The Dynamics of Carbon Supply from Leaves of Barley Plants Grown in Long or Short Days

The role of the mature leaf in supplying carbon for growth inother parts of the plant was examined using a steady-rate ¹⁴CO₂labelling technique. The pattern of events occurring in theleaf during one complete 24 h cycle was compared in plants grownin, and adapted to long and short photoperiods. The rates ofleaf photosynthesis, night respiration and daytime loss of carbonfrom the growing regions of the plant Were similar in long orshort photoperiods. As a percentage of the total carbon fixedduring the photoperiod, total respiration was c. 50% for shortday plants but only 25% for long day plants. Thirty to forty per cent of the carbon fixed during the photoperiodwas retained in the leaf for export during darkness—therest was exported immediately. In leaves of short day plantssucrose and starch were the main form of the stored carbon.By the end of the dark period these compounds had been almostcompletely depleted. In leaves of long day plants there weremuch larger basal levels of sucrose and starch, upon which thediurnal variations were superimposed. These leaves also accumulatedfructosans. The delay in starch remobilization previously foundin leaves of short day plants was also evident in leaves oflong day plants even though large concentrations of sucroseand fructosans were present This suggests the presence of distinctpools of sucrose in the leaf.     

Acclimation of Photosynthate Partitioning and Photosynthetic Rates to Changes in Length of the Daily Photosynthetic Period

Pangola, soya bean and spinach plants were grown in long andshort day photosynthetic periods. Reciprocal shifts betweenlong and short day grown plants were made to study acclimationin the rate of leaf starch synthesis with change in daylength.The rate of leaf starch accumulation is a function of the lengthof the daily photosynthetic period. Acclimation, that is a changein partitioning with a change in length of the photosyntheticperiod, occurs in a variety of species. Acclimation in the rateof starch accumulation occurs rapidly in pangola and is apparentlycomplete the day after a change in length of the daily photosyntheticperiod. Soya bean and spinach leaves require a few days in thenew environment for an acclimation to occur. Digitaria decumbens Stent., Glycine max (L.) Merr., Spinacia oleracea L., pangola, soya bean, spinach, specific leaf weight, starch, photosynthesis     

The Regulation of Carbon Transport and the Carbon Balance of Mature Tomato Leaves

Rates of carbon transport from a single mature tomato leaf inthe light period (day transport) and the dark period (nighttransport) were estimated from the rate of carbon fixation inthe light period, the rate of respiration in the dark periodand the changes in carbon contents over these two periods. Plantswere grown initially at 40 W m^–2 light intensity witheither 350 vpm (nonenriched plants) or 1000 vpm CO₂ (enrichedplants). Various light flux densities or CO₂ concentrationswere then applied to the experimental leaves in the light periodduring the experiment When leaves were temporarily exposed to contrasting light fluxdensities both day transport and night transport were linearlyrelated to the rate of carbon fixation. If leaves were shadedbelow the light compensation point for up to five days, or transferredto contrasting CO₂ concentrations for up to ten days, the linearrelationship between carbon fixation and carbon transport nolonger held. During acclimatization, therate of wbon fixationwas simply related to thecurrent light flux density and CO₂concentration, but the rate of carbon transport changed withtime. Day and night transports responded differently to changesin environment: night transport was more related to the contentof reserve, particularly starch, than to the rate of concurrentwbon fixation. It is concluded that the rate of carbon transport of a maturetomato leaf in a single photoperiod is regulated not merelyby the rate of concurrent carbon fixation but by the contentof reserve in the leaf. The latter results from previous cumulativewbon fixation and carbon transport. As a result of changingthe rate of carbon transport, a balance of carbon input andoutput was achieved within 10 days of acclimatization in a maturetomato leaf.     

Photosynthate Partitioning into Starch in Soybean Leaves: II. IRRADIANCE LEVEL AND DAILY PHOTOSYNTHETIC PERIOD DURATION EFFECTS

Two photosynthetic periods and photosynthetic photon flux densities (PPFD) were used to study the relationship between the rate of photosynthesis and starch accumulation in vegetative soybean leaves (Merr. cv Amsoy 71). Plants grown in short daily photosynthetic periods (7 hours) had higher rates of CO₂ fixation per unit leaf dry weight and of leaf starch accumulation than plants grown in long daily photosynthetic periods (14 hours) irrespective of PPFD. CO₂ fixation rates per unit leaf area were similar in 7-hour and 14-hour plants grown at low PPFD but were highest in 14-hour plants at the high PPFD. When single leaves of 14-hour plants were given 7-hour photosynthetic periods, their rates of starch accumulation remained unchanged. The programming of starch accumulation rate and possibly of photosynthetic rate by the length of the daily photosynthetic period is apparently a whole-plant, not an individual leaf, phenomenon. Programming of chloroplast starch accumulation rate by length of the daily photosynthetic and/or dark periods was independent of PPFD within the ranges used in this experiment.     

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)     

Synthesis and Dissolution of Starch labelled with 14Carbon in Tobacco Leaf Tissue

Tobacco leaves depleted of starch, were detached and allowedto assimilate equal amounts of ¹⁴CO₂ and ¹²CO₂in succession,and vice versa. Distribution of radioactivity in starch, andsugars was determined after assimilation and after disks cutfrom the leaves had been kept in darkness for times up to 40hours. The amount and activity of the CO₂ was also determined.¹⁴C and ¹²C were incorporated in equal amounts into starch independentlyof the order in which they were supplied. In contrast sucrosehad high activity ¹⁴C was given last, and hexose a low one.The reverse was true when ¹²C was given last. Activity of respiratoryCO₂ was slightly higher when ¹⁴C was assimilated last as comparedwith ¹²C. In the dark only ¹⁴C or ¹²C was at first lost fromstarch, in accordance or removal of discrete layers. Analyticalresults show that starch is the main respiratory substrate andto account for the redistribution of radioactivity in passageto CO₂ it is concluded that sucrose occurs at two sites separatedinter-or intra-cellularly, one of which is in equilibrium withthe system intrconverting starch and CO₂ and at the other hexosesare produced by inversion. A starch-like polysaccharide is formedduring assimilation which persists in the dark and there isa significant contribution to respiration of carbon from non-carbohydratesources when leaf disks are kept on the dark.     

Effect of Source-Sink Manipulations on the Crassulacean Acid Metabolism of Kalanchoe pinnata

The effect of manipulations of the sink-source at the above-groundlevel and girdling of source leaves was measured in 4-month-oldplants of the CAM species Kalancho pinnata (Lam.) Pers. At thisage plants developed five pairs of leaves. The upper fourthand fifth leaf pairs were not fully expanded and behaved ascarbohydrate sinks. Removal of the developing leaves induceda progressive accumulation of glucans and sugars in the matureleaves. The titratable acidity increased during the second weekbut accumulation was less than in the control plants three tofour weeks after sink removal. Similar, but more rapid, resultswere observed in mature leaves with girdled petioles. Up tothe second night after girdling dark CO₂ fixation increased,but decreased steadily afterwards. CAM Phase 4 (afternoon CO₂fixation) however, was more sensitive to girdling, being reducedby 38% on the first day, and disappearing completely 3 d aftergirdling. The glucan and sugar contents of girdled leaves increasedcontinuously after treatment, but day-night changes ceased completelyon the fifth day. Girdling also caused a considerable increasein chloroplast area, with up to 80% of their internal spaceoccupied by starch grains, leading to grana distortion. In girdledleaves, or in source leaves in plants lacking aerial carbohydratesinks, dawn-dusk changes in titratable acidity started to decreasewhen the leaf glucan content exceeded 1·0 mol equivalenthexoses kg^–1 dry weight. Increased sink strength throughshading of all leaves except one source leaf did not affectits CAM activity. The titratable acidity and non-structuralcarbohydrate content of the shaded mature leaves was reducedby around 55%. Removal of all the mature source leaves acceleratedthe maturation process of sink leaves, increasing titratableacidity at dawn and synthesis of glucans during the light period.The results support the hypothesis that CO₂ fixation in a CAMplant is controlled by accumulation of glucans in chloroplasts. Key words: CAM, glucan accumulation, sink-source ratio, CO₂ fixation     

Source--sink Relationships and Carbon Metabolism in Tomato Leaves 1. 14C Assimilate Compartmentation

Effects of the interaction between assimilate availability andsink demand on the metabolism of ¹⁴C assimilates in tomato leaveshave been examined in plants where the source—sink relationshipof assimilates was simplified to one leaf and one fruit truss. During experimentation the source leaf was exposed to either80 or 20 W m^–2 (PAR), while the truss was either retainedor removed. Under these four source-sink conditions, a timecourse study was made on ¹⁴C assimilate distribution in thesource leaf over a period of 23 h after pulse feeding with ¹⁴CO₂. While truss removal caused a temporary increase of ¹⁴C sucrosein leaves under both irradiances, the principal assimilatesaccumulated were starch and hexoses. Decreased ¹⁴C export followingtruss removal was observed within a day in well-illuminatedleaves but after 3 days in leaves under low light. The accumulationof ¹⁴C sucrose at the end of the light period was affected bytruss removal in high light leaves only 3 days later. These observations suggest that while the compartmentation ofnewly fixed assimilate was affected rapidly by the change ofsource—sink relationship, carbon export, as measured by¹⁴C loss, was affected only gradually. The possible effect of sucrose accumulation on photosynthesisis discussed.     

Nitrogen Metabolism of Detached Kalanchoe Leaves in the Dark in Relation to Acidification, Deacidification, and Oxygen Uptake

Detached Kalancho leaves were placed in the dark and changesin the amounts of total soluble N, asparagine, glutamine, aminoN, ammonia, and titratable acid were followed during the periodsof acidification and deacidification. Abundant starch was presentthroughout the experiments. The amount of acid which accumulatedin the initial period was increased by placing the leaves in5 per cent. CO₂ and also by decreasing the temperature from25 to 10°. In neither case were the nitrogen fractions affectedin a similar way to the acid levels. No consistent relationshipcould be found between the amount or change in amount of acidand the amounts of the nitrogen fractions. Over prolonged periodsthe drifts in the amounts of the nitrogen fractions were verysimilar to those known in non-succulent leaves. It is concludedthat, despite the large amounts of free acid and starch present,the nitrogen metabolism of detached Kalancho leaves in the darkis similar to that of non-succulent leaves. A possible explanationis suggested for the incorporation of ¹⁴C from ¹⁴CO₂ into nitrogenoussubstances in Kalancho leaves during dark acidification. Changesin the amounts of glutamine, which seemed to be related to oxygenuptake rather than to acid fluctuations, were measured in comparableleaf samples showing different rates of oxygen uptake. The rateof oxygen uptake of leaves and leaf disks in the dark was controlledby placing them in an atmosphere of nitrogen, or 5, 20, or 100per cent, oxygen, by the addition of 10^–3 M. cyanide,or by varying the temperature. At the end of a given periodthe amount of glutamine was always greater in the samples whichhad shown the higher rate of oxygen uptake during this perod.This correlation was found irrespective of whether the amountof glutamine had increased or decreased. With the assumptionsthat glutamine production and consumption were proceeding simultaneouslyand that the rate of consumption was unaffected by the rateof oxygen uptake, the experimental values are consistent withthe hypothesis that the rate of glutamine production and therate of oxygen uptake were directly related. The possible significanceof these findings is discussed.     

Carbon Partitioning in Mature Leaves of Pepper: Effects of Daylength

Grange, R. 1. 1985. Carbon partitioning in mature leaves ofpepper: effects of daylength.—J. exp. Bot. 36: 1749–1759. The partitioning of recently fixed carbon has been examinedin mature pepper leaves grown in 6, 10 or 14 h photoperiodsat different irradiances chosen to give similar radiation integralsand in a 6 h photoperiod at the lowest of these irradiances.The partitioning of carbon into export, starch, sugars and respirationwas followed over the photopenod and the subsequent night ina mature leaf. The maximum export rate during the day (approximately 18 µgC cm^–2 leaf h^–1) was not significantly differentamong the treatments. Net photosynthesis rate was directly relatedto irradiance; the proportion of net photosynthesis exportedduring the day was 33% in 6-h days and 57% in 14-h days. Leafstarch accumulation (as a proportion of net photosynthesis rate)increased slightly when plants were grown in 6-h days. The remobilization of starch and sugars at night allowed exportrates to remain similar over 24 h when plants were grown in10-h or 14-h photoperiods. Leaves grown in 6-h days showed nosignificant changes in export rate during the first few hoursof night but exhausted their starch reserves during the nightand export rates declined. Sucrose and hexose levels decreased at the onset of darkness,but did not fall below 40 µg cm^–2 in plants grownin 10-h or 14-h photoperiods; when this level was reached after3–4 h of darkness, starch breakdown began. In leaves grownin both 6-h treatments, sucrose levels fell below 40 µgcm^–2 when starch reserves were depleted during the nightand the export rate decreased concurrently. The results are discussed in relation to the control of exportand starch metabolism in the leaf. Key words: Pepper, partitioning, daylength     

The Strategy of Carbon Utilization in Uniculm Barley: II. THE EFFECT OF CONTINUOUS LIGHT AND CONTINUOUS DARK TREATMENTS

After a photoperiod of 8.25 h during which the youngest fullyexpanded leaf of uniculm barley plants was allowed to assimilate¹⁴CO₂ for 30 min, groups of plants were transfered either tocontinuous light or to continuous dark. Plants were harvestedover a 72 h period to examine the effect of the treatments (comparedwith control plants growing in normal light/dark cycles) onthe transport of ¹⁴C from the exposed leaf, the distributionof ¹⁴C assimilates to the rest of the plant, and the chemicalfate of assimilated ¹⁴C. In continuous light a substantial quantity (22% at 72 h) ofthe ¹⁴C assimilated by the leaf remained in that leaf in theform of starch and neutral sugars compared with only 4% in thecontrol fed leaf. Also the total amount of ¹⁴C respired fromplants maintained in continuous light was significantly less(c. 18% of the total originally fixed by 24 h) than that respiredfrom control plants (c. 36%). The result was that approximatelyequal amounts of ¹⁴C were accumulated in the growing leavesand roots of plants given continuous light or normal light/darkcycles. In continuous dark the fate of ¹⁴C was similar to that of controlplants. This is probably because the two treatments shared acommon light/dark environment for the first 22 h, during whichtime almost complete distribution and utilization of ¹⁴C occurred.     

Light-enhanced carbon dioxide fixation in isolated chloroplasts

Preillumination of leaves of spinach, soybean and maize in theabsence of CO₂ greatly enhanced the capacity for fixing CO₂in an immediately following dark period. Lightenhanced darkCO₂-fixation was further observed in isolated chloroplasts ofspinach and soybean. When isolated chloroplasts were illuminated,CO₂-fixing capacity in the subsequent dark period increasedrapidly at first and later more slowly attaining a stationaryvalue in about 20 min. When the light was turned off at thisstage, the capacity decreased very rapidly becoming zero inabout 10 min. The magnitude of the enhanced dark fixation andits decay in the dark were not influenced by the presence orabsence of atmospheric oxygen. In both leaves and isolated chloroplasts,no significant change in oxygen (21%) occurred in distributionpatterns of radioactivity in products fixed by photosynthetic,or light-enhanced, dark, ¹⁴CO₂-fixation. In preilluminated leaves¹⁴C was incorporated into sucrose in the subsequent dark period,indicating that the photosynthetic carbon reduction cycle isoperating in light-enhanced dark fixation in higher plants. ¹Present address: Noda Institute for Scientific Research, Noda,Chiba Prefecture (Received August 10, 1970; )     

CO2 Assimilation and Partitioning of Carbon in Maize Plants Deprived of Orthophosphate

In order to study the effects of inorganic phosphate (P1) starvationon C₄plants, 3-week-old maize plants (Zea maysL cv. Brulouis)were grown in a growth chamber on a nutrient solution withoutP1 over 22 d During the first 2 weeks, Pi-starved plants grewas well as control plants The Pi concentration in the planttissue decreased rapidly with time, which suggests that normalbiomass production can be maintained at the expense of internalP1 In addition, photosynthetic CO2 assimilation measured 4-6h after dawn was not affected, but the concentration of glucose,sucrose, and starch in leaves was much higher than in the controls¹⁴CO₂ pulse-chase experiments earned out on the ninth day oftreatment showed that ¹⁴CO₂ assimilation was perturbed duringthis initial period, resulting in a larger flow of carbon toboth starch and sucrose At the beginning of the third week ofP1 starvation (15 d after treatment) ¹⁴C incorporation intosucrose stayed high relative to controls but this was not thecase for starch At the end of the third week of P1-deficiency,shoot growth was considerably reduced and fresh weight was onlyone-third of that of the control plants. The P1 concentrationof both the leaf and root tissues was less than 1.0 µmolg^–1 FW compared to 20-25µmol g¹ FW in the controls.Photosynthetic CO₂ assimilation was reduced and the leaf concentrationof sucrose and starch, which had begun to decrease after theend of the second week of P1 limitation, became lower than inthe controls. These results obtained on maize plants show thatphotosynthesis and carbon partitioning between sucrose and starchwere strongly affected by P1 deficiency, similar to C₃ species. Key words: CO₂ assimilation, corn, orthophosphate deficiency, starch, sucrose     

Effects of Elevated Carbon Dioxide Concentration in the Dark on the Growth of Soybean Seedlings

Previous work has shown that elevated carbon dioxide (CO₂) concentrationsin the dark reversibly reduce the rate of CO₂ efflux from soybeans.Experiments were performed exposing soybean plants continuallyto concentrations of 350 or 700 cm³ m^-3 for 24 h d^-1, or to350 during the day and 700 cm³ m^-3 at night, in order to determinethe importance of the reduced rate of dark CO₂ efflux for plantgrowth. High CO₂ applied only at night conserved carbon andincreased dry mass during initial growth compared with the constant350 cm³ m^-3 treatment. Long-term net assimilation rate was increasedby high CO₂ in the dark, without any increase in daytime leafphotosynthesis. However, leaf area ratio was reduced by thedark CO₂ treatment to values equal to those of plants continuallyexposed to the higher concentration. From days 14-21, leaf areawas less for the elevated night-time CO₂ treatment than foreither the constant 350 or 700 cm³ m^-3 treatments. For the days7-21-period, relative growth rate was significantly reducedby the high night CO₂ treatment compared with the 350 cm³ m^-3continuous treatment. The results indicate that some functionallysignificant component of respiration was reduced by the elevatedCO₂ concentration in the dark.Copyright 1995, 1999 AcademicPress Glycine max L. (Merr.), carbon dioxide, plant growth, respiration     

The Respiration of Mature Field Bean (Vicia faba L.) Leaves During Prolonged Darkness

Dark respiration in attached and detached mature leaves of thefield bean (Vicia faba L.) was studied whilst leaves experiencedup to 60 h of darkness. The results showed: (1) the initialrespiration rate to vary according to the irradiance duringthe previous photoperiod; (2) the dark respiration rate (perunit area) of attached leaves to be essentially constant duringa normal 12 h night although there was a rapid loss in leafd. wt during this time; (3) after 12 h, the respiration rateof attached leaves decayed to an asymptotic value at about 36h; (4) the respiration rate of leaves detached at the end ofthe photoperiod and maintained in the dark on deionised water,decayed only after 36 h of darkness; (5) there was no differencebetween the respiration rate of attached and detached leavesduring the normal 12 h night. It is concluded that the dark respiration of attached fieldbean leaves is intially related to the synthesis and translocationof sucrose in addition to maintenance. After about 36 h, whenthe rate of CO₂ efflux is more or less steady, the CO₂ effluxreflects the intensity of maintenance processes only. The maintenancerespiration rate (determined after 60 h in the dark) rangedfrom 062 to 151 mg CO₂ (g d. wt)^–1 h^–1 but wasrelatively unaffected by several applied treatments. Vicia faba L., field bean, respiration, maintenance, nitrate, non-structural carbohydrate, export     

Mobilization of Starch and Essential Oil Biogenesis during Leaf Ontogeny of Lemongrass (Cymbopogon flexuosus Stapf.)

The levels of starch, soluble sugars, starch mobilizing enzymes(amylases and phosphorylase) and sodium [2-^I4C] acetate incorporationinto essential oil have been examined during leaf ontogeny oflemongrass (Cymbopogon flexuosus Stapf., cv. OD-19). The degradationof starch was predominantly amylolytic and ß-amylasewas the major enzyme involved. Its activity was quite high duringthe period of active leaf growth accompanying active accumulationof essential oil. The activities of a-amylase and phosphorylasewere relatively lower. The change in starch to soluble sugarsratio was inversely related to ß-amylase activity.The time-course (12 h light followed by 12 h dark) monitoringof the [^I4C]-radioactivity in starch and essential oil, afterexposure of the immature (15 days after emergence) leaf to ¹⁴CO₂,revealed a progressive loss of label from starch and a parallelincrease in radioactivity in essential oil. The results havebeen discussed in relation to degradation of transitory starchserving as the source of carbon precursor for essential oil(monoterpene) biogenesis in the tissue. The amount of exogenouslysupplied acetate incorporated into essential oil increased tremendouslywith 5-10 fold decrease in specific activity of the labelledacetate (2,110 GBq mole^–1). The effect was largely manifestedin ‘citral’, the chief (ca. 80%) constituent oflemongrass essential oil. Ontogenetically, the amount of essentialoil synthesized from the exogenously supplied precursor (acetate)was much higher in young (10 days after emergence) than in mature(30 days after emergence) leaf. Thus, the leaf developmentalphase influences the expression of essential oil metabolismand actual synthesis. Only young lemongrass leaves are substantiallyactive to synthesize essential oil. The oil biosynthetic phaseappears to be coordinated/integrated with the development ofelevated levels of certain primary metabolic activities likestarch mobilization. ¹CIMAP Publication No. 706 ²Present address: CSIR Complex, Palampur-176 061, Kangra Distt.Himachal Pradesh, India ^J Present address (until October 10, 1991): Department of Biology,Queen's University, Kingston, Ontario, K7L 3N6, Canada (Received November 30, 1990; Accepted May 31, 1991)     

Prolonged Survival of CAM-Mode Mesembryanthemum crystallinum in Darkness and its Possible Dependence on Malate

A remarkable difference was found in the survival of leavesof Mesembryanthemum crystallinum with plants grown in the C₃versus the CAM mode. With excised leaves (petiole in solution)of C₃-mode plants subjected to 6 days of darkness, there wasa large reduction in the chlorophyll content of the leaf andleaf turgor had decreased. By day 9, the chlorophyll had disappeared,except at the major veins, and the leaf tip had dried and turnedbrown. In contrast, the leaf tissue in the CAM mode showed onlya partial loss of chlorophyll during the same period, and evenafter 17 days of darkness, the tissue at the base was stillalive. Similarly, intact plants grown in the C₃ mode deterioratedmuch faster during 20 days of darkness than did plants grownin the CAM mode. Chlorophyll content, chlorophyll a/b ratio,phosphoenolpyruvate carboxylase, NADP-malic enzyme, malate andstarch content were measured. In both C₃- and CAM-mode plants,the starch content decreased rapidly during the dark periodand was nearly depleted after two days. In the CAM-mode tissue,there was a relatively high level of malate during prolongeddarkness (up to 17 days), with a transitory rise early in thedark period. In contrast, the malate content was low and rapidlydepleted in the C₃-mode leaves kept in darkness. These findingssuggest that malate may be an important source of carbon forsustaining leaves of CAM-mode M. crystallinum during prolongeddarkness. (Received May 20, 1987; Accepted October 23, 1987)     

Endogenous Regulation of Photosynthetic Rate in Primary Leaves of Bean (Phaseolus vulgaris L.)

The rate of photosynthesis and/or dry matter production wasstudied in fully-expanded primary leaves of bean (Phaseolusvulgaris cv. Bulgarian) plants which had been subjected to varioussurgical and hormonal treatments. Between 30 and 40 per centof the assimilates produced by the primary leaves, over a 4-dayperiod starting with expansion of the first trifoliate leaf,were diverted to the growing shoot above the insertion of theprimary leaves. In detopped plants (i.e. lacking all leaves,stem and buds above insertion of primary leaves), both the rateof net photosynthesis (NP) of the primary leaves 4 days afterdetopping, and the mean net assimilation rate (NAR) over thisinterval, did not differ significantly from those of intactplants. The assimilate normally diverted to the top in intactplants was distributed between the remaining organs of the detoppedplant. When translocation of assimilates from the primary leaveswas stopped by girdling their petioles, both NAR and NP wereas in untreated control plants after a 2-day period. The assimilatesproduced during that period accumulated in the mesophyll chlorenchymain the form of starch granules. Intact plants supplied withGA₃, or IAA, through the primary leaves as well as detoppedplants supplied with IAA through the stump, differed from untreatedcontrol plants in the pattern of distribution of the assimilatesproduced: IAA favoured dry-matter accumulation in the roots,while GA₃ favoured the tops. Nevertheless, neither NP, nor NARdiffered significantly from the corresponding controls.     

Translocation Profiles of [11C] and [13N]-Labelled Metabolites after Assimilation of 11CO2 and [13N]-Labelled Ammonia Gas by Leaves of Helianthus annuus L. and Lupinus albus L.

Tracer amounts of atmospheric [¹³N]-Iabelled ammonia gas, wereabsorbed by leaves of Lupinus albus and Helianthus annuus inboth the light and the dark. Exogenous [¹³N]-ammonia was onlyabsorbed in the dark when the feeding occurred shortly aftera period of illumination and the tissue was not depleted ofits carbohydrate reserves (e.g. starch). Incorporation of the[¹³N]-ammonia appeared to occur via the leaf glutamine synthetase/glutamatesynthase (GS/GOGAT) cycle since 2.0 mol m^–3 MSX, an inhibitorof the GS reduced uptake in both the light and dark. Photosyntheticincorporation of ¹¹CO₂ was not affected by this treatment Therate of movement of [¹³N]-assimilates in the petiole of attachedleaves of Helianthus and Lupinus was similar to that of the¹¹Cl-photo assimilates. Export of both [¹³N] and [¹¹C]-Iabelledassimilates from the leaf and movement in the petiole in boththe light and the dark was inhibited by source leaf anoxia (i.e.nitrogen gas). Translocation was re-established at the samerate when the feed leaf was exposed to gas containing more than2% O₂ which permitted dark respiration to proceed. After aninitial feeding of either ¹¹CO₂ or [¹³N]-ammonia at ambient(21%) O₂ exposure of the source leaf to 2% O2, or 50% O² didnot alter the rates of translocation, indicating that changesin photosynthetic activity in the source leaf due to photorespiratoryactivity need not markedly alter, at least during the shortperiod, the loading and translocation of either [¹¹C ] or [¹³N]-labelledleaf products. Key words: Translocation, CO₂, NH₃, Leaves, Helianthus annuus, Lupinus albus     

Diurnal regulation of NO-3 uptake in soybean plants IV. Dependence on current photosynthesis and sugar availability to the roots

The short-term dependence of NO^–₃ uptake upon photosynthesisand sugar supply to the roots of soybean plants was investigatedin a series of experiments where CO₂ availability, light intensityor conduction of phloem sap to the roots were severely limited.Removal of CO₂ from the atmosphere or girdling of the stem equallyprevented the stimulation of NO^–₃ uptake when plants weretransferred from darkness to the light. The effect of thesetwo treatments can be reversed by CO₂ re-supply or by additionof 10 mM glucose in the nutrient solution, respectively. Glucosewas also more effective in stimulating NO^–₃ uptake byintact plants in darkness than in light. Collectively, theseobservations are interpreted as evidence that the diurnal changesin NO^–₃ uptake are due to decreased phloem transport ofphotosynthates in darkness. Accordingly, the magnitude of thesechanges was much dependent on starch accumulation in the leavesat the end of the photo-period. Shading the plants lowered thisaccumulation, and resulted in an amplification of the diurnalchanges in NO^–₃ uptake. These results are discussed inconnection with the hypothesis that the carbon-dependent plasticityof the night/day ratio of NO^–₃ uptake is an importantfeature of the co-ordination of the acquisition of N and C bythe plant. Key words: Glycine max, light/dark cycle, NO^–₃ uptake, C and N acquisition