Far-red light enhances removal of pericarps in tartary buckwheat (Fagopyrum tataricum Gaertn.) sprout production under artificial lighting

• Far-red (FR) light removed pericarps and opened cotyledons in tartary buckwheat.
• Removal of pericarps after FR irradiation was related to low red to FR ratio.
• The longer FR light irradiated, the higher the proportion of pericarps removed.
• FR light didn’t remove pericarps until plants exposed a part of cotyledons.
• Pericarps were removed by FR light at the apical seedling, not the hypocotyl.

When tartary buckwheat (Fagopyrum tataricum Gaertn.) sprouts retain pericarps, their market value decreases. In this study, we investigated whether light conditions can enhance removal of pericarps for sprout production under artificial lighting. Sprouts were grown under consecutive irradiation with white fluorescent lamps (FL), far-red (FR) FL, blue light-emitting diodes (LEDs), green LEDs, or red LEDs at a photosynthetic photon flux density (PPFD) of 70–80 μmol m−2 s−1, except for FR (photon flux density of 24 μmol m−2 s−1) for 6 d. Almost all sprouts grown under FR FL opened cotyledons and lost their pericarps, while those grown under the other lights did not. Since neither the fresh nor dry weight of sprouts irradiated with FR light was significantly heavier than that of sprouts grown under the other lights, removal of pericarps by FR lighting probably results from the opening of the cotyledons. Adding red to FR light irradiation prevented removal of pericarps more than adding other lights to FR. Furthermore, lighting with white FL, which contains red and a little FR light in addition to FR FL, enhanced the removal of pericarps. Thus, a low ratio of red to FR, which is known to be an important light environmental factor in all plant species, is likely to be associated with morphogenesis. Enhancement of pericarp removal by FR light irradiation did not occur in plants grown for less than 2 d after sowing, when they were taking root but had hardly exposed their cotyledons and hypocotyls. On the other hand, plants grown for more than 3 d, which had exposed hypocotyls and a part of the cotyledons, had significantly greater pericarp removal in response to FR light. To investigate the organs that need to be irradiated for removal of pericarps, FR FL and white FL were used to irradiate apical seedlings and hypocotyls, respectively. FR irradiation of the hypocotyl did not enhance pericarp removal compared with irradiation of apical seedlings. The longer the FR irradiation continued, the more pericarp removal was achieved, because this phenomenon is irreversible. Therefore, these results suggest that light of low red to far-red ratio irradiating the apical seedling once it exposes the hypocotyl and at least a part of the cotyledons, for as long as possible will help remove the pericarps efficiently.