Strawberry ‘Tokun’ (2n = 10x = 70) is an interspecific hybrid (Fragaria ×ananassa and Fragaria nilgerrensis) with pale yellowish orange fruit and the fruit has aroma components of caramel, coconuts, and peach (Noguchi, 2011), which attracts a lot of consumers. A major drawback of this cultivar is its late maturity, which hampers its extension. Noguchi and Yamada (2015) reported that the harvest of this variety started in February in Japan. We also found a late-harvest start of this variety in the middle of January when cultivated in Central Yunnan, China (lat. 23°19′–27°03′N; elevation, 1700–2300 m). Studies of the unique cultivar Tokun are needed to find practical ways to advance flowering and fruit harvest, which will benefit both growers and consumers.
Intense studies have been done regarding the effects of temperature and photoperiod on flower induction of octoploid strawberries, and SD and night-chilling treatments have been used to induce flowering and promote early fruit production in autumn (Durner, 2015, 2016; Heide and Sønsteby, 2007; Ruan et al., 2011; Serçe and Hancock, 2005; Sønsteby et al., 2009). For ‘Tokun’, SD combined with night-chilling treatments are also found to be effective at inducing flower buds and promoting early fruit production in Japan (Noguchi and Yamada, 2015). Aside from Japan, however, no reports were found regarding regimes to promote ‘Tokun’ flowering and fruiting. Practical techniques to advance flowering and fruiting of ‘Tokun’ should be developed according to the local climate and production systems.
Plant nutrient status, especially N status, is another factor that influences flower bud induction (Guttridge, 1985). In general, less N can promote flower bud induction (Seo et al., 2014), especially for vigorous cultivars. However, SD treatment and timing of N fertilization have strong interactive effects on flower bud induction. The right timing of N fertilization can promote flower bud initiation for some octoploid strawberries such as ‘Korona’ (Sønsteby et al., 2009, 2013). For the decaploid strawberry ‘Tokun’, the effects of timing of N fertilization relative to start of SD treatments on flower induction need to be studied.
The Central Yunnan area, with its high elevation and low latitude, has the mildest climates in China (Wu et al., 2017). The perpetual spring-like weather provides ideal growth conditions for strawberries (Wang et al., 2018). In this area, 4 weeks of SD treatments in natural summer temperatures is enough for octoploid strawberries ‘Akihime’ and ‘Seolhyang’ to induce flower budding and to harvest in autumn (Ruan et al., 2015). There are no reports of flower bud induction of ‘Tokun’ in the Central Yunnan area regarding SD treatment and N fertilization. Thus, in our study, we investigated the parameters of flowering and fruiting, as well as some morphological traits, of ‘Tokun’ plants conditioned with 5, 6, and 7 weeks of SD treatment supplemented by different timing of N fertilization. A practical regime was developed and recommended to advance flowering and fruiting for decaploid strawberry ‘Tokun’ in the Central Yunnan area.
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