Use of transfer function and compartmental analysis to quantify 11C-labelled photoassimilate export from wheat leaves

Compartmental analysis of tracer loss from a leaf after pulse-labellingwith carbon isotopes has often been used to infer the flow ofphotosynthate through the leaf. Recently, a more general approachhas been suggested based upon estimation of the transfer functionusing data from pulse-labelling as well as continuouslabellingexperiments. A comparison of these two approaches shows thatwith the same data set they give equivalent physiological interpretations.The measured decline of ¹¹C activity from a wheat leaf after¹¹CO₂ pulse-labelling was extrapolated by compartmental as wellas transfer function analysis. Both methods estimated a 66.4%loss of the initially fixed ¹¹C due to export and respiration.The advantage of transfer function analysis, however, is itsapplicability to continuous-labelling experiments. The modelallows the use of the net photosynthetic rate as the reference(100%) value. Data from continuous-labelling experiments withwheat plants indicate diurnal variations in the export of freshlylabelled assimilate of between 32.7% and 43.6% of net photosynthesis. Key words: Triticum aestivum L, ¹¹CO₂, carbon partitioning, transfer function analysis, compartmental analysis     

Assimilation and transport of nitrogen in rice II. 15N-labelled nitrate nitrogen

An investigation was made to study the assimilation and transportof ¹⁵N-labelled nitrate nitrogen in rice plant (Oryza sativaL.). Nitrogen from labelled nitrate at the end of plant feedingwas found mainly in nitrate form, and was more prevalent inroots, stem and leaf sheaths. The nitrite fraction had the nextlargest ¹⁵N enrichment. The ¹⁵NO₃ assimilation in the newlyemerged panicle was mainly in amide and amino acid. The ¹⁵N-incorporation at day 0 was greatest in amino acid andnitrate of roots and decreased towards the stem and leaves.Incorporation in these fractions considerably decreased fromday 0 to day 10. Probably most of the nitrogen from the nitratesource was transported from the roots to the shoot in nitrateand amino acid forms. A decrease of ¹⁵N-incorporation in the soluble N fraction andincrease in the insoluble N fraction from day 0 to day 10 inplant parts, particularly the blades, suggested that proteinsynthesis occurred mostly in young parts of the shoot duringthis period. The marked variation in ¹⁵N distribution in differentparts of the plant during the 10 days indicated that the nitrogenin roots and tillers was probably remobilized and transportedto other parts, particularly the upper leaf blades. Ammonium and nitrate nitrogen transport in rice plant are compared. (Received May 11, 1974; )     

Short-term Effects of Heat-girdles on Source Leaves of Vicia faba: Analysis of Phloem Loading and Carbon Partitioning Parameters

Petiole heat-girdle treatments (followed by a 5 min ¹⁴CO₂ assimilation)were performed on mature leaves of Vicia faba, in order to assesstheir effect on the partitioning of photo-assimilates to theminor vein phloem. Whole leaf autoradiographic evidence indicateda high leaf-to-leaf variation in the image intensity over theminor veins (relative to the mesophyll/epidermal background)in both control and heat-girdled groups of leaves. The averagedegree of minor vein labelling in heat-girdled leaves, however,was found to be significantly lower than that in controls. Comparativeassessment of vein labelling was based on microscopic densityreadings of silver grains over veinal and interveinal regionsin autoradiographic images. Investigations into the cause ofthis alteration in vein labelling indicated no involvement ofan inhibition of apoplasmic phloem loading, as both heat-girdledand control leaves of Vicia were shown to have comparable minorvein uptake of exogenously supplied ¹⁴C-sucrose. Heat-girdlingwas shown, however, to increase significantly the partitioningof recently fixed carbon into the insoluble (mainly starch)fraction relative to the ethanol-soluble fraction, within 12min of the treatment. We suggest that this carbon partitioningchange can primarily account for the change in vein labelling,since an increase in the insoluble fraction would result in(1) more ¹⁴C-activity remaining in the leaf mesophyll and (2)less ¹⁴C-activity going into the mesophyll export pool, andthus, less ¹⁴C-sucrose being transferred to the minor vein region.Additionally, although leaf export was completely halted inheat-girdled leaves, ¹⁴C-activity was found within the majorveins as far as the point of petiole heat-girdling (followinga 5 min assimilation and 4 h chase). Apparently, continued (butlimited) solution flow within the sieve elements is maintainedby transport pathway unloading within the treated leaves. Key words: Phloem loading, carbon partitioning, heat-girdle, Vicia faba     

Nitrate Reduction and Partitioning of Nitrogen in Komatsuna (Brassica campestris L. var. rapa) Plants: Compartmental Analysis in Combination with 15N Tracer Experiments

Komatsuna (Brassica campestris L. var. rapa cv. Misugi) is aleafy vegetable that readily accumulates nitrate in its tissues.Plants grown hydroponically with 2 mM nitrate in a greenhousewere fed ¹⁵N-labeled nitrate for 2 h, followed with nonlabelednitrate for 8 h. At intervals of 2 h, the plants were sampledand analyzed for the distribution of 15N in the nitrate, aminoacids, and proteins in the tissues of roots, petioles plus midribs,and leaves. Nitrate reduction and nitrogen fluxes were examinedusing a compartmental analysis with 19 compartments and 28 transferrate constants. Nitrate existed in the three types of tissues as a large storagepool and a small metabolic pool. Nitrogen reduction was observedin these tissues, but mainly in the leaf tissue. Nitrate uptakeand reduction rates were smaller in the dark than in light,and particularly nitrate reduction in the shoot was less inthe dark. The rate of protein synthesis was much greater inthe light. The simulation, using compartment models and ¹⁵Ndistribution data, may be useful for estimating actual ratesof nitrogen transport and metabolism in the whole plant system. (Received October 15, 1986; Accepted March 26, 1987)     

Transport of 14C-IAA in Cucurbita maxima: A Comparative Study of Rapid, Non-Polar Movement and Slow, Polar Auxin Transport

The transport of ¹⁴C-IAA has been studied in Cucurbita maxima.IAA fed to the leaf of an intact plant moves rapidly in a non-polarfashion in the phloem. Collection and analysis of exudate fromsevered sieve tubes showed that there was no metabolic conversionor complexing of IAA for several hours. Polar movement of ¹⁴C-IAA in isolated internode segments occursat rates an order of magnitude slower than movement in the phloem.The importance of discrete and isolated channels of hormonetransport, that vary in direction and rates, is briefly discussed.     

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     

Assimilation and transport of nitrogen in rice I. 15N-labelled ammonium nitrogen

The assimilation and transport of ¹⁵N-labelled ammonium nitrogenin rice plants (Oryza sativa L.) was studied. Plants assimilatedlarge amounts of nitrogen from labelled ammonium into theiramides and amino acids, particularly in the roots and stem,at the end of a 4-day ¹⁵N feeding and 10 days later in the upperleaves, especially in the blades. Although the incorporationof ¹⁵N into all the nitrogen fractions of the newly emergedpanicle was evident, it was particularly pronounced in the amidesand amino acids of the soluble fractions. The upper leaves hada greater ¹⁵N incorporation in their organic N-fractions thandid the lower ones. Amides and amino acids are considered tobe the main forms of nitrogen transported to the shoot fromthe ammonium assimilated in the roots. The transport of theorganic forms of nitrogen was possibly greater to the upperleaves than to the lower ones. The nitrite fraction had more ¹⁵N than did the nitrate fractionin all parts of the plant, particularly in the upper leaf blades.It appeared that some of the ammonia might have been oxidizedto nitrite, then to nitrate in some parts of the plant; probablyin the upper leaves. The synthesis of protein and nucleic acid occurred rapidly inthe upper leaves, especially in the blades, also in the rootsas evidenced by the considerable incorporation of ¹⁵N in theinsoluble fractions of these parts. The variation in ¹⁵N-distribution,during the 10 days, in the different plant parts suggests thatthe nitrogen incorporated during protein synthesis in the rootsand tillers was remobilized and transported to the upper partsof the shoot. A concept for the transport of organic nitrogenouscompounds from the roots to shoot through the phloem and xylemof the rice plant stem is discussed. (Received May 11, 1974; )     

Cation and Chloride Partitioning through Xylem and Phloem within the Whole Plant of Ricinus communis L. under Conditions of Salt Stress

Uptake and partitioning through the xylem and phloem of K⁺,Na⁺, Mg²⁺ , Ca²⁺ and Cl^– were studied over a 9 d intervalduring late vegetative growth of castor bean (Ricinus communisL.) plants exposed to a mean salinity stress of 128 mol m^–3NaCl. Empirically based models of flow and utilization of eachion within the whole plant were constructed using informationon ion increments of plant parts, molar ratios of ions to carbonin phloem sap sampled from petioles and stem internodes andpreviously derived information on carbon flow between plantsparts in xylem and phloem in identical plant material. Salientfeatures of the plant budget for K⁺ were prominent depositionin leaves, high mobility of K⁺ in phloem, high rates of cyclingthrough leaves and downward translocation of K⁺ providing theroot with a large excess of K⁺ . Corresponding data for Na⁺showed marked retention in the root, lateral uptake from xylemby hypocotyl, stem internodes and petioles leading to low intakeby young leaf laminae and substantial cycling from older leavesback to the root. The partitioning of the anionic componentof NaCl salinity, Cl^–, contrasted to that of Na⁺ in thatit was not substantially retained in the root, but depositedmore or less uniformly in stem, petiole and leaf lamina tissues.The flow pattern for Mg²⁺ showed relatively even depositionthrough the plant but some preferential uptake by young leaves,generally lesser export than import by leaf laminae, and a returnflow of Mg²⁺ from shoot to root considerably less than the recordedincrement of the root. Ca²⁺ partitioning contrasted with thatof the other ions in showing extremely poor phloem mobility,leading to progressive preferential accumulation in leaf laminaeand negligible cycling of the element through leaves or root.Features of the response of Ricinus to salinity shown in thepresent study were discussed with data from similar modellingstudies on white lupin (Lupinus albus L.) and barley (Hordeumvulgare L.) Key words: Ricinus communis L, potassium, sodium, chloride, calcium, magnesium, phloem, xylem, transport, partitioning, salinity     

The Distribution of Nitrate Assimilation Between Root and Shoot in Vicia faba L.

Nitrate assimilation was examined in two cultivars (Banner Winterand Herz Freya) of Vicia faba L. supplied with a range of nitrateconcentrations. The distribution between root and shoot wasassessed. The cultivars showed responses to increased applied nitrateconcentration. Total plant dry weight and carbon content remainedconstant while shoot: root dry weight ratio, total plant nitrogen,total plant leaf area and specific leaf area (SLA) all increased.The proportion of total plant nitrate and nitrate reductase(NR) activity found in the shoot of both cultivars increasedwith applied nitrate concentrations as did NO₃^–: Kjeldahl-Nratios of xylem sap. The cultivars differed in that a greaterproportion of total plant NR activity occurred in the shootof cv. Herz Freya at all applied nitrate concentrations, andits xylem sap NO₃^–: Kjeldahl-N ratio and SLA were consistentlygreater. It is concluded that the distribution of nitrate assimilationbetween root and shoot of V. faba varies both with cultivarand with external nitrate concentration. Vicia faba L., field bean, nitrate assimilation, nitrate reductase, xylem sap analysis     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

The Partitioning of Photosynthetically Assimilated 14C into Amino Acids in Wheat 1. Partitioning During Transport to the Grain

When ¹⁴CO₂ was fed to flag leaf laminae at 20 d post-anthesis,the transport organs between the leaf and the grains containedappreciable ¹⁴C in glutamine, glutamate, serine, alanine, threonineand glycine. Smaller amounts of ¹⁴C were present in gamma-aminobutyricacid (GABA), aspartate and cysteine. Other amino acids whichwere labelled in the source leaf were not labelled in the transportorgans. The export of labelled glutamine, serine, glycine andthreonine from the source leaf was favoured in comparison tothe other amino acids mentioned. Threonine accumulated, andwas subsequently metabolised, in the rachis. [¹⁴C]GABA alsoaccumulated in the rachis. In the grains, the relative amountof soluble [¹⁴C]alanine increased with chase time. This wasprobably due to de novo synthesis and reflected the specialrole of alanine in grain nitrogen metabolism. Wheat, Triticum aestivum, ¹⁴CO₂, amino acids, transport, carbon metabolism     

Translocation from the Flag Leaf of Winter Wheat in the Field

Translocation of assimilate from the flag leaf of winter wheat(Triticum aestivum cv. Maris Huntsman) was studied in the fieldby monitoring the export of photo-assimilated [^l4C]carbon dioxidewith a Geiger-Miiller counter placed under the fed area of leaf.The resulting export curve was analysed as a sum of two exponentialterms, and interpreted as a two-pool compartmental system. Therate constant for export from the leaf increased slightly frommaximum elongation to anthesis, then declined to almost halfits peak value just before the leaf lost all visible chlorophyll.The inter-pool transfer rate constants did not change significantlyover the same period, but all rate constants varied with timeof day. Short-term changes in the environment of the flag leaf had nodiscernible influence on translocation in the field. The timeconstants of the two pools of assimilate agreed with those forother species reported in the literature. These results areconsistent with the suggestion that sucrose is stored in thevacuole of mesophyll cells. The variations in rate constants with time of day, and deviationsof the export data from the two-pool model, suggest that exportand inter-pool transport have saturation kinetics. A model withMichaelis-Menten kinetics was formulated, and simulations ofthis model showed similar deviations from a simple two-poolsystem to those seen in our data.     

Ionic Relations of Garden Orache, A triplex hortensis L.: Growth and Ion Distribution at Moderate Salinity and the Function of Bladder Hairs

The growth of garden orache, A triplex hortensis was studiedunder conditions of mild NaCl or Na₂SO₄ salinity. Growth, drymatter production and leaf size were substantially stimulatedat 10 mM and 50 mM Na⁺ salts. Increased growth, however, appearedto be due to a K⁺-sparing effect of Na⁺ rather than to salinityper se. The distribution of K⁺ and Na⁺ in the plant revealeda remarkable preference for K+ in the roots and the hypocotyl.In the shoot the K/Na ratio decreased strongly with leaf age.However, the inverse changes in K⁺ and Na⁺ content with leafage were dependent on the presence of bladder hairs, which removedalmost all of the Na⁺ from the young leaf lamina. Measurementsof net fluxes of K⁺ and Na⁺ into roots and shoots of growingAtriplex plants showed a higher K/Na selectivity of the netion flux to the root compared to the shoot. With increasingsalinity the selectivity ratio S_{K, Na}^* of net ion fluxes tothe roots and to the shoots was increased. The data suggestthat recirculation of K⁺ from leaves to roots is an importantlink in establishing the K/Na selectivity in A. hortensis plants.The importance of K⁺ recirculation and phloem transport forsalt tolerance is discussed. Key words: Atriplex hortensis, Salinity, Potassium, Sodium, K⁺ retranslocation, Bladder hairs, Growth stimulation     

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.     

The Effect of Clipping on Net Photosynthesis and Dark Respiration Rates of Plants from an Upland Grassland, with Reference to Carbon Partitioning in Festuca ovina

Four co-existing species (Deschampsia flexuosa, Festuca ovina,Juncus squarrosus and Nardus stricta) were subjected to clippingand the net photosynthetic and dark respiration rates were followedafter this treatment for 50 d. Concurrently carbon partitioningin F. ovina plants clipped initially and again at 50 and 100d was examined. An expansion of new leaf lamina was observed with F. ovina,which had a greater net photsynthetic rate per unit leaf areathan unclipped lamina. The remaining leaf lamina (stubble) afterclipping also showed net photosynthetic and dark respirationrates greater than unclipped lamina; these responses were uniqueto F. ovina plants. N. stricta was the only other species toattain a pre-clipping photosynthetic rate within 6 d. Clipped F. ovina plants showed a change in carbon allocationpattern, with a reduction in carbon allocated to roots. ¹⁴Caccumulated in roots and stubble was shown to have a role inregrowth, as was current assimilate via the production of newleaf lamina. Plants initially clipped before exposure to ¹⁴C,redistributed less ¹⁴C to new shoot growth and, therefore, lostless when subsequently clipped. Further redistribution of ¹⁴Ccame from leaf stubble tissue and not at the expense of roots.The variation between species in clipping response are discussedin terms of the implications for coexistence. Carbon partitioning, clipping, gas exchange, grasses     

Salt Tolerance in the Halophyte Suaeda maritima L. Dum.: Intracellular Compartmentation of Ions

The time-course of exchange of sodium and potassium ions fromroot and leaf material of the halophyte Suaeda maritima hasbeen followed and the data analysed according to the phenomenologyof efflux, or compartmental, analysis. Sodium ions were exchangedmuch more slowly (c. 4 times) from the vacuoles of leaf cellsof plants grown in sodium chloride than were potassium ionsfrom the vacuoles of leaf cells of plants grown either in similarconcentrations of potassium chloride or in low concentrationsof potassium. In plants grown in sodium chloride, sodium ionswere exchanged 9 times more slowly from the vacuoles of leafcells than from the vacuoles of root cells. The concentration of sodium ions in the cytoplasm of leaf cellsof plants growing in 340 mol m^–3 sodium chloride was estimatedto be 165 mol m^–3 when the average concentration in theleaf tissue was about 600 mol m^–3. As measured by movement from mature to developing leaves inintact plants; there was less in vivo retranslocation of ²²Naand ³⁶CI in plants growing in sodium chloride than there wasof ⁸⁶Rb in plants growing either in potassium chloride or innon-saline conditions. The results are discussed in terms of the concept and energeticsof compartmentation of ions in the cells of halophytes.     

Short-Term Fluctuations in the Transport of Assimilates to the Ear of Wheat Measured with Steady-State 11C-CO2-Labelling of the Flag Leaf

In order to study the regulation of photosynthate import intothe ear of wheat (Triticum aestivum L. cv. Anza), a system wasdeveloped in which the flag leaf, the major source of photosynthatefor the ear, was provided with steady-state levels of ¹¹C-CO₂for periods of several hours each time. ¹¹C is a short-livedisotope (1? c. 20?3 mm) whose breakdown products (followingpositron annihilation) include paired, high energy -rays (0?5MeV). Consequently, the movement of radioactive photosynthatethroughout the plant and into the ear could be studied in vivoduring multiple labelling sessions over the development of theear with detectors located at various positions around the plant.It was found that import into the ear was not constant in thelight during periods of rapid ear filling, even though flagleaf photosynthesis and export were constant. In more than 50%of all cases, import of ¹¹C-photosynthate into the ear tookthe form of large, regular oscillations with period lengthson the order of 70 mm. Furthermore, oscillations 180? out-of-phasewith those in the ear were observed in portions of the stembelow the point of flag leaf insertion, suggesting that regulationof oscillation may involve shifting the direction of transportbetween ear and lower plant parts. Rhythms in photosynthateimport into the ear do not appear to be synchronized to immediatefluctuations in the environment, since oscillations detectedsimultaneously in neighbouring plants were not synchronizedand had different period lengths. Key words: Triticwn aesrivum L., ¹¹CO₂, radioactive photosynthate, flag leaf, wheat ear     

Modelling Nitrogen Content and Distribution in Cauliflower (Brassica oleracea L.botrytis )

A model of nitrogen uptake and distribution is presented whichdescribes these processes in relation to the amount of availablesoil nitrate and the rate of plant growth. Nitrogen uptake iseither sink or source limited. Sink limitation is based on maximumN-concentrations of plant compartments. The N-uptake model iscombined with a photosynthesis model based on the productivity-nitrogenrelationship at the single-leaf level. The model is parameterizedusing cauliflower as an example crop. Applied to an independentdata set, the combined model was able to predict leaf, stemand inflorescence nitrogen concentrations with correlation coefficientsbetween predicted and simulated values of 0.89, 0.66 and 0.86,respectively. The influence of nitrogen supply and light intensityon leaf nitrate-N could also be predicted with good accuracy(r² = 0.87). Dry matter production based on the productivity-Nrelationship and the partitioning into leaf, stem and inflorescencewas also reproduced satisfactorily (r² = 0.91, 0.93 and 0.92,respectively). Copyright 2000 Annals of Botany Company Brassica oleracea L. botrytis, cauliflower, nitrogen, nitrate, nitrogen supply, nitrogen uptake, nitrogen distribution, model     

The high affinity K+ transporter AtHAK5 plays a physiological role in planta at very low K+ concentrations and provides a caesium uptake pathway in Arabidopsis

Caesium (Cs⁺) is a potentially toxic mineral element that isreleased into the environment and taken up by plants. AlthoughCs⁺ is chemically similar to potassium (K⁺), and much is knownabout K⁺ transport mechanisms, it is not clear through whichK⁺ transport mechanisms Cs⁺ is taken up by plant roots. In thisstudy, the role of AtHAK5 in high affinity K⁺ and Cs⁺ uptakewas characterized. It is demonstrated that AtHAK5 is localizedto the plasma membrane under conditions of K⁺ deprivation, whenit is expressed. Growth analysis showed that AtHAK5 plays arole during severe K⁺ deprivation. Under K⁺-deficient conditionsin the presence of Cs⁺, Arabidopsis seedlings lacking AtHAK5had increased inhibition of root growth and lower Cs⁺ accumulation,and significantly higher leaf chlorophyll concentrations thanwild type. These data indicate that, in addition to transportingK⁺ in planta, AtHAK5 also transports Cs⁺. Further experimentsshowed that AtHAK5 mediated Cs⁺ uptake into yeast cells andthat, although the K⁺ deficiency-induced expression of AtHAK5was inhibited by low concentrations of NH     

The importance of linoleic acid and linolenic acid as precursors of hexanal and trans-2-hexenal in black tea

During tea fermentation, linoleic acid in the neutral fat fraction,and linolenic acid in both the neutral fat and phospholipidfractions from leaves decreased. The addition of linoleic orlinolenic acid to leaf macerates during fermentation resultedin an increase in hexanal or trans-2-hexenal in the volatilefraction. Tracer experiments showed the direct conversion oflinoleic-U-¹⁴C and linolenic-U-¹⁴C acids to labeled hexanaland trans-2-hexenal, respectively, which were identified as2,4-DNPH derivatives. Further conversion of hexanal and trans-2-hexenal into hexanoicand trans-2-hexenoic acids during tea fermentation was suggestedby the increases in these compounds after the addition of hexanaland trans-2-hexenal to leaf macerates. (Received December 21, 1971; )