Influence of Suboptimal Temperature on Flower Quality and Floral Organ Development in Spray-Type Cut Rose ‘Pink Shine’

Low temperatures commonly delay flowering in cut roses but enhance final flower quality, i.e., biomass, petal doubling, and flower size. However, this information remains unclear for spray-type cut roses. This study was conducted to understand the effect of suboptimal temperatures on flower quality in the spray-type cut rose ‘Pink Shine.’ The 6-month-old rooted cuttings were cultivated in environmentally controlled growth chambers at four temperature levels: 25/20 °C (optimal temperature, OT) and 20/20 °C, 20/15 °C, and 15/15 °C (suboptimal temperatures, SOTs). As expected, SOTs significantly delayed the flowering time (11.2–25 days) but enhanced flower quality, with 51% and 160% increases in flower size and biomass, respectively. SOTs did not statistically amplify petal numbers, as expected, compared with OT. Instead, SOTs significantly increased stamen and carpel numbers by 1.3 and 2 times, respectively, resulting in a 1.4-fold increase in total floral organ formation. Moreover, SOTs increased the mRNA levels of A-function genes (RhAP1** and RhFUL**) and C-function genes (RhSHP*) but suppressed the B-function gene (RhPI*), which is linked to the development of plant reproductive structures (stamen and carpel) in spray-type cut roses. Conclusively, the growth temperature was more effective for quantity accumulation than for the number of petals but was similar in carpels. These results suggest that SOTs enhance carpel differentiation during flowering, implying that flowers may choose a reproductive strategy through carpels over petals.