Effect of Cyclic Lighting on Photoperiodic Responses in Onion Plant

Abstract

The experiments were carried out to clarify the effect of cyclic lighting on the bulb formation and the interrelationship between light stimulus and darkness associated with photoperiodism, using the cultivarKaizuka-wase'Sensyu-ki' anSapporo-ki'. The plants were started from seed and grown under short-day in greenhouse for 2 or 3 months prior to experimental use. During treatment periods the plants received 8 or 12 hr of sunlight followed by intermittent lighting (500 lx) in light-dark cycle with incandescent lamp during long dark period. Intermittent light were applied for 50, 33.33 and 25% of time in light-dark cycles of various lengths. The shorter the cycles became, the more the bulb formation was induced. Moreover, it was found that relationship between the bulb formation and the logarithm of length of cycle was inversely proportional. The 15-15 light-dark cycle (min) was as effective as continuous light except foSapporo-ki' when light applied for 50% of time in the cycles. When 1 sec lighting followed by various lengths of darkness, the dark duration less than 3 sec promoted the bulb formation as well as continuous light, whereas those of the dark duration more than 5 sec decreased with length of darkness. From above mentioned results, we made the assumption that the ratio of the duration of light stimulus in darkness to the duration of light applied was inversely proportional to the logarithm of duration of light. With this assumption, the expected values obtained by cyclic lighting were in satisfactory agreement with the observed values.