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Effect of low-temperature treatments on flowering in three cultivars of Hebe Comm. ex Juss.
sclENllA HOllllCULTuM ELSEVIER
Scientia Horticulturae66 ( 1996) 103- 11.5
Effect of low-temperature treatments on flowering in three cultivars of Hebe Comm. ex Juss. Linda E. Noack a,*, Ian J. Warrington b, Julie A. Plummer ‘, Ame Skytt Andersen d aDepartment of Ornamentals, Research Centre Aarslev, Danish Research Service for Plant and Soil Science, Kirstinebjergvej IO. DK-5792 Aarslev, Denmark b Hort + Research, Department of Scientific and Industrial Research, Private Bag, Palmerston North, New Zealand ’ New Zealand Nursery Research Centre, Department of Horticultural Science, Massey University, Palmerston North, New Zealand e Section for Horticulture, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Rolighedsvej 23, DK 1958 Frederiksberg C, Denmark
Accepted 19 January 1996
Abstract The genus Hebe is mainly native to New Zealand, and a number of species and cultivars are used as ornamental garden and balcony plants in many countries. The ornamental value is increased by controlled flowering. In this experiment chilling (15.5/9.5”C or 9/3”C day/night) treatments of different durations (0, 3, 6, 9 or 12 weeks) were tested on three cultivars of Hebe, ‘Inspiration’, ‘Variegata’ and ‘Waikiki’. After chilling the plants were forced under 25/19”C conditions for 12 weeks. Flowering was promoted by the chilling treatments, and complete flowering in the apical shoots occurred after 9-12 weeks chilling in 15.5/9.5”C for ‘Inspiration’ and ‘Waikiki’. Increased flowering of ‘Variegata’ (75% of the plants) was observed after 12 weeks 15.5/9.5”C treatment followed by forcing. The more induced plants had become under the chilling treatments, the shorter time was necessary under the forcing conditions for flower development. With all cultivars, cool conditions were more effective for flower induction than cold conditions. Flowering was more rapid where the low temperature conditions lasted longest. Larger ‘Inspiration’ plants were induced more readily than the smaller plants. The importance of size of the plants at the start of chilling was tested for ‘Inspiration’, where plants with 22 nodes flowered after shorter time under forcing conditions than plants with 12 nodes.
Corresponding author. Present address: Team Grow How, J. Benzous Gade 54, DK 5000 Odeuse C, Denmark. Tel: 45 66 12 04 50; Fax: 45 66 12 84 50. l
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A cold treatment, at 9/3”C, reduced the flowering intensity for ‘Inspiration’and ‘Variegata’, while ‘Waikiki’ reacted unchanged. Without chilling only ‘Waikiki’ was able to develop flowers but in less than 50% of the plants. Keywords: chilling; Cold treatment; Flowering; Hebe X franciscana; Ploriculture; Pot plants; Leaf variegation
1. Introduction The genus Hebe Comm. ex Juss. (Scrophulariaceae) is native to New Zealand (34’46’5”s) and outlying islands, except for two species which are also found in South America and in the Falkland Islands. The exact number of taxa is not known but is probably close to 100 species and 15 varieties, or according to Heenan (19931, 70 species, 28 varieties and 26 undescribed taxa. Depending on taxa, flowering can occur at any time of the year in New Zealand, but the peak flowering occurs from November to January (Kristensen, 1990). Taxa growing in lowland region have the longest flowering season, while taxa growing in alpine and subalpine altitudes have short seasons (Moore, 1961). Plantings of a wide range of Hebe species in Invercargill, Christchurch and Wellington, New Zealand, (annual daylength range 9-16 h and mean annual air temperatures of 9.7, 11.6 and 13.3”C, respectively) all flowered, while less than 80% of a similar range of species planted in Auckland (annual daylength range lo-15 h and mean annual air temperature 155°C) flowered yearly on a regular basis (Kristensen, 1990). Commercial production of Hebe species and cultivars takes place in New Zealand, Australia, America and Europe. In Denmark Hebe is an important pot and bedding plant crop for late summer plantings, and the flowering of some 20 cultivars in production has been investigated (Kristensen and Adriansen, 1988; Bjarke, 1993). The production time is 8-12 months, and the growing takes place in glasshouses during the winter and in the open from spring to selling (Kristensen and Adriansen, 1988). Growth for 5 months under a combination of daylength (8 and 20 h photoperiod) and temperature treatments indicated, in Hebe ‘Inspiration’ and ‘Variegata’, that 10 or 25°C delayed and reduced the ability to flower compared with 15-20°C (L.N. Kristensen, unpublished data, 1988). A Hebe hybrid, incorrectly named Hebe-Andersonii-hybrid ‘Variegata’ (according to Kristensen and Adriansen, 19881, has been investigated by Reimherr (1989). He showed that early propagation and low (2-5°C) winter temperatures or later propagation and higher (lo-15°C) winter temperatures tended to result in earlier flowering. The aim of the present experiments was to investigate the influence of different low temperature treatments on the timing and degree of flowering in three Hebe cultivars with diverse natural flowering periods.
2. Materials and methods The experiment was carried out in controlled environment rooms at the National Climate Laboratory of the Hort + Research Batchelor Research Centre, Palmerston North, New Zealand.
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2.1. Culture conditions One clone of each of three commercially important cultivars of Hebe was selected for the experiment. The cultivars have considerable variation in flowering time in the field (Kristensen and Adriansen, 1988). The names, parentage, natural flowering times and sources of the cultivars are listed in Table 1. Vegetative propagation took place in trays containing peat/pumice (3:2 v/v> growing medium with 1.5 kg Osmocote@ (14:6.1:11.6 N:P:K, 3-4 months release), 3 kg dolomite and 0.6 kg Micromax @ incorporated per cubic metre. Dates of propagation were 26 July (‘large’) and 30 August (‘small’) for ‘Inspiration’; 28 October for ‘Waikiki’ and 2 November for ‘Variegata’. Cuttings of ‘Waikiki’ propagated on 28 October were taken from stock plants that had previously been maintained in a greenhouse (minimum temperature 15°C) for 1 month, while all other cuttings were taken from outdoor stock beds. Uniform 3-4 cm long tip-cuttings which had two or three leaf pairs were selected from soft immature shoots. Rooting time was approximately 4 weeks under closed mist in a ventilated greenhouse (min. I%, max. 22°C). All rooted cuttings were acclimated for 2-4 days under open mist before transplanting. Transplanting was carried out l-2 days before the start of the treatments. Uniform rooted cuttings were placed in 9 cm diameter (500 ml> pots containing peat/pumice (3:2 v/v> growing medium with 1.2 kg Osmocote” (146.1: 11.6 N:P:K, 3-4 months release), 1.8 kg Osmocote@ (18:2.6: 10 N:P:K, 9-10 months release), 3 kg dolomite and 0.9 kg Micromax @ incorporated per cubic meter. All cuttings were intended to be vegetative at the start of the experiment. ‘Variegata’ and ‘Waikiki’ were apparently vegetative, whereas microscopical examinations of apices of ‘Inspiration’ showed that they were initially floral. As a consequence, ‘Inspiration’ plants were grown for 3 weeks in the forcing temperature conditions (25/19”C) with a 12 h photoperiod and a photosynthetic photon flux (PPF) of 210 kmol mm2 s- ’ prior to imposing the temperature treatments in order to enhance vegetative development. Inflorescences and lower axillary branches were removed as they appeared during this 3 week period.
Table 1 Detailsof names, parentage, flowering times in natural habitats, and sources of the three Hebe cultivars used in the experiment Name
Flowering time
(Parentage “)
Christchurch a
Auckland b
Hebe ‘Inspiration’ (H. speciosa X H. diosmifolia) Hebe ‘Waikiki’
Nov.-Dec. and Jul.-Nov. Dec.-Jan.
Nov.-Jan. Nov.Feb.
(Not known) Hebe X franciscana ‘Variegata’ ( H. speciosa X H. elliptica)
’ Metcalf, 1993. b J. Hobbs (personal communication, 1989).
Dec.-Jan. and Mar.-Jun.
Dec.-Feb.
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2.2. Experimental procedures Three controlled environment rooms were used when the experiment started on 5 December. The day/night temperature treatments were: forcing (25/19”(J), cool (15.5/9.5”C) and cold (9/3 f 0.3”C). The durations of the cool and cold treatments for all cultivars were 0, 3, 6, 9 or 12 weeks. After each of the cool and cold treatments, all plants were forced at 25/19”C for at least a further 12 weeks. Within each treatment there were two replicates of seven plants arranged in a randomised block design. At the start of the experiment the replicates were placed on trollies with a density of 100 plants m-*. Plants were spaced at 50 plants m-* and 30 plants mm2 by 3 April and 5 May, respectively. As the plants grew they were supported with stakes to ensure that the apical shoots stayed dominant. 2.3. Environmental conditions Within the cold and cool temperature regimes the day/night air vapour pressure deficit (VPD) was 4.0/2.3 kPa; the corresponding relative humidities (RH) were 65/70 f 5% at 9/3”C and 75/81 f 5% at 15.5/9.5”C. In the 25/19”C forcing treatment the day/night VPD conditions were 7.0/2.3 kPa (78/90 f 5% RH). The lengths of the day and night temperature and humidity periods were all 10 h. Temperature and relative humidity changeovers from day to night and vice versa occurred over 120 min. Carbon dioxide concentration was maintained at 600 f 30 I.L.~ 1-l continuously in all treatment and forcing conditions. Air flow downward through the plants was at a velocity of 0.3-0.5 m s-t. Lighting was provided by four 1000 W high-pressure multivapour lamps (Sylvania ‘Metalarc’) and four 1000 W Philips quartz halogen lamps separated from the plant growth room by a plate glass and water thermal barrier. The energy flux density of photosynthetically active radiation at initial pot surface height, 2 m below the thermal barrier, was 711 f 20 kmol me2 s- ‘. Fuller details of these light gradients and the lighting system are provided in Warrington et al. (1978). Every 7 days, the trollies were rotated within the rooms, to ensure even light conditions over time and to minimise any other positional effects within each room. Water and mineral nutrients were supplied to individual pots via microtubes at regular intervals throughout each day. Initially, pots received four 30 ml applications per day, and as the plants grew this was increased to seven 50 ml applications per day. Modified Hoagland’s A mineral nutrient solution (Brooking, 1976), at half strength concentration, was applied at each application. Pots were maintained on capillary matting to minimise any moisture deficits. 2.4. Data collection and statistical analysis Anthesis time, defined as the day on which the first flower opened on an inflorescence arising from the main shoot, was recorded. At that time the first set of development data was collected. Data were also collected after 10 and 12 weeks in the
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forcing temperature conditions. Data collection terminated after 12 weeks of forcing, except for ‘Waikiki’ where plants were kept for 2 weeks longer to allow complete development of the inflorescences. Data collected included: the number of vegetative and floral nodes; plant height; the number of visible inflorescences and their position on the main shoot; the number of branches per inflorescence and the length of the inflorescences when fully developed. In addition, the number of florets per inflorescence on the inflorescences arising at the first and second floral nodes was recorded from 4/7 of all plants in the 6 and 12 weeks cool and cold treatments. The results were tested according to a split plot design analysis of variance, and differences were tested at LSD(O.05).
3. Results 3.1. Plant morphology Vegetative growth occurred in all low temperature and forcing conditions, whereas obvious floral growth only occurred in the forcing conditions. Vegetative branch development occurred in all treatments and in all cultivars, although only at every third or fourth node in ‘Variegata’. The node preceding the first floral node did not develop vegetative branches in either ‘Inspiration’ or ‘Variegata’. When floral nodes developed, they were followed distally by other floral nodes, and two inflorescences usually developed at each node. With both ‘Inspiration’ and ‘Waikiki’ that had received 12 weeks of cool treatment, plants continued to develop floral nodes throughout the forcing period. In other treatments that resulted in flowering, the floral nodes were eventually succeeded in the apical region by vegetative nodes which had few or no branches. Growth during the forcing period occurred continuously at a high rate with 1.5 to 2 vegetative or reproductive nodes emerging per week from the apex. Internode length varied among cultivars but not by treatment and was 1.1 cm for ‘Inspiration’, 1.2 cm for ‘Variegata’ and 2.1 cm for ‘Waikiki’. After 12 weeks in the forcing conditions plants from the cool treatments were 2-5 cm taller than plants that had been subjected to the cold treatments. The average height at this time was 24 cm in ‘Variegata’, 28 cm in ‘Inspiration’ (small plants), 34 cm in ‘Inspiration’ (large plants) and 42 cm in ‘Waikiki’. Floral growth from the apical end of basal vegetative branches occurred frequently in plants growing in the most inductive treatments. Usually the vegetative branches in all of the lower parts of the plants flowered, while the branches in the four to six nodes proximal to the floral nodes remained vegetative. 3.2. Injlorescence morphology In general, inflorescences at the two to three lowest floral nodes were longest. The average length of an inflorescence was 7 cm for ‘Inspiration’ and 8 cm for ‘Variegata’
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Table 2 Branching of inflorescences in Hebe ‘Inspiration’, ‘Variegata’ and ‘Waikiki’ after cool (S/PC) or cold (9/3”C) treatments followed by forcing (25/ 19°C). Data are averages from floral plants on the basis of 2 X 7 plants and are presented as the number of branches per inflorescence Cultivar a
Treatment
‘Inspiration’ Small
‘Variegata’
‘Waikiki’
Large
Control 0 weeks
1.4bcd
9/3”C (cold) 3 weeks 6 weeks 9 weeks 12 weeks
3.2a
2.Ob 1.9b 1.7b
15.5/9.PC 3 weeks 6 weeks 9 weeks 12 weeks
3.9a 2.3a 3.2a
3.lab 2.3a 3.6a 4Sa
1.Oa
1.7ab 1.3b 1.2b O&d
0.3a 1.Oa
2.6a 1.6abc OSd 0.4d
(cool)
’ Means followed by different letters are statistically different at the 0.05 level.
and was not influenced by plant age or treatment. Inflorescences of ‘Waikiki’ were longest (18-20 cm) on plants in the cold or the 3 week cool treatment. Plants grown in the 6, 9 or 12 week cool treatments had shorter inflorescences (16, 12 and 10 cm, respectively). Inflorescences on the control plants of ‘Waikiki’ were 16 cm long. Lateral branch development on the inflorescences occurred in all cultivars and in all treatments to some extent. The least branching cultivar was ‘Variegata’ with 0.8 branches per inflorescence. Branching on the inflorescences of ‘Waikiki’ was intermediate in the control plants but increased following 3 weeks of cold or cool treatments and decreased with 6 to 12 weeks of treatment (Table 2). Most branching occurred in larger ‘Inspiration’ plants which had received a 12 week cool treatment. Branching of inflorescences in ‘Inspiration’ mostly occurred in the lowest two floral nodes, where 8.2 (larger plants) or 6.6 (smaller plants) branches were recorded after 12 weeks of cool treatment and 7.7 branches (larger plants) after 9 weeks of cool treatment. In other treatments ‘Inspiration’ plants had fewer than 3.5 branches per inflorescence at the two lowermost nodes (Fig. 1). The number of florets per inflorescence recorded after the 6 or 12 week cold or cool treatments showed no obvious correlations to treatments, as it was primarily determined by the length and branching of the inflorescences. The average number of florets per inflorescence at the first floral node was 135, 128 and 234 for small ‘Inspiration’, large ‘Inspiration’ and ‘Waikiki’, respectively, after 6 weeks of cool treatment and 168, 208 and 96 after 12 weeks of cool treatment. The ‘Waikiki’ control plants had on average 168 florets per inflorescence at the first floral node.
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Fig. 1. Branching of inflorescences in Hebe ‘Inspiration’. Proportion of flowering plants (%) 100
Proportion of flowering plants (“Yo)
a
80604020I-IA “0
3
6
9
12
Duration of chilling treatment (weeks)
Proportion of flowering plants (%)
Proportion of flowering plants (%) 100 I
0 3 6 9 12 Duration of chilling treatment (weeks)
I
C
80. 6040200. 0
3
6
9
12
Duration of chilling treatment (weeks)
0
3
6
9
12
Duration of chilling treatment (weeks)
Fig. 2. Proportion of flowering plants after chilling (15.5/9.5 or 9/3”C) within 12 weeks of forcing. (a) Hebe ‘Inspiration’, small plants; (b) Hebe ‘Inspiration’, large plants; (c) Hebe ‘Variegata’; (d) Hebe ‘Waikiki’. Bars on selected data points indicate SD.
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Table 3 Number of days to the first open floret (anthesis) in inflotescences positioned at the earliest flowering node of ‘Inspiration’ (small and large plants), ‘Variegata’ and ‘Waikiki’ following 0,3,6,9 or 12 weeks under cool or cold temperature conditions and subsequent development in forcing (25/19oC) conditions. The results include only the floral plants in each treatment of 14 plants, and the statistical analyses are restricted to within cultivars Treatment
Cultivar 1 ‘Inspiration’
‘Waikiki’
Large
N
N
N
97h
N N N 135a
N 97d 115bc 134d
N N N 139a
lllde 10&f 126c 14la
N 97b 115bc 133a
74e 94d 1lOc 12lb
N N 120a 13la
9ftgh 103fg 116d 134b
Control 0 weeks 9/3”C
‘Variegata’
Small
(cold)
3 weeks 6 weeks 9 weeks 12 weeks 15.5/9.5”c (cool) 3 weeks 6 weeks 9 weeks 12 weeks
N, no flowering within duration of experiment. ’ Means followed by different letters am statistically different at the 0.05 level.
3.3. Proportion of plants flowering After the 9 or 12 week cool treatment all ‘Inspiration’ (larger plants) and ‘Waikiki’ plants developed flowers under the forcing conditions (Fig. 2). All small ‘Inspiration’ plants that had received 6 weeks or more of cool treatment and all small plants given 12 weeks of cold also flowered. Treatments that resulted in 50-93% of the plants flowering included the 6 week cool and 12 week cold treatments with small ‘Inspiration’, the 3 week cool and 9 week cold with large ‘Inspiration’, 9 and 12 week cool with ‘Variegata’, 3 and 6 week cool and 3, 6 and 9 week cold with ‘Waikiki’. In all other treatments less than half of the population flowered. 3.4. Time to jlowering Most plants of all cultivars flowered under the forcing conditions after being grown under either the cool or cold treatment conditions for 12 weeks (Fig. 2). The total number of days to flowering was highest for the longest period of cold treatment and, depending on cultivar, was 2-13 days shorter after similar periods of cool treatment (Table 3). The most rapid flowering (74 days) occurred in the larger plants of ‘Inspiration’ which had been in the cool treatment for 3 weeks. These plants also flowered in less
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100 days under both the cool and cold 6 week exposure treatments. In contrast, the small plants of ‘Inspiration’ flowered in less than 100 days only in the 6 week cool treatment. Plants of ‘Waikiki’ flowered in all treatments, but took less than 100 days to do so only in the continuous forcing and cool 3 week exposure conditions. When assessed on the basis of time in the forcing conditions the fastest flowering (37 days) occurred in large ‘Inspiration’ plants which had been held in the cool treatment for 12 weeks. Both small and large ‘Inspiration’ plants took 10-l 1 days longer to flower when they had been grown previously under either the 9 or 12 weeks cool treatments. The slowest flowering under the forcing conditions occurred with ‘Waikiki’ where plants took 90-97 days to reach anthesis under either continuous forcing conditions or after 3 weeks of cold conditions. Flowering did not occur within the experimental period in any plants of ‘Inspiration’ or ‘Variegata’ if they had not received any cool or cold treatment (i.e. continuous forcing). Anthesis of the two inflorescences subtended by each floral node usually occurred on the same day. Inflorescences in the next, more distal node reached anthesis 2-3 days after anthesis at the above node. Inflorescences opened their florets in succession towards the apex. The flowering period for the plants in a treatment varied with the than
No. of inflorescences 12 I
No. of inflorescences
Duration of chilling treatment (weeks)
Duration of chilling treatment
No. of inflorescences
No. of inflorescences
(weeks)
12‘ lOA 864
Cl 3 6 9 12 Duration of chilling treatment (weeks)
6 9 &o 3 Duration of chilling treatment
12 (weeks)
Fig. 3. Number of inflorescences on the main shoot after chilling (15.5/9.5 or 9/3”C) within 12 weeks of forcing (at 25/ 19°C). Data include flowering plants only and are based on 2 X 7 plants per treatment. (a) Hebe ‘Inspiration’, large plants; (b) Hebe ‘Inspiration’, small plants; (c) Hebe ‘Variegata’; (d) Hebe ‘Waikiki’. Bars on selected data points indicate. SD.
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number of inflorescences per plant, but on the average it lasted three to four weeks. The inflorescences that developed were normal under all treatments except for ‘Variegata’ grown under 12 weeks of cold, where the florets on the only flowering plant developed five to six petals instead of the usual four. 3.5. Flower intensity The number of inflorescences per plant progressively increased with increasing periods of low temperature treatment and was higher in forcing after the cool than in the cold temperature treatment (Fig. 3). The highest numbers of inflorescences were observed in the 9 week cool treatments of larger ‘Inspiration’ plants and of ‘Waikiki’, where about 10 inflorescences were recorded per plant after 9 weeks of cool treatment. The flower development was partial (i.e. vegetative nodes developed after the floral nodes within 12 weeks of forcing) in all treatments which were of less than 12 weeks duration for ‘Inspiration’ (both sizes) and ‘Waikiki’.
4. Discussion 4.1. All cultivars Low temperature promoted subsequent flowering in all Hebe cultivars tested. Flowering in many other woody plant genera is controlled by temperature (Lavee, 1985). In some plants flower promotion is affected by a combination of chilling and short photoperiod as in the temperate and subtropical Leptospermum species (Zieslin, 19851, or intermediate temperature and short photoperiod which accelerates flower bud differentiation in Hydrangea macrophylla (Wallerstein and Rlinger, 1985). The present results showed that chilling at a constant 12 h photoperiod promoted flower initiation in the tested Hebe cultivars, while the possible effects of photoperiod on flowering have only been investigated to a limited extent by L.E. Noack (unpublished data, 1988). An interaction of the photoperiod with temperature regimes could exist in a similar way as in Hydrangea (Link and Shanks, 1954) and Azalea (Pettersen, 1972). Newly rooted cuttings of Hebe ‘Variegata’ and ‘Waikiki’ were used in the experiment, while the cuttings of ‘Inspiration’ were either 2 or 3 months older. As the rooted plants of ‘Inspiration’ were initially floral, probably induced prior to propagation, the inflorescences, lower branches and sometimes the leaves on the main stem were removed during a 3 week pretreatment at 25/19”C under low light intensity conditions. The leaf area and the number of nodes were therefore limited when the experiment started. Although the Hebe cuttings were taken from adult mother plants, the small plants may have been in a stage not able to form flowers when only just rooted or with only a small leaf area. In biological terms, efficient reproduction is dependent on age and size of the plants where fecundity is high and mortality low (Lacey, 1986). With ‘Waikiki’ and ‘large’ ‘Inspiration’ the maximum number of inflorescences was reached after 9 weeks of chilling in 15.5/9.5”C (Fig. 3).
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The total time to flowering increased with the duration of chilling, but when the time in the chilling treatment was excluded, the number of days to flower in forcing conditions was reduced (Table 3). This phenomenon is also seen in Hydrangea where the length of the forcing period depends on the number of cold units (Wallerstein and Rtinger, 1985) given during the cold treatment (Link and Shanks, 1954).
Both parents of Hebe ‘Inspiration’ originate in a mild coastal climate with a natural daylength ranging from 9.5 to 14.5 h and a mean annual surface temperature of 12.5”C (Macaulay and Beavis, 1983). Plants had the fastest and greatest intensity of flowering following the 15.5/9.5”C treatments compared with the 9/3”C treatments, and did not flower without chilling (Table 3, Fig. 2). An earlier phytotron experiment (Noack, 1991) examined combinations of higher temperatures and 8 or 20 h photoperiods. After 5 months, 100% flowering was obtained in ‘Inspiration’ at both 15 and 20°C at both photoperiods. In the 10°C treatment at both photoperiods and in the 25”C/20 h temperature/photoperiod combination none of the plants flowered, and only 60% of the 25”C/8 h photoperiod plants flowered within the 5 month treatment period. This indicates a strong influence of temperature on growth and flowering, while a short photoperiod might promote the flower formation and/or development at higher temperatures. Propagation time and subsequent growth prior to chilling had a major effect on the ability to flower after the different chilling temperatures and durations. The larger ‘Inspiration’ plants, propagated in July, flowered earlier and more abundantly than the smaller plants propagated in August which needed a longer chilling period. Plant height, the number of leaf pairs and the number of nodes on the main stem at the start of the chilling period were greater for the plants propagated in July compared with those in August. Schwabe (1987) suggested that a high number of nodes between the apical meristem and the root system might permit flowering at a certain size. The branches of the ‘Inspiration’ plants did not flower until they had reached a minimum of six nodes, and more inflorescences were produced in those treatments where the required six nodes were reached quickly. This phenomenon might be caused by the overall maturity of the leaves on the branches, as the ratio between mature (sources) and immature (sinks which may produce floral inhibitors) leaves within branches has been shown to influence floral induction (Bemier et al., 1981). Low temperatures often lead to the accumulation of carbohydrates in the shoot apex which may affect the response obtained (Evans, 1971). 4.3. Hebe X fransiscana ‘Varieguta’ The period of chilling required for flowering was longer in ‘Variegata’ than in the other cultivars tested. One of the parents of ‘Variegata’, Hebe speciosa, originates from the eastern coastal region of the central North Island, while the geographical origin of the other parent, Hebe elliptica, is uncertain and could be native to all parts of New Zealand, South America or the Falkland Islands (Heenan, 1993). In most of these areas
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the climate is coastal and temperate with a mean annual surface temperature of 10°C and a daylength ranging from 8 to 15 h (Macaulay and Beavis, 1983). In addition, the natural growth season is shorter than that for the parents of ‘Inspiration’ and a long chilling period would be experienced in the natural environment compared with the parents of ‘Inspiration’. Plants grew better and initiated flowering faster in response to chilling at 15.5/9.Y than at 9/3”C. Node and leaf pair number was low (2.4 and 4.8, respectively) at the start of the experiment, and there may be a need to attain a certain minimum size before flower initiation is possible. This certain size was not determined, but plants grew faster in the 15.5/9.5”C treatment than in the 9/3”C treatment. The low photosynthetic activity of the achlorophyllous sections of the variegated leaves probably slows down the growth rate and therefore delays the effect of chilling. Kristensen and Adriansen (1988) support the present results by suggesting that flower initiation may occur when plants of ‘Variegata’ have six to eight open leaf pairs and are placed in inductive environmental conditions. For Hydrangea a minimal number of leaves is required before flower bud formation will occur (Shanks and Link, 195 1). In Ribes nigmm vegetatively propagated plants require a minimum of 20 nodes before floral initiation is possible (Wright, 1985). It is suspected that the number of leaf pairs needs to be higher in this variegated cultivar than in the closely related green leaved cultivar ‘Blue Gem’ because of the lack of potential photosynthesis in a high proportion of the leaf area. 4.4. Hebe ‘Waikiki The parentage of this natural hybrid ‘Waikiki’ is not known (Moore, 1961; J. Hobbs and P. Gamock-Jones, personal communication, 1989). The ease of flower development, even under the continuous forcing temperature, indicates that the need for chilling is low and that the parents are probably from a mild climate such as the northern region of the North Island of New Zealand. Moore (1961) suggests that one parent of ‘Waikiki’ could be Hebe obtusatawhich originates from a limited area north of Auckland. ‘Waikiki’ did not have an absolute need of chilling in order to flower. However, without chilling only 40% of the control plants flowered within 14 weeks of forcing. Chilling increased the proportion of plants which flowered, so that after 6 weeks of chilling (15.5/9.5”C or 9/3”C) more than 90% of the plants, which were very small at the start of the experiment, flowered within 12 weeks of forcing. ‘Waikiki’ grew taller than any of the other cultivars tested, but node number was similar to that of the other cultivars. This might have influenced the development of the upper inflorescences at upper nodes which were shorter than those lower on the main stem. Bemier et al. (1981), however, suggest that root growth decreases when plants enter the floral stage and root restrictions may even promote continuous flower development. Lacey (1986) supports this suggestion by indicating that slow plant growth promotes flowering. Acknowledgements
We are grateful to R. Southward and Jill Stanley, for technical assistance, Dr. K. Krlstensen for statistical assistance, and O.V. Christensen for criticism of the manuscript.
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Financial support was supplied by the DSIR Fruit and Trees, the New Zealand Nursery Research Centre, the Danish Research Academy and The Joint Committee for Agricultural Research and Experiments.
References Bemier, G., Kinet, J.-M. and Sachs, R.M., 1981. The Physiology of Flowering, Vol. 1: CRC Press, Boca Raton, PL. Bjarke, L., 1993. Hebe til hundtede iir. Gartner Tidende, 109: 1066-1067. Brooking, I.R., 1976. Soilless potting media for controlled environment facilities. N.Z. J. Exp. Agric., 4: 203-208. Evans, L.T., 1971. Plower induction and the florigen concept. Ann. Rev. Plant Physiol., 22: 365-394. Heenan, P.B., 1993. Chromatographical, cytological, morphological and nomenclatural studies on Hebe X andersonii, H. X franciscana (Scrophulariaceae) and their cultivars. M.Sc. Thesis, University of Missouri, St. Louis. Kristensen, L.N. and Adriansen, E., 1988. Growth and flowering in Hebe X franciscana ‘Variegata’ treated with plant growth regulators. Sci. Hortic., 36: 139- 149. Kristensen, L.N., 1990. The genus Hebe, S-bereming, S 2034, Statens Planteavesforsog, Lyngby, DK, 64 pp. Lacey, E.P., 1986. Onset of reproduction in plants: size versus age-dependency. Trends Ecol. Evol., 1: 72-75. Lavee, S., 1985. Olea europea. In: Halevy, A.H. (Editor), CRC Handbook of Flowering, Vol. 3. CRC Press, Boca Raton, PL. pp. 423-434. Link, C.B. and Shanks, J.B., 1954. Studies of the factors involved in terminating the rest period of hydrangeas. Proc. Am. Sot. Hortic. Sci., 64: 519-525. Macaulay, J.U. and Beavis, E.J., Senior Atlas for New Zealand, 1983: Collins (NZ) Ltd. and Longman Ltd., Auckland. Metcalf, L., 1993. The Cultivation of New Zealand Plants, Gcdwit, Auckland Moore, L.B., l%l. Hebe Comm. ex Juss., 1789. In: H.H. Allan (Editor), Flora of New Zealand, Vol. 1, Owen, Wellington, NZ, pp. 885-952. Noack, L., 1991. Environmental control of flower induction in horticultural plants. Case sNdy: Hebe. Ph.D. Thesis, Royal Veterinary and Agricultural University, Denmark, 102 pp. Pettersen, H., 1972: The effect of temperature and daylength on shoot growth and bud formation in azaleas. J. Am. Sot. Hortic. Sci., 97(l): 17-24. Reimherr, P., 1989. Blbtefirderung bei Hebe. Versuche mit der Hebe-Andersonii- hybride ‘Variegata’. Deutsch. Gartenbau, 2: 76-77. Schwabe, W.W., 1987. The flowering problem. In: J.G. Atherton (Editor), Manipulation of Flowering, Buttenvorth, London, pp. 3-13. Shanks, J.B. and Link, C.B., 1951. Some studies on the effects of temperature and photoperiod on growth and flower formation in Hydrangea. Proc. Am. Sot. Hortic. Sci., 58: 357-366. Wallerstein, I. and Rlinger, W., 1985. Hydrangea macrophylia. In: Halevy, A.H. (Editor), CRC Handbook of Flowering, Vol. 3. CRC Press, Boca Raton, FL. pp. 173- 177.. Warrington, I.J., Dixon, T., Robotham, R.W. and Rook, D.A., 1978. Lighting systems in major New Zealand controlled environment facilities. J. Agric. Bng. Res., 23: 23-36. Wright, C.J., 1985. Ribes. In: Halevy, A.H., (Editor) CRC Handbook of Flowering, Vol. IV. CRC Press, Boca Raton, PL. pp. 198-203. Zieslin, N., 1985. Leptospertnum scoparium. In: Halevy, A.H. (Editor), CRC Handbook of Flowering, Vol. III, CRC Press, Boca Raton, PL, pp. 280-282.
Scientia Horticulturae66 ( 1996) 103- 11.5
Effect of low-temperature treatments on flowering in three cultivars of Hebe Comm. ex Juss. Linda E. Noack a,*, Ian J. Warrington b, Julie A. Plummer ‘, Ame Skytt Andersen d aDepartment of Ornamentals, Research Centre Aarslev, Danish Research Service for Plant and Soil Science, Kirstinebjergvej IO. DK-5792 Aarslev, Denmark b Hort + Research, Department of Scientific and Industrial Research, Private Bag, Palmerston North, New Zealand ’ New Zealand Nursery Research Centre, Department of Horticultural Science, Massey University, Palmerston North, New Zealand e Section for Horticulture, Department of Agricultural Sciences, Royal Veterinary and Agricultural University, Rolighedsvej 23, DK 1958 Frederiksberg C, Denmark
Accepted 19 January 1996
Abstract The genus Hebe is mainly native to New Zealand, and a number of species and cultivars are used as ornamental garden and balcony plants in many countries. The ornamental value is increased by controlled flowering. In this experiment chilling (15.5/9.5”C or 9/3”C day/night) treatments of different durations (0, 3, 6, 9 or 12 weeks) were tested on three cultivars of Hebe, ‘Inspiration’, ‘Variegata’ and ‘Waikiki’. After chilling the plants were forced under 25/19”C conditions for 12 weeks. Flowering was promoted by the chilling treatments, and complete flowering in the apical shoots occurred after 9-12 weeks chilling in 15.5/9.5”C for ‘Inspiration’ and ‘Waikiki’. Increased flowering of ‘Variegata’ (75% of the plants) was observed after 12 weeks 15.5/9.5”C treatment followed by forcing. The more induced plants had become under the chilling treatments, the shorter time was necessary under the forcing conditions for flower development. With all cultivars, cool conditions were more effective for flower induction than cold conditions. Flowering was more rapid where the low temperature conditions lasted longest. Larger ‘Inspiration’ plants were induced more readily than the smaller plants. The importance of size of the plants at the start of chilling was tested for ‘Inspiration’, where plants with 22 nodes flowered after shorter time under forcing conditions than plants with 12 nodes.
Corresponding author. Present address: Team Grow How, J. Benzous Gade 54, DK 5000 Odeuse C, Denmark. Tel: 45 66 12 04 50; Fax: 45 66 12 84 50. l
0304-4238/96/$15.00
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A cold treatment, at 9/3”C, reduced the flowering intensity for ‘Inspiration’and ‘Variegata’, while ‘Waikiki’ reacted unchanged. Without chilling only ‘Waikiki’ was able to develop flowers but in less than 50% of the plants. Keywords: chilling; Cold treatment; Flowering; Hebe X franciscana; Ploriculture; Pot plants; Leaf variegation
1. Introduction The genus Hebe Comm. ex Juss. (Scrophulariaceae) is native to New Zealand (34’46’5”s) and outlying islands, except for two species which are also found in South America and in the Falkland Islands. The exact number of taxa is not known but is probably close to 100 species and 15 varieties, or according to Heenan (19931, 70 species, 28 varieties and 26 undescribed taxa. Depending on taxa, flowering can occur at any time of the year in New Zealand, but the peak flowering occurs from November to January (Kristensen, 1990). Taxa growing in lowland region have the longest flowering season, while taxa growing in alpine and subalpine altitudes have short seasons (Moore, 1961). Plantings of a wide range of Hebe species in Invercargill, Christchurch and Wellington, New Zealand, (annual daylength range 9-16 h and mean annual air temperatures of 9.7, 11.6 and 13.3”C, respectively) all flowered, while less than 80% of a similar range of species planted in Auckland (annual daylength range lo-15 h and mean annual air temperature 155°C) flowered yearly on a regular basis (Kristensen, 1990). Commercial production of Hebe species and cultivars takes place in New Zealand, Australia, America and Europe. In Denmark Hebe is an important pot and bedding plant crop for late summer plantings, and the flowering of some 20 cultivars in production has been investigated (Kristensen and Adriansen, 1988; Bjarke, 1993). The production time is 8-12 months, and the growing takes place in glasshouses during the winter and in the open from spring to selling (Kristensen and Adriansen, 1988). Growth for 5 months under a combination of daylength (8 and 20 h photoperiod) and temperature treatments indicated, in Hebe ‘Inspiration’ and ‘Variegata’, that 10 or 25°C delayed and reduced the ability to flower compared with 15-20°C (L.N. Kristensen, unpublished data, 1988). A Hebe hybrid, incorrectly named Hebe-Andersonii-hybrid ‘Variegata’ (according to Kristensen and Adriansen, 19881, has been investigated by Reimherr (1989). He showed that early propagation and low (2-5°C) winter temperatures or later propagation and higher (lo-15°C) winter temperatures tended to result in earlier flowering. The aim of the present experiments was to investigate the influence of different low temperature treatments on the timing and degree of flowering in three Hebe cultivars with diverse natural flowering periods.
2. Materials and methods The experiment was carried out in controlled environment rooms at the National Climate Laboratory of the Hort + Research Batchelor Research Centre, Palmerston North, New Zealand.
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2.1. Culture conditions One clone of each of three commercially important cultivars of Hebe was selected for the experiment. The cultivars have considerable variation in flowering time in the field (Kristensen and Adriansen, 1988). The names, parentage, natural flowering times and sources of the cultivars are listed in Table 1. Vegetative propagation took place in trays containing peat/pumice (3:2 v/v> growing medium with 1.5 kg Osmocote@ (14:6.1:11.6 N:P:K, 3-4 months release), 3 kg dolomite and 0.6 kg Micromax @ incorporated per cubic metre. Dates of propagation were 26 July (‘large’) and 30 August (‘small’) for ‘Inspiration’; 28 October for ‘Waikiki’ and 2 November for ‘Variegata’. Cuttings of ‘Waikiki’ propagated on 28 October were taken from stock plants that had previously been maintained in a greenhouse (minimum temperature 15°C) for 1 month, while all other cuttings were taken from outdoor stock beds. Uniform 3-4 cm long tip-cuttings which had two or three leaf pairs were selected from soft immature shoots. Rooting time was approximately 4 weeks under closed mist in a ventilated greenhouse (min. I%, max. 22°C). All rooted cuttings were acclimated for 2-4 days under open mist before transplanting. Transplanting was carried out l-2 days before the start of the treatments. Uniform rooted cuttings were placed in 9 cm diameter (500 ml> pots containing peat/pumice (3:2 v/v> growing medium with 1.2 kg Osmocote” (146.1: 11.6 N:P:K, 3-4 months release), 1.8 kg Osmocote@ (18:2.6: 10 N:P:K, 9-10 months release), 3 kg dolomite and 0.9 kg Micromax @ incorporated per cubic meter. All cuttings were intended to be vegetative at the start of the experiment. ‘Variegata’ and ‘Waikiki’ were apparently vegetative, whereas microscopical examinations of apices of ‘Inspiration’ showed that they were initially floral. As a consequence, ‘Inspiration’ plants were grown for 3 weeks in the forcing temperature conditions (25/19”C) with a 12 h photoperiod and a photosynthetic photon flux (PPF) of 210 kmol mm2 s- ’ prior to imposing the temperature treatments in order to enhance vegetative development. Inflorescences and lower axillary branches were removed as they appeared during this 3 week period.
Table 1 Detailsof names, parentage, flowering times in natural habitats, and sources of the three Hebe cultivars used in the experiment Name
Flowering time
(Parentage “)
Christchurch a
Auckland b
Hebe ‘Inspiration’ (H. speciosa X H. diosmifolia) Hebe ‘Waikiki’
Nov.-Dec. and Jul.-Nov. Dec.-Jan.
Nov.-Jan. Nov.Feb.
(Not known) Hebe X franciscana ‘Variegata’ ( H. speciosa X H. elliptica)
’ Metcalf, 1993. b J. Hobbs (personal communication, 1989).
Dec.-Jan. and Mar.-Jun.
Dec.-Feb.
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2.2. Experimental procedures Three controlled environment rooms were used when the experiment started on 5 December. The day/night temperature treatments were: forcing (25/19”(J), cool (15.5/9.5”C) and cold (9/3 f 0.3”C). The durations of the cool and cold treatments for all cultivars were 0, 3, 6, 9 or 12 weeks. After each of the cool and cold treatments, all plants were forced at 25/19”C for at least a further 12 weeks. Within each treatment there were two replicates of seven plants arranged in a randomised block design. At the start of the experiment the replicates were placed on trollies with a density of 100 plants m-*. Plants were spaced at 50 plants m-* and 30 plants mm2 by 3 April and 5 May, respectively. As the plants grew they were supported with stakes to ensure that the apical shoots stayed dominant. 2.3. Environmental conditions Within the cold and cool temperature regimes the day/night air vapour pressure deficit (VPD) was 4.0/2.3 kPa; the corresponding relative humidities (RH) were 65/70 f 5% at 9/3”C and 75/81 f 5% at 15.5/9.5”C. In the 25/19”C forcing treatment the day/night VPD conditions were 7.0/2.3 kPa (78/90 f 5% RH). The lengths of the day and night temperature and humidity periods were all 10 h. Temperature and relative humidity changeovers from day to night and vice versa occurred over 120 min. Carbon dioxide concentration was maintained at 600 f 30 I.L.~ 1-l continuously in all treatment and forcing conditions. Air flow downward through the plants was at a velocity of 0.3-0.5 m s-t. Lighting was provided by four 1000 W high-pressure multivapour lamps (Sylvania ‘Metalarc’) and four 1000 W Philips quartz halogen lamps separated from the plant growth room by a plate glass and water thermal barrier. The energy flux density of photosynthetically active radiation at initial pot surface height, 2 m below the thermal barrier, was 711 f 20 kmol me2 s- ‘. Fuller details of these light gradients and the lighting system are provided in Warrington et al. (1978). Every 7 days, the trollies were rotated within the rooms, to ensure even light conditions over time and to minimise any other positional effects within each room. Water and mineral nutrients were supplied to individual pots via microtubes at regular intervals throughout each day. Initially, pots received four 30 ml applications per day, and as the plants grew this was increased to seven 50 ml applications per day. Modified Hoagland’s A mineral nutrient solution (Brooking, 1976), at half strength concentration, was applied at each application. Pots were maintained on capillary matting to minimise any moisture deficits. 2.4. Data collection and statistical analysis Anthesis time, defined as the day on which the first flower opened on an inflorescence arising from the main shoot, was recorded. At that time the first set of development data was collected. Data were also collected after 10 and 12 weeks in the
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forcing temperature conditions. Data collection terminated after 12 weeks of forcing, except for ‘Waikiki’ where plants were kept for 2 weeks longer to allow complete development of the inflorescences. Data collected included: the number of vegetative and floral nodes; plant height; the number of visible inflorescences and their position on the main shoot; the number of branches per inflorescence and the length of the inflorescences when fully developed. In addition, the number of florets per inflorescence on the inflorescences arising at the first and second floral nodes was recorded from 4/7 of all plants in the 6 and 12 weeks cool and cold treatments. The results were tested according to a split plot design analysis of variance, and differences were tested at LSD(O.05).
3. Results 3.1. Plant morphology Vegetative growth occurred in all low temperature and forcing conditions, whereas obvious floral growth only occurred in the forcing conditions. Vegetative branch development occurred in all treatments and in all cultivars, although only at every third or fourth node in ‘Variegata’. The node preceding the first floral node did not develop vegetative branches in either ‘Inspiration’ or ‘Variegata’. When floral nodes developed, they were followed distally by other floral nodes, and two inflorescences usually developed at each node. With both ‘Inspiration’ and ‘Waikiki’ that had received 12 weeks of cool treatment, plants continued to develop floral nodes throughout the forcing period. In other treatments that resulted in flowering, the floral nodes were eventually succeeded in the apical region by vegetative nodes which had few or no branches. Growth during the forcing period occurred continuously at a high rate with 1.5 to 2 vegetative or reproductive nodes emerging per week from the apex. Internode length varied among cultivars but not by treatment and was 1.1 cm for ‘Inspiration’, 1.2 cm for ‘Variegata’ and 2.1 cm for ‘Waikiki’. After 12 weeks in the forcing conditions plants from the cool treatments were 2-5 cm taller than plants that had been subjected to the cold treatments. The average height at this time was 24 cm in ‘Variegata’, 28 cm in ‘Inspiration’ (small plants), 34 cm in ‘Inspiration’ (large plants) and 42 cm in ‘Waikiki’. Floral growth from the apical end of basal vegetative branches occurred frequently in plants growing in the most inductive treatments. Usually the vegetative branches in all of the lower parts of the plants flowered, while the branches in the four to six nodes proximal to the floral nodes remained vegetative. 3.2. Injlorescence morphology In general, inflorescences at the two to three lowest floral nodes were longest. The average length of an inflorescence was 7 cm for ‘Inspiration’ and 8 cm for ‘Variegata’
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Table 2 Branching of inflorescences in Hebe ‘Inspiration’, ‘Variegata’ and ‘Waikiki’ after cool (S/PC) or cold (9/3”C) treatments followed by forcing (25/ 19°C). Data are averages from floral plants on the basis of 2 X 7 plants and are presented as the number of branches per inflorescence Cultivar a
Treatment
‘Inspiration’ Small
‘Variegata’
‘Waikiki’
Large
Control 0 weeks
1.4bcd
9/3”C (cold) 3 weeks 6 weeks 9 weeks 12 weeks
3.2a
2.Ob 1.9b 1.7b
15.5/9.PC 3 weeks 6 weeks 9 weeks 12 weeks
3.9a 2.3a 3.2a
3.lab 2.3a 3.6a 4Sa
1.Oa
1.7ab 1.3b 1.2b O&d
0.3a 1.Oa
2.6a 1.6abc OSd 0.4d
(cool)
’ Means followed by different letters are statistically different at the 0.05 level.
and was not influenced by plant age or treatment. Inflorescences of ‘Waikiki’ were longest (18-20 cm) on plants in the cold or the 3 week cool treatment. Plants grown in the 6, 9 or 12 week cool treatments had shorter inflorescences (16, 12 and 10 cm, respectively). Inflorescences on the control plants of ‘Waikiki’ were 16 cm long. Lateral branch development on the inflorescences occurred in all cultivars and in all treatments to some extent. The least branching cultivar was ‘Variegata’ with 0.8 branches per inflorescence. Branching on the inflorescences of ‘Waikiki’ was intermediate in the control plants but increased following 3 weeks of cold or cool treatments and decreased with 6 to 12 weeks of treatment (Table 2). Most branching occurred in larger ‘Inspiration’ plants which had received a 12 week cool treatment. Branching of inflorescences in ‘Inspiration’ mostly occurred in the lowest two floral nodes, where 8.2 (larger plants) or 6.6 (smaller plants) branches were recorded after 12 weeks of cool treatment and 7.7 branches (larger plants) after 9 weeks of cool treatment. In other treatments ‘Inspiration’ plants had fewer than 3.5 branches per inflorescence at the two lowermost nodes (Fig. 1). The number of florets per inflorescence recorded after the 6 or 12 week cold or cool treatments showed no obvious correlations to treatments, as it was primarily determined by the length and branching of the inflorescences. The average number of florets per inflorescence at the first floral node was 135, 128 and 234 for small ‘Inspiration’, large ‘Inspiration’ and ‘Waikiki’, respectively, after 6 weeks of cool treatment and 168, 208 and 96 after 12 weeks of cool treatment. The ‘Waikiki’ control plants had on average 168 florets per inflorescence at the first floral node.
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Fig. 1. Branching of inflorescences in Hebe ‘Inspiration’. Proportion of flowering plants (%) 100
Proportion of flowering plants (“Yo)
a
80604020I-IA “0
3
6
9
12
Duration of chilling treatment (weeks)
Proportion of flowering plants (%)
Proportion of flowering plants (%) 100 I
0 3 6 9 12 Duration of chilling treatment (weeks)
I
C
80. 6040200. 0
3
6
9
12
Duration of chilling treatment (weeks)
0
3
6
9
12
Duration of chilling treatment (weeks)
Fig. 2. Proportion of flowering plants after chilling (15.5/9.5 or 9/3”C) within 12 weeks of forcing. (a) Hebe ‘Inspiration’, small plants; (b) Hebe ‘Inspiration’, large plants; (c) Hebe ‘Variegata’; (d) Hebe ‘Waikiki’. Bars on selected data points indicate SD.
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Table 3 Number of days to the first open floret (anthesis) in inflotescences positioned at the earliest flowering node of ‘Inspiration’ (small and large plants), ‘Variegata’ and ‘Waikiki’ following 0,3,6,9 or 12 weeks under cool or cold temperature conditions and subsequent development in forcing (25/19oC) conditions. The results include only the floral plants in each treatment of 14 plants, and the statistical analyses are restricted to within cultivars Treatment
Cultivar 1 ‘Inspiration’
‘Waikiki’
Large
N
N
N
97h
N N N 135a
N 97d 115bc 134d
N N N 139a
lllde 10&f 126c 14la
N 97b 115bc 133a
74e 94d 1lOc 12lb
N N 120a 13la
9ftgh 103fg 116d 134b
Control 0 weeks 9/3”C
‘Variegata’
Small
(cold)
3 weeks 6 weeks 9 weeks 12 weeks 15.5/9.5”c (cool) 3 weeks 6 weeks 9 weeks 12 weeks
N, no flowering within duration of experiment. ’ Means followed by different letters am statistically different at the 0.05 level.
3.3. Proportion of plants flowering After the 9 or 12 week cool treatment all ‘Inspiration’ (larger plants) and ‘Waikiki’ plants developed flowers under the forcing conditions (Fig. 2). All small ‘Inspiration’ plants that had received 6 weeks or more of cool treatment and all small plants given 12 weeks of cold also flowered. Treatments that resulted in 50-93% of the plants flowering included the 6 week cool and 12 week cold treatments with small ‘Inspiration’, the 3 week cool and 9 week cold with large ‘Inspiration’, 9 and 12 week cool with ‘Variegata’, 3 and 6 week cool and 3, 6 and 9 week cold with ‘Waikiki’. In all other treatments less than half of the population flowered. 3.4. Time to jlowering Most plants of all cultivars flowered under the forcing conditions after being grown under either the cool or cold treatment conditions for 12 weeks (Fig. 2). The total number of days to flowering was highest for the longest period of cold treatment and, depending on cultivar, was 2-13 days shorter after similar periods of cool treatment (Table 3). The most rapid flowering (74 days) occurred in the larger plants of ‘Inspiration’ which had been in the cool treatment for 3 weeks. These plants also flowered in less
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100 days under both the cool and cold 6 week exposure treatments. In contrast, the small plants of ‘Inspiration’ flowered in less than 100 days only in the 6 week cool treatment. Plants of ‘Waikiki’ flowered in all treatments, but took less than 100 days to do so only in the continuous forcing and cool 3 week exposure conditions. When assessed on the basis of time in the forcing conditions the fastest flowering (37 days) occurred in large ‘Inspiration’ plants which had been held in the cool treatment for 12 weeks. Both small and large ‘Inspiration’ plants took 10-l 1 days longer to flower when they had been grown previously under either the 9 or 12 weeks cool treatments. The slowest flowering under the forcing conditions occurred with ‘Waikiki’ where plants took 90-97 days to reach anthesis under either continuous forcing conditions or after 3 weeks of cold conditions. Flowering did not occur within the experimental period in any plants of ‘Inspiration’ or ‘Variegata’ if they had not received any cool or cold treatment (i.e. continuous forcing). Anthesis of the two inflorescences subtended by each floral node usually occurred on the same day. Inflorescences in the next, more distal node reached anthesis 2-3 days after anthesis at the above node. Inflorescences opened their florets in succession towards the apex. The flowering period for the plants in a treatment varied with the than
No. of inflorescences 12 I
No. of inflorescences
Duration of chilling treatment (weeks)
Duration of chilling treatment
No. of inflorescences
No. of inflorescences
(weeks)
12‘ lOA 864
Cl 3 6 9 12 Duration of chilling treatment (weeks)
6 9 &o 3 Duration of chilling treatment
12 (weeks)
Fig. 3. Number of inflorescences on the main shoot after chilling (15.5/9.5 or 9/3”C) within 12 weeks of forcing (at 25/ 19°C). Data include flowering plants only and are based on 2 X 7 plants per treatment. (a) Hebe ‘Inspiration’, large plants; (b) Hebe ‘Inspiration’, small plants; (c) Hebe ‘Variegata’; (d) Hebe ‘Waikiki’. Bars on selected data points indicate. SD.
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number of inflorescences per plant, but on the average it lasted three to four weeks. The inflorescences that developed were normal under all treatments except for ‘Variegata’ grown under 12 weeks of cold, where the florets on the only flowering plant developed five to six petals instead of the usual four. 3.5. Flower intensity The number of inflorescences per plant progressively increased with increasing periods of low temperature treatment and was higher in forcing after the cool than in the cold temperature treatment (Fig. 3). The highest numbers of inflorescences were observed in the 9 week cool treatments of larger ‘Inspiration’ plants and of ‘Waikiki’, where about 10 inflorescences were recorded per plant after 9 weeks of cool treatment. The flower development was partial (i.e. vegetative nodes developed after the floral nodes within 12 weeks of forcing) in all treatments which were of less than 12 weeks duration for ‘Inspiration’ (both sizes) and ‘Waikiki’.
4. Discussion 4.1. All cultivars Low temperature promoted subsequent flowering in all Hebe cultivars tested. Flowering in many other woody plant genera is controlled by temperature (Lavee, 1985). In some plants flower promotion is affected by a combination of chilling and short photoperiod as in the temperate and subtropical Leptospermum species (Zieslin, 19851, or intermediate temperature and short photoperiod which accelerates flower bud differentiation in Hydrangea macrophylla (Wallerstein and Rlinger, 1985). The present results showed that chilling at a constant 12 h photoperiod promoted flower initiation in the tested Hebe cultivars, while the possible effects of photoperiod on flowering have only been investigated to a limited extent by L.E. Noack (unpublished data, 1988). An interaction of the photoperiod with temperature regimes could exist in a similar way as in Hydrangea (Link and Shanks, 1954) and Azalea (Pettersen, 1972). Newly rooted cuttings of Hebe ‘Variegata’ and ‘Waikiki’ were used in the experiment, while the cuttings of ‘Inspiration’ were either 2 or 3 months older. As the rooted plants of ‘Inspiration’ were initially floral, probably induced prior to propagation, the inflorescences, lower branches and sometimes the leaves on the main stem were removed during a 3 week pretreatment at 25/19”C under low light intensity conditions. The leaf area and the number of nodes were therefore limited when the experiment started. Although the Hebe cuttings were taken from adult mother plants, the small plants may have been in a stage not able to form flowers when only just rooted or with only a small leaf area. In biological terms, efficient reproduction is dependent on age and size of the plants where fecundity is high and mortality low (Lacey, 1986). With ‘Waikiki’ and ‘large’ ‘Inspiration’ the maximum number of inflorescences was reached after 9 weeks of chilling in 15.5/9.5”C (Fig. 3).
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The total time to flowering increased with the duration of chilling, but when the time in the chilling treatment was excluded, the number of days to flower in forcing conditions was reduced (Table 3). This phenomenon is also seen in Hydrangea where the length of the forcing period depends on the number of cold units (Wallerstein and Rtinger, 1985) given during the cold treatment (Link and Shanks, 1954).
Both parents of Hebe ‘Inspiration’ originate in a mild coastal climate with a natural daylength ranging from 9.5 to 14.5 h and a mean annual surface temperature of 12.5”C (Macaulay and Beavis, 1983). Plants had the fastest and greatest intensity of flowering following the 15.5/9.5”C treatments compared with the 9/3”C treatments, and did not flower without chilling (Table 3, Fig. 2). An earlier phytotron experiment (Noack, 1991) examined combinations of higher temperatures and 8 or 20 h photoperiods. After 5 months, 100% flowering was obtained in ‘Inspiration’ at both 15 and 20°C at both photoperiods. In the 10°C treatment at both photoperiods and in the 25”C/20 h temperature/photoperiod combination none of the plants flowered, and only 60% of the 25”C/8 h photoperiod plants flowered within the 5 month treatment period. This indicates a strong influence of temperature on growth and flowering, while a short photoperiod might promote the flower formation and/or development at higher temperatures. Propagation time and subsequent growth prior to chilling had a major effect on the ability to flower after the different chilling temperatures and durations. The larger ‘Inspiration’ plants, propagated in July, flowered earlier and more abundantly than the smaller plants propagated in August which needed a longer chilling period. Plant height, the number of leaf pairs and the number of nodes on the main stem at the start of the chilling period were greater for the plants propagated in July compared with those in August. Schwabe (1987) suggested that a high number of nodes between the apical meristem and the root system might permit flowering at a certain size. The branches of the ‘Inspiration’ plants did not flower until they had reached a minimum of six nodes, and more inflorescences were produced in those treatments where the required six nodes were reached quickly. This phenomenon might be caused by the overall maturity of the leaves on the branches, as the ratio between mature (sources) and immature (sinks which may produce floral inhibitors) leaves within branches has been shown to influence floral induction (Bemier et al., 1981). Low temperatures often lead to the accumulation of carbohydrates in the shoot apex which may affect the response obtained (Evans, 1971). 4.3. Hebe X fransiscana ‘Varieguta’ The period of chilling required for flowering was longer in ‘Variegata’ than in the other cultivars tested. One of the parents of ‘Variegata’, Hebe speciosa, originates from the eastern coastal region of the central North Island, while the geographical origin of the other parent, Hebe elliptica, is uncertain and could be native to all parts of New Zealand, South America or the Falkland Islands (Heenan, 1993). In most of these areas
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the climate is coastal and temperate with a mean annual surface temperature of 10°C and a daylength ranging from 8 to 15 h (Macaulay and Beavis, 1983). In addition, the natural growth season is shorter than that for the parents of ‘Inspiration’ and a long chilling period would be experienced in the natural environment compared with the parents of ‘Inspiration’. Plants grew better and initiated flowering faster in response to chilling at 15.5/9.Y than at 9/3”C. Node and leaf pair number was low (2.4 and 4.8, respectively) at the start of the experiment, and there may be a need to attain a certain minimum size before flower initiation is possible. This certain size was not determined, but plants grew faster in the 15.5/9.5”C treatment than in the 9/3”C treatment. The low photosynthetic activity of the achlorophyllous sections of the variegated leaves probably slows down the growth rate and therefore delays the effect of chilling. Kristensen and Adriansen (1988) support the present results by suggesting that flower initiation may occur when plants of ‘Variegata’ have six to eight open leaf pairs and are placed in inductive environmental conditions. For Hydrangea a minimal number of leaves is required before flower bud formation will occur (Shanks and Link, 195 1). In Ribes nigmm vegetatively propagated plants require a minimum of 20 nodes before floral initiation is possible (Wright, 1985). It is suspected that the number of leaf pairs needs to be higher in this variegated cultivar than in the closely related green leaved cultivar ‘Blue Gem’ because of the lack of potential photosynthesis in a high proportion of the leaf area. 4.4. Hebe ‘Waikiki The parentage of this natural hybrid ‘Waikiki’ is not known (Moore, 1961; J. Hobbs and P. Gamock-Jones, personal communication, 1989). The ease of flower development, even under the continuous forcing temperature, indicates that the need for chilling is low and that the parents are probably from a mild climate such as the northern region of the North Island of New Zealand. Moore (1961) suggests that one parent of ‘Waikiki’ could be Hebe obtusatawhich originates from a limited area north of Auckland. ‘Waikiki’ did not have an absolute need of chilling in order to flower. However, without chilling only 40% of the control plants flowered within 14 weeks of forcing. Chilling increased the proportion of plants which flowered, so that after 6 weeks of chilling (15.5/9.5”C or 9/3”C) more than 90% of the plants, which were very small at the start of the experiment, flowered within 12 weeks of forcing. ‘Waikiki’ grew taller than any of the other cultivars tested, but node number was similar to that of the other cultivars. This might have influenced the development of the upper inflorescences at upper nodes which were shorter than those lower on the main stem. Bemier et al. (1981), however, suggest that root growth decreases when plants enter the floral stage and root restrictions may even promote continuous flower development. Lacey (1986) supports this suggestion by indicating that slow plant growth promotes flowering. Acknowledgements
We are grateful to R. Southward and Jill Stanley, for technical assistance, Dr. K. Krlstensen for statistical assistance, and O.V. Christensen for criticism of the manuscript.
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Financial support was supplied by the DSIR Fruit and Trees, the New Zealand Nursery Research Centre, the Danish Research Academy and The Joint Committee for Agricultural Research and Experiments.
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