Maltose metabolism by pea chloroplasts

The aim of this work was to investigate the origin of maltose formed during starch breakdown in the dark by chloroplasts of Pisum sativum. The maximum catalytic activities of maltose phosphorylase and maltase in pea leaves were shown to be low, relative to those of enzymes known to be involved in starch breakdown. Fractionation of pea leaves indicated that the chloroplasts lack maltase but have enough maltose phosphorylase to synthesize the amounts of maltose formed when isolated chloroplasts breakdown starch. The absence of exogenous phosphate markedly reduced starch breakdown and maltose accumulation by isolated chloroplasts. When [¹⁴C]glucose was supplied to chloroplasts that were breaking down starch in the dark, maltose was labelled and most of the label was in the glucose moeity. It is suggested that maltose phosphorylase, using glucose-1-phosphate formed from starch by α-glucan phosphorylase, is responsible for, at least some of, the synthesis of maltose during starch breakdown by pea chloroplasts in vitro.     

Changes of Starch Synthetase Activity of Cucumber Leaves during Ammonium Toxicity

The changes of granule bound starch synthetase activity in cucumber leaves (Cucumis sativus L. cv. Suisei No. 2) were investigated during ammonium toxicity. Generally speaking the quantity of starch granules of injured plants were less than that of normal plants. ADPG is a more effective glucose donor to starch synthesis than UDPG. It was found that the starch synthetase activity of injured plants was decreased compared to the normal plants. This variation of enzyme activity was higher when UDPG was used as glucose donor. The addition of K⁺ and NH₄⁺ generally inhibited the enzyme activity when UDPG was used as glucose donor, but stimulated it when ADPG was used. This stimulation was found to be more effective in enzymes prepared from injured plants than from normal plants. The level of potassium bound to starch granules was not changed markedly between normal and injured plants.     

去果河套蜜瓜源叶碳水化合物及其相关酶昼夜变化特征

以河套蜜瓜为试材,在果实迅速膨大期通过去果处理改变库源关系,研究源叶净光合速率,蔗糖、还原糖和淀粉含量及其代谢相关酶活性的昼夜变化规律。结果表明:(1)源叶的净光合速率为单峰曲线,无明显的"光合午休"现象,去果处理对其无影响。(2)源叶中蔗糖和还原糖含量的昼夜变化为单峰曲线,蔗糖磷酸合成酶和蔗糖合成酶合成方向活性的昼夜变化为双峰曲线,蔗糖合成酶分解方向、酸性转化酶和中性转化酶活性的昼夜变化无明显规律,改变库源关系对这些指标均无显著影响;蔗糖含量升高受蔗糖磷酸合成酶和蔗糖合成酶合成方向正调控,而蔗糖含量降低则受多种酶的共同调节。(3)源叶中淀粉含量和腺苷二磷酸葡萄糖焦磷酸化酶活性的昼夜变化为单峰曲线,去果处理可以显著提高淀粉含量和腺苷二磷酸葡萄糖焦磷酸化酶活性,淀粉含量升高受腺苷二磷酸葡萄糖焦磷酸化酶正调控。     

Photosynthate metabolism in the source leaves of n(2)-fixing soybean plants

Soybean plants (Glycine max [L.] Merr. cv Williams), which were symbiotic with Bradyrhizobium japonicum, and which grew well upon reduced nitrogen supplied solely through N₂ fixation processes, often exhibited excess accumulation of starch and sucrose and diminished soluble protein in their source leaves. Nitrate and ammonia, when supplied to the nodulated roots of N₂-fixing plants, mediated a reduction of foliar starch accumulation and a corresponding increase in soluble protein in the source leaves. This provided an opportunity to examine the potential metabolic adjustments by which NO₃⁻ and NH₄⁺ (N) sufficiency or deficiency exerted an influence upon soybean leaf starch synthesis. When compared with soybean plants supplied with N, elevated starch accumulation was focused in leaf palisade parenchyma tissue of N₂-fixing plants. Foliar activities of starch synthesis pathway enzymes including fructose-1,6-bisphosphate phosphatase, phosphohexoisomerase, phosphoglucomutase (PGM), as well as adenosine diphosphate glucose pyrophosphorylase (in some leaves) exhibited highest activities in leaf extracts of N₂-fixing plants when expressed on a leaf protein basis. This was interpreted to mean that there was an adaptation of these enzyme activities in the leaves of N₂-fixing plants, and this contributed to an increase in starch accumulation. Another major causal factor associated with increased starch accumulation was the elevation in foliar levels of fructose-6-phosphate, glucose-6-phosphate, and glucose-1-phosphate (G1P), which had risen to chloroplast concentrations considerably in excess of the K_m values for their respective target enzymes associated with starch synthesis, e.g. elevated G1P with respect to adenosine diphosphate glucose pyrophosphorylase (ADPG-PPiase) binding sites. The cofactor glucose-1,6-bisphosphate (G1,6BP) was found to be obligate for maximal PGM activity in soybean leaf extracts of N₂-fixing as well as N-supplemented plants, and G1,6BP levels in N₂-fixing plant leaves was twice that of levels in N-supplied treatments. However the concentration of chloroplastic G1,6BP in illuminated leaves was computed to be saturating with respect to PGM in both N₂-fixing and N-supplemented plants. This suggested that the higher level of this cofactor in N₂-fixing plant leaves did not confer any higher PGM activation and was not a factor in higher starch synthesis rates. Relative to plants supplied with NO₃⁻ and NH₄⁺, the source leaf glycerate-3-phosphate (3-PGA) and orthophosphate (Pi) concentrations in leaves of N₂-fixing plants were two to four times higher. Although Pi is a physiological competitive inhibitor of leaf chloroplast ADPG-PPiase, and hence, starch synthesis, elevated chloroplast 3-PGA levels in N₂-fixing plant leaves apparently prevented interference of Pi with ADPG-PPiase catalysis and starch synthesis.     

Properties of α-glucan phosphorylase from pea chloroplasts

A partially purified preparation of α-glucan phosphorylase was obtained from chloroplasts of Pisum sativum by ion-exchange chromatography and gel filtration. The preparation, in which no other enzyme that metabolized starch or glucose 1 -phosphate could be detected, was characterized. The optimum for phosphorolysis was pH 7.2; at pH 8.0 the activity was reduced by 50%. The preparation showed normal hyperbolic kinetics with the substrates, and catalysed the formation of [¹⁴C]glucose 1-phosphate from ¹⁴C-labelled starch grains from pea chloroplasts. None of the following, generally at 5 and 10 mM, significantly altered the rate of phosphorolysis: glucose, fructose, sucrose, fructose 6-phosphate, fructose 1,6-bisphosphate, dihydroxyacetone phosphate, 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate, pyruvate, ATP, ADP, AMP, 6-phosphogluconate, 2-phosphoglycollate, Mg²⁺, dithiothreitol. However, phosphorolysis was inhibited by ADPglucose. Measurements of ADPglucose in leaves and in isolated chloroplasts showed that none could be detected in the dark and suggested that the concentration in the light was high enough to cause a modest inhibition of the phosphorylase. The control of the breakdown of chloroplast starch is discussed.     

The role of the hexose transporter in the chloroplasts of Arabidopsis thaliana L.

The metabolism of wild-type Arabidopsis thaliana L. and its mutant TC265 were compared in order to reveal the role of the chloroplast glucose transporter. Plants were grown in a 12-h photoperiod. From 20 to 40 days after germination, starch per gram fresh weight of shoot in the mutant was four times that in the wild type. The extent of this difference did not alter during this period. Stereological analysis showed that the chloroplasts in the mutant were larger than those in the wild type; the thylakoids appeared to be distorted by the high starch content. [U-¹⁴C]Glucose and [U-¹⁴C]glycerol were supplied, separately, to excised leaves in the dark. [U-¹⁴C]Glucose was a good precursor of sucrose in the wild type and mutant; [U-¹⁴C]glycerol was a poor precursor of sucrose in both. The distribution of ¹⁴C in the wild type was used to calculate that the net flux was from hexose monophosphates to triose phosphates, not vice versa. During the first 4 h of the night the sugar content (75% sucrose, 20% glucose) of the leaves of the mutant dropped sharply, and at all times during the night it was less than that of the wild-type leaves. This drop in sugar coincided with a decrease in the rate of respiration. The growth rate of the mutant was less than that of the wild type. Addition of sucrose restored the rate of respiration at night and increased the rate of growth. It is argued that a major function of the glucose transporter in Arabidopsis chloroplasts is export of the products of starch breakdown that are destined for sucrose synthesis at night.We thank Professor C.R. Somerville for his generous gift of seed of the Arabidopsis mutant TC265. We are also grateful to Mr B. Chapman for assistance with the preparation of the sections for electron microscopy. R.N.T. thanks the Science and Engineering Research Council for a studentship.     

Carbohydrate and carbon metabolite accumulation responses in leaves of ozone tolerant and ozone susceptible spinach plants after acute ozone exposure

The objective of this study was to determine whether exposure of plants to ozone (O₃) increased the foliar levels of glucose, glucose sources, e.g., sucrose and starch, and glucose-6-phosphate (G6P), because in leaf cells, glucose is the precursor of the antioxidant, L-ascorbate, and glucose-6-phosphate is a source of NADPH needed to support antioxidant capacity. A further objective was to establish whether the response of increased levels of glucose, sucrose, starch and G6P in leaves could be correlated with a greater degree of plant tolerance to O₃. Four commercially available Spinacia oleracea varieties were screened for tolerance or susceptibility to detrimental effects of O₃ employing one 6.5 hour acute exposure to 25O nL O₃ L^-1 air during the light. One day after the termination of ozonation (29 d post emergence), leaves of the plants were monitored both for damage and for gas exchange characteristics. Cultivar Winter Bloomsdale (cv Winter) leaves were least damaged on a quantitative grading scale. The leaves of cv Nordic, the most susceptible, were approximately 2.5 times more damaged. Photosynthesis (Pn) rates in the ozonated mature leaves of cv Winter were 48.9% less, and in cv Nordic, 66.2% less than in comparable leaves of their non-ozonated controls. Stomatal conductance of leaves of ozonated plants was found not to be a factor in the lower Pn rates in the ozonated plants. At some time points in the light, leaves of ozonated cv Winter plants had significantly higher levels of glucose, sucrose, starch, G6P, G1P, pyruvate and malate than did leaves of ozonated cv Nordic plants. It was concluded that leaves of cv Winter displayed a higher tolerance to ozone mediated stress than those of cv Nordic, in part because they had higher levels of glucose and G6P that could be mobilized during diminished photosynthesis to generate antioxidants (e.g., ascorbate) and reductants (e.g., NADPH). Elevated levels of both pyruvate and malate in the leaves of ozonated cv Winter suggested an increased availability of respiratory substrates to support higher respiratory capacity needed for repair, growth, and maintenance.Abbreviations ADPG-PPiase ADPglucose pyrophosphorylase - ASC L-ascorbic acid - APX ascorbate peroxidase - Ce CO₂ concentration in air in the measuring cuvette during photosynthesis measurements - Ci CO₂ concentration in the leaf intercellular spaces during photosynthesis measurement - Chl chlorophyll - DHA dehydroascorbic acid - DHA reductase dehydroascorbate reductase - DHAP dihydroxyacetone phosphate - GAP glyceraldehyde-3-phosphate - Gluc glucose - GR glutathione reductase - G_sw stomatal conductance with units as mmol H₂O m^-2 s^-1 - GSSG oxidized glutathione - GSH reduced glutathione - G1P glucose-1-phosphate - G6P glucose-6-phosphate - G6P dehydrogenase glucose-6-phosphate dehydrogenase - 6PG 6-phosphogluconate - 6PG dehydrogenase 6-phosphogluconate dehydrogenase - F6P fructose-6-phosphate - FBP fructose-1,6-bisphosphate - MAL malate - MDHA reductase monodehydroascorbate reductase - PE post-emergence - PEP phosphoenolpyruvate - PGA 3-phosphoglycerate - Pi orthophosphate - PYR pyruvate - Pn net CO₂ photoas-similation in leaves - PPFD photosynthetic photon flux density with units of mol photons m^-2 s^-1 - PPRC pentose phosphate reductive cycle - RuBP ribulose-1,5-bisphosphate - rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - SLW specific leaf weight - TCA cycle tricarboxylic acid cycle - Triose-P DHAP+GAP     

Changes of Sugar Levels in Cucumber Leaves during Ammonium Toxicity

Toxic effects of high concentrations of ammonia were studied in the cucumber plant (Cucumis sativus cv. Suisei No. 2). When the cucumber plant was cultured with 200 mg/l NH₃-N (as NH₄Cl), some characteristic symptoms, probably due to ammonium toxicity, appeared in the leaves after about 1 week, while no such symptoms were observed in the plants cultured with 20 mg/1 NH₃-N. The level of free sugars in 20 mg/l NH₃-N treated plants decreased with time and was lower than that in plants treated with 200 mg/l NH₃-N. Specially distinct differences were found as regards the levels of fructose and glucose. After 9 days' culture the content of glucose in 200 mg/l NH₃-N plants was 17 times higher than that of 20 mg/l NH₃-N plants. From the results of an incorporation of photosynthesized ¹⁴CO₂ for 3 hours into newly synthesized glucose it is evident that this accumulation of glucose can not be the degradative product of a glucose polymer such as starch. The levels of starch were also studied, and it was found that the starch level decreased due to ammonium toxicity. These results suggest that the translocation of glucose after its synthesis is inhibited by ammonium toxicity, at least up to starch synthesis.     

Large-scale photosynthetic production of carbon-13 labeled sugars: the tobacco leaf system

A system is described for the production of carbon-13 uniformly labeled starch, glucose, fructose, and sucrose from kilogram quantities of tobacco leaves. Nearly 60% of the CO₂ administered (at 80 atom % carbon-13 or above) was found in these four compounds; the loss of enrichment between carbon-13 administered and isolated sugars was 3% or less. About 25 g ¹³C-glucose can be produced in a 40-hr incubation from 1.2 kg of leaves. Label uniformity in the products was established by ¹³C-nmr spectroscopy.     

Alterations in carbohydrate levels in leaves of Populus due to ambient air pollution

Cuttings of Populus nigra L. cv. Loenen and Populus maximowiczii Henri × Populus nigra L. cv. Rochester were exposed for 6 weeks in open-top chambers to investigate effects of realistic mixtures of ozone, sulphur dioxide, and nitrogen oxides on carbohydrate levels. Whereas the total main nonstructural carbohydrates in the leaves were reduced, those in the roots were nearly not affected. The reduction in leaf carbohydrates was associated with a significant decrease in starch. In contrast, sucrose and especially glucose were increased, causing a shift in the starch to sucrose and the starch to glucose ratios. The effects were strongest in the older leaves of both cultivars, with cv. Loenen, especially, responding to ozonic mixtures. In the petioles, sucrose and glucose were unchanged or, in some cases, decreased. The alterations in carbohydrate levels were coupled to a reduction in both total fresh weight of leaves and total dry matter of roots, and were attributed to an air pollution-dependent demand for energy and carbon in leaves.     

Carbon partitioning into sorbitol,sucrose, and starch in source and sink apple leaves as affected by elevated CO2

Experiments were conducted in controlled growth chambers to evaluate how increases in CO₂ concentration ([CO₂]) affected carbon metabolism and partitioning into sorbitol, sucrose, and starch in various ages of apple leaves. Apple plants (Malus domestica), 1 year old, were exposed to [CO₂] of 200, 360, 700, 1000, and 1600 μl l⁻¹ up to 8 days. Six groups of leaves (counted from the shoot apex): leaves 1–5 (sink), 6–7 (sink to source transition), 8–9 (sink to source transition), 10–11 (nearly-matured source), 21–22 (mid-age source), and 30–32 (aged source), were sampled at 1, 2, 4, and 8 days after [CO₂] treatments for carbohydrate analysis. Increases in [CO₂] from a sub-ambient (200 μl l⁻¹) to an ambient level (360 μl l⁻¹) significantly increased the concentrations of sorbitol, sucrose, glucose, and fructose tested in all ages of leaves. Continuous increase in [CO₂] from ambient to super-ambient levels up to 1600 μl l⁻¹ also increased sorbitol concentration by ≈50% in source leaves, but not in sink and sink to source transition leaves. Increases in [CO₂] from 360 to 1600 μl l⁻¹, however, had little effect on sucrose content in all ages of leaves. Starch concentrations increased in all ages of leaves as [CO₂] increased. Rapid starch increases (e.g. 5-, 6-, 20-, and 50-fold increases for leaf groups 1–5, 6–7, 10–11, and 21–22, respectively) occurred from 700 to 1600 μl l⁻¹ [CO₂] during which increases in sorbitol concentration either ceased or slowed down. Our results indicate that changes in carbohydrates were much more responsive to CO₂ enrichment in source leaves than in sink and sink to source transition leaves. Carbon partitioning was favored into starch and sorbitol over sucrose in all ages of leaves when [CO₂] was increased from 200 to 700 μl l⁻¹, and was favored into starch over sorbitol from 700 to 1600 μl l⁻¹ [CO₂].     

Effect of mycorrhizal-enhanced leaf phosphate status on carbon partitioning, translocation and photosynthesis in cucumber

An assessment of the effects of arbuscular mycorrhizal (AM) infection on photosynthesis, carbon (C) allocation, translocation and biomass production of cucumber, grown in sand culture, was made using a previously determined phosphorus (P) supply (0·13 mol m^?3 P) which had a significant impact on AM infection. Separation of a direct effect of AM infection from an indirect one due to an enhanced leaf P status was achieved using a comparable non‐mycorrhizal treatment (NAM + P) supplemented with extra P (0·19 mol m^?3 P). Total leaf P concentration, specific leaf mass, photosynthetic capacity, and incorporation of ¹⁴C into non‐structural carbohydrate pools were dependent on leaf age. Both maximum and ambient photosynthetic rates were significantly higher in the youngest fully expanded leaves from AM and NAM + P plants which also had the higher leaf P concentrations. There were no differences in the total concentrations of starch, sucrose, raffinose or stachyose in young or old leaves among AM, non‐mycorrhizal (NAM) and NAM + P treatments. However, younger leaves of NAM plants showed a shift in ¹⁴C‐partitioning from stachyose and raffinose synthesis to starch accumulation. Determination of ADP‐glucose pyrophosphorylase (AGPase), sucrose synthase and sucrose phosphate synthase enzyme activities revealed that only AGPase activity was correlated with the increased incorporation rate of ¹⁴C into starch in young leaves of NAM plants. Although there were significant AM‐specific effects on C translocation to the root system, AM plants had similar rate of photosynthesis to NAM + P plants. These results suggest that the increase in photosynthetic rate in leaves of AM‐infected cucumber was due to an increased P status, rather than a consequence of a mycorrhizal ‘sink’ for assimilates.     

On the presence of starch bound phosphate in potato leaf starch

The presence of starch-bound phosphate in potato leaves collected late afternoon in the middle of July when the starch content is high (12.9% dry matter basis) was studied. Starch was extracted from the leaves with dimethylsulphoxide and fragments of starch were purified by ultrafiltration in two steps in combination with an -amylase hydrolysis. The fragments were analysed with ³¹P-NMR and no signals corresponding to phosphate monoesters linked to glucose at the C₃ and/or C₆ positions were detected. The results show that starch in potato leaves does not contain any detectable amounts of phosphate monoesters.     

Responses to dietary starch or sucrose on protein and glucose kinetics in goats fed a high-concentrate diet

Abstract

Responses of whole body protein synthesis (WBPS) and glucose irreversible loss rate (ILR) were compared between dietary starch and sucrose in four male goats. Diets were fed at 1.2 times maintenance requirements of ME and CP with 30% of the ME as starch, starch plus sucrose or sucrose, twice daily. The diets consisted of 33, 32, 11 and 24% of alfalfa hay, corn, soybean meal and the carbohydrates, respectively. The WBPS and glucose ILR during 5 – 7 h after feeding were determined by an isotope dilution method of [²H₅]phenylalanine, [²H₂]tyrosine, [²H₄]tyrosine and [¹³C₆]glucose. Sucrose elevated ammonia nitrogen and lowered acetate concentrations in the rumen, but did not differ from starch in nitrogen retention. Glucose ILR and WBPS were similar between the carbohydrates. It was concluded that dietary sucrose would have effects similar to starch on WBPS and glucose kinetics in the absorptive state in goats fed a high-concentrate diet.     

NDP-sugars, floridoside and floridean starch levels in relation to activities of UDP-glucose pyrophosphorylase and UDP-glucose-4-epimerase in Solieria chorda l is (Rhodophyceae) under experimental conditions

Under limited nutrient availability (i.e. unenriched sea‐water) and under 75 mol photons m^–2 s^–1 irradiance 12:12 LD, thalli of Solieria chordalis J. Agardh accumulated floridean starch and floridoside. When they were transferred into nutrient‐enriched seawater (150 umol L^?1 NO3¹‐ and 7 umol L^?1 P0₄³i at 35 umol photons m^?2 s^?1 in irradiance 12:12 LD, starch and floridoside levels decreased. The main nucleotide diphosphate (NDP) sugars (i.e. UDP‐glucose, UDP‐galactose and ADP‐glucose) and the activities of UDP‐glucose pyrophosphorylase [Enzyme Code (EC) 2.7.7.9] and UDP‐glucose‐4‐epimerase (EC 5.1.3.2) were measured under these controlled culture conditions. Both UDP‐glucose and UDP‐galactose in the thal l i increased under conditions known to favor the accumulation of floridean starch and floridoside, whereas they decreased under conditions leading to floridean starch and floridoside breakdown. On the other hand, ADP‐glucose level only varied slightly. Although UDP‐glucose pyrophosphorylase activity rose under conditions of floridean starch synthesis, little variation was observed in UDP‐glucose‐4‐epimerase activity. These results suggest a possible enzymatic regulation of the NDP‐sugar and carbohydrate pool in which UDP‐glucose pyrophosphorylase would play a major role.     

Primary partitioning and storage of photosynthate in sucrose and starch in leaves of C4 plants

A procedure involving pulse labelling of leaves with ¹⁴CO₂ was developed to measure the primary (initial) partitioning of photosynthate between sucrose and starch. Partitioning of photosynthate into sucrose and starch was determined in leaves of C₄ plants and compared with the patterns of storage of carbon in these products during the light period. The ratio of primary partitioning into sucrose and starch varied from about 0.5 in those species that accumulated mostly starch in the leaves (Amaranthus edulis L., Atriplex spongiosa F. Muell. and Flaveria trinervia (Spreng.) C. Mohr) to about 8 in Eleusine indica (L.) Gaertn., which accumulated mostly sucrose. No label was detected in free glucose or fructose. Generally there was a reasonable link between the primary partitioning of photosynthate and the type of carbohydrate stored in the leaf during the day. However, the ratio of carbon initially partitioned into sucrose versus starch was about 3 to 4 times higher in leaves of NADP-malic enzyme-type monocotyledonous species compared with phosphoenolpyruvate carboxykinase-type species, although the ratio of sucrose to starch accumulated in leaves during the day was very similar in the two groups. Sucrose and starch were the principal carbohydrates accumulated in leaves during the day. None of the species examined contained significant amounts of fructan and only one species, Atriplex spongiosa, contained substantial amounts of hexose sugars. In most of the species studied, the proportion of photosynthate partitioned into starch was greater at the end of the day than at the beginning. With the exception of Flaveria trinervia, the rate of CO₂ assimilation did not decline during the day, showing that, under our conditions, accumulation of carbohydrate in the leaves did not lead to feedback inhibition of photosynthesis in these C₄ species.Abbreviations Chl chlorophyll - NAD-ME NAD-malic enzyme - NADP-ME NADP-malic enzyme - PCK phosphoenolpyruvate carboxykinase We thank Prof. H.W. Heldt (Pflanzenphysiologisches Institut, Universität Göttingen) for discussions and advice during the course of this work.     

Leaf Phosphate Status, Photosynthesis, and Carbon Partitioning in Sugar Beet: III. Diurnal Changes in Carbon Partitioning and Carbon Export

The effect of low phosphate supply (low P) was determined on the diurnal changes in the rate of carbon export, and on the contents of starch, sucrose, glucose, and fructose 2,6-bisphosphate (F2,6BP) in leaves. Low-P effects on the activities of a number of enzymes involved in starch and sucrose metabolism were also measured. Sugar beets (Beta vulgaris L. cv. F58-554H1) were cultured hydroponically in growth chambers and the low-P treatment induced nutritionally. Low-P treatment decreased carbon export from the leaf much more than it decreased photosynthesis. At growth chamber photon flux density, low P decreased carbon export by 34% in light; in darkness, export rates fell but more so in the control so that the average rate in darkness was higher in low-P leaves. Low P increased starch, sucrose, and glucose contents per leaf area, and decreased F2, 6BP. The total extractable activities of enzymes involved in starch and sucrose synthesis were increased markedly by low P, e.g. adenosine 5-diphosphoglucose pyrophosphorylase, cytoplasmic fructose-1,6-bisphosphatase, uridine 5-diphosphoglucose pyrophosphorylase, and sucrose-phosphate synthase. The activities of some enzymes involved in starch and sucrose breakdown were also increased by low P. We propose that plants adapt to low-P environments by increasing the total activities of several phosphatases and by increasing the concentrations of phosphate-free carbon compounds at the expense of sugar phosphates, thereby conserving Pi. The partitioning of carbon among the various carbon pools in low-P adapted leaves appears to be determined in part by the relative capacities of the enzymes for starch and sucrose metabolism.     

Comparison of the metabolic properties of plastids isolated from developing leaves or embryos of Brassica napus L

Plastids isolated from developing leaves and embryos of oilseed rape (Brassica napus L.) were incubated with substrates in the light or the dark, with or without exogenous ATP. Incorporation of HCO^-3, and carbon from a range of substrates into fatty acids and/or starch by leaf chloroplasts was absolutely light-dependent and was unaffected by provision of ATP. Incorporation of HCO^-3 into fatty acids and/or starch by embryo plastids was also light-dependent. However, the light-dependent rates attained, when expressed on a comparable basis, were less than 32% of those from Glc6P (plus ATP), which was the most effective substrate for starch and fatty acid synthesis. In the light alone the rates of carbon incorporation from Glc6P, pyruvate and acetate into fatty acids, and from Glc6P into starch by embryo plastids were less than 27% of the respective ATP-dependent (dark) rates. Light had no effect on these ATP-dependent rates of synthesis by embryo plastids. While transporter activities for both glucose and Glc6P were present in embryo plastids, leaf chloroplasts did not have the latter activity. It is concluded that light at in vivo levels can contribute energy to carbon metabolism in embryo plastids. However, this contribution is likely to be small and these plastids are therefore largely dependent upon interaction with the cytosol for the ATP, reducing power and carbon precursors that are required for maximal rates of starch and fatty acid synthesis.     

Starch synthesis by isolated amyloplasts from wheat endosperm

The aim of this work was to discover which compound(s) cross the amyloplast envelope to supply the carbon for starch synthesis in grains of Triticum aestivum L. Amyloplasts were isolated, on a continuous gradient of Nycodenz, from lysates of protoplasts of endosperm of developing grains, and then incubated in solutions of ¹⁴C-labelled: glucose, glucose 1-phosphate, glucose 6-phosphate, fructose 6-phosphate, fructose-1,6-bisphosphate, dihydroxyacetone phosphate and glycerol 3-phosphate. Only glucose 1-phosphate gave appreciable labelling of starch that was dependent upon the integrity of the amyloplasts. Incorporation into starch was linear with respect to time for 2 h. At the end of the incubations, 98% of the ¹⁴C in the soluble fraction of the incubation mixture was recovered as [¹⁴C]glucose 1-phosphate. Thus it is unlikely that the added [¹⁴C glucose 1-phosphate was extensively metabolized prior to uptake by the amyloplasts. It is argued that the behaviour of the isolated amyloplasts, and previously published data on the labelling of starch by [¹³C]glucose, are consistent with the view that in wheat grains it is a C-6, not a C-3, compound that enters the amyloplast to provide the carbon for starch synthesis.Abbreviations PPase alkaline inorganic pyrophosphatase - UDPglucose uridine 5-diphosphoglucose     

Glyphosate inhibits photosynthesis and allocation of carbon to starch in sugar beet leaves

Application of glyphosate (N-[phosphonomethyl] glycine) to exporting leaves of sugar beet (Beta vulgaris, L.) during the day lowered stomatal conductance and carbon fixation. Allocation of newly fixed carbon to foliar starch accumulation was nearly completely inhibited, being decreased by the same amount as net carbon fixation. In contrast, decreasing net carbon fixation in untreated leaves by lowering CO₂ concentration caused starch accumulation to decrease, but only in the same proportion as net carbon fixation. Shikimate level increased 50-fold in treated leaves but the elevated rate of carbon accumulation in shikimate was only 4% of the decrease in the rate of starch accumulation. Application of steady state labeling with ¹⁴CO₂ to exporting leaves confirmed the above changes in carbon metabolism, but revealed no other major daytime differences in the ¹⁴C-content of amino acids or other compounds between treated and control leaves. Less ¹⁴C accumulated in treated leaves because of decreased fixation, not increased export. The proportion of newly fixed carbon allocated to sucrose increased, maintaining export at the level in control leaves. Returning net carbon exchange to the rate before treatment restored starch accumulation fully and prevented a decrease in export during the subsequent dark period.