Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol87.Issue1-2.Jan2001:
Scientia Horticulturae 87 (2001) 53±63
Endogenous cytokinin distribution patterns at
budburst in Granny Smith and Braeburn
apple shoots in relation to bud growth
Nigel C. Cooka,*, Dirk U. Bellstedtb, Gerard Jacobsa
a
Department of Horticultural Science, University of Stellenbosch, Private Bag X1,
Matieland 7602, Stellenbosch, South Africa
b
Department of Biochemistry, University of Stellenbosch, Private Bag X1,
Matieland 7602, Stellenbosch, South Africa
Accepted 11 April 2000
Abstract
The possible relationship of branching habit to cytokinin content of apple shoots
(Malusdomestica Borkh.) was investigated. One-year old apple shoots are acrotonic (distal
branching), more strongly so in Granny Smith than in Braeburn. In the ®rst trial, long, 1-year old
Granny Smith and Braeburn apple shoots were sprayed on 29 August 1995 to break rest with
dinitro-o-cresol (DNOC) oil (5%). The cytokinin contents of the xylem sap, the combined bark and
buds, and the wood were determined in distal and proximal shoot halves over the next 6 weeks.
Budburst (terminal and lateral buds) was ®rst visible (green tip) in both cultivars on 20 September
1995. A greater increase in cytokinin content of distal xylem sap, coupled with elevated cytokinin
in the distal wood, re¯ect the overall acrotony of both cultivars. The strong acrotony of Granny
Smith is re¯ected in the higher cytokinin concentration in distal portion 1 week before the proximal
portion of the shoot. The differential distribution of cytokinin re¯ects the pattern of budburst and
may be correlated with growth habit. In a subsequent trial, Granny Smith shoots chilled and forced
in the absence of roots showed an increase in cytokinin content of the bark and buds, and the wood
as growth resumed. This was roughly comparable in magnitude to the increase observed under ®eld
conditions. The cytokinin increase in rootless shoots and differential distribution of cytokinin prior
to sprouting, support the hypothesis that shoot-derived, rather than root-derived, cytokinins act to
trigger spring budburst. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Acrotonic branching; Prolepsis; Malusdomestica Borkh.
*
Corresponding author. Tel: 27-21-808-4763; fax: 27-21-808-2121.
E-mail address: nc@land.sun.ac.za (N.C. Cook).
0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 0 0 ) 0 0 1 6 1 - 8
54
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
1. Introduction
Acrotony, mesotony and basitony describe distal branching, branching that is more
or less evenly distributed over the shoot axis, and branching from the proximal buds,
respectively (Rauh, 1939; CrabbeÂ, 1987). Proleptic branching, i.e. from previously
dormant buds, in spring on 1-year old apple shoots, is acrotonic (Halle et al., 1978).
The expression of acrotony in apple varies among varieties, resulting in a continuum
from strongly acrotonic to almost mesotonic (Lespinasse and Delort, 1986). The
current understanding is that acrotony is established under the correlative in¯uences
that are active during the dormant period and appears to be mediated via a dominance
of the apical bud that is established prior to budburst in spring (Cook et al., 1998).
Cytokinins play a central role in budburst in spring (Faust et al., 1997). The
exogenous application of cytokinins promotes budburst during late dormancy and
spring (Arias and CrabbeÂ, 1975; Steffens and Stutte, 1989). Endogenous
cytokinins increase just prior to budburst in spring in the xylem sap of shoots
and in the bleeding sap from roots (Skene, 1975; Tromp and Ovaa, 1990; Cutting
et al., 1991). This cytokinin peak in spring is believed to originate from the roots,
as shown by the increase in cytokinin in the bleeding sap (Skene, 1975), and from
the shoot, as shown by the increase in cytokinin in the xylem sap of shoots that
had been arti®cially chilled following excision (Hewett and Wareing, 1973).
Trees exhibiting delayed foliation show a delay in the increase in the xylem sap
cytokinin concentration compared to trees treated with rest breaking agents,
which exhibit a more rapid increase in cytokinin (Cutting et al., 1991). While the
importance of root-derived cytokinin cannot be excluded, shoot-derived
cytokinins may be more directly involved with spring budburst in apple (Tromp
and Ovaa, 1990; Cutting et al., 1991; Faust et al., 1997).
With acrotony the terminal buds burst ®rst, followed closely by the lateral buds.
In our study, cytokinin content changes of the distal and proximal halves of 1-year
old, dinitro-o-cresol (DNOC) oil treated, Granny Smith and Braeburn apple shoots
were determined to assess whether cytokinin content changes within the shoots
correlate with the pattern of budburst. Cytokinin content of the xylem sap, wood, and
pooled bark and bud material was analysed shortly before and after budburst. In a
subsequent trial, the cytokinin content of fall-harvested Granny Smith shoots was
determined after chilling and subsequent forcing (not DNOC oil treated) in an
attempt to con®rm that cytokinins involved in spring budburst are shoot-derived.
2. Materials and methods
2.1. Plant material Ð 1995 trial
Granny Smith trees on M793 rootstock were topworked to Braeburn in the
winter of 1994. Both the Granny Smith and Braeburn shoots were allowed to
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
55
develop in the 1994/1995 growing season such that an ample supply of shoots
from both cultivars existed on the same trees. DNOC oil (5%) was sprayed on 29
August 1995 to break rest, and the ®rst shoots were harvested on 30 August 1995.
Thereafter, shoots were harvested weekly until 4 October 1995 giving a total of
six harvesting dates. One-year old, upright shoots (145058 mm in length) were
harvested in the early morning. At each harvesting date, 10 shoots of each cultivar
were harvested at random from ®ve adjacent trees. Five replicate shoots were
used for the xylem sap analysis, and ®ve replicate shoots were used for both the
bark and buds and the wood cytokinin analysis.
2.2. Cytokinin determination
Shoots were cut in half and the xylem sap was vacuum-extracted from both the
distal and proximal halves as described by Belding and Young (1989). The xylem
sap was extracted, rapidly frozen and stored at ÿ808C until analysis. The t-zeatin
riboside (ZR) concentration of the xylem sap was determined by means of
radioimmunoassay (Hofman et al., 1986) using a monoclonal ZR speci®c
antibody (Eberle et al., 1986). The bark and buds were peeled from the wood
separately from the distal and proximal shoot halves of each of the remaining ®ve
shoots. Bark and bud (pooled) samples as well as wood samples were cut into
1 cm segments, freeze-dried and milled. All samples were stored at ÿ808C until
analysis.
ZR and ZROG (t-zeatin riboside-O-glucoside) were extracted from the samples
by adding 10 ml of extraction solvent (80% methanol, 20% distilled water, BHT
(16 mg/l) and ascorbic acid (10 mg/l)) to 0.5 g freeze-dried material and
constantly stirring for 24 h at 48C. Undissolved solids were removed by
centrifugation (12 000 gn for 10 min at F
Wood
Bark and buds
0.0004
0.0001
LSD
13
82
Cytokinin content (ZR ng/g dry mass)
0 days forced
(dormant)
6 days forced
(green tip)
8 days forced
(leaves unfolding)
50
153
64
520
83
579
a
The shoots were forced at 258C with continuous illumination (215 mmol mÿ2 sÿ1 PAR). Mean
separation by LSD at 5% level; n5; d.f.2.
60
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
increases observed in DNOC treated shoots that burst buds in the ®eld (Figs. 2
and 3).
4. Discussion
Acrotony in M.9 apple shoots growing in Belgium, a region with an extended
ecodormant period, appears to be mediated via dominance of the distal buds reestablished during ecodormancy (Cook et al., 1998). The dominance of the distal
buds (terminal and/or laterals) associated with acrotony is possibly an effect of
polar auxin transport (Bangerth, 1989). Acrotony has been explained in terms of a
positional advantage of distal buds on orthotropic shoots, mediated via an auxin
effect (Faust et al., 1997). Similarly, gravimorphic responses of shoots inclined
from the normal orthotropic orientation have been ascribed to the effects of a
redistribution of endogenous auxins (Mullins and Rogers, 1971). It has also been
shown that during dormancy a distal disbudded shoot piece maintains an
inhibitory in¯uence on lateral buds under forcing conditions (Cook et al., 1998),
an effect that can be overcome by girdling (Champagnat, 1983). Notching, known
to disrupt polar auxin transport, is a commonly used practice to release lateral
buds from the inhibitions associated with acrotony (Greene and Autio, 1994). In
short, auxin undoubtedly plays a central role in correlative phenomena involved
in the determination of acrotony and thus tree architecture.
The inhibition of lateral bud outgrowth can also be overcome by locally applied
cytokinins alone (Steffens and Stutte, 1989), and in combination with gibberellins
(Promalin1 [N-(phenylmethyl)-1H-purine 6-amine and gibberellins A4A7] is
used commercially in South Africa to promote lateral branching). Endogenous
cytokinin increases in spring just prior to and during budburst in poplar shoots
removed from the plant during dormancy (Hewett and Wareing, 1973); in
dormant almond shoots stored at low temperatures for prolonged periods (Van
Staden and Dimalla, 1981); in apple trees exhibiting delayed foliation (Cutting
et al., 1991); and in non-chilled apple trees (Young, 1989). Provided that buds are
receptive, i.e. not endodormant, and conditions are conducive to growth, buds can
respond to this cytokinin peak, overcome correlative inhibitions and grow. This
cytokinin peak appears, thus, to act as a trigger for resumed growth (Faust et al.,
1997).
Both the cytokinin increase in shoots harvested after leaf drop and arti®cially
chilled in the absence of roots, and the accentuated cytokinin increase in the distal
wood, bark and buds, and xylem sap of DNOC treated shoots, indicate that these
cytokinins originate from within the shoot. These ®ndings con®rm the
postulations of Tromp and Ovaa (1990), Cutting et al. (1991) and Faust et al.
(1997). As cytokinins are mainly synthesised in the root tips (Dieleman et al.,
1997), it should be considered that cytokinins associated with budburst in spring
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
61
are stored in the shoot prior to becoming dormant, as has previously been
suggested by Van Staden and Dimalla (1981).
The higher rate of cytokinin increase in the distal xylem sap (Fig. 1), and the
higher distal wood ZR content (Fig. 2), re¯ect the overall acrotonic growth habit
of both cultivars. The distribution of cytokinins in pea and bean stems has been
reported to be under the control of polar auxin transport (Li et al., 1995). Auxins
appear to prevent the movement of cytokinins into the lateral buds under the
in¯uence of apical dominance. It is possible that auxins exported by the shoot
apex before and/or after endodormancy may be associated with the differential
distribution of cytokinins after chilling.
Branching in Granny Smith is strongly acrotonic. Shoot growth is restricted to
the dominant distal shoots with the development of few spurs and a latency of
proximal lateral buds, i.e. ``blind wood'' (Oosthuyse et al., 1992). This was
re¯ected by a large, initial difference between the distal and proximal wood ZR
content (distal contained more ZR, Fig. 2), and an enhanced ZR peak in the
proximal wood and bark/buds (Figs. 2 and 3). In the more ``spur type'' Braeburn,
distal shoots and proximal spurs are more numerous than in Granny Smith. In
Braeburn, which exhibits less dominance, a simultaneous increase of wood ZR of
both shoot halves (Fig. 2), and generally less bark/buds ZR with a greater increase
at budburst in the proximal half was observed (Fig. 3). The differential
distribution of cytokinin in the shoot may be involved in the pattern of budburst
and thus growth habit.
Delayed heading is commonly used to reduce the effects of acrotony. Heading
1-year old Granny Smith shoots from budburst to shortly after full-bloom
increases the number of shoots formed with a subsequent reduction in the total
length of new growth (Oosthuyse et al., 1992). The resultant shoots originate
from buds that would otherwise remain latent. Heading allows buds to burst in a
portion of the shoot with lower available cytokinin levels, the result of the gradient
observed in these data, at a time generally associated with less cytokinin availability, i.e. after the cytokinin peak. Possibly, buds are released under endogenous
conditions less favourable for the development of dominance phenomena.
In conclusion, while auxins play a central role in acrotony and thus tree
architecture, evidence presented here suggests that shoot-derived cytokinins may
also be involved in the acrotonic branching habit of apple. The cytokinin increase
in rootless shoots and differential distribution of cytokinin prior to sprouting,
support the hypothesis that shoot-derived, rather than root-derived, cytokinins act
to trigger spring budburst.
Acknowledgements
We thank Pia Nel, Department of Horticultural Science, University of
Stellenbosch for the cytokinin determinations, Prof. Elmar W. Weiler, Lehrstuhl
62
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
fuÈr P¯anzenphysiologie, Ruhr UniversitaÈt, Bochum, Germany for supplying the
monoclonal ZR speci®c antibody, and both the FRD and the Deciduous Fruit
Industry for ®nancial support.
References
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ligneux, en repos apparent. DonneÂes compleÂmentaires fournies par l'eÂtude de Prunus avium L.
Physiol. VeÂg. 13, 69±81.
Bangerth, F., 1989. Dominance among fruits/sinks and the search for a correlative signal. Physiol.
Plant. 76, 608±614.
Belding, R.D., Young, E., 1989. Shoot and root temperature effects on cytokinin levels during
budbreak in apple trees. HortScience 24, 115±117.
Champagnat, P., 1983. Bud dormancy, correlation between organs, and morphogenesis in woody
plants. Soviet Plant Physiol. 30, 458±471.
Cook, N.C., Rabe, E., Keulemans, J., Jacobs, G., 1998. The expression of acrotony in deciduous
fruit trees: a study of the apple rootstock M. 9. J. Am. Soc. Hort. Sci. 123, 30±34.
CrabbeÂ, J., 1987. Aspects particuliers de la morphogeneÂse caulinaire des veÂgeÂtaux ligneux et
introduction aÁ leur eÂtude quantitative. Centre d'Etude de la Reproduction VeÂgeÂtale, Brussels.
Cutting, J.G.M., Strydom, D.K., Jacobs, G., Bellstedt, D.U., Van Der Merwe, K.J., Weiler, E.W.,
1991. Changes in xylem constituents in response to rest-breaking agents applied to apple before
budbreak. J. Am. Soc. Hort. Sci. 116, 680±683.
Dieleman, J.A., Verstappen, F.W.A., Nicander, B., Kuiper, D., Tillberg, E., Tromp, J., 1997.
Cytokinins in Rosa hybrida in relation to bud break. Physiol. Plant. 99, 456±464.
Eberle, J., Arnscheidt, A., Klix, D., Weiler, E.W., 1986. Monoclonal antibodies to plant growth
regulators. III. Zeatin and dihydrozeatin riboside. Plant Physiol. 81, 516±521.
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fruit trees: physiological basis for dormancy induction, maintenance, and release. HortScience
32, 623±629.
Greene, D.W., Autio, W.R., 1994. Notching techniques increase branching of young apple trees. J.
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bud burst. Physiol. Plant. 28, 393±399.
Hofman, P.J., Featonby-Smith, B.C., Van Staden, J., 1986. The development of ELISA and RIA for
cytokinin estimation and their application to a study of lunar periodicity in Ecklonia maxima
(Osbeck) Papenf. J. Plant Physiol. 122, 455±466.
Lespinasse, J.M., Delort, J.F., 1986. Apple tree management in vertical axis: appraisal after 10 years
of experiments. Acta Hort. 160, 139±155.
Li, C.-J., Guevara, E., Herrera, J., Bangerth, F., 1995. Effect of apex excision and replacement by 1naphthylacetic acid on cytokinin concentration and apical dominance in pea plants. Physiol.
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Van Staden, J., Dimalla, G.G., 1981. The production and utilisation of cytokinins in rootless,
dormant almond shoots maintained at low temperature. Z. P¯anzenphysiol. 103, 121±129.
Endogenous cytokinin distribution patterns at
budburst in Granny Smith and Braeburn
apple shoots in relation to bud growth
Nigel C. Cooka,*, Dirk U. Bellstedtb, Gerard Jacobsa
a
Department of Horticultural Science, University of Stellenbosch, Private Bag X1,
Matieland 7602, Stellenbosch, South Africa
b
Department of Biochemistry, University of Stellenbosch, Private Bag X1,
Matieland 7602, Stellenbosch, South Africa
Accepted 11 April 2000
Abstract
The possible relationship of branching habit to cytokinin content of apple shoots
(Malusdomestica Borkh.) was investigated. One-year old apple shoots are acrotonic (distal
branching), more strongly so in Granny Smith than in Braeburn. In the ®rst trial, long, 1-year old
Granny Smith and Braeburn apple shoots were sprayed on 29 August 1995 to break rest with
dinitro-o-cresol (DNOC) oil (5%). The cytokinin contents of the xylem sap, the combined bark and
buds, and the wood were determined in distal and proximal shoot halves over the next 6 weeks.
Budburst (terminal and lateral buds) was ®rst visible (green tip) in both cultivars on 20 September
1995. A greater increase in cytokinin content of distal xylem sap, coupled with elevated cytokinin
in the distal wood, re¯ect the overall acrotony of both cultivars. The strong acrotony of Granny
Smith is re¯ected in the higher cytokinin concentration in distal portion 1 week before the proximal
portion of the shoot. The differential distribution of cytokinin re¯ects the pattern of budburst and
may be correlated with growth habit. In a subsequent trial, Granny Smith shoots chilled and forced
in the absence of roots showed an increase in cytokinin content of the bark and buds, and the wood
as growth resumed. This was roughly comparable in magnitude to the increase observed under ®eld
conditions. The cytokinin increase in rootless shoots and differential distribution of cytokinin prior
to sprouting, support the hypothesis that shoot-derived, rather than root-derived, cytokinins act to
trigger spring budburst. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Acrotonic branching; Prolepsis; Malusdomestica Borkh.
*
Corresponding author. Tel: 27-21-808-4763; fax: 27-21-808-2121.
E-mail address: nc@land.sun.ac.za (N.C. Cook).
0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 0 0 ) 0 0 1 6 1 - 8
54
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
1. Introduction
Acrotony, mesotony and basitony describe distal branching, branching that is more
or less evenly distributed over the shoot axis, and branching from the proximal buds,
respectively (Rauh, 1939; CrabbeÂ, 1987). Proleptic branching, i.e. from previously
dormant buds, in spring on 1-year old apple shoots, is acrotonic (Halle et al., 1978).
The expression of acrotony in apple varies among varieties, resulting in a continuum
from strongly acrotonic to almost mesotonic (Lespinasse and Delort, 1986). The
current understanding is that acrotony is established under the correlative in¯uences
that are active during the dormant period and appears to be mediated via a dominance
of the apical bud that is established prior to budburst in spring (Cook et al., 1998).
Cytokinins play a central role in budburst in spring (Faust et al., 1997). The
exogenous application of cytokinins promotes budburst during late dormancy and
spring (Arias and CrabbeÂ, 1975; Steffens and Stutte, 1989). Endogenous
cytokinins increase just prior to budburst in spring in the xylem sap of shoots
and in the bleeding sap from roots (Skene, 1975; Tromp and Ovaa, 1990; Cutting
et al., 1991). This cytokinin peak in spring is believed to originate from the roots,
as shown by the increase in cytokinin in the bleeding sap (Skene, 1975), and from
the shoot, as shown by the increase in cytokinin in the xylem sap of shoots that
had been arti®cially chilled following excision (Hewett and Wareing, 1973).
Trees exhibiting delayed foliation show a delay in the increase in the xylem sap
cytokinin concentration compared to trees treated with rest breaking agents,
which exhibit a more rapid increase in cytokinin (Cutting et al., 1991). While the
importance of root-derived cytokinin cannot be excluded, shoot-derived
cytokinins may be more directly involved with spring budburst in apple (Tromp
and Ovaa, 1990; Cutting et al., 1991; Faust et al., 1997).
With acrotony the terminal buds burst ®rst, followed closely by the lateral buds.
In our study, cytokinin content changes of the distal and proximal halves of 1-year
old, dinitro-o-cresol (DNOC) oil treated, Granny Smith and Braeburn apple shoots
were determined to assess whether cytokinin content changes within the shoots
correlate with the pattern of budburst. Cytokinin content of the xylem sap, wood, and
pooled bark and bud material was analysed shortly before and after budburst. In a
subsequent trial, the cytokinin content of fall-harvested Granny Smith shoots was
determined after chilling and subsequent forcing (not DNOC oil treated) in an
attempt to con®rm that cytokinins involved in spring budburst are shoot-derived.
2. Materials and methods
2.1. Plant material Ð 1995 trial
Granny Smith trees on M793 rootstock were topworked to Braeburn in the
winter of 1994. Both the Granny Smith and Braeburn shoots were allowed to
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
55
develop in the 1994/1995 growing season such that an ample supply of shoots
from both cultivars existed on the same trees. DNOC oil (5%) was sprayed on 29
August 1995 to break rest, and the ®rst shoots were harvested on 30 August 1995.
Thereafter, shoots were harvested weekly until 4 October 1995 giving a total of
six harvesting dates. One-year old, upright shoots (145058 mm in length) were
harvested in the early morning. At each harvesting date, 10 shoots of each cultivar
were harvested at random from ®ve adjacent trees. Five replicate shoots were
used for the xylem sap analysis, and ®ve replicate shoots were used for both the
bark and buds and the wood cytokinin analysis.
2.2. Cytokinin determination
Shoots were cut in half and the xylem sap was vacuum-extracted from both the
distal and proximal halves as described by Belding and Young (1989). The xylem
sap was extracted, rapidly frozen and stored at ÿ808C until analysis. The t-zeatin
riboside (ZR) concentration of the xylem sap was determined by means of
radioimmunoassay (Hofman et al., 1986) using a monoclonal ZR speci®c
antibody (Eberle et al., 1986). The bark and buds were peeled from the wood
separately from the distal and proximal shoot halves of each of the remaining ®ve
shoots. Bark and bud (pooled) samples as well as wood samples were cut into
1 cm segments, freeze-dried and milled. All samples were stored at ÿ808C until
analysis.
ZR and ZROG (t-zeatin riboside-O-glucoside) were extracted from the samples
by adding 10 ml of extraction solvent (80% methanol, 20% distilled water, BHT
(16 mg/l) and ascorbic acid (10 mg/l)) to 0.5 g freeze-dried material and
constantly stirring for 24 h at 48C. Undissolved solids were removed by
centrifugation (12 000 gn for 10 min at F
Wood
Bark and buds
0.0004
0.0001
LSD
13
82
Cytokinin content (ZR ng/g dry mass)
0 days forced
(dormant)
6 days forced
(green tip)
8 days forced
(leaves unfolding)
50
153
64
520
83
579
a
The shoots were forced at 258C with continuous illumination (215 mmol mÿ2 sÿ1 PAR). Mean
separation by LSD at 5% level; n5; d.f.2.
60
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
increases observed in DNOC treated shoots that burst buds in the ®eld (Figs. 2
and 3).
4. Discussion
Acrotony in M.9 apple shoots growing in Belgium, a region with an extended
ecodormant period, appears to be mediated via dominance of the distal buds reestablished during ecodormancy (Cook et al., 1998). The dominance of the distal
buds (terminal and/or laterals) associated with acrotony is possibly an effect of
polar auxin transport (Bangerth, 1989). Acrotony has been explained in terms of a
positional advantage of distal buds on orthotropic shoots, mediated via an auxin
effect (Faust et al., 1997). Similarly, gravimorphic responses of shoots inclined
from the normal orthotropic orientation have been ascribed to the effects of a
redistribution of endogenous auxins (Mullins and Rogers, 1971). It has also been
shown that during dormancy a distal disbudded shoot piece maintains an
inhibitory in¯uence on lateral buds under forcing conditions (Cook et al., 1998),
an effect that can be overcome by girdling (Champagnat, 1983). Notching, known
to disrupt polar auxin transport, is a commonly used practice to release lateral
buds from the inhibitions associated with acrotony (Greene and Autio, 1994). In
short, auxin undoubtedly plays a central role in correlative phenomena involved
in the determination of acrotony and thus tree architecture.
The inhibition of lateral bud outgrowth can also be overcome by locally applied
cytokinins alone (Steffens and Stutte, 1989), and in combination with gibberellins
(Promalin1 [N-(phenylmethyl)-1H-purine 6-amine and gibberellins A4A7] is
used commercially in South Africa to promote lateral branching). Endogenous
cytokinin increases in spring just prior to and during budburst in poplar shoots
removed from the plant during dormancy (Hewett and Wareing, 1973); in
dormant almond shoots stored at low temperatures for prolonged periods (Van
Staden and Dimalla, 1981); in apple trees exhibiting delayed foliation (Cutting
et al., 1991); and in non-chilled apple trees (Young, 1989). Provided that buds are
receptive, i.e. not endodormant, and conditions are conducive to growth, buds can
respond to this cytokinin peak, overcome correlative inhibitions and grow. This
cytokinin peak appears, thus, to act as a trigger for resumed growth (Faust et al.,
1997).
Both the cytokinin increase in shoots harvested after leaf drop and arti®cially
chilled in the absence of roots, and the accentuated cytokinin increase in the distal
wood, bark and buds, and xylem sap of DNOC treated shoots, indicate that these
cytokinins originate from within the shoot. These ®ndings con®rm the
postulations of Tromp and Ovaa (1990), Cutting et al. (1991) and Faust et al.
(1997). As cytokinins are mainly synthesised in the root tips (Dieleman et al.,
1997), it should be considered that cytokinins associated with budburst in spring
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
61
are stored in the shoot prior to becoming dormant, as has previously been
suggested by Van Staden and Dimalla (1981).
The higher rate of cytokinin increase in the distal xylem sap (Fig. 1), and the
higher distal wood ZR content (Fig. 2), re¯ect the overall acrotonic growth habit
of both cultivars. The distribution of cytokinins in pea and bean stems has been
reported to be under the control of polar auxin transport (Li et al., 1995). Auxins
appear to prevent the movement of cytokinins into the lateral buds under the
in¯uence of apical dominance. It is possible that auxins exported by the shoot
apex before and/or after endodormancy may be associated with the differential
distribution of cytokinins after chilling.
Branching in Granny Smith is strongly acrotonic. Shoot growth is restricted to
the dominant distal shoots with the development of few spurs and a latency of
proximal lateral buds, i.e. ``blind wood'' (Oosthuyse et al., 1992). This was
re¯ected by a large, initial difference between the distal and proximal wood ZR
content (distal contained more ZR, Fig. 2), and an enhanced ZR peak in the
proximal wood and bark/buds (Figs. 2 and 3). In the more ``spur type'' Braeburn,
distal shoots and proximal spurs are more numerous than in Granny Smith. In
Braeburn, which exhibits less dominance, a simultaneous increase of wood ZR of
both shoot halves (Fig. 2), and generally less bark/buds ZR with a greater increase
at budburst in the proximal half was observed (Fig. 3). The differential
distribution of cytokinin in the shoot may be involved in the pattern of budburst
and thus growth habit.
Delayed heading is commonly used to reduce the effects of acrotony. Heading
1-year old Granny Smith shoots from budburst to shortly after full-bloom
increases the number of shoots formed with a subsequent reduction in the total
length of new growth (Oosthuyse et al., 1992). The resultant shoots originate
from buds that would otherwise remain latent. Heading allows buds to burst in a
portion of the shoot with lower available cytokinin levels, the result of the gradient
observed in these data, at a time generally associated with less cytokinin availability, i.e. after the cytokinin peak. Possibly, buds are released under endogenous
conditions less favourable for the development of dominance phenomena.
In conclusion, while auxins play a central role in acrotony and thus tree
architecture, evidence presented here suggests that shoot-derived cytokinins may
also be involved in the acrotonic branching habit of apple. The cytokinin increase
in rootless shoots and differential distribution of cytokinin prior to sprouting,
support the hypothesis that shoot-derived, rather than root-derived, cytokinins act
to trigger spring budburst.
Acknowledgements
We thank Pia Nel, Department of Horticultural Science, University of
Stellenbosch for the cytokinin determinations, Prof. Elmar W. Weiler, Lehrstuhl
62
N.C. Cook et al. / Scientia Horticulturae 87 (2001) 53±63
fuÈr P¯anzenphysiologie, Ruhr UniversitaÈt, Bochum, Germany for supplying the
monoclonal ZR speci®c antibody, and both the FRD and the Deciduous Fruit
Industry for ®nancial support.
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