Effects of low light levels during the dark period on flower development in Euphorbia pulcherrima

Euphorbia pulcherrima is an important floricultural crop that naturally flowers in response to long night lengths. We observed that earlier flowering cultivars have a shorter critical night length and flower initiation occurs earlier under natural photoperiods. We compared cultivar responses to low levels of light during the long dark period. Four cultivars were grown under natural photoperiods or covered with black cloth for a 16-hour dark period. During the dark period, plants were given 0, 1, 7, or 35 lux of light continuously from an incandescent lamp. The 7 and 35 lux light levels prevented flowering of all cultivars under both natural photoperiods and the 16-hour dark periods. Under 16-hour dark periods, 1 lux delayed flowering of 'Orion', 'Red Splendor', 'Freedom Red' and 'Peterstar Red' by 11, 20, 25, and 39 days, respectively. Under natural photoperiods the delay was increased by 1 to 9 days. This study indicated that low light delayed flower initiation, but did not affect rate of bud development following initiation. Also, the low light affected cultivars that naturally flower later more than it did the early flowering cultivars, such as Orion.