Effects of Local CO2 Enrichment on Strawberry Cultivation during the Winter Season

Carbon dioxide (CO2) concentration is one of the most important environmental factors in crop production. Within the range of suitable CO2 concentration for crops, an increase of CO2 concentration results in an increase in net crop photosynthesis (Cure and Acock, 1986; Keutgen et al., 1997) and thereby promotes photosynthate translocation, which directly controls the crop yield and quality (Troughton and Currie, 1977; David et al., 2014). Therefore, in greenhouse horticulture, CO2 enrichment is commonly used to realize high crop productivity (Kawashima et al., 2008). In strawberry production, CO2 enrichment has been practiced since the 1970s (Kato et al., 2015), and is used widely today in strawberry-cultivation greenhouses (Wada et al., 2010). In areas of scant solar radiation during the winter season, daytime enrichment of CO2 increased crop yield by 40% (Kawashima, 1991), while in areas of abundant solar radiation, early morning enrichment of CO2 increased crop yield by 16% (Shigeno et al., 2001). The effect of CO2 enrichment on increasing yield is well established; however, a normative CO2 enrichment strategy has not been determined (Mizukami et al., 2011). Moreover, a previous study revealed the ineffectiveness of industrystandard methods for enriching CO2 (Miyoshi et al., 2013). At our experimental site in the northern Kyushu area during the winter strawberry production season, CO2 was generally enriched in the greenhouse using a fuel-burning CO2 generator only early in the morning. CO2 concentrations inside the greenhouse began to increase with the beginning of CO2 enrichment and reached a maximum of approximately 2,000 ppm. However, beginning greenhouse ventilation to cool the greenhouse rapidly decreases the CO2 concentration inside the greenhouse. Consequently, the CO2 concentration decreases to outdoor levels before noon, when photosynthetic photon flux density (PPFD) becomes sufficient for strawberry crop photosynthesis. Thus, greenhouse ventilation renders CO2 enrichment ineffective with regard to increasing photosynthesis in the strawberry crop. In this study, we propose a new local CO2 enrichment system. The system enriches CO2 in close proximity to the crops and thereby enables efficient control of CO2 concentration even when the greenhouse ventilation system is active. We applied the system to a strawberry greenhouse with ventilation windows to examine the effect of this local CO2 enrichment. We also examined the effects of CO2 enrichment system on physiological functions such as photo-