Effects of Planting Density and Air Current Speed on the Growth and Uniformity of Qing-geng-cai and Spinach Plug Seedlings in a Closed Transplant Production System

Qing-geng-cai (Brassica chinensis L., cv. Seitei) and spinach (Spinacia oleracea L., cv. Sunlight) seedlings were grown in plug trays at 3 planting densities and 2 air current speeds either in a closed transplant production system (CTPS) or in a greenhouse in which the air current speed was not controlled. The growth parameters such as dry mass, plant height, and leaf area; uniformity of qing-geng-cai; and growth of spinach seedlings decreased with the increase in planting density in the CTPS. At higher air current speeds, the growth of the qing-geng-cai seedlings decreased. However, the growth of the spinach seedlings was not affected by the air current speed. Based on these results, the differences in the extent of mutual shading and space in the seedling populations caused by the differences in canopy height and leaf angle appear to be responsible for differences in the effect of air current speed on the growth of seedlings of each species. The growth and uniformity of qing-geng-cai and the growth of spinach seedlings were greater in the CTPS than in the greenhouse. Based on the abovementioned results, we conclude that growth in the CTPS facilitates denser seedling production and shorter production period in comparison to that in the greenhouse.