Potted plant production of Leptospermum scoparium hybrids as affected by temperature, photoperiod, irradiance and fertiliser treatments

ABSTRACT

Leptospermum scoparium ‘Kea’, ‘Kiwi’ and ‘Ruru’ were evaluated for production as potted plants. Following environmental effects were investigated; (a) Temperatures regimes for 40 days each for two periods; (b) Photoperiods for 25 and 50 days; (c) Combined effects of temperature and duration; (d) Combined effects of temperature and irradiance and (e) Combined effects of day length, irradiance and fertilisers treatments. Floral initiation was completed following 25 days of a 9–12 h short-day (SD) photoperiod at 15/13°C (day/night). Floral development was accelerated at 20/18°C during the first period, and at 25/23°C during the second period. Flower buds were initiated at 14–15/11–14°C and SD. Although the irradiance and nutrition effects were less prominent than temperature and photoperiod, 25/22°C, high irradiance discharge (HID), and 100 ppm of nitrogen from a liquid fertiliser (LF) with 0.5 g of slow-release fertiliser (SRF) are suggested optimum for flowering. The following conditions are recommended for production of compact plants with maximum number of flowers year round starting from small propagules of three L. scoparium hybrids: 9–12 h SD under 15/13°C (day/night) for 25 days, at 20/17°C for 30 days, under 42 Wm⁻² HID, and fertilisation of 100 ppm N in LF with 0.5 g of SRF.

KEYWORDS:

• Forcing temperature
• irradiance
• liquid fertiliser
• potted plant production
• short-day photoperiod
• slow-release fertiliser

Introduction

The genus Leptospermum, Myrtaceae, commonly referred to as Tea Tree, comprises about 83–86 species and is distributed in Australia, New Zealand and escaped to other countries including Africa and Europe (https://www.anbg.gov.au/leptospermum/, accessed on October 27, 2019; https://www.cabi.org/isc/datasheet/30097#tosummaryOfInvasiveness, accessed on October 27, 2019; Dawson 1997). Several cultivars with compact growth characteristics have been developed from Manuka (Leptospermum scoparium J. R. Forst. & G. Forst) (http://www.nzpcn.org.nz/flora_details.aspx?ID=2302, accessed on March 27, 2019), and suitability for production as a pot plant (Bicknell 1985; Dawson 2009, 2010).

L. scoparium indigenous to New Zealand (Williams 1981; Yin et al. 1984; Stephens et al. 2005) is largely distributed near the coastal areas (between 0 and 10 km) at altitudes less than 100 m and temperatures ranging between 0.5 below zero and 4.2°C (coldest month), and between 11.9 and 14.5°C (warmest month) where manuka communities were studied (Yin et al. 1984). Manuka was listed as a noxious weed until 1900 but is now considered to be a valuable ornamental plant and essential oil based on its natural history and human perception (Derraik 2008). Flowering occurs from mid-November to the end of March, with a full bloom in spring or early summer in Australia (https://www.anbg.gov.au/leptospermum/leptospermum-scoparium.html. Accessed on October 27, 2019).

Zieslin and Gottesman (1986) reported that L. scoparium plants grown in a greenhouse flowered under a photoperiod shorter than 12 h at both 20/26°C and 10/20°C (day/night). However, over a 12–14 h photoperiod, flowering occurred only at low temperatures, indicating that the effect of photoperiod is temperature-dependent. Poor flowering has been reported at lower temperatures (below 16°C) (Dawson and King 1993). Bicknell and Jaksons (2018) also reported that short photoperiods less than 9.5 h for 4 weeks or longer were inductive treatments for flowering for ‘Crimson Glory’ and ‘Nanum Ruru’. However, plants failed to flower at constant 20°C or developed poorly at constant 10°C showing interaction with photoperiod. For floral initiation and full flowering of Western Australian flower, Chamelaucium uncinatum Schauer (Myrtaceae), 4-weeks of short-day (SD) conditions at either 20/14°C or 20/18°C (day/night) are required, although fewer flowers were reported to be initiated under long-day (LD) conditions at 20/14°C (Shillo et al. 1985). For C. uncinatum ‘Purple Pride’, the critical photoperiod for flowering for 3–4-month-old plants was reported to be shorter than 13 and 13.5 h. Cool (12–15°C) temperatures imposed for 3–6 weeks under SD conditions inhibited the induction of flowering. Temperatures of 20–25°C and high irradiance favoured flowering of ‘Purple Pride’.

However, the temperature, photoperiod, irradiance and nutrition requirements for controlled flowering of L. scoparium to produce compact potted plants in small pots starting from small propagules, are not available, especially starting from small propagules produced under controlled environments. Therefore, the objectives of this study were to investigate several environmental effects on the growth and flowering of ‘Kea’, ‘Kiwi’, or ‘Ruru’ for the production of potted plants starting from small propagules. These include: (a) Photoperiod (9, 12 and 15 h) for 25 and 50 days. (b) Temperature [15/13, 20/18 and 25/21°C (08:00–16:00 h/16:00–08:00 h; day/night)] for 40 days from May 18 to June 26, and from June 27 to August 2. (c) Temperature treatments at 14/11, 17/14, 20/17 and 23/20°C (day/night) in growth chambers for 30, 45 and 60 days arranged in a factorial design. (d) Temperature treatments at 15/13, 18/16, 21/19 and 25/22°C (day/night), combined with two irradiance levels under natural day (ND) length and high irradiance discharge (HID) treatment at 42 W^.m⁻² from 08:00 to 16:00 h, provided by supplemental lighting from 400 W high-pressure sodium lamps and shade cloth from Jan. 19 to Mar. 19. (e) Two irradiance levels (ND and HID from 08:00 to 16:00 h), combined with three rates of slow-release fertiliser (SRF) (14N-6.0P-8.1 K) and three levels of a liquid fertiliser (LF) (20N-8.6P-11.4 K water-soluble fertiliser) arranged in a 2 × 3 × 3 factorial design.

Materials and methods

Plant materials and common cultural practices

Mother plants of L. scoparium (Nanum Group) ‘Kea’, ‘Ruru’ and ‘Kiwi’ were propagated by rooting of stem tip cuttings from stock plants grown in the greenhouse in all experiments. Five-cm long stem tip cuttings, without using rooting promoting hormones, were propagated using an Oasis 102 count wedge tray and medium foam cubes (Griffin Greenhouse Supply, Kent, OH., USA) as specified in each experiment. Rooted cuttings (Figure 1A) were transplanted into 7.5- or 10 cm pots filled with ProMix BX (ProMix BX Mycorrhizae, Premier Tech Horticulture, Quakertown, PA., USA) for 2–3 weeks. During culture, slow-release fertiliser (SRF) was applied at the beginning of the treatment; 0.3 g/7.5 cm pot and with 0.5 g/10 cm pot plus 200 ppm N liquid fertiliser (LF), once a month, unless otherwise indicated for each experiment.

Figure 1. Rooted cuttings of L. scoparium ready to be transplanted into 10 cm pots (frame A), and appearance of ‘Ruru’ (left) and ‘Kea’ (right) in 10 cm pots at full flowering (frame B).

Expt 1. The effect of temperature on the growth and flowering of ‘Kea’ and ‘Ruru’

Stem tip cuttings obtained from stock plants grown in a greenhouse maintained at 21/15.6°C (08:00–16:00/16:00–08:00 h, day/night) were propagated on 1 February 1996. Rooted propagules were transplanted into 7.5 cm pots filled with ProMix BX on March 8, and pruned, leaving 3–4 long shoots on April 24, and grown in a greenhouse maintained at 18/12.8°C.

Plants were treated with three different temperatures at two stages under a 3 × 3 combination treatment in growth chambers; the first stage was from May 18 to June 26 for 40 days and the second stage from June 27 to August 2 for 40 d at 15/13°C, 20/18 and 25/21°C (08:00–16:00 h, day/night). Irradiance during the day was 35 W m⁻² (400–700 nm), provided by incandescent bulbs and cool white fluorescent tubes. Data were analysed following a three-way factorial design (factor A, cultivar; factor B, first-stage temperature; factor C second-stage temperature) with 25 plants per temperature treatment. The days to flowering of the first bud were counted from May 18 when the temperature treatment began.

Ten days after anthesis of the first flower bud, the degree of open flowers and flower buds prior to anthesis were rated, due to the difficulty in counting the total number of flowers. The following scales were defined: scale 0, no open flower or flower buds; scale 1, one-third of plants covered with flowers or flower buds; scale 2, two-thirds of the plant covered with flowers or flower buds; and scale 3, the entire canopy of the plant covered with flowers or flower buds. Scales of 0.5, 1.5 and 2.5 were also noted between scales 0, 1, 2 and 3. Scales were also recorded 30 days after anthesis of the first flower. Means were compared by Duncan’s multiple range test at P ≤ .01.

Expt 2. The effect of photoperiod on the growth and flowering of ‘Kea’ and ‘Ruru’

L. scoparium ‘Kea’ and ‘Ruru’ propagated on 21 January 1995 were transplanted into 7.5 cm pots, grown in a greenhouse maintained at 21/15.5°C under ND, pruned leaving 5 cm shoots on 25 April 1996, and transplanted into 10 cm pot on July 30. From August 2, plants were grown under three photoperiods (9, 12 and 15 h) for 25 and 50 days in growth chambers maintained at 21/15.5°C (07:00–16:00/16:00–07:00 h, day/night). Incandescent light and two sets of fluorescent tubes were used during the day to provide 35 and 29 W m⁻² with only one set of fluorescent tubes (Table 1).

Table 1. Definition of photoperiod, temperature and light treatments regimes provided by incandescent and fluorescent light sources in the growth chambers (Expt. 2).

Photoperiod (h)	Temperature and time of treatment^a	Light sources and time of lighting^b
		Incandescent and fluorescent 1	Fluorescent 1 and 2
9	07:00–16:00 h	07:00–16:00 h	07:00–16:00 h
12	07:00–16:00 h	07:00–19:00 h	07:00–19:00 h
15	07:00–16:00 h	07:00–22:00 h	06:00–21:00 h

^aDay/night temperature was 21/15.5°C (07:00–16:00/16:00–07:00 h).

^bIncandescent light and two sets of fluorescent 1 and 2 tubes were used under irradiance of 35 W m⁻², and when only one set of fluorescent 1 tubes was used the irradiance was 29 W m⁻².

After the photoperiod treatments, plants were transferred to a greenhouse maintained at 21/15.5°C (day/night) until completion of the experiment on 4 February 1997. Ten plants were used for each photoperiod × treatment duration. The average monthly photoperiods over the year ranged from 9 h 30 min (December 21) to 14 h 60 min (June 21). The number of days to flowering was recorded from August 2. The flowering dates were recorded when three flowers reached anthesis, and the total number of flowers that opened was recorded 5 days after the flowering date.

Expt 3. The effect of temperature and duration on the growth and flowering of ‘Kea’ and ‘Ruru’

L. scoparium ‘Ruru’ and ‘Kea’ plants propagated on 30 January 1997 were grown in a greenhouse maintained at 20/15.5°C and grown at 25–28/21–23°C from June 15 to October 1. From 15 February 1998, all plants received 200 ppm N from LF at 2-week intervals until the end of the experiment. On 1 October plants were pruned leaving 6 cm shoots, and the photoperiod treatments began. Four temperature regimes, 14/11°C, 17/14°C, 20/17°C and 23/20°C (day/night) were administered in growth chambers for 30, 45 and 60 days in a factorial design with 10 plants per temperature × duration treatment. After the respective temperature treatments, plants were transferred to a greenhouse maintained at 21/15.5°C (day/night) and under HID conditions. When three flowers opened, the date of flowering was recorded, and the number of days to flowering were counted from October 1. Flowers and flower buds were rated according to the defined scales described for Expt. 1.

Expt 4. The effect of temperature, irradiance and fertiliser treatment on the growth and flowering of ‘Kea’, ‘Kiwi’ and ‘Ruru’

Five-cm-long stem tip cuttings of ‘Ruru’, ‘Kea’ and ‘Kiwi’ were propagated on 31 October 1997, transplanted into 7.5 cm pots and pruned, leaving 3 cm-long main shoots. From 19 January to 19 March 1998, plants were exposed to temperatures of 15/13°C, 18/16°C, 21/19°C and 25/22°C (day/night). At each temperature regime, plants were exposed to two irradiance levels of an ND and HID at 42 W m⁻² provided by supplemental lighting from 400 W high-pressure sodium lamps and a shade cloth from 08:00 h to 6:00 h. During temperature and irradiance treatments, plants were fertilised with 0, 100, or 200 ppm N by LF.

After the treatments, plants were transplanted to 10 cm pots, grown in a greenhouse maintained at 21/19°C (day/night) and received SRF at transplanting. Plants were fertilised with 100 ppm N by LF every 2 weeks until the end of the experiment. Ten plants were used per treatment, with each plant considered as a sampling unit. When three flowers opened, the flowering date and the length of the longest shoot were recorded. The number of days to flowering was recorded from January 19 and the experiment was terminated in 153 days.

Expt 5. Effect of light irradiance, slow release and liquid fertiliser treatment on flowering of ‘Ruru’

Stem tip cuttings from L. scoparium ‘Ruru’ were rooted on 26 January 1997, and transplanted into 10 cm pots on March 15; the treatments began on March 19. Plants were exposed to two irradiance levels (ND and HID, from 08:00 h to 16:00 h), three rates of SRF (0, 0.5 and 1.0 g per pot) and three levels of LF (0, 100 and 200 ppm N) arranged in a 2 × 3 × 3 factorial design, until 19 May in a greenhouse maintained at 21/15.5°C (day/night). Ten plants were used per treatment. Fertilisation was applied by an SLF to the surface of the growth medium along with a liquid feed every 2 weeks. The greenhouse temperature was maintained at 26/21°C (day/night) from 1 June and at 21/15.5°C (day/night) from 1 September until completion of the experiment on 29 November. Flowers and flower buds were rated using the defined scale described in Expt. 1.

Results

Expt 1. The effect of temperature on the growth and flowering of ‘Kea’ and ‘Ruru’

When data were analysed by cultivar and temperature for the two treatment periods included in the model, there was a significant difference between cultivars (data not presented). Therefore, data were reanalysed for each cultivar. ‘Ruru’ flowered earlier ‘Kea’ regardless of temperature treatments, ranging from 82 to 131 days, and from 93 to 138 days, respectively (Table 2). In both cultivars, low temperature (15/13°C) during the first period accelerated flowering regardless of the temperature during the second period. High temperature during the first period (25/23°C) delayed flowering taking longer than 124–131 days in ‘Ruru’ and 130–138 days in ‘Kea’. Both cultivars can be grown at 15/13°C to induce early flowering compared with 25/23°C during the first period.

Table 2. The effect of temperature treatments on the growth and flowering of L. scoparium ‘Ruru’ and ‘Kea’ (Expt. 1).

Temperature (°C)		No. of days to flower	10 days after anthesis of the first flower				30 days after anthesis of the first flower
First period (FPT)^a	Second Period (SPT)		No of flowers	Flower scale ^b	Flower buds	Ratio of FS/FS + FB	Flower scale	Flower buds
‘Ruru’
15/13^c	15/13	91 ef ^d	14.3 c	1.3 b	1.2 c	0.52 c	2.6 a	2.4 a
	20/18	87 f	9.4 de	1.5 a	1.7 b	0.47 d	1.7 d	2.3 a
	25/23	82 f	5.3 g	1.1 c	0.6 e	0.65 b	1.1 f	2.3 a
20/18	15/13	103 d	10.3 d	0.9 d	2.1 a	0.30 e	2.2 b	1.0 d
	20/18	96 e	7.2 f	0.9 d	1.0 d	0.47 d	1.6 d	1.4 c
	25/23	117 c	22.0 a	1.4 ab	1.2 c	0.54 c	2.1 bc	2.0 b
25/23	15/13	124 b	15.7 b	0.9 d	0.6 e	0.60 b	1.4 e	1.4 c
	20/18	131 a	9.4 de	0.5 f	0.6 e	0.45 d	0.5 g	0.6 e
	25/23	129 a	16.0 b	0.7 e	0.2 f	0.78 a	0.4 g	0.4 f
Level of significance^e
FPT		**	**	**	**	**	**	**
SPT		**	ns	ns	**	**	**	ns
FPT × SPT		**	**	**	**	*	**	**
‘Kea’
15/13	15/13	105 e	21.2 d	1.4 d	1.2 b	0.54 c	0.2 f	0.2 e
	20/18	93 g	15.4 e	1.6 cd	1.5 a	0.52 c	0.2 f	0.2 e
	25/23	117 d	22.5 d	1.1 e	1.5 a	0.42 d	0.8 c	0.8 c
20/18	15/13	106 e	27.1 c	1.7 bc	1.4 a	0.55 c	0.2 f	0.2 e
	20/18	100 f	20.5 d	1.8 b	0.8 c	0.69 b	0.2 f	0.2 e
	25/23	125 c	62.6 a	2.1 a	1.5 a	0.58 c	1.7 a	1.8 a
25/23	15/13	130 b	52.2 b	1.7 bc	0.5 d	0.77 b	1.5 b	1.6 b
	20/18	138 a	11.7 f	0.4 f	0.0 e	1.00 a	0.5 d	0.4 d
	25/23	136 a	13.7 ef	0.4 f	0.0 e	1.00 a	0.3 e	0.5 d
Level of significance
FPT		**	**	**	**	**	**	**
SPT		**	**	**	ns	**	**	**
FPT × SPT		**	**	**	ns	*	**	**

^aThe first and second temperature stages for 40 days per period were given from May 18 to June 26 and from June 27 to August 2, respectively.

^b Scale 0, no open flower or flower buds; scale 1, one-third of plant covered with flowers or flower buds; scale 2, two-third of the plant covered with flowers or flower buds; and scale 3, the entire canopy of the plant covered with flowers or flower buds. Scales 0.5, 1.5 and 2.5 were also noted between scales 0, 1, 2 and 3.

^cTo facilitate the readers understanding, the rows which represent the best treatment are highlighted in bold face letters.

^dMean separation by Duncan’s multiple test at P ≤ .01, F-test. Means with different letters in each column are significantly different from each other.

^ens, *, **; non-significant, significant at P ≤ .05 and <.01, F-test.

The number of flowers 10 days after anthesis of the first flower was the highest when ‘Ruru’ (22.0 flowers) and ‘Kea’ (62.6 flowers) were grown at 20/18°C during the first period and 25/23°C during the second period, followed by 25/22 and 15/13°C, especially with ‘Kea’ (52.2 flowers) (Table 2). Rather than counting small flowers and flower buds, which was not easy, flower scales were introduced to estimate their quantities. Flower scales and flower bud scales were highest at 20/18°C during the first period followed by 25/23°C during the second period. Flower bud scales were generally higher when plants were grown at low temperature (15/13°C) during the first and the second period. Thirty-days after the first flowers appeared, higher scales for both flowers and flower buds were recorded when ‘Ruru’ was grown at 15/13°C during the first period and at all three temperature regimes during the second period. However, ‘Kea’ did not show this trend.

Expt 2. The effect of photoperiod treatment on the growth and flowering of ‘Kea’ and ‘Ruru’

Flowering was the earliest when ‘Ruru’ received 25 days of a 9 h photoperiod during the first period, and 12 h of photoperiod during the second period (97 ± 5.9 days) or 50 days during the same photoperiods (96 ± 4.5 days) (Table 3). Flowering of ‘Kea’ was the earliest when plants received a 9 and 12 h photoperiod during the first and second photoperiod for 25 (98 ± 3.3 days) or 50 days (95 ± 3.8 days), followed by 9 and 12 h for 25 (102 ± 3.8 days) or 50 (102 ± 5.8 days) days. Under these photoperiod conditions, flowering was 100% observed at both 25 and 50 days.

Table 3. Effect of photoperiod treatments for two different durations on growth and flowering of L. scoparium ‘Ruru’ and ‘Kea’ (Expt. 2).

Cultivar	Photoperiod (h)^a		No. of days to flower		Flowering percentage		No. of flowers that reached anthesis
	1st	2nd	25 days^b	50 days	25 days	50 days	25 days	50 days
‘Ruru'	9^d	9	136 ± 8.8^c	143 ± 18.8	100	100	12.5 ± 10.65	5 ± 3.50
		12	97 ± 5.0	101 ± 6.9	100	100	27.8 ± 8.78	8.3 ± 5.52
		15	132 ± 7.4	132 ± 24.8	90	80	15.4 ± 12.73	11± 8.55
	12	9	125 ± 21.3	134 ± 22.2	100	100	14.2 ± 11.47	4.3 ± 4.35
		12	97 ± 5.9	96 ± 4.5	100	100	10.7 ± 4.05	11.8 ± 7.48
		15	121 ± 15.1	127 ± 20.8	100	100	9.2 ± 11.07	5.5 ± 5.55
	15	9	133 ± 7.7	160 ± 16.5	100	50	15.4 ± 9.03	4.4 ± 2.30
		12	112 ± 4.9	111 ± 4.2	100	70	17.7 ± 6.53	14.3 ± 7.74
		15	137 ± 10.2	169 ± 18.5	100	30	24.1 ± 10.39	1.7 ± 1.15
‘Kea'	9	9	154 ± 2.7	152 ± 13.8	100	100	15.1 ± 9.18	5.7 ± 6.41
		12	102 ± 3.8	102 ± 5.2	100	100	14.7 ± 12.31	17.3 ± 10.52
		15	142 ± 3.6	103 ± 17.1	100	100	13.7 ± 7.83	10.2 ± 7.55
	12	9	142 ± 20.7	143 ± 28.9	100	100	16.2 ± 12.22	8.5 ± 7.27
		12	98 ± 3.3	95 ± 3.8	100	100	21.3 ± 10.93	24.3 ± 10.74
		15	140 ± 3.7	154 ± 3.0	100	100	18.9 ± 6.45	15.1 ± 10.64
	15	9	149 ± 4.4	151 ± 5.6	80	20	5.3 ± 3.69	4.3 ± 2.52
		12	123 ± 4.6	124 ± 6.9	100	60	10.2 ± 5.37	13.6 ± 9.70
		15	146 ± 5.4	152 ± 1.2	100	40	5.8 ± 3.12	3.0 ± 2.16

^a,bThree photoperiod (9, 12 and 15 h) treatments for 25 and 50 days starting from 2 August.

^cMean and standard deviation from 10 plants per treatment.

^dTo facilitate the readers understanding, the rows which represent the best treatment are highlighted in bold face letters.

Flowering in both cultivars was delayed with a photoperiod of 9 h over the two periods for 50 days, and took longer than 136 days for ‘Ruru’ and 154 days for ‘Kea’ or under a 9 h photoperiod during the second photoperiod (Table 3). Furthermore, flowering was delayed when plants were subjected to a 15 h photoperiod during the first period, particularly over 25 days in both cultivars, and was further delayed when forced at 15 h during the first and second period for ‘Ruru’ (169 days) and ‘Kea’ (152 days). When ‘Ruru’ was subjected to photoperiods of 25 days, all plants flowered at 100%. ‘Kea’ also flowered at 100%; however, those that received a 15 h photoperiod followed by a 9 h photoperiod flowered at 80%. When both cultivars received 50 days of a 15 h photoperiod, flowering percentages ranged from 30% to 70% for ‘Ruru’ and 20%–60% for ‘Kea’ depending on the temperature (Table 3).

The number of flowers 5 days after anthesis for ‘Ruru’ was the highest (27.8 flowers) when grown under photoperiods of 9 and 12 h for 25 days, and the second highest (24.1 flowers) when grown under two photoperiods of 15 h; however, only 8.3 and 1.7 flowers were counted, respectively, when treatments lasted for 50 days. For ‘Kea’, the number of flowers when treated for 50 days was 17.3 with two photoperiods of 9 and 12 h, which was higher than the number of flowers when treated for 25 days. Additionally, the number of flowers was higher (24.3) when the plants received 50 days of two 12 h photoperiods compared with 25 days (21.3). The number of flowers was less than 5.8 when plants received a 15 h photoperiod during the first and second treatment periods. The standard deviations were very high, sometimes exceeding the mean values, regardless of the photoperiod and treatment durations.

Expt 3. The effect of temperature and duration of temperature treatment on growth and flowering of ‘Kea’ and ‘Ruru’

Flowering in L. scoparium ‘Ruru’ and ‘Kea’ counted from 1 October (when 182-day-old plants were pruned leaving 6 cm shoots and temperature treatments began) took less than 46 days in both cultivars when plants were grown at 24/21°C for 30 days; extended temperature treatment did not further accelerate flowering (Table 4). Flowering took less than 55 days regardless of treatment durations lasting for up to 60 days at 20/17°C. Flowering was delayed when temperatures were lowered to 17/14°C and 14/11°C, especially when plants were treated for 45 days, taking 73 and 70 days, respectively.

Table 4. Effect of temperature and treatment duration for L. scoparium ‘Kea’ and ‘Ruru’ on flowering (Expt. 3).

Temperature^a [Temp. day/night (°C)]	No. of days of temp. treatment duration^a	No. of days to flower		Scales of^b				Ratio of flower buds/flower bud + bud
				Flower		Buds
		‘Kea’	‘Ruru’	‘Kea’	‘Ruru’	‘Kea’	‘Ruru’	‘Kea’	‘Ruru’
14/11	30	65 b^c	60 b	1.5 a	2.0 a	1.4 b	1.3 b	0.52 a	0.61 a
	45	73 a	62 b	1.8 a	2.2 a	1.4 b	1.3 b	0.56 a	0.63 a
	60	71 a	61 b	1.7 a	2.2 a	1.5 ab	1.3 b	0.53 a	0.63 a
17/14	30	65 b	60 b	1.3 ab	1.8 a	1.3 b	1.5 ab	0.50 ab	0.55 b
	45	70 a	64 ab	1.0 b	2.0 a	1.0 b	1.1 b	0.50 ab	0.65 a
	60	67 b	68 a	1.8 a	2.0 a	1.4 b	1.1 b	0.56 a	0.65 a
20/17^d	30	53 c	49 cd	1.2 b	1.7 b	1.9 a	1.7 a	0.39 c	0.50 b
	45	55 c	50 c	1.3 ab	1.3 b	1.9 a	1.4 ab	0.41 c	0.48 b
	60	52 c	51 c	1.5 a	1.8 a	1.6 ab	1.3 b	0.48 b	0.58 ab
24/21	30	46 d	46 cd	1.1 b	1.6 b	1.4 b	1.2 b	0.44 bc	0.57 ab
	45	44 d	45 cd	1.2 b	1.4 b	1.7 ab	1.5 ab	0.41 c	0.48 b
	60	44 d	45 cd	1.1 b	1.4 b	1.5 ab	1.6 a	0.42 c	0.47 b
Level of significance^e
Temp		**	**	**	**	**	**	**	**
Duration		ns	ns	ns	ns	ns	ns	ns	ns
Temp × duration		ns	ns	ns	ns	ns	ns	ns	ns

^aTemperature treatments started from October 1 for 35, 45 and 60 days.

^bScale 0, no open flower or flower buds; scale 1; one-third of plant covered with flowers or flower buds; scale 2, two-thirds of plant covered with flowers or flower buds; and scale 3, the entire canopy of the plant covered with flowers or flower buds. Scales 0.5, 1.5 and 2.5 were also noted between scales 0, 1, 2 and 3.

^cMean separation by Duncan’s multiple test at P ≤ .01, F-test. Means with different letters in each column are significantly different from each other.

^dTo facilitate the readers understanding, the rows which represent the best treatment are highlighted in bold face letters.

^ens, *, **; non-significance, significant at P  ≤ .05 and P ≤ .01, respectively, according to F-test.

Scale ratings of open flowers were less than 1.2 for ‘Kea’ and 1.6 for ‘Ruru’ when plants were grown at 24/21°C and did not differ greatly when grown at 20/17°C (Table 4). As the temperatures were low, the scale ratings for open flowers were increased in both cultivars to 1.8 for ‘Kea’ at 14/11°C for 45 days and 17/14°C for 60 days. For ‘Ruru’, scale ratings of flower buds were generally higher than 1.8 when plants were grown at 14/11 and 17/14°C, regardless of the treatment duration. However, scale ratings of flower buds did not differ between cultivars and temperatures and treatment durations, although ratings were higher than 1.4 for ‘Kea’ and 1.2 for ‘Ruru’ when plants were grown at 20/17°C and 24/21°C, and 1.0 for ‘Kea’ and 1.1 for ‘Ruru’ when grown at 14/11°C and 17/14°C, respectively.

Expt 4. The effect of temperature, irradiance and nutrition treatment on the growth and flowering of ‘Kea’, ‘Ruru’ and ‘Kiwi’

Among the three cultivars evaluated, ‘Ruru’ did not flower for 153 days (2 June) when data collection was terminated following the start of treatment on 19 January (Table 5). ‘Kea’ flowered after 143–151 days; however, due to high variation, there was no significant difference in the number of days to flower between temperature and irradiance treatments. Flowering rates were higher, reaching 93% when grown under 21/19°C and 25/22°C compared with those when grown under 15/13°C, especially when grown under HID as compared to ND. Flowering rates of ‘Kiwi’ were also increased when grown under higher temperatures compared with those under 15/13°C; however, HID had no effect on flowering rates when grown at 25/22°C. The length of the longest shoot in ‘Kiwi’ was >20.6 cm when grown at 15/13°C under HID, which was longer than those of ‘Ruru’ (10.3–13.3 cm) and ‘Kea’ (13.0–14.9 cm).

Table 5. Effect of temperature and irradiance on the growth and flowering of L. scoparium ‘Kea’ and ‘Kiwi’, and ‘Ruru’ (Expt. 4)^a

Temperature (°C, day/night)	Irradiance^b	‘Ruru’			‘Kea’			‘Kiwi’
		No. of days to flower^c	Flowering rates	Shoot length (cm)	No. of days to flower	Flowering rates	Shoot length (cm)	No. of days to flower	Flowering rates	Shoot length (cm)
15/13	ND	–^d	0	10.3 ± 0.46^e	144 ± 4.0	16.7	13.2 + 0.89	146 ± 0.0	3.3	23.7 ± 0.96
	HID	–	0	11.5 ± 0.93	145 ± 6.6	26.7	13.5 ± 1.42	142 ± 13.6	16.7	20.7 ± 3.92
18/16	ND	–	0	12.3 ± 0.60	151 ± 0.0	6.7	13.0 ± 0.69	136 ± 9.9	26.7	20.6 ± 3.43
	HID	–	0	12.3 ± 1.56	147 ± 5.3	16.7	13.9 ± 0.70	139 ± 8.9	30.0	20.9 ± 2.55
21/19^f	ND	–	0	11.9 ± 1.02	143 ± 1.7	57.7	13.6 ± 0.67	145 ± 1.5	43.3	23.0 ± 0.50
	HID	–	0	13.2 ± 0.45	143 ± 1.5	93.3	13.8 ± 0.55	137 ± 12.7	60.0	24.3 ± 1.55
25/22	ND	–	0	12.1 ± 0.44	146 ± 4.7	70.1	13.7 ± 0.56	145 ± 3.2	50.0	23.4 ± 1.70
	HID	–	0	13.3 ± 0.95	147 ± 0.6	93.3	14.9 ± 0.80	146 ± 1.7	56.7	24.0 ± 2.21

^aData on fertiliser treatments were pooled due to a lack of significance among fertiliser treatments due to high variation in each cultivar.

^bNatural photoperiod and irradiance (ND) and high irradiance treatment at 42 W m⁻² from 08:00 to 16:00 h (HID) from January 19 to March 19.

^cNumber of days to flower was counted from January 19.

^d‘Ruru’ failed to reach anthesis until the experiment was completed on June 2.

^eMean and standard deviation from 10 plants per treatment.

^fTo facilitate the readers understanding, the rows which represent the best treatment are highlighted in bold face letters.

Expt 5. Effect of light irradiance, slow release and liquid fertiliser treatments on the flowering of ‘Ruru’

Flowering of ‘Ruru’ only influenced by irradiance (IR) was delayed by HID treatment; ranging from 218 to 226 days under ND compared to from 221 to 231 days under HID. There were no interactions between irradiance, SRF and LF treatments (Table 6).

Table 6. The effect of light irradiance, slow release and liquid fertiliser treatment on the growth and flowering of L. scoparium ‘Ruru’ (Expt. 5).

Treatment			No. of		Flowering scale^d	Plant height (cm)
irradiance^a	Fertiliser^b
	Slow release (g)	Liquid (ppm)	days to flower^c	flowers
ND^e	0	0	221 bc^f	8.6 abc	1.7 ab	20.6 bc
	0	100	220 bc	7.5 bc	1.7 ab	20.5 bc
	0	200	224 abc	8.4 abc	1.6 ab	23.3 abc
	0.5	0	218 c	6.4 c	1.25 bc	21.5 bc
	0.5	100	224 abc	12.1 a	2.0 a	21.3 abc
	0.5	200	220 bc	7.6 bc	1.75 ab	23.2 abc
	1	0	225 ab	8.9 abc	1.7 ab	24.7 ab
	1	100	226 ab	12.0 a	1.85 ab	23.8 abc
	1	200	225 ab	11.1 a	1.45 abc	22.8 abc
HID	0	0	227 a	9.1 abc	1.35 bc	20.0 c
	0	100	225 ab	10.3 a	1.35 bc	23.1 abc
	0	200	231 a	7.0 bc	1.1 c	23.7 abc
	0.5	0	229 a	8.6 abc	1.4 bc	21.6 bc
	0.5	100	230 a	10.9 a	1.65 ab	24.4 ab
	0.5	200	221 bc	9.9 abc	1.7 ab	25.1 a
	1	0	224 abc	11.4 a	1.7 ab	25.5 a
	1	100	230 a	9.7 abc	1.55 abc	25.4 a
	1	200	230 a	6.2 c	1.0 c	24.7 ab
Level of significance^g
Irradiance (IR)			**	ns	*	**
Slow-release fertiliser (SRF)			ns	ns	ns	**
Liquid fertiliser (LF)			ns	**	ns	*
IR × SRF			ns	ns	ns	**
IR × LF			ns	ns	ns	ns
SR × LF			ns	ns	ns	ns
IR × SRF × LF			ns	*	ns	ns

^aNatural photoperiod and light irradiance (ND), and high irradiance treatment (HID) from 08:00 to 16:00 h.

^bSlow-release fertiliser (SRF) and liquid fertiliser (LF).

^cNumber of days to flower was counted from 19 March.

^dRefer to Table 4 for scales.

^eTo facilitate the readers understanding, the rows which represent the best treatment are highlighted in bold face letter.

^fMean separation by Duncan’s multiple test at P ≤ .01. Means with different letters in each column are significantly different from each other.

^gns, *, **; non-significant, significant at P ≤ .05 and at P < .01, respectively, according to F-test.

The number of flowers was significantly affected by LF, and its interaction with IRL and SRF (LF × IR × SRF) (Table 6). Generally, the highest number of flowers (12.1) was obtained following treatment with 100 ppm LF when 0.5 g of SRF under ND was applied, with more than 6.2 flowers (HID × 1 g SRF × 200 ppm LF). The number of flowers was reduced when 200 ppm LF was applied with SRF treatment, especially under HID. Flower bud scales were also high when plants were grown under the condition that produced the most flowers; plants treated with 0.5 g SLF and 100 ppm LF under ND produced the 12.1 flowers with a 2.0 flowering scale rating and 6.2 flowers with a 1.0 flowering scale rating with 1 g SLF and 200 ppm LF. Plant height was also significantly affected by IR, SRF, LF and the interaction between IR × LF, producing plants >24.7 cm when plants were grown under HID with 1 g SRF or 0.5 g SRF with 200 ppm LF. The width of plants ranged from 22.2 cm (ND + 0.5 g SRF + 100 ppm LF) to 25.4 cm (HID + 0.5 g SRF + 200 ppm LF), and was not affected by any treatment nor their interaction (data not presented).

Discussion

Although Leptospermum was listed as a noxious weed in 1900, it is now considered as a valuable ornamental plant (Derraik 2008). To produce quality Leptospermum potted plants with many flowers after anthesis and flower buds (Figure 1B), the requirements for environmental factors, such as temperature, photoperiod, irradiance and nutrition were investigated. Uniform in size of the propagules under a vegetative growth stage obtained from the rooting of cuttings and testing extensive environmental conditions as described in each experiment to produce compact and quality potted plants is considered a major difference from the recent work (Bicknell and Jaksons 2018). Flowering of three L. scoparium cultivars over 3 years following the propagation of plants at different times of the year (October in Expt. 2 and January and February in other experiments) revealed that the number of days to flowering was generally dependent on the age of plants and when the experiments were performed. ‘Ruru’ did not flower for 153 days (2 June) following the start of treatment on 19 January.

Some treatments in a given experiment did not lead to flowering, while others led to 100% flowering. Therefore, the means and standard deviations were calculated to demonstrate significant differences among means. However, the standard deviation is influenced by extreme values in each treatment, which does not follow a pattern of normal distribution means lying outside of one standard deviation is considered different.

Overall, ‘Ruru’ was observed to be late flowering, self-branching and short stemmed; ‘Kea’, medium branching; and ‘Kiwi’, less branching with small flower buds largely formed from the middle to the top of the shoot (data not presented). These characteristics may not require the use of plant growth retardants to control plant height, since these hybrids, derived from the dwarf ‘Ruru’, were evaluated as the most floriferous, probably the best cultivar with occasionally grown ‘Kea’ and ‘Nanum’ (Dawson 2010).

Effect of temperature on growth and flowering (Expt. 1 and 3)

Flowering in ‘Ruru’ and ‘Kea’ was the earliest, ranging from 82 to 91 and 93 to 117 days, respectively, when grown under low temperatures (15/13°C) during the first period starting on 24 April, and grown in a greenhouse maintained at 18/12.8°C, regardless of the temperatures during the second period (Table 2; Expt. 1). This suggests that flowering is initiated at temperatures near 15/13°C, which are close to those during the warmest months at indigenous sites in New Zealand (Williams 1981; Yin et al. 1984). High temperatures (25/23°C) should be avoided since they delayed flowering in both cultivars. L. scoparium cultivars failed to flower at 20°C even under an inductive 9.5 h or SD (Bicknell and Jaksons 2018), or grown in a greenhouse flowered under a photoperiod shorter than 12 h at both 20/26 and 10/20°C (day/night). Therefore, the upper temperature limit to induce floral induction would be around 20°C.

After floral initiation, development can be accelerated under reasonably high temperatures at 20/18°C during the first period, followed by 25/23°C during the second period, resulting in an increased number of flowers – 22.0 for ‘Ruru’ and 62.6 for ‘Kea’ at anthesis. This suggests that flower bud development is considered normal without the induction of florets and inflorescence blast at 25/23°C, as reported in Lachenalia aloides (L.f.) Engl. (Roh 2005).

Counting very small flowers from numerous shoots of compact plants is difficult, and the increased number of flowers was also confirmed by the scale ratings of flowers that reached anthesis at the time of anthesis, and 30 days after anthesis at 20/18°C during the first period, followed by 25/23°C during the second period, especially in ‘Ruru’. Therefore, the flowers were not counted (Expt. 3). The difficulty in counting flowers may be related to the numerous shoots or the co-presence of shoots with reproductive development and vegetative growth. This was observed in mango (Mangifera indica L.) (Nunez-Elisea et al. 1996), whereby transferring plants initiating floral shoots from 18°C/10°C to 30°C/25°C (day/night) increased the ratio of floral shoots over vegetative shoots.

Low temperatures promoted flowering and high temperatures delayed flowering with 90-day-old ‘Ruru’ and ‘Kea’ plants (Table 4; Expt. 3). The earliest flowering of L. scoparium occurred in 182-day-old ‘Ruru’ and ‘Kea’ plants propagated on January 30, treated at 20/17°C or 24/21°C for 30 days, and was delayed at 17/14°C and 14/11°C. This suggests that floral development is accelerated at high temperatures, taking less than 46 days in both cultivars. This is supported by the increased scales of open flowers in both cultivars when plants were grown at 20/17°C or at higher temperatures, and the increased scales of flower buds at 14/11°C for 45 days and 17/14°C for 60 days.

Summarising the effects of temperature from the two experiments, it is clear that floral buds initiate at low temperatures; 15/13°C during the first 40 days and 14/11°C for 45 days, indicating that floral initiation occurs between 14°C and 15°C during the day, and between 11°C and 14°C during the night. This suggests that Leptospermum hybrids may be considered as a cool season crop and that high temperatures (25/23°C and 24/21°C) delay flowering. Once the initiation of flower buds is completed, high temperature accelerates floral development for early flowering. Maintaining greenhouse temperatures during the late spring to early fall between 14°C and 15°C during the day and between 11°C and 14°C during the night when the photoperiod is longer than the short photoperiod (9 and 12 h), will limit the production of Leptospermum between these months.

Would it be possible to find an optimal temperature treatment period during floral initiation and development to accelerate floral initiation at low temperature (15/13°C) and to increase the number of flowers promoting the development of initiated flower buds at 20/18°C or 25/23°C (Expt. 1)? One possible way is to manipulate the treatment duration during the first and second periods by treating them for 20 or 30 days at various temperatures (15/13°C, 20/18°C or 25/23°C), or by intercalating at 20/18°C or 25/23°C during 15/13°C. These treatments were demonstrated in the forcing of the Easter Lily (Lilium longiflorum Thunb) for increasing the number of flowers without delaying flowering (Roh and Wilkins 1977).

Effects of photoperiod on growth and flowering (Expt. 2)

In Australia, full bloom occurs between December and February or in the spring and early summer (https://www.anbg.gov.au/leptospermum/leptospermum-scoparium.html. Accessed on April 1, 2019). This implies that floral initiation occurs under an SD photoperiod in the spring, and develops under an increasing photoperiod in the early summer. As Expt. 2 was started during the summer from 30 July, plants should be under vegetative growth when cuttings were propagated on 21 January and pruning on 25 April. As flowering occurred earliest in ‘Ruru’ and ‘Kea’ with a 9 h photoperiod followed by a 12 h photoperiod at 21/15.5°C during 25 days of each of these photoperiods, this was sufficient to accelerate flowering, which takes 98 days for both cultivars. duration of 50 days of treatment did not further accelerate flowering (Table 3, Expt. 2). Under these photoperiods, there was 100% flowering after either 25 or 50 days. At least four weeks of short photoperiod (11.5 h) was found to be required for the floral induction of two L. scoparium ‘Crimson Glory’ and ‘Nanum Ruru’ (Bicknell and Jaksons 2018). Zieslin and Gottesman (1986) reported that L. scoparium plants flowered under a photoperiod shorter than 12 h at 20/26°C (day/night) and 10/20°C; however, flowering only occurred at low temperatures under a photoperiod of 12–14 h. The temperature regime of 20/26°C (day/night), used by Zieslin and Gottesman (1986), was higher than the temperature regime of 21/15.5°C (day/night) tested in the present study. The requirement for only 25 days of an SD photoperiod will make it easier to control flowering in Leptospermum.

Information on the photoperiod and temperature requirements for the flowering of C. uncinatum Schauer, endemic to Western Australia, is comparable to the results observed for Leptospermum in the present study. In C. uncinatum, the major factor influencing floral initiation and full flowering was a 4-week SD photoperiod, under either 20/14°C or 20/18°C (day/night), although fewer flowers were initiated under an LD at 20/14°C (Shillo et al. 1985). Flowering was delayed when forced at 15 h during the first and second periods for ‘Ruru’ (169 days) and ‘Kea’ (152 days). This indicates that floral initiation is inhibited under a 15 h LD photoperiod, as evaluated in this study, and also as previously reported by Bicknell and Jaksons (2018), showing that a 16 h LD photoperiod inhibited floral initiation. This is further supported by the observation that 50 days of a 15 h photoperiod results in 30%–70% flowering for ‘Ruru’ and 20%–60% flowering for ‘Kea’ depending on the temperature during the second treatment period (Table 3). Photoperiod after the completion of treatment may not inhibit floral initiation and development, as the monthly photoperiod over the year ranged from 9 h 30 min (21 December) to 14 h 60 min (21 June).

Differences in the number of days to flower among treatment means were less than 60–70 days, which is beyond the range of one standard deviation range indicating a difference. The number of flowers, which was highest with a 9 and 12 h photoperiod during both treatment periods for 25 days, supports the finding that an SD or intermediate photoperiod favours floral initiation and development. However, great variation was determined by the range of standard deviations, which sometimes exceeded the value of the mean, regardless of the photoperiod and treatment durations. Thus, a photoperiod that exceeds 50 days may be required to confirm that a 15-h photoperiod during the treatment period and after the completion of the experiment (an LD photoperiod) is inhibitory for floral initiation. The requirement of an SD photoperiod for floral initiation will certainly limit year-round production, since SD treatments during the summer may increase the temperature above the optimum, perhaps higher than a constant 20°C under a 9.5 h photoperiod (Bicknell and Jaksons 2018).

The effect of temperature, irradiance and nutrition treatments on growth and flowering (Expt. 4 and 5)

Among the three cultivars evaluated, ‘Ruru’ propagated on 31 October and treated from 19 January to 19 March did not flower when data collection was terminated on 2 June (Table 5, Expt. 4), although ‘Ruru’ is considered to be the most floriferous cultivar, perhaps in the natural environment, due to late flowering characteristics (Dawson 1997). Poor flowering of Correa reflexa (Labill.) Vent. in response to favourable photoperiod and temperature conditions for flowering was observed with Correa ‘Mannii’ Andr. (Lee and Roh 2011). ‘Kea’ flowered after 143–151 days, and the days to flower were not significantly different among temperature and irradiance treatments due to great variations based on the standard deviation. However, the flowering rates of ‘Kea’ and ‘Kiwi’ were higher (>93%) when grown at 21/19°C and 25/22°C, especially under HID, implying that the optimum temperature should exceed 15/13°C under HID. In studies using constantly light growth chambers, time to floral initiation following cession of leaf initiation was reduced as irradiance was increased to 50 W m⁻² (Cockshull 1979)

Based on the scales of open flowers and flower buds of ‘Kea’ and ‘Kiwi’, 25/22°C and HID are the optimum conditions for the development of initiated flower buds (Table 5, Expt. 4). Flowering of C. uncinatum ‘Purple Pride’ responding to the high temperature of 20–25°C and HID favoured flowering, and poor flowering at lower temperatures (below 16°C) (Dawson and King 1993) could be related to the results obtained with Leptospermum in the present study. Cockshull (1979) reported that Chrysanthemum morifolium Ramat ‘Polaris’ initiated flower buds in continuous irradiances from 7.5 to 120 W m⁻², and the transition to reproductive development began earlier after leaves were initiated before flower buds, as irradiance increased. However, due to its self-branching morphological characteristics, the number of leaves from all shoots at flowering could not be determined. Pearson et al. (1993) reported that photoperiod is the most important factor controlling flowering in chrysanthemum (Dendranthema grandiflora) and ‘the rate of progress of flowering increases linearly with increasing light integral and effective temperature’.

While flowering and flower scales were not affected by the application of SRF and LF or their combination, only plant height was significantly affected by SRF, LF and irradiance, and the interaction between SRF and irradiance (Table 6, Expt. 5). The increases in height was not considered excessive to produce Leptospermum hybrids as a potted plant. These three cultivars were compact and dwarf selections derived from ‘Nanum’ (Dawson 2010). As the number of plants producing the highest number of flowers (12.1) with flower scales (2.0) following treatment with 100 ppm LF when 0.5 g of SRF under ND, and increasing LF to 200 ppm and 1 g SRL under HID, 100 ppm LF, ND and 0.5 g SRF is suggested to be optimum for growth and flowering. It is possible that these treatments could be administered during early vegetative growth, and then the concentration of LF could be reduced to 100 ppm prior to floral initiation which requires further study. In Correa ‘Mannii’, flowering and the number of flowers increased as the concentration of LF increased to 400 ppm when SRL was applied at 1 g per pot, but not when SRL was applied 2 g per pot (Lee and Roh 2011). Our results indicate that Leptospermum may not require high levels of nutrition for growth and flowering.

Conclusions

The Leptospermum hybrids evaluated in this study are classified as SD plants. Floral initiation can be completed in 25 days under a short photoperiod treatment. This will make it easier to force plants to flower following treatment for 25 days at an optimum photoperiod which covers the floral initiation (9 h SD) and the development of initiated flower buds to reach anthesis (12 h of intermediate photoperiod; intermediate stage from initiation to development). These conditions were confirmed in the present study, which demonstrated that floral initiation of L. scoparium plants occurs at temperature near 15/13°C, close to the temperatures during the warmest months at the indigenous sites in New Zealand. After floral initiation, development can be accelerated by increasing the number of flowers that reach anthesis at reasonably high temperatures of 20/18°C during the first period, followed by 25/23°C during the second period. The effects of irradiance and nutrition were considered less prominent than the temperature regimes when plants were grown under an increasing photoperiod provided by HID, compared to plants grown under a decreasing photoperiod. Based on ratings from the open flower and flower bud scales of ‘Kea’, ‘Kiwi’ and ‘Ruru’, 25/22°C and HID may be suggested as optimum conditions for the development of flowers. Fertiliser treatment with 100 ppm LF combined with 0.5 g of SRF when grown under either ND or HID is therefore suggested. These are the optimum requirements for the production of short and compact potted plant hybrids with many flowers and flower buds.

Acknowledgements

This research was performed when the corresponding author (MS Roh) worked at the US Dept. of Agriculture, Agricultural Research Service, Beltsville, MD, USA.

Disclosure statement

No potential conflict of interest was reported by the authors.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by the Project of Ministry-Province Co-Construct [grant number C176280108] awarded to Xuewei Wu.