Effects of Low Oxygen on Photosynthesis, Translocation and Growth in Green Pepper (Capsicum annuum)

Green pepper (Capsicum annuum cv. Bell Boy) plants were exposedin chambers to low (2%) oxygen and controlled carbon dioxideconcentrations. Vegetative and fruiting plants showed short-termincreases in net photosynthesis in low oxygen or elevated carbondioxide (up to 900 µl CO₂ l^–1). Photosynthesis ofyoung vegetative plants increased in low oxygen in the short-termbut there was no long-term benefit. Low oxygen enhancement ofphotosynthesis declined with time and after 10 d, leaf areaand root dry weight were less than in plants grown in normalair. Labelled assimilates were translocated from leaves to otherregions at similar rates in low oxygen and normal air. Low oxygenreduced respiratory losses from leaves and reduced the proportionof soluble carbohydrate converted to polysaccharide in all plantparts. Thus, low-oxygen environments decrease the utilisationof assimilates which then may lead to inhibition of photosynthesis. Capsicum annuum, photosynthesis, photorespiration, translocation, utilization of assimilates     

The Pathway of Assimilate Flow from Source to Sink in Urtica dioica L., Studied with14C under Ambient Atmospheric Conditions

The contribution of individual vascular bundles of the stemto the flow of assimilates from a selected source leaf to thesink regions was investigated inUrtica dioica L., a plant witha decussate leaf arrangement. Two homologous sets of eight vascularstrands were recognized, arranged in mirror symmetry in thestem internodes. In each set, three of the bundles were identifiedas traces of one leaf merging into the vascular system of thestem one node below the origin of the leaf. The main bundleof a stem-half bifurcates at each end of the internode intotwo subdominant bundles, which combine in the next but one nodeto form the dominant bundle again. Each set of vascular strandsalso contains two minor bundles which pass more or less withoutinterruption through the whole stem. The uppermost mature source leaf (leaf number 5 as counted fromthe tip) was exposed to¹⁴CO₂in a closed gas circuit. The concentrationof the carbon-labelled CO₂was maintained at the ambient CO₂levelto maintain the natural source strength of the leaf. By theend of the usual nocturnal dark phase, carbon from the sourceleaf had been imported predominantly by sink leaves of the sameorthostichy. Lesser, but significant amounts of radiocarbonwere also incorporated into the sink leaves of the adjacenttwo orthostichies via the marginal leaf traces. In spite ofthe junction of the vascular strands in the nodes and an interfascicularconnection of the stem bundles, randomization of the photosynthatesfrom individual leaves was minimal in the vascular system ofthe stem in the upward direction, and also low in the flux tothe roots. Substantial amounts of radioactivity were also foundin the lately-formed xylem elements of the vascular strandsand their interfascicular connections, indicating active secondarygrowth. Assimilate distribution; source–sink connections; Urtica dioica ; vascular architecture     

Aspects of the Comparative Physiology of Ranunculus bulbosus L. and Ranunculus repens L.: II. Carbon Dioxide Assimilation and Distribution of Photosynthates

Detailed analysis of the interrelationships between sourcesof photosynthate production and sites of utilization in thetaxonomically closely related species Ranunculus bulbosus L.and R. repens L. showed that leaves whether present on rosette,stem, or stolon had similar levels of ¹⁴CO₂-fixation but thepattern of distribution of radiocarbon to the rest of the plantdiffered. Fruits of R. bulbosus had a lower fixation rate thanleaves but were characterized by total retention of the fixedradiocarbon. Rosette leaves of R. bulbosus supplied the youngleaves, developing apices in the rosette, roots, and corms,whereas the labelled assimilates from cauline leaves were evenlydistributed between reproductive and vegetative parts. The cormwas the major sink both at the flowering and fruiting stages.When plants were treated with ¹⁴CO₂ in the field even higherlevels of radiocarbon moved into the corm than in comparableexperiments under greenhouse conditions. The rosette leaf ofR. repens exported mainly to actively growing stolons in plantswith many stolons bearing rooted ramets although growth of astolon was also substantially supported by photosynthates producedby its own ramets. A proportion of the radiocarbon fixed byleaves of mature ramets was exported and moved in a predominantlyacropetal direction into the stolon apex, stolon axis, and youngramets of the same stolon. The stock in R. repens had a muchlower demand for assimilates than the corm in R. bulbosus. The results are consistent with the concept that R. bulbosusoperates a conservative policy involving the replacement ofthe parent in situ by a daughter from the corm, coupled withextensive fruit production. In R. repens the emphasis is onlateral spread and exploitation of substantial areas of groundby vegetative spread and replacement of the parent by daughtersmany of which may occupy sites some distance from the parent.     

Distribution of Assimilates from Various Source Leaves During the Development of Gladiolus grandiflorus

The distribution pattern of the products of photosynthesis wasstudied in gladiolus plants (Gladiolus grandiflorus cv Eurovision)in four stages of development I, plants having a very younginflorescence still enclosed between the leaves; II, plantswith a young inflorescence just emerged from the leaves, III,plants at full bloom, IV, plants with young fruits. The first,third or sixth foliage leaf was labelled with ¹⁴CO₂, and subsequentdistribution in the plant was determined Results were expressedas a percentage of translocated ¹⁴C accumulated by each partof the plant which gives a measure of its ‘sink strength’,or as ‘relative sink activity’ (RSA) which is independentof the size of the indicated organ. There are two competing sinks in the developing gladiolus—theinflorescence and the new corm. When RSA is the criterton theinflorescence constitutes the main sink irrespective of thesource leaf from the first stage until flowering. With the subsequentwilting of the flowers and fruit set RSA of the inflorescencedeclines rapidly and the new corm becomes the main sink When‘sink strength’ is the criterton it appears thatthe inflorescence acts as a very weak sink when it is youngand becomes increasingly stronger until flowering and then declinessteeply. Sink strength of the corm declines during the developmentof the inflorescence and then increases again steeply with wiltingof the flowers and fruit set. There are small differences betweenthe various source leaves. The young new corm acts as a strongsink for the lower foliage leaf and progressively weaker forupper leaves. Gladiolus grandiflorus, flower development, corm, assimilates distribution, sink strength, relative sink activity     

Carbon Translocation in the Tomato: Carbon Import and Fruit Growth

The rates of carbon import by fruits were measured over 48 has the sum of the change in the total organic carbon contentof the fruit and the respiratory loss of carbon. Over a rangeof fruit sizes from 20–90 per cent of the maximum volumethe smaller fruits imported carbon at an absolute rate (mgCfruit^–1 h^–1) nearly twice that of the larger fruits.The imported leaf assimilates, identified as the ¹⁴C-compoundsalong the pathway between a ¹⁴CO₂-fed leaf and a young fruit,comprised 90 per cent sucrose and 10 per cent glutamic acid,aspartic acid and malic acid. Within the fruit the imported¹⁴C-sucrose was hydrolysed into hexoses. The changes in thelevels of starch and insoluble residue in the fruit were positivelycorrelated with the carbon import rates. In the largest fruitswith the lowest import rates, there was breakdown of insolubleresidue and less accumulation of starch, but a significant increasein the level of sucrose. The sink strength of a tomato fruitis dependent more on sink activity than on sink size.     

Effects of elevated CO2 concentrations on three montane grass species: III. Source leaf metabolism and whole plant carbon partitioning

Agrostis capillaris L.⁵, Festuca vivipara L. and Poaalpina L.were grown in outdoor open-top chambers at either ambient (340 3µmol mol^–1) or elevated (6804µmol mol^–1)concentrations of atmospheric carbon dioxide (CO₂) for periodsfrom 79–189 d. Photosynthetic capacity of source leaves of plants grown atboth ambient and elevated CO₂ concentrations was measured atsaturating light and 5% CO₂. Dark respiration of leaves wasmeasured using a liquid phase oxygen electrode with the buffersolution in equilibrium with air (21% O₂, 0.034% CO₂). Photo-syntheticcapacity of P. alpina was reduced by growth at 680 µmolmol^–1 CO₂ by 105 d, and that of F. vivipara was reducedat 65 d and 189 d after CO₂ enrichment began, suggesting down-regulationor acclimation. Dark respiration of successive leaf blades ofall three species was unaltered by growth at 680 relative to340 µmol mol^–1 CO₂. In F. vivipara, leaf respirationrate was markedly lower at 189 d than at either 0 d or 65 d,irrespective of growth CO₂ concentration. There was a significantlylower total non-structural carbohydrate (TNC) concentrationin the leaf blades and leaf sheaths of A. capillaris grown at680µmol mol^–1 CO₂. TNC of roots of A. capillariswas unaltered by CO₂ treatment. TNC concentration was increasedin both leaves and sheaths of P. alpina and F. vivipara after105 d and 65 d growth, respectively. A 4-fold increase in thewater-soluble fraction (fructan) in P. alpina and in all carbohydratefractions in F. vivipara accounted for the increased TNC content. In F. vivipara the relationship between leaf photosyn-theticcapacity and leaf carbohydrate concentration was such that therewas a strong positive correlation between photosynthetic capacityand total leaf N concentration (expressed on a per unit structuraldry weight basis), and total nitrogen concentration of successivemature leaves reduced with time. Multiple regression of leafphotosynthetic capacity upon leaf nitrogen and carbohydrateconcentrations further confirmed that leaf photosynthetic capacitywas mainly determined by leaf N concentration. In P. alpina,leaf photosynthetic capacity was mainly determined by leaf CHOconcentration. Thus there is evidence for down-regulation ofphotosynthetic capacity in P. alpina resulting from increasedcarbohydrate accumulation in source leaves. Leaf dark respiration and total N concentration were positivelycorrelated in P. alpina and F. vivipara. Leaf dark respirationand soluble carbohydrate concentration of source leaves werepositively correlated in A. capillaris. Changes in source leafphotosynthetic capacity and carbohydrate concentration of plantsgrown at ambient or elevated CO₂ are discussed in relation toplant growth, nutrient relations and availability of sinks forcarbon. Key words: Elevated CO₂, Climate change, grasses, carbohydrate partitioning, photosynthesis, respiration     

The Distribution and Identity of Assimilates in Tomato with Special Reference to Stem Reserves

Much of the work on the distribution of ¹⁴C-labelled assimilatesin tomato has been done in winter under low light intensities,and consequently the reported distribution patterns of ¹⁴C maynot be representative of plants growing in high light. Further,there are several somewhat conflicting reports on patterns ofdistribution of ¹⁴C-assimilates in young tomato plants. We soughtto clarify the situation by studying the distribution of ¹⁴C-assimilatesin tomato plants of various ages grown in summer when the lightintensity was high. In addition, the role of the stem as a storageorgan for carbon was assessed by (a) identifying the chemicalfractions in the stem internode below a fed leaf and monitoring¹⁴ C activity in these fractions over a period of 49 d, and(b) measuring concentrations of unlabelled carbohydrates inthe stem over the life of the plant. The patterns of distribution of ¹⁴C-assimilates we found fortomato grown under high light intensity confirmed some of thosedescribed for plants grown under low light, but export of ¹⁴Cby fed leaves was generally higher than reported for much ofthe earlier work. Lower leaves of young plants exported over50% of the ¹⁴C they fixed, although export fell sharply as theplants aged. Initially, the roots and apical tuft were strongsinks for assimilates, but they had declined in importance bythe time plants reached the nine-leaf stage. On the other hand,the stem became progressively more important as a sink for ¹⁴C-assimilates.Older, lower leaves exported more of their ¹⁴C-assimilates tothe upper part of the plant than to the roots, whereas youngleaves near the top of the plant exported more of their assimilatesto the roots. The stem internode immediately below a fed leafhad about twice the ¹⁴C activity of the internode above theleaf. Mature leaves above and below a fed leaf rarely importedmuch ¹⁴C, even when in the correct phyllotactic relationshipto the fed leaf. In the first 3 d after feeding leaf 5 of nine-leaf plants, theorganic and amino acid pools and the neutral fraction of theinternode below the fed leaf had most of the ¹⁴C activity, butby 49 d after feeding, the ethanolic-insoluble, starch and lipidfractions had most of the ¹⁴C activity. Glucose, fructose andsucrose were the main sugars in the stem. Although concentrationsof these sugars and starch declined in the stem as the plantsmatured, there was little evidence to indicate their use infruit production. Stems of plants defoliated at the 44-leafstage had lower concentrations of sugars and starch at maturity,and produced less fruit than the controls. It was concludedthat tomato is sink rather than source limited with respectto carbon assimilates, and that the storage of carbon in thestem for a long period is possibly a residual perennial traitin tomato.Copyright 1994, 1999 Academic Press Lycopersicon esculentum, tomato, assimilate distribution, ¹⁴C, internode storage, sink-source relationships, starch, stem reserves, sugars     

Pattern and Control of Distribution of 14C-assimilates in Reproductive Plants of Lolium multiflorum Lam. var. Westerwoldicum

The distribution of ¹⁴C-assimilates was examined in reproductiveplants of Lolium multiflorum Lam. var. Westerwoldicum (cv. Tama)from which all emerged tillers had been removed, leaving themain tiller with two expanding leaves, one of them the flagleaf, and two expanded leaves. Export of ¹⁴C from the lowerexpanded leaf was mainly to the tiller in its axil, the steminternode below its node and the roots, whereas the upper expandedleaf supplied predominantly the expanding leaves, the ear, steminternodes, roots and the tiller bud in the axil of the lowerleaf. Defoliation and root-pruning showed that expanding leaveswere able to compete successfully for assimilates, probablythrough the production of substances capable of mobilizing supply.Local application of 1-naphthaleneacetic acid (NAA), gibberellicacid (GA₃) and 6-benzylaminopurine (BAP) to small tiller budsshowed that GA³ and BAP promoted bud growth and ¹⁴C accumulation,but that addition of NAA reduced these effects.     

Assimilate Distribution in Poa annua L.

The carbon economy of a flowering tiller of Poa annua L. hasbeen examined over the period from inflorescence emergence tograin shedding. The total import of ¹⁴C by the inflorescencereached a maximum at late grain filling but the relative importof assimilate was greatest 14 days after its appearance andrepresented 20–25 per cent of that assimilated by theinflorescence itself. The inflorescence continued to be an importantassimilatory organ after grain ripening when it exported morethan 50 per cent of its assimilate to the stem, roots and othertillers. The patterns of distribution of assimilates from the youngestuppermost and the oldest green leaf of the reproductive tillerwere largely determined by the stage of development of the inflorescence.The youngest leaf mainly supported the inflorescence up to theend of the grain-filling stage but then supplied assimilatesbasally to the roots and adjacent tillers. The oldest greenleaf supported the growth of the stem and the inflorescenceup to anthesis but after this supplied assimilates mainly tothe roots and tillers. Removal of grains or the entire inflorescence only 1 h beforesupplying ¹⁴CO₂ greatly reduced the rate of fixation of ¹⁴CO₂and the export of radiocarbon, as well as changing the patternof distribution of assimilates within the plant. The significanceof these results is discussed and comparisons made with cerealsand perennial grasses.     

Partitioning of 14C Labelled Assimilates in Arctium tomentosum

Plants of the biennial Arctium tomentosum were grown from seedto seed-set in an open field under three different treatments:control plants receiving full light intensity, plants with aleaf area reduced by 45 per cent, and shaded plants receivingonly 20 per cent of natural illumination. At various stagesof development the youngest fully expanded leaf of one plantin each treatment was exposed to ¹⁴CO₂ for half an hour. Subsequentdistribution of labelled assimilates in various plant partswas determined after eight hours. In the first year, the mostdominant sink was the tap root irrespective of variation inassimilate supply. During the production of new vegetative growthin the second season, a larger amount of radioactive photosynthatewas recovered from above ground parts, especially during formationof lateral branches. Seed filling consumed 80–90 per centof labelled carbon exported from the exposed leaf. In the secondyear, the most pronounced difference between treatments wasin the degree of apical dominance, being highest in shaded plantsand lowest in the plants with cut leaves. Results from ¹⁴C experimentsagreed fairly well with a ‘partitioning coefficient’derived from a growth analysis of plants grown independentlyunder the same experimental conditions. Reasons for discrepanciesbetween the ¹⁴C results and the partitioning coefficient arediscussed. Arctium tomentosum, burdock, variation in assimilate supply, assimilate distribution, ¹⁴CO₂, labelling, growth analysis     

Influence of Sink-Source Interaction on Dry Matter Production in Tomato

Sink-source ratio in tomato was manipulated, in six glasshouseexperiments, by fruit pruning (trusses pruned to two to sevenfruits immediately after fruit set of each truss), truss pruning(removal of every other truss at anthesis) and truss pruningin plants with two shoots. Periodic destructive harvest wereconducted for about 100 d after flowering of the first truss.Dry matter production was not influenced by sink-source ratio,whereas dry matter distribution between fruits and vegetativeparts was greatly affected. The fraction of dry matter distributedto the fruits at the end of the fruit pruning experiments (F_fruits)could be described accurately as a saturation-type functionof number of fruits retained per truss (N_f): F_fruits = 0.660(l-e^-0.341N_f). Specific leaf area and internode length decreasedand plant leaf area increased when sink-source ratio was reduced.Removal of every other truss at anthesis did reduce dry matterpartitioning into the fruits, but it did not influence internodelength. Plant development (number of visible leaves at the endof the experiments) was not influenced by sink-source ratio.In four experiments some plants were pruned to one fruit pertruss. Final dry matter production was 8-24% lower for theseplants, compared with plants with more than one fruit per truss.This was, at least party, the result of less light interceptionby these plants, which had strongly curled leaves pointing downwards. Results indicate that effects of sink demand on dry matter productionper unit of intercepted radiation and probably on leaf photosyntheticrate in commercial tomato production can be ignored.Copyright1995, 1999 Academic Press Dry matter production, feedback control, glasshouse, growth analysis, Lycopersicon esculentum, pruning, sink demand, sink-source ratio, tomato     

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.     

The Export and Distribution of 14C-labelled Assimilates from each Leaf on the Shoot of Lolium temulentum during Reproductive and Vegetative Growth

In both reproductive and vegetative plants of Lolium temulentumL., the export of ¹⁴C-labelled assimilates from each healthyleaf on the main shoot to terminal meristem, stem, tillers,and roots was measured each time a new leaf was expanded, fora period of 5 to 6 weeks. Some labelled assimilates moved fromeach leaf on the main shoot to every meristem in the same shoot,as well as to the tops and roots of adjacent organically attachedtillers. The terminal meristem of the reproductive shoot, which includedthe developing inflorescence, received 70–80 per centof the carbon assimilated by the emerged portion of the growingleaf, 15–25 per cent of the carbon assimilated by thetwo youngest expanded leaves, and 5–10 per cent of thatfrom each of the older leaves. A similar pattern of carbon supplyto the terminal meristem was found in vegetative shoots, exceptthat older leaves on young vegetative shoots supplied even lessof their carbon to the terminal meristem. The general conclusionis that developing leaves at the tip of the shoot receive aboutthe same proportion of carbon from each leaf as does a developinginflorescence. Young expanded leaves provided most labelled assimilates forstem growth; during both reproductive and vegetative growth,expanded leaves increased their export of labelled carbon tostem, and exported less of their ¹⁴C to roots and sometimesto tillers. In these reproductive and vegetative shoots, grown in a constantexternal environment, the major changes in the pattern of distributionof labelled assimilates appeared to be the result of increasedmeristematic activity in stem internodes; the development ofan inflorescence had no obvious direct effect on the carboneconomy of shoots.     

The Influence of Plant Water Deficit on Distribution of 14C-labelled Assimilates in Cacao Seedlings

The relationship between plant water status and distributionof ¹⁴C-labelled assimilates in cacao (Theobroma cacao L.) wasevaluated after ¹⁴CO₂ pulse labelling leaves of seedlings subjectedto varying levels of water deficiency. The proportion of ¹⁴Cexported by source leaves was strongly affected by seedlingwater status. An increasing proportion of labelled assimilatesremained in source leaves at both 24-h and 72-h harvests aswater stress intensity increased. Water stress reduced the distributionof exported label to leaves and to the expanding flush in particularbut increased the proportion of label in stems and roots. Theresults suggest that current photoassimilates may be temporarilystored in source leaves and stems of cacao seedlings duringperiods of plant water deficit. The stress-induced changes inpartitioning of labelled carbon were in concordance with changesin shoot to root biomass ratios, which was likely due to greaterreduction in growth of above-ground organs to that of roots. Theobroma cacao L, assimilate partitioning, cacao, ¹⁴C-photoassimilate, water stress, water potential     

Effect of Gibberellic Acid on the Distribution of Products of Photosynthesis in Sunflower

Single leaves on growing sunflower plants were allowed to assimilate¹⁴CO₂. Gibberellic acid was applied to the same leaf or to theterminal bud or the roots, and the distribution of assimilated¹⁴C was determined at intervals of 1–96 h. Gibberellicacid had no significant effect on initial distribution of ¹⁴Cduring the period of rapid export from the leaf, but enhancedre-export from the roots after translocation from the leaf hadvirtually ceased. Most of the ¹⁴C exported from the roots accumulatedin the shoot tip. The site of application of the hormone wasof relatively minor importance. Wherever it was applied themajor effect was enhancement of movement from the roots to theshoot tip. Application to the terminal bud was most effectivein this respect. There was no evidence that gibberellic aciddirectly affected the transport system, but the data supportthe hypothesis that it increases the strength of the sink inthe shoot tip. Helianthus annuus L., sunflower, assimilate transport, gibberellic acid, phloem transport     

Assimilation and Translocation in Perennial Grasses

Single leaves, ears, or shoots of timothy (Phleum pratense L.)were exposed in light to ¹⁴CO₂, then left overnight, after whichthe plants were autoradiographed. The following conclusionswere drawn. Actively growing leaves retain all their assimilatesand import from older ones. Fully expanded leaves export butdo not import assimilates. Export begins before leaf expansionis complete, so import and export may for a time be simultaneous.Exports go at first to younger leaves and to roots, accumulatingat meristems. At later stages, exports move downwards ratherthan upwards. Buds and small tillers import from older shoots,but large tillers do not import from other shoots or exportto other large ones. Ears assimilate while still green, andimport assimilates from their associated flag leaves. Exportsfrom other leaves on flowering stems move downwards. These findings agree in general with those from other plants:they are discussed in relation to the vascular system of thegrass plant, and the need for further studies, particularlyquantitative ones, is emphasized.     

The Translocation of 14C-Photoassimilate from Normal and Mutant Leaves to the Pods of Pisum sativum L.

In experiments using radioactive carbon dioxide (¹⁴CO₂) a comparisonwas made of the ¹⁴C-photoassimilate translocation potentialsof two normal leaved (genotype AfAfTlTl) and two mutant formsof Pisum sativum (pea). A ¹⁴CO₂ administration method is describedthat permitted ¹⁴C-translocation studies to be conducted underfield conditions. One of the mutants available produced tendrils in place of leaves(afafTlTl). The other mutant studied was without tendrils buthad a much branched petiole with numerous relatively minuteleaflets (afaftltl). These mutants and the normal-leaved cultivarswith which they were compared were not isogenic lines. Lengthybackcrossing would be required before full assessment couldbe made of the possible agronomic value of such mutations. An interim evaluation of these mutants was based on ¹⁴C-distributionassays that were conducted 48 h after feeding ¹⁴CO₂, to specifiedleaves. The indication was that in translocation terms the leafand pod had a well defined respective source and sink relationshipthat was independent of leaf morphology. In each case the podswhich constituted the major ¹⁴C sinks depended on which leafhad been fed ¹⁴CO₂. With regard to sink specific activity asdefined by the quantity of ¹⁴C incorporated per unit dry weightof pod, the mutants were not significantly different from normal. The implication of these findings was that fundamental changesin pea leaf morphology could be made genetically without a markedeffect on the photoassimilate export potential of the leaf.     

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.     

Acclimation of Lolium temulentum to enhanced carbon dioxide concentration

Acclimation of single plants of Lolium temulentum to changing[CO₂] was studied on plants grown in controlled environmentsat 20°C with an 8 h photoperiod. In the first experimentplants were grown at 135 µ;mol m^–2 s^–1 photosyntheticphoton flux density (PPFD) at 415µl l^–1 or 550µll^–1 [CO₂] with some plants transferred from the lowerto the higher [CO₂] at emergence of leaf 4. In the second experimentplants were grown at 135 and 500 µmol m^–2 s^–1PPFD at 345 and 575 µl l^–1 [CO₂]. High [CO₂] during growth had little effect on stomatal density,total soluble proteins, chlorophyll a content, amount of Rubiscoor cytochrome f. However, increasing [CO₂] during measurementincreased photosynthetic rates, particularly in high light.Plants grown in the higher [CO₂] had greater leaf extension,leaf and plant growth rates in low but not in high light. Theresults are discussed in relation to the limitation of growthby sink capacity and the modifications in the plant which allowthe storage of extra assimilates at high [CO₂]. Key words: Lolium, carbon dioxide, photosynthesis, growth, stomatal density     

Short and Long Term Fluctuations of the Leaf Mass Per Area of Tomato Plants--Implications for Growth Models

The leaf mass per unit leaf area (LMA) is a key variable inmany growth models, since it is often used to predict leaf areaexpansion from leaf dry weight increase, orvice versa. Influencesof source-sink balance on leaf area, leaf dry weight, LMA, andleaf content in non-structural carbohydrates were investigatedin glasshouse tomato crops. The source-sink balance was manipulatedby artificial shading, CO₂enrichment or fruit removal usingdifferent tomato cultivars. Leaf area was hardly affected bycompetition for assimilates except under extreme conditions.In contrast, leaf dry weight, and consequently LMA, underwentlarge and rapid fluctuations in response to any factor thatchanged source and sink activities. A 60% reduction of photosyntheticallyactive radiation involved a 24% decrease in LMA after 10 d.Carbon dioxide enrichment and fruit removal induced about a45% and 15% increase in LMA, respectively, on plants with twofruiting trusses, but hardly affected LMA of producing plants.No significant cultivar effect could be identified. Changesin starch and soluble sugar content in leaves accounted foronly 29% of diurnal variations in LMA, suggesting regular fluctuationsof other components. We propose that structural LMA varies betweena maximum and a minimum value according to the ratio of assimilatesupply and demand during leaf development. Leaf area is independentof the supply of assimilates when the minimum structural LMAis realised. When the maximum structural LMA is attained, astorage pool of assimilates may accumulate in leaves duringperiods of high supply and low demand. We present a model includingthese hypotheses, which predicts structural and non-structuralLMA variations of plants with different source-sink ratios.Copyright1998 Annals of Botany Company Tomato,Lycopersicon esculentumMill., SLA, SLW, leaf growth, vegetative sink strength, assimilate competition, source-sink ratio, non-structural carbohydrate, models.