Changes in the Nitrogen Compounds of Xylem Sap of Mulberry (Morus alba L.) during Regrowth after Pruning

Changes in fresh and dry weights and total nitrogen in stemsections of mulberry (Morus alba L.) and seasonal fluctuationin the amounts of exudate, pH and nitrogen compounds in xylemsap from cut stems of the plant after pruning have been studied.The amount and concentrations of nitrogen compounds in the sapchanged during the experimental period, but nitrate-nitrogenand ammonia-nitrogen were constant constituents, and asparaginewas quantitatively the major organic nitrogen compound. Smalleramounts of glutamine, aspartic acid and glutamic acid were alsofound, but no ureides or alkaloids were detected. Relationshipsbetween growth and nitrogen in xylem sap of mulberry and therole and importance of asparagine in nitrogen metabolism arediscussed. Morus alba L., mulberry, asparagine, exudates, nitrogen, amino-acids, xylem sap, stem growth, aspartic acid, glutamic acid, ureides     

Abscission in Coleus: Light and Phytohormone Control

Light control of leaf abscission in Coleus (Coleus blumei Benthcv. Ball 2719 Red) appears to be regulated by the quantity ofendogenous auxin transported from the leaf blade to the abscissionzone. Gas chromatographic—mass spectrophotometric analysisindicated that diffusate collected from leaf tissue treatedwith red light contained significantly higher levels of auxinthan dark and far-red light-treated leaf tissue. In addition,diffusate from red light-treated tissue inhibited abscissionof leafless petioles while diffusate from far-red light-treatedtissue promoted abcission when compared with diffusate fromdark-treated tissue. The effect of red light on abscission couldbe mimicked by IAA, but not by other phytohormones. An auxintransport inhibitor, 2, 3, 5-triiodobenzoic acid (TIBA), appliedeither as a lanolin ring around the petiole or vacuum infiltratedinto tissue, could completely eliminate any red light effecton abscission. The data are consistent with a phytochrome-mediatedlight regulation of endogenous auxin level in the leaf whichthen controls abscission. Key words: Abscission, Coleus, IAA, plant hormones, red (far-red) light, TIBA     

Modelling of the Partitioning, Assimilation and Storage of Nitrate within Root and Shoot Organs of Castor Bean (Ricinus communis L.)

An experimentally-based modelling technique was developed todescribe quantitatively the uptake, flow, storage and utilizationof NO₃-N over a 9 d period in mid-vegetative growth of sandcultured castor bean (Ricinus communis L.) fed 12 mol m^–3nitrate and exposed to a mean salinity stress of 128 mol m^–3NaCl. Model construction used information on increments or lossesof NO₃-N or total reduced N in plant parts over the study periodand concentration data for NO₃-N and reduced (amino acid) Nin phloem sap and pressure-induced xylem exudates obtained fromstem, petiole and leaf lamina tissue at various levels up ashoot. The resulting models indicated that the bulk (87%) of incomingnitrate was reduced, 51% of this in the root, the remainderprincipally in the laminae of leaves. The shoot was 60% autotrophicfor N through its own nitrate assimilation, but was oversuppliedwith surplus reduced N generated by the root and fed to theshoot through the xylem. The equivalent of over half (53%) ofthis N returned to the root as phloem translocate and, mostly,then cycled back to the shoot via xylem. Nitrate comprised almosthalf of the N of most xylem samples, but less than 1% of phloemsap N. Laminae of leaves of different age varied greatly inN balance. The fully grown lower three leaves generated a surplusof reduced N by nitrate assimilation and this, accompanied byreduced N cycling by xylem to phloem exchange, was exportedfrom the leaf. Leaf 4 was gauged to be just self-sufficientin terms of nitrate reduction, while also cycling reduced N.The three upper leaves (5–7) met their N balance to varyingextents by xylem import, phloem import (leaves 6 and 7 only)and assimilation of nitrate. Petioles and stem tissue generallyshowed low reductase activities, but obtained most of theirN by abstraction from xylem and phloem streams. The models predictedthat nodal tissue of lower parts of the stem abstracted reducedN from the departing leaf traces and transferred this, but notnitrate, to xylem streams passing further up the shoot. As aresult, xylem sap was predicted to become more concentratedin N as it passed up the shoot, and to decrease the ratio ofNO₃-N to reduced N from 0·45 to 0·21 from thebase to the top of the shoot. These changes were reflected inthe measured N values for pressure-induced xylem exudates fromdifferent sites on the shoot. Transfer cells, observed in thexylem of leaf traces exiting from nodal tissue, were suggestedto be involved in the abstraction process. Key words: Ricinus communis, nitrogen, nitrate, nitrate reduction, partitioning, phloem, xylem, flow models     

Flower-inducing and -inhibiting activities of Phloem Exudate from Cotyledons of Pharbitis Seedlings

The flower-inducing and -inhibiting activities of phloem exudate (PE) prepared from cotyledons of Pharbitis seedlings were examined, using apex cultures in vitro from Pharbitis as a bioassay system.The PE was prepared from photoperiodically-induced cotyledons (SD-PE). The SD-PE was subjected to the following fractionations: When the SD-PE was extracted with CHCl₃ and then ethyl acetate, the inducing activity was located in the final aqueous fraction. The activity was localized in the diffusate when the aqueous fraction was dialyzed (molecular weight cut off was 10,000). The diffusate was fractionated by ion exchange chromatography, and flower-inducing activity was found in the fraction adsorbed onto anion exchange resin. When the fraction was applied to a Sep-Pak C18 cartridge, the activity eluted with 25% MeOH. As a result of the above fractionation, activity was increased about 30-fold.The nature of the flower-inhibiting activity of the PE taken from cotyledons exposed to continuous-light conditions was examined (CL-PE). The inhibiting activity was decreased as the cotyledons were exposed to longer dark periods; it appeared to be heat-stable. The CL-PE also inhibited flowering in Lemna. The CL-PE was subjected to the following fractionations: When the CL-PE was extracted with CHCl₃ and ethyl acetate, activity was located in the final aqueous fraction. Activity was localized in the diffusate when the aqueous fraction was dialyzed (molecular weight cut off was 10,000). When the diffusate was fractionated by ion exchange chromatography, the activity was found in the flow-through fraction. When the fraction was applied to a hydroxyapatite cartridge, the activity eluted with 25 mM sodium phosphate buffer. When the fraction was re-dialyzed (molecular weight cut off was 1,000), the diffusate contained the activity. As a result of the above fractionation, activity was increased about 10-fold.     

Phyllody Malformation in Flowers of Rosaxhybrida cv. Motrea: Effects of Rootstocks, Flower Position, Growth Regulators and Season

Phyllody, a common malformation of Rosaxhybrida cv. Motrea,is characterized by the formation of vegetative structures inplace of stamens and carpels. The incidence of phyllody in thiscultivar is dependent on the type of the root system. The highestproportion of malformed flowers was found in ungrafted plants,this decreased in plants grafted on R. indica major and waslowest on R. canina inermis. Flowers formed on shoots, whichwere located closer to the roots, were more prone to phyllodythan those grown on the upper part of the plants. The incidenceof phyllody was lower in winter than in summer, while shadingof the plants reduced the proportion of malformed flowers regardlessof their root system and the season. Application of gibberellicacid (GA₃) or benzylaminopurine (BAP) greatly promoted the incidenceof phyllody, while paclobutrazol (PBZ) reduced it. The activityof endogenous cytokinins in the xylem exudates from ungraftedplants was higher than in exudates from those grafted on R.indica major, and the lowest activity was found in exudatesfrom plants grafted on R. canina inermis. The involvement ofplant hormones in the development of floral organs in rose flowersis discussed. Key words: Cytokinins, flower malformation, gibberellic acid, homeosis, phyllody, roses     

The Resumption of Cambial Activity and the Change of Peroxidase Isozymes in Broussonetia papyrifera

Five Broussonetia papyrifera (L.) Vent. trees were selected in a natural stand located on the campus of Peking University, Beijing, China. The trees were ca. 5-6 years old, 3-4 m tall,and had diameters of about 3 cm measured 1.2 m above ground level. They were samplied at monthly intervals between January 28 and March 25, then at ten-day intervals between March 25 and May 20,1991. On each occasion, one 3-year-old shoot was cut from the tree. Two blocks (about 1 cm ×1 cm) contained peridern,phloem,cambium and wood with more than one annual ring were cut from every shoot,fixed in FAA,and then were prepared for anatomical studies. And on each occasion,7 layers of tissues (from periderm to mature xylem)were scraped off from the shoots and 100 mg of separate tissues were randomly extracted in 0.1 ml of 20% sucrose. The extracts were used for isoelectric-focusing in polyacrylamide gel slabs (85 mm × 60 mm × 1 mm). Benziding and odianisidine was used as substrate. After electrophoresis the gel slabs were placed in the substrate buffer until the isozyme bands were visible. Owing to the ring-porous structure of the wood of Broussonetia papyrifera, the cambial activity was comparable with that in the most ring-porous dicots. The cambium activity started about ten days before bud sprouting. On April 4,the dormant cambial zone consisted of ca. 4 cell layers. The trees did not sprout until April 16,but ca. 2 cell layers of immature xylem and phloem were formed concomitantly. Ten days later, 8-9 cell layers of xylem and ca. 5 cell layers of phloem were formed. The formation of immature phloem cells continued to increase slowly between April 4 and May 20, whereas that of immature xylem cells increased rapidly between April 4 and April 26,and then decreased between April 26 and May 20. It was suggested that differentiation of immature xylem into mature xylem lasted ca. 10 days,whereas that of immature phloem into mature one lasted ca. 20 days. There were totally 6 peroxidase isozyme bands in dormant cambial region and functional phloem. Variation of zymogram in cambial region occurred before cambial activity activated which is followed by more or less minor changes of bands in all other tissues. These indicated that several significant changes were related to the level of endogenous IAA and differentiation of vascular tissues.     

Translocation of copper and other micronutrients in tomato plants (Lycopersicon esculentum Mill.): nicotianamine-stimulated copper transport in the xylem

The influence of the endogenous micronutrient chelator, nicotianamine(NA), and of Cu nutrition on the distribution of Cu, Fe, Mn,Zn, and NA was investigated in eight different shoot organs,roots, and in xylem exudates of the NA-containing tomato wildtype Lycopersicon esculentum Mill. cv. Bonner Beste and itsNA-less mutant chloronerva. Contrary to the other heavy metals, copper transport in thexylem was inefficient in the mutant and was enhanced by an applicationof NA to the roots or leaves in proportion to the applied NAconcentration. Also, with NA application, the Cu concentrationin mutant roots decreased significantly, and increased in theshoot. Fe and Mn transport in the xylem was greater in the mutantthan in the wild type, and was decreased in the mutant by theapplication of NA to the leaves. Zn transport in the xylem wasthe same in both genotypes and was unaffected by NA application.After application of NA to leaves and roots of the mutant itwas possible to detect NA in the xylem exudate (up to 2nmolNA(g^–1 root FWh^–1). High Cu supply (3 µM) resulted in higher Cu and Mn concentrationsin all organs of the wild type as compared to mutant organs,but Fe concentrations were not influenced. Under high Cu supply(3µM) the NA concentrations of roots and the three youngestleaves of the wild type were higher than under normal Cu supply(0.3 µM). The highest concentrations were found in theshoot apex under both Cu conditions (up to 361 nmol NAg^–1FW). It is concluded from our experiments and from the high stabilityconstant of the NA-Cu-complex (log K= 18.6) that NA is involvedin Cu translocation whereas for the translocation of Fe, Mn,and Zn, NA is not essential. Key words: Copper transport, micronutrients, mobilization, nicotianamine, xylem     

Fibres and Fibre-sclereids in Wild-typeArabidopsis thaliana

Examination of the fibre system in roots and shoots ofArabidopsisthaliana(L.) Heynh. revealed three types of fibres, distinguishedaccording to their site of differentiation: (1) short fibre-sclereidsin the secondary phloem of roots and shoots at the rosette level;(2) long fibres in the secondary xylem of the main root; and(3) very long fibres in the xylem of the inflorescence stems.These three types are in addition to the small number of primaryphloem fibres that are formed even in smallA. thalianaindividuals.These findings provide a basis for the use ofArabidopsis thalianaasa model system to study the processes leading to fibre differentiationin dicotyledons. Arabidopsis thaliana; differentiation; fibre-sclereids; inflorescences; phloem fibres; xylem     

Effect ofLupinus seed diffusates onBradyrhizobium sp. growth and nodulation of lupine

Seeds of three species of lupine (Lupinus termis, L. triticale andL. albus) were tested to determine if the seed contains diffusable substances toxic to bradyrhizobia.L. albus seeds were less toxic to bradyrhizobia, followed byL. triticale. Six strains ofBradyrhizobium were evaluated for their resistance to the toxic substances in lupine seeds. Zones of growth inhibition were determined on yeast-mannitol-agar medium surrounding surface-sterilized seed. The effect of surface sterilization of seeds by different chemical treatments on seed toxicity was assessed. Seeds soaked in water for 1 h before placing on agar surface significantly decreased the inhibition zone. Also, the effect of soaking seeds in water for 4 h before planting and inoculation on nodulation, nitrogen fixation and plant growth were investigated. Addition of seed diffusate to soaked seeds significantly decreased nodulation and plant growth. Autoclaving the seed diffusate had no effect on the toxicity of the seed diffusate. Addition of the absorbent polyvinylpolypyrrolidone (PVPP) to seed diffusates significantly decreased the inhibitory effect of seed diffusate on nodulation and plant growth. Seed diffusate substances were water-soluble, heat-stable and partially bound to PVPP.     

Boron Mobility and Nutrition in Broccoli (Brassica oleracea var. italica

Broccoli (Brassica oleracea var. italica cv Premium Crop) plantswere germinated in soil, transferred to vermiculite three weekslater and grown in the glasshouse, then either supplied continuouslywith boron levels ranging from 0.0 (deficient) to 12.5 (toxic)mg l^–1 of nutrient solution or transferred from 2.5 to0.0 mg B l^–1 at the initiation of inflorescence development.At commercial maturity the concentrations of various inorganicand organic solutes in phloem exudates and xylem saps, as wellas plant characteristics and elemental composition of the variousplant parts, were determined. Under deficient B levels leaf midrib and stem corkiness wereevident, together with signs of stem pith breakdown, symptomswhich resemble the initiation of the hollow stem disorder. Thexylem sap B concentration declined by about 50 % when B wasnot supplied or was removed after a period of adequate supply;the phloem concentration was unaffected. Also, the decreasingB concentration gradients from mature transpiring tissues toyoung developing sinks disappeared. Therefore, it is concludedthat when B is deficient, it is retranslocated from source leavesin the phloem stream supplying the developing leaves and inflorescence.The data also suggested that at toxic levels B undergoes extensivelateral transfer, probably from xylem to xylem, thereby enhancingthe B concentration of developing sinks. The B regime influenced dry-matter partitioning, retranslocationof some elements, and the synthesis and distribution of aminoacids and sugars, reflecting the general nature of B involvementin plant processes. Brassica oleracea var., italica, broccoli, phloem, mobility, retranslocation, boron nutrition, transport fluids, concentration gradients     

Xylem Sap from Actinidia chinensis: Gradients in Sap Composition

In early spring, just prior to leaf break, there were substantialgradients in the concentration of many solutes in xylem sapcollected by vacuum extraction from shoots of different ages,from the branches, trunk and roots of mature Chinese gooseberryvines (Actinidia chinensis Planchon) For most nutrients therewas a progressive fall in concentration from youngest shootsdown to the roots and nitrate was the only nutrient to showa trend in the opposite direction These gradients in concentrationare probably sufficient to account for most of the changes observedin the composition of bleeding xylem sap during the first fewhours of bleeding after a branch is cut or wounded Actinidia chinensis, Chinese gooseberry, kiwifruit, xylem sap, nutrient gradients     

Water Flow Through Cotton Roots in Relation to Xylem Anatomy

The effect of root anatomy on water flow was studied in 7-d-oldcotton (Gossypium hirsutum L.) seedlings grown in solution culture.The total water flux of the intact root system was measuredusing a pressure chamber. Then successive terminal root sectionswere removed at 2,6,10 and 12 cm behind the root tip and theflux was remeasured after each successive cut was made. Xylemdevelopment at different distances behind the root apex wasstudied with a microscope using sections cut free-hand and stainedwith toluidine blue. Water flux increased with the removal ofsuccessive terminal root sections and this coincided with thedegree of basipetal primary xylem development. The large increasein water flux at 10 to 12 cm was associated with secondary xylemdevelopment and increased xylem vessel number. A comparison of water flow and xylem anatomy between roots withtetrarch (Stoneville 506 and Deltapine 41) and pentarch (T25strain) vascular bundle arrangements showed no significant differencesin the measured values of water flux for the primary root. Waterflux, estimated using Poiseuille's equation and measured xylemdimensions, was greater for the tetrarch roots, primarily becauseof the larger diameter of individual vessel elements. The increasednumber of vessel elements in the pentarch primary root of T25did not result in any apparent decrease in axial resistanceto water flow. Key words: Gossypium hirsutum L., roots, vascular bundle, xylem water flux, xylem     

Composition of Amino Compounds Transported in Xylem of Casuarina sp.

Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N₂ fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.     

Deep supercooling xylem parenchyma cells of katsura tree (Cercidiphyllum japonicum) contain flavonol glycosides exhibiting high anti-ice nucleation activity

Xylem parenchyma cells (XPCs) of boreal hardwood species adapt to sub-freezing temperatures by deep supercooling to maintain a liquid state of intracellular water near −40 °C. Our previous study found that crude xylem extracts from such tree species exhibited anti-ice nucleation activity to promote supercooling of water. In the present study, thus, we attempted to identify the causative substances of supercooling. Crude xylem extracts from katsura tree ( Cercidiphyllum japonicum ), of which XPCs exhibited deep supercooling to −40 °C, were prepared by methanol extraction. The crude extracts were purified by liquid–liquid extraction and then by silica gel column chromatography. Although all the fractions obtained after each purification step exhibited some levels of anti-ice nucleation activity, only the most active fraction was retained to proceed to the subsequent level of purification. High-performance liquid chromatography (HPLC) analysis of a fraction with the highest level of activity revealed four peaks with high levels of anti-ice nucleation activity in the range of 2.8–9.0 °C. Ultraviolet (UV), mass and nuclear magnetic resonance (NMR) spectra revealed that these four peaks corresponded to quercetin-3- O - β -glucoside (Q3G), kaempferol-7- O - β -glucoside (K7G), 8-methoxykaempferol-3- O - β -glucoside (8MK3G) and kaempferol-3- O - β -glucoside (K3G). Microscopic observations confirmed the presence of flavonoids in cytoplasms of XPCs. These results suggest that diverse kinds of anti-ice nucleation substances, including flavonol glycosides, may have important roles in deep supercooling of XPCs.     

Regeneration of Tissues in Wounded Stems: A Quantitative Study

When a dicotyledonous stem is wounded by longitudinally splittinga young internode into halves, cells near the cut surface proliferateto form a callus within which vascular tissues differentiateand tend to restore a vascular cylinder in each half. Threephases of regeneration after wounding were identified and quantifiedin stems of three Solanaceous species. (1) In an initial ‘lag’phase, lasting about 2 d, neither cell division nor enlargementwere detected, but mitotic figures were observed within about300 µm of the cut surface. (2) Throughout a second, ‘division’phase, from about days 2–10, cell division and enlargementoccurred. Both were initiated mainly in the two cell layersnearest the surface. A mass of callus formed, with new cellwalls mostly parallel to the surface. Cell enlargement laggedbehind cell division for the first few days, so that mean radialcell diameter decreased until day 6, thereafter remaining almostconstant at 30–40 µm. Towards the end of this phase,mitoses ceased within the callus except in the positions ofthe future vascular and cork cambia, where radial cell diameterfell towards a constant 15–20 µm. (3) During a third,‘differentiation’ phase, cell division was restrictedto the cambial zones, and derivatives differentiated into cork,phloem or xylem according to position. The rate of increasein cell number per transect was 1.5–2.0 cells d^–1,of which more than half was xylem. Capsicum annuum L., sweet pepper, Lycopersicon esculentum Mill., tomato, cambium, cell division, differentiation, regeneration, wounding of stems, xylem     

Cutting stems before relaxing xylem tension induces artefacts in Vitis coignetiae,as evidenced by magnetic resonance imaging

It was recently reported that cutting artefacts occur in some species when branches under tension are cut, even under water. We used non‐destructive magnetic resonance imaging (MRI) to investigate the change in xylem water distribution at the cellular level in Vitis coignetiae standing stems before and after relaxing tension. Less than 3% of vessels were cavitated when stems under tension were cut under water at a position shorter than the maximum vessel length (MVL) from the MRI point, in three of four plants. The vessel contents remained at their original status, and cutting artefact vessel cavitation declined to <1% when stems were cut at a position farther than the MVL from the MRI point. Water infiltration into the originally cavitated vessels after cutting the stem, i.e. vessel refilling, was found in <1% of vessels independent of cutting position on three of nine plants. The results indicate that both vessel cavitation and refilling occur in xylem tissue under tension following stem cutting, but its frequency is quite small, and artefacts can be minimized altogether if the distance between the monitoring position and the cutting point is longer than the MVL.     

Carbon Flow from Nodulated Roots to the Shoots of Soybean (Glycine max L. Merr.) Plants: An Estimation of the Contribution of Current Photosynthate to Ureides in the Xylem Stream

Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulatedroots to the shoots of soybean {Glycine max L. Merr.) plants:An estimation of the contribution of current photosynthate toureides in the xylem stream.–J. exp. Bot. 39: 1015–1023. Well-nodulated, water-cultured soybean plants were allowed toassimilate ¹³CO₂ at a constant specific activity for 10 h andthe ¹³C-labelling of total carbon and ureides in xylem sap wasinvestigated. Labelled carbon appeared very rapidly in the xylem stream. Percentageof labelled carbon (relative specific activity, RSA) in xylemsap was 18% at 2 h after the start of ¹³CO₂ assimilation andreached 53% at the end of the 10 h assimilation. The amountof labelled carbon exported from nodulated roots to the shootsvia the xylem during the 10 h labelling period accounted for33% of total labelled carbon imported into the nodulated roots.Ureides (allantoin and allantoic acid) in xylem sap were stronglydependent on currently assimilated carbon. The RSA of ureidesin xylem sap had reached 83% at the end of the assimilationperiod. Labelled carbon in ureides accounted for 51% of totallabelled carbon returned from nodulated roots to the shootsvia the xylem during the 10 h assimilation period. A treatmentwith 20 mol m^–3 nitrate in the culture medium for 2 ddecreased the ureide concentration in the xylem sap slightly,but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylemsap of nodule-detached plants, it was concluded that the majorityof labelled carbon exported to the xylem stream from noduleswas in ureide form. A considerable amount of carbon was alsoreturned from roots to shoots via the xylem stream but it wasmore dependent on (non-labelled) carbon reserved in the roottissues. Key words: Soybean(Glycine max L.), root nodule, carbon partitoning, ¹³CO₂ assimilation, xylem     

Changes in the Chlorophyll Contents of Leaves and in Levels of Cytokinins in Root Exudates during Ripening of Rice Cultivars Nipponbare and Akenohoshi

Changes in fluxes of cytokinins in exudates transported viathe xylem from roots of rice plants cvs. Nipponbare (a standardJapanese cultivar) and Akenohoshi (a slowly senescing cultivar)were measured by mass spectrometry with deuterium-labeled standards.The fluxes of zeatin (Z), trans-ribosylzeatin (trans-RZ), N⁶-isopentenyladenine(iP), and "conjugated Z" (Z in the hydrolysates of highly polarfractions) decreased from heading to the late ripening stagein both cultivars. At the late ripening stage, iP and Z couldno longer be detected, while the flux of N⁶-isopentenyladenosine(iPA) increased slightly. In Akenohoshi, conjugated Z was thepredominant cytokinin from heading to the middle of the ripeningstage. The flux of each of the cytokinins in Akenohoshi washigher than that in Nipponbare at every time point, with theexception of the flux of iPA just after heading. The total concentrationof cytokinins in the xylem exudate of Akenohoshi was higherthan that of Nipponbare after the middle of ripening stage.The chlorophyll content of the third leaves, which were senescingrapidly, was significantly correlated with the flux of totalcytokinins per plant or per unit leaf area. These results suggestthat the larger amounts of cytokinins, in particular conjugatedZ, transported from the roots to the shoots caused the slowsenescence of leaves in Akenohoshi during the ripening stage. (Received May 9, 1994; Accepted July 1, 1995)