Two-year-old ginseng (Panax ginseng C. A. Meyer, Araliacease family) seedling roots used in this study were obtained from a specialized grower. Selected seedling roots of 10 - 12 g were transplanted into a field of clay loam soil at a density of 33 plants • m^–2 (6 plants × 9 rows, plot size 1.62m²). Plants were grown under a blue polyethylene shade net with 5% light transmission rate, following Rural Development Administration (RDA, 2011) ginseng cultivation standard farming method. The blue polyethylene shading net was opened from 17:00 pm the day before to 10:00 am the next morning for enhancing light transmission inside the shading structures without removing the blue shading sheets that reflected sunlight and protected from the rain. This treatment was conducted for about 60 days from April 23 to June 23 in 2014 at the Agricultural Experimental station of the National Institute of Horticultural and Herbal Science, Soi-myeon, Emsoung, Chungbuk Province, Korea.

Data on growth parameters of the aerial parts including in plant height, stem length, stem diameter, leaf length, and leaf width were collected on July 20. Data on root growth parameters, such as root length, diameter, weight, and the number of lateral roots were collected after harvest on October 20. All growth characteristics were examined in 10 plants and replicated 4 times per treatment plot.

Ginseng leaves were collected between 9:00 am and 10:00 am, when the photosynthetic ability is the highest, on June 20. Leaf sections (4 × 4mm) were put in 2.5% glutaraldehyde solution and stored for 90 min 4°C for the first tissue fixation. Then, they were rinsed with 0.1M phosphate buffer (pH 7.2), for 4 - 5 times at intervals of 15min. Leaf sections were put in 1% osmium tetroxide solution and kept for 90min at 4°C for the second tissue fixation. They were rinsed with 0.1M phosphate buffer (pH 7.2) for 4 - 5 times at intervals of 20min, and treated in 0.1M phosphate buffer overnight. The surface of leaf sections were dehydrated 2 times for 45 min each with 40, 60, 80, 90, 95%, and 100% ethanol solution and dried in a critical point dryer. Before scanning electron microscopic (SEM, S- 2460N, Hitachi, Tokyo, Japan) observation, the mounted samples were gold coated using Ion-Sputter (K-450, Emitech, Ashford, England). Surface pattern and stomatal aperture of ginseng leaves were observed by SEM (S- 2460 N, Hitachi, Tokyo, Japan).

Light quantity was measured about 60cm above the ground between 8:30 am and 9:00 am by a portable illuminometer (CL-200A, Konica Minolta, Tokyo, Japan) on June 20. The light transmission ratios were calculated by the following equation:

Light transmission ratio (%) = (light quantity under shading net / external light quantit ) × 100

Soil moisture and temperature were measured at a depth of 15cm between 9:00 am and 10:00 am on June 20 by an electronic soil-temperature and moisture measuring instrument (T1000H, Mirae sensor, Seoul, Korea). Chlorophyll content of ginseng leaves was analyzed using a portable chlorophyll-measuring instrument (SPAD-502Plus, Konica Minolta, Tokyo, Japan). Net photosynthetic ratio and stomatal conductance were measured on June 20 between 8:30 am and 9:30 am by an LI-6000 portable photosynthesis system (Li-Cor, Lincoln, NE, USA) under natural light with an air influx rate of 500mol• s^–1, a CO₂ concentration of 400mol• s^–1, a relative humidity of 40%, and a temperature of 20°C.

For identifying ginseng pathogens, the infected plants were put on water agar medium. Conidia and conidiophores were identified using stereoscopic and optical microscopes. Severity of leaf blight disease was scored on a 1 - 5 scale based on the proportion of leaf spot disease lesions (1; no disease symptoms observed, 2; only pencilpointed or arrested spots on leaf, 3; symptoms on 25% of leaf area, 4; symptoms on 50% of leaf area, and 5; symptoms on more than 50% of leaf area). Severity of anthracnose blight disease was also scored on a 1 - 5 scale (1; no symptom observed, 2; more than 5 spots with lesions less than 2mm in diameter, 3; less than 10 spots with lesions less than 4mm in diameter, 4; less than 10 spots with lesions less than 6mm in diameter, and 5; more than 10 spots with lesions larger than 6mm in diameter).

Severity of rusty root was scored on a 1 - 5 scale based on the proportion of reddish brown to black root lesions (1; 0%, 2; less than 10%, 3; 11 - 29%, 4; 30 - 49% and 5; more than 50%). The percentage of inside cracking and rough skin were calculated as the number of inside cracking and rough skin to the total number of plants, respectively.

The experiment was analyzed as a randomized block design using SAS (SAS institute Inc., Cary, NC, USA). Analysis of variance in conjunction with student’s t-test was applied to test the significance of mean differences among treatments at p < 0.05 and p < 0.01.

Stomatal aperture of ginseng (Panax ginseng C. A. Meyer, Araliacease family) leaves from plants grown under a shade net with 15% light transmission rate is shown Fig. 1. Enhanced light transmission rate stimulated stomatal opening, results that were in agreement with previously reported studies (Casson and Gray 2008; Roelfsema et al., 2006). Ginseng, as an understory herb plant, has a lower compensation point and appears to be very susceptible to high light intensity and high temperature compared to other full-sun plants (Hwang and Hyun, 1989). Stomatal movement and development is not only influenced by light conditions, but also by water status, CO₂ concentration, and abscisic acid (Casson and Gray, 2008; Roelfsema et al., 2006; Royer, 2001; Xu and Zhou, 2008).

Fig. 1.  Stomatal aperture of ginseng leaves.

A; stomatal aperture of ginseng leaf grown under a shade net with 15% light transmission rate, B; stomatal aperture of ginseng leaf grown under a shade net with 5% light transmission rate (control). Ginseng leaves were collected between 8:30 am and 9:00 am on June 20.

In our study, stomatal aperture was increased with increasing light intensity and temperature. Stomatal frequency (number of stomata • mm^–2) and the number of guard cells are considerably affected by light intensity (Casson and Gray, 2008). In particular, those are increased with increasing light transmission and reach a peak at 15% light transmission ratio (Park, 1980). Guard cells in albino leaf patches do not respond to photosynthetic active radiation (PAR) but are sensitive to blue light, CO₂, and abscisic acid (Roelfsema et al., 2006). Stomatal opening can be stimulated more by blue light than by PAR, which is associated with the photosynthetic ability of mesophyll cells and involved to stomatal opening (Assmann and Shimazaki, 1999; Roelfsema and Hedrich, 2005).

Changes in photosynthetic ability of plants grown under a shade net with 15% light transmission rate and the control plants are presented in Table 1. On June 20, light transmission rates of treatment plot and control plot were 15% and 5%, respectively, a difference that was highly significant. Jo et al. (1986) reported that ginseng plants grown under 10 - 15% light transmission ratios showed the highest photosynthetic ability.

In this study, soil moisture content and temperature of the treatment plot were lower compared to the control plot. Reduction of soil moisture content and increase of soil temperature in the treatment plot was probably due to the enhanced light transmission rate that accelerated water evaporation and increased soil temperature, result that were in agreement with those reported by Cheon et al. (1991).

Net photosynthetic ratio (3.8 μmol• m^–2s^–1) of treated plants was higher than that of the control plants (1.1 μmol• m^–2s^–1), indicating a high value (0.07 μmol• m^–2s^–1) of stomatal conductance compared to the control (0.05 μmol• m^–2s^–1). The effect of environmental factors, such as light intensity, soil moisture content, and temperature of soil and air on photosynthesis in ginseng well studied. In particular, temperature and light intensity have been shown to be highly associated with increased photosynthetic ability (Kim, 1991). Lee et al. (1987) and Lee (2012) reported that most of the cultivars of Korean ginseng, including ‘Chunpoong’, ‘Yunpoong’, ‘Gopoong’, and ‘Jakyungjong’ reach the highest photosynthetic ration 15,000 lux and 20°C. Leaf temperature is also major activator of photosynthetic capacity and its optimal level is 18°C (Hwang and Hyun, 1989; Hyun et al., 1993). However, temperature and light intensity do not act separately, but interact to enhance the photosynthetic ratio in ginseng. Lee (2007) reported that the net photosynthetic ratio was largely influenced by photosynthetically action radiation (PAR) and transpiration rate was more increased in the afternoon than in the morning.

Ginseng plant grown under a shade net with 15% light transmission rate showed a good diversity of certain growth parameters, including stem diameter, yield, fresh root weight, root diameter, and root length compared to the control (Table 2 and 3). In previous studies, enhanced light transmission rate decreased stem length and leaf length, increased stem diameter, and decreased chlorophyll content (Cheon et al., 1991, 2003; Cho et al., 2008; Kim et al., 2012). Cheon et al. (2003) reported that leaf weight was increased with increasing light transmission rate but in general, the growth of the aerial parts shows a decreasing tendency. By contrast, Proctor et al. (2010) reported that the growth of the aerial parts continues to increase up to 30% light transmission rate. Kim et al. (1982) reported that the leaves of ginseng grown in the front row were lager than the ones in the back row because of a large amount of light. In our study, the growth parameters of the aerial parts, such as stem diameter and chlorophyll content, were significantly higher in the treatment plot than in the control plot. Stem height and leaf length decreased with increasing light intensity, while plant height and leaf length and width had no significant differences between treated plants and the control, results that were not in agreement with previous studies (Kim et al., 2012). That was probably because the unique growth parameters of ginseng develop very slowly (Jin et al., 2009); therefore, further study may be necessary to accurately investigate the same parameters at aged roots.

As the light transmission ratio increases, leaf scorches and chlorosis are rapidly accelerated in ginseng leaves, leading to early defoliation before root enlargement (Cho et al., 2008). This may occur because as the leaves are consistently exposed to high light intensity, the cuticle and the wax layers, which protect the leaf tissues from ultraviolet rays and prevent moisture loss, respectively, collapse as a result of high temperature injury, due to intense evaporation on the leaf surfaces (Lee et al., 2011; Samdur et al., 2003). Chlorophyll decomposition in ginseng leaves is caused by singlet oxygen, which is produced during photosynthesis and increasingly progresses with excessive light intensity (Yang et al., 1991).

In this study, the growth of underground parts was remarkably better in the treatment plot than control plot reported by Cheon et al. (1991, 2003), Lee et al. (1982). They showed that the development of ginseng roots increases with increasing light intensity and suggested that the optimum light transmission rate for root growth is between 10% and 15% of full sunlight. In this study, root parameters, such as root weight, root diameter, and root length of treated plants were significantly increased compared to the control plants. Yield performance (187.4kg• 10 a^–1) of treated plants was 55.5% higher than that of the control plants (150.4kg• 10 a^–1). The results showed that the enhanced light transmission rate appositively affected the photosynthetic ability also reported by Cheon et al. (1991) and Kim et al. (2012). However, the number of lateral roots was not significantly different between the treatment and the control plot. Our results suggested that proper management of light transmission rate in the morning hours might increase yield performance of ginseng plants.

Ginseng is highly susceptible to plant diseases compared to other crops. Two major diseases in ginseng, anthracnose blight disease, caused by C. gloeosporioides, and leaf blight disease, caused by A. panax, decrease yield performance by defoliation before root enlargement (Choi et al., 2011; Han et al., 2004). These two highly destructive pathogens are air-borne and greatly stimulated by warm rainy weather conditions year round (Kang et al., 2007; Kim et al., 2008). Therefore, growers must apply fungicides, such as azoxystrobin and pyraclostrobin many times during growing season to protect the plants against defoliation and subsequent death.

The effect of enhanced light transmission rate on diseases severity is presented in Table 4 and on plant survival rate is shown in Fig. 2. Anthracnose blight and leaf blight severity on leaves and stems was associated with light intensity and leaking rate. Infection rate s also increased with increasing light intensity under warm rainy weather conditions (Cheon et al., 1991), because pathogens easily enter into the plants through the opened stomata (Chung and Bae, 1979). However, our results were not in agreement with previous studies, except for the incidence rate of leaf blight. Cheon et al. (2003) reported that ginseng showed high survival rate and tolerance to plant diseases when light intensity was enhanced during the early growth stage (April to June) and the late growth stage (September to October), but not during the summer (July to August). Cho et al. (2008) observed that cuticle layers on leaves surface, which have a protective function against pathogen infections and insect damages, are developed at higher light intensity and then the stem tissue is hardened by the increase in hypodermic cells formed inside the cortex. Ginsenoside, which is the most important secondary metabolites in ginseng plant, has been reported to be largely involved in plant defense against pathogen’s attack and in enhancement of plant growth (Nicol et al., 2002; Zhang et al., 2011). Their contents increase with an increase of light intensity (Kim et al., 2012). Thus, plant survival rate was higher in the treatment plot than the control plot, indicating a low infection rate.

Fig. 2.  Severity of leaf blight caused by Alternaria panax and anthracnose caused by Colletotrichum gloeosporioides in ginseng.

A; ginseng plants grown under a shade net with 5% light transmission rate (control), B; ginseng plants grown under a shade net with 15% light transmission rate, C; representative symptoms on caused by Alternaria panax, D; representative symptom on ginseng leaves caused by Colletotrichum gloeosporioides.

In our study, leaf blight severity was slightly higher in the treatment plot compared to control plot, but the difference was not statistically significant. This was probably because weather conditions were unfavorable for pathogen development (Kim et al., 1990). The disease causes blighting of stem and leaf from mid-May to the end of June (Oh et al., 1987) but during these periods, amount of rainfall was very low. Becktell et al. (2005) reported that temperature and leaf wetness are requirements for disease development. Outbreaks of disease are greatly affected by a relative humidity, temperature and wetness periods (Arauz and Sutton, 1989; Fitt et al., 1989). Therefore, disease development was insufficient for evaluating plant tolerance against leaf blight in this study.

The effect of enhanced light transmission on physiological disorders in ginseng is presented in Table 5. The severity of physiological disorders, such as inside cracking root and rough skin, resulting in low quality red ginseng, significantly differed between the treatments plot and the control plot. The severity of rusty root, resulting in low quality, root rot, and decreased yield performance was not significantly different between the treatment plot and the control plot, results that were not in agreement with previous studies (Kim et al., 1990). The severity of physiological disorders, including rusty root and cracking root, was higher in the treatment plot than that in the control plot. Otherwise, rough skin and root rot caused by Pythium ultimum were lower in the treatment plot than in the control plot. Kim et al. (1990) mentioned that physiological disorders, such as inside root cavity and inside root cracking were increased with increasing light intensity. The differences between anabolism and catabolism are because high light intensity accelerates respiration. Thus, the accumulation of starch granules inside the root was inhibited by the significant difference between the two metabolic processes during root enlargement stage. Cho et al. (2008) observed that an increase in the light transmission rate over 15% leads to the destruction of root epidermis and the inhibition of cambium development and phloem development inside the vascular cylinder, which is the channel of mass transportation in plants.