Introduction

Cut rose plants are heliophytes and thus sufficient quantity of light must be secured for good growth and flowering. The light requirement of greenhouse roses was more or less photosynthetically active radiation (PAR) 900 μmol • m⁻² • s⁻¹ (RDA 2001). In winter cut rose greenhouse production used to decrease in yield of cut flowers due to the low light intensity and short day-length (Kim and Lieth 2012). As roses have characteristics of displaying less physiological disorder under the continuous light conditions unlike other crops, supplemental lighting secured in this season their quantity of harvest is secured by supplementing the insufficient daylight through lighting during nocturnal period.

By late 1960s, it has been revealed that the supplemental lighting with fluorescent lamps was effective for the increase of yield. Since 1975, advancement in the technology enabled commercial supplemental lighting with high pressure sodium lamps (HPS), which produces light intensity that ranges from PAR 60-70 to PAR 150-300. While the effect of supplemental lighting appears differently in different cultivars, but lighting generally increases stem length and fresh weight (Zieslin and Tsujita 1990), production rate (Bredmose 1994), shoot occurrence and flower promotion (Sarkka and Christian 2003), enhances photosynthesis (Zieslin and Tsujita 1990), and improves yield by reducing bud blindness (Khosh-Khui and George 1977). However, due to the relatively low height of the greenhouses used for rose cultivation in Korea, light sources for supplemental lighting are installed close to the beds, the distance between the lamps and the plants being 100-120 cm in average. Due to this, light distribution over the beds is uneven, and heat from the light source affects the flower and results in the increase of floral malformation (Doi et al. 1991). Despite this, majority of the studies, both domestic and international, focuses on the benefits of supplemental lighting, and thus the studies on the limits of supplemental lighting and solutions for problems occurring from it are required. Therefore, this study was conducted to investigate the effect of supplementary lighting intensity (SLI) of high pressure sodium lamps on the shoot growth and the flower quality of the cut rose plants, which will be useful for heat injury reduction and flower quality improvement in cut rose production.

Materials and Methods

Plant materials and SLI treatments

Bare-rooted cuttings of Rosa hybrida ‘Pink Bell’ were transplanted on 25^th August 2010 in rockwool slabs (100 cm long, 15 cm wide, and 7.5 cm deep, UR Rockwool, Pocheon, Korea) with five plants per slab. After twice flowering cycles, the experiment was conducted from 17^th November 2010 to 30^th January 2011 in an environmental-controlled glasshouse at the University of Seoul. During experiment, the rose plants were treated with five SLIs: PAR 0 (control), 30, 50, 70, and 90 respectively. The supplemental lighting periods were 16:00-22:00 and 02:00-10:00. High-pressure sodium lamps (GEO-NH 400W-L/P, Daekwang, Yeosu, Korea) were used as the light source and installed at 1.4 m height above the rockwool slabs. The treated supplemental lighting intensity was controlled by shading curtain. Ten plants were replicated for each treatment. The nutrient solution was composed of Ca(NO₃)₂ • 4H₂O, KNO₃, EDTA-Fe, Mg(NO₃)₂ • 6H₂O, (NH₄)₂PO₄, MnSO₄ • 5H₂O, ZnSO₄ • 7H₂O, H₃BO₃, CuSO₄ • 5H₂O, and (NH₄)₆M_O7O₂₄ • 4H₂O provided 1841, 2323, 64.5, 204.8, 575, 12.05, 8.63, 9.27, 1.25, and 0.88 g •m⁻³, H₂SO₄ provided 281 mL, respectively (EC 1.0 ds •m⁻¹, pH 6.0 ± 0.2). Pesticides were applied as needed throughout the growing period.

Shoot growth and leaf photosynthesis

Shoot length was scaled every two days for one month since treatment. When two outer petals were unfolded, the shoot was harvested as a cut flower and measured the shoot growth and flower quality: its length and weight, number of nodes, peduncle weight, and number and length of petals. In addition, the color of 4^th to 6^th outer petals was determined by colorimeter (JX-777, MINOLTA Co., Japan). Lightness (L value), redness (a value), and yellowness (b value) were calculated.

The photosynthesis was measured for the first fully unfolded leaf (three-leaflet) from the peduncle by portable photosynthesis system (LI-6400, Li-Cor, Lincoln, NE, USA). The photosynthetic photon flux density (PPFD) in the LI-6400 leaf chamber was set at the value of 900 μmol • m⁻² • s⁻¹, the CO₂ concentration at 400 μmol CO₂ • mol⁻¹ air, the leaf temperature at 25℃, and the relative humidity of the incoming air at 65-70%. The leaf chlorophyll and carotenoid contents were detected in the 2^nd or 3^rd leaf (five-leaflet) from peduncle by spectrophotometer (UV-2450, Shimadzu, Kyoto, Japan). As described by Wellburn (1994), leaf samples were cut thinly and grinded, and then the mixture of 0.1 g was put into a test tube, and 10 mL of 100% methanol was added. Then the test tubes were stored for 24 hours in the dark. Absorbance was analyzed at the wave lengths of 470, 652, and 655 nm.

Heat stress and injury

On 11^th January 2011, the surface temperature of flowering shoots under different SLI during night time (20:00-22:00) was detected by Infrared camera (Testo 875-2, Testo, Korea). In order to observe the petal heat injury, the upper part of the third outer petal was sampled in the size of 5 × 5 mm as soon as flowering shoots harvested. The samples were fixed in fixing agent prepared by mixing 2% paraformaldehyde and 2% glutaraldehyde solution (0.05 M sodium cacodylate buffer, pH 7.2). They were washed 3 times with sodium cacodylate buffer (0.05 M, pH 7.2) at 4℃ at the interval of 10 minutes, and then were fixed again in fixing agent of 2% osmium tetroxide (0.1 M sodium cacodylate buffer, pH 7.2) for 2 hours at 4℃. After then the samples were washed again two times with distilled water at room temperature. After this procedure was done, the samples were dehydrated by applying ethanol (30, 50, 70, 80, 90, 100, 100, and 100%) every 10 minutes, soaked 2 times into 100% isoamyl acetate for 15 minutes, and dried with critical point dryer. The dried samples were coated on the surface and were observed through scanning electronic microscopy (SEM) (AURIGA, Carl Zeiss, Germany).

Statistical analysis

The statistical analysis including analysis of variance (ANOVA) was conducted using the statistical analysis system (version 9.3, SAS Institute Inc., Cary, NC, USA). The significance was verified by Duncan's multiple test methods (DMRT P = 0.05).

Results and Discussion

Shoot growth and flower quality

Compared to no supplemental lighting condition (control group), shoot elongation was promoted as SLI increased, highest at PAR 90 (Table 1). It was consistent with the reports by Kim and Lee (2008). Brand (1970) also reported that excessively low light intensity caused serious decrease in shoot length, lateral shoot number, leaf growth, and shoot dry weight. Due to rapid shoot growth, however, the distance between the floral buds of shoots and the light source became closer and on the 30^th day, the distance was as close as 80 cm or less in SLI PAR 90 treatment group. Shoot growth indicators such as shoot weight, peduncle length, and petal length was increased while significantly shortening the average days to flowering. On the other hand, flower quality indicators such as petal number, biomass distribution to the petals, and petal pigmentation were as worse as SLI increased to PAR (Table 2). The color of the petals became gradually brightening and reddish as SLI PAR 70 and then soon faded out PAR 90. In fact, the standard color of 'Pink Bell' was most similar to the petals treated with PAR 50. It was reported that the color of petals in New Guinea Balsam also had faded out under either strong light intensity or weak intensity (Lee et al. 2007).

Thus, the intensity of light could be one of the important external factors for petal pigmentation with the negative effect by insufficient or excessive lighting intensity.

Photosynthesis and chlorophyll content

Photosynthetic rate and chlorophyll content increased in proportion to SLI (Table 3). In general, the chlorophyll content in light sensitive plants was decreased when light level was insufficient and the photosynthetic rate was declined accordingly (Bjarkman and Holmgran 1963). In this study, the plants without supplemental lighting (control group) also showed the same response to low light intensity condition where the photosynthetic rate and chlorophyll content were reduced about 30% and 47% respectively. The carotenoid content also increased as SLI increased from no supplemental lighting to PAR 70, 8.7 mg • g⁻¹ to 16.33 mg • g⁻¹ respectively. Meanwhile at PAR 90 the carotenoid content dropped to 44% compared to PAR 70. It is known that carotenoid acts as a kind of photoprotection agents in green plants, rapidly quenching excited state of chlorophyll (Taiz and Zeiger 1991). This result showed that photoprotection in plants might have been disrupted by excessively high SLI with heat stress induced by continuous high SLI PAR 90. Therefore it could be said that the carotenoid had been more sensitive or responsive to the change in light intensity or temperature rather than chlorophyll in cut rose plants.

Heat stress and injury

The shoots during supplemental lighting by HPS lamps showed the temperature gradient in stems in accordance with the height (Fig. 1). The temperature gradient was extended in proportion to SLI. The lower part of stems was maintained about 14-15℃ nevertheless of SLI, but the upper part, especially floral buds, showed up to 10℃ difference among all SLIs. As to PAR 90 (Fig. 1E), the stem temperature was kept high in whole plant. Among shoot parts, the floral buds were most sensitive to heat stress and changeable as SLI. In rose plants continuous lighting with excessive intensity was possible to induce water stress in leaves or petals and influence significantly shoot growth and flower quality, reducing yield up to 70% (Chimonidou-Pavlidou 1999). This result means that water stress at the transition phase of shoot apical meristem to floral bud was negative to flower development (Dela et al. 2003). In addition, the petals treated by PAR 90 showed morphological distortion in the surface compared to those of the control group (Fig. 2). In order to find anatomically tissue injury by supplemental lighting accompanied with heat stress or water loss, the surface of the petal was observed through SEM. As a result, the epidermal cells of the petals which were treated with PAR 90 were partly damaged unlike those of the control group (Fig. 2). This malformation in the petals was a kind of physiological disorder occurred due to unfavorable environment in certain floral development stage (Moe 1971). Considering that excessively high temperature in stamen organogenesis stage caused malformation in floral buds and decreased cut flowers quality in Gypsophila paniculata (Doi et al. 1991), it is likely that high nocturnal temperature by supplemental lighting has greatly affected floral development in roses, leading to malformed flowers (Shi and Kim 2014).

In conclusion, our research has found the fact that supplemental lighting promoted shoot growth in proportion to SLI, meanwhile excessive SLI of PAR 90 dropped the flower quality such as floral bud development by biomass distribution, petal number and color in cut rose plants. This negative effect of supplemental lighting by HPS lamps was due to domestic greenhouse structure which has relatively low height inappropriate to supplemental lighting. The shoot top is able to be closer than necessary to the lamps, which causes heat stress with water loss, eventually tissue injury in petals. Therefore, it is necessary to avoid heat injury by switching light intensity according to shoot growth condition and greenhouse structure.