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.     

Effect of the Photosynthetic Light Period on the Carbon Budget of Young Tomato Leaves

This study was carried out to elucidate the carbon budget inyoung tomato plants in photosynthetic light periods of 8, 12and 16 h after being acclimated to an 8-h light period. Thephotosynthetic rate was nearly constant for 16 h: thereforethe amount of ¹⁴C fixed was proportional to the light period(13·77 mg C per 8 h, 20·2 mg C per 12 h, 30·5mg C per 16 h). The amounts exported, lost by respiration and accumulated whenexpressed as percentages of the carbon fixed in the day differedlittle between the light periods. The leaf continued to exportcarbon at a nearly constant rate during the light periods and,for example, exported approximately twice as much during a 16-hperiod as in an 8-h light period, even though it was not acclimatedto the long light period. The amount of starch breakdown affected the amount of carbonexported and carbon lost by respiration at night, although itwas not sufficient to account for these losses altogether. Thepossible roles of carbon accumulation and respiration are alsodiscussed. Carbon budget, steady state feeding, ¹⁴C, photosynthesis, respiration, translocation, carbon metabolism, tomato, Lycopersicon esculentum Mill     

The Effect of Temperature and Carbon Metabolism on Sucrose Uptake by Detached Tomato Fruits

The effect of temperature on sucrose uptake, and changes inlevels of starch, hexoses and sucrose in detached tomato fruitswas used to investigate the role of the sink in regulation ofcarbon import. Sucrose uptake was lower at 5 °C and greaterat 40 °C than at 25 °C. Conversion of radioactive componentsto starch was lower at both 5 °C and 40 °C than at 25°C, while the levels of non-radioactive starch was similarat all three temperatures. There was a depletion of glucoseand fructose in fruits at 40 °C. Uptake of sucrose froman agar medium by detached tomato fruits was negatively correlatedwith initial sucrose content of the fruit. The results indicatethat carbon import by tomato fruits is largely determined bysucrose levels which can be affected by metabolic activity. Lycopersicon esculentum L., tomato, fruit, sucrose uptake, temperature, carbon metabolism     

The Effect of Current Photosynthesis on the Origin of Translocates in Old Tomato Leaves

The rate of carbon transport from an old tomato leaf (54 days),grown at 80 W m^–2, was measured under light flux densitiesbetween 7 and 90 W m^–2. Under low light, the rate of carbontransport over a 6 h period was about 1 mg C dm^–2 h^–1,well in excess of the concurrent photosynthetic rate. The lossfrom these leaves of ¹⁴C-leaf assimilate which was fixed beforethe experimental period amounted to 62 per cent of the totalinitial uptake and was higher than that from leaves with higherconcurrent photosynthetic rates. The higher loss of ¹⁴C fromleaves with low photosynthetic rates was due to a greater contributionof ¹⁴C from the starch and residue fractions. The rate of transportappeared to be determined by the concentration of the labilesucrose, not the total sucrose concentration. In comparisonwith young fully-expanded tomato leaves (Ho, 1976) the sizeof the labile sucrose pool appeared to decrease with age. Thephotosynthesistranslocation coefficient was low (k₁k₂=0•21)for an old tomato leaf. Based on these results a scheme of carbonpartitioning in relation to translocation is proposed. Criteriafor assessing the efficiency of translocation in leaves arediscussed.     

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     

The Relationship Between the Rates of Carbon Transport and of Photosynthesis in Tomato Leaves

The rate of carbon transport based on the carbon balance overa 6-h period from a mature tomato leaf was measured overa rangeof net photosynthetic rates from 0.1 to 4.9 mg C dm^–2h^–1 under light flux densities from 4 to 140 W m^–2.A proportional relationship was demonstrated between the rateof carbon transport and carbon fixation when the carbon fixationrate was higher than 2 mg C dm^–2 h^–1.Below a carbonfixation rate of 1 mg C dm^–2 h^–1, the rate of carbonexport was maintained at 1 mg C dm^–2 h^–1 at theexpense of the breakdown of starch. A highly significant correlationwas observed between sucrose concentration and the rate of carbontransport. The sucrose concentration in the leaf appears tobe the factor controlling carbon export.     

Effects of Temperature and Light on the Carbon Budget of Young Cucumber Plants Studied by Steady-State Feeding with 14CO2

Shishido, Y., Challa, H. and Krupa, J. 1987. Effects of temperatureand light on the carbon budget of young cucumber plants studiedby steady-state feeding with ¹⁴CO°₂J. exp. Bot. 38: 1044–1054. The effect of temperature on the fate of ¹⁴C assimilated insteady-state by the expanding third leaf of cucumber seedlingswas studied at irradiances of either 30 or 75 W m^–2 (PAR)with a daylength of 8 h. The irradiance did not affect the relativedistribution of ¹⁴C assimilated by the source leaf between growth,respiration and export. In the range 15–30°C risesin temperature generally increased the proportion of carbonexported. The average rate of carbon exported during the nightwas about half the rate in the day. About 45% of the exportedcarbon was lost by respiration. The distribution pattern ofcarbon exported during the day differed considerably from thatof carbon exported during the night. The intensity of irradiance did not affect the proportion oflabelled carbon recovered from the roots. Thus the decreasedshoot/root ratio generally observed with increased irradianceis not directly controlled by carbohydrate supply. We found that the distribution patterns of exported ¹⁴C do notnecessarily represent the real carbon distribution, due to differencesin specific activity of imported carbon of individual organs.Consequently distribution patterns of ¹⁴C observed in experimentswith one source leaf have to be considered with caution. Key words: Carbon budget, ¹⁴C, ¹⁴C steady-state feeding, translocation, respiration, assimilate distribution, cucumber, temperature     

Investigations of Carbon Transport in Plants: II. THE EFFECTS OF LIGHT AND DARKNESS AND SINK ACTIVITY ON TRANSLOCATION

The use of ¹¹C as a tracer has allowed repetitive measurementsof the speed of assimilate translocation to be made on singlemaize plants throughout prolonged periods of light and darkness.The speed appeared to double when the light was switched on.The time required to achieve a maximum speed, usually about3·5 cm min^–1, depended on the duration of the previousdark period. When the plant was transferred to darkness thespeed immediately decreased by about 20 per cent and continuedto decrease over the next 20 h to values of 0·5 to 0·9cm min^–1. The mean speed of translocation in tomato in the light, andother C3 plants, was usually about 1 cm min^–1. It wasreduced by 15–30 per cent when the fruit was removed orcooled from 26 to 10°C.     

Maintenance and Growth Components of Carbon Dioxide Efflux from Growing Pea Fruits

Carbon dioxide efflux from 5- to 20-day-old pea fruits was measuredfor plants grown in controlled environment at 15 °C and600 µmol s^–1 m^–2 photon flux density in a16 h photoperiod. The rate of CO₂ output per fruit increasedquickly from 0.005 to 0.018 mg CO₂ min^–1 during fruitelongation and subsequently more slowly to 0.030 mg CO₂ min^–1as the fruits inflated. On a d. wt basis the rate was highest,0.175 mg CO₂ g^–1 min^–1, in the youngest fruits anddeclined curvilinearly with increasing fruit weight to 0.02mg CO₂ g^–1 min^–1. Separation of maintenance andgrowth components was achieved by starvation methods and bymultiple regression analysis. From the latter method estimatesof the maintenance coefficient declined hyperbolically from150±8.7 mg carbohydrate g^–1 d. wt day^–1 inthe very young fruits (0.05 g) to 10.4±0.36 mg carbohydrateg^–1 d. wt day^–1 in older fruits (2.0 g). On a nitrogenbasis maintenance costs decreased from 2240 to 310 mg carbohydrateg^–1 nitrogen day^–1 while nitrogen concentrationfell from 6.7 to 3 per cent d. wt. A simple linear relationshipbetween maintenance cost per unit d. wt and nitrogen concentrationwas not observed. A growth coefficient of 50±6.7 mg carbohydrate g^–1growth (equivalent to a conversion efficiency, Y_G, of 0.95)was estimated for all fruits examined. The overall efficiency, Y, increased from a mean of 0.70 to0.85 during fruit elongation and subsequently declined to 0.80.For a given fruit weight, efficiency increased asymptoticallywith relative growth rate; both asymptote and slope of the relationshipincreased as the fruits grew. Pisum sativum L., garden pea, legume fruit, carbon dioxide efflux, maintenance respiration, growth respiration     

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.     

Distribution of 14C Assimilates in the Cowpea (Vigna unguiculata L.) in Relation to Fruit Abscission and Treatment with Benzyladenine

The distribution of ¹⁴C assimilates from ¹⁴C-sucrose was studiedin relation to premature fruit abscission in two cowpea cultivars,Adzuki and Mala. In both cultivars most of the radioactivitywas recovered in the fruits, constituting 63–85 per centof the total ¹⁴C imported from the fed leaflet. This was followedby the root, leaves and stem in descending order, except thatin Mala, import by the stem was greater than that by the leaves.Adzuki imported 56 per cent more ¹⁴C than Mala, from the fedleaflet. In Adzuki, which exhibits a relatively low degree ofabscission of young fruits, the ratio of ¹⁴C accumulated bypeduncle 1 (oldest) fruits to that of peduncle 3 (youngest)fruits was 0·31; while in Mala it was 0·61. Ratiosof the combined accumulation by peduncles 1 and 2 fruits topeduncle 3 fruits were 0·81 for Adzuki, and 1·88for Mala. The more mature fruits of Mala thus constituted amore potent sink for ¹⁴C assimilates than those of Adzuki. In Adzuki, benzyladenine treatment of young fruits at each pedunclewas not significantly effective in reversing or modifying thenormal gradient of assimilates in fruits of different ontogeny.However, in Mala, BA treatment of the youngest fruits caused43 per cent increase in ¹⁴C import, when compared with correspondingfruits of control plants. In Adzuki, BA had no significant effecton total fruit weight, whereas in Mala the weight was increasedby about 36 per cent.     

Carbon Budget in Tomato Plants as Affected by Night Temperature Evaluated by Steady State Feeding with 14CO2

A semi-closed system to label with ¹⁴C and trace photoassimilatesunder steady state conditions is described. It was used to elucidatethe effects of night temperature on the carbon budget of tomato The third leaf kept at 25 °C in 8 h light of 36 W m^–1(PAR) assimilated 13·77 mg C . d^–1. By the endof the photoperiod, 46% of the carbon assimilate was exportedto the sinks, out of which 27% was respired and 19% was accumulatedin the sinks, respectively The plants were then kept in the dark for 16 h at 15, 20, 25and 30%C. The export in the night-time increased with nighttemperature, reaching 18–27% of the carbon assimilated.Thus, the total export in a whole day amounted to 63–72%of the carbon assimilated, out of which 35 and 42% were lostby respiration and 29 and 31% was accumulated in the sinks at15 and 30 °C, respectively. Thus, accumulation in the sinksdiffered little with night temperature, while that in the sourceleaf, and hence the total accumulation, decreased with increasingnight temperature The export started early in the morning and was much greaterin the light than in the dark. In addition, the day/night ratioof export was lower at higher night temperatures The percentage distribution of ¹⁴C-assimilates to the lowerparts decreased, while that to the upper parts increased withincreasing night temperature. The calculated respiratory lossin individual sinks seemed to correspond to the distributionpattern of ¹⁴C-assimilates Carbon budget, 14C, ¹⁴C steady state feeding, translocation, respiration, assimilate distribution, temperature, tomato     

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.     

Distribution of 14C-labelled Products of Photosynthesis during the Commercial Life of the Tomato Crop

On seven occasions measured amounts of ¹⁴CO₂ were supplied toindividual leaves on tomato plants grown as a crop under commercialconditions. Twenty-four hours later the distribution of radiocarbonproducts was determined using counting procedures. In the vegetativephase lower leaves exported more carbon up than down, and upperleaves a larger proportion downwards. During the developmentof the crop upper leaves continued to export more carbon downthan up, and lower leaves developed a similar tendency. In theyoung fruiting crop all leaves supplied all the trusses, butas the number of trusses increased groups of leaves came tohave primary (but not absolute) responsibility for supplyingsingle trusses. At the time of fruit development the leavesretained for 20–24 hours more than 80 per cent of thecarbon they fixed. Mature leaves and stems both above and belowthe test leaves were significant sinks for ¹⁴C products.     

The Carbon Economy of Rubus chamaemorus L. II. Respiration

Respiratory activity and seasonal changes in carbohydrate contentof the storage organs of Rubus chamaemorus L. have been investigated.Leaf dark respiration rate increases in a non-linear mannerfrom 0·7 mg CO₂ evolved dm^–2 h^–1 at 0 °Cto 4·6 rng CO₂ evolved dm^–2 hh^–1 at 30 °C.Root and rhizome respiration rates increase from 1 µ1O₂ uptake g^–1 fresh weight h^–1 at 0.7 ° C to10 µ10, uptake g^–1 f. wt h^–1 at 20 °C.Rhizome carbohydrate reserves decline from a September peakof 33 per cent alcohol insoluble d. wt to 16 per cent in May. The circumpolar distribution of R. chamaemorus is discussedin relation to the evidence presented here and in the precedingpaper of the series.     

Germination as an Unreliable Indicator of the Effectiveness of Cryopreservative Procedures for Imbibed Seeds

Tomato seeds of 49 per cent moisture content have been thawedfrom liquid nitrogen (—196 °C) with no loss of germinationfollowing both slow (12 °C min^–1) and rapid (850 °Cmin^–1) cooling, using 35 per cent dimethyl sulphoxide(DMSO) as a cryoprotectant. Lower concentrations of DMSO resultedin reduced germination. High proportions of seedlings from frozen/thawedseed failed to survive transplanting into soil, especially ifthey had been cooled rapidly. Seedlings grown to maturity afterthe slow cooling treatment showed a greater proportion of morphologicalabnormalities than a control population or those that had beenrapidly cooled. It is suggested that if cryopreservative techniques are to beevaluated for the storage of recalcitrant seeds, false impressionsof efficacy might be gained by reliance solely on germinationtesting. Cryopreservation, germination, recalcitrant seed     

Translocation of 14Carbon in Tobacco following Assimilation of 14Carbon Dioxide by a Single Leaf

¹⁴CO₂ was assimilated by single leaves (presentation leaves)of tobacco plants for periods of 2–3 hours. The plantswere then kept in air in continuous light and the redistributionof radioactivity determined at various times up to 96 hours.There was a complete turnover of sucrose in the presentationleaf in about 24 hours without change in amount. Starch turnedover more slowly and simultaneously increased in amount. 20–30per cent. of the radioactivity appeared to be irreversibly incorporatedinto the presentation leaf. Of the material exported from thepresentation leaf some 3 per cent. reached the upper leavesand stem apex. Import into leaves above the presentation leafwas completed in about 6 hours. No activity appeared in leavesbelow the presentation leaf, therefore the balance of the exportedactivity was retained in the stem and roots. The distribution of radioactivity in the leaves followed a well-definedpattern determined by the vascular interconnexions. Radioautographs of stem sections provided some information concerningdistribution of radioactivity in the stem.     

The Interdependence of Tillers in Lolium multiflorum Lam.--a Quantitative Assessment

Experiments were made to determine the extent of reciprocaltransfer of products (derived from the assimilation of ¹⁴CO₂)between various parts of the young vegetative grass plant (Loliummultiflorum Lam.). When individual laminae on different tillerswere supplied with ¹⁴CO₂, 47–64 per cent of the fixedcarbon was exported after 24 h. The principal sinks were theroot system and the shoot or tiller to which the fed leaf wasattached. Other tillers also received significant quantitiesof radiocarbon. When whole tillers were supplied with ¹⁴CO₂the percentage of fixed radiocarbon exported within 24 h rangedfrom 14–31 per cent. Of this, 50–74 per cent wasrecovered from the root system (except in the case of exportfrom the youngest tiller) but exchange of material between tillersalso occurred.A reciprocity diagram is presented and it is concludedthat despite the magnitude of the exchange no tiller showedan over-all net gain or loss. The main shoot and the tillersdiffered in the extent of their carbon exchange and in theirdegree of independence. The oldest daughter tiller of the mainshoot was the most independent and the main shoot most interdependent.     

Carbon Economy and Translocation of 14C in Leaflets of the Seventh Leaf of Tomato During Leaf Expansion

The growth pattern (leaf expansion, carbon metabolite accumulation)and the assimilation and translocation of leaf assimilate inthe leaflets of the seventh leaf of tomato were examined onsix occasions during a period of 35 days beginning 10 days afterthe leaf was first visible. Leaf expansion progressed basipetally within the leaf. The primarypartitioning of the currently-fixed ¹⁴C and the 24 h incorporationof both self-fixed and imported ¹⁴C were closely related tothe degree of leaf expansion. Therefore, leaflets of up to halffinal size within the same leaf, were markedly different inphysiological function. Maximum import of ¹⁴C-leaf assimilate from the six basal leavesinto the seventh leaf was recorded at 15 per cent expansion.The terminal and top-pair of leaflets reached one quarter fullsize first and exported ¹⁴C leaf assimilate to the basal-pairof leaflets and to the rest of the plant when the seventh leaf,as a whole, was only one tenth expanded. The transition fromimport to export in a given leaflet was gradual and overlapping. During the early part of leaflet expansion, more imported ¹⁴C-leafassimilate was used for protein synthesis than for the synthesisof sugars and starch. Meanwhile, the capacity for sucrose synthesisfrom the self-fixed carbon increased significantly precedingthe onset of export. Each leaflet resembles a whole leaf in functional development.     

Carbon and Nitrogen Transfer from Vegetative Organs to Ripening Seeds of Field Pea (Pisum arvense L.)

The fate of the carbon from photosynthetically assimilated ¹⁴CO₂and the nitrogen of rootapplied ¹⁵NO₃ was studied by harvestingplants at intervals after a week's feeding period, either early(34–41 d after sowing) during vegetative growth, or late(112–19 d) as plants were fruiting. Only 23 per cent of the early fed ¹⁴C but more than 90 per centof the early-fed ¹⁵N survived in plants until maturity (155d). Efficiency of transfer of early-fed ¹⁴C to seeds was 2 percent, that for early-fed ¹⁵N, 51 per cent. Assimilatory activitybefore flowering had little direct relevance to the carbon nutritionof seeds but provided approximately one-fifth of the seeds'requirement for nitrogen. Early or late-fed ¹⁵N was mobilized slowly for reproductivedevelopment, seeds at all 15 reproductive nodes deriving benefit,late-forming ones more so than those developing earlier. Vegetative organs on a young shoot lost previously acquired¹⁵N at rates within the range 0.5–2.4 per cent per day.The released nitrogen was first taken up by the root and youngerparts of the shoot, but much was later traced to the seeds. Late feeding of ¹⁴C or ¹⁵N resulted in a high (74–6 percent) efficiency of transfer to seeds, most of this appearingto be donated by, or cycled through, vegetative parts of thereproductive zone of the shoot. The significance of past and current assimilation in fruit nutritionis discussed.