Comparative Basipetal Transport of 6-Benzylaminopurine-8-14C, Gibberellin A3-3H, IAA-2-14C, and Sucrose-14C in the Root of Intact Citrus aurantium Seedlings

The transport and distribution of IAA-2-¹⁴C, gibberellin A₃-³H, 6-benzylaminopurine-8-¹⁴C and sucrose-¹⁴C (U) were studied in whole seedlings of Citrus aurantium L. after “replacing” the root tip with the solution of radiochemicals. All four substances were transported basipetally in the root and were distributed to the stem and leaves. The pattern of distribution of the label from 6-benzylaminopurine was similar to that of sucrose, while a considerably larger amount of gibberellin A₃ was transported to basal regions of the root, away from the tip, and into the shoot. Contrary to these three substances, the basipetal transport of IAA in the root was very low, and the majority of the label was retained in the terminal sections of the root. It is suggested that the different efficiencies at which various hormones move in the transpiration stream in the root may be an important factor in the attainment of a certain balance of hormones in the shoot.     

Dependence of cytokinin distribution in plants on their physical and chemical properties and transpiration rate

The effect of transpiration on cytokinin accumulation and distribution in 7-day-old wheat (Triticum durum Desf.) seedlings grown on nutrient medium supplemented with zeatin or its riboside was studied. The content of cytokinins in plants and nutrient medium was measured by the immunoenzyme analysis; cytokinin distribution between root cells was assessed immunohistochemically using antibodies against zeatin derivatives. The rate of transpiration was reduced 20-fold by plant placing in humid chamber. At normal transpiration, after 6 h of plant incubation on the solution of zeatin, the level of cytokinins in plant tissues increased stronger than after incubation on the solution of zeatin riboside (by 7.3 and 3.5 times, respectively, as compared with control), although the rates of both cytokinin uptake were equal. Most portions of cytokinins were retained in the roots, which was stronger expressed in the case of free zeatin uptake. A decrease in the rate of transpiration did not affect substantially the zeatin absorption from nutrient medium and the total level of cytokinin accumulation in plants, but these indices were sharply decreased in the case of zeatin riboside. In the zone of absorption of both control roots and roots treated with cytokinins, more intense cytokinin immunostaining was observed in the cells of the central cylinder. The interrelation between cytokinin distribution between the cells and apoplast, their inactivation, and transport over the plant and their form (zeatin or zeatin riboside) used for treatment is discussed.     

The effect of zeatin and iso-pentenyladenine on IAA transport from the shoot to the root of Pinus pinea seedlings

The tips of the tap roots of Pinus pinea seedlings were dipped in zeatin or iso-pentenyladenine solutions. Immediately after cytokinin application to the root tip or after a 24 h lag phase, [2-¹⁴C]IAA was applied to the shoot apex. Treating with zeatin resulted in an increase in [2-¹⁴C]IAA transport from the shoot to the root. Iso-pentenyladenine also caused a slight increase in transport of radioactivity to the root but this was less pronounced compared to the results obtained with zeatin. With zeatin treatment increasing amounts of radioactivity accumulated in the lateral root emerging zone of the tap root (Section III). This was in sharp contrast to the treatment with iso-pentenyladenine where little radioactivity accumulated in this section of the root. Recovery of radioactivity 48 h after applying [2-¹⁴C]IAA showed that 33% of the recovered radioactivity co-chromatographed with authentic IAA. The implications of the effect of different cytokinins on the distribution of radioactivity along the tap root of Pinus pinea following [2-¹⁴C]IAA application to the shoot are discussed.Abbreviations Z zeatin - iP iso-pentenyladenine - TCL thin-layer chromatography     

Quantification of the Daily Cytokinin Transport from the Root to the Shoot of Urtica dioica L.*

Cytokinins are predominantly root-born phytohormones which are distributed in the shoot via the xylem stream. In the hormone message concept they are considered as root signals mediating the transport of the photosynthates to the various sinks of a plant. In this paper the cytokinin relations of Urtica dioica L., the stinging nettle, are described, based on the daily flux from the roots to the shoot. Trans-zeatin-type cytokinins predominate in the various tissues of Urtica (Wagner and Beck, 1993), and accordingly trans-zeatin riboside and trans-zeatin are the forms transported by the xylem sap. The daily time-course of cytokinin concentration in root pressure exudates and in xylem sap collected from a petiole after pressurizing the root bed showed high concentrations in the morning, followed by a substantial drop to a level of 15–30% of the initial concentration which was then maintained during the afternoon. This time-course is interpreted as resulting from continuous synthesis and exudation of cytokinins into the xylem fluid of the roots whose cytokinin concentration is then modified by the dynamics of the transpiration stream. Loading of cytokinins into the xylem sap could be enhanced several times by increasing the flux rate of the xylem stream to the maximal transpiration rate when a maximum export rate was reached. The total daily cytokinin gain by the shoot depended on the nitrogen status of the plant. Roots of Urtica plants grown on a sufficient nitrogen supply had a significantly higher cytokinin content and exuded more cytokinins into the shoot than those of plants raised under nitrogen shortage. A positive correlation was found between the steady rates of cytokinin export measured during the afternoon and the shoot to root-ratios of biomass which, in turn, corresponded to the nitrogen status of the plants.     

Ontogenetic Changes in the Transport of Indol-3yl-acetic Acid into Maize Roots from the Shoot and Caryopsis

The quantities of endogenous indol-3yl-acetic acid (IAA) in endosperms and scutella of 6-day-old maize seedlings (Zea mays L. cv Giant White Horsetooth) were determined by a fluorimetric method. Endosperms were found to contain 33.4 nanograms IAA per plant, and scutella 7.5 nanograms IAA per plant. [5-³H]IAA applied to endosperms of 6-day-old seedlings moved into the roots and radioactivity accumulated at the apex of the primary root within 8 hours. Two to 7-day-old seedlings were treated simultaneously with [5-³H]IAA in the endosperm and [2-¹⁴C] IAA on the shoot apex. The patterns of transport into the root were found to change during ontogeny: in successively older plants, transport from the shoot into the roots increased relative to transport from the endosperm into the roots. The auxin required for the growth of maize roots could, therefore, partially be contributed by the shoot and endosperm. Ontogenetic changes in the relative importance of these two supplies could be of significance for the integration of growth and development between shoot and root.     

Untersuchung des Einflusses der Temperatur auf die Photosynthese-Induktion bei Chlorella vulgaris mit radioaktivem CO2

Summary The lateral bud of Solanum andigena has the potentiality to develop as a stolon or as a leafy, orthotropic shoot. Natural stolons are normally only produced from underground nodes, but aerial stolons can be induced to form by the application of a combination of indole-3-acetic acid and gibberellic acid (IAA/GA₃) paste to the cut surface; under some conditions both natural or induced stolons are converted to upright, leafy shoots. The presence of roots was found to be necessary for the conversion of a natural stolon to a leafy shoot, but this root effect could be replaced by the synthetic cytokinin, 6-benzylaminopurine (BAP). By using -¹⁴C-BAP it was demonstrated that cytokinin accumulates in the tip of an induced stolon, prior to its conversion to a leafy shoot caused by withdrawal of the IAA/GA₃ paste. The application of IAA/GA₃ to decapitated plants was shown to influence both the distribution and metabolism of the cytokinin. The possibility that the role of auxin in apical dominance, at least in part, is to control the distribution and metabolism of cytokinins is discussed.     

Root-Shoot Interactions in Peach: The Function of the Root

Peach seedlings were grown in aerated nutrient solution in small(15 cm³) or large (500 cm³) containers. Subsequently, some plantswere removed from the small containers to the large, or werestressed by root pruning. Half the plants received a foliarspray of synthetic cytokinin 6-benzylaminopurine (BAP) every4 days. Analysis of all the results enabled correlations between functionand growth measurements to be made. Irrespective of the treatmentthe total water and the total nutrient uptake by the roots wasdirectly related to the total dry weight increment. This suggeststhat for both water and nutrient uptake a functional equilibriumexisted between the root and the shoot. Where BAP treatmentincreased the top: root ratio there was a corresponding increasein the rate of water uptake per unit length of root. The relationship between root number and leaf number and thecomplementary action of BAP suggests that the root tip and itsproduction of cytokinins exerts considerable control over topgrowth.     

Cytokinin producing bacteria enhance plant growth in drying soil

Cytokinins can promote stomatal opening, stimulate shoot growth and decrease root growth. When soil is drying, natural cytokinin concentrations decrease in association with stomatal closure and a redirection of growth away from the shoots to the roots. We asked if decreased cytokinin concentrations mediate these adaptive responses by lessening water loss and promoting root growth thereby favouring exploration for soil water. Our approach was to follow the consequences for 12-d-old lettuce seedlings of inoculating the growing medium with cytokinin-producing bacteria under conditions of water sufficiency and deficit. Inoculation increased shoot cytokinins as assessed by immunoassay and mass spectrometry. Inoculation also promoted the accumulation of shoot mass and shortened roots while having a smaller effect on root mass. Inoculation did not raise stomatal conductance. The possible promoting effect of these cytokinins on stomatal conductance was seemingly hampered by increases in shoot ABA that inoculation also induced. Inoculation lowered root/shoot ratios by stimulating shoot growth. The effect was greater in non-droughted plants but remained sufficiently strong for shoot mass of inoculated droughted plants to exceed that of well-watered non-inoculated plants. We conclude that compensating for the loss of natural cytokinins in droughted plants interferes with the suppression of shoot growth and the enhancement of root elongation normally seen in droughted plants.     

Quantification of the export of cytokinins from roots to shoots of Rosa hybrida and their degradation rate in the shoot

Cytokinins from the roots may be involved in regulating rose ( Rosa hybrida ) shoot growth and development. The objective of this study was to estimate the export of cytokinins from the roots and their degradation rate in the shoot, which were expected to be correlated with plant development. Hence, the total cytokinin content of the shoot, the concentration of zeatin riboside (ZR) in bleeding sap, and the transpiration rates in three stages of development were determined. The estimations performed are based on the assumption that the cytokinin concentration in bleeding sap is representative for the cytokinin concentration in xylem sap in situ. This was verified by comparing the ZR concentration in bleeding sap and in sap obtaíned after pressurizing the root system to a level equivalent to the leaf water potential; no significant differences could be found. The import of cytokinins could not be correlated with plant development, as it increased linearly with time. The estimated relative degradation rate of cytokinins in the shoot decreased as the plants matured. The half-life of cytokinins in the shoot was found to be approximately 1 day, indicating that cytokinins are rapidly metabolized in the shoot.     

The Effect of Abscisic Acid on the Uptake and Distribution of Ions in Intact Seedlings of Phaseolus vulgaris cv. Redland Pioneer

Abscisic acid (ABA) was found to increase the accumulation of ³⁶Cl, total Cl, ²²Na and total Na⁺ in roots of intact bean seedlings. After an initial promotion. ABA inhibited longdistance transport of these ions from the root to the shoot. However, it consistently inhibited both uptake and transport of ⁴²K and total K⁺ in intact bean seedlings. A promotion of net ³⁶Cl influx (ψ_oc) and its accumulation in the root (Q*_v) concomitant decrease in transport index (long-distance transport as percentage of total influx) showed that ABA stimulates -³⁶Cl transport at the tonoplast. It inhibited H⁴ extrusion and net ⁸⁶Rb influx which agrees with a cation exchange theory K⁺/Rb⁺ transport.     

Localization of cytokinin biosynthetic sites in pea plants and carrot roots

The biosynthesis of cytokinins was examined in pea (Pisum sativum L.) plant organs and carrot (Daucus carota L.) root tissues. When pea roots, stems, and leaves were grown separately for three weeks on a culture medium containing [8-¹⁴C]adenine without an exogenous supply of cytokinin and auxin, radioactive cytokinins were synthesized by each of these organs. Incubation of carrot root cambium and noncambium tissues for three days in a liquid culture medium containing [8-¹⁴C]adenine without cytokinin demonstrates that radioactive cytokinins were synthesized in the cambium but not in the noncambium tissue preparation. The radioactive cytokinins extracted from each of these tissues were analyzed by Sephadex LH-20 columns, reverse phase high pressure liquid chromatography, paper chromatography in various solvent systems, and paper electrophoresis. The main species of cytokinins detectable by these methods are N⁶-(Δ²-isopentyl_adenine-5′-monophosphate, 6-(4-hydroxy-3-methyl-2-butenyl-amino)-9-β-ribofuranosylpurine-5′- monophosphate, N⁶-(Δ²-isopentenyl)adenosine, 6-(4-hydroxy-3-methyl-2-butenylamino)-9-β-ribofuranosylpurine, N⁶-(Δ²-isopentenyl)adenine, and 6-(4-hydroxy-3-methyl-2-butenylamino)purine. On the basis of the amounts of cytokinin synthesized per gram fresh tissues, these results indicate that the root is the major site, but not the only site, of cytokinin biosynthesis. Furthermore, cambium and possibly all actively dividing tissues are responsible for the synthesis of this group of plant hormones.     

Cytokinin metabolism in tomato plants. II. Metabolites of kinetin and benzyladenine in decapitated roots

Decapitated tomato plants were supplied via the roots with [8-¹⁴C]-kinetin or [8-¹⁴C]-benzyladenine in a nutrient solution for a period of 24 h. After this time the root material, the root sap produced during the 24 h period and the nutrient solution remaining at the end of the experiments were analysed for cytokinins. HPLC techniques and chemical treatments were used to tentatively identify radioactive metabolites formed. Uptake of kinetin and benzyladenine by the roots was found to be limited but once within the root tissues metabolism was both rapid and extensive.At least 14 metabolites of kinetin were recovered from root tissue and root sap. Many of these appeared to be degradation products. There was, however, some evidence of formation of zeatin-like derivatives. Side-chain cleavage of the original kinetin which occurs rapidly is suggested as a possible route for the eventual production of these endogenous cytokinin forms.The benzyladenine taken up by the roots was apparently both ribosylated and glucosylated. No unmetabolized benzyladenine was detected in the root tissues after 24 h. Only very low levels of radioactivity were associated with the retention time of adenine, suggesting that in the case of benzyladenine side-chain cleavage is of limited importance.The significance of these reactions in relation to the potential use of cytokinins in the regulation of plant growth is discussed.     

Effects of Root Cutting on Cytokinin Content in the Shoot Apex Cells of Arabidopsis Plants

The dependence of cytokinin accumulation in the shoot apexes of Arabidopsis plants on the delivery of these hormones from the roots was studied. For the estimation of cytokinin content in the cells, the immunohistochemical localization method using antibodies against zeatin riboside was used. Differential conjugation of free cytokinin bases and their ribosides was used to prevent washing of cytokinins during the dehydration process. Root cutting decreased the immunostaining of zeatin in the cells of the shoot apical meristem, thereby supporting the hypothesis about dependence of cytokinin accumulation in these cells on the hormone delivery from the roots. The level of cytokinins in the cells of the shoot apex decreased under the effect of protonophore, indicating the important role of the secondary-active transmembrane transport process of cytokinins in the maintenance of their level in the cells of the shoot apex.     

Role of roots in regulating the growth rate and cytokinin content in leaves

The role of roots in the enhancement of cytokinin content and leaf growth of Phaseolus vulgaris plants after decapitation and partial defoliation was investigated. Partial excision of the roots of plants which were decapitated above the primary leaf node resulted in a reduction of leaf growth and soluble proteins accumulation in the primary leaves. Roots excision was done at time of decapitation and repeated 8 days later. Endogenous cytokinins, known to be involved in enhancing shoot growth, accumulated in the leaves and stems of decapitated and partially defoliated plants. Lower levels of cytokinins were detected in the leaves of decapitated plants with only a partial root system. The level of cytokinins in the roots of decapitated plants was reduced by partial root excision. The growth and accumulation of cytokinins in leaves were, however, not totally suppressed by removing a large proportion of the roots. At the commencement of the experiment the stem had a higher cytokinin content than both the leaves and roots. This suggests that the stem could be an alternative source of cytokinins to the leaves. The cytokinin complement in the leaves of decapitated plants is not identical to that in the roots. It appears that cytokinins supplied by the roots are metabolized in the leaves, or that alternatively certain cytokinins are synthesized in the leaves themselves.     

Distribution and metabolism of root-applied cytokinins in Pisum sativum

When N ⁶ [8–¹⁴C] furfuryladenine was applied to the intact root system of Pisum sativum L. cv. Meteor seedlings it was almost completely metabolised to other compounds within 24 h. Of the total activity recovered from the plants 94.5% was retained in the root system itself. ¹⁴C was recovered in a number of ethanol-soluble compounds and in ribonucleic acid, deoxyribonucleic acid and protein fractions of roots, stems, leaves and axillary buds. In rapidly growing axillary buds released from apical dominance by removal of the shoot apex the combined nucleic acid fractions accounted for 63.3% of the total ¹⁴C recovered from these organs. Xylem exudate collected from decapitated plants 0 to 12 h after supplying N ⁵[8–¹⁴C]furfuryladenine to the roots consistently contained a single major ¹⁴C-labelled compound which, in three different solvent systems, had the same Rf values as a major endogenous cytokinin isolated from the xylem of unlabelled plants. The content of N ⁶ [8–¹⁴C] furfuryladenine itself in the xylem exudate was always low and in some experiments it could not be detected.
It is suggested that part of the label from N ⁶ [8- ¹⁴CJfurfuryladenine taken up by the intact root system may have become incorporated in an endogenous cylokinin before export to the shoot.     

Exogenous zeatin accumulation in wheat root cells shows its role in regulation of cytokinin transport

Here we have shown that 24 hours after addition of zeatin to the nutrient solution the cytokinin content in xylem sap of wheat plants appears to be about 10 times lower that in the nutrient solution. Cytokinins accumulated mostly in roots and not in shoots of treated plants. These data demonstrate the existence of some barrier on cytokinin pathway from the nutrient solution to the plant shoot. With the help of Sudan III an increase in lignin and suberin deposition in the endodermis could be detected, being stronger with the increase in the distance from the root tip. The increase in deposition of suberin and lignin coincided with the decrease in cytokinin immunolabeling in root cells revealed with the help of monoclonal cytokinin antibodies and the second gold-labelled antibodies. Simultaneously exogenous cytokinins accumulated in root stele cells showing that the Casparian band was not only barrier on cytokinin pathway to plant shoot. It is concluded that high cytokinin immunolabe ling in the stele parenchyma cells around the stele vessels demonstrated accumulation of cytokinins by these cells, which could be important in regulation of cytokinin loading to the xylem vessels during there transport to the shoot. The role of cytokinin transporters is discussed.     

32P uptake and transport to shoots in Pinuus serotina seedlings under aerobic and hypoxic growth conditions

We examined the effects o long-term hypoxic growth conditions on net uptake and transport of P to shoots of pond Pine (Pinus serotina Michx.), a moderately flood-tolerant southern pine. Seedlings were grown under aerobic orhypoxic solution conditions for 4–5 weeks in continuously flowing solution culture containing 100 μM P. Short – and long-term ³²P. experiments were then concluded with intact seedlings to determine rates of ³²P influx, efflux and net transport to the shoot. Shoot fresh weight/root fresh weight ratios were significantly higher under hypoxic gorwth conditions, reflecting the larger reduction in root growth than shoot growth, despite extensive aerechyma formation in roots. Estimates for the unidirectional influx of ³²P in aerobic and hypoxic seedlings were 1.43 and 3.20 μmol P (g_FW root)_?1 h_?1, respectively. However, ³²P accumulation between the two treatments became similar within 8 h, suggesting that efflux was also higer in seedlings from the hypoxic treatment. Indeed in a separate experiment, hypoxic growth conditions increased efflux by over 60%. Transport of ³²P to shoots was significantly reduced under hypoxic growth conditions, despite higher root P concentrations and lower shoot P concentrations. After 48 h, ³²P accumulation in roots was similar between the two treatments. Yet total accumulation of seedling ³²P decrcased by 31% under the hypoxic treatment, largely because of reduced transport of ³²p to the shoot. The lower accumulation of ³² by shoots of seedlings in the hypoxic treatment may be the result of a direct inhibition on the transport process in O²-defident tissues, but could also reflect a slower turnover or labeling of the ool available for transport. Indeed, the percentage of total ³²P in. roots present in the soluble P. (or transportable form of P) was about 33% lower in seedlings from the hypoxic treatment, probably reflecting increased assimilation into organic compounds as well as chelation with iron. Our results suggest that P transport to the shoots of acclimated seedlings may be more sensitive to hypoxic solution conditions than influx at the root Plasmalemma.     

Nitrite reduction and carbohydrate metabolism in plastids purified from roots of Pisum sativum L.

Nitrate reduction in roots and shoots and exchange of reduced N between organs were quantitatively estimated in intact 13-d-old seedlings of two-row barley (Hordeum vulgare L. cv. Daisengold) using the ¹⁵N-incorporation model (A. Gojon et al. (1986) Plant Physiol. 82, 254–260), except that NH ₊ ⁴ was replaced by NO _- ² . N-depleted seedlings were exposed to media containing both nitrate (1.8 mM) and nitrite (0.2 mM) under a light-dark cycle of 12:12 h at 20°C; the media contained different amounts of ¹⁵N labeling. Experiments were started either immediately after the beginning (expt. 1) or immediately prior to the end (expt. 2) of the light period, and plants were sampled subsequently at each light-dark transition throughout 36 h. The plants effectively utilized ¹⁵NO _- ³ and accumulated it as reduced ¹⁵N, predominantly in the shoots. Accumulation of reduced ¹⁵N in both experiments was nearly the same at the end of the experiment but the accumulation pattern in roots and shoots during each 12-h period differed greatly depending on time and the light conditions. In expt. 1, the roots accounted for 31% (light), 58% (dark), and 9% (light) of nitrate reduction by the whole plants, while in expt. 2 the contributions of the root were 82% (dark), 20% (light), and 29% (dark), during each of the three 12-h periods. Xylem transport of nitrate drastically decreased in the dark, but that of reduced N rather increased. The downward translocation of reduced ¹⁵N increased while nitrate reduction in the root decreased, whereas upward translocation decreased while nitrate reduction in the shoot increased. We conclude that the cycling of reduced N through the plant is important for N feeding of each organ, and that the transport system of reduced N by way of xylem and phloem, as well as nitrate reduction by root and shoot, can be modulated in response to the relative magnitude of reduced-N demands by the root and shoot, with the one or the other predominating under different circumstances.Symbols Anl accumulation of reduced ¹⁵N from ¹⁵NO _- ³ in ¹⁴NO _- ³ -fed roots of divided root system - Ar accumulation in root of reduced ¹⁵N from ¹⁵NO _- ³ - As accumulation in shoot of reduced ¹⁵N from ¹⁵NO _- ³ - Rr ¹⁵NO _- ³ reduction in root - Rs ¹⁵NO _- ³ reduction in shoot - Tp translocation to root of shoot-reduced ¹⁵N from ¹⁵NO _- ³ in phloem - Tx translocation to shoot of root-reduced ¹⁵N from ¹⁵NO _- ³ in xylem     

The effect of root cooling on hormone content, leaf conductance and root hydraulic conductivity of durum wheat seedlings (Triticum durum L.)

Root cooling of 7-day-old wheat seedlings decreased root hydraulic conductivity causing a gradual loss of relative water content during 45 min (RWC). Subsequently (in 60 min), RWC became partially restored due to a decrease in transpiration linked to lower stomatal conductivity. The decrease in stomatal conductivity cannot be attributed to ABA-induced stomatal closure, since no increase in ABA content in the leaves or in the concentration in xylem sap or delivery of ABA from roots was found. However, decreased stomatal conductance was associated with a sharp decline in the content of cytokinins in shoots that was registered shortly after the start of root cooling and linked to increases in the activity of cytokinin-oxidase. This decrease in shoot cytokinin content may have been responsible for closing stomata, since this hormone is known to maintain stomatal opening when applied to plants. In support of this, pre-treatment with synthetic cytokinin benzyladenine was found to increase transpiration of wheat seedlings with cooled roots and bring about visible loss of turgor and wilting.     

Regulation of cytokinin oxidase activity as a factor affecting the content of cytokinins

In the shoots of 7-day-old seedlings of wheat (Triticum durum Desf., cv. Bezenchukskaya 139), ion deficiency in Hoagland-Arnon nutrient solution induced a decrease in the cytokinin content by the end of the first day, whereas the excision of four out of five primary roots brought about an opposite response (accumulation of cytokinins) as early as during the first hour. It was assumed that changes in the content of cytokinins might depend on the activity of cytokinin oxidase (CKO) and the expression of gene encoding this enzyme. It was shown that tenfold dilution of the nutrient solution activated CKO and raised the level of CKO gene expression, whereas excision of some roots brought about a quick decrease in enzyme activity and gene expression. The role of ABA and arrival of cytokinins from the roots to the shoot as factors affecting CKO activity in the shoot is discussed; arguments for the priority of hormonal signal over the influx of nitrates from the roots are offered. It was concluded that the regulation of CKO activity might be one of the important mechanisms determining plant response to treatments via the changes in the cytokinin concentration.