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     

Carbon Partitioning in Mature Leaves of Pepper: Effects of Transfer to High or Low Irradiance

Grange, R. I. 1987. Carbon partitioning in mature leaves ofpepper: Effects of transfer to high or low irradiance.—J.exp. Bot. 38: 77–83. Pepper plants were grown at an irradiance of either 55 W m^–2or 90 W m^–2 PAR. Changes in net photosynthesis, carbonexport, starch and sugar contents in a single mature leaf weremeasured at intervals for 8 d following transfer of plants betweenthe two irradiances. On transfer from low to high irradiance,the net photosynthesis rate increased immediately but exportrate increased only slowly, to a maximum after 3 d. While assimilationexceeded export more starch and sucrose accumulated in the dayand remained in the leaf at the end of each night. Hexose contentsat the end of night remained low and constant, but the daytimemaximum rose during the first 2 d from transfer, thereafterreturning to pre-transfer contents. Following transfer of leaves from high to low irradiance starchpresent in the leaf provided sufficient reserves to maintainthe rate of export for one day. Subsequently, the sucrose contentfell and the export rate declined to near that in leaves grownin low irradiance. Sucrose and hexose accumulation following transfer from lowto high irradiance suggests a limitation to export ‘downstream’from sucrose synthesis, probably in the loading step from mesophyllto phloem. Key words: Pepper, export, starch, loading     

The Extent of Starch Turnover in Mature Pepper Leaves in the Light

The extent of starch turnover in pepper leaves in the lighthas been estimated. After leaves were labelled with ¹⁴CO₂ atconstant specific activity for 4–7 h, the irradiance wasreduced to a level which caused no net change in the starchcontent of the leaf, and the supply of ¹⁴CO₂ was removed. Therewas no significant change in specific activity of starch overthe following 6–10 h, thus there was no exchange of ¹⁴C-starchwith ¹²C-assimilates entering the chloroplasts. Starch, turnover, ¹⁴C-labelling, pepper, Capsicum annuum L.     

Regulation of Assimilate Translocation Between Leaves and Fruits in the Tomato

Simultaneous measurement of export from leaves and import tofruits were made on tomato plants reduced to one fully expandedleaf and one fruit. Experimental leaves were exposed to sixlight flux densities (0.5–100 W m^–2) for 24 h whilerapidly growing fruits were kept in the dark at 22 °C. The rates of export of assimilate from these leaves varied from70 to 120 mg C leaf^–1 day^–1 corresponding with ratesof carbon fixation from 3 to 290 mg C leaf^–1 day^–1.Export from leaves with the lowest carbon fixation rates weremaintained by a loss of up to one-sixth of their initial carbon.In contrast, leaves with the highest carbon fixation rates exportedonly half the newly fixed carbon. The rates of import of assimilate to similar-sized fruits (c.16 cm³) were between 80 and 110 mg C fr^–1 day^–1but differed from the export rates of the source leaves. Thespecific growth rates and the specific respiration rates ofthe fruits were related to their initial carbon content at thebeginning of the experiment. Thus, over 24 h, the rate of importwas predetermined by the developmental stage of the fruit unalteredby the rate of current carbon fixation in the source leaf. Translocationof assimilate was regulated by sink demand under both source-and sink-limiting conditions in this short-term situation. The dynamic relationship between assimilate production in leavesand its utilization in fruits is discussed together with therole of sucrose concentration in these organs in regulatingtransport. Lycopersicon esculentumL, tomato assimilate translocation, source-sink relationships     

Diurnal Production and Utilization of Photosynthate in Nodulated White Clover

Gordon, A. J., Mitchell, D. F., Ryle, G. J. A. and Powell, C.E. 1987. Diurnal production and utilization of photosynthatein nodulated white clover.—J. exp. Bot. 38: 84–98. A steady-state ¹⁴C-labelling technique was used to examine thediurnal carbon fixation, storage and export characteristicsof white clover leaves. Approximately 70% of fixed carbon wasexported to other organs during the photoperiod. The remainingcarbon was stored mainly as starch (80% at the end of the photoperiod)with smaller amounts of sucrose, hexoses and charged compounds.Carbon export from the leaf at night was provided by remobilizationof starch. During the photoperiod it was estimated that c.60% of carbonexported from the leaf was directed towards the nodulated root;45% to nodules and 15% to roots. The 40% directed towards theshoot was supplemented by a further 11% of carbon (in the formof amides) re-exported from the nodules. During the photoperiod, all organs of the plant accumulatedcarbohydrate which was available for use during darkness, inconjunction with a diminished supply of exported carbon fromleaves. Nodules exhibited a striking pattern of carbohydratestorage and depletion. The levels of sucrose and starch in thenodules at the end of the photoperiod were sufficient to maintainN₂ fixation for 8–9 h of the 12 h dark period. We proposethat continued import from leaves provided the additional sucrosenecessary to support undiminished nodule function throughoutthe entire dark period. Key words: White clover, photosynthate, starch, carbohydrate, nodules, N₂ fixation     

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     

Irradiance and daylength effects on growth and chemical composition of Gyrodinium aureolum Hulburt in culture

For Gyrodinium aureolum significant irradiance and daylengtheffects were found on the division rate and on the growth-relevantChla-normalized photosynthetic rate (^gP^B). Optimum conditionsof irradiance and daylength were found at 230 µmol m^–2s^–1 and 14 h for the division rate, and at >260 µmolm^–2 s^–1 and <6 h for ^gP^B.^gP^B showed no photoinhibition,while the division rate decreased markedly at irradiances abovesaturation. This difference and the difference in optimum irradiancebetween the division rate and ^gP^B are explained by a decreasein cellular Chla/carbon ratio with increasing irradiance. Thecellular content of carbon and nitrogen decreased significantlywith increasing irradiance. Total phosphorus was independentof irradiance and daylength. Below the saturation irradiancefor ^gP^B the daily Chla-normalized carbon yield may be describedas an exponential function of the daily irradiance (irradiancex daylength).     

The Effect of Cooling on Photosynthesis, Amounts of Carbohydrate and Assimilate Export in Sunflower

Sunflower plants (Helianthus annuus L.) grown at 30°C werecooled to 13°C in the light in atmospheric CO² or low CO²,or in darkness. Photosynthetic rate at 30°C after coolingwhole plants in atmospheric CO² for 12 h during a photoperiodwas significantly lower than at the start of the photoperiodcompared to plants cooled at low CO², those cooled in the darkand those maintained at 30°C. Amounts of sucrose, hexosesand starch in leaves at 13°C increased throughout a 14 hphotoperiod to levels higher than in leaves at 30°C, whereamounts of sucrose and hexoses were stable or falling after4 h. Carbohydrate accumulation at 13°C during this photoperiodwas more than twice that at 30°C. After three photoperiodsand two dark periods at 13°C carbohydrate levels in leaveswere still as high as at the end of the first photoperiod, butless carbohydrate accumulated during the photoperiods than duringthe first photoperiod, and more was partitioned as starch. Amountsof soluble carbohydrate in roots were greater after 14 h at13°C than in roots of plants at 30°C. Loss of ¹⁴C fromleaves at 30°C as a proportion of ¹⁴CO2 fixed by them at30°C, decreased after exposure of plants to 13°C inthe light for 30 min prior to ¹⁴CO²feeding. Results indicatean effect of cold on the transport process that was light-dependent.It is inferred that the reduction in the proportion of ¹⁴C lostfrom leaves after 10 h cooling was due to reduced sink demand,whereas the rise in the proportion of ¹⁴C lost from leaves after24 h reflects reduced photosynthetic rate. The coincidence ofreduced photosynthetic rate with raised carbohydrate levelsin leaves maintained at 30°C throughout, whilst the restof the plant was cooled to 13°C in the light implies feedbackinhibition of photosynthesis. This may reduce the imbalancebetween source and sink in sunflower during the first days oflong-term cooling. Key words: Temperature, carbon export, carbohydrates, photosynthesis, sunflower     

The Growth and Development of the Wheat Apex: The Effects of Photoperiod on Spikelet Production and Sucrose Concentration in the Apex

Spring wheat (Triticum aestivum cv. Warimba) plants were grownin a controlled environment (20°C) in two photoperiods (8or 16 h). In the first instance, plants were maintained in eachof the photoperiods from germination onwards at the same irradiance(375 µE m^–2 s^–1). In the second case, allplants were grown in a long photoperiod until 4 days after double-ridgeinitiation when half the plants were transferred to a shortphotoperiod with double the irradiance (16 h photoperiod at225 or 8 h at 475 µE ^–2 s^–1). The rates of growth and development of the apices were promotedby the longer photoperiod in both experiments. Shoot dry weightgain was proportional to the total light energy received perday whereas the dry weight of the shoot apex increased withincreasing photoperiod even when the total daily irradiancewas constant. The principal soluble carbohydrate present in the shoot apexwas sucrose, although low concentrations of glucose and fructosewere found in the apices of long photoperiod plants late indevelopment. Sucrose concentration was invariably greater inthe slow-growing apices of short photoperiod plants, but roseto approach this level in the long photoperiod plants when theterminal spikelet had been initiated. Triticum aestivum, wheat, apex, spikelet initiation, photoperiod, flower initiation     

Growth Analysis of Sweet Pepper (Capsicum annuum L.): 2. Interacting Effects of Irradiance, Temperature and Plant Age in Controlled Conditions

A growth analysis was carried out with sweet pepper plants grownin a phytotron. Irradiance conditions were: 0.84 or 3.25 MJm^–2 in 8 h, 1.67 MJ m^–2 in 16 h and 2.51 MJ m^–2in 24 h. Temperatures applied were 25 or 21 °C during thephotoperiod in combination with 25, 21 and 17 or 21, 17 and13 °C respectively during the nyctoperiod. Highest values for leaf area and total dry weight were foundwhen applying 1.67 MJ m^–2 in 16 h, followed by 3.25 MJm^–2 in 8 h, irrespective of the temperature regime. Continuousirradiance ultimately resulted in leaf drop. A reduction inthe day temperature decreased leaf area and total dry weight.At a day temperature of 25 °C the dry weight increased withdecreasing night temperature when applying 3.25 MJ m^–2in 8 h. At a day temperature of 21 °C leaf area and dryweight were reduced when 17 or 13 °C were applied duringa 16 h nyctoperiod. Values for relative growth rate, net assimilation rate, leafarea ratio and leaf weight ratio strongly decreased with advancingplant age. The effects of irradiance treatment on RGR and NARwere analogous to those on total dry weight while the reversepattern was observed for the LAR. A decrease in day temperaturedecreased the RGR. The effects of night temperature exhibitedstrong interactions with day temperature and photoperiod. Theinfluence of temperature on RGR was largely mediated throughchanges in the LAR. The latter parameter was highly correlatedwith the specific leaf weight. Capsicum annuum L., sweet pepper, growth analysis, irradiance, temperature, plant age     

Export, Mobilization, and Respiration of Assimilates in Uniculm Barley during Light and Darkness

The respiratory losses and the pattern of carbon supply froma leaf of unicuim barley were examined during a complete diurnalperiod using a steady state ¹⁴C-labelling technique. After a delay of c. 1 h a portion of the ¹⁴C exported from acontinuously assimilating leaf was lost in respiration in thelight. This respiratory loss amounted to c. 20% of the total¹⁴C fixed. A further 28% of the total ¹⁴C fixed was respiredduring the dark period. In the light, carbon was fixed at a rate of c. 8·9 mgC dm^{–2 h–1} and exported from the leaf at ^c. 5·3mg C dm^{–2 h–1}. Dark export averaged ^c. 31% of theday-time rate. Carbohydrate was stored in the leaf during the day and was almostcompletely remobilized during the dark. Sucrose, the major reservecarbohydrate, was exported first whilst the starch level remainedconstant. After some 9 h of darkness, sucrose declined to alow level and starch remobilization began.     

Characteristics of Starch Grains Isolated from Mature Pepper Leaves Grown under Different Irradiances and Daylengths

The size, shape and number of starch grains have been determinedin mature pepper leaves taken from plants grown under definedconditions of daylength and irradiance. Starch grains were 0.2–7.0 µm diameter and 02–1.5µm in thickness. Grain diameter was positively relatedto daylength and the number of grains per unit leaf area inverselyrelated to daylength. Mean grain diameter was also positivelyrelated to leaf area. Analysis of starch grains from leaves having a wide range ofstarch contents showed that grain diameter was linearly relatedto leaf starch content. However, mean diameter only doubledwith a 10-fold increase in starch content. The number of grainsincreased from approximately 5 ? 10¹⁰ m^–2 of leaf to over200 ? 10¹⁰ m^–2 with increasing starch content. The totalsurface area of grains increased from less than 1.0 m² m^–2leaf to over 20 m² m^–2 leaf. Leaf starch grain shape and size are compatible with both efficientstorage as disc-shaped chloroplasts and the maintenance of hightotal grain surface area by increasing grain number more thandiameter. Possible mechanisms for the control of grain initiation,growth and degradation are suggested. Key words: Starch grains, size, shape, pepper leaves     

Growth Responses of Two Solanum Species to Contrasting Temperatures and Irradiance Levels: Relations to Photosynthesis, Dark Respiration and Chlorophyll Fluorescence

Potato production in the tropical lowlands during the rainyseason is constrained by high temperature and low irradiance.This study examined the effect of these two variables on drymatter production and allocation, using plant growth, leaf anatomy,gas exchange and chlorophyll fluorescence measurements. Plantsof two clones, Solanum goniocalyx cv. Garhuash Huayro (GH) andDTO-33, a heat tolerant clone of S. tuberosum x S. phureja,were grown in growth chambers at 33/25 °C or 20/10 °Cday/night temperature. At each temperature, plants were grownin either 12 h high irradiance (430–450 µmol m^–2s^–1 PAR) or 12 h low irradiance (250–280 µmolm^–2 s^–1) both with a 6–h photoperiod extensionof 6 µmol m^–2 s^–1. Plants were harvested after10 d (initial harvest) and after 20 d (final harvest). By theend of the study DTO-33 had produced more dry matter and hadtuberized, whereas GH had a greater leaf area ratio (LAR) andspecific leaf area (SLA). The highest relative growth rate (RGR)was at low temperature and low irradiance, possibly due to acombination of thin leaves with a large surface area. At thehigh temperature, low irradiance had the opposite effect, producingthe lowest net assimilation rate (NAR) and lowest RGR. Bothtuber number and weight were markedly reduced by high temperature.Low irradiance, in combination with high temperature, producedvirtually no tubers. Stomatal density, which was greater onGH than in DTO-33, was increased at high temperature. When measuredat 30 °C both clones, especially DTO-33, showed heat-adaptationin terms of ability to maintain a high rate of net photosynthesisat 30 °C. Plants grown at high irr-adiance and low temperaturehad the lowest net photosynthetic rate at 30 °C. Concurrentmeasurements of chlorophyll fluorescence indicated that onlythe initial (O) fluorescence parameter was affected. The dataconfirm the field observation that reduction in potato growthat high temperature can be aggravated by lower irradiance. Thisreduction is associated with a reduced leaf area and NAR. Growth analysis, heat adaptation, light     

Leaf Development in Lolium temulentum: Photosynthesis and Photosynthetic Proteins in Leaves Senescing under Different Irradiances

Changes in carbon fixation rate and the levels of photosyntheticproteins were measured in fourth leaves of Lolium temulentumgrown until full expansion at 360 µmol quanta m^–2s^–1 and subsequently at the same irradiance or shadedto 90 µmol m^–2 s^–1. Ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco), light-harvesting chlorophylla/b protein of photosystem II (LHCII), 65 kDa protein of photosystemI (PSI), cytochrome f (Cytf) and coupling factor 1 (CF₁) declinedsteadily in amount throughout senescence in unshaded leaves.In shaded leaves, however, the decrease in LHCII and the 65kDa protein was delayed until later in senescence whereas theamount of Cyt f protein decreased rapidly following transferto shade and was lower than that of unshaded leaves at the earlyand middle stages of senescence. Decreases in the Rubisco andCF₁ of shaded leaves occurred at slightly reduced rates comparedwith unshaded leaves. These results indicate that chloroplastproteins in fully-expanded leaves are controlled individually,in a direction appropriate to acclimate photosynthesis to agiven irradiance during senescence. (Received August 20, 1992; Accepted January 5, 1993)     

Influence of Carbon Dioxide Concentration on Growth, Carbohydrate Content, Translocation and Photosynthesis of White Clover

White clover ramets were grown at various carbon dioxide concentrations(200, 350 and 1000 µl 1^–1), defoliated and regrownat the same concentrations. Morphological characteristics, dryweights and non-structural carbohydrate contents of plant organs,diurnal variation of sugar and starch content of leaves, translocationof assimilates and photosynthesis were determined. Carbon dioxide concentration influenced the dry weights, butnot the number and size of the plant organs. However, defoliationof plants at low carbon dioxide concentration resulted in decreasedleaf size and stolon length. Carbon dioxide concentration influencedthe content and diurnal variation of starch and sugar in theleaves. Starch was accumulated at medium carbon dioxide concentrationand sugar at a higher concentration when the storage capacityfor starch seemed to be exceeded. Starch was preferentiallyaccumulated in the first and sugar in the second half of thelight period. Translocation was decreased during the periodsof accumulation. Sugar accumulation in the leaves seemed tobe a consequence of the imbalance between sink and source, whereasstarch accumulation seemed to follow an in-built diurnal pattern.Accumulation of both starch and sugar during the photoperiodwas followed by degradation and export during the dark period.Decreased dark export occurred at low carbon dioxide concentrationwhen neither starch nor sugar was accumulated during the photoperiod. Carbon dioxide, white clover, Trifolium repens L., growth, carbohydrates, starch, sugar, translocation, photosynthesis     

Carbon partitioning and export from mature cotton leaves

The partitioning of carbon in intact, mature cotton (Gossypium hirsutum L.) leaves was examined by steady-state ¹⁴CO₂ labeling. Plants were exposed to dark periods of varying lengths, followed by similar illuminated labeling periods. These treatments produced leaves with a range of starch and soluble sugar contents, carbon exchange, and carbon export rates. Export during the illuminated periods was neither highly correlated with photosynthesis nor was export during the illuminated periods significantly different among the treatments. In contrast, the rate of subsequent nocturnal carbon export from these leaves varied widely and was found to be highly correlated with leaf starch content at the end of the illumination period (r = 0.934) and with nocturnal leaf respiration (r = 0.954). Leaves which had accumulated the highest levels of starch (about 275 micrograms per square centimeter) by the end of the illumination period exhibited nocturnal export rates very similar to those during the daylight hours. Leaves which accumulated starch to only 50 to 75 micrograms per square centimeter virtually ceased nocturnal carbon export. For leaves with starch accumulations of between 50 and 275 micrograms per square centimeter, nocturnal export was directly proportional to leaf starch at the end of the illumination period. After the nocturnal export rate was established, it continued at a constant rate throughout the night even though leaf starch and sucrose contents declined.     

The Uptake of 14CO2 by Developing First Leaves of Barley and Partition of the Labelled Assimilates

The uptake of ¹⁴CO₂ by developing barley leaves of three ageswas followed using short presentation periods at the beginningor the end of the photoperiod. Partition of labelled carboninto ethanol-soluble and insoluble compounds, and movement oflabel within the plant were also examined. Young expanding leaves (day 6) retained most of the assimilatedcarbon and within 24 h 75–80 per cent of this was in ethanol-insolublecompounds. Leaves that were fully expanded took up rather more¹⁴CO₂ but exported a substantial amount of this to roots, leafbases including the stem apex, and to the developing secondleaf. Export occurred over periods up to 24 h, and by that time8- and 10-day-old leaves retained only 35 per cent and 15 percent respectively of the total label taken up. The label retainedin these leaves was predominantly in ethanol-soluble forms,whereas 75 per cent or more of the labelled carbon which wasexported from the leaves was found in ethanol-insoluble compounds.     

Use of the Plastochron Index to Evaluate Effects of Light, Temperature and Nitrogen on Growth of Soya Bean (Glycine max L. Merr.)

The plastochron index (PI) has been compared with leaf growthand biomass accumulation in young soya bean plants of severalcultivars that were grown in controlled environments with differentirradiance levels and durations, temperatures, and nitrogen(N) regimes. Increasing the photoperiod from 10 to 16 h day^–1 increasedthe plastochron rate (PR) and the proportion of axillary growth.Doubling the photosynthetic photon flux density (PPFD) to 1000µmol m^–2S^–1, increased PR and the proportionof roots to total plant weight, but decreased the proportionof stems plus petioles to total. In a series of experiments,the plants were grown in an 8 h photoperiod at constant temperaturesof 17, 20, 26 or 32 °C. As temperature increased, PR increased,but the duration of leaf expansion decreased. Leaves were largestat 20 and progressively smaller at 26, 32 and 17 °C. Biomasswas greatest for a given PI at 20 °C and decreased in theorder of 26, 32, and 17 °C. The proportion of axillary growthalso was greatest at 20 °C. When plants were grown in a15 h photoperiod at temperatures from 17.1 to 26.6 °C, leafsize continued to increase up to the highest temperature. At17 °C, the PR in the 15 h photoperiod (PPFD 390 µmol;m^–2S^–1) was about threefold greater than in 8 h(500 µmol m^–2 S^–1); biomass accumulation perday was about fivefold greater. Increasing N from 3 to 36 mMincreased PR about 10 per cent, altered biomass partitioningamong plant parts, and increased the biomass of the plants.The NO₂ form of N markedly stimulated axillary growth as comparedwith the NH₄⁺ form. Environment or cultivar had little influenceon the duration of leaf expansion in terms of PI. Cultivarsdid not differ consistently in biomass production and allocationin the different environments. Glycine max (L.) Merrill, soybean, soya bean, plastochron index, leaf development, growth analysis, partitioning, light, nitrogen, temperature     

The Effect of Shade During Leaf Expansion on Photosynthesis by White Clover Leaves

In three experiments measurements of photosynthesis were madeon single leaves of white clover (Trifolium repens L.) on threecultivars grown in a controlled environment. Plants which had grown under an irradiance of 30 J m^–2s^–1, or in shade within a simulated mixed sward, producedleaves with photosynthetic capacities some 30 per cent lowerthan did plants grown at 120 J m^–2 s^–1 without shade.There were no differences between treatments either in photosynthesismeasured at 30 J m^–2 s^–1, or in respiration ratesper unit leaf dry weight. Respiration per unit leaf area washigher in the plants grown at 120 J m^–2 s^–1, reflectingthe lower specific leaf area of these leaves. There were nodifferences between the three cultivars examined. Leaves which were removed from the shade of a simulated swardshortly after becoming half expanded achieved photosyntheticcapacities as high as those which were in full light throughouttheir development. It is suggested that it is this characteristicwhich enables clover plants growing in an increasingly densemixed sward to produce a succession of leaves of high photosyntheticcapacity, even though each lamina only reaches the top of thesward at a relatively late stage in its development. Trifolium repens L., white clover, photosynthesis, leaf expansion, shade, specific leaf area, stomatal conductance     

The Influence of Applied Nitrogen on Export and Partitioning of Current Assimilate by Field-grown Potato Plants

Field-grown potatoes were subjected to N deficiency (no appliedN) or received high levels of N (240 kg N ha^–1) at planting.The effects of these treatments were monitored at five stagesduring growth in terms of the allocation of photosynthate withinthe leaf, and the export and partitioning of carbon to differentsinks. N deficiency significantly raised the starch concentrationin all organs of the plants, particularly in leaves and stems,and as a consequence the total amount of starch in the canopyof the low N plants remained greater than that of the high Nplants until approx. 100 days after planting (DAP). The totalamounts of carbohydrates, protein and amino acids were calculatedfor each treatment and these values were used to derive a balancesheet for major reserves. Net losses of reserves occurred fromthe canopy in both treatments in the period 97–133 DAP,although these were shown to represent < 3 per cent of thetotal gain in tuber dry weight for the season. Partitioning of ¹⁴C assimilates was examined in whole plantsand also in single leaves. Reduced partitioning to the tubers,seen in high N plants throughout their growth, was shown tobe due to decreased percentage export by the leaf and accumulationof exported ¹⁴C by the stems. Partitioning to the tubers inlow N plants increased prior to senescence when 87 per centof the fixed ¹⁴C was exported within 24 h, 80 per cent of thisto the tubers. The equivalent values for the high N plants were77 and 60 per cent respectively. Increased percentage exportcoincided with decreased allocation to starch in the leaf, anda link between these processes is suggested. N also significantlyaltered the allocation of ¹⁴C within the leaf and may have influencedthe degradation of starch in the dark to a greater degree thanits synthesis in the light. The enzymes sucrose phosphate synthase (SPS), and starch synthasewere measured concurrently with partitioning. High N plantsshowed higher rates of activities of each of the enzymes althoughboth enzymes showed a similar pattern of development over theseason, irrespective of N treatment. The data are discussed in the light of conflicting reports concerningthe influence of N on translocation and partitioning. ¹⁴C assimilates, carbohydrates, nitrogen, potato (Solanum tuberosum L.), protein