The role of phytohormones in flowering and bolting of spinach (Spinacia oleracea L.) under mid-night lighting

Supplemental lighting during the middle of the night (mid-night lighting) promotes growth of some leafy vegetables. In this study, we investigated the effects of different intensities of mid-night lighting on the growth of spinach, and the correlation between phytohormones and bolting under mid-night lighting. Plants were exposed to lighting from 23:00 to 7:00 the next morning. The top fresh weight of spinach under high (100 μmolm^-2s^-1) and low (10 μmolm^-2s^-1) light intensities was approximately 3 to 5 times higher than the control (no lighting). In addition to a high growth rate, bolting was also induced with mid-night lighting. However, bolting was delayed under high light irradiance compared to low light irradiance, accompanied with inhibition of flower bud development under the mid-night lighting at high light irradiance treatment. IAA content in spinach under high light irradiance treatment became about twice that of spinach under low light irradiance treatment or control. However, applications of IAA and IAA inhibitors to spinach plants had no clear effect on flower bud development and bolting in any treatment. On the other hand, application of GA₃ to spinach plants rapidly induced bolting under all treatments. Additionally, bolting was delayed by prohexadione-calcium or uniconazole applications under the mid-night lighting. There was a significant difference in GA contents among mid-night lighting treatments, GA₂₀ and GA₁ levels were about 80% higher under low light irradiance treatment than under high light irradiance treatment. These results indicate that a high light irradiance of mid-night lighting inhibits GA biosynthesis as compared to low light irradiance treatment, and that a relative lower GA level in the high light irradiance treatment may retard bolting and flowering.