Uptake and Transport of Calcium and the Possible Causes of Blossom-end Rot in Tomato

The possible causes of blossom-end rot (BER) in tomato fruitwere investigated by comparing the uptake of calcium by theroots, the distribution of ⁴⁵Ca within the fruit and the vascularbundle network in the fruit of susceptible cultivars (Calypsoand Spectra) with those of a less susceptible cultivar (Counter)grown in a range of salinities (electrical conductivity of 5,10 and 15 mS cm^–1). The daily calcium uptake rates at5 mS cm^–1 as estimated from the xylem exudation of thedecapitated stem stump in young plants of Calypso and old plantsof Spectra, were lower than that of Counter. The uptake of ⁴⁵Caby, and the transport to, the distal part of the detached fruitof susceptible cultivars, especially Calypso, were less thanin Counter at 10 mS cm^–1. The number of vascular bundlesin both proximal and distal fruit tissues was similar in allcultivars and was only slightly reduced by salinity. However,the number of bundles containing lignified xylem vessels, asdetected by safranin staining, was reduced substantially bysalinity, particularly in Calypso. The estimated area of thefruit tissue served by individual xylem bundles in the BER susceptiblefruit grown at high salinity was greater than in Counter. Theincidence of BER in all trusses was linearly related to theproduct of average daily irradiance and daily temperature throughoutthe year. Temperature appears to be the major environmentalfactor which induces BER, regardless of cultivars and salinitytreatment. The most likely causes of BER in susceptible cultivarsare the interactions of (a) light and temperature on fruit enlargement,(b) inadequate xylem tissue development in the fruit and (c)competition between leaves and fruit for the available Ca. Key words: Lycopersicon esculentum, calcium transport, susceptibility to blossom-end rot, root exudation, xylem     

Translocation of Calcium in Relation to Tomato Fruit Growth

Regulation of the uptake and distribution of calcium in thetomato plant was investigated in plants grown in recirculatingnutrient solutions at electrical conductivities of 2,7,12 and17 millisiemens (mS). Despite an increased calcium content inthe nutrient solution at high conductivity (7–17 mS),the accumulation of calcium by fruit was progressively reducedby increasing salinity, particularly in the distal half. Theincidence of blossom-end rot in fruit (BER) also increased withsalinity. The uptake of water and ⁴⁵Ca by plants was substantially reducedin the high salinity treatment (17 mS) and, to a lesser extent,by high relative humidity (90 per cent r.h. at 20 °C). Further,the translocation of ⁴⁵Ca from roots to shoots was reduced byhigh salinity, while the percentage distribution of ⁴⁵Ca tothe apex was reduced by high humidity. Only approx. 2 per centof the ⁴⁵Ca taken up by a plant was imported by the truss. The uptake of ⁴⁵Ca and its distribution among pedicel, calyxand berry by detached fruit in 24 h showed that fruit from highsalinity plants had a reduced uptake and a lower accumulationof ⁴⁵ Ca in the berry than in the calyx. In addition, plants grown at high conductivity had a lower rateof xylem sap exudation from decapitated plants. The fruit ofthese plants had a smaller xylem cross-sectional area in thefruit pedicel and a smaller calyx than those of the low conductivitytreatment. Calcium, translocation, tomato, fruit, blossom-end rot     

Environmental Effects on the Diurnal Accumulation of 45Ca by Young Fruit and Leaves of Tomato Plants

Diurnal uptake and distribution of ⁴⁵Ca in young fruiting tomatoplants were assessed 12 or 24 h after ⁴⁵Ca was applied to thenutrient solution at the beginning of either the light (12 h)or the dark (12 h) period. During the experiment, the salinityof the nutrient solution (measured as electrical conductivity,EC) was either 2·5 or 17 mS cm^–1 and the relativehumidity (measured as vapour pressure deficit, VPD) was either0·2 or 0·6 kPa The uptake of ⁴⁵Ca by a tomato plant over 12 h was higher inthe light than in the dark but the difference was less at lowhumidity. More ⁴⁵Ca was transported from the roots to the shootin the light than in the dark. More than half of the ⁴⁵Ca inthe shoot was accumulated by the stem; the proportion of ⁴⁵Cain the stem was greater in the dark and was further enhancedby high humidity to more than 80% of the ⁴⁵Ca in the shoot.The accumulation of ⁴⁵Ca by the fruit truss in the dark wasgreater than in the light in all experimental conditions. Underlow humidity the accumulation of ⁴⁵Ca by young leaves was similarin both light and dark. In high humidity there was considerablyless accumulation of ⁴⁵Ca by the young leaves in the dark The uptake of ⁴⁵Ca continued over 24 h but the transport of⁴⁵Ca to individual organs in the second 12 h period was affectedby both light and humidity. Some of the ⁴⁵Ca accumulated byyoung leaves and fruit in the second period appears to havebeen derived from ⁴⁵Ca released from the xylem wall along thetransport pathway in the stem The roles of root pressure and transpiration in the diurnalaccumulation of calcium in young fruit and leaves are discussed Calcium, diurnal translocation, tomato, young fruit and leaves     

Regulation of the Partitioning of Dry Matter and Calcium in Cucumber in Relation to Fruit Growth and Salinity

The regulation of the partitioning of dry matter and calciumin relation to fruit growth was investigated in cucumber plantsgrown in the salinity range of 3-8 mS cm^-1 in NFT (NutrientFilm Culture), with or without a fruit pruning treatment. Thedry weight gain of the plants was proportional to the outdoorintegral irradiance, with a common daily rate of 1 g MJ^-1 m^-2in two crops grown under summer (18 MJ m^-2 d^-1) and autumn (7MJ m^-2 d^-1) conditions. Within the salinity range studied, thereduction of plant dry weight was 9% mS^-1 cm^-1. However, fruitdry weight was only reduced at salinities above 5·5 mScm^-1, although the daily dry matter accumulation by fruit, asa percentage of total dry matter accumulation, was increased.Salinity reduced the dry matter accumulation in the young shootproportionally more than in the fruit. Although the total plantCa content was reduced by 13% mS^-1 cm^-1, the Ca content of theyoung shoot was reduced by 16·6%, compared to 11% inthe fruit. Pruning fruit reduced neither plant dry weight norCa uptake. The growth of the remaining fruit, and to a lesserdegree of the young shoot, accounted for all surplus assimilates.Thus, fruit were the dominant sinks for assimilates whilst themature leaves were the strongest sinks for Ca. Nevertheless,the fruit sustained the capacity to import Ca better than theyoung shoot, when supplies of both assimilates and Ca were reducedby high salinity.Copyright 1994, 1999 Academic Press Cucumber, Cucumis sativus L., salinity, fruit pruning, dry matter and calcium     

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     

Rates of Photosynthesis in Characean Cells: II. PHOTOSYNTHETIC 14CO2 FIXATION AND 14C-BICARBONATE UPTAKE BY CHARACEAN CELLS

Photosynthetic ¹⁴C fixation by Characean cells in solutionsof high pH containing NaH¹⁴CO₃ gave a measure of the abilityof these cells to take up bicarbonate (H¹⁴CO₃^–). Whereascells of Nitella translucens from plants collected and thenstored in the laboratory absorbed bicarbonate at 1–1.5µµmoles cm^–2 sec^–1, rates of 3–8µµmoles cm^–2 sec^–1 were obtained withN. translucens cells from plants grown in the laboratory. Influxesof 5–6 µµmoles cm^–2 sec^–1 wereobtained with Chara australis, 3–8 µµmolescm^–2 sec^–1 with Nitellopsis obtusa, and 1–5µµmoles cm^–2 sec^–1 with Tolypella intricata.It is considered that these influxes represent the activityof a bicarbonate pump, which may be an electrogenic process. In solutions of lower pH, H¹⁴CO₃^– uptake would be maskedby rapid diffusion of ¹⁴CO₂ into the cells: the four Characeanspecies fixed ¹⁴CO₂ at maximum rates of 30–40 µµmolescm^–2 sec^–1 (at 21° C).     

Soil Water Extraction, Measured by Computer-Assisted Tomography, in Seedling Lupinus angustifolius cv. Yandee when Healthy and Infected with Phytophthora cinnamomi

The water-mould fungus Phytophthora cinnamomi Rands causes drought-likesymptoms on many hosts, and yet the mechanisms by which infectionleads to wilting are not fully understood. This is the firststudy to describe in detail changes in soil water around theroot with infection. Computer-assisted tomography (CAT) wasused with Lupinus angustifolius L. cv. Yandee to examine drawdowns(removal of soil water) around a central root infected by P.cinnamomi in a white sand. No growth differences in roots or shoots were found betweenhealthy and diseased plants during the 8 d of the experiment.However,drawdowns failed at high levels of inoculum (8–16 /Pc-infectedmillet seeds/plant) by 8 d. Water contents in pots with uninfectedplants were in the range 0·09–0·12 cm³ watercm^–3 soil in the centre of the pot, while water contentsin pots with infected plants at 16 millet seeds applied werein the range of 0·16–0·19 cm³ water cm^–3soil in the centre of the pot. A higher transpirational demand produced lower soil water contentsnear the root but this effect was confounded with infection:disease was more pronounced with higher transpirational demand,and disease led to an increase in water content. Key words: Root disease, Phytophthora cinnamomi, water uptake, soil-root interface, computer-assisted tomography     

Influence of the Form of Nitrogen Supply on Root Uptake and Translocation of Cations in the Xylem Exudate of Maize (Zea mays L)

Concentrations of inorganic cations are often lower in plantssupplied with NH₄⁺ as compared with NO₃^–. To examine whetherthis is attributable to impaired root uptake of cations or lowerinternal demand, the rates of uptake and translocation of K,Mg, and Ca were compared in maize plants (Zea mays L.) withdifferent growth-related nutrient demands. Plants were grownin nutrient solution with either 1·0 mol m^–3 NO₃^–or NH₄⁺ and the shoot growth rate per unit weight of roots wasmodified by varying the temperature of the shoot base (SBT)including the apical shoot meristem. The shoot growth rate per unit weight of roots, which was takenas the parameter for the nutrient demand imposed on the rootsystem, was markedly lower at 12°C than at 24°C SBT.As a consequence of the lower nutrient demand at 12°C SBT,uptake rates of NO₃^– and NH₄⁺ declined by more than 50%Compared with NO₃^– supply, NH₄⁺ nutrition depressed theconcentrations of K and particularly of Ca in the shoot, bothin plants with high and with low nutrient demand. This indicatesa control of cation concentration by internal demand ratherthan by uptake capacity of the roots. Translocation rates of K, Mg and Ca in the xylem exudate werelower in NH₄⁺- than in NO₃^–-fed plants. Net accumulationrates of Ca in the shoot were also decreased, whereas net accumulationrates of K in the shoot were even higher in NH₄⁺-fed plants.It is concluded that reduced cation concentrations in the xylemsap of plants supplied with NH₄⁺ are due to the lower demandof cations for charge balance. The lower K translocation tothe shoot is compensated by reduced retranslocation to the roots.For Ca, in contrast, decreased translocation rates in NH₄⁺-fedplants result in lower shoot concentration. Key words: Nitrogen form, cation nutrition, charge balance, xylem exudate, recirculation     

An analysis of the accumulation of water and dry matter in tomato fruit

Abstract Previously published data from tomato plants grown in nutrient solutions having one of three electrical conductivities (2, 12 and 17 mS cm^?1) were analysed. The rate of water import into the fruit, and the proportion of this conducted by the xylem stream were calculated from the daily rates of transpiration and the net accumulation of water and calcium. The rate of water import decreased as the conductivity of the nutrient solution rose, the maximum daily import rates in the third week after pollination being 3.2, 3.0 and 1.8 g fruit^?1 d^?1 for fruit grown at 2, 12 and 17 mS cm^?1, respectively. During fruit development, the proportion of water imported via the xylem fell from 8–15% to 1–2% at maturity. The principal source of water for tomato fruit growth was phloem sap. Based on the daily rates of net dry matter accumulation, respiration and phloem water import, the calculated dry matter concentration of the phloem sap declined from 7 to 3%, or from 12.5 to 7.8% during fruit development in low or high salinity, respectively. The similar dry matter accumulation of fruit grown at different salinities was due to changes in both volume and concentration of phloem sap. Potassium salts in tomato fruit were calculated lo have contributed –0.29, –0.48 and –0.58 MPa to total fruit osmotic potential in the 2, 12 and 17 mS cm^?1 treatments, respectively, which accounted for 38% or 49% of the measured total osmotic potential of the 2 mS cm^?1 or 17 mS cm^?1 treatments. The contribution of hexoses to total fruit osmotic potential in the young fruit was from about –0.1 to –0.2 MPa at all salinities. The osmotic potential of tomato fruit is regulated more by potassium salts than by hexoses.     

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     

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

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

Effect of Abscisic Acid on Sorbitol Uptake in Growing Apple Fruits

Levels of abscisic acid (ABA) in the fruit flesh of developingapples (cv. Golden Delicious) were measured by electron capturegas chromatography. ABA content of the tissue, calculated ona fresh weight basis, decreased at a constant rate from 200µg g^–1 in young fruit to 10 µg g^–1 inolder fruit and then increased when the ripening process commenced.On a whole fruit basis, the ABA level increased during the initialphase of fruit growth, remained constant during the linear growthphase and increased again when fruits started to ripen. During fruit development the ABA content correlated with therate of sorbitol uptake, when measured after discs of fruittissue were incubated in [¹⁴C]sorbitol. Sorbitol uptake washigh during the initial growth phase and declined at a constantrate during fruit development. ABA present in the incubation medium stimulated sorbitol uptakeinto fruit tissue at concentrations higher than 10^–8 M,whereas indolyl-3-acetic acid had no effect on uptake. When comparing sorbitol uptake in different zones of young fruit,it was found that uptake was higher in discs of outer fruitlayers than in discs from inner fruit zones. Key words: Pyrus malus, Apple, Abscisic acid, Sorbitol     

Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesisthat the inhibition of NO₃^– uptake in darkness is dueto feedback control by NO₃^– and/or Asn accumulating inthe roots. Xylem export of N compounds was shown to depend onwater flux in both excised root systems and ¹⁵N-labelled intactplants, suggesting that the shortage of transpiration in darknessmay be responsible for the retention of NO₃^– and Asn inthe roots. This was verified in experiments where the light/darkpattern of transpiration was modulated in intact plants by changingthe relative humidity of the atmosphere. Any decrease of transpirationat night was associated with a concurrent stimulation of NO₃^–and Asn accumulations in the roots. However, the light/darkrhythmicity of NO₃^– uptake was only marginally affectedby these treatments, and thusappeared quite independent fromtranspiration and root NO₃^– or Asn levels. Typically,the maintainance of a constant transpiration during the day/nightcycle did not suppress the inhibition of NO₃^– uptake indarkness, whereas it almost prevented the dark increase in rootNO₃^– and Asn contents. These data strongly support theconclusion that the effect of light on NO₃^– uptake isnot mediated by changes in translocation and accumulation ofN compounds. Key words: Glycine max, light/dark, cycles, nitrate uptake, transpiration, transport of N compounds, accumulation of N compounds     

The Regulation of Citric Acid Accumulation and Carbon Recycling During CAM in Ananas comosus

The carbon balance of shade-grown Ananas comosus was investigatedwith regard to nitrogen supply and responses to high PAR. Netdark CO₂ uptake was reduced from 61.2 to 38.5 mmol CO₂ m^–2in N limited (–N) plants grown under low PAR (60 µmolm^–2 s^–1) and apparent photon yield declined from0.066 to 0.034 (mol 0².mol^–1 photon), although photosyntheticcapacities (measured under 5% CO₂) were similar. Following transferfor 7 d to high PAR (600. µmol m^–2 s^–1), netCO² uptake at night increased by 14% in +N plants, and daytimephotosynthetic capacity was higher, with a maximum value of7.8 µmol m^–2 s^–1. The magnitude of dark CO₂ fixation during CAM was measured asdawn—dusk variations in leaf-sap titratable acidity (H+)and as the proportion of malic and citric acids. The contributionfrom re-fixation of respiratory CO₂ recycling (measured as thedifference between net CO₂ uptake and malic acid accumulation)varied with growth conditions, although it was generally lower(30%) than reported for other bromeliads. Assuming a stoichiometryof 2H+: malate and 3H+: citrate, there was a good agreementbetween titratable protons and enzymatically determined organicacids. The accumulation of citric acid was related to nitrogensupply and PAR regime, increasing from 7.0 mol m–3 (+Nplants) to 18 mol m^–3 (–N plants) when plants weretransferred to high PAR; malate: citrate ratios decreased from13.1 to 2.5 under these conditions. Under the low PAR regime, leaf-sap osmotic pressure increasedat night in proportion to malic acid accumulation. However,following the transfer to high PAR for 7 d, there was a muchgreater depletion of soluble sugars at night which correspondedto a decrease in leaf-sap osmotic pressure. Although a rolefor citric acid in CAM has not been properly defined, it appearsthat the accepted stoichiometry for CAM in terms of gas exchange,titratable acidity, malic acid and osmotic pressure may nothold for plants which accumulate citric acid. Key words: Ananas comosus, CAM, citric acid accumulation, carbon recycling     

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.     

Growth and Phosphate Transport in Barley and Tomato Plants During the Development of, and Recovery from, Phosphate-stress

Barley and tomato plants were cultured in nutrient solutionsincluding 0.15 mol m^–3 H²PO^–₄. The phosphate supplywas discontinued and the subsequent effects on growth, internalphosphorus concentrations, phosphate absorption and translocationwere measured at frequent intervals. Growth rates were at firstunchanged and the internal phosphorus concentration decreased.During this phase the rate of phosphate transport by the rootssometimes increased significantly. Growth slowed more in shootsthan in roots during a second phase of stress development andvisual symptoms of deficiency appeared in tomato but not inbarley. During this phase, enhancement of phosphate uptake capacityreached a maximum in both species. The subsequent decline inuptake capacity was associated with visible symptoms of deficiencydeveloping in barley and intensifying in tomato. When stressedplants were returned to a solution containing 0.15 mol m^–3H₂PO^–₄ rapid absorption continued for several days afterthe internal phosphorus concentration had returned to the levelof the controls. Phosphate toxicity may have been the causeof leaf lesions and necrosis during the ‘recovery’phase. Stomatal conductance in tomato was decreased at an early stageof stress development. Foliar-applied phosphate was absorbedmore rapidly by P-stressed barley leaves than by their controlsand much larger amounts were translocated from the leaves tothe roots.     

Induction and Repression of CAM in Sedum telephium L. in Response to Photoperiod and Water Stress

Lee, H. S. J. and Griffiths, H. 1987. Induction and repressionof CAM in Sedurn relephluni L. in response to photopcnod andwater stress.—J. exp. Bot. 38: 834–841. The introduction and repression of CAM in Sedurn telephiunmL, a temperate succulent, was investigated in watered, progressivelydrouglited and rewatered plants in growth chambers. Measurementswere made of water vapour and CO₂ exchange, titratable acidity(TA) and xylem sap tension. Effects of photoperiod were alsostudied. CAM was induced by drought under long or short days,although when watered no CAM activity was expressed. C₃-CAM intermediate plants were used for the investigation ofwater supply. Those which had received water and those drought-stressedboth displayed a similar nocturnal increase in TA, with a day-nightmaximum (H+) of 69 µmol g^–1 fr. wt. The wateredplants took up CO₂ at a maximum rate of 2?2 µmol m^–2s^–1 only in the light period, while the droughted plantsshowed a maximum nocturnal CO₂ uptake rate of 0?69 µmolm^–2 s^–1. Subsequently, as CAM was repressed, thewatered S. telephiwn displayed little variation in TA, withconstant levels at 42 µmol g^–1 fr. wt. (day 10).After 10 d of drought stress, the CAM characteristics of S.telephiurn were aLso affected, with reduced net CO₂ uptake andH+. The transition between C₃ and CAM in S. telephium can be describedas a progression in terms of the proportion of respiratory CO₂which is recycled and refixed at night as malic acid, in comparisonwith net CO₂ uptake. Recycling increased from 20% (day 1) to44% (day 10) as a result of the drought stress and was highin both the CAM-C₃ stage (no net CO2 uptake at night) and alsoin the drought-stressed CAM stage (reduced net CO₂ uptake atnight). The complete C₃-CAM transition occurred in less than8 d, and the stages could be characterized by xylem sap tensionmeasurements: CAM = 0?50 MPa C₃-CAM = 0?36 MPa C₃ = 0?29 MPa. Key words: CAM, Sedum telephium L., recycling     

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.     

N2 Fixation and Nitrate Uptake by White Clover Swards in Response to Root Temperature in Flowing Solution Culture

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown as simulated swards for 71 d in flowing nutrientsolutions with roots at 11 C and shoots at 20/15 C, day/night,under natural illumination. Root temperatures were then changedto 3, 5, 7, 11, 13, 17 or 25 C and the total N₂, fixation over21 d was measured in the absence of a supply mineral N. Alltreatments were subsequently supplied with 10 mmol m^–2NO₂ ^– in the flowing solutions for 14 d, and the relativeuptake of N by N₂, fixation and NO₃ ^– uptake was compared.Net uptake of K⁺ was measured on a daily basis. Root temperature had little effect on root d. wt over the 35-dexperimental period, but shoot d. wt increased by a factor of3.5 between 3 and 25 C, with the sharpest increase occurringat 7–11 C. Shoot: root d. wt ratios increased from 25to 68 with increasing temperature at 7–25 C. N₂-fixationper plant (in the absence of NO₂ ^–) increased with roottemperature at 3–13C, but showed little change above13 C. The ratios of N₂ fixation: NO₂ ^– uptake over 14d (mol N: mol N) were 0.47–0.77 at 3–7 C, 092–154at 11–17 C, and 046 at 25 C, reflecting the dominanceof NO₃ ^– uptake over N₂ fixation at extremes of high andlow root temperature. The total uptake of N varied only slightlyat 11–25 –C (095–110 mmol N plant^–1),the decline in N₂ fixation as root temperature increased above11 C was compensated for by the increase in NO^– ₃ uptake.The % N in shoot dry matter declined with decreasing root temperature,from 32% at 13 C to 15% at 3 C. In contrast, concentrationsof N expressed on a shoot water content basis showed a modestdecrease with increasing temperature, from 345 mol m^–3at 3 C to 290 mol m^–3 at 25 C. Trifolium repens L, white clover, root temperature, N₂ fixation, potassium uptake, nitrate uptake, flowing solution culture     

The Effect of Temperature and Light on Gas Exchange and Acid Accumulation in the C3-CAM Plant Clusia minor L

Gas exchange and organic acid accumulation of the C₃-CAM intermediateClusia minor L. were investigated in response to various day/nighttemperatures and two light regimes (low and high PAR). For bothlight levels equal day/night temperatures between 20°C and30°C caused a typical C₃ gas exchange pattern with all CO₂uptake occurring during daylight hours. A day/ night temperatureof 15°C caused a negative CO₂ balance over a 24 h periodfor low-PAR-grown plants while high-PAR-grown plants showeda CAM gas exchange pattern with most CO₂ uptake taking placeduring the dark period. However, there was always a considerablenight-time accumulation of malic acid which increased when thenight-time temperature was lowered and had its maximum (54 mmolm^–2) at day/night temperature of 30/15°C. A significantamount of malic acid accumulation (23 mmol m^–2) in low-PAR-grownplants was observed only at 30/15°C. Recycling of respiratoryCO₂ in terms of malic acid accumulation reached between 2·0and 21·5 mmol m_–2 for high-PAR-grown plants whilethere was no significant recycling for low-PAR-grown plants.Both low and high-PAR-grown plants showed considerable night-timeaccumulation of citric acid. Indeed under several temperatureregimes low-PAR-grown plants showed day/night changes in citricacid levels whereas malic acid levels remained approximatelyconstant or slightly decreased. It is hypothesized that lowand high-PAR-grown plants have different requirements for citrate.In high-PAR-grown plants, the breakdown of citrate preventsphotoinhibition by increasing internal CO₂ levels, whereas inlow-PAR-grown plants the night-time accumulation of citric acidmay function as an energy and carbon saving mechanism. Key words: C. minor, C₃, CAM, citric acid, light intensity