Effect of low temperature and solar radiation on dry-matter production, fruit yield and emergence of malformed fruit in strawberry (Fragaria × ananassa Duch.)

In Japan, most strawberries are grown by forcing culture, and fruit is harvested from November to June. Low temperature and solar radiation are major factors behind the unstable yield in winter. To clarify the effect of substrate heating and shading in winter in Japan, we investigated dry-matter production, fruit yield and causes of malformed fruit emergence in strawberry. We compared strawberry 'Mouikko' plants grown under combinations of substrate heating (25°C) and no heating, and 55% shading and no shading under forcing culture. Substrate heating and shading started on 8 December 2015. Total above-ground dry weight in the substrate-heating+no-shading treatment was significantly higher than that in other treatments, except on 15 January. In both shading and no shading treatments, substrate heating increased total above-ground dry weight significantly. There were significant differences in total above-ground dry weight as a result of substrate heating and shading. Leaf area showed a similar tendency to total above-ground dry weight. Total fresh-fruit yield was highest in substrate heating+no shading, and significantly lowest in no substrate heating+shading of the four treatments. On the other hand, the rate of malformed fruit was significantly highest in no substrate heating+shading and lowest in substrate heating+no shading. There were significant differences in the number of normal fruit, total number of fruit and malformed fruit rate in relation to substrate heating and shading. Fruit yields with substrate heating were significantly higher than those without substrate heating at the end of January and the beginning of April. These results show that, at low temperature and low solar radiation, emergence of malformed fruit may be influenced by lower levels of assimilation products caused by the decreased leaf area. Thus, substrate heating may decrease the rate of malformed fruit and increase fruit yield through an increase in total above-ground dry weight.