Sprout and stem development from potato tubers of differing physiological age: the role of apical dominance

Field Crops Research, 27 ( 1991 ) 1-16

1

Elsevier Science Publishers B.V., Amsterdam

Sprout and stem development from potato tubers of differing physiological age: the role of apical dominance R.K.M. Hay ~and J. Hampson Plant Sciences Department. The Scottish Agricultural College, Auchincruive, Ayr KA6 5HW, UK (Accepted 18 July 1990)

ABSTRACT Hay, R.K.M. and Hampson, J., 1991. Sprout and stem development from potato tubers of differing physiological age: the role of apical dominance. Field Crops Res., 27: 1-16. A series of experiments over three years ( 1984-87 ) is described in which tubers of a range of early and maincrop varieties were exposed to two different storage-temperature regimes to give different physiological ages. Tubers exposed to 10-15 °C from the break of dormancy and then held at 5 ° had a final age at planting of ca. 500 day-degrees > 4°C. Tubers held at 5°C throughout storage were physiologically younger, having accumulated only 100 day-degrees > 4°C. These temperature regimes, in the light, were considered to be representative of the treatment of tubers for commercial production of early and maincrop potatoes. For each treatment, the number and length of all sprouts longer than 2 mm at planting, and the number of stems produced in the field, were recorded. In a parallel study in 1985-86, tubers of a single variety, aged in the same way, intact or with the apical sprout removed, were held in the laboratory to give full expression of sprout production. The use of physiologically younger tubers did not necessarily give more sprouts at planting, nor mainstems after planting in the field. However, the younger tubers held in the laboratory did carry significantly more sprouts, and sprout production from additional eyes was induced by desprouting. Overall, younger tubers produced at least one more mainstem than older tubers in only three out of twelve comparisons and, when the total number of stems per tuber was assessed, the younger tubers carried the same number or fewer stems in all cases. It is concluded from this investigation that, although apical dominance plays an important role in determining the number of sprouts produced per tuber, there is a high degree of variation between varieties and seasons, and within batches of tubers, in its expression. Extrapolation from a single variety, and in general from experiments confined to the laboratory, may not be justified in the prediction of numbers of stems in the field.

INTRODUCTION

The total tuber yield of a potato crop increases with increasing population density up to a plateau at moderate stem numbers (e.g. around 2 × 105 stems ha-~; Wurr, 1974; Hay and Walker, 1989). However, because the number of t Present address: Agricultural Scientific Services, 50 AFD, East Craigs, Edinburgh E H I 2 8N J, Great Britain.

0378-4290/91/$03.50

© 1991 Elsevier Science Publishers B.V. All rights reserved.

2

R.K.M. HAY AND J. HAMPSON

tubers initiated continues to rise with increasing population density, mean tuber weight begins to decline once this plateau has been achieved, and the proportion of the total tuber yield which falls within the saleable-ware fraction (45-80 mm) also decreases continuously. It is important, therefore, to be able to manipulate planting rate (number of tubers per unit area) and the number of stems produced per tuber, to give stem population densities which are no greater than those required to achieve the yield plateau. The number of stems produced per tuber is determined primarily by the degree of apical dominance exercised by the sprout or sprouts at the apical complex over the other eyes of the tuber (Appleman, 1924). A detailed study of the early variety Arran Pilot (Goodwin, 1967) indicated that tubers exposed to high temperatures (relative to the threshold of 4°C, i.e. > 15°C) immediately after the break of dormancy tended to produce a single sprout from the apical complex/eye, whereas apical dominance was broken at lower temperatures, giving multisprouted tubers. Broadly similar results have been obtained from subsequent investigations with other varieties (e.g., Allen et al., 1978; Hartmans and van Loon, 1987 ). However, all of these findings were based on tubers which were given the appropriate storage-temperature treatment and then transferred for several weeks to favourable temperatures for sprout expansion (10-20°C) in the laboratory or glasshouse. In contrast, tubers to be planted in the field are either stored at low temperatures until planting (giving 'unsprouted' seed tubers for maincrop production), or 'sprouted' at higher temperatures, mainly for early production. Thus, for example, many of the eyes of 'unsprouted' tubers which would produce sprouts under favourable conditions may not have the opportunity to do so before planting. It is important to know whether these potential stems do develop in the field and, in general, to what extent the degree of apical dominance suggested by experiments confined to the laboratory does hold in the field (Holmes and Gray, 1971; Wurr, 1975 ). This paper reports the results of a three-year (1984-87) study of the influence of temperature on the number and position of sprouts developing from stored potato tubers. These data were compared with the number, position and origin (primary or secondary ) of stems growing from the same tubers in the field, to establish whether the patterns of sprout production in storage were maintained after planting. The temperature treatments were designed to give tubers whose physiological ages (O'Brien et al., 1983 ) were representative of seed tubers used in commercial early and maincrop production in Great Britain. Because preliminary work indicated substantial differences in the pattern of apical dominance amongst varieties, the work reported here took the form of a broad survey over nine contrasting varieties, rather than an intensive study of one variety. A supplementary experiment, in which similarly pretreated tubers of one variety were held at laboratory temperature to give full expression of sprout production, was carried out to aid in the interpretation of the apical dominance of the planted tubers.

DEVELOPMENT OF POTATO OF DIFFERING PHYSIOLOGICAL AGE

3

MATERIALS AND METHODS

The experiments were carried out at Auchincruive over three seasons, 1984-87. In 1984, tubers of Pentland Javelin and Maris Piper were purchased from commercial storage in September and November, respectively. In 1985, however, four varieties - - Home Guard, King Edward, Maris Piper, Pentland Javelin - - and in 1986, six varieties - - Arran Pilot, D6sir6e, Estima, Maris Piper, Pentland Crown and Record - - were obtained directly from the field at harvest between August and October, so that the conditions during storage could be completely controlled. Mean tuber size, in terms of number of eyes, is given for each batch in Figs. 1-4. The tubers were elite grade in all experiments. On arrival in the laboratory, batches of each variety (from 80-250 tubers in 1986/87 to 600 in 1984/85 ) were divided into two lots by visually matching individual tubers; batches were then placed in single layers on plastic trays, with the apical complex upwards. Damaged and infected tubers were removed at this stage, and each sound tuber was numbered with waterproof ink, which remained legible after planting until tuber senescence. One lot of tubers was then transferred to an illuminated cold-room at 5 + 1 °C until planting. Tubers treated in this way are referred to as physiologically 'young'. The other lot of tubers was held in the light in the laboratory at 10-15 °C until they had accumulated 400 day-degrees (°C d) > 4°C (O'Brien et al., 1983 ) from the break of dormancy, at which time they were transferred to the same coldroom; these tubers are referred to as physiologically 'old'. The break of dormancy under each treatment was taken to be the date at which the length of the longest expanding sprout per tuber exceeded 2 mm. In each year, the physiological ages of the young and old tubers at planting were 100 and 500°C d. As in previous experiments (e.g. O'Brien et al., 1983 ), it is assumed that the imposition of lower temperature at the end of the high-temperature treatment of the old tubers did not affect the subsequent pattern of sprout production. Two weeks before planting, the following measurements were taken for each tuber: fresh weight; length; number of eyes; and length of each sprout >12 mm at each eye (including multiple sprouts at some eyes ). It should be noted that determination of the number of eyes per tuber requires a degree of judgement as to how many buds should be included in the apical complex (Wurr and Morris, 1979); as a result, the mean number of eyes per cm of tuber length (e.g., Fig. 1 ) may differ slightly from other reports. From these results, the number and total length of sprouts carried by each tuber were determined. After these measurements were complete, all marked tubers were carefully planted out in the field (14 March 1985; I I April 1986; 21 April 1987) by hand, to avoid physical damage to sprouts. Each variety was planted in a single block 6 m wide, without replication, in a uniform well-drained soil (sandy clay loam limed to pH 6.5 ). The crops were established under standard man-

LL PENTLAND

100 <4,0cm(5.6 eyes)

JAVELIN

5.1 - 6.0cm(6.3 eyes)

o .o

R . K . M . H A Y A N D J. H A M P S O N

>7.0cm(7.7

eyes)

4O .

20 0

A2

3456

78910

MARIS

o

A 2

34

5

6

7 8

910

A 2

3 4

5 6

A 23

456

7"8910

PIPER

7

8 910

A2

3 4

5 6 7 8

910

eye position (A=splcsl complex)

Fig. 1. The frequencyof sprouts longer than 2 mm at different eyes of potato tubers of different physiologicalage ([~, old; II, young; see text) and size class (longest axis, cm) immediately before planting in 1985. The numbers in brackets indicate mean numbers of eyesper tuber. For clarity, two size-classesof Pentland Javelin have been omitted. agement (ridges 71 cm apart; 22.5-cm spacing between tubers; 150 kg N, 66 kg P and 174 kg K ha-1, broadcast at planting). Several weeks after emergence of all stems of each crop was complete, but before the start of senescence of the early crop canopies (15 July 1985, first week of June 1986, 1987, respectively), a randomly selected subsample of marked seed tubers (5% in 1985, 25% in 1987) or all marked tubers (1986), was carefully dug up and transferred to the laboratory, where each plant was washed and dissected to determine the number and position of primary (growing directly from eyes) and secondary (below-ground branches of primary) stems. In 1985/6, two matched lots of 420 tubers each of Pentland Javelin were physiologically aged as described above, and on 15 January, each lot (physiologically old and young) was sub-divided into two sub-lots by visual matching of tubers. The tubers of one sub-lot of each ageing treatment were left intact ('old, intact' and 'young, intact'), whereas the sprouts at the apical complex of the other two sub-lots were removed, either by knocking off the sprouts ('old, desprouted' ) or by dissecting out the apical complex ('young, dissected'). At the time of desprouting, virtually all ( > 98%) of the physiologically older Pentland Javelin tubers carried sprouts at the apical complex, whereas less than 50% of the younger tubers did (Fig. 2). The tubers of all treatments were then held at room temperature ( 10-15 ° C ) in the light. The

DEVELOPMENT OF POTATO OF DIFFERING PHYSIOLOGICAL AGE HOME

GUARD

PENTLAND

5

JAVELIN

(8.2eyes) 80 60 40

Q

20

¢=

0

A 2 3 4 5 6

7 8

910

A 2 3 4 5 6 7

8 910

O KING

EDWARD

MARLS

PIPER

J:: m 8O

W Q

60 4O 2O 0

A

2 3 4 5 6 7

8

910

A 2

3 4 5

6 7 8 910

e y e position (A=apical c o m p l e x )

Fig. 2. The frequencyof sprouts longerthan 2 mm at different eyes of potato tubers of different physiological age ([3, old; II, young; see text) immediatelybefore plantingin 1986. The numbers in brackets indicate mean numbersof eyes per tuber. numbers and lengths of the sprouts at each eye were measured on three dates ( 17 January, 16 February and 20 March). The statistical significance of the effects observed in this laboratory experiment were assessed by analysis of variance and, between appropriate pairs of values in the remaining experiments, by standard t-tests. RESULTS

Seed tuber characteristics at planting, 1985-8 7 Number and length of sprouts The only consistent effect of physiological ageing across nine varieties and three seasons was on the length of the longest sprout; as found in most investigations, the longest sprout (which was not necessarily the apical) carried by the older tubers was significantly longer ( P < 0.001 ) than that of the younger tubers (data not presented). However, there was no such consistency in the effects on the number of sprouts produced per tuber; in eight out of twelve comparisons, the younger tubers carried more sprouts but, in the early variety Pentland Javelin in 1985, the effect was reversed (Table 1 ). Only with Arran Pilot ( + 3 . 5 sprouts) and Maris Piper in 1987 ( + 1.6) did continuous cool storage result in a mean increase of more than one sprout per tuber.

6

R.K.M. HAY AND J. HAMPSON

TABLE 1 Numbers a and lengths a per tuber of sprouts ( > 2 m m ) carried by potato tubers of differing physiological age at planting in three seasons Season/Variety

1984/85 Pentland Javelin Maris Piper 1985/86 Home Guard Pentland Javelin King Edward Maris Piper 1986/87 Arran Pilot Estima D6sir6e Maris Piper Pentland Crown Record

Treatment

Number

Total length (mm)

Proportion b (%) 1

2

3

4

>5

Old Young Old Young

3.7+0.09 1.5 _+0.05*** 3.9 _+0.11 3.8_+ 0.10ns

81 + 1.9 11 _+0.4*** 73 _+2.2 39_+ 1.6"**

7 54 5 8

19 27 14 13

20 9 22 22

29 3 24 17

25 1 35 40

Old Young Old Young Old Young Old Young

2.5_+0.11 3.3_+0.11"** 2.6 + 0.24 2.2 + 0.23ns 1.6 _+0.08 1.9 _+0.09* 3.5+0.18 4.2-+0.16"*

60+2.0 68+2.1"* 23-+ 1.5 11 _+ 1.2"** 25 _+0.9 13 _+0.5*** 33-+ 1.5 32-+ 1.2ns

11 3 30 33 61 44 11 0

42 12 25 28 22 28 9 9

36 50 20 21 11 18 35 13

9 27 15 5 5 7 26 40

1 8 10 8 1 1 19 38

Old Young Old Young Old Young Old Young Old Young Old Young

1.7-+0.10 5.2_+0.22*** 1.9_+0.11 2.1 _+0.16ns 1.6_+0.14 2.5_+0.17"** 2.3-+0.13 3.9-+0.24*** 2.2_+0.22 2.7_+0.20* 1.7-+ 0.07 1.9_+0.09"

48_+ 1.0 60_+ 1.7"** 23_+ 1.1 7_+0.6*** 13_+ 1.0 12+_0.8ns 25-+ 1.2 20-+ 1.4"* 19+ 1.1 18-+ 1.5ns 22 + 0.8*** 14+0.6

62 5 39 21 39 22 21 5 50 16 44 35

18 7 43 30 33 29 45 24 13 39 38 41

15 13 12 19 14 29 21 19 18 18 15 17

3 17 4 13 4 16 12 14 8 13 2 4

2 58 2 4 0 4 1 36 11 13 0 1

aValues are means + SE, and differences between pairs of old and young values within columns are significant at P<0.001 (***), P<0.01 (**), P < 0 . 0 5 (*), or not significant (ns). bTubers with different numbers of sprouts ( 1 to >- 5 ).

As a result of these two effects, the difference in total sprout length per tuber associated with differences in physiological age were not consistent between varieties. For example, although the longest sprout of the older Arran Pilot tubers was longer than that of the corresponding younger tubers, the latter carried a significantly greater total length of sprout because of the proliferation of sprouts (Table 1 ). For each of the other varieties except Home Guard, total length of sprouts per tuber was greater for the older treatment, or was unaffected by physiological ageing. In each year, the older tubers of the early varieties tended to produce longer sprouts than did those of the maincrop varieties, and the same effect was

7

D E V E L O P M E N T O F P O T A T O O F D I F F E R I N G P H Y S I O L O G I C A L AGE

shown for the younger tubers of Home Guard and Arran Pilot in comparison with the maincrop varieties used in 1986 and 1987 (total lengths in Table l; the results for the longest sprout per tuber confirm this effect). There were also marked overall differences in the average numbers of sprouts produced by the tubers of different varieties, with Maris Piper, Pentland Javelin and Arran Pilot being capable of producing four or more sprouts per tuber, whereas the numbers for King Edward and Record did not exceed two (Table 1 ). These differences can be accounted for only in part by different mean tuber sizes and number of eyes per tuber (Figs. 1-3 ). However, restriction of the study to mean number of sprouts per seed tuber can give a misleading impression of varietal response to physiological ageing. For example, the older Arran Pilot tubers carried, on average, 1.7 sprouts longer than 2 mm, but 20% of the tubers treated in this way carried three or more sprouts (Table l ). In contrast, 12% of the young tubers produced one or two sprouts, but the overall mean was 5.2 per seed tuber. For each variety studied, there was similar evidence of the heterogeneous nature of the response of a batch of tubers to storage temperature.

Distribution of sprouts amongst the eyes Several aspects of the distribution of sprouts amongst the eyes of the treated tubers are revealed by Figs. 1-3. First, for all varieties except Pentland Javelin

ARRANPILOT 100 '~9.4eyes)

~

ESTIMA (5.0eyes)

/

/

DESIREE (6.0

eyes)

60 4o

~

0

A 2345678910 A 2345678 910 A 2345678910 MARLSPIPER PENTLANDCROWN RECORD ~. 08001 ~P15.3eyes) ~ ~ (6.1

eyes)

J~ ~

5o

"~

4o 20 0

A 2345 67 8910 A 234 56 78 910 A 2 345678910

eyepositio(A=spl n ccompl sl ex) Fig. 3. The frequency of sprouts longer than 2 mm at different eyes of potato tubers of different physiological age. ( U3, old; B, young, see text) immediately before planting in 1987. The numbers in brackets indicate mean numbers of eyes per tuber.

8

R.K.M. HAY AND J. HAMPSON

(old, 1986 ) and Arran Pilot, a proportion of the tubers did not show any bud development from the apical complex. This proportion varied from 5-10% for Pentland Crown to nearly 90% for Record (young). There was no evidence to suggest that this was the consequence of physical damage or (macroscopically visible) disease, and since the majority of the tubers were regular, both in shape and in the phyllotaxis of their eyes, there were few difficulties in the identification of the apical complex. Furthermore, Figs. 1-3 indicate that continuous low-temperature storage can reduce sprout production from the apical complex of three varieties, Pentland Javelin, Estima and Record. Secondly, for old and young tubers of Pentland Javelin, King Edward, Pentland Crown and Record, there was evidence of a depression of sprout production at eyes 2 a n d / o r 3, with a second peak from eye 4 towards the heel end of the tuber (bearing in mind the varying mean size of the tubers of the different varieties; Figs. 1-3 ). However, in no case were eyes 2 and 3 suppressed in all tubers. In contrast, sprout production from the eyes of the other six varieties tended to decrease progressively from apex to heel. Thirdly, the results in Figs. 1-3 tend to further emphasise the variability amongst the tubers of a given batch in their response to variation in storage temperature. For example, although each of the older Arran Pilot tubers carried, on average, 1.7 sprouts, the second sprout, after that at the apex, could be located at any one of eyes 2 to 7. The variation in activity at the apical complex has already been described, but what is not revealed by either Table 1 or Figs. 1-3 is that the low-temperature treatment tended to stimulate multiple sprouting at a single eye. This phenomenon was particularly marked in the physiologically young Arran Pilot and Maris Piper tubers, between 64 and 79% of which carried two or more sprouts at the apical complex.

Sprout production by seed tubers of Pentland Javelin held under favourable conditions for sprout growth in the laboratory, 1986 The numbers and lengths of sprouts carried by tubers of Pentland Javelin aged in the same way as the preceding experiments, but transferred to higher temperatures in the laboratory to give full expression of sprout production, are shown in Table 2. These tubers were particularly suitable for this investigation because multiple sprouts at a given eye were relatively uncommon, and all of the older tubers produced a sprout at the apical complex (Figs. 2,4 ). By 20 March, the young intact tubers had produced nearly twice as many sprouts longer than 2 m m as had the old intact tubers. This indicated that high-temperature treatment at the break of dormancy did lead to the inhibition (or, at least, severe reduction in the rate of expansion) of two sprouts per tuber, compared with tubers aged continuously at 5 °C. However, this difference was expressed only if, as in this experiment, the tubers were transferred to favourable temperatures for sprout growth. Where - - as in commer-

9

DEVELOPMENT OF POTATOOF DIFFERING PHYSIOLOGICALAGE TABLE 2

Numbers a and lengths a per tuber, of the sprouts (>-2 m m ) carried by Pentland Javelin tubers of different physiological ages held in the laboratory from 15 January, 1986 Date/Treatment

NumbeI ~

17 January Old, intact desprouted Young, intacl dissected 16 February Old, intact desprouted Young, intact dissected 20 March Old. intact desprouted Young, intact dissected

Total length a (ram)

Proportion b (%) 1

2

3

4

>-5

1.5_+0.05 0.6_+0.06 0.1 _+0.03 0.2_+0.03

10_+0.3 3_+0.3 1 _+O. 1 1 _+0.1

60 26 8 14

26 13 1 2

9 2 0 1

1 1 0 9

0 0 0 0

2.1 _+0.10 4.3_+0.09 3.8_+0.11 4.9_+0.09

21 _+0.6 34_+0.7 23_+0.6 29_+0.5

44 0 9 1

25 4 13 3

18 24 23 10

7 34 22 27

6 37 33 59

2.2_+0.10 4.6_+0.10 4.0_+0.11 5.0_+0.10

32_+0.9 60_+ 1.1 41 _+ 1.0 54_+0.9

47 0 7 0

21 3 12 3

17 18 22 9

7 33 23 27

8 45 36 61

aValues are the means of 210 tubers +_SE. (On 20th March, the effects of ageing and desprouting treatments on number and length of sprouts were significant at P < 0.001. hTubers with different numbers of sprouts ( 1 to >_5 ).

PENTLAND JAVELIN old

om- •

young

BO 6O

"~ ~,~

•

20

0

A 2 3 4

5

6

7

8

9 10

A2

3

4

5 6

7

8

9 !0

eye position (A=apical complex)

Fig. 4. The frequency of sprouts longer than 2 mm at different eyes of potato tubers of different physiological age (old or young; see text) which were held in the laboratory until measurement on 20 March 1986. The tubers were intact (O), apical sprout removed (11, old) or apical bud dissected out (11, young). Mean number of eyes per tuber was 6.2. cial practice - - tubers o f different p h y s i o l o g i c a l ages were held at l o w temperatures until planting, this higher p o t e n t i a l for stem p r o d u c t i o n by p h y s i o l o g i c a l l y - y o u n g e r tubers was n o t e v i d e n t at planting (Table 1 ). Table 2 also s h o w s the results o f a standard procedure in the study o f apical d o m i n a n c e in a b o v e - g r o u n d stems, in w h i c h the apical b u d is r e m o v e d and

10

R.K.M. HAY A N D J. H A M P S O N

the subsequent expansion of axillary buds recorded. In the case of the older tubers, removal of the apical bud resulted, by 20 March, in sprout expansion at 4-5 sub-apical eyes, only one of which, on average, would have developed in the intact tuber. (Note that there was no regrowth at the apical complex after desprouting.) However, as noted above, the response of tubers to the experimental treatments was highly heterogeneous. For example, 32% of the (apparently apically dominant ) old intact tubers carried three or more sprouts on 20 March (Table 2), and the frequency of occurrence of the subsidiary sprouts increased between eye 2 and the heel (Fig. 4 ); in contrast, 21% of the old desprouted tubers carried three or fewer sprouts, and there was a broadly similar frequency of sprouts from eyes 3 to 6. Similar measurements of the physiologically younger tubers showed that removal of the apical complex by dissection released one further eye from inhibition compared with the ( multisprouted ) intact young tubers (Table 2 ). Taking into account the fact that the apical complex had been removed completely and could not regrow, these dissected tubers showed sprout growth at every eye (5.0 out of 5.2, Table 2; Fig. 4).

Fate of sprouts after planting in the field (1985-8 7) The subsequent development, into above-ground stems, of the sprouts induced to extend in storage by different temperature regimes is shown in Table 3. For the physiologically older tubers of the early varieties (Arran Pilot, Home Guard, Pentland Javelin) the results were consistent across varieties and seasons; for each, the number of primary stems was significantly lower than the number of sprouts longer than 2 m m at planting. Detailed study of the positions of the primary stems in 1986 and 1987 (Table 4 ), shows that this effect was caused simply by the lack of further expansion of sprouts, generally at eyes 2-4, in 15-25% of the tubers. In the current experiment it is not possible to exclude sprout damage during measurement and planting as a factor in this decrease. In contrast, the very much larger loss of potential primary stems in 1985 (Table 3 ) can be attributed to the defoliation of the crop by frost shortly after emergence. The results were less consistent for the physiologically younger tubers of the early varieties. With Pentland Javelin (1985) and Home Guard, a very high proportion of the sprouts at planting gave primary stems in the field, with no recruitment of further primary stems from previously undeveloped eyes (e.g., for Home Guard, Table 4 ). The profusely sprouted younger tubers of Arran Pilot showed a substantial loss of potential primary stems after planting, whereas, in 1986, Pentland Javelin produced two to three additional primary stems per tuber from eyes near the apical end which had not produced sprouts longer than 2 m m by the time of planting (Tables 3,4). The second early, Estima, showed a broadly similar response, with the older tub-

D E V E L O P M E N T O F P O T A T O O F D I F F E R I N G P H Y S I O L O G I C A L AGE

11

TABLE 3 Numbers a, per tuber, of sprouts >- 2 mm, and above-ground stems, produced by potato tubers in three seasons Season/Variety

1984/85 Pentland Javelin

Maris Piper 1985/86 Home Guard Pentland Javelin King Edward

Maris Piper 1986/87 Arran Pilot Estima

D6sir6e Maris Piper Pentland Crown Record

Treatment

Sprouts at planting (n)

Stems ( n ) Primary

Total

Old Young Old Young

3.8 ± 0.44*** 1.8 ± 0.17ns 4.3 ± 0.39ns 4.1 ± 0.44ns

0.6 + 0.18 b 1.9 ± 0.33 4.4+0.43 5.0 ± 0.41

4.5 + 0.40 4.4 + 0.28 6.1 _+0.59 5.9 ± 0.47

Old Young Old Young Old Young Old Young

2.5 ± 0.11"* 3.3±0.11" 2.6 ± 0.24* 2.2 ± 0.23 ~** 1.6 ± 0.08* 1.9±0.09"** 3.5 +, 0.18*** 4.2±0.16ns

2.1 +_0.09 3.0 ± 0.11 2.0 + 0.17 4.8 ± 0.22 1.9 + 0.10 2.4_+0.11 4.9 ± 0.38 4.4+0.20

7.8 ± 0.33 5.5 ± 0.26 5.1 _+0.26 5.5 ± 0.24 3.5 + 0.16 3.8_+0.13 6.4 ± 0.36 4.6_+0.21

Old Young Old Young Old Young Old Young Old Young Old Young

1.6 +, 0.23ns 5.0 ± 0.46ns 1.6±0.1 Ins 1.8 ± 0.28" 1.8 _+0.22* 2.3±0.18ns 2.3 ± 0.18ns 3.9 i 0.35ns 2.3 ± 0.28ns 2.8 ± 0.26ns 1.4 ± 0.14ns 1.8±0.18ns

1.3 + 0.13 4.3 ± 0.43 1.5+0.16 2.6 +_0.22 2.4 _+0.23 2.4+0.18 2.4 ± 0.20 3.3 + 0.31 2.2 ± 0.30 2.8 + 0.27 1.6 ± 0.14 1.8_+0.10

7.3 + 0.42 8.4 ± 0.72 2.2+0.18 2.6 _+0.22 3.6 _+0.28 2.6+0.17 4.7 ± 0.36 3.7 ± 0.27 4.8_+ 0.23 3.2 + 0.25 3.6 + 0.25 1.9+0.14

aValues are means of 18 (1984/85), 40-150 ( 1 9 8 5 / 8 6 ) and 25 ( 1 9 8 6 / 8 7 ) randomly selected tubers ± SE. The significance of differences between corresponding values in adjacent columns is as indicated in Table 1. bFrost-damaged crop.

ers showing a slight loss of potential sprouts, whereas the younger tubers carried 44% more primary stems than there were sprouts at planting (Table 3 ). The fate of the sprouts carried by physiologically older tubers differed amongst the maincrop varieties. For Maris Piper ( 1985,87 ), Pentland Crown and Record, the number of primary stems formed in the field per tuber was indistinguishable from the number of sprouts at planting (Table 3 ). Detailed study of the origin of the primary stems (Table 4 ) showed that this effect was the result of small losses and gains of sprouts after planting. Under this treatment, most sprouts longer than 2 mm at planting survived to give primary

12

R.K.M. HAY AND J. HAMPSON

TABLE 4 Origins of primary stems growing in the field in 1986 and 1987 Variety

Home Guard Pentland Javelin King Edward Maris Piper (1986) Arran Pilot Estima Drsirre Maris Piper (1987) Pentland Crown Record

Treatment

Old Young Old Young Old Young Old Young Old Young Old Young Old Young Old Young Old Young Old Young

Tubers (n)

67 66 40 39 148 149 57 58 25 25 25 25 25 25 25 25 25 25 25 25

Proportion of tubers (%, eyes) With sprouts a

Producing stems b

Proportion of apical complexesc (%)

25 (2,3) 15 20 10 3 0 9 18 16 (2,4) 44 (2-5) 24 (3-5) 24 (4-6) 16 12 16 (3-5) 20 (3-5) 16 (4-5) 12 8 20 (4-7)

0 0 5 77 (A-5) 17 (4-7) 34 10 7 8 8 8 48 (A-4) 32 (A-4) 20 (2,3) 8 12 8 12 16 24

9 20 0 18 1 0 44 44 0 32 0 0 4 0 12 28 8 20 4 0

aThose which ceased to expand after planting. bFrom undeveloped eyes at planting. cCarrying three or more primary stems.

stems. In contrast, the older tubers of Maris Piper (1986), King Edward and Drsirre showed a significant degree of recruitment of primary stems from eyes which had not produced sprouts by the time of planting (increases of 40, 19 and 33%, respectively). This development of previously inactive eyes occurred in different zones of the tubers (apical complex to eye 4 in Drsirre; towards the basal end in King Edward), but for Maris Piper, the increase was largely the result of a proliferation of primary stems at the apical complex (Table 4). The responses were even more complex for the physiologically younger tubers of the maincrop varieties. Numbers of primary stems per tuber were higher than the numbers of sprouts at planting in only two cases, Marls Piper ( 1985 ) and King Edward. In the latter case, this was the result of the development of additional sprouts, which was not restricted to a single zone of the tubers (Table 4). The two measurements were not significantly different in four cases, Marls Piper ( 1986 ), Drsirre, Pentland Crown and Record, as a result of small gains and losses of sprouts (Table 4). Finally, for Marls Piper in 1987, there

DEVELOPMENT OF POTATO OF DIFFERING PHYSIOLOGICAL AGE

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was a preponderance of sprout loss over gain at subsidiary eyes, associated with a less-marked proliferation of primary stems at the apical complex than was observed in 1986. The net result of these effects was a final number of primary stems per tuber which was significantly lower than the number of sprouts at planting. Thus, Marls Piper showed different responses in each of the three years. For all varieties and seasons, the mean number of secondary stems per primary stem was higher for the physiologically older than for the younger tubers, and the effect was generally more marked for the early varieties (Table 3 ). Excluding the frost-damaged crop of Pentland Javelin, which was composed almost entirely of secondary stems, the extremes are represented by Arran Pilot (4.6 compared with 1.0 secondary stems per primary) and Estima (0.5 and 0). As a result of the heterogeneous and, in some cases, mutually conflicting effects of storage treatment on the production of primary and secondary stems, the physiologically younger tubers produced significantlyfewer stems than did the older tubers in six out of twelve comparisons (Home Guard, Maris Piper 1986 and 1987, D6sir6e, Pentland Crown, Record). In four of the other comparisons (Pentland Javelin 1986, King Edward, Arran Pilot, Estima), the younger tubers did give more stems, but the effects were not statistically significant. Finally, comparing the results in Tables 2 and 3, which were obtained from tubers originating from the same batch of Pentland Javelin tubers, it can be seen that the final number of sprouts produced by the physiologically older tubers under favourable conditions in the laboratory (2.2 _+0.10) was a reasonable guide to the number of mainstems produced in the field (2.0 _+0.17 ). In contrast, the laboratory value for the younger treatment (4.0 _+0. l 1 ) was a significant underestimate of the number of mainstems in the field (4.8+0.22). DISCUSSION

This study has demonstrated no 'typical' responses of sprout development to physiological ageing. First, there was considerable Variation amongst the studied varieties in the responses of their tubers to storage temperature (number, length and origin of sprouts), and these differences were not necessarily related to maturity class. Such differences between varieties have been described before (e.g., Toosey, 1964; Allen, 1978 ) but, in the present work, it has been shown that the responses of Arran Pilot, a variety used in several earlier experiments on apical dominance and stem development (e.g. Goodwin, 1963, 1967; Goodwin et al., 1969), are not typical of a range of older and more recently developed varieties. Secondly, the response of the tubers within each batch was highly variable; for example, in several lots of physiologically-older tubers, for which the mean

14

R.K.M. HAY AND J. HAMPSON

number of sprouts per tuber was two or less, at least 20% of the tubers carried three or more sprouts. Thus even for Arran Pilot, whose tubers responded strongly to physiologically ageing, the coefficient of variation of the number of sprouts per tuber at planting was 57% (old) and 42% (young) (Table 1 ). Clearly, the heterogeneity of tuber response should be taken into account when planning and interpreting experiments in which storage conditions are varied. Thirdly, a substantial proportion of the tubers of each variety studied, excluding only Arran Pilot, did not produce sprouts at the apical complex. The consistency of this effect (Figs. 1-3), and the lack of evidence of damage, disease or incorrect identification, indicates that it is real, and not an artefact of the experimental procedure. Furthermore, Wurr ( 1978 ) reports a similar finding in an investigation of four maincrop varieties (D6sir6e, King Edward, Majestic, Pentland Crown). It appears, therefore, that sprout development does not begin invariably at the apical complex in apparently healthy tubers. The widely held view that high temperature ( >_ 15 oC) at the break of dormancy gives apically dominant tubers, whereas long, cool storage leads to the breakdown of apical dominance and the development of more eyes must, therefore, be more closely examined on several counts. First, this pattern of response appears to be characteristic of certain varieties such as Arran Pilot, rather than of potato varieties in general (e.g., Tables 1,3 ). Secondly, there were many multisprouted tubers in each batch of apparently apically dominant tubers of each variety, and thirdly, in many apparently apically dominant tubers of varieties other than Arran Pilot, the sprouts produced did not occur at the apical complex. A number of investigations have tried to relate final stem numbers in the field to the pattern of sprouting established in storage. The conclusion to be drawn from three experiments using well-sprouted tubers of Arran Pilot (early) and Majestic (main crop ) was that the number of mainstems formed in the field was significantly lower than the number of sprouts at planting (Goodwin et al., 1969; Holmes et al., 1970; Holmes and Gray, 1971 ). The results of the more comprehensive study reported here confirm this finding for early varieties (Arran Pilot, Home Guard and old Pentland Javelin) but show that the pattern of development can vary amongst maincrop varieties. The results in Table 3 indicate that, for some varieties, the development of all sprouts longer than 2 m m can continue after planting, and that there can even be recruitment of sprouts from previously inactive eyes. Examination of the responses of old and young tubers has shown that tubers exposed to high temperatures at the start of storage do not necessarily give fewer mainstems than tubers held at lower temperatures. In several comparisons, the physiologically younger tubers (expected to be multisprouted) did not carry more sprouts than the corresponding older tubers at planting, and in only three out of twelve cases did the use of younger tubers result in an increase of one or more mainstems per tuber after planting. Indeed, the ma-

DEVELOPMENTOF POTATOOF DIFFERINGPHYSIOLOGICALAGE

15

jority of the crops established from physiologically older tubers produced m o r e stems per tuber, if secondaries were included (Table 3 ). Where a direct comparison was made between Pentland Javelin tubers aged in the same way, and then held at either 10-15 °C or 5 °C until planting in the field, there was a good agreement between total number of sprouts produced in the laboratory and the number of mainstems produced by the physiologically older tubers; however, the laboratory results predicted a significantly lower number of mainstems for the younger tubers than was recorded. In general, as found in other experiments (reviewed by Allen, 1978), there were cases of well-developed sprouts at planting which failed to give mainstems, and of eyes which had not sprouted at planting which produced mainstems, and several cases where the two measurements (sprouts and mainstems) were not significantly different. However, these effects did not occur consistently for one ageing treatment only; for example, sprouts failed, and new mainstems were recruited, in both old and young treatments. There are insufficient data to discriminate between damage and the reimposition of inhibition, in the case of sprouts which did not give mainstems. In summary, this investigation confirms that apical dominance does occur in potato tubers (e.g., Table 2), and that its expression can be modified by storage temperature. However, the phenomenon is complex, there is considerable variation in response to storage temperature amongst the tubers of a given population, and extrapolation from a single variety to all potato crops is hazardous. The results presented also indicate the difficulties involved in predicting mainstem populations in the field from measurements made on tubers held in the laboratory. ACKNOWLEDGEMENTS

We thank A.J. Walker for useful discussions throughout the investigation, and E.J. Allen and D.K.L. Mackerron for their detailed criticism of this paper in draft.

REFERENCES Allen, E.J., 1978. Plant density. In: P.M. Harris (Editor), The Potato Crop. Chapman and Hall, London, pp. 278-326. Allen, E.J., Bean, J.N. and Griffith, R.L., 1978. Effects of low temperature on sprout growth of several varieties. Potato Res., 21: 249-255. Appleman, C.O., 1924. Apical dominance in potatoes: an index of seed value. Bull. Univ. MD Agric. Res. Stn., 265. Goodwin, P.B., 1963. Mechanisms and significance of apical dominance in the potato tuber. In: J.D. Ivins and F.L. Milthorpe (Editors), The Growth of the Potato. Butterworths, London, pp. 63-71.

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Goodwin, P.B., 1967. The control of branch growth on potato tubers. II. The pattern of sprout growth. J. Exp. Bot., 18: 87-99. Goodwin, P.B., Brown, A., Lennard, J.H. and Milthorpe, F.L., 1969. Effect of centre of production, maturity and storage treatment of seed tubers on the growth of early potatoes. 2. Field growth. J. Agric. Sci. (Camb.), 73:167-176. Hartmans, K.J. and van Loon, C.D., 1987. Effect of physiological age on growth vigour of seed potatoes of two cultivars. 1. Influence of storage period and temperature on sprouting characteristics. Potato Res., 30: 397-409. Hay, R.K.M. and Walker, A.J., 1989. An Introduction to the Physiology of Crop Yield. Longman, London, 292 pp. Holmes, J.C. and Gray, D., 1971. A comparison of desprouted and late sprouted seed potatoes in relation to apical dominance and tuber production. Potato Res., 14: I 1 1 - l 18. Holmes, J.C., Lang, R.W. and Singh, A.K., 1970. The effect of five growth regulators on apical dominance in potato seed tubers and on subsequent tuber production. Potato Res., 13."342352. O'Brien, P.J., Allen, E.J., Bean, J.N., Griffith, R.L., Jones, S.A. and Jones, J.L., 1983. Accumulated day degrees as a measure of physiological age and the relationships with growth and yield in early potatoes. J. Agric. Sci. (Camb.), 101: 613-631. Toosey, R.D., 1964. The presprouting of seed potatoes: factors affecting sprout growth and subsequent yield. Field Crop Abstr., 17:16 l - 168, 239-244. Wurr, D.C.E., 1974. Some effects of seed size and spacing on the yield and grading of two maincrop potato varieties. I. Final yield and its relationship to plant population. J. Agric. Sci. (Camb.), 82: 37-45. Wurr, D.C.E., 1975. Relationships between sprouting characters and stem development in two maincrop potato varieties. Potato Res., 18: 83-91. Wurr, D.C.E., 1978. Studies of the measurement and interpretation of potato sprout growth. J. Agric. Sci. (Camb.), 90: 335-340. Wurr, D.C.E. and Morris, G.E.L., 1979. Relationships between the number of stems produced by a potato seed tuber and its weight. J. Agric. Sci. (Camb.), 93" 403-409.