Cytokinins in photoperiodic induction of flowering in Chenopodium species

Changes in cytokinin (zeatin – Z, zeatin riboside – ZR, isopentenyladenine – iP, isopentenyladenosine – iPA) levels were determined under light regimes inductive and non-inductive for flowering in leaves, stems, roots and apical parts of short-day Chenopodium rubrum and long-day Chenopodium murale. In leaves. stems and roots of both plant species the level of cytokinins (in C. rubrum of Z and ZR, in C. murale of Z. ZR, iP and iPA) decreased by about 50% during the dark period and increased again during the subsequent light period, No significant changes in cytokinin levels were observed in continuous light. In apical parts of C. rubrum cytokinin level (Z, ZR, iP) was dramatically increased (by 400–500%) at the end of the dark period and decreased to about the original value during the following light period, while no changes were observed in continuous light. In apical parts of C. murale the level of cytokinins doubled during floral induction consisting of 10 days of continuous light. A red (R) break (15 min at the 6th h of darkness), which prevents flowering in C. rubrum , has no significant effect on cytokinin levels in leaves at the end of darkness. Cytokinin levels increased 1 h after R and decreased again rapidly. On the other hand, the increase of cytokinin level in the apical parts of C. rubrum was largely prevented by the R break. These effects of R on cytokinin levels were not reverted by far-red (FR), while the effect on flowering was reverted. It may be concluded that there is no correlation between changes in cytokinin levels in leaves. Stems and roots and photoperiodic flower induction, as both species, representing different photoperiodic types, showed similar changes under the same light regime. The increase of cytokinin levels in apical parts of both photoperiodic species during floral induction suggests a role (increased cell division and branching) for cytokinins in apex evocation.     

Changes in Concentrations of Cytokinins (CKs) in Root and Axillary Bud Tissue of Miniature Rose Suggest that Local CK Biosynthesis and Zeatin-Type CKs Play Important Roles in Axillary Bud Growth

Involvement of cytokinins (CKs) in axillary bud growth of miniature rose was studied. Variation in root formation and axillary bud growth was induced by two indole 3-butyric acid (IBA) pretreatments in two cutting sizes. At six physiological developmental stages around the onset of axillary bud growth, concentrations of CKs were determined in both root and axillary bud tissue by liquid chromatography combined with electrospray tandem mass spectrometry (LC-ESP-MS/MS). Chronological early onset of axillary bud growth occurred in long cuttings pretreated at low IBA concentration, whereas physiological early root formation was associated with long cuttings and high IBA concentration. The CKs zeatin (Z), isopentenyl adenine (iP), zeatin riboside (ZR), dihydrozeatin riboside (DHZR), isopentenyl adenosine (iPA), zeatin O-glucoside (ZOG), zeatin riboside O-glucoside (ZROG), zeatin riboside 5′-monophosphate (ZRMP), and isopentenyl adenosine 5′-monophosphate (iPAMP) were detected. Concentrations of CKs in axillary bud tissue far exceeded those in root tissue. Indole 3-butyric acid pretreatment influenced the concentration of CKs in axillary bud tissue more than did cutting size, whereas pretreatments only slightly affected CKs in root tissue. The dominant CKs found were iPAMP and ZR. An early and large increase in iPAMP indicated rapid CK biosynthesis in rootless cuttings, suggesting that green parts, including the axillary bud, can synthesize CKs. At the onset of axillary bud growth an increase in concentration of Z, ZR, ZRMP, ZOG, and ZROG was largely coincident with a decrease in iPAMP, iPA, iP, and DHZR. After the onset of axillary bud growth, CK content largely decreased. These results strongly indicate a positive role for CKs in axillary bud growth, and presumably ZRMP, ZR, and Z are active in miniature rose.     

Immunocytochemical localization of cytokinins in Craigella tomato and a sideshootless mutant

Post-embedding immunocytochemical techniques using peroxidase-antiperoxidase or immunoglobulin G-gold as markers were used for the localization of cytokinins (CKs) in two isogenic lines, Craigella (C) and Craigella lateral suppressor (Cls), of tomato Lycopersicon esculentum Mill. Terminal buds, nodes, hypocotyl segments and root tips were submitted to a periodate-borohydride procedure, to obtain the coupling of isopentenyladeosine and zeatin riboside to cellular proteins, followed by a fixative step with a paraformaldehyde and glutaraldehyde mixture. Enzyme-linked immunosorbent assay tests performed on ovalbumin-coated microtitration plates have shown that this method was effective for CK riboside and base coupling to proteins. Paraffin-wax- or Spurr's-resin-embedded sections were cleared of wax or resin before incubation with anti-zeatin riboside or anti-isopentenyladenosine antibodies. The procedure was thoroughly investigated and many controls were done in order to eliminate artefacts. The immunostaining patterns observed along the plants showed a basipetally decreasing gradient of CKs along the stem and in the roots. Immunolabelling was higher in the actively growing regions of the stem bud and root apices. Terminal buds of Cls appeared to be less immunoreactive than C, whereas no differences were detected in root-tip immunolabelling. The staining patterns are consistent with the idea that root and bud apices have a different CK metabolism. The absence of axillary bud formation in Cls is correlated with low CK levels in the organogensis sites.Abbreviations C Craigella, isogenic line - CK cytokinin - Cls Craigella lateral suppressor - EDC 1-(3-dimethylaminopropyl)3-ethylcarbodiimide hydrochloride - ELISA enzyme-linked immunosorbent assay - 2iP isopentenyladenine - 2iPA isopentenyladenosine - PAP peroxidase-anti-peroxidase - PFAG paraformaldehyde/glutaraldehyde mixture - Z zeatin - ZR zeatin riboside     

Rapid increases in cytokinin concentration in lateral buds of chickpea (Cicer arietinum L.) during release of apical dominance

We examined the role of cytokinins (CKs) in release of apical dominance in lateral buds of chickpea (Cicer arietinum L.). Shoot decapitation or application of CKs (benzyladenine, zeatin or dihydrozeatin) stimulated rapid bud growth. Time-lapse video recording revealed growth initiation within 2 h of application of 200 pmol benzyladenine or within 3 h of decapitation. Endogenous CK content in buds changed little in the first 2 h after shoot decapitation, but significantly increased by 6 h, somewhat later than the initiation of bud growth. The main elevated CK was zeatin riboside, whose content per bud increased 7-fold by 6 h and 25-fold by 24 h. Lesser changes were found in amounts of zeatin and isopentenyl adenine CKs. We have yet to distinguish whether these CKs are imported from the roots via the xylem stream or are synthesised in situ in the buds, but CKs may be part of an endogenous signal involved in lateral bud growth stimulation following shoot decapitation. To our knowledge, this is the first detailed report of CK levels in buds themselves during release of apical dominance. Received: 12 December 1996 / Accepted: 7 January 1997     

Response of cytokinin concentration in the xylem exudate of bean (Phaseolus vulgaris L.) plants to decapitation and auxin treatment,and relationship to apical dominance

When xylem exudate of previously untreated Phaseolus vulgaris plants was analysed for cytokinins by radioimmunoassay, a low concentration (about 5 ng · ml^–1) was found. However, when the plants were decapitated about 16 h before the xylem exudate was collected, an almost 25-fold increase in cytokinin concentration was observed. Twenty-four hours after decapitation this increase even reached 4000 compared to control plants. Applying naphthaleneacetic acid (NAA) to the shoot of decapitated plants almost eliminated the effect of shoot tip removal on cytokinin concentration, suggesting that cytokinins in the xylem exudate of intact plants are under the control of the polar auxin transport system. Other xylem constituents, such as potassium or free amino acids did not show this strong increase after decapitation and did not respond to NAA application. It is concluded that the observed auxin/cytokinin interaction has an important regulatory role to play, not only in apical dominance but in many other correlative events as well.Abbreviations AD apical dominance - CKs cytokinin(s) - iAde/iAdo isopentenyladenine/iospentenyladenosine - NAA naphthaleneacetic acid - Z/ZR zeatin/zeatin riboside     

Cytokinin metabolism in nondividing and auxin-induced dividing explants ofHelianthus tuberosus L. Tuber tissue

Aqueous solutions of auxin (indole-3-acetic acid,α-naphthalene acetic acid, or 2,4-dichlorophenoxyacetic acid) were active in inducing DNA synthesis and mitosis in prewashed tissue explants of mature Jerusalem artichoke tubers. Explants did not respond in this way to aqueous solutions of cytokinin (zeatin, zeatin riboside, 6-benzylaminopurine, or kinetin). The metabolism of [8-³H]zeatin riboside (ZR) was studied in non-dividing and auxin-induced synchronously dividing explants over the first 36 h of culture. ZR was taken up rapidly and to the same extent by both tissues. Sequential analysis of tissue extracts by thin-layer and high-performance-liquid chromatography identified zeatin nucleotide(s) (ZN), O-glucosyl zeatin riboside (OGZR), adenosine, and adenine nucleotide(s) (AN) as the principal metabolites in both tissues. The proportion of radio-activity due to ZR declined steadily and OGZR accumulated steadily at similar rates in both tissues. ZN was the major metabolite in both tissues at 12 h; thereafter ZN continued to accumulate in nondividing tissue, but its level declined in dividing tissue, and a corresponding increase in the levels of AN and adenosine was observed. These treatment differences in cytokinin metabolism were apparent at least 6 h before the onset of mitosis.     

Cytokinin metabolism in nondividing and auxin-induced dividing explants ofHelianthus tuberosus L. Tuber tissue

Aqueous solutions of auxin (indole-3-acetic acid,-naphthalene acetic acid, or 2,4-dichlorophenoxyacetic acid) were active in inducing DNA synthesis and mitosis in prewashed tissue explants of mature Jerusalem artichoke tubers. Explants did not respond in this way to aqueous solutions of cytokinin (zeatin, zeatin riboside, 6-benzylaminopurine, or kinetin). The metabolism of [8-³H]zeatin riboside (ZR) was studied in non-dividing and auxin-induced synchronously dividing explants over the first 36 h of culture. ZR was taken up rapidly and to the same extent by both tissues. Sequential analysis of tissue extracts by thin-layer and high-performance-liquid chromatography identified zeatin nucleotide(s) (ZN), O-glucosyl zeatin riboside (OGZR), adenosine, and adenine nucleotide(s) (AN) as the principal metabolites in both tissues. The proportion of radio-activity due to ZR declined steadily and OGZR accumulated steadily at similar rates in both tissues. ZN was the major metabolite in both tissues at 12 h; thereafter ZN continued to accumulate in nondividing tissue, but its level declined in dividing tissue, and a corresponding increase in the levels of AN and adenosine was observed. These treatment differences in cytokinin metabolism were apparent at least 6 h before the onset of mitosis.     

Correlation of xylem sap cytokinin levels with monocarpic senescence in soybean

Cytokinins (CKs) coming from the roots via the xylem are known to delay leaf senescence, and their decline may be important in the senescence of soybean (Glycine max) plants during pod development (monocarpic senescence). Therefore, using radioimmunoassay of highly purified CKs, we quantified the zeatin (Z), zeatin riboside (ZR), the dihydro derivatives (DZ, DZR), the O-glucosides, and DZ nucleotide in xylem sap collected from root stocks under pressure at various stages of pod development. Z, ZR, DZ, and DZR dropped sharply during early pod development to levels below those expected to retard senescence. Pod removal at full extension, which delayed leaf senescence, caused an increase in xylem sap CKs (particularly ZR and DZR), while depodding at late podfill, which did not delay senescence, likewise did not increase the CK levels greatly. The levels of the O-glucosides and the DZ nucleotide were relatively low, and they showed less change with senescence or depodding. The differences in the responses of individual CKs to senescence and depodding suggest differences in their metabolism. Judging from their activity, concentrations and response to depodding, DZR and ZR may be the most important senescence retardants in soybean xylem sap. These data also suggest that the pods can depress CK production by the roots at an early stage and this decrease in CK production is required for monocarpic senescence in soybean.     

Transport and metabolism of xylem cytokinins during lateral bud release in decapitated chickpea (Cicer arietinum) seedlings

Although cytokinins (CKs) are widely thought to have a role in promoting shoot branching, there is little data supporting a causative or even a correlative relationship between endogenous CKs and timing of bud outgrowth. We previously showed that lateral bud CK content increased rapidly following shoot decapitation. However, it is not known whether roots are the source of this CK. Here, we have used shoot decapitation to instantaneously induce lateral bud release in chickpea seedlings. This treatment rapidly alters rate and direction of solvent and solute (including CK) trafficking, which may be a passive signalling mechanism central to initiation of lateral bud release. To evaluate changes in xylem transport, intact and decapitated plants were infiltrated with [³H]zeatin riboside ([³H]ZR), a water‐soluble blue dye or [³H]H₂O by injection into the hypocotyl. All three tracers were recovered in virtually all parts of the shoot within 1 h of injection. In intact plants, solute accumulation in the lateral bud at node 1 was significantly less than in the adjacent stipule and nodal tissue. In decapitated plants, accumulation of [³H]ZR and of blue dye in the same bud position was increased 3‐ to 10‐fold relative to intact plants, whereas content of [³H]H₂O was greatly reduced indicating an increased solvent throughput. The stipule and cut stem, predicted to have high evapotranspiration rates, also showed increased solute content accompanied by enhanced depletion of [³H]H₂O. To assess whether metabolism modifies quantities of active CK reaching the buds, we followed the metabolic fate of [³H]ZR injected at physiological concentrations. Within 1 h, 80–95% of [³H]ZR was converted to other active CKs (mainly zeatin riboside‐5′phosphate (ZRMP) and zeatin (Z)), other significant, but unconfirmed metabolites some of which may be active (O‐acetylZR, O‐acetylZRMP and a compound correlated with sites of high CK‐concentrations) and inactive catabolites (adenosine, adenine, 5′AMP and water). Despite rapid metabolic degradation, the total active label, which was indicative of CK concentration in buds, increased rapidly following decapitation. It can be inferred that xylem sap CKs represent one source of active CKs appearing in lateral buds after shoot decapitation.     

Cytokinin translocation: Changes in zeatin and zeatin-riboside levels in the root exudate of tomato plants during their development

Summary The zeatin and zeatin riboside content of tomato (Lycopersicon esculentum Mill.) root exudates were determined at different stages of development. Zeatin riboside was found to be the major translocational form of cytokinin in the xylem during early vegetative growth. During flower bud formation this cytokinin decreased markedly in concentration so that, at anthesis, there was no appreciable difference in the zeatin and zeatin riboside concentration in the root exudate.     

Cytokinin production by fluorescent Pseudomonas in the presence of rice root exudates

Free-living bacteria may trigger the plant growth through production of phytohormones viz. gibberellins, auxins and cytokinins. A total 50 isolates of fluorescent Pseudomonas were screened for their ability to produce cytokinins such as isopentenyladenosine (IPA), dihydroxyzeatin riboside (DHZR) and zeatin riboside (ZR) as plant growth-promoting activity. Pseudomonas fluorescens AK1 and Pseudomonas aeruginosa AK2 were found higher phytohormones producing strains. Of the three cytokinins, IPA was the major cytokinin produced by both isolates in pure culture (5.5 and 2.9 pmol/ml, respectively) and with rice root exudates (5.9 and 3.4 pmol/ml, respectively). Production of ZR and DHZR for both organisms was found after 48 and 72 h. Amount of ZR and DHZR increased with time for both isolates in pure culture conditions. In presence of rice, production of ZR was increased 0.8 and 0.6 pmol/ml for P. fluorescence AK1 and P. aeruginosa AK2, respectively, in comparison with controls. There was no significant difference in the production of DHZR with rice exudates.     

The stability and biological activity of cytokinin metabolites in soybean callus tissue

The activity, uptake and metabolism of cytokinin metabolites was determined in soybean (Glycine max (L.) Merr.) callus tissue. The following activity sequence was established: zeatin riboside (ZR)>zeatin (Z)>O-glucosides of Z, ZR and their dihydro derivatives>lupinic acid (an alanine conjugate of Z)>7- and 9-glucosides of Z which were almost inactive. The 7- and 9-glucosides and lupinic acid were taken up very slowly by the callus tissue and showed great metabolic stability, but some degradation to 7-glucosyladenine, 9-glucosyladenine and the 9-alanine conjugate of adenine occurred. Compared with its aglycone, O-glucosyl-ZR exhibited slow uptake and greatly enhanced stability but gas chromatographic-mass spectrometric analysis showed that appreciable amounts were hydrolyzed to ZR in the tissue. Both ZR and O-glucosyl-ZR were metabolised extensively, with adenine, adenosine, and adenine nucleotide(s) as the major metabolites. A diversity of minor metabolites of ZR were identified, including O-glucosides, lupinic acid and dihydrolupinic acid. The metabolism of ZR was suppressed by 3-isobutyl-1-methylxanthine. When compared with the soybean callus line normally used for cytokinin bioassays (cv. Acme, cotyledonary callus), related callus lines exhibited greatly differing growth responses to cytokinin: however, these were not reflected in marked differences in metabolism.Abbreviations GC-MS gas chromatography-mass spectrometry - HPLC high-performance liquid chromatography - LA lupinic acid - OGZR O--D-glucopyranosylzeatin riboside - TLC thin-layer chromatography - IMX 3-isobutyl-1-methylxanthine - Z zeatin - ZR zeatin riboside     

Zeatin-9-glucoside,a major endogenous cytokinin of Vinca rosea L. crown gall tissue

Cultured crown gall tissue of Vinca rosea L. was found to contain, in addition to the previously reported cytokinins zeatin, zeatin riboside, and the 0-glucosides of these two compounds, relatively high levels of zeatin-9-D-glucopyranoside. This is the first conclusive identification of an endogenous cytokinin 9-glucoside.Abbreviations GC gas chromatography - HPLC high-performance liquid chromatography - I.D. internal diameter - RFE rotary film evaporation - TLC thin layer chromatography - TMS trimethylsilyl - UV ultraviolet - Z zeatin - Z7G zeatin-7-glucoside - Z9G zeatin-9-glucoside - Z0G zeatin-0-glucoside - ZR zeatin riboside - ZR0G zeatin riboside-0-glucoside     

Rapid effects of red light on the isopentenyladenosine content in scots pine seeds

Red light (R) stimulates germination in Scots pine seed (Pinus sylvestris L.). The response is far red (FR) reversible. The dynamics of cytokinin changes following light treatment was investigated. Extracts were purified by immunoaffinity and high performance liquid chromatography. N⁶-(Δ²-Isopentenyl) adenosine (iPA) and trans-zeatin riboside (ZR) were quantified by both UV-absorbance of high performance liquid chromatography peaks and by enzyme-linked immunosorbent assay. Identification of iPA was accomplished by gas chromatography-mass spectrometry. Levels of cytokinins were low in seeds imbibed in the dark. Exposure of seeds imbibed in the dark for 5 hours to R for 15 minutes induced a strong, immediate but transitory increase in iPA content. This increase was not observed when the R treatment was followed by 10 minutes of FR or by storage in darkness before extraction. No ZR was detected during the first 8 hours of imbibition in any treatment. Addition of iPA via acetone enhanced seed germination in the dark. The results suggest that iPA may be involved in the R-mediated release of dormancy of Scots pine seed.     

Changes in Phytohormone Status in Stems and Roots after Decapitation of Pea Seedlings

Experiments were performed on the first and second internodes and 4-cm-long apical segments of main roots of pea (Pisum sativum L.) seedlings, grown in the light and decapitated above the second node on the seventh day after seed germination. Endogenous phytohormones were measured by the enzyme-linked immunosorbent assay during three days after decapitation of seedlings. The IAA level in the internodes decreased 2–3 times on the second day after decapitation of seedlings while the cytokinin level increased 5–6 times for zeatin and zeatin riboside (Z and ZR) and 1.5–2 times for isopentenyl adenine and isopentenyl adenosine (IP and IPA). In contrast to internodes, the IP and IPA contents in the roots of decapitated seedlings did not change, but the levels of Z and ZR increased 1.5–2 times compared to intact plant roots. The IAA level in the apical region of root remained almost unchanged after the removal of shoot apex. It was concluded that the apical meristem of the main root is not the site of the cytokinin response to the auxin signal coming from the stem apex and that a slight accumulation of Z and ZR after decapitation is due to upper zones of the root. There was no difference in the content of gibberellin-like substances between the internodes of intact and decapitated seedlings. However, the content of gibberellins (GA) in the root tip decreased after decapitation of seedling, which suggests an essential role of apical bud in supplying the root with GA and/or intermediates for their biosynthesis.     

Alteration of Hormonal Levels in a Rootless Epiphytic Bromeliad in Different Phenological Phases

Major changes in indole-3-acetic acid (IAA) and cytokinin (CK) levels occur at different phenological phases of Tillandsia recurvata shoots. This epiphytic rootless bromeliad was chosen as suitable material for hormonal analysis because CK synthesis is restricted to the shoots, thus avoiding problems in the interpretation of results caused by translocation and interconversion of CK forms between roots and leaves encountered in plants with both organs. Young plants of T. recurvata have weak apical dominance because side shoots appeared early in development, and branch growth was correlated with a strong increase in the level of zeatin. The flowering phase was characterized by a significant increase in free base CKs, zeatin, and isopentenyladenine compared with the levels found in adult vegetative shoots. In contrast, both free-base CKs declined in the fruiting phenological phase, and the IAA level increased dramatically. It was concluded that in phases characterized by intense organ formation, such as in the juvenile and flowering stages, there was an enhancement of CK content, mainly caused by zeatin, leading to a lower IAA/CK ratio. Higher ratios were correlated with phases that showed no organogenesis, such as adult and fruiting phenologies. Received April, 15, 1999; accepted September 7, 1999     

Effect of partial rootzone drying on the concentration of zeatin-type cytokinins in tomato (Solanum lycopersicum L.) xylem sap and leaves

Decreased cytokinin (CK) export from roots in drying soil might provide a root-to-shoot signal impacting on shoot physiology. Although several studies show that soil drying decreases the CK concentration of xylem sap collected from the roots, it is not known whether this alters xylem CK concentration ([CK(xyl)]) in the leaves and bulk leaf CK concentration. Tomato (Solanum lycopersicum L.) plants were grown with roots split between two soil columns. During experiments, water was applied to both columns (well-watered; WW) or one (partial rootzone drying; PRD) column. Irrigation of WW plants aimed to replace transpirational losses every day, while PRD plants received half this amount. Xylem sap was collected by pressurizing detached leaves using a Scholander pressure chamber, and zeatin-type CKs were immunoassayed using specific antibodies raised against zeatin riboside after separating their different forms (free zeatin, its riboside, and nucleotide) by thin-layer chromatography. PRD decreased the whole plant transpiration rate by 22% and leaf water potential by 0.08 MPa, and increased xylem abscisic acid (ABA) concentration 2.5-fold. Although PRD caused no detectable change in [CK(xyl)], it decreased the CK concentration of fully expanded leaves by 46%. That [CK(xyl)] was maintained and not increased while transpiration decreased suggests that loading of CK into the xylem was also decreased as the soil dried. That leaf CK concentration did not decline proportionally with CK delivery suggests that other mechanisms such as CK metabolism influence leaf CK status of PRD plants. The causes and consequences of decreased shoot CK status are discussed.     

Further studies on the effects of different auxins and cytokinins on flower formation in thin cell layers of Nicotiana tabacum: The effects of zeatin riboside, dihydrozeatin and dihydrozeatin riboside

Organogenesis in thin cell layers of Nicotiana tabacum L. was studied in relation to the effects of natural and synthetic auxins in combination with various cytokinins. All cytokinins tested, benzyladenine (BA), kinetin, zeatin (Z), zeatin riboside (ZR), N⁶-Δ²-isopentenyl) adenine (IPA), dihydrozeatin [(diH)Z] and dihydrozeatin riboside [(diH)ZR], seem to be active in flower bud formation. In addition to the initiation of flower buds, vegetative buds or roots were also formed on the explants in the presence of BA, Z or IPA as exogenous cytokinins. Only dihydrozeatin and its riboside stimulated the initation of flower buds alone (as is known for kinetin), especially if supplemented with indole-3-acetic acid (IAA) as exogenous auxin. A high number of explants with flower buds was also found with high cytokinin/2,4-D ratios. In these conditions the presence of (diH)Z yielded the higest number of flower buds per explant.     

Exposure of Petunia Seedlings to Ethylene Decreased Apical Dominance by Reducing the Ratio of Auxin to Cytokinin

Seedlings of Petunia x hybrida ‘Orchid’ treated with the ethylene-releasing compound ethephon at 0.9, 1.7, and 3.5 mM evolved ethylene at a higher rate as the concentration of ethephon increased. Regardless of the concentration of ethephon applied, ethylene evolution peaked 6 to 8 h following application. Evidence that ethephon application decreased apical dominance included an increase in the number of new nodes on the main stem and a sustained increase in the length of new and existing lateral shoots compared to the control (no ethephon). Plants treated with 3.5 mM ethephon developed mild chlorosis, whereas a concentration of 1.7 mM ethephon decreased apical dominance without phytotoxic effects. The auxin/cytokinin ratio decreased in the apical shoot section as early as 1 h after ethephon treatment. In contrast, a decrease in the ratio in the subapical shoot section was not detected until 24 h after ethephon application. Reduction in auxin/cytokinin ratio was a result of a decrease in indole-3-acetic acid (IAA) and an increase of zeatin riboside (ZR), but not isopentenyladenosine (iPA). These results suggest that exposing ‘Orchid’ petunia seedlings to ethylene via ethephon lowers the auxin/cytokinin ratio, thereby promoting the outgrowth of lateral shoots.     

Involvement of cytokinins in the germination of chick-pea seeds

Eight cytokinins detected in germinated chick-pea (Cicer arietinum L. var. Castellana) seeds were first present in the embryonic axes but appeared in the cotyledons after 12h of germination. The cytokinins detected in the cotyledons originate in the embryonic axes, but no passage of these substances from the cotyledons to the axes was detected, except when the seeds were treated with red light.It is concluded that the role played by the embryonic axis in mobilizating the main reserves of the cotyledons is mainly effected through these cytokinins. Both natural and synthetic cytokinins exert an important regulatory role in the hydrolysis of reserve proteins and calcium could be involved as an intermediate.Abbreviations BA benzyladenine - cot. cotyledon - (diH)Z dihydrozeatin - (diH)ZR dihydrozeatin riboside - GZR glycosyl zeatin riboside - 2iP 277-1 - iPA 277-2 riboside - Kin kinetin - Z zeatin - ZG zeatin glucoside - ZR zeatin riboside