Annals of Botany 82 : 21–27, 1998

Carbon Dioxide, Oxygen and Ethylene Effects on Potato Tuber Dormancy Release and Sprout Growth W A R R E N K. C O L E M A N Potato Research Centre, Agriculture and Agri-Food Canada, 850 Lincoln Road, Fredericton, New Brunswick, E3B 4Z7, Canada Received : 12 November 1997

Returned for revision : 11 February 1998

Accepted : 13 March 1998

The possible roles of oxygen and carbon dioxide treatments in the presence or absence of ethylene on tuber dormancy release in potato (Solanum tuberosum L.) were examined. Using two gas compositions (I : 60 % CO –20 % O –20 % # # N and II : 20 % CO –40 % O –40 % N ), the phase of tuber dormancy and previous storage temperature were # # # # demonstrated to be important parameters for dormancy release by these gas mixtures. Gas I caused decreased abscisic acid (ABA) levels within 24 h regardless of previous storage temperature, although this effect was reversible. Exogenous C H , an effective dormancy release agent, also caused decreased ABA levels within 24 h. It also enhanced # % dormancy release and further promoted ABA losses by gas I. Gas II treatment led to slight reductions in ABA levels that were further decreased by C H . Sprout length was modelled successfully by multiple regression analysis in terms # % of glucose and ABA levels within the apical eye tissues of Russet Burbank tubers immediately after, and regardless of, previous gas treatments or storage temperatures. # 1998 Annals of Botany Company Key words : Solanum tuberosum, potato, abscisic acid, ethylene, carbon dioxide, oxygen, dormancy.

abscisic acid (ABA), in tuber dormancy induction and maintenance (Suttle and Hultstrand, 1994 ; Suttle, 1995). Dormancy release in buds and seeds by high concentrations The latter studies supported earlier work that indicated a of carbon dioxide and oxygen has been observed repeatedly, major role for inhibitors such as ABA in tuber dormancy although the specific physiological mechanisms are un- (Hemberg, 1985). However, the relationship between ABA known (Esashi, 1991 ; Wiltshire and Cobb, 1996). In levels and dormancy release by such effective treatments as potatoes (Solanum tuberosum L.), Thornton (1933) initially partial or complete anaerobiosis, or high CO –O concen# # observed that tuber dormancy could be broken effectively trations, is unknown (Rakitin and Suvorov, 1935 ; Coleman, with 40–60 % CO and 20 % O applied to tubers con- 1987). # # tinuously for 3–7 d at 25 °C. He subsequently demonstrated Burton (1958) demonstrated that 1–2 % CO levels # an enhancement of this effect by high (20–80 %) concen- stimulated sprout growth. Subsequent work (Burton, 1968) trations of O (Thornton, 1939). He hypothesized that indicated a progressive increase in the O requirement (from # # normal termination of tuber dormancy was due to a 2 to 23 %) for initiating and sustaining sprout growth as relatively O -impermeable periderm while CO acted as a tubers aged. Burton (1968) hypothesized ‘ an optimally # # ‘ metabolic regulator ’. Subsequent work, however, did not anaerobic reversible metabolism of a growth inhibitor ’ support the role of O as the major and sole factor that during the hypoxic phase although no data was provided. # regulated dormancy (Sawyer and Smith, 1955). Besides re-emphasizing a possible link between dormancy, In view of the suggested roles of CO –O in ethylene sprout growth and an endogenous inhibitor system, Burton’s # # production (Esashi, 1991) and the role of ethylene (C H ) in studies highlighted the fact that any evaluation of dormancy # % dormancy release of tubers (Rylski, Rappaport and Pratt, release mechanisms must include a study of sprout 1974), it is possible that at least part of the CO –O effect elongation since we define the former developmental event # # resides in the increased production of endogenous C H by in terms of the latter growth feature. Other work has # % promoting, for example, the production of C H from demonstrated a significant negative correlation between # % aminocyclopropane-1-carboxylic acid (ACC ; Esashi, 1991 ; endogenous ABA in tubers of ten potato cultivars and Mattoo and White, 1991 ; Smith and John, 1993). However, ensuing sprout growth rate (Coleman and King, 1984). as noted by Rylski et al. (1974), the limited effectiveness of Consequently, the present study (1) examined the reexogenous C H suggests that it is only one component in lationship between high O and CO treatment levels on # % # # an apparent multi-factor dormancy control system for tuber dormancy release and sprout growth in terms of potato tubers that encompasses both inhibitors and previous storage temperature and tuber age ; and, (2) promoters. evaluated the hypothesis that the CO –O action on # # While gibberellins (GA) may promote sprout growth in dormancy release and sprout growth of potato tubers is due tubers after innate dormancy has been removed (Stallknecht, to an effect on ABA and sugar levels. 1984), previous work has implicated the growth inhibitor, Two CO –O mixtures (60 % CO –20 % O , gas I and # # # # 0305-7364}98}070021­07 $30.00}0 bo980645 # 1998 Annals of Botany Company INTRODUCTION

22

Coleman—Dormancy and Sprout Growth in Potato Tubers

20 % CO –40 % O , gas II with the balance nitrogen), which # # had previously demonstrated pronounced effectiveness in dormancy release and sprout growth of tubers, were studied (Thornton, 1933, 1939 ; Reust and Gugerli, 1984 ; Coleman and McInerney, 1997).

MATERIALS AND METHODS Tuber production Tubers of the cultivar Russet Burbank were produced in New Brunswick at the Potato Research Centre of Agriculture and Agri-Food Canada during the summers of 1991, 1992 and 1993. Plants were top killed after 80 d and dug 2 weeks later, in accordance with normal seed production practices. Tuber periderm matured at 13–15 °C for 10–14 d followed by storage at 3 or 13 °C until required.

Gas deliŠery system A semi-automated gas delivery and flow-through treatment system was designed for application and continuous monitoring by gas chromatography (Model 5890 series II, Hewlett-Packard Canada Ltd., Mississauga, ON, Canada) of concentrations of carbon dioxide, oxygen, nitrogen and ethylene (Coleman and McInerney, 1997).

Tuber experiments Uniform tubers (80–120 g f.wt per tuber ; 20–40 tubers per treatment) were treated in the sample chambers for periods of 1–7 d at 22–24 °C with a range of gas mixtures composed of nitrogen, carbon dioxide, oxygen and}or ethylene. Untreated tubers and tubers treated for 24 h at 22–24 °C with bromoethane (BE) vapour, an effective dormancy release agent (0±22 ml liquid per litre of treatment chamber ; Coleman, 1983) served as reference controls. After treatment, tubers were either placed in controlled environmental facilities (20 °C constant dark), or planted directly in sterile loam (approx. 2 cm soil covering), under glasshouse conditions, with supplemental fluorescent and incandescent lighting (14 h photoperiod) and variable temperature conditions (22–25 °C day and 15–18 °C night).

Sugar analysis Fructose, glucose and sucrose in tuber samples (six tubers per sample) were determined using a high performance liquid chromatograph (Series 4 HPLC ; Perkin-Elmer Canada, Mississauga, ON, Canada). Sugars in aqueous plant extracts were separated on a Sugar Pak I column (6±5¬300 mm ; Waters Ltd., Mississauga, ON, Canada) using an aqueous mobile phase (EDTA, calcium-disodium salt, 50 mg l−") at 80 °C and a flow rate of 0±5 ml min−". Sugars were quantitatively determined with a Waters (model 410) differential refractometer and analysed with a chromatography data system (EZChrom, Shimadzu, c}o ManTech Associates Inc., Guelph, ON, Canada).

Abscisic acid analysis ABA was extracted from potatoes (six tubers per sample) using a solvent extraction method originally developed for gas chromatography (Coleman and King, 1984). The final (chloroform) extract was evaporated to dryness and dissolved in a mobile phase (water-acetonitrile-acetic acid ; 750 : 250 : 15) before injection (1±0 ml sample) into a Shimadzu LC-10 liquid chromatograph. ABA was separated on a Supelco (LC-18, Supelco Canada, Oakville, ON, Canada) column (15 cm¬4±6 cm ; 5 µ particle size) and subsequently identified with a Shimadzu SPD-M6A diode array detector in conjunction with a Shimadzu EZChrom chromatography data system. Dormancy release and associated sprout growth Tuber dormancy release and sprout emergence from the soil surface were recorded two–three times per week using d after planting the ‘ mother ’ tuber (DAP) as a time basis (Cho, Iritani and Martin, 1983). In the present study, three reference stages were distinguished : an early initial growth stage that was characterized in this study by a 1 mm reference threshold, an intermediate stage of linear growth rate (3 mm reference) and a final stage of exponential sprout growth rate (10 mm reference). ‘ Phases ’ of the dormancy period were defined in terms of cultivar and DAP. For example, Russet Burbank seed tubers progressed through an arbitrarily defined early (80–150 DAP) and late (150–220 DAP) dormancy phase. Dormancy release would normally occur in untreated tubers during the latter part of the late phase. Statistical analysis Dormancy release and emergence data were evaluated using survival and probit analysis (Finney, 1971 ; Scott, Jones and Williams, 1984 ; Lee, 1992). The presence of censored observations and skewed data distributions led to the use of the following survival tests for comparing two or more survival distributions : Peto and Peto’s generalization of Wilcoxon’s two-sample rank sum test, Gehan’s generalization of Wilcoxon’s two-sample rank sum test and the log rank test. Cumulative percentages of dormancy release or emergence were transformed to probits and used as dependent variables while DAP was used as the independent variable in a linear regression model of the form : P ¯ a­b²log (DAP)´ "! where p is the probit value, a is the y-intercept and b is the slope. The emerging sprout populations can be characterized as follows (Berrie and Taylor, 1981) : (1) the most likely or ‘ threshold ’ time for dormancy release (G ) or emergence ! (S ; x-intercept) ; (2) rate of dormancy release or emergence ! (b) ; (3) dormancy or emergence ‘ scatter ’ or variability (s.e. of b) ; and, (4) time to 50 % dormancy release (G ) or &! emergence (S ). In the present study, emphasis was placed &! on determining G and S . &! &!

23

Coleman—Dormancy and Sprout Growth in Potato Tubers Probit, multiple regression and survival analyses were carried out using the statistical software NCSS version 5.03 (Dr. Jerry Hintze, Kaysville, Utah, USA) and Systat 6.0 for Windows (SPSS Inc., Chicago, IL, USA).

Effect of tuber age and storage temperature Tubers that were treated at 118 DAP during the early dormancy phase (80–150 DAP) were quite sensitive to the previous storage conditions in terms of responsiveness to gas mixtures I and II. Tubers removed from a 3 °C storage responded to gas II regardless of the reference length chosen. However, when tubers were removed from a 13 °C storage, they were not affected by this treatment (Table 1). When tubers were treated with gas I, an opposite response occurred. While tubers from 3 °C responded significantly at the 3 and 10 mm reference lengths, but not at all at the 1 mm reference length, tubers from 13 °C storage were very responsive to gas I in terms of reduced dormancy duration. Tubers treated at 157 DAP during the late dormancy phase (150–220 DAP) demonstrated that both CO –O # # mixtures significantly reduced dormancy using the 1 and 3 mm reference lengths (Table 1). However, gas I treatment was more effective at the 10 mm reference length than gas II

40 Mean length of longest sprout per tuber (mm)

RESULTS

50 3 °C storage Gas I Gas II Control

30 20 10 0 50 40

13 °C storage Gas I Gas II Control

30 20 10 0 140

150

160

170

180

190

DAP

T     1. Effect of gas composition and preŠious storage temperature on dormancy release of Russet Burbank tubers at three reference sprout lengths during the early (80–150 DAP) or late (150–220 DAP) dormancy phase Mean dormancy release date, G

&!

(DAP)

Sprout reference length (mm) Treatment† Early phase (118 DAP) 3 °C storage Control Gas I Gas II 13 °C storage Control Gas I Gas II Late phase (157 DAP) 3 °C storage Control Gas I Gas II 13 °C storage Control Gas I Gas II

1

3

10

167 169 NS 147**

186 173* 160**

198 180* 180**

170 144** 172 NS

187 147** 185 NS

201 153** 198 NS

177 168** 173**

196 171** 187*

216 181** 210 NS

191 173** 184**

199 176** 192**

217 183** 211 NS

† Control, Ambient atmosphere of 0±03 % CO and 20±9 % O ; gas I, # # 60 % CO –20 % O ; gas II, 20 % CO –40 % O . Treatments applied # # # # for 7 d. NS, Not significant compared to control ; *P ! 0±05 ; **P ! 0±01.

F. 1. Sprout growth from Russet Burbank tubers from the early dormancy phase (80–150 DAP) removed from 3 or 13 °C storage and treated for 7 d with gas I (60 % CO –20 % O ), gas II (20 % CO –40 % # # # O ) or untreated control (0±03 % CO –20±9 % O ). Time zero for the # # # different treatments was 129 DAP.

and this enhanced response was reflected in a more rapid emergence rate (data not shown). Subsequent sprout growth from tubers treated during the early dormancy phase was sensitive to previous storage conditions with maximum growth rate exhibited by gas Itreated tubers after 3 °C storage (Fig. 1). The differential response of dormant tubers to the gas mixtures during the early dormant phase was also observed in sugar changes. Gas I-treated tubers previously stored at 3 or 13 °C and treated during the early dormancy phase (129 DAP) led to increased levels of sucrose, glucose and fructose at the end of the 7 d period when compared to untreated (control) or gas II-treated tubers (Table 2). This effect was more pronounced after 15 d. Removal of tubers after 120 h treatment with gas I led to rapid increases (within 24 h) in sucrose concentrations in the apical region (Fig. 2). After a 24 h lag, sucrose levels also increased in the basal region of the treated tubers.

Effect of CO –O and C H on tuber ABA leŠels # # # % In order to examine whether CO –O could affect ABA # # levels, Russet Burbank tubers at their early dormant phase (80–150 DAP) were treated with gas I or gas II at 129 DAP. Regardless of previous storage temperature, a 7 d gas I treatment caused a pronounced decrease in ABA levels

24

Coleman—Dormancy and Sprout Growth in Potato Tubers

T     2. Effect of gas composition and preŠious storage temperatures on sugar leŠels in apical eye tissues of Russet Burbank tubers from the early dormancy phase (80–150 DAP)

Treatment 3 °C storage Control Gas I Gas II

13 °C storage Control Gas I Gas II

Glucose

µmol g−" d wt (³s.e.)

Day

Treatment

0 7 15 0 7 15 0 7 15

241±4 (3±8) 53±7 (0±1) 37±7 (0±1) — 224±1 (0±7) 160±9 (0±4) — 178±8 (0±4) 62±9 (0±2)

207±3 (6±0) 130±0 (0±3) 125±6 (1±4) — 229±5 (0±4) 177±2 (0±4) — 98±7 (0±5) 83±2 (0±5)

202±7 (5±9) 80±9 (0±4) 71±5 (0±1) — 219±2 (0±0) 199±4 (1±3) — 74±0 (0±6) 58±9 (0±2)

0 7 15 0 7 15 0 7 15

19±3 (0±3) 20±0 (0±3) 19±1 (0±3) — 80±0 (0±0) 192±6 (0±4) — 25±7 (0±1) 26±1 (0±4)

10±6 (0±3) 12±9 (0±1) 6±2 (0±7) — 20±2 (0±0) 123±2 (0±1) — 9±2 (0±3) 11±1 (0±1)

6±4 (0±3) 8±7 (0±1) 6±1 (0±1) — 17±9 (0±1) 101±2 (2±7) — 8±9 (0±2) 8±0 (0±0)

Control, Ambient atmosphere of 0±03 % CO and 20±9 % O ; gas I, # # 60 % CO –20 % O ; gas II, 20 % CO –40 % O . Treatments applied for # # # # 7 d starting at zero (129 DAP).

100 80 60

Sucrose (µmol g–1 d. wt)

Day 15

2±76 (0±1) — —

1±51 (0±2) ! 0±04 1±21 (0±2)

1±51 (0±1) 1±89 (0±6) 1±66 (0±3)

1±34 (0±08) — —

1±82 (0±2) ! 0±04 0±68 (0±08)

1±36 (0±2) 3±10 (0±2) 1±63 (0±3)

4.0 3.5 3.0

Gas I Gas II C2H4 Control

2.5 2.0 1.5 1.0

50

100

150

200 250 Time (h)

300

350

400

F. 3. Time course of changes in ABA content in the apical eye region of Russet Burbank tubers from the late dormancy phase (150– 220 DAP), removed from 13 °C storage and treated for 7 d with gas I (60 % CO –20 % O ), gas II (20 % CO –40 % O ), 1±74 µmol l−" C H , # # # # # % or untreated control (0±03 % CO –20±9 % O ). Time zero for the # # different treatments was 162 DAP. Treatments ended at 168 h.

0 100 Basal Early removal Late removal Untreated Gas I, 7 d

40 20

0

Day 7

Control, Ambient atmosphere of 0±03 % CO and 20±9 % O ; gas I, # # 60 % CO –20 % O ; gas II, 20 % CO –40 % O . Treatments applied for # # # # 7 d starting at zero (129 DAP).

0

20

60

3 °C storage Control Gas I Gas II 13 °C storage Control Gas I Gas II

Day 0

0.5

Apical Early removal Late removal Untreated Gas I, 7 d

40

80

nmol ABA g−" d. wt tissue (³s.e.)

Fructose

ABA (nmol g–1 d. wt)

Sucrose

T     3. Effect of gas composition and preŠious storage temperature on ABA leŠels in apical eye tissues of Russet Burbank tubers from the early dormancy phase (80–150 DAP)

50

100 150 Time (h)

200

250

F. 2. Effect of early (24 h) or late removal (120 h) from a gas I (60 % CO –20 % O ) treatment on sucrose levels in the apical or basal regions # # of Russet Burbank tubers from 13 °C storage. Untreated tubers (0±03 % CO –20±9 % O ) and tubers treated for 7 d with gas I are also noted. # # Time zero for the different treatments was 147 DAP.

(Table 3). Recovery to pre-treatment levels was apparent 8 d after cessation of treatment. Decreased levels were also detected from tubers out of a 13 °C storage after a gas II treatment. After treatment of similar tubers with gas I, a time course study revealed that ABA levels in the apical tuber region decreased to almost zero and remained there until approx. 9 d after the start of the 7 d treatment, at which time there was a rapid increase to a level higher than the control (Fig. 3). Treatment with gas I for up to 14 d maintained suppression of ABA levels in the apical tuber end and, to a lesser extent, the basal end (data not shown). Removal of tubers from gas I at different times demonstrated a slight rise in ABA levels during the initial 6 h of treatment, which was quickly followed by a rapid decrease in ABA levels (Fig. 4). Removal of tubers at 24 h to ambient atmospheric conditions allowed a rapid resumption of ABA levels to near control levels. However, removal after 120 h was

Coleman—Dormancy and Sprout Growth in Potato Tubers

Both gas II and the dormancy release agent, BE (Coleman, 1983), led to decreased ABA levels after 24 h (Table 4). Ethylene also decreased ABA levels after a 24 h treatment (Fig. 3), and enhanced ABA loss by the gas II treatment (Table 4) to levels obtained by the gas I treatment without added C H (Table 3). # %

4.0 Early removal Late removal 7 d treatment Control

ABA (nmol g–1 d. wt)

3.5 3.0 2.5 2.0 1.5 1.0

DISCUSSION

0.5

Although a 2–3 mm sprout length from tuber buds has traditionally been used as the criterion for dormancy release (Emilsson, 1949 ; Reust, 1986 ; Van Ittersum, 1992), previous studies have noted the discontinuous nature of early tuber sprout growth (Goodwin, 1966). If we accept the premise that the termination of dormancy is indicated by the start of continuous sprout growth (Goodwin, 1966), and differences in dormancy duration by conventional standards (for example, the time until sprout length exceeds 2–3 mm) are due to differences in very early growth rate or ‘ the period without bud growth ’ (Van Ittersum, Aben and Keijzer, 1992), then multiple mean reference lengths should allow different sprout growth stages to be characterized. In this context, emergence was viewed as an integration of dormancy release and sprout growth effects. Similarly, the concept of two dormancy phases of approximately equal duration can be found in the work of Macdonald and Osborne (1988) : an early phase of low levels of nucleic acid and protein synthesis, and a late cell expansion phase (‘ white tip ’ phase). Dormancy release would occur due to a resumed cell cycle and cell elongation after the ‘ white tip ’ phase. Differences in effectiveness of gas I compared to gas II in terms of dormancy release and subsequent sprout growth must be considered primarily to be due to the complex effects of, inter alia, previous storage temperature, tuber age and dormancy release stage. For example, gas II treatment of tubers from the early dormancy phase was effective from 3 °C storage, but ineffective when removed from 13 °C storage, regardless of reference length. However, gas II was effective at 1 and 3 mm sprout reference lengths when removed from 3 or 13 °C storage during the late dormancy phase (Table 1). Abscisic acid levels were also affected in a differential manner by CO –O mixtures. Gas I mixture decreased ABA # # levels after both 3 °C and 13 °C storage (Table 3). After 13 °C storage the decrease was rapid (! 48 h) and reversible (Figs 3 and 4). The present observations indicate that the initial effect of a gas I treatment is extremely rapid compared to ABA biosynthesis inhibitors such as fluridone (Suttle and Hultstrand, 1994). After 120 h of CO –O treatment, there # # is a 24–48 h lag period before ABA levels recover. Whether decreased ABA synthesis and}or enhanced breakdown or complexing is responsible for these rapid changes induced by high CO –O is currently unknown. # # Gas II mixture was partially effective in reducing endogenous ABA only from previous storage at 13 °C but not 3 °C. However, the addition of C H to gas II led to # % significant reductions in endogenous ABA. In addition to decreasing tuber dormancy (Rylski et al., 1974), C H # %

0

50

100 Time (h)

150

200

F. 4. Effect of early (24 h) or late removal (120 h) from a gas I (60 % CO –20 % O ) treatment on ABA levels in the apical eye region of # # Russet Burbank tubers, from 13 °C storage and the late dormancy phase. ABA levels are also shown for a 7 d treatment as well as control (0±03 % CO –20±9 % O ) material. Time zero for the different # treatments # was 159 DAP.

T     4. Effect of dormancy release methods on ABA leŠels after 1 or 7 d treatment in apical eye tissues of Russet Burbank tubers remoŠed from a 3 °C storage during the early dormancy phase (80–150 DAP) Treatment Control C H (1±74 µmol l−")

# %

Gas II Gas II­C H

# %

BE

25

Day

nmol ABA g−" d. wt tissue (³s.e.)

1 7 1 7 1 7 1 7 1 7

1±60 (0±2) 1±58 (0±2) 0±42 (0±08) 0±41 (0±08) 1±14 (0±10) 0±60 (0±04) 0±26 (0±02) ! 0±04 1±17 (0±01) 1±64 (0±2)

Control, Ambient atmosphere of 0±03 % CO and 20±9 % O ; gas II, # # 20 % CO –40 % O . Treatments applied for 7 d starting at zero # # (134 DAP).

followed by a 24 h lag before a rapid increase in ABA levels. After removal at 168 h, ABA stayed low for at least another 24 h. Mean sprout length was examined 1 month after the three treatments (two gas mixtures and the control) were applied to Russet Burbank tubers during the early dormancy phase (80–150 DAP). A multivariate model developed by stepwise regression with pooled data from the six conditions (two storage temperatures¬three gas treatments) allowed sprout length to be related to previous glucose and ABA levels (i.e. at the end of the 7 d treatment period) as follows : sprout length ¯ 3±6­0±3 glucose®9±6 ABA with F ratio ¯ 135±5, R# ¯ 0±99 and P ! 0±001.

26

Coleman—Dormancy and Sprout Growth in Potato Tubers

treatment of dormant tubers caused a significant decline in endogenous ABA levels after 24 h treatment. Previous work (Coleman and McInerney, 1997) demonstrated that C H # % was effective in promoting emergence only with CO levels # above ambient levels, when C H was combined with # % different CO concentrations in the presence of 40 % O . No # # enhanced emergence was apparent at ambient CO levels, # and tuber breakdown occurred at 40 % CO in the presence # of C H . Additional reports of C H -ABA interactions in # % # % other plant species (Tittle and Spencer, 1986 ; Tan and Thimann, 1989), suggest the possibility that CO –O # # dormancy release effects may be explained at least partially by their interactions with a hormonally based dormancy control system. The current study links specific CO –O treatments and # # decreased ABA levels. This link may be interpreted in the context of earlier hypotheses of a causal connection between dormancy release, sprout growth and an endogenous inhibitor containing ABA (Burton, 1958 ; Goodwin, 1966 ; Hemberg, 1985). The CO –O treatments were effective in # # reducing ABA levels in dormant tubers, decreasing tuber dormancy duration and increasing sprout growth rate. Previous work has indicated a significant negative correlation between sprout growth rate and initial ABA levels in tuber tissue of ten potato cultivars (Coleman and King, 1984). Exogenous ABA is also capable of inhibiting potato sprout growth when applied repeatedly at high concentrations (El-Antably, Wareing and Hillman, 1967). Differential effects of the CO –O mixtures were also # # observed on sugar levels. Unlike the gas II treatment for 168 h, gas I led to significantly greater levels of sucrose, glucose and fructose, regardless of previous storage temperature (Table 2). Removal of tubers after 120 h from the gas I treatment led to rapidly increased sucrose levels (within 24 h ; Fig. 2). Although increased sugar levels are not causative agents of dormancy release (Emilsson and Lindblom, 1963), the increased availability of soluble sugars could be important for subsequent sprout growth after cell wall loosening due to acidification by high CO levels. The # high CO levels used in the current study would acidify # extracellular water to pH 2–3 while maximum acid-induced cell extension in dicot systems could occur over the pH range of 2±0–3±5 (Taiz, 1984). Since CO levels above 1 % # would also acidify the cytoplasm, which possesses a low buffering capacity (Kurkdjian and Guern, 1989), a possible role of high CO through cell activation (for example, # modulation of ABA levels or enzyme induced wall loosening) cannot be discounted. The present study indicates that sprout length in Russet Burbank tubers can be successfully modelled in terms of initial glucose and ABA levels from the apical eye regions, regardless of previous gas treatments or storage temperatures. Studies have implicated ABA and cytokinins in tuber dormancy and sprout growth control with less well defined roles for GA and C H (El-Antably et al., 1967 ; Bailey, # % Phillips and Pitt, 1978 ; Van Staden and Dimalla, 1978 ; Turnbull and Hanke, 1985 a, b ; Cvikrova et al., 1994) and no direct role for IAA (Sukhova et al., 1993). The multiple aspects, quantitative features and dynamic nature of a control system with possible feedback interactions suggests

that a conceptual model using a dynamic systems approach (Trewavas, 1986) may be helpful in further delineating dormancy control in potato tubers by endogenous plant growth regulators, as well as such exogenous agents as C H , CO and O . The present study has demonstrated that # % # # CO –O mixtures and, to a lesser extent, exogenously # # applied C H , can modify sugar levels, reduce ABA levels or # % reduce dormancy duration in potato tubers. A C K N O W L E D G E M E N TS The author thanks T. Bourque, J. Embleton, M. Howie and J. LeBlanc for excellent technical assistance. LITERATURE CITED Bailey KM, Phillips IDJ, Pitt D. 1978. The role of buds and gibberellin in dormancy and the mobilization of reserve materials in potato tubers. Annals of Botany 42 : 649–657. Berrie AMM, Taylor GCD. 1981. The use of population parameters in the analysis of germination of lettuce seed. Physiologia Plantarum 51 : 229–233. Burton WG. 1958. The effect of the concentrations of carbon dioxide and oxygen in the storage atmosphere upon the sprouting of potatoes at 10C. European Potato Journal 1 : 47–57. Burton WG. 1968. The effect of oxygen concentration upon sprout growth on the potato tuber. European Potato Journal 11 : 249–265. Cho JL, Iritani WM, Martin MW. 1983. Comparison of methods for measuring dormancy of potatoes. American Potato Journal 60 : 169–177. Coleman WK. 1983. An evaluation of bromoethane for breaking tuber dormancy in Solanum tuberosum L. American Potato Journal 60 : 161–167. Coleman WK. 1987. Dormancy release in potato tubers : a review. American Potato Journal 64 : 57–68. Coleman WK, King RR. 1984. Changes in endogenous abscisic acid, soluble sugars and proline levels during tuber dormancy in Solanum tuberosum L. American Potato Journal 61 : 437–449. Coleman WK, McInerney J. 1997. Enhanced dormancy release and emergence from potato tubers after exposure to a controlled atmosphere. American Potato Journal 74 : 173–182. Cvikrova M, Sukhova LS, Eder J, Korableva NP. 1994. Possible involvement of abscisic acid, ethylene and phenolic acids in potato tuber dormancy. Plant Physiology and Biochemistry 32 : 685–691. El-Antably HMM, Wareing PF, Hillman J. 1967. Some physiological responses to D,L abscisin (dormin). Planta 73 : 74–90. Emilsson B. 1949. Studies on the rest period and dormant period in the potato. Acta Agriculturae Suecana 3 : 189–284. Emilsson B, Lindblom H. 1963. Physiological mechanisms concerned in sprout growth. In : Ivins JD, Milthorpe FL, eds. The growth of the potato. London : Butterworths, 45–62. Esashi Y. 1991. Ethylene and seed germination. In : Mattoo AK, Suttle JC, eds. The plant hormone ethylene. Boca Raton : CRC Press, 133–157. Finney D. 1971. Probit analysis. 3rd edn. New York : Cambridge University Press. Goodwin PB. 1966. The effect of water on dormancy in the potato. European Potato Journal 9 : 53–63. Hemberg T. 1985. Potato rest. In : Li PH, ed. Potato physiology. Orlando : Academic Press, 353–388. Kurkdjian A, Guern J. 1989. Intracellular pH : measurement and importance in cell activity. Annual ReŠiew of Plant Physiology and Molecular Biology 40 : 271–303. Lee ET. 1992. Statistical methods for surŠiŠal data analysis. 2nd edn. New York : John Wiley. Macdonald MM, Osborne DJ. 1988. Synthesis of nucleic acids and protein in tuber buds of Solanum tuberosum during dormancy and early sprouting. Physiologia Plantarum 73 : 392–400.

Coleman—Dormancy and Sprout Growth in Potato Tubers Mattoo AK, White WB. 1991. Regulation of ethylene biosynthesis. In : Mattoo AK, Suttle JC, eds. The plant hormone ethylene. Boca Raton : CRC Press, 21–42. Rakitin JV, Suvorov NN. 1935. The effect of temporary anaerobiosis on the sprouting of young potato tubers. Doklady Akademii Nauk SSSR 9 : 295–297. Reust W. 1986. EAPR working group ‘ Physiological age of the potato ’. Potato Research 29 : 268–271. Reust W, Gugerli P. 1984. Oxygen and carbon dioxide treatment to break potato tuber dormancy for reliable detection of potato virus Y (PVY) by enzyme-linked immunosorbent assay (ELISA). Potato Research 27 : 435–439. Rylski I, Rappaport L, Pratt HK. 1974. Dual effects of ethylene on potato dormancy and sprout growth. Plant Physiology 53 : 658–662. Sawyer RL, Smith O. 1955. A study of the oxygen-periderm relationship in potato tubers and the effect of oxygen on the normal breaking of the rest period. American Potato Journal 32 : 15–22. Scott SJ, Jones RA, Williams WA. 1984. Review of data analysis methods for seed germination. Crop Science 24 : 1192–1199. Smith JJ, John P. 1993. Activation of 1-aminocyclopropane-1carboxylate oxidase by bicarbonate-carbon dioxide. Phytochemistry 12 : 1381–1386. Stallknecht GF. 1984. Application of plant growth regulators to potatoes. Production and research. In : Nickell LG, ed. Plant growth regulating chemicals. Vol II. Boca Raton : CRC Press, 161–176. Sukhova LS, Machackova I, Eder J, Bibik ND, Korableva NP. 1993. Changes in the level of free IAA and cytokinins in potato tubers during dormancy and sprouting. Biologia Plantarum 35 : 387–391. Suttle JC. 1995. Postharvest changes in endogenous ABA levels and ABA metabolism in relation to dormancy in potato tubers. Physiologia Plantarum 95 : 233–240. Suttle JC, Hultstrand JF. 1994. Role of endogenous abscisic acid in potato microtuber dormancy. Plant Physiology 105 : 891–896.

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Taiz L. 1984. Plant cell expansion : regulation of cell wall mechanical properties. Annual ReŠiew of Plant Physiology 35 : 585–657. Tan Z-Y, Thimann KV. 1989. The roles of carbon dioxide and abscisic acid in the production of ethylene. Physiologia Plantarum 75 : 13–19. Thornton NC. 1933. Carbon dioxide storage. V. Breaking the dormancy of potato tubers. Contributions of the Boyce Thompson Institute 5 : 471–481. Thornton NC. 1939. Carbon dioxide storage. XIII. Relationship of oxygen to carbon dioxide in breaking dormancy of potato tubers. Contributions of the Boyce Thompson Institute 10 : 201–204. Tittle FL, Spencer MS. 1986. Interactions between ethylene, CO , and # ABA on GA -induced amylase synthesis in barley aleurone tissue. $ Plant Physiology 80 : 1034–1037. Trewavas A. 1986. Understanding the control of plant development and the role of growth substances. Australian Journal of Plant Physiology 13 : 447–457. Turnbull CGN, Hanke DE. 1985 a. The control of bud dormancy in potato tubers. Evidence for a primary role of cytokinins and a seasonal pattern of changing sensitivity to cytokinin. Planta 165 : 359–365. Turnbull CGN, Hanke DE. 1985 b. The control of bud dormancy in potato tubers. Measurement of the seasonal pattern of changing concentrations of zeatin-cytokinins. Planta 165 : 366–376. Van Ittersum MK. 1992. Variation in the duration of tuber dormancy within a seed potato lot. Potato Research 35 : 261–269. Van Ittersum MK, Aben FCB, Keijzer CJ. 1992. Morphological changes in tuber buds during dormancy and initial sprout growth of seed potatoes. Potato Research 35 : 249–260. Van Staden J, Dimalla GG. 1978. Endogenous cytokinins and the breaking of dormancy and apical dominance in potato tubers. Journal of Experimental Botany 29 : 1077–1084. Wiltshire JJJ, Cobb AH. 1996. A review of the physiology of potato tuber dormancy. Annals of Applied Biology 129 : 553–569.