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  • 1
    Electronic Resource
    Electronic Resource
    Developmental changes of enzymes of malate metabolism in relation to respiration, photosynthesis and nitrate assimilation in peach leaves (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 89 (1993), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The developmental profile of the activities of some enzymes involved in malate metabolism, namely phosphoenolpyruvate carboxylase (PEPC; EC 4. 1. 1. 31), NAD+-linked (EC 1. 1. 1. 37) and NADP+-linked (EC 1. 1. 1. 82) malate dehydrosenase (MDH), NAD+linked (EC 1. 1. 1. 39) and NADP+-linked (EC 1. 1. 1. 40) malic enzyme (ME), has been determined in leaves of peach [Prunus persica (L.) Batsch cv. Maycrest], a woody C3 species. In order to study the role of these enzymes, their activities were related to developmental changes of photosynthesis, respiration, and capacity for N assimilation. Activities of PEPC, NAD(P)+-MDH and NADP+-ME were high in young expanding leaves and decreased 2- to 3-fold in mature ones, suggesting that such enzymes play some role during the early stages of leaf expansion. In leaves of peach, such a role did not seem to be linked to C3 photosynthesis or nitrate assimilation, in that photosynthetic O2 evolution and activities of nitrate reductase (EC 1. 6. 6. 1) and glutamine synthetase (EC 6. 3. 1. 2) increased during leaf development. In contrast, leaf respiration strongly decreased with increasing leaf age. We suggest that in expanding leaves of this woody species the enzymes associated with malate metabolism have anaplerotic functions, and that PEPC may also contribute to the recapture of respiratory CO2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1993.tb01788.x
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  • 2
    Electronic Resource
    Electronic Resource
    Changes in carbohydrate and enzyme levels during development of leaves of Prunus persica, a sorbitol synthesizing species (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 83 (1991), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Carbohydrate levels and activities of enzymes associated with starch, sucrose and sorbitol metabolism were assayed in leaves of peach [Prunus persica (L.) Batsch cv. Redhaven] of different ages, in order to examine developmental changes in leaf carbohydrate metabolism. Dry matter, soluble protein, chlorophyll and the activities of key enzymes of the reductive pentose phosphate pathway increased during leaf development. The levels of leaf carbohydrates, especially sorbitol and starch, also increased. Changes of starch levels were related to increases in the activities of enzymes associated to starch metabolism, such as ADPglucose-pyrophosphorylase (E.C. 2.7.7.27) and amylase (E.C. 3.2.1.1. plus E.C. 3.2.1.2). The activities of enzymes involved in sucrose and sorbitol degradation decreased during leaf development, whereas the activities of aldose-6-phosphate reductase (E.C. 1.1.1.200) and cytosolic fructase-1,6-bisphosphatase (E.C. 3.1.3.11) increased. In contrast, the activity of sucrose-phosphate synthase (E.C. 2.4.1.14) did not vary in a significant manner. The results suggest that the ability to synthesize and utilize both sucrose and sorbitol changes as peach leaves mature, and also that there are differences in metabolism of these two transport sugars during leaf development.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1991.tb02478.x
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  • 3
    Electronic Resource
    Electronic Resource
    Effect of decreased irradiance on N and C metabolism in leaves and roots of maize (1994)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 91 (1994), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: The effects of decreased irradiance on fresh and dry weight, root respiration, levels of carbohydrates and N-compounds, and extractable activities of enzymes involved in C and N metabolism were evaluated in maize (Zea mays L. cv. Plauto) seedlings during the 7 days following transfer from 450 to 200 μmol m−2 s−1 PAR. The fresh weight of roots and stems, the initiation of new leaves, root respiration rate, and the accumulation of dry matter, soluble sugars, starch, malate and amino acids in both leaves and roots were strongly reduced at low irradiance. In contrast, the level of nitrate was increased in leaves and only marginally affected in roots. Leaf phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity started to decrease after 24–34 h, whereas ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) activity and chlorophyll content were unaffected or only slightly reduced. In both leaves and roots, the adjustment of N metabolism to low irradiance occurred through a relatively rapid (30% after 10 h) and large (60% after 3 days) decrease of nitrate reductase (NR; EC 1.6.6.1) activity, followed by slower and smaller changes in the activity of nitrite reductase (EC 1.7.7.1), glutamine synthetase (EC 6.3.1.2) and NAD-dependent glutamate dehydrogenase (EC 1.4.1.2). We suggest that the preferential decrease of NR activity relative to other N-assimilating enzymes may be important for preventing the accumulation of toxic N-compounds like ammonia in both leaf and root tissues.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1994.tb00661.x
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  • 4
    Electronic Resource
    Electronic Resource
    Light-modulation of nitrate reductase activity in leaves and roots of maize (1995)
    Oxford, UK : Blackwell Publishing Ltd
    Physiologia plantarum 94 (1995), S. 0 
    Details
    ISSN: 1399-3054
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: In order to examine the light-modulation of nitrate reductase (NR: EC 1. 6.6.1) activity in tissues of maize (Zea mays L. cv. Plauto). the enzyme was assayed under both selective (5 mM, Mg2+) and unselective conditions (2 mM EDTA; Vmax). in leaf and root extracts from seedlings transferred from high irradiance (HI: 450 μmol m−2s−1 PAR) to shade (200 μmol m−2 s−1) or to darkness. The corresponding changes in tissue nitrate were also evaluated. The results show that maize leaf NR activity (NRA) is subjected to reversible light-modulation involving rapid changes in the sensitivity of the enzyme to inhibition by Mg2+, In fact, NR assayed in the presence of Mg2+ lost about 60% and 20% of its activity after 30 min in the dark and in the shade, respectively. Vmax started to decrease only after 1–3 h. depending on the extent of light reduction. Deactivation in the shade was reversed in vivo by returning the plants to HI. the kinetics of reactivation being similar to those of deactivation. Following dark-deactivation in vivo, spontaneous reactivation of leaf NR occurred during in vitro incubation at 25°C. and the process was accelerated by AMP. The NR from HI-leaves was deactivated (half-time 2 min) by incubation with MgATP. but the extent of the in vitro deactivation was small. Maize root NR was more sensitive to Mg2+ than leaf NR, with 5 mM Mg2+ causing a 50% inhibition of the activity in HI-roots. In contrast to leaves, marked changes in the Mg2+-sensitivity of maize root NRA became apparent only after 3 h in the dark or in the shade, when Vmax had also declined significantly.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1399-3054.1995.tb05316.x
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  • 5
    Electronic Resource
    Electronic Resource
    When growing potato tubers are detached from their mother plant there is a rapid inhibition of starch synthesis, involving inhibition of ADP-glucose pyrophosphorylase (1994)
    Springer
    Planta 193 (1994), S. 486-493 
    Details
    ISSN: 1432-2048
    Keywords: ADP-glucose pyrophosphorylase ; Solanum ; Starch synthesis ; Sucrose synthesis ; Tuber (potato)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Labelling experiments in which high-specific-activity [U-14C]sucrose or [U-14C]hexoses were injected into potato (Solanum tuberosum L. cv. Desiree) tubers showed that within 1 d of detaching growing tubers from their mother plant, there is an inhibition of starch synthesis, a stimulation of the synthesis of other major cell components, and rapid resynthesis of sucrose. This is accompanied by a general increase in phosphorylated intermediates, an increase in UDP-glucose, and a dramatic decrease of ADP-glucose. No significant decline in the extracted activity of enzymes for sucrose degradation or synthesis, or starch synthesis is seen within 1 d, nor is there a significant decrease in sucrose, amino acids, or fresh weight. Over the next 7 d, soluble carbohydrates decline. This is accompanied by a decline in sucrose-synthase activity, hexose-phosphate levels, and the synthesis of structural cell components. It is argued that a previously unknown mechanism acting at ADP-glucose pyrophosphorylase allows sucrose-starch interconversions to be regulated independently of the use of sucrose for cell growth.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02411552
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  • 6
    Electronic Resource
    Electronic Resource
    Regulation of sucrose and starch metabolism in potato tubers in response to short-term water deficit (1997)
    Springer
    Planta 201 (1997), S. 502-518 
    Details
    ISSN: 1432-2048
    Keywords: Key words: Adenosine 5′-diphosphoglucose ; Starch synthesis ; Solanum (carbon metabolism ; tuber) ;  Sucrose-phosphate synthase ; Sucrose metabolism (futile cycle) ; Water stress
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract. To investigate the effect of water stress on carbon metabolism in growing potato tubers (Solanum tuberosum L.), freshly cut and washed discs were incubated in a range of mannitol concentrations corresponding to external water potential between 0 and −1.2 MPa. (i) Incorporation of [14C]glucose into starch was inhibited in water-stressed discs, and labeling of sucrose was increased. High glucose overrode the changes at low water stress (up to −0.5 MPa) but not at high water stress. (ii) Although [14C]sucrose uptake increased in water-stressed discs, less of the absorbed [14C]sucrose was metabolised. (iii) Analysis of the sucrose content of the discs confirmed that increasing water deficit leads to a switch, from net sucrose degradation to net sucrose synthesis. (iv) In parallel incubations containing identical concentrations of sugars but differing in which sugar was labeled, degradation of [14C]sucrose and labeling of sucrose from [14C]glucose and fructose was found at each mannitol concentration. This shows that there is a cycle of sucrose degradation and resynthesis in these tuber discs. Increasing the extent of water stress changed the relation between sucrose breakdown and sucrose synthesis, in favour of synthesis. (v) Analysis of metabolites showed a biphasic response to increasing water deficit. Moderate water stress (0–200 mM mannitol) led to a decrease of the phosphorylated intermediates, especially 3-phosphoglycerate (3PGA). The decrease of metabolites at moderate water stress was not seen when high concentrations of glucose were supplied to the discs. More extreme water stress (300–500 mM mannitol) was accompanied by an accumulation of metabolites at low and high glucose. (vi) Moderate water stress led to an activation of sucrose phosphate synthase (SPS) in discs, and in intact tubers. The stimulation involved a change in the kinetic properties of SPS, and was blocked␣by protein phosphatase inhibitors. (vii) The amount of ADP-glucose (ADPGlc) decreased when discs were incubated on 100 or 200 mM mannitol. There was a strong correlation between the in vivo levels of ADPGlc and 3PGA when discs were subjected to moderate water stress, and when the sugar supply was varied. (viii) The level of ADPGlc increased and starch synthesis was further inhibited when discs were incubated in 300–500 mM mannitol. (ix) It is proposed that moderate water stress leads to an activation of SPS and stimulates sucrose synthesis. The resulting decline of 3PGA leads to a partial inhibition of ADP-glucose pyrophosphorylase and starch synthesis. More-extreme water stress leads to a further alteration of partitioning, because it inhibits the activities of one or more of the enzymes involved in the terminal reactions of starch synthesis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004250050095
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  • 7
    Electronic Resource
    Electronic Resource
    Partial purification from potato tubers of three fructokinases and three hexokinases which show differing organ and developmental specificity (1993)
    Springer
    Planta 190 (1993), S. 156-165 
    Details
    ISSN: 1432-2048
    Keywords: Fructokinase ; Hexokinase ; Solanum ; Sucrose metabolism
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract A combination of chromatography on DE-52 cellulose, Cibacron Blue agarose, Mono Q anion exchanger and gel filtration was used to resolve different hexose-phosphorylating enzymes from growing “sink” potato tubers (Solanum tuberosum L.). Three enzymes (fructokinases: FK1, FK2 and FK3) are active with fructose and inactive with glucose, and three (hexokinases: HK1, HK2 and HK3) are active with glucose but not with fructose. Elution from DE-52 columns showed that the relative abundance of the six activities changes, depending on the organ and on the developmental stage. FK1 and FK2 were present at high activities in tubers but at very low activity in leaves; conversely FK3 was present at very low activity in tubers but at high activity in leaves. During storage of potato tuber, and also during sprouting, there was a decrease of FK1 and FK2. In contrast, glucose-phosphorylating activity was very low in growing tubers. During storage and sprouting the activity of the glucose-phosphorylating enzymes rose, until they exceeded FK1 and FK2. This was due particularly to an increase of HK1, whereas HK2 declined relative to HK1, and HK3 was always negligible. These changes in the pattern of hexose-phosphorylating enzyme forms are compared with the changing metabolic fluxes and pools of hexose sugars in potato tubers. It is concluded that organ- and development-specific changes in the abundance of the various enzyme forms contribute to the regulation of hexose metabolism in the potato.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00196607
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  • 8
    Electronic Resource
    Electronic Resource
    Some effects of cadmium on maize plants (1993)
    Springer
    Archives of environmental contamination and toxicology 25 (1993), S. 244-249 
    Details
    ISSN: 1432-0703
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering , Medicine
    Notes: Abstract The growth of the whole plant and the chlorophyll content, oxygen evolution, and chloroplast ultrastructure of leaf tissues have been studied in maize plants grown on a culture medium either without cadmium (Cd) or supplied with increasing concentrations of the metal. The plants treated with high Cd concentrations showed symptoms of heavy metal toxicity, such as length reduction of both roots and shoots, leaf bleaching, ultrastructural alterations of chloroplasts and lowering of photosynthetic activity. Some symptoms appeared at 100 μM Cd, but the strong toxic effects of the metal were found only at 250 μM Cd.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00212136
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