Acceleration of growth in spinach by short-term exposure to red and blue light at the beginning and at the end of the daily dark period

Spectral effects of supplemental light irradiated during the dark period on growth were examined in spinach grown at either 100 (low level) or 300 (medium level) mumol m(-2) s(-1) of photosynthetic photon flux density (PPFD) during the light period. Plants grown under the low level of PPFD during the light period were exposed to 50 mumol m(-2) s(-1) of supplemental light for six hours at the beginning, the end, or the middle of the dark period. Acceleration of growth including increases in total dry matter, plant height and leaf area was caused by exposure to any of the supplemental lights, blue, green, or red. These supplemental lights did not cause different acceleration of growth by irradiating at the middle of the dark period, while more acceleration of growth was caused by exposure to red light rather than blue light at the beginning of the dark period, and blue light rather than red light at the end of the dark period. Under the medium level of PPFD during the light period, plants were exposed to 50 mumol m(-2) s(-1) of supplemental light for 30 minutes at either the beginning or the end of the dark period. As a result, brief exposures to both blue light at the end of the dark period and red light at the beginning of the dark period produced the acceleration of growth including about 20% increase of total dry matter. The other two combinations of quality and lighting hour of supplemental light during the dark period, 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 an increase in total dry matter and other accelerative effects by supplemental lighting at the beginning and the end of the dark period.