Effects of photoperiod and temperature on growth and flowering in the annual (primocane) fruiting raspberry (Rubus idaeus L.

Transcription

1 Journal of Horticultural Science & iotechnology (29) 84 (4) Effects of photoperiod and temperature on growth and flowering in the annual (primocane) fruiting raspberry (Rubus idaeus L.) cultivar Polka y A. SØNSTEY 1 * and O. M. HEIDE 2 1 Arable Crops Division, Norwegian Institute for Agriculture and Environmental Research, NO-2849 Kapp, Norway 2 Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. ox 3, NO-1432 Ås, Norway ( anita.sonsteby@bioforsk.no) (Accepted 22 March 29) SUMMARY Growth and flowering of the annual-fruiting raspberry (Rubus idaeus L.) cultivar Polka were studied under controlled environment conditions in order to facilitate out-of-season production. Vegetatively-propagated plants originating from adventitious root buds were used. Height growth and the rate of leaf formation increased with increasing temperature, up to a broad optimum in the mid-2 C range. While elongation was consistently enhanced by long-day () conditions, photoperiod had no effect on the rate of leaf formation. stimulation of growth thus resulted from increased internode length only. In agreement with earlier reports, it was found that, in contrast to biennial-fruiting cultivars, such annual-fruiting cultivars do not need low temperatures for flower initiation, nor do they appear to have a juvenile phase during which they are un-responsive to flower-inducing conditions. Polka plants responded to inductive conditions as early as the -leaf stage, and flowered freely across the entire range of growth temperatures, even at 3 C. Flowering was advanced and the number of flowers increased with increasing temperature, up to an optimum at 27 C. Flowering was also consistently advanced and occurred at lower nodes under than under short-day () conditions across the whole range of temperatures. Night interruption for 3 h in the middle of the night was also effective, demonstrating that this is a true photoperiodic response and not merely an effect of increased light integral in. It was also confirmed that a distinct vernalisation-type advancement of flowering took place when small, non-dormant plants were exposed to additional chilling at 6 C for several weeks. At low temperatures, a large proportion of the lateral buds were dormant, so that, at 12 C, the plants actually flowered only at their tips. Dissections also revealed that the dormant buds had initiated flowers; but, because of their dormant state, they needed several weeks of chilling before they could flower (biennial-fruiting behaviour). oth types of buds were initiated by the same environmental conditions. Practical applications of the findings are suggested. Red raspberry (Rubus idaeus L.) is a temperate species with short-lived woody shoots borne on a long-lived perennial root system. Two groups of cultivars with different life cycles are commonly recognised. In the common biennial-fruiting cultivars, the shoots (canes) have a 2-year life cycle during which they pass through a sequence of seasonal phases involving vegetative growth, flower formation, and fruiting, as well as induction and breaking of Winter bud dormancy (Hudson, 199; Williams, 199a, b; 196; Sønsteby and Heide, 28). In the so-called primocane-fruiting cultivars, on the other hand, the entire cycle of vegetative growth, flowering, and fruiting is normally completed in a single growing season (Keep, 1988). In addition, a third, intermediate, so-called tip-flowering type is also often recognised (Carew et al., 2; 23). Such cultivars usually produce a few flowers and fruits at the tip of the cane at the end of the first growing season, while the rest of the buds will flower and fruit in the second year. The physiological and horticultural aspects of life-cycle control in these types of plants were reviewed by Carew et al. (2). *Author for correspondence. Annual-fruiting cultivars have variously been referred to as Autumn-fruiting, everbearing, Fall-bearing, primocane-fruiting, or tip-fruiting cultivars (Keep, 1988); whereas biennial-fruiting cultivars, sometimes also termed Summer-cropping cultivars, are often now referred to as floricane-fruiting cultivars (Carew et al., 2; Oliveira et al., 21; Dale et al., 2). Since none of these terms are self-explanatory, and since annual- and biennial-fruiting are the terms that best and adequately describe the essential difference, namely a 1-year or 2-year shoot life-cycle, we prefer to use these terms and suggest that they should be generally adopted. Environmental control of growth and flowering has been studied extensively in both biennial- and annualfruiting raspberry cultivars (for reviews, see Keep, 1988; Carew et al., 2). In biennial-fruiting cultivars flower buds are initiated in short days () if the temperature is below C (Williams, 199a, b; 196; Sønsteby and Heide, 28). However, if the temperature is below 12 C, buds are also initiated under long-day () conditions, whereas flowering can be prevented indefinitely at 18 C and higher temperatures, regardless of daylength conditions (Williams, 196; Sønsteby and Heide, 28). It is important that, in these cultivars, floral initiation is

2 44 Growth and flowering in annual-fruiting raspberry accompanied by a cessation of growth and bud dormancy (Sønsteby and Heide, 28). Therefore, since release from this dormancy requires exposure to low temperatures (Winter chill) over several weeks (Williams, 199a; Sønsteby and Heide, 28), this confers a 2-year life cycle upon the shoots of these cultivars. Accordingly, Sønsteby and Heide (28) concluded that a different linkage between flower initiation and bud dormancy is the crucial feature responsible for the different life-cycles in raspberry. Furthermore, in biennial-fruiting cultivars, shoots originating from adventitious buds formed on the roots of mature plants are juvenile and must develop 2 nodes (leaves) before they are able to respond to low temperatures and with flower formation (Williams, 199b; Sønsteby and Heide, 28). This implies that rejuvenation takes place during the process of adventitious bud formation (Sønsteby and Heide, 28). Shoots arising from such adventitious buds are therefore useful experimental material for studies on the control of flowering. In annual-fruiting cultivars, the environmental control of growth and flowering is less well understood, although progress has been made in recent years (Carew et al., 21; 23; Dale, 28). As in biennialfruiting cultivars, several weeks of chilling are required to break dormancy in the adventitious buds formed on the perennial root system (Keep, 1988; Carew et al., 2). Furthermore, in annual-fruiting cultivars, there is an additional vernalisation effect of low temperatures (chilling) on flowering, which is distinct from that on the release of dormancy (Carew et al., 21). Although Vasilakakis et al. (198) and Takeda (1993) reported that chilling was not a requirement for flowering in annual-fruiting Heritage raspberry; flowering was erratic and took place only after 24 d and the production of 8 9 nodes. Also, after some weeks of chilling, flowering was greatly advanced and the number of nodes was reduced to approx. 3. On the other hand, Dale et al. (2) reported that dormancy in non-chilled shoots was gradually broken as the photoperiod increased during the season, and that annual-fruiting cultivars flowered and fruited on their primocanes for three consecutive years in the absence of chilling (i. e., at temperatures > 16 C). Post-chilling growth temperature also has a marked effect on growth and flowering of annual-fruiting cultivars. Lockshin and Elfving (1981) grew Heritage plants at day/night temperatures of 29 /24 C and 2 /2 C in a 16 h photoperiod and found that plants at the higher temperature flowered approx. 2 weeks earlier and produced more flowers and flowering nodes than those at the lower temperature conditions. Carew et al. (1999) grew plants of Autumn liss at a range of semicontrolled temperature conditions (averages of approx C) with a 16 h photoperiod, and found that the rate of node production increased with temperature, to an optimum at 22 C, and declined thereafter. The rate of progress to flowering and fruiting increased similarly up to approx. 22 C and also declined at higher temperatures. However, the final numbers of leaves produced before the terminal flower were similar (approx. 3) at temperatures of approx. 19., 22., or 23.4 C, but significantly higher at lower temperatures ( C and 17 C). In a later paper, Carew et al. (23) found that both rate of vegetative growth and progress to flowering increased with temperature, with a relatively broad optimum in the low-to-mid 2 C range. The effect of photoperiod on flowering of annualfruiting raspberry cultivars, and its interaction with temperature, have not been researched extensively, and various statements on the effect of photoperiod can be found in the literature. The only true photoperiodic study was reported by Carew et al. (23). Under semicontrolled temperature conditions (approx. º/2ºC day/night temperatures, 8 h day/16 h night) they found that, while photoperiod had no significant effect on vegetative growth, flowering was somewhat advanced by intermediate photoperiods (11 h and 14 h) compared to 8 h and 17 h photoperiods. Quantitative effects on flowering (e.g., the number of flowers or flowering nodes) were not reported. Considering that buds at different positions along the length of a cane usually behave differently and may have different life-cycles, Oliveira and Dale (27) and Dale (28) speculated that these buds might also have different photoperiodic requirements for flower initiation. According to these workers, the upper nodes or so may initiate flower buds at a certain stage of maturity, independent of photoperiodic conditions, and flower in the first season, while buds further down the shoot may require for initiation, as in biennial-fruiting cultivars (cf. Williams, 196; Sønsteby and Heide, 28). However, since the distinction between annual- and biennial-fruiting cultivars is not absolute, but represents a continuum from true biennial-fruiting cultivars, through tip-fruiting, to annual-fruiting cultivars (Carew et al., 2; 23), such a mechanism seems unlikely and warrants further scrutiny. Given this background, we decided to study growth and flowering in a typical annual-fruiting raspberry cultivar under fully-controlled environmental conditions. Special emphasis was placed on the effects of photoperiod and temperature on growth and flowering, and their modifying effect on lateral development (i.e., plant architecture) and on the strength of the annual-fruiting tendency along the length of the shoot. MATERIALS AND METHODS Plant material and cultivation The raspberry (Rubus idaeus L.) cultivar Polka, derived from Autumn liss (Danek, 22), was used for the experiments which were conducted in the Ås phytotron, as described by Sønsteby and Heide (28). For the purpose of propagation, mature plants grown in 3. l plastic pots were cut at soil level after fruiting and the pots were exposed to chilling at 2 C in the dark for 6 weeks. The root systems were then separated from the soil and left to sprout in trays with moist sphagnum peat at 21 C. Emerging shoots with new roots were cut at the base, potted in 12 cm plastic pots, and raised at 21 C in 1 h conditions for 2 weeks, at which time the plants had an average of. ±.2 leaves and the experimental treatments were started.at a height of approx. 3 cm, the plants were transplanted into 3. l plastic pots where they remained until the experiments were terminated. At all stages, plants were grown in a coarse-textured sphagnum peat fertilised with 3 g 8 l 1 of Osmocote

3 A. SØNSTEY and O. M. HEIDE 441 controlled-release fertiliser (14% N, 4.2% P, 11.6% K plus micronutrients; release rate 3 4 months) from Scotts UK Ltd., Nottingham, UK. All plants were watered daily with tap water as required. Throughout the plant raising and experimental periods, all plants were grown in daylight phytotron compartments in natural daylight from h and were then moved into adjacent growth rooms with darkness, or with low-intensity light (6 7 µmol quanta m 2 s 1 provided by 7 W incandescent lamps) for daylength manipulations. Thus the plants received almost the same daily light integral in both daylengths (only 2 3% more radiation in ). Whenever the photosynthetic photon flux density (PPFD) in the daylight compartments fell below µmol quanta m 2 s 1, as on cloudy days, an additional 12 µmol quanta m 2 s 1 were automatically added using Philips HPT-I 4 W lamps. Temperatures were controlled to ± 1 C, and a water vapour pressure deficit of 3 Pa was maintained at all temperatures. Three experiments were conducted. Experiment 1 examined the effect of three constant temperatures (12 C, 18 C, or 24 C) and two photoperiods of 1 h () or 24 h (). In addition a fourth group of plants was grown at 6 C in or for 7 weeks, then transferred to 24 C, the daylengths remaining unchanged. The temperature and daylength treatments were started on 22 January 28. Experiment 2 was similar in structure, except that temperatures of 21 C, 27 C, and 3 C were used, and the treatments were started on 29 April 28. Experiment 3 compared the effect of a 3 h night interruption with that of 1-h and 24-h as described above, at a temperature of 21 C. The night interruption treatment consisted of 3 h with low intensity light (6 7 µmol quanta m 2 s 1 ) given in the middle of the 14 h night with 7 W incandescent lamps. Experimental design, data observation, and analysis All three Experiments were fully factorial, with a splitplot design, with temperature as the main plots, and photoperiod as sub-plots. All Experiments were replicated with three randomised blocks, each consisting of four plants on a separate trolley (i.e., a total of 12 plants per treatment). Growth was monitored by weekly measurements of plant height and counting of leaf (node) numbers. Time of flowering, expressed as the first (terminal) anthesis, was recorded daily. One week after terminal anthesis, each plant was terminated and the developmental state, length, and number of flowers (flowers + buds) of each lateral bud or shoot were recorded. Experimental data were subjected to analysis of variance (ANOVA) by standard procedures using a MiniTab Statistical Software programme package (Release 14; Minitab, Inc., State College, PA, USA). RESULTS Effects of temperature and photoperiod Shoot growth and the production of new nodes exhibited a sigmoid time-course and both were highly temperature-dependent (Figure 1A, ). Long days also consistently increased the rate of shoot growth at all temperatures, but had no effect on the rate of leaf initiation. Thus, stimulation of growth resulted from Plant height (cm) No. of leaves : A : 6º24ºC 12ºC 18ºC 21ºC 24ºC 27ºC 3ºC 6º24ºC 12ºC 18ºC 21ºC 24ºC 27ºC 3ºC Time (weeks) Time (weeks) FIG.1 Effects of temperature and photoperiod on the time-course of shoot elongation (Panel A) and on leaf formation (Panel ) in Polka raspberry plants. Pooled results of Experiments 1 and 2. Values are the means of three replicates, each with four plants. increased internode length only (cf. Figure 2). While growth increased with temperature up to 24 C in Experiment 1 (the highest temperature tested), an optimum was reached at 27 C in Experiment 2. Plants which were initially kept at 6 C for 7 weeks grew very little during this period; but, after transfer to 24 C, they resumed the same growth rate as in plants maintained at 24 C from the start. Under conditions, the shoots developed an arcing (parageotropic) growth habit (Figure 2A). Combining the data from Experiments 1 and 2, an ANOVA of the shoot height and node number data at week 7, at the end of the main growth period, and before flowering, revealed a highly significant (P <.1) main effect of temperature on both growth and node development, while photoperiod had a highly significant effect only on shoot growth (Table I). Higher PPFD during mid-summer in Experiment 2 increased both growth and node development significantly (P =.1) compared with Experiment 1, without changing the general trend in the temperature and daylength responses (Figure 1A, ). While the rate of leaf initiation was unaffected by photoperiod during vegetative growth, flowering was delayed under conditions and, hence, the period of growth and leaf initiation was generally prolonged under conditions (Figure 1). Since flowering terminates vegetative growth and the production of new leaves (Figure 2D), node number at anthesis (i.e., the A

4 442 Growth and flowering in annual-fruiting raspberry A D 39 cm 6 C 39 cm 6 C 12 C D 12 C C 18 C 24 C 18 C 24 C FIG.2 Appearance of Polka raspberry plants after 7 weeks of cultivation under (Panel A) or conditions (Panel ) and a range of temperatures, as indicated. Inserted are close-ups of the shoot tips of plants grown at 24 C under (Panel C) and (Panel D) conditions. Note the termination of growth by a terminal flower (Panel D). number of nodes subtending the terminal flower) was a reliable physiological indicator of earliness of flower initiation. The results revealed that flowering consistently took place at lower nodes in than in at all temperatures, except for the 6 C 24 C shift treatment (Figure 3A, ). Likewise, days to anthesis were also significantly (P <.1) reduced in compared with, at all temperatures except the 6ºC 24 C treatment (Figure 3C). Anthesis was greatly advanced as temperature increased from 12 C to 21 C, with no further advancement at higher temperatures (Figure 3C). The number of flowers per plant was significantly (P <.1) higher in than in under all temperature conditions (Figure 4A). Flowering also increased highly significantly (P <.1) with increasing temperature, reaching an optimum at 27 C and declining Height at anthesis (cm) Final number of nodes Days to anthesis Heigth at antesis No. of nodes days to antesis Expt. 1: Expt. 2: FIG.3 Effects of temperature and daylength on plant height at anthesis (Panel A), the final number of nodes (Panel ), and the number of days to anthesis (Panel C) in Polka raspberry plants. Columns to the left in each Panel represent the results of plants grown at 6 C for 7 weeks, then transferred to 24 C in Experiment 1. Values are the means (± SE) of three replicates, each with four plants. again at 3 C. Higher light fluxes during mid-summer in Experiment 2 also increased flowering markedly without changing the general response trends. Reduced flowering at low temperature was mainly an effect of a reduction in the number of lateral buds that developed A C TALE I Probability levels of significance for main effects and interactions of temperature and photoperiod on flowering and growth variables in Polka raspberry Source of variation Plant height No. of leaves Days to anthesis Flowers per plant Dormant buds Total length of laterals Temperature (A) <.1 <.1 <.1 <.1 <.1 <.1 Photoperiod () <.1 n.s. <.1 <.1 <.1 <.1 A <.1 n.s. <.1.4 <.1 <.1 Data in columns 1 and 2 refer to results after 7 weeks of cultivation, whereas the other data are final results from Experiments 1 and 2.

5 A. SØNSTEY and O. M. HEIDE 443 TALE II Effects of temperature and photoperiod on plant architecture and flowering of Polka raspberry plants (Experiments 1 and 2)* Total no. No. of No. of Flowering Mean no. Lateral Temperature ( C) Photoperiod (h) of nodes flowering laterals dormant buds laterals (%) flowers per lateral length (cm) j 16.8 c 8.4 h 67.4 ab 12.1 bc 14.7 de fgh 7.7 e 22.1 a 23.1 h 3. i 3.9 g ij 8.9 e 23. a 27.9 h 7.7 fg 1.4 f efg 16.3 c 18. c 47.4 g 7.9 fg 11.9 ef fgh 23. a 1. g 69.1 ab 8.1 fg 16.3 cd b 21.8 ab.8 d 7.8 de 13. ab 26.7 a a 22.1 ab 19.3 c 3.4 efg 9.3 ef 2.9 b Mean hi. c 12.3 fg 6.2 def 14.2 a 18. bc gh 7.8 e 21.2 ab 26.9 h.4 h 6.4 g ij 12.9 d 12.4 fg.7 fg 7.7 g.1 de efg 2.4 b 1.8 g 6.1 bc 1. de 21.3 b fgh 21.6 ab 8. h 73. a 1. de 16.6 cd bc 23.1 a 13.1 ef 63.8 bc 13.9 a 27. a b 23.3 a 14.9 de 61. cd 11.3 cd 2.9 a Mean *All data are means of three replicates, each represented by four plants. Mean values within each column followed by different lower-case letters are significantly different (P <.) by Tukey s test. into flower-bearing shoots, resulting in typical tip flowering. (Table II; Figure 4). The number of dormant buds at the lower part of the shoot was particularly high at 12 C and at 18 C in, whereas generally resulted in high numbers of growth-active and flowering buds with a broad temperature optimum in the C range. In both daylengths, the number of dormant buds increased again when the temperature was raised to 3 C (Figure 4). An illustration of the impact of No. of flowers No. of flowers Expt. 1: Expt. 2: A temperature and photoperiod on plant architecture and total flowering is presented in Figure. There was a gradual delay in the earliness of flowering of the laterals from the top towards the base of the shoot, and this was accompanied by a basipetal increase in growth and in the final length of the laterals (Figure ). These lower laterals produced large numbers of flowers that contributed greatly to the total number of flowers. Dissection of a number of non-growing (dormant) lower lateral buds revealed that they all had initiated flower buds which were at an advanced stage at the time that the terminal buds were flowering. These buds remained dormant for several months at 21 C, and required 4 6 weeks of chilling at 2 4 C for growth activation and flowering (data not shown). Low temperature treatment for 7 weeks at an early stage of development reduced plant height at anthesis, lowered the number of nodes before flowering, and reduced the number of days to anthesis at the subsequent high temperature (24 C) compared with continuous 24 C (Figure 3). However, compared to a constant 24 C, the number of flowers per plant was not significantly increased by such early low-temperature 2 2 No. of dormant buds No of dormant buds FIG.4 Effects of temperature and daylength on the total number of flowers per plant (Panel A), and the number of dormant buds per plant (Panel ) in Polka raspberry plants. Columns to the left in each Panel represent the results of plants grown at 6 C for 7 weeks, then transferred to 24 C in Experiment 1.Values are the means (± SE) of three replicates, each with four plants. FIG. Illustration of plant architecture (i.e., plant height, total number of nodes, the number and length of laterals, and the number of dormant buds) of Polka raspberry plants grown under different temperature and daylength conditions, as indicated on the x-axis. For simplification, all laterals are drawn on only one side of the stem. Lateral lengths are drawn on the same scale as plant height (cm). Numbers denote the total number of flowers per plant in the respective treatments. Results are from Experiment 1.

6 444 Growth and flowering in annual-fruiting raspberry TALE III Effects of photoperiod (,, or with a 3 h night interruption) at 21ºC on growth and flowering in Polka raspberry plants* Photoperiod Plant Total number Flowers Days to No. of flowering No. of Flowering Flowers per Lateral (h) height (cm) of nodes per plant anthesis laterals dormant buds laterals (%) lateral length (%) a 34.3 a 12.8 b 64 a 16.3 b 18. a 47.4 b 7.9 a 11.9 b 24 a 31.3 b 21.7 a 3 c 2.4 ab 1.8 b 6.1 a 1. a 21.3 a a 32.4 ab a 6 b 21.3 a 12.8 b 62. a 8.3 a 19.9 a P value n.s..2.1 < n.s..4 *All data are means of three replicates, each represented by four plants. Mean values within each column followed by different lower-case letters are significantly different (P <.) by Tukey s test. treatment (Figure 4A). Whereas continuous cultivation at 12 C resulted in a high proportion of dormant lateral buds, with a corresponding reduction in the number of flowering laterals, this was not the case when plants were first grown at 6 C for 7 weeks, and then transferred to a higher temperature (Figure 4; Table II). Effects of night interruption Night interruption with low-intensity light for 3 h during the middle of the night, significantly promoted flowering compared with 1-h, but was less effective than 24-h (Table III). For most flowering parameters the response to night interruption was intermediate between those of and. The number of flowers per plant was higher, and days to first anthesis was significantly lower with night interruption than in, while the number of nodes subtending the terminal flower was barely significantly different (P =.2) when compared to and. The number of dormant buds was significantly lower in the night interruption than in the treatment, and not significantly different from the treatment. The number and percentage of flowering laterals was similarly and significantly increased by both the and night interruption treatments. Also, and night interruption caused a significant increase in the length of the lateral shoots compared with conditions (Table III). DISCUSSION The results showed that, unlike biennial-fruiting raspberries, the annual-fruiting Polka had no need for low temperatures for flower formation. Thus, Polka flowered freely at temperatures as high as 3 C (Figure 3; Figure 4). However, the number of nodes subtending the terminal flower increased and the total number of flowers decreased markedly when the temperature was increased from 27 C to 3 C (Figure 3; Figure 4A), suggesting an upper temperature limit for flowering also in annual-fruiting cultivars. Also, days to anthesis decreased with increasing temperature up to 21 C, then levelled-off at higher temperatures (Figure 3C). These results are in general agreement with those of Carew et al. (23) using the related cultivar Autumn liss which, likewise, flowered freely at temperatures up to approx. 27 C with a broad temperature optimum in the mid-2 C range. The results also confirm the findings of Carew et al. (21) that flowering in annual-fruiting raspberry was advanced by additional chilling (vernalisation) at 6 7 C for 6 8 weeks, even in plants raised from roots that had previously been chilled to break bud dormancy and initiate vegetative growth. These results show that, although annual-fruiting raspberries are promoted in their flowering by high growth temperatures, they also exhibit a distinct vernalisation-type promotion of flowering at low temperatures. The fact that small plants with only four-to-five leaves did respond to low temperature vernalisation (Carew et al., 21; Figure 3) indicates that annual-fruiting raspberries have no juvenile phase during which their flowering is unresponsive to environmental factors. Furthermore, the present results demonstrate, for the first time, a consistent and significant promotion of flowering by in annual-fruiting raspberry (Figure 3; Figure 4; cf. Carew et al., 23). Significant promotion of flowering by night interruption (Table III) also proves that the effect is a true photoperiodic response and not merely an effect of increased daily light integral in the treatments. All these results demonstrate a remarkably different and, in fact, contrasting environmental control of flowering in biennial-fruiting and annual-fruiting raspberry cultivars. Thus, while the former have an obligatory need for low temperature and/or conditions for the initiation of flower primordia (Williams, 196; Sønsteby and Heide, 28), the annualfruiting cultivars flower freely across the entire range of temperatures and with a marked enhancement of flowering by conditions (Figure 3C; Figure 4A). Furthermore, while biennial-fruiting cultivars have a distinct juvenile phase and do not respond to flowerinducing conditions before they have formed 2 leaves (Williams, 196; Sønsteby and Heide, 28), such a juvenile phase is absent in annual-fruiting cultivars which respond to flower-inducing conditions at the -leaf stage (Carew et al., 21; Figure 3; Table III). It was suggested (Haltwick and Struckmeyer, 196) that the main physiological difference between annualand biennial-fruiting cultivars was that the biennialfruiting types had shorter photoperiod and lower temperature requirements for flower initiation (cf. Carew et al., 2). However, although such a difference is definitely present, the crucial point is whether the shoots have an annual or a biennial life cycle, a matter that is determined by the dormancy control system of the plant. Since it was found that floral initiation was accompanied by dormancy induction in biennial-fruiting cultivars, Sønsteby and Heide (28) concluded that a different linkage between flowering and dormancy induction was the main feature responsible for the different shoot lifecycles in the two groups of raspberry. The present results fully support and verify this conclusion. However, the results in Figure and Table III also demonstrate that the degree of bud dormancy, and hence the degree of tip flowering (see Carew et al., 2), is not only a matter of the genetic constitution of the plant, but is also under environmental control. Thus, both the number and proportion of dormant buds were strongly

7 A. SØNSTEY and O. M. HEIDE 44 influenced by both temperature and photoperiod (Figure 4; Figure ; Table III). In Experiment 1, more than 7% of the buds of plants grown at 12 C did not grow out, but became dormant, while only about 2% of the buds were dormant in plants grown at 24 C (Figure 4; Figure ). While daylength had no effect at these temperatures, both the number and proportion of dormant buds were significantly higher under than under conditions at intermediate temperatures of 18 C and 21 C (Figure 4), with night interruption being almost as effective as daylength extension in this respect (Table III). ud dormancy also tended to increase once again when the temperature was raised above 24ºC. Interestingly, a chilling temperature (6 C) for 7 weeks at an early stage of growth did not significantly increase the number or proportion of dormant buds if the plants were subsequently grown at 24 C (Figure 4). These results are in full agreement with, and may explain, many of the responses observed in commercial production. Thus, in Portugal, where annual-fruiting cultivars such as Autumn liss are widely grown on a commercial scale, tip-flowering in the Autumn is the general rule when plants are over-wintered in the field under natural conditions. The majority of buds go dormant and do not flower until the following Spring (Oliveira et al., 1996; 21, and references therein). The result is a small, off-season Autumn crop, while the major crop occurs in the following Summer. Temperatures during the mild Portuguese Winter are about 12 C during December February (Oliveira et al., 1996). The temperature effects in Experiment 1 (Figure 4; Figure ) strongly suggest that such cool growth temperatures, in combination with natural conditions over a period of 3 months, are the main reasons for this flowering and cropping behaviour. This conclusion was further supported by the fact that greenhouse-propagated planting material imported from UK and planted in the field in Spring, after the temperature has risen to > 2 C, produced abundant Autumn flowering and cropping under the same conditions (P.. Oliveira, personal communication). Likewise, in Norway, Heiberg (26) found that plants of Polka and Autumn liss, transferred from a cold store to a non-heated plastic tunnel on April, grew to a height of more than 2 m and produced typical tip-flowering. Again, we conclude that the low temperatures prevailing during April and May in Norway were the main reason for this growth and flowering behaviour. Thus, it seems clear that temperature during the growing season largely determines the degree of annual flowering and fruiting, even in typical annual-fruiting genotypes such as Autumn liss and Polka. Manipulation of an annual-fruiting cultivar by cultural treatments may, in fact, modify the flowering behaviour to the extent that only a negligible Autumn crop is obtained, with the plants then behaving in effect as biennial cultivars (Oliveira et al., 1996; 21). The present results support the view that the distinction between annual- and biennial-flowering raspberry cultivars is not absolute, but that cultivars represent a continuum from true biennial-flowering, through tip-flowering, to annual-flowering types (cf. Carew et al., 2; 23; Dale, 28). Furthermore, expression of these various flowering types may be modified to a considerable extent by the environment. It is well known that inherently biennial-flowering cultivars such as the classical Lloyd George (Williams, 196), and others such as Glen Moy (Carew et al., 2), commonly produce some flowers at the tips of the annual shoots. On the other hand, the present results, and those discussed above, further demonstrate that typical annual-fruiting cultivars such as Autumn liss and Polka may also perform as tip-flowering cultivars under cool temperatures and/or conditions (Oliveira et al., 1996; 21; Figure 4A; Figure ). Oliveira and Dale (27) and Dale (28) speculated that floral induction in the uppermost annual-flowering buds and in the lower biennial-flowering buds of such tipflowering plants may be controlled by different external conditions. However, our results with Polka clearly demonstrate that this was not the case. Floral initiation took place in both types of buds at both low (12 C) and high temperatures (24 C) under both and conditions. The only difference was that the uppermost buds developed directly into open flowers, whereas the lower buds became dormant and needed Winter chilling to flower (biennial-flowering behaviour). Dissection of these dormant buds revealed that they contained flower buds at an advanced stage. Flowering, and the annual-flowering tendency, also varied along the length of the shoot. Starting at the apex (Figure 2), flowering spread basipetally towards the base. Then, at a certain stage, depending on temperature and daylength conditions, flower development halted and the remaining buds at the lower nodes became dormant, with various degrees of tip-flowering as a result. The length of the lateral shoots, and hence the number of flowers produced per lateral, increased almost linearly from the top to the base of the shoot (Figure ; Table III). Therefore, the key to abundant first-year flowering and fruit yield was to produce a plant with many flowering laterals as far down the shoot as possible, with only a minimum number of dormant buds at the base. The results showed that this type of plant architecture is strongly favoured by relatively high temperatures, with a broad optimum in the mid-2 C range. These findings have the potential to improve commercial production systems of annual-fruiting raspberry. In Mediterranean climates, with mild but cool Winters, Autumn planting should be avoided and Spring planting should be delayed until the temperature has risen to > 2 C. Exposure to cool temperatures (1 C) should be avoided at all times. Either cold-stored or greenhouse-grown planting material should be used, and daylength extension by night interruption should be considered. An extended harvest season can be obtained by early planting in plastic tunnels and late planting directly in the field (cf. Oliveira et al., 1996; 21). Likewise, at higher latitudes, where protected cultivation is needed for these cultivars, temperatures above 2 C should be maintained throughout culture. Natural long days are an advantage at these latitudes.

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