proteins (extractable with alkali and Triton X-100) in the acceptor tissues. In the donor part of the shoot, particularly in the bast, cellulose was mainly synthesized from the new assimilates.     

Effect of Defoliation or Excision of Growing Axillary Shoots on the Composition of Labeled Products of Photosynthesis in the Leaves and Xylem Sap of Kidney Bean

The content of ¹⁴C in the products of photosynthesis of the source leaf and xylem sap was investigated in kidney bean (Phaseolus vulgaris L.) plants during the stage of mass tillering. ¹⁴C partition was measured a day after two-minute photoassimilation of ¹⁴CO₂ by an individual mature leaf located in the middle part of the shoot. The source-sink relations were disturbed by the excision of all mature leaves (except the source leaf) or all growing axillary shoots. The leaves or growing axillary shoots were excised 15 min after leaf feeding with ¹⁴C₂. A day later, in plants with excised growing axillary shoots, the content of ¹⁴C in the source leaf was by 18% higher and in those with removed leaves by 15% lower than in control plants. The next day after the excision of growing axillary shoots, radioactivity of the xylem sap increased; after defoliation, both the volume of the xylem sap and its specific radioactivity decreased. In the xylem sap of defoliated plants, the proportion of ¹⁴C in malate decreased more than six times, whereas the proportion of ¹⁴C in amino acids somewhat increased (1.5 times). In two days, the volume of the xylem sap exuded by treated plants became the same as in control plants and its radioactivity decreased almost by an order of magnitude but essentially did not differ in the both types of treatment. It is concluded that the processes occurring in the roots are governed by photosynthesis but its regulatory effect is limited by a photoperiod and largely depends on changes in the ratio between biosynthesis of amino acids in the roots and leaves.__________Translated from Fiziologiya Rastenii, Vol. 52, No. 4, 2005, pp. 518–521.Original Russian Text Copyright © 2005 by Chikov, Bakirova, Batasheva, Sergeeva.     

Evolution of notions about relationships between photosynthesis and plant productivity

The concepts of photosynthesis role in the production process underwent evolution from the initial assumption of complete identity of these terms to the notion that photosynthesis is only a supplier of assimilates for sink organs. The following issues are discussed as individual stages of problem resolution: whether or not photosynthesis restricts productivity; what is the contribution of chlorophyll-containing non-leaf organs to the production process; what are the roles of photooxidation processes and source-sink relations between photosynthesizing organs and assimilate consumers; how the apoplast is involved in regulation of photosynthetic function of the whole plant; and what is the role of nitrate in control of photosynthesis and assimilate export from the leaf. Finally, the distribution of assimilates among the sink organs and the role of competition among the organs in regulation of photosynthesis and yield formation are considered.     

Effect of insertion of apoplastic invertase gene on photosynthesis of potato plants grown at various light intensities

We studied the influence of yeast invertase gene (inv), with the apoplastic localization of the enzyme, on photosynthesis of potato plants (Solanum tuberosum L., cv. Desiree) grown at various irradiances. Plants were raised in vitro, planted in soil in gauze-insulated stands, and grown at irradiances of 100, 200, and 380 W/m² of photosynthetically active radiation. Wild-type plants (WT) and the plants transformed with yeast invertase gene (B33-inv) were used. In the beginning of flowering stage, assimilation of ¹⁴CO₂ and ¹⁴C incorporation into photosynthates were measured. Irrespective of irradiance, the carbon assimilation was higher in WT plants, than in transformed B33-inv plants. In the plants studied, we observed divergent light dependences of ¹⁴C inclusion into sucrose: the highest labeling was observed at low irradiance in WT plants and at high irradiance in B33-inv transformed plants. The content of ¹⁴C incorporated into amino acids changed in the opposite direction compared to ¹⁴C incorporation into sucrose. Irrespective of the plant type, similar light dependences were observed for ¹⁴C content in the products of glycolate metabolism and in glycerate. At the intermediate irradiance, the patterns of ¹⁴C distribution among photosynthetic products showed minimal differences between the plants of two types. The role of apoplast invertase in sugar export from the leaf and the possible control of plant productivity through this enzyme activity are discussed.     

Metabolism of Labeled Exogenous Glucose in Fiber Flax Tissues

Labeled glucose solution was introduced into cut fiber flax plants (45–50 cm high) under a pressure of 0.1 bar for 30 min, 1, and 2 h using a special device. The highest quantities of labeled carbon were revealed in the woody tissue. Sucrose made up a considerable proportion in low molecular weight products of [2-¹⁴C]-glucose transformation (23.5%). Metabolism of labeled glucose in the leaves exposed to sunlight yielded a set of metabolites similar to products of ¹⁴CO₂ photoassimilation. In the shade, the pattern of ¹⁴C distribution in labeled compounds of the alcohol/water soluble fraction was similar to that in the light in mature leaves; while in juvenile leaves, ¹⁴C content decreased in sucrose and increased in organic and amino acids. In the shade, the incorporation of ¹⁴C into starch and hot water soluble polysaccharides increased at the expense of the acetone fraction (lipids and pigments), water/salt soluble proteins, and cellulose. Low light conditions increased the radioactivity ratio of sparingly soluble (KOH and Triton X-100 soluble) proteins to albumins and globulins. We propose that the synthesis of components of the photosynthetic apparatus in juvenile leaves is directly powered by photosynthesis and the photosynthesis of sucrose and the polymers compete for photosynthetic ATP. Appearance of sucrose in the xylem is due to its release from the phloem to the stem apoplast and the radial transfer to the xylem, where it is transported to the upper part of the shoot with the transpiration stream.__________Translated from Izvestiya Akademii Nauk, Seriya Biologicheskaya, No. 3, 2005, pp. 294–299.Original Russian Text Copyright © 2005 by Chikov, Avvakumova, Bakirova, Khamidullina.     

Apoplastic Transport of 14C-Photosynthates Measured under Drought and Nitrogen Supply

Using water infiltration of the plant and individual shoots with the subsequent intercellular liquid extraction by the pressure chamber, dynamics of the movement ¹⁴C-photosynthates from cell to apoplast, and ¹⁴C distribution among photosynthetic products in mesophyll cells and apoplast were studied. The relative quantity of ¹⁴C-photosynthetes in leaf apoplast depended on growing conditions; drought increased, and nitrate supply decreased it. When the middle leaves absorbed ¹⁴CO₂, photosynthates moving down in stem phloem appeared in intercellular space, where they were transported up by transpiration stream. ¹⁴C-photosynthates entering to the apex and young leaves were utilized a accumulated, and photosynthates transported to the mature leaves were reloaded into the phloem and reexported. Thus, photosynthates circulated through the plant and were redistributed to the plant organs according to their transpiration. In leaf apoplast photosynthetic sucrose was partly hydrolyzed to glucose and fructose. This increased under high nitrogen supply. The result indicate that apoplast sucrose hydrolysis is the basic cause of the reduction of photosynthate flux from leaves when the nitrate concentration in soil increases.     

Effect of nitrates supplied with the transpiration flow on assimilate translocation

Solutions of nitrates (0.5% KNO₃, 0.2% NH₄NO₃) or urea (0.15%) were fed under the pressure of 10⁴ Pa to 50–60-cm-long detached shoots of common flax (Linum usitatissimum L.). One hour after the start of supplying the solutions, an assimilation clip chamber was fastened to the middle part of the shoot (¹⁴C source area), and ¹⁴CO₂ was blown through in the light for 2.5 min. The analysis of distribution of ¹⁴C among the labeled products of photosynthesis produced by source leaves showed that nitrates reduced the incorporation of the label into sucrose. At the same time, the ratio of labeled sucrose to labeled hexoses decreased, and the incorporation of the label into serine greatly increased. Urea did not produce such effects. The pattern of distribution of ¹⁴C within the plant 3 h after the assimilation of ¹⁴CO₂ points to the suppression of assimilate efflux from the leaves of plants fed with nitrates. In plants supplied with water or urea, 17–20% of labeled carbon was found below the ¹⁴C source area of the shoot, in nitrate type of treatment, only 3–5% was found there. In plants supplied with nitrates, the cortex tissue below the source leaf contained more ¹⁴C in proteins and less in low-molecular substances. In the wood tissue, such a correlation was not observed. When the shoot was supplied with water or urea, the content of ¹⁴C in sucrose in the source leaves in 3 h declined from 55–60% to 38–42%. When the shoot was fed with nitrates, the share of label in sucrose increased from 50 to 62–73%. Autoradiography of the source leaves showed that, in plants supplied with water or urea, the label was predominantly accumulated in large vascular bundles, and in nitrate type of treatment, it was accumulated outside large bundles. Electron microscopy showed that, in nitrate plants, the companion cells of phloem endings were very much vacuolated.