Influence of Monochromatic Light Irradiation as Pre-rooting Treatment on Rooting of Cuttings in Spray-type Chrysanthemums

Abstract

Vegetative propagation through cuttings is a common way of reproduction in chrysanthemums. There are two main ways to grow chrysanthemums commercially in Japan. One is to transplant the unrooted cuttings in a propagation bed or soil block, to root the cuttings, and to transplant rooted cuttings in the field (rooted cutting cultivation). The other is to plant the unrooted cuttings directly in the field (direct cutting cultivation). Since the process of rooting cuttings can be omitted in the direct cutting cultivation, labor time can be greatly reduced compared to rooted cutting cultivation (Shigeki et al., 2006). To produce chrysanthemums stably around the year in Japan, it is necessary to transplant them even in hot summer. It was found that rooting of chrysanthemum cuttings initiated more quickly at a temperature of 30°C, but it required over 7 days to initiate rooting (Dykeman, 1976; Ooishi et al., 1978; Takahashi et al., 1981). Especially in direct cutting cultivation in summer with high temperature and high solar irradiation, leaf wilting and rot of cuttings are frequently observed from transplanting to establishment (Sasaki et al., 1996; Nishio and Fukuda, 1998). Therefore, in direct cutting cultivation, it is necessary to root the cuttings as quickly as possible after transplanting, and to establish those. Pre-rooting treatments that promote the rooting of the cuttings had been studied to reduce the period from transplanting to establishment in direct cutting cultivation. Some studies showed the duration, temperature, and light conditions of pre-rooting treatments, as well as the concentration and soaking time of rooting promoters (Nishio and Fukuda, 1998; Yonekura et al., 1999). Nishio and Fukuda (1998) have reported that light as a pre-rooting treatment has little effect on the formation of root primordia. Meanwhile, Yamamura’s research group has improved the previous treatment conditions to a pre-rooting treatment that can be practiced by growers. Namely, the whole cutting is soaked in a 40 mg g 1 of indole-3-butyric acid (IBA) solution for 10 s and then irradiated with light at 300 lx for 24 h at 20°C for 7 d. Additionally, Yamamura et al. (2006) have developed a system that can treat under these conditions. In this system, a fluorescent lamp has been used as a light source to irradiate more than 300 lx near the shoot apices of the cuttings. However, in the previous study, the mechanism of rooting promotion by light irradiation remains to be clarified. Recently, light-emitting diodes (LEDs) have been practically developed as an alternative light source for horticultural lighting to fluorescent and high-pressure sodium lamps. LED lighting has several advantages compared to existing horticultural lighting, including the ability to reduce electrical energy consumption, the ability to produce high light levels with low radiant heat output when cooled properly, and the ability to maintain a useful light output for years without replacement (Morrow, 2008; Davis and Burns, 2016). Since LED lighting can also control the spectral composition and irradiate with narrow spectrum light, the effects of light quality on the growth