Time and Duration Effects of Night Cooling Period on Growth and Flowering of Potted Marguerite Lines under High-temperature Conditions

This study investigated the time and duration effects of night cooling period on plant growth and flowering to optimize cooling techniques for potted marguerite lines, focusing on the widely grown cultivars ‘Sunday Ripple’ and ‘Furenka’, which are known for their early-flowering traits and commercial significance. Three cooling treatments—overnight, end-of-day (EOD, 4 h post-sunset), and end-of-night (EON, 4 h pre-sunrise) cooling—were compared with a non-cooled control. The EON-cooling treatment accelerated flowering and reduced the leaf number in ‘Sunday Ripple’, while the EOD-cooling treatment produced similar effects in ‘Furenka’. Subsequent experiments evaluated the effects of 4-h nightly exposure to 21°C at different time periods on growth, flowering, and flower bud differentiation and development. Both EOD-21°C and EON-21°C treatments promoted flower bud differentiation in ‘Sunday Ripple’, with the EOD-21°C treatment proving most effective for ‘Furenka’. The final experiment investigated the optimal period of time and duration of the nighttime cooling. In ‘Sunday Ripple’, days to bud formation and flowering remained unaffected during both the first (FC, starting 5 weeks post-pinching) and the second (SC, starting 6 weeks post-pinching and lasting for 5 weeks) cooling periods, compared with those of the non-cooled control. However, in ‘Furenka’, EOD-cooling during the first cooling period (EOD-FC) accelerated flower budding and flowering similar to those in the continuous EOD-cooling (EOD-C) treatment and resulted in earlier flowering than that in the control. The EON-cooling and EOD-FC treatments are expected to substantially reduce cooling degree hours (CDH) compared with those needed with overnight cooling. These results show that EON-cooling for ‘Sunday Ripple’ and EOD-FC for ‘Furenka’ effectively promoted flowering. Additionally, these cooling techniques significantly reduced production costs, highlighting their potential as efficient cultivation methods for high-temperature conditions. Our study offers practical solutions for mitigating delayed flowering, a prevalent production challenge in high-temperature conditions.