Effects of quality and lighting period of supplementary light on growth of spinach
人工照明を用いた実験により，補光の光質と照射時間帯がホウレンソウの生長に及ぼす影響を調べた。昼間が低照度の場合（PPFD100μmol m-2 s-1）に青色光を増やす，または遠赤色光を減らすと，草丈および葉柄と茎の乾物割合が抑えられることが明らかになった。一方,昼間の低照度条件を補うために長時間の夜間補光を行うと，乾物重が増加するが，増加量は光質により異なり，夜の終了時に青色光を照射するか，夜の開始時に赤色光を照射すると増加が大きかった。中照度条件下（PPFD300μmol m-2 s-1）で短時間の夜間補光を行うと，夜の終了時に青色光を照射するか，夜の開始時に赤色光を照射した場合にのみ乾物重が24％増加した。これらの結果から，夜の終了時に青色光を照射すると，徒長の抑制と光刺激反応による生長促進の効果が得られることが明らかになった。
This study sought to determine the effects of quality and lighting period of supplementary light on growth of spinach through experiments using artificial light in growth chambers. The effects of increasing blue light and increasing far-red light (reducing red to far-red photon ratio, R: FR ratio) on growth were investigated under the constant photosynthetic photon flux density (PPFD) of 100 μmol m-2 s-1 during the daytime. When blue light increased, dry matter distribution to petioles and stems, and plant height decreased. A reduction in R: FR ratio increased dry matter distribution to petioles and stems, and plant height. Therefore, both increasing blue light and reducing R: FR ratio resulted in suppression of excessive elongation. The spectral effects of supplementary light irradiated at night on the growth were examined in spinach grown at 100 (low level) and 300 (medium level) μmol m-2 s-1 of PPFD during the daytime. Plants grown under the low level of PPFD during the daytime were exposed to 50 μmol m-2 s-1 of supplementary light for six hours at the beginning, the middle, or the end of the night. Acceleration of growth including increases in total dry matter, plant height and leaf area was caused by exposure to any of the supplementary lights, blue, green, or red. These supplementary lights did not cause any difference in acceleration of growth when the lights were irradiated at the middle of the night, while more acceleration of growth was caused by exposure to both red light rather than blue light at the beginning of the night, and blue light rather than red light at the end of the night. Under the medium level of PPFD during the daytime, plants were exposed to 50 μmol m-2 s-1 of supplementary light for 30 minutes at either the beginning or the end of the night. As a result, brief exposures to both blue light at the end of the night and red light at the beginning of the night induced the acceleration of growth including 24% increase of total dry matter. The other two combinations of quality and lighting period of supplementary light during the night, however, did not produce any accelerative effects on growth. These results suggest that not only photosynthesis but also low-energy responses to blue and red light bring about acceleration of dry matter production by supplementary lighting at the beginning and the end of the night. Supplementary blue-light can suppress excessive elongation of spinach as well as kidney bean. In addition, blue light at the end of the night can induce more increases of total dry matter and dry matter ratio of eatable leaves while keeping the increase of plant height less, consequently being very effective in improvement of growth.