Botany School, Downing Street, Cambridge CB2 3EA, UK. Abstract. ... persisted for 9 to 12 weeks in storage at 10 C, but only 3 to 4 weeks when the tubers were ...
Planta
Planta (1985) 165:359-365
9 Springer-Verlag 1985
The control of bud dormancy in potato tubers. Evidence for the primary role of cytokinins and a seasonal pattern of changing sensitivity to cytokinin C.G.N. Turnbull* and D.E. Hanke Botany School, Downing Street, Cambridge CB2 3EA, UK
Abstract. Potato (Solanum tuberosum L.) tuber buds normally remain dormant through the growing season until several weeks after harvest. In the cultivar Majestic, this innate dormancy persisted for 9 to 12 weeks in storage at 10~ C, but only 3 to 4 weeks when the tubers were stored at 2~ At certain stages, supplying cytokinins to tubers with innately dormant buds induced sprout growth within 2d. The growth rate was comparable to that of buds whose innate dormancy had been lost naturally. Cytokinin-treatment did not accelerate the rates of cell division and cell expansion in buds whose innate dormancy had already broken naturally. Gibberellic acid did not induce sprout growth in buds with innate dormancy. We conclude that cytokinins may well be the primary factor in the switch from innate dormancy to the non-dormant state in potato tuber buds, but probably do not control the subsequent sprout growth. Key words: Bud dormancy - Chilling - Cytokinin and dormancy - Solanum (bud dormancy) - Tuber storage.
Introduction Although there is extensive literature on dormancy seeds and buds, very little of it concerns the internal mechanisms by which dormancy is regulated and this aspect of the subject is not well understood. * Present address and address for correspondence: Department
of Botany, University, Glasgow G12 8QQ, UK Abbreviations: tio 6ade = 6-(4-hydroxy-3-methylbut-trans-2-enyl amino)purine, zeatin; tio6ado =6-(4-hydroxy-3-methylbut-trans-
2-enyl amino)-9-13-D-ribofuranosyl purine, zeatin riboside
The word 'dormancy' has been used to describe different states of growth arrest (Davidson 1958; Rappaport and Wolf 1968; Hemberg 1970; Taylorson and Hendricks 1977; Saunders 1978; Harkett 1981), and it is important to distinguish clearly the two main reasons for a dormant seed or bud failing to grow. In 'innate' dormancy, germination or growth is prevented by internal inhibition even in a favourable external environment. Once innate dormancy is broken, germination or growth is possible given suitable conditions. However, if the requirements for growth, usually water and warmth, are not met, a state of 'imposed' dormancy will be entered (see discussion in Saunders 1978). The superficial result (i.e. lack of growth) is the same in both cases, but the underlying, internal physiological states are very different. This study concerns the release of the buds of the potato tuber from innate dormancy into either imposed dormancy or sprout growth. Various parameters of potato tuber dormancy have been described. The duration of innate dormancy depends largely on the cultivar and to some extent on the conditions during tuber growth (Davidson 1958; Lindblom 1970; Wurr and Allen 1976; Burton 1978; Harkett 1981). Of the environmental conditions affecting dormant tubers, only temperature seems to have a major influence. There is evidence that chilling (1-3 ~C) shortens dormancy in some varieties with lengthy dormant periods (Wurr and Allen 1976; Harkett 1981), but Davidson (1958) and Burton (1978) reported that only high storage temperatures (approx. 30~ could prematurely terminate dormancy. Major effects of applied plant growth substances on potato tuber dormancy have been obtained, leading inevitably to the notion that dormancy is regulated in vivo by these substances.
360
C.G.N. Turnbull and D.E. Hanke: Cytokinins and potato bud dormancy
Ethylene, gibberellins and abscisic acid have each been cast in a major role, but so far the evidence is unsatisfactory. Although ethylene could induce sprouting in stored tubers (Denny 1926a, b; Rosa 1923, 1924, 1928), 2-chloroethanol ('ethylenechlorhydrin') was much more effective (Madec and Perennec 1969; Rylski et al. 1974) and the amount of ethylene released by tubers was so small compared with that needed to induce sprouting (Poapst et al. 1968) that it seems unlikely that ethylene release alone could regulate potato bud dormancy. From the results of several years' work, Rappaport and co-workers concluded that gibberellins and abscisic acid were probably not primary regulators of dormancy (Rappaport and Wolf 1968; Shih and Rappaport 1970, 1971), in agreement with Doorenbos (1958). Cytokinins are apparently involved in the process of tuber initiation (Palmer and Smith 1969; Claver 1970; Okazawa 1970; Forsline and Langille 1975). There is also evidence that they play a role in the regulation of tuber bud dormancy but this aspect has received surprisingly little attention. Hemberg (1970) convincingly demonstrated that innately dormant potato tuber buds could be induced to sprout by the application of cytokinins. Although Engelbrecht and Bielinska-Czarnecka (1972) found an increase in endogenous cytokinin levels in tubers towards the end of innate dormancy, Van Staden and Dimalla (1978) reported changes only after the end of innate dormancy. Relying on bioassays for identification and quantification, neither of these results can be regarded as definitive. We present here the results of a more detailed investigation of the effects of exogenous cytokinins on potato tuber buds. Materials and methods
gibberellic acid. Chemicals for electron microscopy were from Agar Aids, Stanstead, UK.
Induction of sprouting by cytokinins. Only two of several techniques tested gave consistent results 1. Dipping excised pieces of tubers. Washed tubers were surface-sterilised by 10 min immersion in NaC10 solution (1% available C12) and rinsed in running tap water until no C12 odour remained. Cylinders centred on single lateral eyes were cut with a cork-borer, diameter 22 ram, and trimmed to 10 mm height. The explants were rinsed in three changes of distilled water to wash off the contents of wounded cells, then blotted dry. Sprouting was induced by immersion for 5 rain in a cytokinin solution (most frequently 50 laM tio6ade) or distilled water for controls, then the cylinders were transferred to a dark incubator, 20~ relative humidity approx. 95%. Gibberellic acid was always supplied at 50 gM. Using this method, sprout growth showed little variation between replicate samples probably because initial rates were related to the amount of attached storage tissue. The technique was particularly suited to the production of samples for microscopy. Its drawback is the wounding caused by excision which could have effects on sprouting. 2. Injecting whole tubers. Except during the very early stages of tuber development, surface application of cytokinins, as described above, to whole tubers was ineffective, probably because of the impervious, suberised cuticle, and so the growth substance was injected under pressure, a method by which wounding is minimised. The needle of a syringe containing 0.3 ml cytokinin solution (distilled water for controls) was inserted through a point near the apex down the long axis of the tuber, then withdrawn 5 mm to ensure the aperture was not blocked. The elasticity of the tuber tissue around the needle gave an effective seal such that pressure could be exerted on the plunger high enough to infiltrate the solution into adjacent tissue. Although most of the solution was forced back out of the tuber when the needle was withdrawn, 10-20 gl (estimated as described in Turnbull and Hanke i985) did remain inside. As judged by similar injections of red ink, immediate infiltration to a radius of 3 mm was found around the whole length of the needle channel. Injecting iodine solution revealed that the blackening of starch grains associated with cell wounding was restricted to the surface of the channel, i.e. to where the needle point had cut the tissue. Injecting 400 gM cytokinin or gibberellic acid solution, an estimated dose of 4-8 nmol was supplied to the centre of each tuber.
Measurement of sprout growth. The percentage of growing Plant material. For all experiments, tubers of Solanum tuberosum L,. cv. Majestic were used, as this cultivar has a naturally long dormant period. Certified seed tubers were obtained locally each year and planted in the Cambridge University Botanic Garden during the first week of April. Main harvest was approx. 22 weeks later in mid-September, two weeks after haulm die-back. Some experiments were carried out on immature tubers, from the time of their initiation at the beginning of June right through to the end of the growing season. Damaged, diseased and green tubers were rejected. The remainder of the mature tubers were stored in the dark at constant temperature (2 ~C or 10~C) until required.
Chemicals. The cytokinins 6-(4-hydroxy-3-methylbut-trans-2enyl amino)purine (tio6ade) and 6-(4-hydroxy-3-metylbut-trans2-enyl amino)-9-[3-D-ribofuranosyl purine (tio6ado) were obtained from Sigma Chemical Co., Poole, Dorset U K as was
sprouts was scored by observation under a stereoscopic dissecting microscope. Dormant, non-growing bud tissue is matt, brownish-yellow; new, growing sprout tissue is glistening, translucent, pale yellow-green. Sprout growth rates were estimated by daily measurement using a graticule eye-piece (accuracy • 0.1 mm) in a steroscopic dissecting microscope. Alternatively, growth was estimated by excising sprouts into liquid N2 and weighing immediately to the nearest 0.1 mg. Very early rates of cell division and expansion in cytokinin-induced sprouts were determined by cell counting and light microscopy.
Cell counting. The height and basal diameter of each sprout were measured to the nearest 0.1 ram. The shape of the sprout corresponded approximately to half a prolate ellipsoid and the tissue volume was, therefore, estimated (ignoring small spaces between leaf primordia) as 2/3 rc r 2 h. Sprouts were then excised at the base and immersed for 24h in 5% (w/v) chromium
C.G.N. Turnbull and D.E. Hanke: Cytokinins and potato bud dormancy trioxide solution. This macerate was flushed repeatedly through a 22-gauge syringe needle to improve cell separation. Cell counts per unit volume of suspension were made using an Improved Neubauer Haemocytometer slide.
Light microscopy. Using single-eye cut cylinders, a series of sprouts were harvested at the following times after excision and cytokinin (50 gM tio6ade) treatment: 0, 8, 17, 25, 32, 42, 48 h. They were fixed for two weeks in a standard FAA mixture (formalin, CH3OH, C2HsOH, H20, 2:2:25:11, by vol.). At least five randomly selected sprouts were taken for each time point and the same number for (water-treated) controls. After dehydration in a C2HsOH series, the tissue blocks were infiltrated and embedded in Paraplast wax. Serial longitudinal sections were cut at 10 gm thickness. Those through the median of the apex were selected and stained with O-Safranin and Fast Green. An average of 14 approximately median sections were scored for mitoses for each apex. The elongation rates of the sprouts were estimated by measuring the height of each apex above a fixed plane in the non-growing tuber tissue, in this case the vascular strands. Any tissue shrinkage was assumed to be constant, and was not corrected for.
Results
The influence of tuber age, storage temperature and cytokinin treatment on dormancy and sprouting in tuber buds was studied over three seasons. The results of 27 experiments are summarised in Table 1. Detailed results from some of these are presented in Figs. 1-4. All of the work was on whole tubers, except for the microscopy which was carried out on excised buds.
Growth in the field. Seed tubers were planted in early April and new tubers of cv. Majestic were initiated at the end of May. Three weeks later, dormant buds had developed which normally remained innately dormant until well after the termination of tuber growth in late August. Experiments with immature tubers detached .from growing plants. Normally there was very little sprout growth during four to six weeks at 20 ~C, but treatment with cytokinin induced sprouting in apical and lateral buds within 2 d. At this stage ("new potatoes"), immersion in cytokinin solution for 5 min was effective, perhaps because the cuticle of the tuber was not yet sufficiently developed to bar cytokinin entry. By 7 d after immersion in 50 gM tio6ade or tio6ado solution, sprout length had usually increased 2- to 3-fold, but growth was not sustained beyond this time. Water (controls) or 50 gM gibberellic acid solution resulted in negligible growth over 14 d. Gibberellin treatment of tubers at other stages likewise had no effect on innate dormancy.
361
Table 1. Summary of results of experiments on sprouting of whole tubers. Experiments were carried out over three seasons and are arranged in chronological order. Cytokinin treatment was by injection except for small tubers which were dipped (indicated by*) Time Time since in tuber storage initiation a
Storage temp.
(weeks)
(~
(weeks)
Bud CKdormancy induced in untreated sprouting c tubers at 20~ b
1980: seed tubers planted 3 April, tubers lifted and stored 22 September 3 8 10
2 2 2
+ ++ 4-+
+ 4,
1981 : seed tubers planted 3 April, tubers lifted and stored 16 September 3 4 8 12 13 14 15
--
1
2
2 3 4 4 6 7 7 10 10 1l 12
2 2 2 10
10
+4-*
--
4-4-*
--
4-4-
--
4.
-4. + +4-
4, 4. +
--
+
--
--
--
4-+
2 10
--
2
--
+4-
10
--
+
10 10
+ +
+4++
1982: seed tubers planted 10 April, tubers lifted and stored 2 September 2 3 4 5 8
-
11
-
3
4 4
2 2 10
-
+ 4-+ --
4-4-* 4. ++ + 4. 4-
+
a Age of immature tubers b Measure of innate dormancy: - , no sprouting; + , slow or low % sprouting; + + , all sprouting fast c Measure of ability to respond to CK: - , no growth; + , transient growth, 2-3 d; + 4-, growth sustained > 4 d
Experiments with mature tubers, September to November. As tuber development progressed, the extent of cytokinin-inducible sprouting decreased. For mature tubers stored for up to three weeks, sprout length increased for only 2-3 d at most
362
C.G.N. Turnbull and D.E. Hanke: Cytokinins and potato bud dormancy
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140
Fig. 1 a, b. A comparison of the effects of four weeks storage at 2~ or 10~ on the state of dormancy of apical (a) and lateral (b) buds of whole potato tubers after transfer to 20~ on 29 September 1982. Tubers were stored: at 2~ (m); at 10~ and injected with 400 gM tio6ade 5 d after transfer to 20~ ( ~ ) ; at 10~ and injected with HzO 5 d after transfer to 20~ (O). The a r r o w indicates the time of injection. Each point is taken from a sample of 14 tubers
Fig. 2a, b. A comparison of the effects of nine weeks storage at 2~ ( i , [ ~ ) or 10~ ( O , 9 on the growth of apical ( i , O ) and lateral ( 9 9 buds of whole potato tubers after transfer to 20 ~C on 24 November 1981. a Cell number; b total sprout volume. Each point is the mean of 5-10 buds. Bars are 95% confidence limits from Student's t-tables
(Table 1). Insensitivity to cytokinin lasted about 10 weeks in tubers stored at 10~ and the buds on these tubers stayed innately dormant (Fig. 1). It was repeatedly observed that when tubers were stored at 2 ~C, innate dormancy disappeared within three to four weeks (typical result: Fig. 1). All such cold-treated tubers showed sprout growth within 4 d of transfer to 20 ~C. After nine weeks at 2 ~C, sprouting was initiated even sooner on transfer to 20 ~C: within 2 d, cell division and cell expansion had begun, and after 6 d the lateral sprouts had increased more than 400% in volume with an increase in ceil number of more than 200% (Fig. 2). This indicates that the increase in size is not due solely to cell expansion but also involves cell division. Buds of tubers stored at 10~ over the same period showed no cell division or expansion.
20 ~C, and then examining the sprouts for mitosis and extension growth. No cell division or extension growth could be detected in buds stored at 2~ Both processes began around 20 h after transfer to 20~ were independent of cytokinin treatment (Fig. 3) and may have been slightly accelerated as a result of treatment with tio6ade. Separate measurements of the height of leaf primordia and the stem apex showed that both began to increase simultaneously, and followed the pattern in Fig. 3 b.
The effect of cytokinin on the growth of non-dormant buds. Tubers stored for eight weeks at 2~ had buds with no innate dormancy. The effect of cytokinin on sprout growth after the break of innate dormancy was investigated by immersing single-eye cylinders from such tubers in 50 gM tio6ade or water for 5 min, transferring them to
Experiments with mature tubers, December onwards. During December, tubers stored at 10~ began to sprout, indicating that innate dormancy had disappeared, though at widely differing times for different individual tubers. Figure 4 shows the proportion of tubers with growing sprouts at different times in a period of observation during December. At 47 h into the period of observation, non-sprouting (i.e. still innately dormant) tubers were selected and injected with 400 gM tio6ade solution or water. Cytokinin treatment rapidly induced apical and lateral sprouting in 60% and 100% of the tubers, respectively, whereas controls
C.G.N. Turnbull and D.E. Hanke: Cytokinins and potato bud dormancy
363
c 0
]00 O. u~
10
u~ 0
E
5
r~ u~
~
13_
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133
.c
r-
131
s r
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io ~o 3'0 ~:o ~o time at 20 ~
0
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50 time at 20~
i
i
i
i
,
100
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140
(hours)
Fig. 3a, b. The effect of cytokinin-treatment on cell division (a) and whole-sprout elongation (b) of non-dormant potato tuber buds at 20~ Single-eye cylinders were taken on 16 November 1980 from tubers stored eight weeks at 2~ to break innate dormancy. Cylinders were dipped in 50 gM tio6ade ([~) or H20 ( 9 For each point, estimates of cell division were made from a minimum of fifty 10-gm-thick sections taken from at least four apices. Bars are 95% confidence limits from Student's t-tables
Fig. 4 a, b. Cytokinin-induced sprouting of apical (a) and lateral (b) buds of potato tubers near the end of innate dormancy. After 12 weeks storage at 10~ tubers were transferred to 20~ (10 December 1981) and the % of tubers with growing sprouts was determined for all tubers ( I ) and for selected dormant tubers injected at the time indicated (arrow) with 400 gM tio6ade ([Z) or H20 ( 9 Each point is taken from a sample of at least six tubers
showed minimal sprouting (Fig. 4). Data obtained at different times in the same experiment showed that cell division and cell expansion in cytokinintreated sprouts commenced within 48 h and the rates were similar to those of sprouts whose innate dormancy had ended naturally. There was no increase in cell number in buds which did not grow out. Buds on tubers kept at 10~ after the end of innate dormancy grew out rapidly to sprouts 5-10 cm in length by early January. In contrast, those at 2~ remained non-growing (0.5 • 0.1 mm long for lateral buds) but viable, for long periods: in one experiment, when transferred to 20~ sprouts grew from tubers which had been stored for 21 months at 2 ~C.
formed in the axils of the scale leaves of young developing tubers, the buds cease growing and do not grow out, even in conditions favourable to growth, until several weeks after harvest of the mature tubers. We consider this phase to be caused by innate dormancy of the buds themselves, and not maintained by influences outside the bud, since buds isolated during this period (but still attached to storage tissue) remained innately dormant. Also, apical dominance is unlikely to be involved, since the apical bud of the tuber is dormant from the earliest stages after tuber initiation, after which the stolon tip grows by cell divisions in the subapical meristem and enhanced, radial cell expansion (Leshem and Clowes 1972). For the first six weeks or so after tuber initiation, these innately dormant buds could be induced to grow out by supplying cytokinins to the tuber. Radioimmunoassay of extracts of buds from injected tubers, has indicated that a significant proportion of the dose moved into the buds (Turnbull and Hanke 1985). The ability to sprout in response to cytokinin-treatment disappeared
Discussion
The characteristics of dormancy in buds on potato tubers, cultivar Majestic, were studied over a threeyear period. These characteristics may be different for other cultivars. As soon as the buds have
364
C.G.N. Turnbull and D.E. Hanke: Cytokinins and potato bud dormancy
from immature tubers older than approx, six weeks. Cytokinin-treatment was ineffective for the remainder of the period of tuber growth and, in tubers stored at 10~C, until ten weeks after harvest. In tubers stored at 10~ the ability to respond to cytokinin reappeared for the last three weeks of innate dormancy. Cytokinins induced cell division and cell expansion within 48 h of injection, whereas control buds showed no such changes. Stored at 2~ tubers changed from innate dormancy unresponsive to cytokinin to imposed dormancy within three to four weeks of harvest. An intermediate phase of cytokinin-responsive innate dormancy was never found, either because this phase was too short to detect or because it was absent. A similar pattern of changing response to exogenous growth substances at different times within the dormant period has been reported for winter buds in other species. Leike (1967) found that innately dormant buds of Acer negundo would s p r o u t in response to gibberellin-treatment during October and then again at the end of December, but not during the middle section of the period of innate dormancy. Similar results were reported for Acer pseudoplatanus and Populus nigra var. italica. However, for Alnus glutinosa and Fagus sylvatica, gibberellin-treatment served only to promote growth after the end of innate dormancy (Leike 1967). Weinberger (1969) found that innately dormant buds of Prunus persica would sprout in response to treatment with solutions of cytokinin only towards the end of the period of innate dormancy, but not in the middle section of this period. The loss and re-establishment of cytokinininducible sprouting takes place in potato buds which remain innately dormant, i.e. no growth is associated with these transitions, and the phenomenon can best be described as changes in 'tissue sensitivity' as outlined by Trewavas (1981). As yet, nothing is known about the molecular basis of this sensitivity. The phase of cytokinin-inducible sprouting early in tuber development is anomalous and may be due to persistence of cytokinin sensitivity in the tissue from the immediately preceding phase of tuber initiation. Stolon tips cultured in vitro require cytokinin for tuber initiation (Palmer and Smith 1969; Claver 1970). The effects of cytokinin on sprouting of potato tubers can be contrasted with the effect of gibberellins on this process. There is good evidence that gibberellins have a functional role in the growth of potato tuber buds (Rappaport and Wolf
1968). Our results, for cold-treated, completely non-dormant buds, indicated that cytokinins did not enhance cell division or cell expansion rates (Fig. 3). Similarly, Weinberger (1969) found that cytokinin treatment of non-dormant peach buds had no effect on growth. In contrast, cytokinins were able to trigger the end of innate dormancy in potato tubers whereas gibberellins were not, indicating that cytokinins may be a primary factor in the control of dormancy in this tissue. The existence of a cytokinin-sensitive phase of innate dormancy indicates that there is a stage when the endogenous cytokinin concentration is too low to initiate this transition. That this phase is followed by natural dormancy-break indicates either that the endogenous concentration of cytokinins increases, or that sensitivity increases to a level which allows the endogenous cytokinins to become effective. To understand the system better, an analysis of the endogenous cytokinins and cytokinin metabolism of the buds has been carried out (Turnbull and Hanke 1985). The general conclusion from this work is that changes in tissue sensitivity to cytokinin are an important component of the control of dormancy in potato tuber buds. This work forms part of a Ph.D. dissertation submitted to the University of Cambridge by C.G.N.T. who was supported by a Post-Graduate Agricultural Studentship from the Department of Agriculture and Fisheries for Scotland. We thank the National Institute of Agricultural Botany, Huntingdon Road, Cambridge, for a generous gift of tubers of the cv. Majestic which were used in preliminary work during 1979 and 1980.
References Burton, W.G. (1978) The physics and physiology of storage. In: The potato crop, pp. 545-606, Harris, P.M. ed. Chapman and Hall, London Claver, F.K. (1970) The effects of abscisic acid on tuberisation of potato sprouts in vitro. Phyton 27, 25-29 Davidson, T.M.V. (1958) Dormancy in the potato tuber and the effects of storage conditions on initial sprouting and on subsequent sprout growth. Am. Potato J. 35, 451M65 Denny, F.E. (1926a) Hastening the sprouting of dormant potato tubers. Am. J. Bot. 13, 118 125 Denny, F.E. (1926 b) Second report on the use of chemicals for hastening the sprouting of dormant potato tubers. Am. J. Bot. 13, 38(~396 Doorenbos, J. (1958) Effect of gibberellic acid on sprouting of potatoes. Neth. J. Agric. Sci. 6, 267-270 Engelbrecht, L., Bielinska-Czarnecka, M. (1972) Increase of cytokinin activity in potato tubers near the end of dormancy. Biochem. Physiol. Pflanz. 163, 499-504 Forsline, P.L., Langille, A.R. (1975) Endogenous cytokinins in Solanum tuberosum as influenced by photoperiod and temperature. Physiol. Plant. 34, 75-77 Harkett, P.J. (1981) External factors affecting length of dormant period in potatoes. J. Sci. Food Agric. 32, 102-103 Hemberg, T. (1970) The action of some cytokinins on the rest-
C.G.N. Turnbull and D.E. Hanke: Cytokinins and potato bud dormancy period and the content of acid growth-inhibiting substances in potato. Physiol. Plant. 23, 850-858 Leike, H. (1967) Wirkung yon Gibberellins/iure auf ruhende Knospen verschiedener Geh61ze. Wiss. Z. Univ. Rostock 16, 607-608 Leshem, B., Clowes, F.A.L. (1972) Rates of mitosis in shoot apices of potatoes at the beginning and end of dormancy. Ann. Bot. 36, 687 691 Lindblom, H. (1970) Sprouting tendencies of stored potatoes. Potato Res. 13, 159-166 Madec, P., Perennec, P. (1969) Lev+e de dormance de tubercules de pomme de terre d'fige different: action de la rindite, de la gibberelline et de l'ceilleternage. Eur. Potato J. 12, 96-115 Okazawa, Y. (1970) Physiological significance of endogenous cytokinin occurred in potato tubers during their developmental period. Proc. Crop Sci. Soc. Japan 39, 171-176 Palmer, C.E., Smith, O.E. (1969) Effect of abscisic acid on elongation and kinetin-induced tuberization of isolated stolons of Solanum tuberosurn L. Plant Cell Physiol. 10, 657-664 Poapst, P.A., Durkee, A.B., McGugan, W.A., Johnston, F.B. (1968) Identification of ethylene in gibberellic-acid-treated potatoes. J. Sci. Food Agric. 19, 325-327 Rappaport, L., Wolf, N. (1968) The problem of dormancy in potato tubers and related structures. Symp. Soc. Exp. Biol. 23, 219-240 Rosa, J.T. (1923) Abbreviation of the dormant period in potato tubers. Proc. Soc. Hort. Sci. 20, 180 187 Rosa, J.T. (1924) Report on potato dormancy abbreviation experiments. Proc. Potato Assoc. Am. 11, 48-52 Rosa, J.T. (1928) Effects of chemical treatments on dormant potato tubers. Hilgardia 3, 125 142 Rylski, I., Rappaport, L., Pratt, H.K. (1974) Dual effects of
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ethylene on potato dormancy and sprout growth. Plant Physiol. 53, 658-662 Saunders, P. (1978) Phytohormones and bud dormancy. In: Phytohormones and related compounds - a comprehensive treatise, vol. 2, pp. 423-445, Letham, D.S., Goodwin, P.B., Higgins, T.J.V., eds. Elsevier, Amsterdam Shih, C.Y., Rappaport, L. (1970) Regulation of bud rest in tubers of potato, Solanum tuberosurn L. VII. Effect of abscisic acid and gibberellic acid on nucleic acid synthesis in excised buds. Plant Physiol. 45, 33-36 Shih, C.Y., Rappaport, L. (1971) Regulation of bud rest in tubers of potato, Solanurn tuberosurn L. VIII. Early effects of gibberellin A 3 and abscisic acid on ultrastructure. Plant Physiol. 48, 31-35 Taylorson, R.B., Hendricks, S.B. (1977) Dormancy in seeds. Annu. Rev. Plant Physiol. 28, 331-354 Trewavas, A.J. (1981) How do plant growth substances work? Plant Cell Environ. 4, 203-228 Turnbull, C.G.N., Hanke, D.E. (1985) The control of bud dormancy in potato tubers. Measurement of the seasonal pattern of changing concentrations of zeatin-cytokinins. Planta 165, 366-376 Van Staden, J., Dimalla, G.G. (1978) Endogenous cytokinins and the breaking of dormancy and apical dominance in potato tubers. J. Exp. Bot. 29, 1077-1084 Weinberger, J.H. (1969) The stimulation of dormant peach buds by a cytokinin. Hort. Sci. 4, 125-126 Wurr, D.C.E., Allen, E.J. (1976) Effects of cold treatments on the sprout growth of three potato varieties. J. Agric. Sci. 86, 221 224 Received 5 July 1984; accepted 8 January 1985
