• Introduction
• Materials and Methods
• Plant materials
• Stenting and treatments
• Experimental conditions
• Growth measurements and design
• Statistical analysis
• Results and Discussion
• Acknowledgements

Introduction

Successful adventitious rooting during stenting propagation depends upon several factors, including the physiological condition of the stock plants and the environmental conditions during adventitious root formation. Factors affecting rooting in the stenting propagation are node position, number of leaflets left and picking time on the cuttings, light intensity, temperature, humidity, medium, and plant growth regulators. Also, it is well known that the original leaf left on single node softwood stem cuttings of roses has a strong effect on survival and rooting of cuttings (Moe, 1973). 

The effect of light on flower production in roses is two-fold. First, light affects the number of buds developing from the base of the plant and the remaining part of a branch after harvesting the flowers. Second, light influences flower development. Although initiation of the flower primordium is independent of light intensity and photoperiod (Zieslin and Moe, 1985), further development of the flower bud may stop, resulting in a blind shoot. The quantity and quality of the light may affect vegetative and flower bud development. By increasing photosynthetic photon flux density (PPFD), number of days from excision until flowering of  roses is reduced (Moe, 1972). There are indications that increased PPFD may have a more pronounced effect on production time from the onset of bud growth through flowering than from visibility of the flower bud until anthesis (Berninger, 1994). Bredmose (1998) reported that increasing the daily quantum integral from 17.8 to 21.0 mol ·m^-2 · d^-1 increased fresh biomass efficiency, stem diameter, and specific fresh mass, while decreasing number of nodes, number of five-leaflet leaves, plastochron value, and stem length at anthesis.

There have been only a few studies (Park and Jeong, 2010; Park and Jeong, 2012) on the rooting and growth of stenting-propagated cut rose. This study was conducted to investigate the effect of light intensity during a winter season on rooting and subsequent growth of two stenting-propagated domestic cut rose (Rosa hybrida Hort.) ‘Pink Aurora’ and ‘Yellow King’ in an effort to develop an efficient stenting propagation method for rose cultivars. 

Materials and Methods

Plant materials

 Plant materials, grown in a commercial rose farm (Dowon Rose Farm, Gimhae, Korea), consisted of flowering stems with full-grown leaves and just opening flowers. After normal harvesting, each individual stem was kept apart and cut into sections with a five-leaflet leaf and a dormant bud. First grade flowering shoots were harvested at the stage when two sepals were free from the flower bud (Jensen and Hansen, 1971). Two cultivars of domestic cut rose used in this study were a standard cultivar ‘Pink Aurora’ and a spray cultivar ‘Yellow King’. Rosa indica ‘Major’ was grown as a rootstock material in a commercial greenhouse (Borame Rose Farm, Gimhae, Korea). The softwood material was harvested at a stage when leaves are well developed and thorns can be broken off easily (van de Pol et al., 1986).

Stenting and treatments

 As a rootstock, a piece of stem consisting of a single internode without a bud was used. The rootstock cuttings were removed of all leaves and buds (Fig. 1). Top of the rootstock internodes and the basal part of the scions were cut at an angle of 45^˚ for grafting. For a good development of the graft, partners were in close contact. Scionrootstock unions were stuck in rockwool cubes (5 cm × 5 cm× 5 cm, Delta, Grodan, Denmark) and were placed in a graft-take chamber for five days before being placed on misted greenhouse bench in Boramae Rose Farm, Gimhae, Korea. Scion-rootstock unions of a standard cultivar ‘Pink Aurora’ and a spray cultivar ‘Yellow King’ were planted in rockwool cubes on Dec. 29, 2008. Three shading levels used were 0, 35, and 55% of the incident sunlight measured in the greenhouse. Shading treatments were used 35 and 55% of the shading screen (G 301 and G 305, Danong Co. Ltd., Korea) placed on greenhouse bench.

Fig. 1. The course of stenting used in Rosa hybrida: A and B, preparation of harvested scion (A) and rootstock (B); C, both base of the scion and top of the stock cut simultaneously at a 45˚ angle for grafting; D, uniting the cut surface of a scion and a rootstock using a piece of split tube; E, wrapping the united and tubed area with parafilm; and F, grafted tissues stuck in a rockwool cube and placed on a misted propagation bench.

Experimental conditions

 Mean daily air temperature and relative humidity measured during the experimental period by a digital thermometer (Thermo Recorder TR-72U, T&D Corp., Japan) were 23.9^˚C and 76.1% in the greenhouse. The outdoor PPFD measured with a digital photometer (HD2102.1, Delta OHM, Italy) was 764.8 μmol ·m^-2 · s^-1, and 0, 35, 55% of the shading PPFD was 650, 228, 125 μmol ·m^-2 · s^-1,  respectively.

Growth measurements and design

 At 62 days after stenting, percent rooting, percent graft-take, shoot length, number of roots, length of the longest root, fresh weight, and dry weight were measured. The propagation experiment included two cultivars and 36 plants per replication with three replications.

Statistical analysis

 The treatment plot was laid out in a completely randomized design. Data collected were analyzed for statistical significance by the SAS (Statistical Analysis System, V. 9.1, Cary, NC, USA) program. The experimental results were submitted to an analysis of variance (ANOVA) and Duncan’s multiple range tests. Graphing was performed with Sigma Plot 10.0 (Systat Software, Inc., San Jose, CA, USA).

Results and Discussion

 Figure 2 shows effect of light intensity on the shoot and root growth of cut roses measured at 62 days after stenting. The shading used in stentingpropagated roses was significantly affected to shoot length, number of roots, length of the longest root, fresh weights of shoot and root, and root dry weight (Table 1). The greatest shoot length (9.2 cm) was found in the no shading treatment and the least one (6.7 cm) in the 55% shading treatment. Number of roots increased with increasing light intensity in ‘Yellow King’. These results correspond well with results showing advanced axillary bud growth and shoot development of rose plants at low plant density or high PPFD integral (Bredmose, 1997). In roses, axillary shoot length was positively correlated with root weight of cuttings (Bredmose et al., 2004). In this study, fresh and dry weights of the root were the greatest in the no shading treatment, where the length of the developing axillary shoot was the greatest. However, whether the onset of axillary bud growth was caused by root formation is not clear as Dubois and de Vries (1991) also noticed.

Fig. 2. Effect of light intensity on the shoot and root growth of two rose cultivars measured at 62 days after stenting. A, ‘Pink Aurora’; and B, ‘Yellow King’.

Table 1. Effect of light intensity during propagation on the growth of two rose cultivars measured at 62 days after stenting.

 In both cultivars, length of the longest root was the greatest in no shading and it decreased under lower light intensities. However, chlorophyll content was not affected significantly by shading. Root to shoot relationships in cuttings has been reported with respect to fresh weight (Kuris et al., 1980) and growth rate (Mertens and Wright, 1978). The effect of number of adventitious roots on axillary bud growth and subsequent shoot growth has been studied. Gad et al. (1987) reported for Pelargonium that number of roots per cutting was positively related to the subsequent growth of the cutting. According to Halevy (1996) and Marcelis-van Acker (1994), rose shoot growth is mainly dependent on the assimilate supply available during growth, and developing flowers make intensive use of carbohydrates and other metabolites. Mor et al. (1980) showed that PPFD received by the rose shoot tip promoted shoot sink activity by increasing the sucrose unloading process.

 In both cultivars, rooting and root growth were accelerated and percent rooting increased under higher light intensities (Fig. 3). Similar results were reported by Moe (1973) who described that rooting of ‘Roswytha’ rose cuttings was enhanced at increased irradiance. The results implese a relation between rooting ability and stored and/or supplied carbohydrates (including photosynthates) or phytohormones (Druege et al., 2000; Hoad and Leakey, 1996). Choi et al. (2000) reported that time for root development decreased and percent rooting increased under higher light intensities. Bredmose (1998) also reported an enhanced response resulting from increased photosynthetic photon flux density (PPFD) for rose cuttings. Generally, high light intensities promoted photosynthesis necessary for root development (Veierskov et al., 1982), while excessively high light intensities were not good for rooting because of water stress (Mudge, 1995).

Fig. 3. Percent rooting of stenting-propagated Rosa hybrida as affected by light intensity.

 Therefore, in this experiment the greatest rooting and subsequent growth of stenting-propagated roses shown in the no shading was probably due to increased assimilate supply and translocation supported by higher PPFDs. These results suggested that additional research is needed on reproductive growth of the crop after transplanting.

Acknowledgements

 This study was carried out with the support of “On-Site Cooperative Agriculture Research Project (Project No. 006330)”, RDA, Republic of Korea. “Yoo Gyeong Park was supported by a scholarship from the BK21 Program, the Ministry of Education, Science, & Technology, Korea.”