Crown-cooling Treatment Induces Earlier Flower Bud Differentiation of Strawberry under High Air Temperatures

Over 90% of Japanese strawberry farmers employ forcing to enable harvest from winter to the following spring (Yamasaki, 2013). However, because available production area continues to decline, new techniques to obtain consistently high yields are required. Many factors contribute to fruit yield in strawberry production (Hidaka et al., 2014a). Fruit yield per plant is influenced by factors including per unit fruit weight, fruit number, flower budding, photosynthate partitioning, leaf photosynthesis, and water and nutrient uptake by roots. These factors are affected by the growing environment (e.g., light intensity, photoperiod, temperature, CO2 concentration, humidity, and wind velocity) and the genetic potential of each cultivar. In our previous studies, we explored the development of environmental control techniques, such as supplemental lighting and CO2 enrichment, to achieve high increases in fruits yield through acceleration of leaf photosynthesis (Hidaka et al., 2013; 2014b; 2015; 2016). However, seeking to increase yields through environmental controls relies on the assumption that flower bud differentiation will be induced normally. Global warming has recently been reported to have serious potential impacts on water resources, ecosystems, food production and other aspects of life. The Japanese Ministry of Agriculture, Forestry and Fisheries has reported on agricultural issues already known to result from global warming, including high-temperature-related injuries to rice (cracked rice), abnormal fruit coloration, changes in fruit growing zones, and increased incidences of pests and disease (2008). Further, effects of recent warming on agricultural production have been observed throughout the whole of Japan (Sugiura et al., 2012). Japanese strawberry producers usually use Junebearing cultivars, and flower bud differentiation in these cultivars is induced by short days and low temperatures (Ito and Saito, 1962). However, recently there have been concerns that rising air temperatures in August and September will cause delayed flower bud differentiation in first inflorescences. Many types of localized temperature control systems have been developed to stabilize flower bud differentiation under high-temperature conditions (Mukai and Ogura, 1988; Ikeda et al., 2007; Yamazaki et al., 2007; Miyoshi et al., 2013). Our research group also developed a technique to control the temperature of the strawberry crown, which is the organ containing the shoot apical meristem (Dan et al., 2015). However, few studies have examined the effect of such cooling systems under the high temperatures expected with future global warming. We calculated likely future air temperatures in the study area based on past recorded temperatures and predictions of future global warming and reproduced these temperature conditions in a greenhouse. We examined the effect of crown-cooling treatments on flower bud differentiation, flowering characteristics and yield under high air temperature with the aim of achieving stable future production of strawberry.