Effect of Carbohydrate Demand on the Remobilization of Starch in Stolons and Roots of White Clover (Trifolium repens L.) after Defoliation

White clover plants were grown from stolon tips in growth cabinetsand then defoliated. Thereafter, changes in the contents ofnon-structural carbohydrates such as starch, sucrose, glucose,fructose, maltose, and pinitol in stolons and roots were monitored.Initial contents of carbohydrate reserves, photosynthetic supplyof new carbohydrates and carbohydrate demand after defoliationwere varied by growing the plants at various CO₂ partial pressures,by varying the extent of defoliation and by removing eitherroots or stolon tips at the time of defoliation. Remobilization of carbohydrate reserves in stolons increasedproportionally to their initial contents and was greater whenplants had been severely defoliated, suggesting that carbohydrateswere remobilized according to availability and demand. Starchwas the predominant reserve carbohydrate. Starch degradationwas associated with decreased contents of sucrose, glucose andfructose in young stolon parts and roots but not in old stolonparts suggesting that starch degradation was not strictly controlledby the contents of these sugars. A decrease in the demand forcarbohydrates by removal of roots did not decrease starch degradationbut increased the contents of sucrose, glucose, and fructose.Removal of stolon tips decreased starch degradation and contentsof sucrose, glucose, and fructose. The results suggest thatstarch degradation was controlled by a factor other than sucrose,glucose, and fructose which was exported from stolon tips, e.g.gibberellin. Key words: White clover, storage carbohydrates, remobilization, regrowth     

Sugar and organic Acid constituents in white clover

Major ethanol-soluble carbohydrate and organic acid constituents of white clover (Trifolium repens) have been identified by use of high-performance liquid chromatography and gas chromatography. In leaves, petioles, roots, and nodules, pinitol (3-O-methyl chiro-inositol) is the predominant sugar, with sucrose present in lower concentration. In leaves and petioles there are significant levels of α- and β-methyl glucosides, linamarin, glucose, and fructose. In the nodules glucose is rarely present at detectable levels. The concentration of pinitol is generally greater than 25 millimolar in each tissue examined whereas the level of sucrose varies depending on the time of day. Sucrose is the major sugar significantly labeled during 1 hour administration of ¹⁴CO₂ and accounts for more than 99% of all the radioactivity detected in the nodules at early times. Between 3 and 7 hours after labeling, 6% of the radioactivity is found in the organic acids fraction and 5% in the basic fraction of nodules. Malonic acid does not appear to be present in unusually high concentrations in either leaves or nodules of white clover.     

Pinitol occurrence in soybean plants as affected by temperature and plant growth regulators

Unsuitable temperatures are frequently encountered by soybean(Glycine max L. Merr.) plants grown in the field. Certain polyolshave been reported to protect plants from high temperature orfrost damage. Controlled environment studies were conductedto investigate the effect of stressful temperature regimes onthe content of pinitol (3-O-methyl-D-chiro-inositol) in soybeanplants. Hydroponically-grown soybean plants were subjected tohigh (35/30 C) or low (15/10 C) day/night temperature stresses,and pinitol content in different plant parts was determinedusing high performance liquid chromatography (HPLC). A syntheticplant growth regulator, PGR-IV, was foliarly applied to theplants to evaluate its effect on pinitol content in differentplant components. Uniformly-labelled ¹⁴C-glucose was fed intothe leaves via the transpiration stream, and the effects ofhigh temperature and EXP-S1089, another synthetic plant growthregulator, on the incorporation of ¹⁴C-glucose into pinitolwas evaluated using HPLC separation and scintillation spectrometry.High-temperature stress significantly increased plant pinitolcontent and the incorporation of ¹⁴C-glucose into pinitol, butdecreased the content of sucrose, glucose and fructose. Underlow-temperature stress, there was hardly any change in pinitolcontent, but a drastic increase in soluble sugars. PGR-IV enhancedpinitol translocation from leaves to stems and roots, whileEXP-S1089 increased pinitol/sucrose ratio. Accumulation of pinitolmay be an adjustment mechanism of the plant to reduce high-temperaturedamage, but not low-temperature injuries. Key words: Pinitol, soybean, temperature, plant growth regulator     

Rhythms as photoperiodic timers in the control of flowring in Chenopodium rubrum L.

Summary In C. rubrum, the amount of flowering that is induced by a single dark period interrupting continuous light depends upon the duration of darkness. A rhythmic oscillation in sensitivity to the time that light terminates darkness regulates the level of flowering. The period length of this oscillation is close to 30 hours, peaks of the rhythm occurring at about 13, 43 and 73 h of darkness.Phasing of the rhythm by 6-, 12- and 18-h photoperiods was studied by exposing plants to a given photoperiod at different phases of the free-running oscillation in darkness. The shift in phase of the rhythm was then determined by varying the length of the dark period following the photoperiod; this dark period was terminated by continuous light.With a 6-h photoperiod the timing of both the light-on and light-off signals is shown to control rhythm phasing. However, when the photoperiod is increased to 12 or 18 h, only the light-off signal determines phasing of the rhythm. In prolonged periods of irradiation-12 to 62 h light—a durational response to light overrides any interaction between the timing of the light period and the position of the oscillation at which light is administered. Such prolonged periods of irradiation apparently suspend or otherwise interact with the rhythm so that, in a following dark period, it is reinitiated at a fixed phase relative to the time of the light-off signal to give a peak of the rhythm 13 h after the dusk signal.In daily photoperiodic cycles rhythm phasing by a 6-h photocycle was also estimated by progressively increasing the number of cycles given prior to a single dark period of varied duration.In confirmation of Bünning's (1936) hypothesis, calculated and observed phasing of the rhythm controlling flowering in c. rubrum accounts for the photoperiodic response of this species. Evidence is also discussed which indicates that the timing of disappearance of phytochrome P_fr may limit flowering over the early hours of darkness.     

Growth and Dormancy in Lunularia cruciata (L.) Dum: VI. GROWTH REGULATION BY DAYLENGTH, BY RED, FAR-RED, AND BLUE LIGHT, AND BY APPLIED GROWTH REGULATORS AND CHELATING AGENTS

Experiments with photoperiods ranging from 2 to 24 h confirmthat 8 h light per day is optimal for Lunularia: there is nogrowth in the dark or in continuous light, which causes therapid onset of dormancy. Short-day cycles intercalated amonga series of continuous light cycles promote growth; in cycleslonger than 24 h very long dark periods are detrimental. Withvery short photoperiods (5 min) red light promotes growth moreeffectively than white light at higher intensity; far-red actsas dark. The growth effects of red and far-red light breaks(3 min) depended on the time of application; red light inhibitedin the middle but promoted at the beginning of the 16-h darkperiod of a short day; far-red light had the opposite effect;in each case red and far-red effects were reversible by theother wavelength. Blue light gave the same response as red includingthe reversibility of far-red effects and vice versa. Surprisingly,significant effects of 5 min red, blue, and far-red irradiationwere also found in the middle of the main high-intensity white-lightperiod, red and blue promoting growth, far-red reducing it;again there was ready reversibility of the effects. Growth promoters of higher plants are generally inhibitory toLunularia or have little effect; among growth retardants TIBA,Phosphon D, and CCC gave a slight promotion of growth. EDTApromoted growth (cell numbers) very significantly while 8-hydroxyquinolinewas initially inhibitory, but had a marked latent promotingeffect when subsequently washed from the thalli.     

Photocontrol of flowering and stem extension of the intermediate-day plant Echinacea purpurea

Intermediate-day plants (IDP) flower most rapidly and completely under intermediate photoperiods (e.g., 12–14 h of light), but few species have been identified and their flowering responses are not well understood. We identified Echinacea purpurea Moench as an IDP and, based on our results, propose a novel mechanism for flowering of IDP. Two genotypes of E. purpurea ('Bravado' and 'Magnus') flowered most completely (≥79%) and rapidly and at the youngest physiological age under intermediate photoperiods of 13–15 h. Few (≤14%) plants flowered under 10- or 24-h photoperiods, indicating E . purpurea is a strongly quantitative IDP. Plants were also induced to flower when 15-h dark periods were interrupted with as few as 7.5 min of low-intensity lighting (night interruption, NI). Flowering was progressively earlier as the NI increased to 1 h, but was delayed when the NI was extended to 4 h. Stem length increased by ≥230% as the photoperiod or NI duration increased, until plants received a saturating duration (at 14 or 1 h, respectively). Flowering was inhibited when 16-h photoperiods were deficient in red (R, 600–700 nm) light, and was promoted when photoperiods were deficient in far-red (FR, 700–800 nm) light. Because of our results, we propose the flowering behavior of IDP such as E . purpurea is composed of two mechanisms: a light-dependent response operating through light-labile (type I) phytochrome in which flowering is inhibited by an LD, and a light-stable (type II) phytochrome (i.e., phyB, D and E) response in which flowering is promoted by a short-night.     

Carbohydrate Metabolism in Drought-Stressed Leaves of Pigeonpea (Cajanus cajan)

Pigeonpea is a tropical grain-legume, which is highly dehydrationtolerant. The effect of drought stress on the carbohydrate metabolismin mature pigeonpea leaves was investigated by withholding waterfrom plants grown in very large pots (50 kg of soil). The moststriking feature of drought-stressed plants was the pronouncedaccumulation of D-pinitol (1D-3-methyl-chiro-inositol), whichincreased from 14 to 85 mg g^–1 dry weight during a 27d stress period. Concomitantly, the levels of starch, sucroseand the pinitol precursors myo-inositol and ononitol all decreasedrapidly to zero or near-zero in response to drought. The levelsof glucose and fructose increased moderately. Drought stressinduced a pronounced increase of the activities of enzymes hydrolysingsoluble starch (amylases) and sucrose (invertase and sucrosesynthase). The two anabolic enzymes sucrose phosphate synthase(sucrose synthetic pathway) and myo-inositol methyl transferase(pinitol synthetic pathway) also showed an increase of activityduring stress. These results indicate that pinitol accumulatedin pigeonpea leaves, because the carbon flux was diverted fromstarch and sucrose into polyols. Key words: Drought, polyols, pinitol, sucrose, starch, pigeonpea     

Photosynthetic carbohydrate metabolism in wheat (Triticum aestivum L.) leaves: optimization of methods for determination of fructose 2, 6-bisphosphate

The accurate measurement of fructose 2,6-bisphosphate from plants such as wheat is fraught with difficulty. Extraction and assay methods for fructose 2,6-bisphosphate that give near 100% recovery of the metabolite, and a linear response with volume have therefore been developed for extracts prepared from wheat leaves of different ages. Amounts of fructose 2,6-bisphosphate in different regions of leaves generally showed a positive correlation with chlorophyll content. Measurements of sucrose and starch in third leaves harvested at different times of the diurnal cycle demonstrated that sucrose is the major form in which photosynthate is stored in the leaf, but starch can account for up to about 30% of the stored carbohydrate. Virtually all of the carbohydrate accumulated as starch and sucrose during the day was degraded at night. Amounts of fructose 2,6-bisphosphate were generally lower in extracts prepared from leaves harvested in the light than in the dark. Additionally, there was no change in either the amount of fructose 2, 6-bisphosphate or the ratio of sucrose to starch in samples prepared from leaves harvested at different times of the day. These results are broadly consistent with a role for fructose 2,6-bisphosphate in the regulation of sucrose synthesis and the partitioning of carbohydrate between sucrose and starch in wheat leaves.     

The effect of sugars on inverse relation between the growth and chlorophy11 synthesis in tobacco tissue

Cytokinin-dependent and cytokinin-autonomous strains of tobacco callus tissue (Nicotiana tabacum L. cv. ‘Wisconsin 38’) were grown on media containing sucrose, glucose and fructose, respectively. The tissues were kept 14 days in darkness and then transferred for 9 days to continuous light after which time the fresh weight and chlorophyll content were estimated. The highest chlorophyll concentration was recorded at sugar levels which were either suboptimal (sucrose in the case of cytokinin-dependent strain) or supraoptimal (all other sugars for both strains and sucrose for the cytokinin-autonomous strain) for tissue growth. The chlorophyll concentration was increased when the tissue was cultured on media containing glucose or fructose,i.e. sugars whioh did not support the growth as well as sucrose. Chlorophyll synthesis in the cytokinin-autonomous strain is significantly lower than in the cytokinin-dependent strain. This difference was independent of either sugar source or concentration. These results support the observed inverse relationship between tissue growth and plastid development and the limited metabolic activity of plastids in cytokinin-autonomous tissues.     

Radioimmunoassay of phytochrome content in green, light-grown oats

Abstract. We measured phytochrome content by radioimmunoassay in green, light-grown oats ( Avena sativa L., cv. Garry) grown under several light regimes. Phytochrome content increased linearly to six times the light-grown level during a 24-h dark incubation and to fifty times the light-grown level after 48 h darkness. Phytochrome content, in response to a cycle of 12-h light, 12-h dark photoperiods, increased to three times the light-grown level in darkness and returned to the light-grown level during the subsequent light period. Plants given far-red light immediately prior to a 12-h dark period contained about four-fold more phytochrome than light-grown oats, compared with a three-fold increase in plants treated identically except given no far-red light. We also attempted, unsuccessfully, to eliminate a general nonspecific interference by crude plant extracts with the radioimmunoassay.     

Effects of feeding schedules on diurnal periodicity of Leucocytozoon smithi gametocytes in the peripheral blood of domestic turkeys

Eighteen domestic turkeys naturally infected with Leucocytozoon smithi Laveran & Lucet were maintained on restricted feeding schedules under conditions of either continuous light or natural light (light 13 h:darkness 11 h) photoperiods. Peripheral gametocyte numbers of L. smithi in all turkeys were determined every 2 h over a 36-h period. Peripheral gametocyte numbers of turkeys maintained under continuous light and restricted to either a 10-h feeding period (9:30 p.m. to 7:30 a.m.) once a day or a 2-h feeding period twice a day (7:30 a.m. to 9:30 p.m. and 7:30 p.m. to 9:30 p.m.) increased at or near the time of feed availability. Under natural photoperiod, gametocyte periodicity was not affected by restricting feed availability to the dark phase of the light-dark cycle. Mean parasite numbers were highest during the light phase when feed was not available, and lowest during the dark period when feed was accessible.     

Influence of Carbohydrates on Photosynthesis in Single, Rooted Soybean Leaves Used as a Source-Sink Model

The single rooted leaf of soybean (Glycine max L. Merr.) wasused to study source-sink relationships in photosynthesis. Whenthe leaves were kept under a regime of 10 h light (410–480µmol photons m^–2, 400–700 nm)–14 h dark,they did not expand, the increase in leaf dry weight almoststopped, and photosynthetic activity remained at a high andconstant level for 8 d while the dry weight of the roots increasedat a constant rate throughout the period. Thus, under this conditionthe leaf and the root system served as the only source and sinkorgans, respectively. When leaves grown for 7 d under this conditionwere placed under continuous light to alter the source/sinkbalance in photosynthate, the root dry weight increased at aconstant rate equal to that found under the 10 h light–14h dark condition. The leaf dry weight markedly increased andby day 5 of continuous light had increased 1.6-fold, mainlyas a consequence of accumulation of starch and sucrose, whichwere not translocated for root growth. The continuous lightcaused an abrupt decrease in the photosynthetic activity (40%of initial value by day 5). However, the activity recoveredalmost completely after a 32-h transfer to darkness. Significantnegative correlations existed between photosynthetic activityand the sucrose and starch contents in the rooted leaves placedunder continuous light. When the plants were treated with variouslight conditions, there was no significant difference (p<0.01)among the regression line slopes for photosynthetic activityon the sucrose content, but there was some deviation among thosefor the photosynthetic activity on the starch content. Thisresult suggests that sucrose accumulated in the leaf has a moredirect influence on photosynthetic activity when the source/sinkbalance was altered. (Received September 9, 1985; Accepted February 21, 1986)     

Properties of phosphoenolpyruvate carboxylase in rapidly prepared,desalted leaf extracts of the Crassulacean acid metabolism plant Mesembryanthemum crystallinum L.

Properties of phosphoenolpyruvate (PEP) carboxylase, obtained from leaves of Mesembryanthemum crystallinum L. performing Crassulacean acid metabolism (CAM), were determined at frequent time points during a 12-h light/12-h dark cycle. Leaf extracts were rapidly desalted and PEP carboxylase activity as a function of PEP concentration, malate concentration, and pH was measured within 2 min after homogenization of the tissue. Maximum velocity of PEP carboxylase was similar in the light and dark at pH 7.5 and pH 8.0. However, PEP carboxylase had as much as a 12-fold lower K _m for PEP and as much as a 20-fold higher K _i for malate during the dark than during the light periods, the magnitude of these differences being dependent on the assay pH. Assuming that enzyme properties immediately after isolation reflect the approximate state of the enzyme in vivo, these differences in enzyme properties reduce the potential for CO₂ fixation via PEP carboxylase in the light. A small decrease in cytoplasmic pH in the light would greatly magnify the above differences in day/night properties of PEP carboxylase, because the sensitivity of PEP carboxylase to inhibition by malate increased with decreasing pH. Properties of PEP carboxylase were also studied in plants exposed to short-term perturbations of the normal 12-h light/12-h dark cycle (e.g., prolonged light period, prolonged dark period). Under all light/dark regimes, there was a close correlation between change in properties of PEP carboxylase and changes of the tissue from acidification to deacidification, and vice versa. Changes in properties of PEP carboxylase were not merely light/dark phenomena because they were also observed in plants exposed to continuous light or dark. the data indicate that, during CAM, PEP carboxylase exists in two stages which differ in their capacity for net malate synthesis. The physiologically-active state is distinguished by a low K _m for PEP and a high K _i for malate and favors malate synthesis. The physiologically-inactive state has a high K _m for PEP and a low K _i for malate and exists during periods of deacidification and other periods lacking synthesis of malic acid.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEPC PEP carboxylase - RuBP ribulose 1,5-bisphosphate - RH relative humidity     

Patterns of pinitol accumulation in soybean plants and relationships to drought tolerance

Previous studies indicate that methylated cyclitols are potentially important osmolytes in plants. In a search for genetic diversity for pinitol (D -3-O-methyl-chiro-inositol) accumulation in soybean (Glycine max (L.) Merr.), two- to three-fold differences in pinitol accumulation in leaf blades were found among Chinese plant introductions. Furthermore, it was found that genotypes that accumulated high concentrations of pinitol, when grown under well-watered conditions, had been selected for performance in regions of China having low rainfall. Among the carbohydrates analysed, pinitol accumulation was uniquely associated with adaptation to dry areas of China. A detailed study of pinitol accumulation in the soybean plant showed two- to three-fold gradients in pinitol concentration from the bottom to the top of the plant. The gradient shifted during plant development, with consistently higher concentrations of pinitol in the uppermost leaves. Pinitol accumulation was not correlated with activity of the key biosynthetic enzyme, inositol methyl transferase. This result and other lines of evidence indicated that shifting patterns of pinitol accumulation were due to translocation of the cyclitol from lower to upper nodes. Pinitol, proline, and sugars accumulated in leaf blades on soybean plants subjected to drought, but the molar concentration of pinitol in stressed plants was greater than the concentrations of proline or sugars. Although the mechanism by which pinitol participates in drought tolerance is not fully known, our results provide additional correlative evidence linking pinitol and drought tolerance in soybean.     

Fluxes in central carbohydrate metabolism of source leaves in a fructan-storing C3 grass: rapid turnover and futile cycling of sucrose in continuous light under contrasted nitrogen nutrition status

This work assessed the central carbohydrate metabolism of actively photosynthesizing leaf blades of a C3 grass (Lolium perenne L.). The study used dynamic (13)C labelling of plants growing in continuous light with contrasting supplies of nitrogen ('low N' and 'high N') and mathematical analysis of the tracer data with a four-pool compartmental model to estimate rates of: (i) sucrose synthesis from current assimilation; (ii) sucrose export/use; (iii) sucrose hydrolysis (to glucose and fructose) and resynthesis; and (iv) fructan synthesis and sucrose resynthesis from fructan metabolism. The contents of sucrose, fructan, glucose, and fructose were almost constant in both treatments. Labelling demonstrated that all carbohydrate pools were turned over. This indicated a system in metabolic steady state with equal rates of synthesis and degradation/consumption of the individual pools. Fructan content was enhanced by nitrogen deficiency (55 and 26% of dry mass at low and high N, respectively). Sucrose content was lower in nitrogen-deficient leaves (2.7 versus 6.7%). Glucose and fructose contents were always low (<1.5%). Interconversions between sucrose, glucose, and fructose were rapid (with half-lives of individual pools ranging between 0.3 and 0.8 h). Futile cycling of sucrose through sucrose hydrolysis (67 and 56% of sucrose at low and high N, respectively) and fructan metabolism (19 and 20%, respectively) was substantial but seemed to have no detrimental effect on the relative growth rate and carbon-use efficiency of these plants. The main effect of nitrogen deficiency on carbohydrate metabolism was to increase the half-life of the fructan pool from 27 to 62 h and to effectively double its size.     

Effects of apoplastic sucrose on carbohydrate pools and sucrose efflux of mesophyll protoplasts of pea (Pisum sativum)

The effects of the apoplastic, i.e. external, concentration of sucrose (0–30 m M ) on O₂ evolution, O₂ consumption, starch, sucrose, glucose and fructose content, and uptake and efflux of sucrose in mesophyll protoplasts of Pisum sativum L. cv. Fenomen were studied. Neither photosynthesis, dark respiration, sucrose nor starch content change with increased apoplastic sucrose concentration. The contents of glucose and fructose in the protoplasts increase with increased apoplastic sucrose concentration. The sucrose efflux increases with increased external sucrose concentrations between 1 and 5 m M , but above this range the efflux decreases with increased external sucrose concentrations. These findings indicate that although external sucrose does not enter the protoplasts, there is a relationship between the external sucrose pool and the internal pools of sugars in the mesophyll protoplasts. The results suggest an active sucrose efflux from the protoplasts at physiological concentrations of apoplastic sucrose (max 5 m M ) and a simple diffusion mechanism at higher concentrations.     

Circadian Rhythms in Stomatal Responsiveness to Red and Blue Light

Stomata of many plants have circadian rhythms in responsiveness to environmental cues as well as circadian rhythms in aperture. Stomatal responses to red light and blue light are mediated by photosynthetic photoreceptors; responses to blue light are additionally controlled by a specific blue-light photoreceptor. This paper describes circadian rhythmic aspects of stomatal responsiveness to red and blue light in Vicia faba. Plants were exposed to a repeated light:dark regime of 1.5:2.5 h for a total of 48 h, and because the plants could not entrain to this short light:dark cycle, circadian rhythms were able to "free run" as if in continuous light. The rhythm in the stomatal conductance established during the 1.5-h light periods was caused both by a rhythm in sensitivity to light and by a rhythm in the stomatal conductance established during the preceding 2.5-h dark periods. Both rhythms peaked during the middle of the subjective day. Although the stomatal response to blue light is greater than the response to red light at all times of day, there was no discernible difference in period, phase, or amplitude of the rhythm in sensitivity to the two light qualities. We observed no circadian rhythmicity in net carbon assimilation with the 1.5:2.5 h light regime for either red or blue light. In continuous white light, small rhythmic changes in photosynthetic assimilation were observed, but at relatively high light levels, and these appeared to be attributable largely to changes in internal CO2 availability governed by stomatal conductance.     

Changes in Stolon Carbohydrates During the Winter in Four Varieties of White Clover (Trifolium repensL.) with Contrasting Hardiness

Miniswards of white clover (Trifolium repensL.) varieties Menna,Huia, AberCrest and AberHerald, with contrasting winter hardiness,were established from cloned material in flat boxes. The miniswardswere placed in an unheated glasshouse with natural lightingfor the winter of 1995/96. During this time there were severalperiods of cold weather. The plants experienced temperaturesat least as severe as those experienced by unprotected plantsduring preceding mild winters. Growth measurements carried outmonthly suggested the cloned genotypes were representative ofthe varieties from which they were derived. Growth continuedthroughout the experiment, particularly when expressed as appearanceof new internodes. During the coldest weather these internodeswere short, and the amount of tissue produced was small. Starchconcentration showed a complex pattern of changes during thewinter. Older internodes were the major site of starch storagein the autumn; starch was lost from these internodes duringthe winter and early spring. Starch synthesis also occurred,even during the coldest periods. Starch accumulated in all newstolon tissue. There were significant differences between varieties.AberCrest and AberHerald retained higher concentrations of starchin older stolon tissue, synthesized higher concentrations ofstarch in young tissue and retained these for longer. Therewere few differences in soluble sugar concentrations betweenvarieties, but there were significant changes during the winter.In mid-winter, sucrose and pinitol were at their highest concentrations,whereas reducing sugars were at their lowest concentrations.This study further supports the important role of starch reservesin supporting winter survival.Copyright 1998 Annals of BotanyCompany Trifolium repens; white clover; winter hardiness; starch; sugar; pinitol.     

Phloem unloading following reactivation in predarkened mature maize leaves

Mature leaf blades of 48-h predarkened maize plants (Zea mays L. cv. Prior) were excised, and treated apically as the source (light, normal air) and basally as the sink (light or dark, air without CO₂). After providing the source portion with ¹⁴CO₂, the sink portions were harvested after 2, 7 or 14 h by freezing with liquid nitrogen, grinding, and freeze-drying. Extracts, fractionated by ionexchange resins into neutral, basic and acid fractions, were chromatographed on thin cellulose layers, and autoradiographed. Identification of labeled compounds was carried out by co-chromatography with authentic labeled substances. Activities of enzymes pertaining to the metabolism of sucrose were checked. Results show that the source supplies sucrose to the sink, where it is unloaded and metabolized by acid invertase (EC 3.2.1.26) in both the light and the dark. Starch appearing in the sink only in the light, after 7 h of re-illumination, yields labeled glucose upon hydrolysis. Although sucrose-phosphate synthetase (EC 2.4.1.14) is active in sinks and in isolated vascular-bundle fragments, it remains questionable whether sucrose unloaded from sieve tubes is metabolized by a method other than inversion. Sucrose synthetase (EC 2.4.1.13) was found to be inactive. Obviously, the main metabolite of unloaded sucrose is glucose-6-phosphate, giving access to the glycolytic pathway. The main difference between the sinks in the light and the dark is the lack of labeled glycine and serine in the dark. This indicates that in the light decarboxylation of glycine yields CO₂, which is recycled photosynthetically.Abbrevations Glc1P glucose-1-phosphate - Glc6P glucose-6-phosphate - TLC thin-layer chromatography - UDPGlc uridine 5-diphosphate glucose