Effects of Temperature and Timing/Duration of Night Cooling Treatments on Flowering Time and Quality of Cut Flowers of Standard Type Carnation (<i>Dianthus caryophyllus</i>)

Abstract

The aim of this study was to develop efficient night cooling technology to produce high-quality carnation cut flowers. The effects of temperature and timing of night cooling treatments (applied using a heat pump) on the flowering time and quality of cut flowers were investigated for standard type carnation flowers grown in a greenhouse in hot conditions. In the first experiment, rooted cuttings of the carnation 'Exceria' were planted in a greenhouse on July 6, 2012. Night cooling treatments at 18°C, 21°C, and 24°C were carried out, and the harvested cut flowers were compared with those of a non-cooled control group. For flowers harvested in November, those from plants subjected to the night-cooling treatments had harder stems compared with those of the non-cooled control. In the next experiment, to determine the best timing and duration of the night cooling treatment, we applied three treatments to rooted cuttings planted on June 18, 2013; overnight cooling from sunset to sunrise, cooling for 4 h from sunset (End of Day cooling: EOD-cooling), and cooling for 4 h before sunrise (End of Night cooling: EON-cooling), and compared them with a non-cooled control. For flowers harvested in December, the node order of flowering was the same in the EOD-cooling treatment and the overnight cooling treatment, and was lower than those in the EON-cooling treatment and the non-cooled control. The number of days to flowering was shorter in the EOD-cooling treatment than in the non-cooled control. In October and November in the harvest season when the high temperature influenced the early growth stage, the stem quality of cut flowers was improved by night-cooling treatments, as indicated by a significantly smaller stem weeping angle than that in the non-cooled control group. These results suggested that an EOD-cooling treatment at 21°C for 4 h from sunset was as effective as an overnight cooling treatment to improve the quality of cut flowers by promoting stem hardness and flowering. This method is a costeffective and efficient thermal management strategy to produce high-quality carnation cut flowers in hot conditions.

Verification of the Effects of End-of-Day-cooling on Floral Differentiation and Cut-flower Characteristics in Carnation (<i>Dianthus caryophyllus</i> L.)

“…Further-more, the developmental stages of flower buds at the same node positions proceeded through three stages under EOD-cooling conditions compared with under either non-cooling or EON-cooling conditions. Thus, EOD-cooling in glass greenhouses promotes both bud initiation and flower development, which may increase flower blooming as previously reported (Higashiura et al, 2020). Dahab (1967) reported that the lower the night temperature, the lower the node position of flowering in 'William Sim' carnation.…”

“…We reported previously that an EOD-cooling treatment reduces both the days to flowering and the number of nodes at flowering compared with the control (non-cooling) in carnation (Higashiura et al, 2020). This may result from the promotion of flower differentiation by short-term cooling.…”

“…As a countermeasure to these problems, the use of heat pumps to cool greenhouses at night while growing the standard carnation 'Excerea' has been investigated. In our previous report, an overnight cooling at 21°C improved the quality of cut flowers by hardening stems and promoting flowering (Higashiura et al, 2020). Furthermore, an End-of-Day (EOD)-cooling treatment for 4 h after sunset was as effective as overnight cooling, thereby reducing electricity consumption.…”

“…Moreover, the effects of an End-of-Night (EON) heating at 30°C for 3 h were limited compared with those of the EOD treatment. Similarly, the acceleration in flowering time was greater after EOD-cooling than after EON-cooling (Higashiura et al, 2020). However, it is unknown whether this promotive effect is a result of accelerated floral differentiation.…”

“…The optimum growing temperature for carnation (Dianthus caryophyllus L.) is 18-24°C, and they prefer relatively cooler climates (Higashiura et al, 2020). Therefore, it is difficult to grow high-quality carnations systematically for cutting during the hot season in Japan.…”