Root Development and Source-Sink Relations in Carrot, Daucus carota L: II. EFFECTS OF ROOT PRUNING ON CARBON ASSIMILATION AND THE PARTITIONING OF ASSIMILATES

Physiological responses to root pruning were investigated bycomparing ¹⁴CO₂ fixation rates, the partitioning of ¹⁴C-labelledassimilate, and soluble and insoluble carbohydrate levels inthe leaves of carrot plants following the removal of some ofthe fibrous roots, or fibrous roots and part of the tap root.Root pruning reduced ¹⁴CO₂ fixation by 28–45% but leafspecific activity (¹⁴C assimilation g-¹ leaf fresh weight) wasunchanged. The proportion of total assimilate exported to theroot system increased following root pruning and this was atthe expense of the developing leaves. In younger plants (wherethe tap root received 10% of the assimilate) the supply of ¹⁴Cto the tap root was maintained in spite of root pruning. However,shortening the tap root to 3 cm in older plants (in which 30%of the fixed 14C was normally exported to the developing storageorgan), reduced its sink capacity and resulted in slightly greaterretention of ¹⁴C in the mature leaves. Greater concentrationsof insoluble carbohydrate were found in the mature leaves followingroot pruning but soluble sugar content was unaffected. Onlysmall differences were observed in the distribution of ¹⁴C betweensoluble and insoluble carbohydrate fractions when plants werefed ¹⁴CO₂ several days after the root pruning operations. Thesephysiological responses were mainly associated with the removalof fibrous roots and support the view that the fibrous rootsystem is more important than the developing storage organ inregulating growth in young carrot plants.     

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.     

The Sources of Carbon for Developing Leaves of Barley

The contribution of carbon from various sources to leaves 2,3 and 4 of Proctor barley as they expanded, was determined usingdata from photosynthetic measurements and ¹⁴CO₂ feeding. Bythe time that it was half expanded, the growing leaf providedabout half its carbon requirement from current photosynthesis,and this proportion increased further during the later phasesof expansion. In all, about half of the carbon in a fully expandedleaf was derived from local photosynthesis. The pattern of supplyfrom older leaves changed with time, depending on their ageand photosynthetic activity. Shading the third or expandingfourth leaf led to enhanced photosynthetic activity in the unshadedleaf, but this was not sufficient to compensate fully for thereduction due to shade treatment, and carbon supply for thegrowing leaf was reduced. Key words: Photosynthetic rate, Assimilate partitioning, Leaf growth, Hordeum vulgare     

青海省三江源区人工草地生态系统CO2通量

 了解三江源人工草地净生态系统CO₂交换(Net ecosystem CO₂ exchange, NEE)的季节变化规律和主要生物因子及环境因子对这些过程的影响将有助于认识青藏高原人工草地生态系统碳循环、生态价值、功能,以及对三江源区的生态安全的重要意义。该研究利用涡度相关技术,于2005年9月1日至2006年8月31日对位于青海腹地的垂穗披碱草（Elymus nutans）人工草地的NEE及生物和环境因子进行观测, 阐明NEE及其组分的动态变化特征和影响因子。三江源区人工草地生态系统的日最大吸收量为2.38 g C·m^－2·d^－1,出 现在7月30日。日间最大吸收率和最大排放率都出现在8月,分别为-6.82和2.95μmol CO₂·m^－2·s^－1。在生长季, 白天的NEE主要受光合有效辐射(Photosynthe tically active rad iation, PAR)变化控制,同时又与叶面积指数和群落多样性交互作用,共同调节光合速率和光合效率的强度。最大光合同化速率为2.46～10.39μmol CO₂·m^－2·s^－1,表观初始光能利用率为0.013～0.070μmol CO₂·μmol^－1 PAR。 在碳交换日过程中,NEE并不完全随着 PAR的增加而增大,当PAR超过某一值（>1 200μmol ·m^－2·s^－1）时,NEE随PAR的增加而降低。受温度的影响,生长季的生态系统的呼吸商Q₁₀（1.8）小于非生长季节的 2.6）。 生态系统呼吸主要受温度的控制,同时也受到叶面积指数的显著影响。生长季昼夜温差大并不利于生态系统的碳获取。 三江源区人工草地生态系统是一个较强的碳汇,为-49.35 g C·m^－2·a^－1。     

Effect of Elevated CO2 on the Photosynthesis, Respiration and Growth of Perennial Ryegrass

Single, seed-grown plants of ryegrass (Lolium perenne L. cv.Melle) were grown for 49 d from the early seedling stage ingrowth cabinets at a day/night temperature of 20/15 C, witha 12 h photoperiod, and a CO₂ concentration of either 340 or680µI 1^–1 CO₂. Following complete acclimation tothe environmental regimes, leaf and whole plant CO₂ effluxesand influxes were measured using infra-red gas analysis techniques.Elevated CO₂ increased rates of photosynthesis of young, fullyexpanded leaves by 35–46% and of whole plants by morethan 50%. For both leaves and whole plants acclimation to 680µI^–1 CO₂ reduced rates of photosynthesis in bothCO₂ regimes, compared with plants acclimated to 340µll^–1. There was no significant effect of CO₂ regime onrespiration rates of either leaves or whole plants, althoughleaves developed in elevated CO₂ exhibited generally lower ratesthan those developed in 340µI I^–1 CO₂. Initially the seedling plants in elevated CO₂ grew faster thantheir counterparts in 340µI I^–1 CO₂, but this effectquickly petered out and final plant weights differed by onlyc. 10%. Since the total area of expanded and unexpanded laminaewas unaffected by CO₂ regime, specific leaf area was persistently13–40% lower in elevated CO₂ while, similarly, root/shootratio was also reduced throughout the experiment. Elevated CO₂reduced tissue nitrogen contents of expanded leaves, but hadno effect on the nitrogen contents of unexpanded leaves, sheathsor roots. The lack of a pronounced effect of elevated CO₂ on plant growthwas primarily due to the fact that CO₂ concentration did notinfluence tiller (branch) numbers. In the absence of an effecton tiller numbers, any possible weight increment was restrictedto the c. 2.5 leaves of each tiller. The reason for the lackof an effect on tillering is not known. Key words: Lolium perenne, ryegrass, elevated CO₂, photosynthesis, respiration, growth, development     

Whole Plant and Leaf Steady State Gas Exchange during Ethylene Exposure in Xanthium strumarium L

The effects of ethylene evolved from ethephon on leaf and whole plant photosynthesis in Xanthium strumarium L. were examined. Ethylene-induced epinasty reduced light interception by the leaves of ethephon treated plants by up to 60%. Gas exchange values of individual, attached leaves under identical assay conditions were not inhibited even after 36 hours of ethylene exposure, although treated leaves required a longer induction period to achieve steady state photosynthesis. The speed of translocation of recently fixed ¹¹C-assimilate movement was not seriously impaired following ethephon treatment; however, a greater proportion of the assimilate was partitioned downward toward the roots. Within 24 hours of ethephon treatment, the whole plant net carbon exchange rate expressed on a per plant basis or a leaf area basis had dropped by 35%. The apparent inhibition of net carbon exchange rate was reversed by physically repositioning the leaves with respect to the light source. Ethylene exposure also inhibited expansion of young leaves which was partially reversed when the leaves were repositioned. The data indicated that ethylene indirectly affected net C gain and plant growth through modification of light interception and altered sink demand without directly inhibiting leaf photosynthesis.     

The Effects of Leaf Age and Senescence on the Distribution of Carbon in Lolium temulentum

Assimilate distribution in leaves of Lolium temulentum was establishedby root absorption of [¹⁴C]sucrose and after exposure to ¹⁴CO₂.Age determined the amount of carbon assimilated, with more labelbeing incorporated during expansion than at maturity. Duringsenescence ¹⁴C assimilation was much lower. Ethanol-solubleextracts from various tissues of root-labelled plants containedmost of the radioactivity chiefly in basic and acidic compounds.The neutral fraction was composed predominantly of sucrose. Sucrose was comparably labelled in leaves from plants fed equalamounts of either [¹⁴C]sucrose, glucose, or fructose and onlytraces of labelled monosaccharides appeared in extracts. Radioactive sucrose was translocated rapidly from mature leaveswhereas, in the expanding leaf, carbon incorporation was directedtowards growth and the greater proportion of label present atligule formation was in ethanol-insoluble material. Induced senescence, of a mature leaf fed during expansion, produceda rapid loss from the pool of insoluble ¹⁴C. This was accompaniedby a reduction in the contents of chlorophyll and soluble proteinand an accumulation of amino acids. The onset of senescencecaused changes in leaf sugar levels which were correlated withincreased rates of respiration.     

The Effect of Lenient Defoliation on the Nitrogen Economy of White Clover: The Contribution of Mineral Nitrogen to Plant Nitrogen Accumulation During Regrowth

Single plants of white clover (Trifolium repens L.) were grownfrom stolon cuttings rooted in sand. All plants were inoculatedwith Rhizobium trifolii, and for 14 weeks received nutrientsolution containing 0.5 mg N each week, as either ammonium ornitrate. Plants were then leniently defoliated or were leftintact and a ¹⁵N-labelled N source was applied at intervalsof 4 d to replace the unlabelled N. Lement defoliation removedfully expanded leaves only; the remaining immature leaves accountedfor 39–44% of the total. At harvests over the following21 d, leaf numbers were counted and dry matter (DM), N contentsand ¹⁵N enrichments of individual plant organs were determined. Rates of leaf emergence and expansion were accelerated in defoliatedplants; numbers of young leaves were similar in defoliated andintact plants. Total DM and N content were less in defoliatedthan intact plants and were not affected by form of N supplied.DM of young leaves, growing points and stolons and N contentof young leaves were, however, greater when ammonium ratherthan nitrate N was supplied. Rates of increase in the contentof plant total N were 8.2 ± 1.36 mg N d^-1 and 10.2±1.82 mg N d^-1 in defoliated and intact plants respectively.The increases were predominantly due to N₂ fixation, since recoveryof ¹⁵N showed that less than 1% of the increment in plant totalN was assimilated mineral N. Nevertheless, the contributionof mineral N to plant total N was 50% more in defoliated thanin intact plants; higher amounts of mineral N were found particularlyin young leaves and growing points. Partitioning of mineralN to nodulated roots increased over time and was greater whenammonium rather than nitrate N was present. White clover, Trifolium repens L. cv. S184, lenient defoliation, N accumulation, N₂ fixation     

Effect of the age of tobacco leaves on photosynthesis and photorespiration

Relationships among the activities of enzymes related to photosynthesisand photorespiration, and ¹⁴CO₂ photosynthetic products wereinvestigated with individual tobacco leaves attached to thestalk from the bottom to the top. P-glycolate phosphatase ofthe chloroplasts and glycolate oxidase of the peroxisomes hadtheir maximum activities in the 25th leaf from the dicotyledons.Maximum photorespiration was similarly distributed. The highestratio of serine-¹⁴C to glycine-¹⁴C in the photosynthesates andmaximum glycolate formation were also observed in the 25th leaf.Glutamateglyoxylate aminotransferase, serine hydroxymethyltransferaseand glycine decarboxylase were more active in the upper leaves.RuDP carboxylase had nearly constant activity in all leaves,except for the youngest in which activity decreased. MaximumCO₂ photosynthesis and enzyme activity for the C₄ dicarboxylicacid cycle occurred in the upper, youngest leaf. Distributionof photosynthetic CO² fixation among the leaves did not coincidewith RuDP carboxylase activity. The photosynthetic capacityappeared to be better related to the distribution pattern forenzymes of the C₄ dicarboxylic acid pathway, i.e. PEP carboxylase,pyruvate Pi dikinase and 3-PGA phosphatase in the upper leaves.The results suggest that the C₄ dicarboxylic acid pathway participates,to some extent, in photosynthesis in young leaves of tobacco,a dicotyledonous plant. ¹This work was reported at the Annual Meeting (1970) of theJapanese Plant Physiologists in Kobe. ²The Central Research Institute, Japan Monopoly Corporation1-28-3, Nishishinagawa, Shinagawaku, Tokyo, 141 Japan. (Received November 2, 1972; )     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

The Distribution Pattern of 14Carbon Assimilated by the Third Leaf of Wheat

The third leaf of wheat, variety Jufy I, was allowed to assimilate¹⁴CO₂ for 2 hrs.; after a further hour the distribution patternof the assimilates was determined. Uptake of ¹⁴CO₂ and assimilatesleaving the leaf increased until the leaf was fully expanded,then slowly decreased. High proportions of labelled translocates were recorded in boththe stem and the the root system, that in the roots increasinggreatly as movement of translocates to the leaves decreased.The two fully grown leaves, L₁ and L₂, imported only slightamounts of labelled translocates. Movement of labelled translocateto each of the younger leaves in turn occurred in a strikingpattern, such that import into a given leaf reached a maximumwhich coincided with its maximum rate of growth, subsequentlyfalling rapidly and reaching a very low level by the time theleaf is fully grown. The results are discussed in relation to what is known of thegeneral pattern of growth and translocation in the wheat plant.     

STUDIES ON THE RE-ASSIMILATION OF RESPIRATORY CO2 IN ILLUMINATED LEAVES

Experiments were performed, using rice, barley and Hydrangealeaves, to examine the re-assimilation of respiratory ¹⁴CO₂while photosynthesis is going on in an open air flow system. It was found that the leaves which had assimilated ¹⁴CO₂ beforehandevolved, when kept under photosynthesizing conditions, threeto four tenths (variable according to plant species and externalconditions) of the amount of ¹⁴CO₂ to be produced in the dark.Such an incomplete re-utilization of ¹⁴CO₂ was observed alsoin spinach leaf homogenate as well as in the leaves which hadpreviously absorbed ¹⁴C-glucose. The ¹⁴CO₂ output in rice leaves was found to be acceleratedby the light of high intensity. A possibility of light stimulationon the respiration was suggested. (Received October 7, 1961; )     

Ribulose Bisphosphate Carboxylase Activity and Photosynthesis during Leaf Development in the Tomato

Besford, R. T., Withers, A. C. and Ludwig, L. J. 1985. Ribulosebisphosphate carboxylase activity and photosynthesis duringleaf development in the tomato.—J. exp Bot. 36: 1530–1541. The carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenaseand of phosphoenolpyruvate carboxylase, and the light saturatedrate of net photosynthesis were measured in the developing 5thleaf of tomato plants. Values for light saturated net photosynthesiswere also calculated from the measured carboxylase activitiesand estimates of internal CO₂ and oxygen concentrations. Thecalculated rate using the activity of ribulose bisphosphatecarboxylase alone for net CO₂ assimilation in 300 mm³ dm^–3CO₂ was greater than the measured rate at 80% and full expansionbut less than the measured rate in younger leaves. When theactivities of both the carboxylases were taken into accountbetter agreement was evident for young leaves but the rate wasfurther overestimated for older leaves The calculated rate forphotosynthesis in 1200 mm³ dm^–3 CO², assuming saturationof ribulose bisphosphate carboxylase with RuBP, was an overestimatefor young leaves but was close to the observed values for leavesnear full expansion. The results are discussed in terms of measuredconductances for CO₂ and the availability of RuBP in the leaf Key words: Tomato, leaf development, photosynthesis, RuBP carboxylase, oxygenase     

The Influence of Carbon Dioxide and Daily Photon-flux Density on Optimal Leaf Nitrogen Concentration and Root: Shoot Ratio

Using a cost-benefit model, the leaf nitrogen concentrationand root : shoot ratio that maximize whole-plant relative growthrate are determined as a function of the above-ground environment(integrated daily photon flux density and the concentrationof carbon dioxide at the site of fixation within the leaf).The major advantage of this approach is that it determines theadaptive significance of leaf physiology by considering thefunctional integration of leaves and roots. The predicted responseto increasing daily photon flux densities is an increase inoptimal leaf N concentration (N_opt) and a concomitant increasein root: shoot ratio. Increased carbon dioxide concentrations,on the other hand, reduce N_opt and only slightly change root:shoot ratio. The observed increase in leaf nitrogen concentrationfound in plants growing at high altitudes (low CO₂ partial pressure)is also predicted. Since these responses to light and CO₂ maximizethe whole-plant relative growth rate, the observed adjustmentsthat plants make to light and carbon dioxide concentration appearto be adaptive. We show that the relationship between photosynthesis and leafnitrogen concentration is complex and depends on the light andCO₂ levels at which photosynthesis is measured. The shape ofthis function is important in determining N_opt and the oppositeresponse of leaf nitrogen to light and carbon dioxide is shownto be the result of the different effects of light and CO₂ onthe photosynthesis-leaf nitrogen curve. Plant growth, photosynthesis, leaf nitrogen, biomass allocation, optimization, carbon dioxide light     

Changing Sink Demand Affects the Area but not the Specific Activity of Assimilate Sources in Cantaloupe (Cucumis meloL.)

To better understand source-sink interactions, this work focusedon the influence of fruit number on leaf area and photosyntheticactivity in cantaloupe. To this end, flowers were removed over2 years on two Charentais cultivars to obtain single-fruit plantsand plants with an unrestricted fruit load (which set two tofive fruits and constituted control plants). At the whole plantscale, net photosynthesis was reduced by about 30% under highfruit load. At the leaf scale, a submodel of stomatal conductancewas fitted to the data and was included in a rectangular hyperbolamodel of leaf photosynthesis. Maximum leaf net photosynthesisaveraged 14.83 µmol CO₂m^-2s^-1at 1000 µmol quantam^-2s^-1. Light use efficiency was not affected by fruit loadand equalled 0.040 mol CO₂mol^-1quanta. Leaf area of plants withunrestricted fruit load decreased after 24 days from pollination,while the leaf area of single-fruit plants was still increasing.The decrease was due to production of fewer new leaves per day,whereas the number of senescent leaves and the size of individualleaves were not affected by the treatment. Under high fruitload, cultivar Galoubet developed a larger projected leaf areathan cultivar Talma. Thus it is concluded that: (1) large cantaloupefruits may divert a large amount of assimilates away from, andgrow at the expense of, the canopy; and (2) photosynthesis ofthe canopy was lowered because leaf area was reduced whereasphotosynthetic rate of leaves was not altered.Copyright 1998Annals of Botany Company. Cucumis meloL., fruit load, source-sink interactions, leaf photosynthesis, canopy photosynthesis, leaf area, SLA, source strength.     

Effect of elevated CO2 and nutrient status on growth, dry matter partitioning and nutrient content of Poa alpina var. vivipara L.

Poa alpina var. vivipara L. was grown in an atmosphere containingeither 340 or 680 µmol CO₂ mol^–1 within controlledenvironment chambers. The available nutrient regime was variedby altering the supply of nitrogen and phosphorus within a completenutrient solution. At a high, but not low, N and P supply regime,elevated CO₂ markedly increased growth. Differences betweennutrient supply, but not atmospheric CO₂ concentration, alteredthe allometric relations between root and shoot. Net photosynthesisof mature leaf blades and leaf N and P concentration were reducedin plants grown at the elevated CO₂ concentration. The question was asked: is it possible to ascribe all of theseeffects to elevated CO₂ or are some due to nutrient deficiencycaused by dilution with excess carbon? Several criteria, includingthe nutrient content of sink tissue, root:shoot allometry andthe use of divalent cations to estimate integrated water flowsare suggested in order to make this distinction. It is concludedthat only at a low supply of N and P₁ and elevated CO₂ concentration,was low leaf N concentration due to induced nutrient deficiency.The data are consistent with a model where the capacity of sinksto use photosynthetically assimilated carbon sets both the rateof import into those sinks (and thus rate of export from sourceleaves) and the rate of photosynthesis of source leaves themselves. Key words: Poa alpina L., growth, photosynthesis, carbohydrate, export, nitrogen, phosphorus     

The Seasonal Pattern of Growth and Production of a Temperate C4 Species, Cyperus longus

Growth and production of the temperate C₄ species Cyperus longusL. was measured throughout a growing season in an establishedplot in Eastern Ireland. The maximum standing live biomass reachedwas 2·5 kg m^–2. Estimates of unit leaf rate (ULR)and leaf area index (LAI) were made. The product of these quantitiesgave the crop growth rate (CGR) each week. C. longus was foundto maintain high values of LAI throughout the summer, with amaximum value of about 13 in early August. CGR reached a peakin early July. The optimum LAI was 11·6. Temperaturesat five levels in the plant canopy, and the amount of solarradiation intercepted by the canopy were measured continuouslyduring the summer. The mean daily rate of leaf extension waspositively correlated with the mean daily air temperature abovethe canopy but the temperature coefficient of the process waslow compared with other temperate species. The percentage ofsolar radiation intercepted by the canopy increased rapidlyin early summer, and canopy closure had occurred by mid-June.Rates of net photosynthesis were measured on young and old leafmaterial in situ at the time of peak LAI. In young leaves themaximum rates of net photosynthesis were higher than those publishedfor a range of temperate C₃ species, but similar to those foundin another temperate C₄ species, Spartina townsendii. Key words: C₄ photosynthesis, leaf growth, productivity     

Changing Patterns of Distribution of the Products of Photosynthesis in the Tomato Plant with Respect to Time and to the Age of a Leaf

Two series of experiments were conducted with tomato plantsgrown in a glasshouse. In the first series the second leaf ofa young plant was exposed to ¹⁴CO₂ for periods up to a maximumof 4 h and the distribution of the ¹⁴C-products was determinedeither at intervals of 10 and 30 min, 1, 2, 3, 4, 5, 6, 12,and 24 h, or 1, 2, 3, 5, 7, 11, and 15 days after the ¹⁴CO₂was initially supplied. Seventeen per cent of the containedradioactive carbon was exported during the first day and a further23 per cent during the following week. At the death of the leaf44 per cent of the carbon originally fixed was present in thedead structure. There is evidence that the roots re-export alarge proportion of the carbon products originally imported. In the second series the tenth leaf of a plant was suppliedwith ¹⁴CO₂, and 24 h later the relative amounts of radioactivitywere determined for every leaf, internode, and fruit truss andfor the roots and shoot apex. The plants were selected so thatleaf 10 was always immediately below truss 1. The experimentwas performed seven times, each time at a different stage bothin the life-cycle of the leaf and the development of the plant.There were differences in the total amounts of radioactivityfound at the end of each experiment but these differences werenot random. The pattern of distribution of the ¹⁴C exportedby leaf 10 changed as the plant developed. The percentage ofthe ¹⁴C initially fixed which was exported by leaf 10 rose toa peak early in its life and then fell sharply but the distributionof ¹⁴C exported followed a different pattern. For a long periodthere was a heavy accumulation of ¹⁴C in the internode immediatelybelow leaf 10 but later in development this was less evidentand the proportions reaching the roots increased. The internodeabove leaf 10 never showed the same degree of accumulation.Truss I dominated the distribution patterns during the phaseof its active growth. These results are discussed and a hypothesisto account for the changing patterns of distribution is presented.     

The Transport of Photosynthetic Products from the Chloroplasts of Tobacco Leaves

Tobacco leaves were fractionated by the non-aqueous method afterphotesynthesis for short periods in ¹⁴CO₂ and with or withoutsubsequent photosynthesis in ¹²CO₂ or respiration in the dark.Phosphate esters became labelled only slowly in the non-chloroplastparts of the leaf; glycine and serine which became rapidly labelledin the non-chloroplast leaf fraction appear to be concernedin the transport of newly assimilated carbon from the chloroplast.     

Changes in Photosynthesis, Carbon Budget and Mineral Content during the Growth of the First Leaf of Cucumber

The relationships between photosynthesis, dry matter accumulationand translocation have been studied during the development ofthe first true leaf of cucumber. The leaf was grown in an irradianceof 50W m^–2 photosynthetically active radiation for 10h^–1 at 20 C and 2 g m^–3 CO₂. The maximum rate of net photosynthesis, on a leaf area basis,occured at full expansion. Photochemical efficiency, based onincident radiation, also increased up to this stage and wasrelated to the concentration of chlorophyll in the leaf. Darkrespiration and the light compensation point fell over the wholeperiod of leaf expansion. A carbon budget analysis showed that the rate of carbon accumulationin the leaf reached a peak at 70 percent expansion. The leafchanged from a net importer to a net exporter of carbon whenit was about 30 percent expanded. The rate of export increasedwith leaf expansion (and with net photosynthesis) and was twiceas high in the day an in the night at full expansion. At fullleaf expansion there was a reduction in the amount of starchlost overnight, and the carbon exported amounted to 80 per centof the daily net carbon fixed. Cucumber, Cumic satinu L., leaf development, photosynthesis, translocation, carbon budget, mineral content