The Reaction to Daylength of Sunflower Cultivars Grown as Cut Flowers

Sunflower cultivars grown as cut flowers typically flower in 60 to 80 days after sowing when planted in the field during summer.  In conditions of short daylength, flowering of some cultivars is speeded up by as much as 3 weeks, indicating that they are facultative short day plants.  The work of Blacquiere et al. (2002), indicating that sunflowers are sensitive to daylength during the first 3 weeks after emergence suggests that a pre-transplant screening test could be used to classify cultivars on their daylength response.  Accordingly, a total of 42 cultivars was sown in 72-cell seedling trays in 2006 to 2009, exposed to either 12 or 16 hr photoperiod on greenhouse benches for 3 weeks, and  then transplanted to the field, where they were harvested at flowering, and plant height, flower diameter and leaf number on the main stem measured.  In addition, in each screening trial, ‘Sunrich Orange’ and ‘Procut Lemon’ were included as standard cultivars.  The detailed results for the 42 cultivars screened indicate that the reaction to the daylength treatment imposed ranged from a facultative short day response to a facultative long day response, with 18 cultivars (43%) that showed less than a 7-day difference between long and short day treatments, and thus were considered daylength neutral.  Of the 31 cultivars with a 7 day or longer delay in flowering in long days, 6 were delayed from 7 to 13 days, and were considered moderately short day sensitive.  Seventeen were delayed more than two weeks, and were classed as extreme short day types.  One cultivar, ‘Procut Bicolor’ flowered 6 days earlier after long than short-day treatment, and was considered to show a moderate long-day response.  A correlation analysis of daylength for the two weeks after transplanting with flowering times of the two standard cultivars indicated that ‘Procut Lemon’ may show a short-day response during that time.