Pre-Harvest Night-Interruption on Everbearing Cultivars in Out-of-Soil Strawberry Cultivation in Belgium

Abstract

Trials were conducted in 2009 and 2010 to examine the relationship between photoperiod and temperature in commercial production of two everbearing strawberry cultivars. Specifically, two experiments were conducted to determine: 1) the extent of flowering and runnering in the everbearing strawberry cultivar Charlotte as influenced by night-interruption using incandescent lights, or no night-interruption; and 2) the influence of temperature and night-interruption on flowering and runnering of the everbearing cultivar Portola in 2010. The 2009 trial was conducted in a glasshouse, while the 2010 trial was conducted on a table-top system. In both trials, the influence of pre-harvest night-interruption lighting or night-interruption lighting + temperature on fruit yield, cropping pattern, and runnering were investigated. The results demonstrate that use of night-interruption can result in increased fruit production and earlier fruit production of the second fruit cycle and a reduced stolon development for both cultivars. Both cultivars were sensitive to long-day treatments. It appears that an appropriate lighting strategy is required for optimizing yields of everbearing cultivars in Northern European environments.

Keywords:

• Fragaria × ananassa
• day-neutral
• photoperiod
• long-day
• temperature
• runners
• inflorescences
• incandescent lamp
• cyclic lighting

INTRODUCTION

The so-called short-day (single cropping, Junebearing) strawberry cultivars are still the most important cultivars in commercial strawberry production in Belgium, with Elsanta as the most commonly grown short-day cultivar. In climates such as Belgium, short-day cultivars produce a single flush of flowers and have a relatively short harvest period. Considerable research has been done to understand the flowering of these short-day strawberry cultivars (e.g., Darrow and Waldo, 1934; Guttridge, 1985; Taylor, 2002). While they are generally considered short-day plants, the relation between photoperiod and temperatures may vary among cultivars (Ito and Saito, 1962; Heide, 1977). Due to the natural conditions in Belgium, floral initiation and induction takes place in autumn when temperature and day length are decreasing. The use of cold storage of plants and appropriate production systems and planting dates, makes it possible to harvest strawberries from the cultivar Elsanta almost year round in Belgium.

Distinct from Junebearers, Darrow and Waldo (1934) called everbearing cultivars, which induce flowers due to long day lengths when appropriate temperatures predominate (i.e., temperatures between ca. 10–27°C), “long-day” plants. Everbearing strawberry cultivars are capable of producing numerous flower flushes during the growing season. Around 1980, publications appeared with another name for everbearing strawberries: day-neutrals (e.g. Bringhurst and Voth, 1980 Nicoll and Galletta, 1987) This new term was presumed to be more accurate because it was stated that these varieties flower under both long- and short-day conditions. However, other researchers distinguish between everbearers (long-day plants) and day-neutrals, and this terminology is currently accepted by many researchers (e.g. Durner et al., 1984; Dale et al., 2002; Serçe and Hancock, 2005) Most scientists who use the term ‘day-neutral’ recognize the existence of photoperiod response of DN cultivars and distinguish weak, intermediate and strong day neutral cultivars (e.g. Serçe and Hancock, 2005) In the past decade, new insights revealed an incompleteness of the terminology of day-neutrals next to everbearers. Nishiyama and Kanahama (2002)concluded that “although day-neutrals are inherited [sic.] their flowering trait from Fragaria virginiana subsp. glauca (Bringhurst and Voth, 1980) flowering responses to temperature and photoperiod are not different between everbearing and day-neutral cultivars” (p. 254). They suggest that everbearing strawberry cultivars (including the so called ‘day-neutrals’) are qualitative long-day plants under high temperature and quantitative long-day plants under low temperatures. Sønsteby and Heide (2007b)completed this theory, concluding that “everbearing strawberry cultivars, in general, are qualitative long-day plants at high temperature (27°C) and quantitative long-day plants at intermediate temperatures (21°C and 15°C). This applies to modern Californian cultivars as well as older cultivars mainly of European origin. Only at temperatures below 10°C are these cultivars day-neutral” (p. 884). Sønsteby and Heide (2008)stated that there is an identical interaction between long-day and temperature for flower induction for the octoploid strawberry Fragaria x ananassa and the diploid Fragaria vesca ssp. semperflorens. Despite this recent research, there is still no agreement about the terminology for everbearing cultivars. In recent publications the authors still keep the old, confusing terminology by consequently using ‘day-neutral’ for all everbearing varieties (e.g. Stewart and Folta, 2010) In this article all multiple cropping cultivars will be called “everbearers”, in agreement with the terminology described by Sønsteby and Heide (2007b)

By imposing artificial long day lengths everbearers should be capable of initiating and developing more flowers due to their quantitative response to intermediate temperatures (Nishiyama et al., 2006; Sønsteby and Heide, 2007b) In this study, the influence of pre-harvest temperature and night-interruption (via use of incandescent lights) on the yield, fruit cropping pattern, and runner production of two everbearing strawberry cultivars was investigated. The objective of these trials was to stimulate floral induction by day length extension in order to increase fruit production and decrease stolon development.

MATERIALS AND METHODS

Two trials were conducted at the Research Centre Hoogstraten (51° 27′ N, 4° 47′ E; altitude 16 m) to investigate the effect of cyclic night-break lighting on yield, cropping pattern, and runner production of two everbearing strawberry cultivars. For both trials, incandescent lamps (100W, 10W m⁻²) were used for night-interruption (NI) prior to the period of fruit harvest. Two trials were conducted using two strawberry cultivation systems: the first in a glasshouse while the second was a table-top system. The trials (with different cultivars) were not meant for comparison, but rather for developing improved methods for commercial, out-of-soil strawberry cultivation for everbearing cultivars in Belgium.

Experiment 1: ‘Charlotte’ in 2009

The first experiment was done in 2009 with the French everbearing cultivar Charlotte. On 4 Feb. 2009, cold-stored (frigo) plants (A+) were planted in a peat substrate on support structures in a glasshouse. There were four replicate groups of 30 plants each, with a plant density of 5.2 plants·m⁻². During a 38-day period from 20 Feb. to 30 Mar., a subset of the experimental plants were subjected to NI treatment by providing light from incandescent lamps (10W·m⁻²) for 15 min per hour from sunset to sunrise. In the same glasshouse, a second subset of plants was subjected to complete darkness (no NI) throughout the night by use of a curtain that blocked light from the incandescent lamps. Each week the number of runners on all plants were counted and removed. Berries were weighed and graded (large, small, misshapen, and rotten fruits), and large fruit had diameters of 35 mm.

Experiment 2: ‘Portola’ in 2010

The second experiment was conducted in 2010 on the cultivar Portola. Cold-stored plants (A+) were planted in a peat substrate on support structures in a glasshouse on 11 Mar. 2010, where temperature and lighting could be controlled. The lighting treatment consisted of cyclic lighting (15 min per hour) at 10W·m⁻² with incandescent lamps. After 50 days of temperature and light treatments, all plants were moved on 30 Apr. to an outside table-top system, covered with plastic tunnels. Plant density was 4.3 plants m⁻².

One group of plants was subjected to incandescent lighting from planting date (11 Mar.) to 6 Apr., for a total of 26 nights. A second group of plants received incandescent light from 11 Mar. to 30 Apr., for a total of 50 nights with NI treatment. A third group of plants were maintained in the dark without light at night for a total of 50 nights (no NI). Light treatments were imposed in two glasshouses: one heated and one unheated, resulting in six different treatments. During the first period (11 Mar. to 6 Apr.), the average day/night temperatures were 16°C/11°C and 12°C/9°C, in the heated- and the unheated glasshouse, respectively. During the second period (6 Apr. to 30 Apr.), the heated glasshouse was maintained at an average day/night temperature of 18°C/11°C while the cold glasshouse was at an average temperature of 14°C/9°C (day/night). Thus, while the average night temperatures were identical during the entire treatment period, the day temperatures increased by 2°C halfway through the treatment period. At the end of the light treatment periods, all plants were moved to outside table-tops covered with plastic tunnels.

RESULTS AND DISCUSSION

Experiment 1: ‘Charlotte’ in 2009

For the period 27 Mar. to 26 Aug., plants cv. Charlotte exposed to NI produced 10.9 runners·plant⁻¹ while plants not exposed to NI produced 22.1 runners per plant. This difference was most pronounced for the period 23 Apr. to mid June (Fig. 1).

FIGURE 1 Effects of night light interruption on runner (no./100 plants) and fruit production (kg·m⁻²) from Mar. to Sept. 2010 (cv. Charlotte) (color figure available online).

Fruit harvest commenced 16 Apr. and ended 2 Sept. Total yields for NI and no NI treatments were statistically similar, with 7.9 and 6.9 kg·m⁻² for NI and non-NI plants, respectively (Table 1). The plants of cv. Charlotte exposed to either NI or no NI had equal fruit production patterns during the period 15 Apr. to 12 May; this early harvest period was the result of flowers initiated the previous autumn. After planting in the greenhouse, flowers continued to develop and produce fruit for the first fruiting cycle (15 Apr. to 12 May). During the period 13 May to 17 June, plants subjected to NI treatment in February and March had greater flowering and fruiting than non-NI plants (Fig. 1). The fruit production during this second fruiting cycle was statistically different with 5.11 kg·m⁻² and 4.41 kg·m⁻² for the NI and non-NI plants, respectively (Table 1). In the third fruiting cycle, the production was similar for both treatments.

TABLE 1 Effect of Night Light Interruption on Fruit Production (kg·m⁻²) of ‘Charlotte’ Strawberry during the Three Main Fruiting Cycles in 2009

	Total production		Production 1st crop, 15 April–12 May		Production 2nd crop, 13 May–31 July	Production 3rd crop, 1 Aug.–2 Sept.
NI treatment	7.91	ns	1.00	ns	5.11a ^z	1.79	ns
Non-NI treatment	6.92		0.96		4.41b	1.56
^zMean separation within a column by t-test (p = 0.05).

The mid-harvest date was similar for the first and the third fruit cycle (data not presented). For the second fruit cycle, NI treatment advanced the mid-harvest date by 7 days.

Experiment 2: ‘Portola’ in 2010

In both temperature treatments, there was a strong significant reduction of runners for all NI treatments (Table 2). The difference in the number of runners between the short lighting period (26 nights) and the long lighting period (50 nights) was not significant. For the unheated plants, the average number of runners per 100 plants was 405, 211, and 223 for plants subjected to non-NI, to 26 nights of NI, and to 50 nights of NI, respectively. This represented a reduction of 48% and 45% for the plants exposed to 26 and 50 nights of NI, respectively. For the heated plants, the average number of runners per 100 plants was 432, 233, and 229 for the plants exposed to non-NI, to 26 nights of NI, and to 50 nights of NI, respectively. This represents a reduction of 46% and 47% for the plants exposed to 26 and 50 nights of NI.

TABLE 2 Effects of Night Light Interruption and Heating on Runner Production in ‘Portola’ Strawberry in 2010

		Percentage of runners compared to the control
Treatment	Total number of runners per 100 plants	Heat (%)	No heat (%)
Non-NI, unheated	405a ^z	100	—
NI 26 days, unheated	211b	52	—
NI 50 days, unheated	223b	55	—
Non-NI, heated	431a	—	100
NI 26 days, heated	233b	—	54
NI 50 days, heated	229b	—	53
^zMean separation by Mann-Whitney test (p = 0.05).

The fruit was harvested from 21 May to 6 Oct. Fruit production ranged from 3.7 kg·m⁻² to 4.2 kg·m⁻², with no statistical differences among treatments (Table 3). The first fruit cycle (21 May to 17 June) was similar for all treatments (Fig. 2). Although the productions of this first fruit cycle were low and the yield of non-NI and long NI plants differed by only 170 g·m⁻², the difference was significant (data not presented). The reason for this higher yield is unclear. Mid-harvest dates of the first crop were similar for all treatments (data not presented).

FIGURE 2 Effects of night light interruption and heating on cropping pattern of ‘Portola’ strawberry (color figure available online).

After the first crop, the NI plants initiated the subsequent fruit production earlier (18 June to 5 Aug.) at both temperatures (Fig. 2). For the unheated treatments, the mid-harvest date was 4 days earlier for 26- and 50-nights NI treatments than the non-NI plants (data not presented). For the heated treatments, the 26- and 50-nights NI treatments led to mid-harvest dates 5 and 6 days earlier than non-NI, respectively. In the third fruit cycle, temperature and NI treatments had no effect on yield or cropping pattern (Table 3 and Fig. 2), except the mid-harvest date for the heated 50-nights NI-plants was seven days later than for non-NI and 26-nights NI plants.

TABLE 3 Effect of Night Light Interruption and Heating on Fruit Production (kg·m⁻²) in ‘Portola’ Strawberry during Three Main Cropping Cycles in 2010

	Total	Production 1st crop	Production 2nd crop	Production 3rd crop
Treatment	production	21 May–17 June	18 June–5 Aug.	6 Aug.–6 Oct.
Non-NI, unheated	4.04 ns	0.37b ^z	1.99b	1.69a
NI 26 days, unheated	3.92	0.54a	2.10b	1.28ab
NI 50 days, unheated	3.72	0.37b	2.29ab	1.07ab
Non-NI, heated	4.18	0.33b	2.36ab	1.49ab
NI 26 days, heated	4.33	0.35b	2.64a	1.34ab
NI 50 days, heated	3.84	0.30b	2.72a	0.82b
^zMean separation within columns by a Tukey-test (p = 0.05).

The first trial started at the beginning of Feb. 2009 and was done in a glasshouse, the second started 11 Mar. 2010 and after the treatments the trial was moved outside to a table-top system. As the trials were conducted under different photoperiod, temperature, and cultivation systems conditions, the results of the two trials may not be compared directly with each other.

The trial conducted in a glasshouse with cv. Charlotte in 2009 showed that NI treatment had a strong influence on both runner and fruit production. Total runner production was reduced 50%. The first crop was not affected by the NI treatment since these flowers were formed in autumn during plant growth. The yield of the second crop increased remarkably due to NI treatments. The lighted plants initiated inflorescences instead of runners during the lighting treatment, which led to increased second cycle. The differences in the number of runners produced per week were evident from the end of April until the end of May. The NI treatment increased plant yield during the second cropping cycle (starting 13 May). This seems to confirm that inflorescences are initiated instead of runners. There was also an advancement of the second cropping cycle by seven days as a consequence of the NI treatment. The yield and mid-harvest date in the third cycle were similar. The increase of yield and cropping advancement during the second fruit cycle, in combination with the reduction of runners, can be a real opportunity for glasshouse cultivation with everbearing cultivars.

The light treatments on cv. Portola in 2010 on a table-top system in the glasshouse reduced runner development by 45% to 48%. There were no significant differences in total fruit production between the various pre-harvest treatments. Nevertheless, the differences in the cropping pattern and mid-harvest dates showed that there was a small effect of the NI treatment, which was more explicit at higher temperatures. The second crop of the lighted plants started earlier than the non-NI plants: mid-harvest date was 4 days earlier for 26- and 50-nights NI treatment on unheated plants and for heated plants it was 5 and 6 days earlier for 26- and 50-nights NI treatments, respectively. The production during this second fruit cycle appeared to increase with NI treatment, but these differences are not significant. Remarkably, the 50-nights NI treatment had a larger relapse after the second crop than the 26-nights NI. A possible explanation for this is that they were overloaded during the second crop, which led to a stronger relapse during the third fruit cycle. Because the 26-nights NI plants had less second crop, the lapse appears to be limited. This trial confirmed that everbearing varieties react to long-day conditions, but the overall yield for all of these treatments was similar. There was an effect of NI treatment on flower initiation and fruit development in cv. Portola, which was more obvious at higher temperatures. For strawberry cultivation, the temperature/light treatment should be well balanced in this system to avoid the lapse after the second crop. The shorter lighting period of 26 days appears to be suitable for ‘Portola’ in this system.

Although the two experiments cannot be compared, it was clear that two “multiple-cropping” cultivars responded similarly to night-interruption treatments. A reduction in runnering and an increase in yield during the second cropping cycle due to long days are in agreement with others (Nishiyama et al., 2003; Sønsteby and Heide, 2007a, 2007b). Moreover, an advancement of the second cropping cycle was reached due to NI treatments, which is in agreement with the results of Sønsteby and Heide (2007a) However, greater fruit production during the second cropping cycle caused a reduction in yield during the third cropping cycle, mainly in ‘Portola’. Sønsteby and Heide (2007a) also noticed that the formation of new flowers leveled-off with high fruit load. For the tested cultivars, the differences in the lapse between the second cropping cycle with high yields and the third cropping cycle is a result of differences in photoperiod-temperature threshold correlations for these cultivars. Also, extremely high temperatures during the second cropping cycle may have depressed flower development. The results of both experiments support the hypothesis that the everbearing cultivars behave like qualitative long-day plants, quantitative long-day plants or day-neutrals at respectively high, intermediate, and low temperatures (Sønsteby and Heide, 2007b).

The findings from this study suggest the possibilities for improvement of the yield of everbearing cultivars in Belgium by artificial lighting. This research showed that the pre-harvest NI treatments have an effect on the second fruit cycle by promoting flower induction and development. Also, overloading of plants with fruit during the second fruiting cycle reduced yield during the third fruiting cycle. The temperature during the lighting period, the length of the lighting period, and also the timing of the lighting period affected plant response. Additional studies are needed to determine the proper balance between day length and temperature in order to avoid excessive flower bud development.