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Bud Respiration and Dormancy of Kiwifruit (Actinidia deliciosa)
Annals of Botany 80 : 411–418, 1997
Bud Respiration and Dormancy of Kiwifruit (Actinidia deliciosa) H. G. M c P H E R S ON, W. P. S N E L G AR, P. J. M A N S ON and A N G E L A M. S N O W B A L L Mt Albert Research Centre, The Horticulture and Food Research Institute of New Zealand Ltd, Priate Bag 92 169, Auckland, New Zealand Received : 29 September 1996
Accepted : 30 April 1997
Kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang & A. R. Ferguson) is a perennial vine which requires winter chilling during the dormant period before it will flower and crop adequately. Quantifying the impact of winter temperatures is difficult because the first detectable responses of the vine are floral initiation and bud break which do not occur until spring. We believed that the respiration rate (RD) of buds of kiwifruit during the dormant period might provide an early indication of the changes in bud state associated with chilling. RD was measured during autumn, winter and spring on buds collected from three contrasting regions of New Zealand. The RD of buds collected from these regions during the autumn were consistently different. Buds from the coolest region had the highest RD. These differences which were small, but consistent, were apparent by the time measurements started at the beginning of May. The autumn RD was correlated with the proportion of floral shoots and numbers of flowers per winter bud, and to a lesser extent, the amount and spread of bud break. These correlations focus attention on the importance of this period in determining ‘ winter ’ chilling responses. It raises the possibility of RD being used as a predictor of vine performance as early as 17 weeks before bud break. A rapid increase in RD was detected during spring. This occurred 3–6 weeks prior to bud break and coincided with developmental changes within the buds. Changes in RD in early spring were correlated with the timing and spread of bud break and the proportion of shoots that bore flowers, but not flower numbers. To provide a comparison of the potential use of RD as an index of vine response to chilling, bud break and flowering parameters were regressed against mean air temperature during the period from May to July. The correlations with air temperature were as useful as those based on the autumn or spring RD but were available only 6–9 weeks before bud break. The autumn RD can be obtained at least 8 weeks earlier than mean winter temperature data and this may be an important advantage if this information can be used for forecasting vine responses to chilling. # 1997 Annals of Botany Company Key words : Actinidia deliciosa, kiwifruit, respiration, chilling, dormancy.
INTRODUCTION Winter dormancy occurs in many fruit trees and woody perennials and is a state in which visible growth is suspended, but in which developmental changes can still occur (Saure, 1985). At least some of these changes are promoted by cool winter temperatures and are important because they are required for release from dormancy. They also have a major affect on the timing and amount of bud break and flowering. Bud break and flowering are particularly important in kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang & A. R. Ferguson) because productivity is frequently limited by flower numbers. Flower initiation does not occur until about 3 weeks before bud break (Linsley-Noakes and Allan, 1987 ; Snowball and Walton, 1992). Field studies have shown variation in the timing and amount of bud break and flowering among regions and years in New Zealand (Davison, 1990 ; Salinger, Kenny and Morley-Bunker, 1993 ; McPherson, Hall and Stanley, 1994) and in South Africa (Linsley-Noakes and Allan, 1987). Cooler regions tend to have the highest proportion of bud break and largest number of flowers. Controlled environment studies (Warrington and Stanley, 1986 ; McPherson et al., 1988 ; McPherson, Stanley and Warrington, 1995 ; Stanley, McPherson and Plummer, 1995), field studies (Salinger et al., 1993) and experiments with excised canes (Snelgar, 0305-7364}97}10041108 $25.00}0
Manson and McPherson, 1997) have helped to quantify the effects of temperature, but reliable predictive indices have proved elusive. Quantitative modelling of these winter chilling responses is hampered by the lack of indicators of the changes of state that are occurring in the buds during the period when the environmental influences are having their greatest impact. A number of studies have shown that changes in respiration rate (RD) can be associated with chilling responses. This suggested to us that respiration might provide a useful index of changes in bud state with chilling.
Respiration rate and chilling responses During winter, the flower bud RD of Pyrus calleryana, an early-blooming pear, was twice as high as that of P. communis which blooms late (Cole, Solomos and Faust, 1982). Young, Motomura and Unrath (1987) measured the respiration of entire shoots and roots of apple and peach at 23 °C following various chilling treatments. RD was higher the greater the amount of chilling that occurred, but only where the chilling requirements for bud break were surpassed. However, Hatch and Walker (1969) found no correlation between the respiration rate of peach and apricot flower buds and rest intensity.
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McPherson et al.—Respiration and Dormancy of Kiwifruit
Changes in respiration rates during winter and spring Hatch and Walker (1969) found that peach and apricot flower bud RD remained relatively stable during the winter then increased rapidly during spring. The increase in leaf bud RD occurred 2–4 weeks after the corresponding increase in the flower buds. Likewise, Fielder and Owens (1992) observed that the RD of Douglas fir shoot tips was stable during the winter and there was a gradual rise in rates during February and March when the buds became mitotically active. The RD of buds of Japanese pear was found to increase gradually after rest completion and then increase rapidly at bud break (Tamura, Tanabe and Banno, 1992). In contrast, Cole et al. (1982) found the RD of Pyrus calleryana buds increased more or less linearly during winter, when RD was measured at 25 °C. Measurements made at 5 °C showed a different pattern : rates were relatively stable until mid-winter, then increased steadily until spring. The objective of the present study was to determine whether the RD of dormant kiwifruit buds might provide an index of changes in their state with chilling. We were particularly interested in the possibility that this indication might be available earlier than the alternative technique of dissection, and that it would be non-destructive. RD was measured during autumn, winter and spring on buds which had developed in conditions that span the temperature range found in the major commercial kiwifruit growing areas in New Zealand. MATERIALS AND METHODS Selection of canes Canes were collected from mature (over 6 years old) kiwifruit vines (Actinidia deliciosa cv. Hayward (A. Chev) C. F. Liang & A. R. Ferguson) on New Zealand Research Orchards at Kerikeri (35° 10« S, 173° 55« E), Kumeu (36° 47« S, 174° 33« E) and Riwaka (41° 06« S, 172° 58« E), at 2 to 4 week intervals during the winters of 1992 and 1993. At each site, one cane for each collection time was identified on each of ten vines during April. A further two canes on each vine were reserved for assessing bud break and flowering behaviour on the vines. The preferred cane size was approx. 15–18 mm basal diameter, and canes were pruned to a length of 1±5 m. Self-terminating canes, and those which had fruited in the previous season were avoided wherever possible. Canes were excised from the vine at each sampling date during the season. The cut ends of the canes were wrapped in moist paper towels and the entire cane was wrapped in protective polythene film to minimize water loss. They were then sent by courier to the laboratory in Auckland. On arrival, the canes were unwrapped, stood upright with their cut proximal ends in water and maintained at 20 °C air temperature in the laboratory. The initial measurement of bud RD was made within 30 h of the canes being removed from the vine. Growth enironment and ine performance Air temperatures (1±3 m above ground level inside a Stevenson screen) were obtained from the National Institute
of Water and Atmospheric Research for the meteorological sties at Kerikeri (A53191) and Riwaka (G12191). Equivalent data from a similar meteorological site at Kumeu Research orchard were supplied by W. R. Henshall (HortResearch). All meteorological sites were within 300 m of the experimental vines. Bud break was recorded at 3–7 d intervals on 20 canes at each site. Flowering records were obtained in late spring, when bud break was complete and flower buds were approx. 2–6 mm in diameter. The day of bud break was calculated as the mid point of a cumulative normal curve fitted to the data. The spread of bud break was defined as four times the standard deviation of this fitted distribution ; this includes 95 % of occurrences. Experimental conditions Respiration measurements were made in a temperaturestabilized laboratory at 20 °C and ambient photosynthetic photon flux of approx. 10 µmol m−# s−". Tests showed no detectable difference between respiration rates measured at this flux and in the dark. Consequently we have used the abbreviation RD throughout. Air temperature was measured by a thermistor and recorded every 300 s on a Delta T datalogger (Delta T Instruments, Cambridge, United Kingdom). Bud temperature was measured on a cane using a copper-constantan thermocouple (wire diameter 0±15 mm) inserted into the bud. This cane was not used for the measurement of RD or development. The mean daily air temperature in the laboratory at each sampling date ranged from 19±9 to 21±5 °C in 1992. In 1993 mean daily bud temperatures were measured and these ranged from 19±7 to 21±4 °C. These variations in mean daily temperatures could affect comparisons between days. However, assuming a Q of 2, a range of ³0±8 °C would "! change the rate of respiration by only approx. ³6 %, so corrections were considered unnecessary. The potential for variation of temperature within each day to affect comparisons between sites was eliminated by alternately sampling canes from each site. In 1993, in addition to measuring RD 1 d after excision of the canes, a second measurement was made a week later. Between these measurements canes were stored at 20 °C to promote bud development. The response of respiration to these ‘ forcing ’ conditions was used as an index of bud state. Measurement of respiration rates A closed configuration gas exchange system was used (McPherson, Manson and Snelgar, 1997). The system consisted of : a small bud chamber with an orifice which could be sealed against the bud aperture of an individual bud ; a syringe pump for sampling a set volume of air which had passed through the bud chamber ; and an infrared gas analyser. Air of known CO concentration was drawn # through the bud chamber by the syringe pump and collected in a gas-tight syringe. This was injected directly into the infrared gas analyser and the peak response was recorded on a digital multimeter. The technique was found to be reliable even at rates of respiration as low as 20 pmol s−" and gave
McPherson et al.—Respiration and Dormancy of Kiwifruit repeatable measurements on individual buds (CV approx. 5 %). The ability to make non-destructive measurements was a considerable advantage because it enabled us to make repeat measurements on individual buds. Respiration measurements were made on the apical bud on all canes because this is normally the first bud to develop into a shoot and it usually produces the most flowers (McPherson, Hall and Stanley, 1992). For comparative purposes a second, more proximal bud, was also measured. This was normally the fifth bud from the apex. On the few occasions where the fifth bud was unsuitable in some way (e.g. because of damage or its proximity to a neighbouring bud) the fourth or sixth bud was measured. This position is referred to as the ‘ middle bud ’. Measurements of RD were discontinued once buds had passed a stage of development designated as ‘ bud swell ’ (Brundell, 1975). At this stage the bud was expanding rapidly and it became increasingly difficult to seal the bud chamber in place. The rates of respiration of buds at the bud swell stage were found to be at least 1200 pmol s−". This value was assumed for any buds that had passed the bud swell stage. This rate is so much higher than the rate of buds which were at the stable, winter dormant rate, that estimates made of the time of rapid increase in bud RD in spring were not sensitive to the exact value used in this assumption.
413
Fresh and dry weights of buds were determined by the following procedure. The protective bark covering the bud was removed to expose the dome-shaped bud structure which is attached at its base to the cane (Ferguson, 1984). The bud was then removed by cutting its base in line with the surface of the exposed cane. Fresh weights were measured immediately, and dry weights determined after drying to constant weight at 60 °C. RESULTS AND DISCUSSION Seasonal changes in respiration Dormant bud RD was stable and in the range 20 to 110 pmol s−" from May to July in both 1992 and 1993 (Fig. 1 A and B). By the time measurements were started, RD was higher on buds from the coolest site (Riwaka) for both bud positions. There was no marked difference in RD between apical and middle buds during autumn and winter (data for middle buds not shown). Early in spring, RD increased rapidly.
June 1200
July
August
September
A 1992
Buds were dissected and observed under a light microscope (¬40 magnification) to determine their stage of development, so that this could be related to the observed changes in RD. Dissections were only undertaken in 1993. The first measurements were on 17 August (day 229), prior to any observable external changes in the buds. Measurements were made on five occasions, the last being on 28 September (day 271). Buds were dissected by removing each bract or leaf initial individually, exposing the axillary structure for examination and identification. Each axillary structure within each bud was classified as falling in one of seven identifiable stages. At stage 0 the axillary structure was a simple, undeveloped primordium. At stage 1 the primordium was swollen with a flat top and at stage 2 the two lateral flower bracts were differentiated. By stage 3 the first sepals on the ‘ king ’ flower were visible. This stage corresponds to the stage VI of Linsley-Noakes and Allan (1987). Stage 4 was reached when the lateral flower primordia were visible and the king flower was swollen. Three sepals were visible on the king flower at stage 5, and six sepals were present at stage 6. These stages were used to calculate an arbitrary score to compare the stage of bud development with the changes in RD. As a variable number of structures at different stages of development could be observed in any one bud, a weighted index was calculated. This was given for each bud, by the sum of the products of the number of structures in each stage of development by the value of each stage. Therefore, as the first stage was designated as zero, only those structures which had a visible change indicating they were undergoing reproductive development contributed to the index.
600 300 0 B 1993 900 600 300 0 C 1993
Developmental score
Bud deelopment
Respiration rate (pmol CO2 s–1)
900
20 15 10 Kerikeri Kumeu Riwaka
5 0
140
160
180
200 220 Day of the year
240
260
F. 1. Changes in the RD of dormant apical buds 1992 (A) and 1993 (B). Canes were excised from vines grown in three contrasting regions and measured at 20 °C the following day. Vertical bars indicate l.s.d. (P ¯ 0±05). C, Changes in the state of development of reproductive structures within buds. The score was weighted according to the number of buds at each stage of development. See text for details.
McPherson et al.—Respiration and Dormancy of Kiwifruit August
Kerikeri
June
September
Respiration rate (pmol CO2 s–1)
414
A 1992
Kumeu Riwaka Kerikeri
B 1993
Kumeu
1200
Kerikeri Kumeu Riwaka
August
September
1993
900
600
300
0
Riwaka Kerikeri
July
140
160
180
200 220 240 Day of the year
260
280
F. 3. RD of apical buds before and after ‘ forcing ’ at 20 °C for 1 week. Canes were excised from vines grown in three contrasting regions. Vertical bars indicate l.s.d. (P ¯ 0±05).
C 1993
Kumeu Apical Middle
Riwaka 200
210
220
230 240 250 Day of the year
260
270
F. 2. A and B, The day of the year on which bud RD reached an average of 200 pmol CO s−" on canes excised from vines growing in # three contrasting regions and measured the following day. C, The day of the year on which RD increased by 5 pmol CO s−" d−" in response # to forcing at 20 °C (1993 only). Data are shown for both the apical bud, and one more remote from the effects of apical control (bud 4, 5 or 6) which was designated as the ‘ middle ’ bud.
Canes from cooler sites showed this increase earlier than those from warmer sites in both 1992 and 1993. The RD of apical buds increased earlier than middle buds. The time of rapid increase in RD was quantified by estimating the day on which it reached 200 pmol s−". This threshold value was chosen so that it was clearly higher than the winter RD, but would detect the rise in RD during spring as early as possible. The choice was arbitrary, but selection of alternative threshold values between 100 and 300 pmol s−" would not substantially alter our analyses (Fig. 1). In 1992, apical buds from Riwaka reached this rate 29 d before apical buds from Kerikeri (Fig. 2). At all sites, middle buds reached this state 12 to 19 d later than apical buds. The changes in dormant bud RD in 1993 were similar to the pattern found in 1992. However, the time at which RD increased was between 4 and 20 d later in 1993 (Fig. 2). This pattern of a stable RD during winter with a rapid increase in spring is similar to findings for peach and apricot (Hatch and Walker, 1969) and Douglas fir shoot tips (Fielder and Owens, 1992). It contrasts though, with the pattern observed in Pyrus spp. where there is an approximately linear increase in RD during the winter (Cole et al., 1982). Response to forcing The response of buds to forcing in warm temperatures was assessed because it was possible that it would indicate a change of state of the bud earlier than the RD of buds
measured soon after being removed from field conditions. During autumn and winter, forcing buds at 20 °C for 1 week reduced RD by between 0 and 5 pmol s−" d−". By early spring, this pattern had reversed and forcing increased, rather than decreased, respiration (Fig. 3). The response to forcing became progressively greater as bud break approached, and the colder the site, the earlier the change. Rates of increase as high as 77 pmol s−" d−" were observed. The timing of this change in response was quantified by estimating the day on which the rate of change in respiration reached 5 pmol s−" d−", by linear interpolation of the data in Fig. 3. This threshold value was chosen to represent the earliest time that we could be confident the forcing response had changed from negative to positive. The choice was arbitrary, but again the conclusions were insensitive to the exact value chosen. Buds from the coolest site reached the 5 pmol s−" d−" change in RD earliest (day 207), and apical buds generally showed an earlier, or larger, response than middle buds (data not shown). This alteration in forcing response of apical buds preceded the increase in RD by 9 d at Kerikeri, which is the warmest site, and by 26 d at Riwaka, the coolest site (Fig. 2 B and C). Bud size Bud fresh weights and dry weights were determined following the 7 d forcing treatment for the canes measured on May 25 (day 145), June 16 (day 167), July 6 (day 187) and July 27 (day 208) 1993. At the subsequent sampling dates the buds were dissected to determine any changes visible by light microscopy, and it was not possible to weigh them accurately. There were few changes in bud fresh and dry weight during this period (data not shown). Walton, Clark and Boldingh (1991) found that bud dry weights in kiwifruit were stable from early August until mid-September, 10 d before bud break. Hatch and Walker (1969) also found that peach and apricot flower bud dry weight did not vary significantly during winter. In the case of pears (Zimmerman and Faust, 1969 ; Cole et al., 1982) there was a gradual increase in fresh weight during winter.
McPherson et al.—Respiration and Dormancy of Kiwifruit
415
T 1. Mean bud fresh weight, dry weight and moisture content from samples measured on four dates between May 25 (day 145) and July 27 (day 208) 1993 after buds had been forced for 7 d at 20 °C Apical buds
Site Kerikeri Kumeu Riwaka Mean l.s.d. (P ¯ 0±05)
Middle buds
Fresh weight (mg)
Dry weight (mg)
Moisture content (%)
Fresh weight (mg)
Dry weight (mg)
Moisture content (%)
50±0 53±0 45±4 49±6 6±9
31±7 33±6 28±3 31±2 4±3
37±2 36±7 37±9 37±3 2±1
55±1 55±0 50±2 53±5 6±1
35±9 35±9 31±7 34±6 4±1
34±8 34±7 37±1 35±5 2±1
The dry weight of apical buds from Riwaka was 16 % lower than those from Kumeu (Table 1). At this time of year the RD of the apical buds was 45 % higher on average (Fig. 1 A and B). The rate of respiration per unit dry weight was therefore 72 % higher in the Riwaka buds over the May to July period. As such large differences are established before the beginning of winter, this implies that autumn temperatures contribute to chilling. Phenological models have also emphasized the importance of autumn temperatures, with April temperatures alone accounting for 74 % of the observed variation in the date of bud break (Hall and McPherson, 1997). Measurements made on 12 Aug. 1992 (day 224) showed differences in the distribution of bud moisture content among sites. This was the first sampling date on which marked differences in RD among sites were becoming apparent (Fig. 1 A and B). At this time, Kerikeri bud RD were still at their wintertime rate, moisture contents averaged 38 % and the range was narrow (s.d. ¯ 3 %). The RD of Kumeu buds were about to increase rapidly. Their mean moisture content was similar to the Kerikeri buds (39 %), but the range of moisture content was greater (s.d. ¯ 5 %). The substantial increase in the RD of the Riwaka buds was accompanied by a substantial increase in both average moisture content (49 %) and range of moisture content (s.d. ¯ 11 %). Others have noticed an increase in bud moisture content during bud development in spring. Zimmerman and Faust (1969) found that the terminal buds of Pyrus calleryana (pear) had a rapid increase in fresh weight in spring but the dry weight remained constant, indicating a movement of water into the buds. The water content of the terminal buds of birch trees increased from 37 to 49 % during the first 10 d of forcing under controlled conditions (Rinne, Tuominen and Junttila, 1994). Assuming a typical fresh weight of 50 mg for dormant kiwifruit buds (Table 1), the RD was approx. 1±2 nmol s−" g−" f. wt. These are similar to values reported for peach flower buds (0±9 nmol s−" g−") and apricot flower buds (0±8 nmol s−" g−") (Hatch and Walker, 1969), but about 30 % of those reported for Pyrus calleryana (3±5 nmol s−" g−") and P. communis (3±3 nmol s−" g−") (Cole et al., 1982) and Japanese pear (3±5 nmol s−" g−") (Tamura et al., 1992). Immediately prior to the stage of bud swell, when measurements were ceased, RD in kiwifruit had increased 20-fold to approx. 24 nmol s−" g−".
Bud deelopment Microscopic examination of the axillary structures of dissected buds enabled us to score their stage of development. At the time of the first measurement in midAugust, the buds from the coolest site, Riwaka, showed readily detectable reproductive development (Fig. 1 C). There were no visible changes at this stage in material from the other two sites. By the beginning of September, reproductive structures were visible in the buds from Kumeu and Kerikeri. The rate of bud development, as measured by our score, soon matched that at Riwaka. Comparison of Fig. 1 B and C shows that the marked change in RD occurred at about the same time that the developmental score increased. The buds from Riwaka reached 200 pmol s−" on about day 230, by which time they were showing a weighted developmental score of 5. The buds from Kerikeri and Kumeu reached an RD of 200 pmol s−" by approx. day 248 and they reached a developmental score of 5 on day 250. A comparison of RD and the developmental score on a bud-to-bud basis showed a substantial correlation, though there was considerable variation (R# ¯ 0±62). Bud break and flowering in the field The mid point of bud break was earliest and the spread of bud break shortest in Riwaka (day 258, 19 d, respectively) and latest and most prolonged in Kerikeri (day 286, 36 d). This is in accord with the regional variation previously reported (McPherson et al., 1994). During spring 1993 two screen frosts occurred during bud break at Riwaka (day 251 and 272). These frosts caused considerable damage to many shoots and this promoted a second wave of bud break which resulted in a high (0±65) and an extended bud break. The late-breaking buds were clearly discernible from the cumulative bud break plots, so these data were excluded from the results presented in Table 2. The proportion of buds which developed in spring varied from a minimum of 0±38 at Kerikeri to 0±57 at Riwaka (Table 2), and these values are typical of those reported previously in these regions (McPherson et al., 1994). However, a high proportion of shoots (0±85 to 0±94) at each site bore flowers, and this led to an unusually high number of flowers per winter bud. This effect was particularly
416
McPherson et al.—Respiration and Dormancy of Kiwifruit T 2. Bud break and flowering data for kiwifruit growing in three regions of New Zealand
Year
Site
Time of 50 % bud break (day of year)
1992
Kerikeri Kumeu Riwaka Kerikeri Kumeu Riwaka
274 263 264 286 278 258
1993
Spread of bud break (d)
Proportion of bud break
No. reproductive shoots per shoot
No. flowers per reproductive shoot
No. flowers per winter bud
34 29 19 36 26 25
0±46 0±50 0±57 0±38 0±46 0±56
0±87 0±89 0±94 0±85 0±88 *
5±6 4±3 7±6 5±1 4±5 *
2±2 1±9 4±1 1±6 1±9 *
* Data not determined as frosts during bud break killed shoots and flower buds.
T 3. Regressions of bud break and flowering data against either respiration indices, or mean temperature for May, June and July (T )
DBB SpreadBB ShootWB R.shootshoot FlowerR.shoot FlowerWB
RD during May–July
R#
Timing of RD increase in spring
R#
Mean May–July temperature
R#
49®9 RD 0±260±1 RD 0±770±06 RD
n.s. 0±69 0±68 0±84 n.s. 0±84
321±0 (DA) for DA ¯ 230 to 251* ®1061±1 (DM®DA)0±51 DA 0±56®(2±1¬10−"#) e(DA/"!) 1±5®0±0025 DA®0±003 (DM–DA)
0±84 0±94 0±85 0±98 n.s. n.s.
1987 T for 9 to 12 °C† ®0±33 T 0±59®(48¬10−') e!.&T 1±05®0±016 T
0±73 0±78 0±99 0±95 n.s. 0±93
®1±31±7 RD
7±2®0±47 T
T ranged from 6±8 to 12±1 °C. Only regressions which were statistically significant (P ! 0±05) are presented. DA and DB are the days on which the RD of the apical buds and middle buds, respectively, reached 200 pmol s−". For all bud break data n ¯ 6, and for all flowering data, n ¯ 5. DBB, day of bud break ; SpreadBB, spread of bud break ; ShootWB, no. of shoots per winter bud ; R.shootshoot, no. reproductive shoots per shoot ; FlowerR.shoot, no. of flowers per reproductive shoot ; FlowerWB, no. of flowers per winter bud. * DBB ¯ 263 for DA below 230. † DBB ¯ 260 for T below 9 °C.
obvious in Kerikeri where the values of 2±2 and 1±6 flowers per winter bud are considerably higher than the mean of 0±9 observed by McPherson et al. (1994) during 1986 to 1989. However, higher numbers of flowers per winter bud have been recorded at Kerikeri more recently (Manson and Snelgar, 1995). In 1992, the flowering data for Riwaka was based on only nine canes as many shoots had been damaged by frost, and thus complete flowering records could not be obtained for these canes. In 1993 frost-damage at Riwaka was so extensive that no flowering records were collected. Bud respiration during autumn and winter as a predictor of ine performance From early May until about the end of July, there was little change in RD at any of the study sites. Buds at Riwaka had consistently higher RD throughout this period during both 1992 and 1993. We used the mean RD for this period as a predictor of the timing of bud break and the amount of bud break and flowering (Table 3). Since RD was stable through this period and could be measured in autumn, we refer to this as ‘ autumn RD ’. The timing of bud break was not significantly correlated with RD, but there was a negative correlation with the spread of bud break. The number of flowers per winter bud (FlowerWB) is the most commonly used index of flower production in kiwifruit vines. It is the product of several components : FlowerWB ¯ ShootWB¬R.ShootShoot¬FlowerR.Shoot,
where ShootWB is the number of shoots (‘ laterals ’) that develop per winter bud (i.e. ‘ bud break ’) ; R.ShootShoot is the number of reproductive shoots that develop per shoot and FlowerR.shoot is the number of flowers per reproductive shoot. The relationship between dormant RD and each of these components was investigated separately. RD was positively correlated with the proportion of buds that broke, the proportion of shoots that bore flowers, and the number of flowers per winter bud. These high correlations should be accepted with caution because of the poor distribution of data which resulted from the loss of flowering data from Riwaka. The correlation with the number of flowers per shoot was not significant (Table 3). Bud respiration during spring as a predictor of ine performance The time at which the RD began to increase rapidly in spring was quantified by estimating the day on which RD reached 200 pmol s−" (DA for apical buds and DM for middle buds) by linear interpolation between the data shown in Fig. 1 A and B. DA was positively correlated with the time of bud break over the range DA ¯ 230 to 251 (Table 3). When DA was earlier than day 230, then bud break occurred at the earliest date recorded, approx. day 263. To predict the spread of bud break we regressed it against both DA and DM–DA (Table 3). We used the
McPherson et al.—Respiration and Dormancy of Kiwifruit latter parameter to provide a measure of the variation in development within each cane. A late rise in respiration led to a greater spread of bud break. The greater the difference between the timing of the apical and middle buds, the greater was the spread of bud break. The proportion of buds that broke was highest when DA was small i.e. when bud break was early. The relationship was exponential, with the upper asymptote for bud break being 0±56. Small changes in DA late in the season were associated with large changes in bud break (Table 3). Thus if buds commenced development early in the season (as indicated by a small DA) bud break was high. The proportion of shoots that bore flowers decreased as DA increased, and also as DM–DA increased. That is, late bud break, and a large variation in the time of bud break within the cane, was associated with a low proportion of shoots bearing flowers. This confirms other reports that late bud break is associated with poor flowering (Grant and Ryugo, 1982 ; Snelgar, 1988 ; McPherson et al., 1992), and factors which increase the spread of bud break decrease flowering (Grant and Ryugo, 1982 ; Linsley-Noakes and Allan, 1987). These reductions are likely to be due to the between-bud interactions that occur within a cane during bud break which can lead to low flower numbers (Grant and Ryugo, 1982 ; Snowball and Considine, 1986 ; McPherson et al., 1992). The number of flowers on each shoot was not significantly correlated with either the time of development of the apical bud, or the spread of development along the cane. Consequently, it is not surprising that the product of the three components of flowering, the number of flowers per winter bud, was not significantly correlated with the time at which the apical bud breaks. Temperature during autumn and winter as a predictor of ine performance To provide a comparison of the use of RD as an index of vine response to chilling, the bud break and flowering parameters were regressed against the mean monthly air temperature during the period from May to July (Table 4). Since this mean can be established only by the end of July, we refer to the mean as ‘ winter temperature ’. This period has been used by Salinger et al. (1993) to model the end of winter dormancy. Although bud break and flower production in kiwifruit are believed to be enhanced by cool temperatures during winter, it is likely that cool spring temperatures can impair bud break and flowering T 4. Mean monthly air temperatures (°C ) for Kerikeri, Kumeu and Riwaka meteorological sites for 1992 and 1993
1992
Kerikeri Kumeu Riwaka 1993 Kerikeri Kumeu Riwaka
May
June
July
12±3 11±4 8±0 13±6 12±4 10±0
10±1 9±6 5±5 12±3 11±7 8±6
11±6 10±8 6±8 10±5 9±2 6±5
417
(Warrington and Stanley, 1986 ; McPherson et al., 1988 ; McPherson, Warrington and Stanley, 1989). It is important, therefore, that the period selected to define winter chilling does not include early spring. More complex indexes, which are derived from winter temperatures, have been used in other studies of winter chilling in fruit trees (Richardson, Seeley and Walker, 1974). However, since this chill-unit produces inconsistent predictions for kiwifruit (McPherson et al., 1995), we chose to use mean temperature as a measure of winter chilling. Warm temperatures during winter resulted in bud break being delayed and spread over a longer period. The midpoint of bud break was delayed by 7 d for each 1 °C increase in mean winter temperature above 9 °C (Table 3). Temperatures below 9 °C did not affect the timing of bud break. Both these results are in agreement with responses found with excised canes (Snelgar et al., 1997). The spread of bud break increased by 3 d for each 1 °C increase in mean winter temperature. This shows the same sensitivity to temperature as previous reports have shown for flowering (McPherson et al., 1988). The proportion of over-wintered buds which developed into shoots (ShootWB or bud break) decreased exponentially as the mean winter temperature increased from 7 to 12 °C (R# ¯ 0±99). The changes in bud break were small between 7 and 10 °C, declining from 0±57 to 0±52, but they were much larger above 10 °C, being reduced to 0±40 at 12 °C. The proportion of shoots which produced flowers (R.shootshoot) decreased linearly as winter temperature increased, but the rate of change was only 0±016 per °C (R# ¯ 0±95). The proportion of shoots which bore flowers was reduced from 0±94 to 0±85 over the range of 5±4 °C recorded in the present study. The number of flowers on each flowering shoot (FlowerR.Shoot) was not significantly correlated with mean winter temperature. The product of all three flowering components, the number of flowers per winter bud (FlowerWB), decreased linearly as mean winter temperature increased. Although this correlation is statistically highly significant (R# ¯ 0±93) the data set was poorly distributed. This finding should be viewed with caution unless it can be verified against other data. CONCLUSIONS Buds collected from different regions showed differences in RD during autumn. Our measurements did not start early enough to determine when these differences were established, but they were apparent as early as the beginning of May. This was approx. 17 weeks prior to bud break. The autumn RD was useful for predicting the proportion of floral shoots and number of flowers per winter bud, and to a lesser extent, the amount and spread of bud break. This finding corroborates recent suggestions (Hall and McPherson, 1997 ; Snelgar et al., 1997) the autumn temperatures need to be included in ‘ winter ’ chilling models. The rates of RD did not change appreciably during the period from May to July. This may have been because there were no major changes in the state of the buds, or because any changes that occurred did not affect respiration rates.
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The changes in RD during early spring could be used to predict the timing and spread of bud break, and the amount of bud break, but not flower numbers. These changes occurred at about the same time that the formation of reproductive structures could be detected by dissection. Increases in RD were accompanied by an increase in the moisture content of the bud, so this is an alternative method of detecting changes in bud state though, like dissection, it is destructive. All of these changes occurred only 3 to 6 weeks before the mid point of bud break. Mean winter temperatures were available 6 to 9 weeks before the mid point of bud break and were more effective, in general, than either spring or autumn RD for predicting vine performance. However, the autumn RD can be obtained at least 8 weeks before mean winter temperature data and this may enable earlier forecasts of vine performance, even if those forecasts are less reliable. A C K N O W L E D G E M E N TS We wish to thank Roger Hamilton and Greg Lupton for assessing vine performance, and Sally Roughan and Deborah Hutching for their assistance with the dormant bud respiration measurements. This work has benefitted from many useful discussions with Alistair Hall on chilling. LITERATURE CITED Brundell DJ. 1975. Flower development of the Chinese Gooseberry (Actinidia chinensis Planch.) I. Development of the flowering shoot. New Zealand Journal of Botany 13 : 473–483. Cole ME, Solomos T, Faust M. 1982. Growth and respiration of dormant flower buds of Pyrus communis and Pyrus alleryana. Journal of the American Society of Horticultural Science 107 : 226–231. Davison RM. 1990. The physiology of the kiwifruit vine. In : Warrington IJ, Weston GC, eds. Kiwifruit science and management. Auckland : Ray Richards, 127–154. Ferguson AR. 1984. Kiwifruit : A botanical review. In : Janick J, ed. Horticultural reiews. Westport : AVI Publishing Co 6 : 1–64. Fielder P, Owens JN. 1992. Shoot-tip respiration of 1st-year interior and coastal Douglas-fir seedlings during bud development. Canadian Journal of Forest Research 22 : 765–768. Grant JA, Ryugo K. 1982. Influence of developing shoots on flowering potential of dormant buds of Actinidia chinensis. HortScience 17 : 977–978. Hall AJ, McPherson, HG. 1997. Modelling the influence of temperature on the timing of bud break in kiwifruit. In : Proceedings of the Third International Symposium on Kiwifruit, Thessaloniki, Greece, September 1995. Acta Horticulturae 444 (in press). Hatch AH, Walker DR. 1969. Rest intensity of dormant peach and apricot leaf buds as influenced by temperature, cold hardiness, and respiration. Journal of the American Society of Horticultural Science 94 : 304–307. Linsley-Noakes GC, Allan P. 1987. Effects of winter temperatures on flower development in two clones of kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson). Scientia Horticulturae 33 : 249–260.
McPherson HG, Hall AJ, Stanley CJ. 1992. The influence of current temperature on the time from bud break to flowering in kiwifruit Actinidia deliciosa. Journal of Horticultural Science 67 : 509–519. McPherson HG, Hall AJ, Stanley CJ. 1994. Seasonal and regional variation in bud break and flowering of kiwifruit vines (Actinidia deliciosa) in New Zealand. New Zealand Journal of Crop and Horticultural Science 22 : 263–276. McPherson HG, Manson PJ, Snelgar WP. 1997. Non-destructive measurement of dormant bud respiration rates. Photosynthetica 33 : 125–138. McPherson HG, Stanley CJ, Warrington IJ. 1995. The response of bud break and flowering to cool winter temperatures in kiwifruit (Actinidia deliciosa). Journal of Horticultural Science 70 : 737–747. McPherson H, Stanley J, Warrington I, Jansson D. 1988. Dynamics of bud break and flowering. New Zealand Kiwifruit Special Publication No. 2 : 9–11. McPherson H, Warrington I, Stanley J. 1989. Weathering the weather. New Zealand Kiwifruit April : 14–15. Manson PJ, Snelgar WP. 1995. Regional variations in the response of kiwifruit vines to time of cane tipping. New Zealand Journal of Crop and Horticultural Science 23 : 67–71. Richardson EA, Seeley SD, Walker DR. 1974. A model for estimating the completion of rest for Redhaven and Elberta peach trees. HortScience 9 : 331–332. Rinne P, Tuominen H, Junttila O. 1994. Seasonal changes in bud dormancy in relation to bud morphology, water and starch content, and abscisic acid concentration in adult trees of Betula pubescens. Tree Physiology 14 : 549–561. Salinger MJ, Kenny GJ, Morley-Bunker MJ. 1993. Climate and kiwifruit cf. Hayward 1. Influences on development and growth. New Zealand Journal of Crop and Horticultural Science 21 : 235–245. Saure MC. 1985. Dormancy release in deciduous fruit trees. Horticultural Reiews 7 : 239–300. Snelgar WP. 1988. The effect of cane orientation on flower production in kiwifruit (Actinidia deliciosa). Journal of Horticultural Science 63 : 341–347. Snelgar WP, Manson PJ, McPherson HG. 1997. Evaluating winter chilling of kiwifruit using excised canes. Journal of Horticultural Science 72 : 305–315. Snowball AM, Considine JA. 1986. Flower development and shoot growth. New Zealand Kiwifruit July : 20. Snowball AM, Walton EF. 1992. Flowering in kiwifruit. New Zealand Kiwifruit Special Publication No. 4 : 25–28. Stanley CJ, McPherson HG, Plummer JA. 1995. The use of unrooted cuttings for studying the effects of chilling in kiwifruit (Actinidia deliciosa). Journal of Horticultural Science 70 : 749–756. Tamura F, Tanabe K, Banno K. 1992. Effect of chilling treatment on intensity of bud dormancy, respiration and endogenous growth regulators in Japanese Pear ‘ Nijisseiki ’. Journal of the Japanese Society of Horticultural Science 60 : 763–769. Walton EF, Clark CJ, Boldingh HL. 1991. Effect of hydrogen cyanamide on amino acid profiles in kiwifruit buds during budbreak. Plant Physiology 97 : 1256–1259. Warrington IJ, Stanley CJ. 1986. The influence of pre- and postbudbreak temperatures on flowering in kiwifruit. Acta Horticulturae 175 : 103–107. Young E, Motomura Y, Unrath CR. 1987. Influence of root temperature during dormancy on respiration, carbohydrates, and growth resumption in apple and peach. Journal of the American Society of Horticultural Science 112 : 514–519. Zimmerman RH, Faust M. 1969. Pear bud metabolism : seasonal changes in glucose utilization. Plant Physiology 44 : 1273–1276.
Bud Respiration and Dormancy of Kiwifruit (Actinidia deliciosa) H. G. M c P H E R S ON, W. P. S N E L G AR, P. J. M A N S ON and A N G E L A M. S N O W B A L L Mt Albert Research Centre, The Horticulture and Food Research Institute of New Zealand Ltd, Priate Bag 92 169, Auckland, New Zealand Received : 29 September 1996
Accepted : 30 April 1997
Kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang & A. R. Ferguson) is a perennial vine which requires winter chilling during the dormant period before it will flower and crop adequately. Quantifying the impact of winter temperatures is difficult because the first detectable responses of the vine are floral initiation and bud break which do not occur until spring. We believed that the respiration rate (RD) of buds of kiwifruit during the dormant period might provide an early indication of the changes in bud state associated with chilling. RD was measured during autumn, winter and spring on buds collected from three contrasting regions of New Zealand. The RD of buds collected from these regions during the autumn were consistently different. Buds from the coolest region had the highest RD. These differences which were small, but consistent, were apparent by the time measurements started at the beginning of May. The autumn RD was correlated with the proportion of floral shoots and numbers of flowers per winter bud, and to a lesser extent, the amount and spread of bud break. These correlations focus attention on the importance of this period in determining ‘ winter ’ chilling responses. It raises the possibility of RD being used as a predictor of vine performance as early as 17 weeks before bud break. A rapid increase in RD was detected during spring. This occurred 3–6 weeks prior to bud break and coincided with developmental changes within the buds. Changes in RD in early spring were correlated with the timing and spread of bud break and the proportion of shoots that bore flowers, but not flower numbers. To provide a comparison of the potential use of RD as an index of vine response to chilling, bud break and flowering parameters were regressed against mean air temperature during the period from May to July. The correlations with air temperature were as useful as those based on the autumn or spring RD but were available only 6–9 weeks before bud break. The autumn RD can be obtained at least 8 weeks earlier than mean winter temperature data and this may be an important advantage if this information can be used for forecasting vine responses to chilling. # 1997 Annals of Botany Company Key words : Actinidia deliciosa, kiwifruit, respiration, chilling, dormancy.
INTRODUCTION Winter dormancy occurs in many fruit trees and woody perennials and is a state in which visible growth is suspended, but in which developmental changes can still occur (Saure, 1985). At least some of these changes are promoted by cool winter temperatures and are important because they are required for release from dormancy. They also have a major affect on the timing and amount of bud break and flowering. Bud break and flowering are particularly important in kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang & A. R. Ferguson) because productivity is frequently limited by flower numbers. Flower initiation does not occur until about 3 weeks before bud break (Linsley-Noakes and Allan, 1987 ; Snowball and Walton, 1992). Field studies have shown variation in the timing and amount of bud break and flowering among regions and years in New Zealand (Davison, 1990 ; Salinger, Kenny and Morley-Bunker, 1993 ; McPherson, Hall and Stanley, 1994) and in South Africa (Linsley-Noakes and Allan, 1987). Cooler regions tend to have the highest proportion of bud break and largest number of flowers. Controlled environment studies (Warrington and Stanley, 1986 ; McPherson et al., 1988 ; McPherson, Stanley and Warrington, 1995 ; Stanley, McPherson and Plummer, 1995), field studies (Salinger et al., 1993) and experiments with excised canes (Snelgar, 0305-7364}97}10041108 $25.00}0
Manson and McPherson, 1997) have helped to quantify the effects of temperature, but reliable predictive indices have proved elusive. Quantitative modelling of these winter chilling responses is hampered by the lack of indicators of the changes of state that are occurring in the buds during the period when the environmental influences are having their greatest impact. A number of studies have shown that changes in respiration rate (RD) can be associated with chilling responses. This suggested to us that respiration might provide a useful index of changes in bud state with chilling.
Respiration rate and chilling responses During winter, the flower bud RD of Pyrus calleryana, an early-blooming pear, was twice as high as that of P. communis which blooms late (Cole, Solomos and Faust, 1982). Young, Motomura and Unrath (1987) measured the respiration of entire shoots and roots of apple and peach at 23 °C following various chilling treatments. RD was higher the greater the amount of chilling that occurred, but only where the chilling requirements for bud break were surpassed. However, Hatch and Walker (1969) found no correlation between the respiration rate of peach and apricot flower buds and rest intensity.
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Changes in respiration rates during winter and spring Hatch and Walker (1969) found that peach and apricot flower bud RD remained relatively stable during the winter then increased rapidly during spring. The increase in leaf bud RD occurred 2–4 weeks after the corresponding increase in the flower buds. Likewise, Fielder and Owens (1992) observed that the RD of Douglas fir shoot tips was stable during the winter and there was a gradual rise in rates during February and March when the buds became mitotically active. The RD of buds of Japanese pear was found to increase gradually after rest completion and then increase rapidly at bud break (Tamura, Tanabe and Banno, 1992). In contrast, Cole et al. (1982) found the RD of Pyrus calleryana buds increased more or less linearly during winter, when RD was measured at 25 °C. Measurements made at 5 °C showed a different pattern : rates were relatively stable until mid-winter, then increased steadily until spring. The objective of the present study was to determine whether the RD of dormant kiwifruit buds might provide an index of changes in their state with chilling. We were particularly interested in the possibility that this indication might be available earlier than the alternative technique of dissection, and that it would be non-destructive. RD was measured during autumn, winter and spring on buds which had developed in conditions that span the temperature range found in the major commercial kiwifruit growing areas in New Zealand. MATERIALS AND METHODS Selection of canes Canes were collected from mature (over 6 years old) kiwifruit vines (Actinidia deliciosa cv. Hayward (A. Chev) C. F. Liang & A. R. Ferguson) on New Zealand Research Orchards at Kerikeri (35° 10« S, 173° 55« E), Kumeu (36° 47« S, 174° 33« E) and Riwaka (41° 06« S, 172° 58« E), at 2 to 4 week intervals during the winters of 1992 and 1993. At each site, one cane for each collection time was identified on each of ten vines during April. A further two canes on each vine were reserved for assessing bud break and flowering behaviour on the vines. The preferred cane size was approx. 15–18 mm basal diameter, and canes were pruned to a length of 1±5 m. Self-terminating canes, and those which had fruited in the previous season were avoided wherever possible. Canes were excised from the vine at each sampling date during the season. The cut ends of the canes were wrapped in moist paper towels and the entire cane was wrapped in protective polythene film to minimize water loss. They were then sent by courier to the laboratory in Auckland. On arrival, the canes were unwrapped, stood upright with their cut proximal ends in water and maintained at 20 °C air temperature in the laboratory. The initial measurement of bud RD was made within 30 h of the canes being removed from the vine. Growth enironment and ine performance Air temperatures (1±3 m above ground level inside a Stevenson screen) were obtained from the National Institute
of Water and Atmospheric Research for the meteorological sties at Kerikeri (A53191) and Riwaka (G12191). Equivalent data from a similar meteorological site at Kumeu Research orchard were supplied by W. R. Henshall (HortResearch). All meteorological sites were within 300 m of the experimental vines. Bud break was recorded at 3–7 d intervals on 20 canes at each site. Flowering records were obtained in late spring, when bud break was complete and flower buds were approx. 2–6 mm in diameter. The day of bud break was calculated as the mid point of a cumulative normal curve fitted to the data. The spread of bud break was defined as four times the standard deviation of this fitted distribution ; this includes 95 % of occurrences. Experimental conditions Respiration measurements were made in a temperaturestabilized laboratory at 20 °C and ambient photosynthetic photon flux of approx. 10 µmol m−# s−". Tests showed no detectable difference between respiration rates measured at this flux and in the dark. Consequently we have used the abbreviation RD throughout. Air temperature was measured by a thermistor and recorded every 300 s on a Delta T datalogger (Delta T Instruments, Cambridge, United Kingdom). Bud temperature was measured on a cane using a copper-constantan thermocouple (wire diameter 0±15 mm) inserted into the bud. This cane was not used for the measurement of RD or development. The mean daily air temperature in the laboratory at each sampling date ranged from 19±9 to 21±5 °C in 1992. In 1993 mean daily bud temperatures were measured and these ranged from 19±7 to 21±4 °C. These variations in mean daily temperatures could affect comparisons between days. However, assuming a Q of 2, a range of ³0±8 °C would "! change the rate of respiration by only approx. ³6 %, so corrections were considered unnecessary. The potential for variation of temperature within each day to affect comparisons between sites was eliminated by alternately sampling canes from each site. In 1993, in addition to measuring RD 1 d after excision of the canes, a second measurement was made a week later. Between these measurements canes were stored at 20 °C to promote bud development. The response of respiration to these ‘ forcing ’ conditions was used as an index of bud state. Measurement of respiration rates A closed configuration gas exchange system was used (McPherson, Manson and Snelgar, 1997). The system consisted of : a small bud chamber with an orifice which could be sealed against the bud aperture of an individual bud ; a syringe pump for sampling a set volume of air which had passed through the bud chamber ; and an infrared gas analyser. Air of known CO concentration was drawn # through the bud chamber by the syringe pump and collected in a gas-tight syringe. This was injected directly into the infrared gas analyser and the peak response was recorded on a digital multimeter. The technique was found to be reliable even at rates of respiration as low as 20 pmol s−" and gave
McPherson et al.—Respiration and Dormancy of Kiwifruit repeatable measurements on individual buds (CV approx. 5 %). The ability to make non-destructive measurements was a considerable advantage because it enabled us to make repeat measurements on individual buds. Respiration measurements were made on the apical bud on all canes because this is normally the first bud to develop into a shoot and it usually produces the most flowers (McPherson, Hall and Stanley, 1992). For comparative purposes a second, more proximal bud, was also measured. This was normally the fifth bud from the apex. On the few occasions where the fifth bud was unsuitable in some way (e.g. because of damage or its proximity to a neighbouring bud) the fourth or sixth bud was measured. This position is referred to as the ‘ middle bud ’. Measurements of RD were discontinued once buds had passed a stage of development designated as ‘ bud swell ’ (Brundell, 1975). At this stage the bud was expanding rapidly and it became increasingly difficult to seal the bud chamber in place. The rates of respiration of buds at the bud swell stage were found to be at least 1200 pmol s−". This value was assumed for any buds that had passed the bud swell stage. This rate is so much higher than the rate of buds which were at the stable, winter dormant rate, that estimates made of the time of rapid increase in bud RD in spring were not sensitive to the exact value used in this assumption.
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Fresh and dry weights of buds were determined by the following procedure. The protective bark covering the bud was removed to expose the dome-shaped bud structure which is attached at its base to the cane (Ferguson, 1984). The bud was then removed by cutting its base in line with the surface of the exposed cane. Fresh weights were measured immediately, and dry weights determined after drying to constant weight at 60 °C. RESULTS AND DISCUSSION Seasonal changes in respiration Dormant bud RD was stable and in the range 20 to 110 pmol s−" from May to July in both 1992 and 1993 (Fig. 1 A and B). By the time measurements were started, RD was higher on buds from the coolest site (Riwaka) for both bud positions. There was no marked difference in RD between apical and middle buds during autumn and winter (data for middle buds not shown). Early in spring, RD increased rapidly.
June 1200
July
August
September
A 1992
Buds were dissected and observed under a light microscope (¬40 magnification) to determine their stage of development, so that this could be related to the observed changes in RD. Dissections were only undertaken in 1993. The first measurements were on 17 August (day 229), prior to any observable external changes in the buds. Measurements were made on five occasions, the last being on 28 September (day 271). Buds were dissected by removing each bract or leaf initial individually, exposing the axillary structure for examination and identification. Each axillary structure within each bud was classified as falling in one of seven identifiable stages. At stage 0 the axillary structure was a simple, undeveloped primordium. At stage 1 the primordium was swollen with a flat top and at stage 2 the two lateral flower bracts were differentiated. By stage 3 the first sepals on the ‘ king ’ flower were visible. This stage corresponds to the stage VI of Linsley-Noakes and Allan (1987). Stage 4 was reached when the lateral flower primordia were visible and the king flower was swollen. Three sepals were visible on the king flower at stage 5, and six sepals were present at stage 6. These stages were used to calculate an arbitrary score to compare the stage of bud development with the changes in RD. As a variable number of structures at different stages of development could be observed in any one bud, a weighted index was calculated. This was given for each bud, by the sum of the products of the number of structures in each stage of development by the value of each stage. Therefore, as the first stage was designated as zero, only those structures which had a visible change indicating they were undergoing reproductive development contributed to the index.
600 300 0 B 1993 900 600 300 0 C 1993
Developmental score
Bud deelopment
Respiration rate (pmol CO2 s–1)
900
20 15 10 Kerikeri Kumeu Riwaka
5 0
140
160
180
200 220 Day of the year
240
260
F. 1. Changes in the RD of dormant apical buds 1992 (A) and 1993 (B). Canes were excised from vines grown in three contrasting regions and measured at 20 °C the following day. Vertical bars indicate l.s.d. (P ¯ 0±05). C, Changes in the state of development of reproductive structures within buds. The score was weighted according to the number of buds at each stage of development. See text for details.
McPherson et al.—Respiration and Dormancy of Kiwifruit August
Kerikeri
June
September
Respiration rate (pmol CO2 s–1)
414
A 1992
Kumeu Riwaka Kerikeri
B 1993
Kumeu
1200
Kerikeri Kumeu Riwaka
August
September
1993
900
600
300
0
Riwaka Kerikeri
July
140
160
180
200 220 240 Day of the year
260
280
F. 3. RD of apical buds before and after ‘ forcing ’ at 20 °C for 1 week. Canes were excised from vines grown in three contrasting regions. Vertical bars indicate l.s.d. (P ¯ 0±05).
C 1993
Kumeu Apical Middle
Riwaka 200
210
220
230 240 250 Day of the year
260
270
F. 2. A and B, The day of the year on which bud RD reached an average of 200 pmol CO s−" on canes excised from vines growing in # three contrasting regions and measured the following day. C, The day of the year on which RD increased by 5 pmol CO s−" d−" in response # to forcing at 20 °C (1993 only). Data are shown for both the apical bud, and one more remote from the effects of apical control (bud 4, 5 or 6) which was designated as the ‘ middle ’ bud.
Canes from cooler sites showed this increase earlier than those from warmer sites in both 1992 and 1993. The RD of apical buds increased earlier than middle buds. The time of rapid increase in RD was quantified by estimating the day on which it reached 200 pmol s−". This threshold value was chosen so that it was clearly higher than the winter RD, but would detect the rise in RD during spring as early as possible. The choice was arbitrary, but selection of alternative threshold values between 100 and 300 pmol s−" would not substantially alter our analyses (Fig. 1). In 1992, apical buds from Riwaka reached this rate 29 d before apical buds from Kerikeri (Fig. 2). At all sites, middle buds reached this state 12 to 19 d later than apical buds. The changes in dormant bud RD in 1993 were similar to the pattern found in 1992. However, the time at which RD increased was between 4 and 20 d later in 1993 (Fig. 2). This pattern of a stable RD during winter with a rapid increase in spring is similar to findings for peach and apricot (Hatch and Walker, 1969) and Douglas fir shoot tips (Fielder and Owens, 1992). It contrasts though, with the pattern observed in Pyrus spp. where there is an approximately linear increase in RD during the winter (Cole et al., 1982). Response to forcing The response of buds to forcing in warm temperatures was assessed because it was possible that it would indicate a change of state of the bud earlier than the RD of buds
measured soon after being removed from field conditions. During autumn and winter, forcing buds at 20 °C for 1 week reduced RD by between 0 and 5 pmol s−" d−". By early spring, this pattern had reversed and forcing increased, rather than decreased, respiration (Fig. 3). The response to forcing became progressively greater as bud break approached, and the colder the site, the earlier the change. Rates of increase as high as 77 pmol s−" d−" were observed. The timing of this change in response was quantified by estimating the day on which the rate of change in respiration reached 5 pmol s−" d−", by linear interpolation of the data in Fig. 3. This threshold value was chosen to represent the earliest time that we could be confident the forcing response had changed from negative to positive. The choice was arbitrary, but again the conclusions were insensitive to the exact value chosen. Buds from the coolest site reached the 5 pmol s−" d−" change in RD earliest (day 207), and apical buds generally showed an earlier, or larger, response than middle buds (data not shown). This alteration in forcing response of apical buds preceded the increase in RD by 9 d at Kerikeri, which is the warmest site, and by 26 d at Riwaka, the coolest site (Fig. 2 B and C). Bud size Bud fresh weights and dry weights were determined following the 7 d forcing treatment for the canes measured on May 25 (day 145), June 16 (day 167), July 6 (day 187) and July 27 (day 208) 1993. At the subsequent sampling dates the buds were dissected to determine any changes visible by light microscopy, and it was not possible to weigh them accurately. There were few changes in bud fresh and dry weight during this period (data not shown). Walton, Clark and Boldingh (1991) found that bud dry weights in kiwifruit were stable from early August until mid-September, 10 d before bud break. Hatch and Walker (1969) also found that peach and apricot flower bud dry weight did not vary significantly during winter. In the case of pears (Zimmerman and Faust, 1969 ; Cole et al., 1982) there was a gradual increase in fresh weight during winter.
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T 1. Mean bud fresh weight, dry weight and moisture content from samples measured on four dates between May 25 (day 145) and July 27 (day 208) 1993 after buds had been forced for 7 d at 20 °C Apical buds
Site Kerikeri Kumeu Riwaka Mean l.s.d. (P ¯ 0±05)
Middle buds
Fresh weight (mg)
Dry weight (mg)
Moisture content (%)
Fresh weight (mg)
Dry weight (mg)
Moisture content (%)
50±0 53±0 45±4 49±6 6±9
31±7 33±6 28±3 31±2 4±3
37±2 36±7 37±9 37±3 2±1
55±1 55±0 50±2 53±5 6±1
35±9 35±9 31±7 34±6 4±1
34±8 34±7 37±1 35±5 2±1
The dry weight of apical buds from Riwaka was 16 % lower than those from Kumeu (Table 1). At this time of year the RD of the apical buds was 45 % higher on average (Fig. 1 A and B). The rate of respiration per unit dry weight was therefore 72 % higher in the Riwaka buds over the May to July period. As such large differences are established before the beginning of winter, this implies that autumn temperatures contribute to chilling. Phenological models have also emphasized the importance of autumn temperatures, with April temperatures alone accounting for 74 % of the observed variation in the date of bud break (Hall and McPherson, 1997). Measurements made on 12 Aug. 1992 (day 224) showed differences in the distribution of bud moisture content among sites. This was the first sampling date on which marked differences in RD among sites were becoming apparent (Fig. 1 A and B). At this time, Kerikeri bud RD were still at their wintertime rate, moisture contents averaged 38 % and the range was narrow (s.d. ¯ 3 %). The RD of Kumeu buds were about to increase rapidly. Their mean moisture content was similar to the Kerikeri buds (39 %), but the range of moisture content was greater (s.d. ¯ 5 %). The substantial increase in the RD of the Riwaka buds was accompanied by a substantial increase in both average moisture content (49 %) and range of moisture content (s.d. ¯ 11 %). Others have noticed an increase in bud moisture content during bud development in spring. Zimmerman and Faust (1969) found that the terminal buds of Pyrus calleryana (pear) had a rapid increase in fresh weight in spring but the dry weight remained constant, indicating a movement of water into the buds. The water content of the terminal buds of birch trees increased from 37 to 49 % during the first 10 d of forcing under controlled conditions (Rinne, Tuominen and Junttila, 1994). Assuming a typical fresh weight of 50 mg for dormant kiwifruit buds (Table 1), the RD was approx. 1±2 nmol s−" g−" f. wt. These are similar to values reported for peach flower buds (0±9 nmol s−" g−") and apricot flower buds (0±8 nmol s−" g−") (Hatch and Walker, 1969), but about 30 % of those reported for Pyrus calleryana (3±5 nmol s−" g−") and P. communis (3±3 nmol s−" g−") (Cole et al., 1982) and Japanese pear (3±5 nmol s−" g−") (Tamura et al., 1992). Immediately prior to the stage of bud swell, when measurements were ceased, RD in kiwifruit had increased 20-fold to approx. 24 nmol s−" g−".
Bud deelopment Microscopic examination of the axillary structures of dissected buds enabled us to score their stage of development. At the time of the first measurement in midAugust, the buds from the coolest site, Riwaka, showed readily detectable reproductive development (Fig. 1 C). There were no visible changes at this stage in material from the other two sites. By the beginning of September, reproductive structures were visible in the buds from Kumeu and Kerikeri. The rate of bud development, as measured by our score, soon matched that at Riwaka. Comparison of Fig. 1 B and C shows that the marked change in RD occurred at about the same time that the developmental score increased. The buds from Riwaka reached 200 pmol s−" on about day 230, by which time they were showing a weighted developmental score of 5. The buds from Kerikeri and Kumeu reached an RD of 200 pmol s−" by approx. day 248 and they reached a developmental score of 5 on day 250. A comparison of RD and the developmental score on a bud-to-bud basis showed a substantial correlation, though there was considerable variation (R# ¯ 0±62). Bud break and flowering in the field The mid point of bud break was earliest and the spread of bud break shortest in Riwaka (day 258, 19 d, respectively) and latest and most prolonged in Kerikeri (day 286, 36 d). This is in accord with the regional variation previously reported (McPherson et al., 1994). During spring 1993 two screen frosts occurred during bud break at Riwaka (day 251 and 272). These frosts caused considerable damage to many shoots and this promoted a second wave of bud break which resulted in a high (0±65) and an extended bud break. The late-breaking buds were clearly discernible from the cumulative bud break plots, so these data were excluded from the results presented in Table 2. The proportion of buds which developed in spring varied from a minimum of 0±38 at Kerikeri to 0±57 at Riwaka (Table 2), and these values are typical of those reported previously in these regions (McPherson et al., 1994). However, a high proportion of shoots (0±85 to 0±94) at each site bore flowers, and this led to an unusually high number of flowers per winter bud. This effect was particularly
416
McPherson et al.—Respiration and Dormancy of Kiwifruit T 2. Bud break and flowering data for kiwifruit growing in three regions of New Zealand
Year
Site
Time of 50 % bud break (day of year)
1992
Kerikeri Kumeu Riwaka Kerikeri Kumeu Riwaka
274 263 264 286 278 258
1993
Spread of bud break (d)
Proportion of bud break
No. reproductive shoots per shoot
No. flowers per reproductive shoot
No. flowers per winter bud
34 29 19 36 26 25
0±46 0±50 0±57 0±38 0±46 0±56
0±87 0±89 0±94 0±85 0±88 *
5±6 4±3 7±6 5±1 4±5 *
2±2 1±9 4±1 1±6 1±9 *
* Data not determined as frosts during bud break killed shoots and flower buds.
T 3. Regressions of bud break and flowering data against either respiration indices, or mean temperature for May, June and July (T )
DBB SpreadBB ShootWB R.shootshoot FlowerR.shoot FlowerWB
RD during May–July
R#
Timing of RD increase in spring
R#
Mean May–July temperature
R#
49®9 RD 0±260±1 RD 0±770±06 RD
n.s. 0±69 0±68 0±84 n.s. 0±84
321±0 (DA) for DA ¯ 230 to 251* ®1061±1 (DM®DA)0±51 DA 0±56®(2±1¬10−"#) e(DA/"!) 1±5®0±0025 DA®0±003 (DM–DA)
0±84 0±94 0±85 0±98 n.s. n.s.
1987 T for 9 to 12 °C† ®0±33 T 0±59®(48¬10−') e!.&T 1±05®0±016 T
0±73 0±78 0±99 0±95 n.s. 0±93
®1±31±7 RD
7±2®0±47 T
T ranged from 6±8 to 12±1 °C. Only regressions which were statistically significant (P ! 0±05) are presented. DA and DB are the days on which the RD of the apical buds and middle buds, respectively, reached 200 pmol s−". For all bud break data n ¯ 6, and for all flowering data, n ¯ 5. DBB, day of bud break ; SpreadBB, spread of bud break ; ShootWB, no. of shoots per winter bud ; R.shootshoot, no. reproductive shoots per shoot ; FlowerR.shoot, no. of flowers per reproductive shoot ; FlowerWB, no. of flowers per winter bud. * DBB ¯ 263 for DA below 230. † DBB ¯ 260 for T below 9 °C.
obvious in Kerikeri where the values of 2±2 and 1±6 flowers per winter bud are considerably higher than the mean of 0±9 observed by McPherson et al. (1994) during 1986 to 1989. However, higher numbers of flowers per winter bud have been recorded at Kerikeri more recently (Manson and Snelgar, 1995). In 1992, the flowering data for Riwaka was based on only nine canes as many shoots had been damaged by frost, and thus complete flowering records could not be obtained for these canes. In 1993 frost-damage at Riwaka was so extensive that no flowering records were collected. Bud respiration during autumn and winter as a predictor of ine performance From early May until about the end of July, there was little change in RD at any of the study sites. Buds at Riwaka had consistently higher RD throughout this period during both 1992 and 1993. We used the mean RD for this period as a predictor of the timing of bud break and the amount of bud break and flowering (Table 3). Since RD was stable through this period and could be measured in autumn, we refer to this as ‘ autumn RD ’. The timing of bud break was not significantly correlated with RD, but there was a negative correlation with the spread of bud break. The number of flowers per winter bud (FlowerWB) is the most commonly used index of flower production in kiwifruit vines. It is the product of several components : FlowerWB ¯ ShootWB¬R.ShootShoot¬FlowerR.Shoot,
where ShootWB is the number of shoots (‘ laterals ’) that develop per winter bud (i.e. ‘ bud break ’) ; R.ShootShoot is the number of reproductive shoots that develop per shoot and FlowerR.shoot is the number of flowers per reproductive shoot. The relationship between dormant RD and each of these components was investigated separately. RD was positively correlated with the proportion of buds that broke, the proportion of shoots that bore flowers, and the number of flowers per winter bud. These high correlations should be accepted with caution because of the poor distribution of data which resulted from the loss of flowering data from Riwaka. The correlation with the number of flowers per shoot was not significant (Table 3). Bud respiration during spring as a predictor of ine performance The time at which the RD began to increase rapidly in spring was quantified by estimating the day on which RD reached 200 pmol s−" (DA for apical buds and DM for middle buds) by linear interpolation between the data shown in Fig. 1 A and B. DA was positively correlated with the time of bud break over the range DA ¯ 230 to 251 (Table 3). When DA was earlier than day 230, then bud break occurred at the earliest date recorded, approx. day 263. To predict the spread of bud break we regressed it against both DA and DM–DA (Table 3). We used the
McPherson et al.—Respiration and Dormancy of Kiwifruit latter parameter to provide a measure of the variation in development within each cane. A late rise in respiration led to a greater spread of bud break. The greater the difference between the timing of the apical and middle buds, the greater was the spread of bud break. The proportion of buds that broke was highest when DA was small i.e. when bud break was early. The relationship was exponential, with the upper asymptote for bud break being 0±56. Small changes in DA late in the season were associated with large changes in bud break (Table 3). Thus if buds commenced development early in the season (as indicated by a small DA) bud break was high. The proportion of shoots that bore flowers decreased as DA increased, and also as DM–DA increased. That is, late bud break, and a large variation in the time of bud break within the cane, was associated with a low proportion of shoots bearing flowers. This confirms other reports that late bud break is associated with poor flowering (Grant and Ryugo, 1982 ; Snelgar, 1988 ; McPherson et al., 1992), and factors which increase the spread of bud break decrease flowering (Grant and Ryugo, 1982 ; Linsley-Noakes and Allan, 1987). These reductions are likely to be due to the between-bud interactions that occur within a cane during bud break which can lead to low flower numbers (Grant and Ryugo, 1982 ; Snowball and Considine, 1986 ; McPherson et al., 1992). The number of flowers on each shoot was not significantly correlated with either the time of development of the apical bud, or the spread of development along the cane. Consequently, it is not surprising that the product of the three components of flowering, the number of flowers per winter bud, was not significantly correlated with the time at which the apical bud breaks. Temperature during autumn and winter as a predictor of ine performance To provide a comparison of the use of RD as an index of vine response to chilling, the bud break and flowering parameters were regressed against the mean monthly air temperature during the period from May to July (Table 4). Since this mean can be established only by the end of July, we refer to the mean as ‘ winter temperature ’. This period has been used by Salinger et al. (1993) to model the end of winter dormancy. Although bud break and flower production in kiwifruit are believed to be enhanced by cool temperatures during winter, it is likely that cool spring temperatures can impair bud break and flowering T 4. Mean monthly air temperatures (°C ) for Kerikeri, Kumeu and Riwaka meteorological sites for 1992 and 1993
1992
Kerikeri Kumeu Riwaka 1993 Kerikeri Kumeu Riwaka
May
June
July
12±3 11±4 8±0 13±6 12±4 10±0
10±1 9±6 5±5 12±3 11±7 8±6
11±6 10±8 6±8 10±5 9±2 6±5
417
(Warrington and Stanley, 1986 ; McPherson et al., 1988 ; McPherson, Warrington and Stanley, 1989). It is important, therefore, that the period selected to define winter chilling does not include early spring. More complex indexes, which are derived from winter temperatures, have been used in other studies of winter chilling in fruit trees (Richardson, Seeley and Walker, 1974). However, since this chill-unit produces inconsistent predictions for kiwifruit (McPherson et al., 1995), we chose to use mean temperature as a measure of winter chilling. Warm temperatures during winter resulted in bud break being delayed and spread over a longer period. The midpoint of bud break was delayed by 7 d for each 1 °C increase in mean winter temperature above 9 °C (Table 3). Temperatures below 9 °C did not affect the timing of bud break. Both these results are in agreement with responses found with excised canes (Snelgar et al., 1997). The spread of bud break increased by 3 d for each 1 °C increase in mean winter temperature. This shows the same sensitivity to temperature as previous reports have shown for flowering (McPherson et al., 1988). The proportion of over-wintered buds which developed into shoots (ShootWB or bud break) decreased exponentially as the mean winter temperature increased from 7 to 12 °C (R# ¯ 0±99). The changes in bud break were small between 7 and 10 °C, declining from 0±57 to 0±52, but they were much larger above 10 °C, being reduced to 0±40 at 12 °C. The proportion of shoots which produced flowers (R.shootshoot) decreased linearly as winter temperature increased, but the rate of change was only 0±016 per °C (R# ¯ 0±95). The proportion of shoots which bore flowers was reduced from 0±94 to 0±85 over the range of 5±4 °C recorded in the present study. The number of flowers on each flowering shoot (FlowerR.Shoot) was not significantly correlated with mean winter temperature. The product of all three flowering components, the number of flowers per winter bud (FlowerWB), decreased linearly as mean winter temperature increased. Although this correlation is statistically highly significant (R# ¯ 0±93) the data set was poorly distributed. This finding should be viewed with caution unless it can be verified against other data. CONCLUSIONS Buds collected from different regions showed differences in RD during autumn. Our measurements did not start early enough to determine when these differences were established, but they were apparent as early as the beginning of May. This was approx. 17 weeks prior to bud break. The autumn RD was useful for predicting the proportion of floral shoots and number of flowers per winter bud, and to a lesser extent, the amount and spread of bud break. This finding corroborates recent suggestions (Hall and McPherson, 1997 ; Snelgar et al., 1997) the autumn temperatures need to be included in ‘ winter ’ chilling models. The rates of RD did not change appreciably during the period from May to July. This may have been because there were no major changes in the state of the buds, or because any changes that occurred did not affect respiration rates.
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McPherson et al.—Respiration and Dormancy of Kiwifruit
The changes in RD during early spring could be used to predict the timing and spread of bud break, and the amount of bud break, but not flower numbers. These changes occurred at about the same time that the formation of reproductive structures could be detected by dissection. Increases in RD were accompanied by an increase in the moisture content of the bud, so this is an alternative method of detecting changes in bud state though, like dissection, it is destructive. All of these changes occurred only 3 to 6 weeks before the mid point of bud break. Mean winter temperatures were available 6 to 9 weeks before the mid point of bud break and were more effective, in general, than either spring or autumn RD for predicting vine performance. However, the autumn RD can be obtained at least 8 weeks before mean winter temperature data and this may enable earlier forecasts of vine performance, even if those forecasts are less reliable. A C K N O W L E D G E M E N TS We wish to thank Roger Hamilton and Greg Lupton for assessing vine performance, and Sally Roughan and Deborah Hutching for their assistance with the dormant bud respiration measurements. This work has benefitted from many useful discussions with Alistair Hall on chilling. LITERATURE CITED Brundell DJ. 1975. Flower development of the Chinese Gooseberry (Actinidia chinensis Planch.) I. Development of the flowering shoot. New Zealand Journal of Botany 13 : 473–483. Cole ME, Solomos T, Faust M. 1982. Growth and respiration of dormant flower buds of Pyrus communis and Pyrus alleryana. Journal of the American Society of Horticultural Science 107 : 226–231. Davison RM. 1990. The physiology of the kiwifruit vine. In : Warrington IJ, Weston GC, eds. Kiwifruit science and management. Auckland : Ray Richards, 127–154. Ferguson AR. 1984. Kiwifruit : A botanical review. In : Janick J, ed. Horticultural reiews. Westport : AVI Publishing Co 6 : 1–64. Fielder P, Owens JN. 1992. Shoot-tip respiration of 1st-year interior and coastal Douglas-fir seedlings during bud development. Canadian Journal of Forest Research 22 : 765–768. Grant JA, Ryugo K. 1982. Influence of developing shoots on flowering potential of dormant buds of Actinidia chinensis. HortScience 17 : 977–978. Hall AJ, McPherson, HG. 1997. Modelling the influence of temperature on the timing of bud break in kiwifruit. In : Proceedings of the Third International Symposium on Kiwifruit, Thessaloniki, Greece, September 1995. Acta Horticulturae 444 (in press). Hatch AH, Walker DR. 1969. Rest intensity of dormant peach and apricot leaf buds as influenced by temperature, cold hardiness, and respiration. Journal of the American Society of Horticultural Science 94 : 304–307. Linsley-Noakes GC, Allan P. 1987. Effects of winter temperatures on flower development in two clones of kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson). Scientia Horticulturae 33 : 249–260.
McPherson HG, Hall AJ, Stanley CJ. 1992. The influence of current temperature on the time from bud break to flowering in kiwifruit Actinidia deliciosa. Journal of Horticultural Science 67 : 509–519. McPherson HG, Hall AJ, Stanley CJ. 1994. Seasonal and regional variation in bud break and flowering of kiwifruit vines (Actinidia deliciosa) in New Zealand. New Zealand Journal of Crop and Horticultural Science 22 : 263–276. McPherson HG, Manson PJ, Snelgar WP. 1997. Non-destructive measurement of dormant bud respiration rates. Photosynthetica 33 : 125–138. McPherson HG, Stanley CJ, Warrington IJ. 1995. The response of bud break and flowering to cool winter temperatures in kiwifruit (Actinidia deliciosa). Journal of Horticultural Science 70 : 737–747. McPherson H, Stanley J, Warrington I, Jansson D. 1988. Dynamics of bud break and flowering. New Zealand Kiwifruit Special Publication No. 2 : 9–11. McPherson H, Warrington I, Stanley J. 1989. Weathering the weather. New Zealand Kiwifruit April : 14–15. Manson PJ, Snelgar WP. 1995. Regional variations in the response of kiwifruit vines to time of cane tipping. New Zealand Journal of Crop and Horticultural Science 23 : 67–71. Richardson EA, Seeley SD, Walker DR. 1974. A model for estimating the completion of rest for Redhaven and Elberta peach trees. HortScience 9 : 331–332. Rinne P, Tuominen H, Junttila O. 1994. Seasonal changes in bud dormancy in relation to bud morphology, water and starch content, and abscisic acid concentration in adult trees of Betula pubescens. Tree Physiology 14 : 549–561. Salinger MJ, Kenny GJ, Morley-Bunker MJ. 1993. Climate and kiwifruit cf. Hayward 1. Influences on development and growth. New Zealand Journal of Crop and Horticultural Science 21 : 235–245. Saure MC. 1985. Dormancy release in deciduous fruit trees. Horticultural Reiews 7 : 239–300. Snelgar WP. 1988. The effect of cane orientation on flower production in kiwifruit (Actinidia deliciosa). Journal of Horticultural Science 63 : 341–347. Snelgar WP, Manson PJ, McPherson HG. 1997. Evaluating winter chilling of kiwifruit using excised canes. Journal of Horticultural Science 72 : 305–315. Snowball AM, Considine JA. 1986. Flower development and shoot growth. New Zealand Kiwifruit July : 20. Snowball AM, Walton EF. 1992. Flowering in kiwifruit. New Zealand Kiwifruit Special Publication No. 4 : 25–28. Stanley CJ, McPherson HG, Plummer JA. 1995. The use of unrooted cuttings for studying the effects of chilling in kiwifruit (Actinidia deliciosa). Journal of Horticultural Science 70 : 749–756. Tamura F, Tanabe K, Banno K. 1992. Effect of chilling treatment on intensity of bud dormancy, respiration and endogenous growth regulators in Japanese Pear ‘ Nijisseiki ’. Journal of the Japanese Society of Horticultural Science 60 : 763–769. Walton EF, Clark CJ, Boldingh HL. 1991. Effect of hydrogen cyanamide on amino acid profiles in kiwifruit buds during budbreak. Plant Physiology 97 : 1256–1259. Warrington IJ, Stanley CJ. 1986. The influence of pre- and postbudbreak temperatures on flowering in kiwifruit. Acta Horticulturae 175 : 103–107. Young E, Motomura Y, Unrath CR. 1987. Influence of root temperature during dormancy on respiration, carbohydrates, and growth resumption in apple and peach. Journal of the American Society of Horticultural Science 112 : 514–519. Zimmerman RH, Faust M. 1969. Pear bud metabolism : seasonal changes in glucose utilization. Plant Physiology 44 : 1273–1276.