Introduction

Asparagus (Asparagus officinalis L.) is a dioecious perennial vegetable that is nutritionally beneficial for human (Anastasiadi et al., 2020). It is a rich source of bioactive phytochemicals such as asparagusic acid, saponins, folic acid, rutin, amino acids, carotenoids, and polyphenols (Mitchell and Waring, 2014; Hamdi et al., 2018; Ku et al., 2018a, 2018b; Chitrakar et al., 2019), which have antioxidant, antiviral, antifungal, and anti-inflammatory activities (Ji et al., 2012; Lee et al., 2015; Jaramillo et al., 2016; Iqbal et al., 2017; Ku et al., 2018a). In addition, pharmacological studies have demonstrated health benefits of asparagus that it can prevent cardiovascular disease, rheumatism, diarrhea, and diabetes (Iqbal et al., 2017; Jiménez-Moreno et al., 2019). Consequently, the consumption of asparagus has increased worldwide, particularly in South Korea (Lee et al., 2015; Ku et al., 2018a).

Asparagus is cultivated in open fields in many countries, but in South Korea, it is mostly grown in rain-shelter houses (Seo et al., 2016; Ku et al., 2018b). In both open field and rain-shelter house conditions in South Korea, plants are subjected to high light irradiance stress during the summer (from June to August). Several studies have reported that extremely high light intensity induced photoinhibition, resulting in a constrained plant growth and a decreased plant or fruit yield, therefore, shade treatment has been applied to reduce adverse effects of high light intensity on plant growth (Jeeatid et al., 2017; Zeng et al., 2017). Similarly, Onggo (2012) reported that shade treatment increased the yield and quality of asparagus by protecting the plants from high irradiance levels. Plants require an optimal light intensity for their normal growth and photosynthesis, which depend on cultivar (Jeeatid et al., 2017; Zeng et al., 2017). Therefore, it is important to determine the effects of shade treatment on different cultivars to identify suitable high yield cultivars for shade conditions.

Growing conditions and cultivar selection are important for asparagus cultivation because asparagus plants remain productive for at least 15 years (González and Pozo, 2002). The growth and biochemical properties of asparagus are significantly influenced by cultivar and the surrounding environment (Kim et al., 2016; Fukuda et al., 2018). Cultivation systems, temperature, soil water content, and light intensity affect plant growth characteristics, bioactive compounds, and bioactivities of asparagus (Motoki et al., 2012; Kim et al., 2016; Ku et al., 2018b). Asparagus spear yield is associated with photosynthetic activity, which greatly depends on the light irradiance (Guo et al., 2002).

There is limited information about cultivars suitable for growth under high light irradiance in Korean summers. Moreover, the effects of shade on bud break and spear number of asparagus have not been thoroughly investigated in the country. Therefore, the objective of this study was to investigate the effect of shade treatment on plant growth characteristics, bud break, and spear number of five asparagus cultivars (Apollo, Atlas, Grande, UC157, and Walker Deluxe) grown in a rain-shelter house.

Materials and Methods

Growing Conditions and Experimental Treatments

The experiment was conducted in a rain-shelter house located at Wonkwang University, Iksan, South Korea (35°56'N, 126°57'E) from May 2018 to April 2019. The seeds of five asparagus cultivars (Apollo, Atlas, Grande, UC157, and Walker Deluxe) were sown in plug trays filled with a commercial growing medium containing 68.51% coco-peat, 10% peat moss, 10% vermiculite, 5% zeolite, 6% pearlite, 0.18% fertilizer, 0.01% wetting agent, and 0.3% pH regulator (Alpha-Plus; Sang-Lim Company, Iksan, South Korea). After germination, one-month-old seedlings were transplanted into plastic pots (9 cm diameter × 9 cm high) containing a commercial growth medium and grown for one month in the rain-shelter house. The seedlings were then transplanted into larger pots (22 cm diameter × 25 cm high) containing growth medium. Seedlings with uniform height and number of shoots were used for the experiment. The shade treatment was 30% reduced light irradiance by using a polyethylene film (Nakta Jangsu film; 0.07 mm thick, 10 m long × 5.7 m wide), while the control plants were grown in a rain-shelter house without the additional layer of polyethylene. The light intensity in control and shade treatments was monitored daily at 2 pm at 1 m above ground level using a SpectroPen-SP-110 spectrometer (Photon Systems Instruments, Brno, Czech Republic) (Fig. 1). Temperatures under both control and shade treatments were measured at 30-min intervals using data loggers (Micro Logger, Dallas Semi-conductor, USA) at 1.1 m above ground level (Fig. 2).

Fig. 1.

Sunlight intensity under control and shade treatments in a rain-shelter house during the experiment. A) Irradiance (W·m^-2), B) photon flux density (µmol·m^-2·s^-1), and C) illuminance (lux).

Fig. 2.

Temperature under control (A) and shade (B) conditions in a rain-shelter house during the experiment.

() Maximum temperature in a day; () average temperature of whole day; () minimum temperature in a day.

Measurement of Plant Growth Parameters

The shoot number per plant was counted weekly. Shoot number was measured once it reached 1 cm in length. The shoot number per week (A) was calculated using equation 1:

Asparagus plant height was measured from the top of the growing media to the plant tip using a tape ruler (EGT-7525; Ex-Power). The plants were then uprooted from the pots and the shoots and roots were separated. Roots were carefully washed with tap water to remove soil and adhering materials. Shoot and root diameter were measured at 1 cm from the base of the plant using a digital caliper (CD-20APX; Mitutoyo Corp., Kanagawa, Japan). The number of buds and roots was counted manually. Root length was measured using a tape ruler. Fresh weights of shoots and roots were determined using an electronic balance. Dry weights of the shoots and roots were determined after drying the samples in an oven at 60°C. The root:shoot ratio was calculated after measuring the dry weight.

Measurement of Bud Break

Asparagus seedlings become dormant at low temperature conditions. Dead asparagus ferns were removed, and the pots of the five asparagus cultivars were placed under the control and shade treatments. The increase in temperature in spring encouraged the buds to sprout (Fig. 2). The number of days to the first bud break was recorded.

Measurement of Spear Number

Spear number per plant was counted for the seven weeks of spear harvest. Spear height was measured daily from the soil surface to the spear tip using a ruler. The spears were harvested at 20 cm length. Spear diameter and spear weight were not measured because the spears were very thin and light in weight (less than 3–4 g).

Determination of the Ratio of Spear Number to Bud Number

The ratio of spear number to bud number (B, %) was determined according to equation 2:

Statistical Analysis

The experiment was set up in a completely randomized design with five replicates for each cultivar. The effects of the treatments were evaluated, and the level of significance was analyzed using ANOVA with IBM SPSS Statistics for Windows, version 24 (IBM Corp., Armonk, NY, USA). Significant differences between the means and correlation coefficients among the treatments, plant growth characteristics, and cultivars were estimated using Duncan’s multiple range test at p ≤ 0.05.

Results and Discussion

Plant Growth Characteristics and Their Correlations

From the fourth week after treatment, the shade treatment significantly increased the asparagus shoot number compared with the control (Table 1). During summer, an average of 50,000 lux light was received in the control section (Fig. 1). However, the intensity of light (150–200 W/m²; 700–900 µmol m^-2s^-1; 30,000–40,000 lux) in the 30% shade treatment may be more favorable for a higher number of shoots compared with that of the control. Previous research has shown that 27,000–48,600 lux light levels were good for asparagus grown in South Korea (RDA). Onggo (2012) reported that the shoot number of asparagus seedlings was higher under a 40–50% shade treatment during the dry season in Indonesia. Research on another Liliaceae member, Sandersonia aurantiaca, has shownthat 700 µmol m^-2s^-1 was beneficial for the growth of this plant based on leaf area, flower number, and flower size (Catley et al., 2002; Davies et al., 2002). Liu and Zhao (2013) reported that Hosta ensata, another Liliaceae member, flowered best under 50% shade. In this study, the shoot number after three weeks of treatment differed significantly by cultivar (Table 1). From six weeks after treatment, the shoot numbers of UC157 and Walker Deluxe were significantly higher than those of other cultivars. Similarly, UC157 seedlings have been found to produce more shoots than Atlas in a pot experiment (Pertierra et al., 2006). Shoot number was significantly affected by the interaction between shade treatment and cultivar.

The shade treatment significantly increased the shoot number per week (Fig. 3A). At week 8, shoot numbers per week in the shaded and control plants were 1.15 and 1.01, respectively. UC157 and Walker Deluxe produced 1.25 and 1.28 shoots per week, respectively, which was significantly more than the other cultivars (Fig. 3B). The shoot number per week for Grande under control condition was 67% higher than that under shade treatment. However, the shoot numbers per week of UC157 and Walker Deluxe with shaded treatment were approximately 40% higher than those of control condition. Shade treatment stimulated plant height but the cultivars had no significant effect on plant height (Table 2). Under the shade treatment, Grande and Atlas were significantly taller than UC157, whereas the plant heights of Apollo, UC157, and Walker Deluxe were not significantly different. The plant heights of Grande and Atlas with the shade treatment were approximately 25% higher than those under the control condition. Shoot diameter was unaffected by the shade treatment or cultivar, however, the interaction between treatment and cultivar had a significant effect on the shoot diameter. The shoot diameter of Atlas was significantly higher than that of UC157 under shade treatment. The shoot diameter of Atlas significantly increased by 56% under the shade treatment, whereas the shoot diameters of Apollo, Grande, UC157, and Walker Deluxe were not significantly different between the shade treatment and the control.

Fig. 3.

The effect of shade treatment on shoot number per week (A), and the shoot number of each cultivar per week (B) in a rain-shelter house. The vertical bars represent the standard deviation (A, n = 25; B, n = 5). Different lowercase letters above bars within the figure indicate significant differences based on Duncan’s multiple range test at p ≤ 0.05.

The number of asparagus buds influences the spear yield in the following season (Gasecka et al., 2009). In this study, the shade treatment significantly increased the number of buds compared to the control (Table 2). The increased bud number in the shade treatment indicated that the yield of asparagus spears would be greater in the subsequent year. The enhanced bud number of shade-treated asparagus may have resulted from the high shoot numbers (Table 1). Other studies have shown that the number of buds in asparagus plants is correlated with the number of shoots (Drost and Wilcox-Lee, 1997; Kim et al., 2016; Siomos, 2018). Cultivar had a significant effect on the number of buds. Walker Deluxe showed a significantly higher number of buds than the other cultivars. The significant interaction between treatment and cultivar was also found in the bud number of the asparagus. Under the shade treatment, Walker Deluxe produced the highest number of buds. The number of buds on Walker Deluxe grown in the shade was significantly increased (56%) with respect to that of the control. Under the control condition, the bud number of Grande was significantly higher than that of other cultivars, and 48% higher than that of Grande grown in the shade treatment.

As a perennial crop, asparagus yield also depends on root growth and development (Elmer, 1995; Wilson et al., 2002b). The number of roots of Grande under the control condition was higher than that under shade treatment (Table 2). The number of roots of Atlas, UC157, and Walker Deluxe with the control condition did not differ significantly. However, under shade treatment, Walker Deluxe produced a higher number of roots than under control condition. Both shade treatment and cultivar had significant effects on the root length of the asparagus. The average root length of shade-treated plants was 37.76 cm, which was 2.84 cm higher than that of the control plants. Under control condition, the root lengths of Apollo, UC157, and Walker Deluxe were not significantly different, whereas the root length of UC157 was higher than that of Atlas and Grande. However, under the shade treatment, root length did not differ significantly between cultivars.

Compared to the control condition, shade treatment significantly increased the fresh weight of shoots (Table 2). However, cultivar had no significant effect on shoot fresh weight, whereas cultivar and treatment had a significant interaction effect. The shoot fresh weight of Walker Deluxe was significantly greater under shade treatment (56.60 g) than under control condition (33.20 g). Shoot dry weight, root fresh and dry weights, total fresh and dry weights, and root:shoot ratio were unaffected by shade treatment or cultivar. However, there was a significant interaction effect between treatment and cultivar, except in the case of the root:shoot ratio. Under shade treatment, root fresh weight, total fresh weight, shoot dry weight, root dry weight, and total dry weight of Walker Deluxe were significantly higher than those of Apollo. There were increases in root fresh and dry weight, shoot dry weight, and total fresh and dry weights of shade-treated Walker Deluxe compared with those under control condition.

The asparagus plant height showed a positive significant correlation with shoot diameter (p ≤ 0.01, r = 0.49, Table 3). Machon et al. (1995) also showed a significant positive correlation between the height of asparagus and shoot diameter (r = 0.60). There was a significant positive relationship between the number of shoots and buds (p ≤ 0.01, r = 0.46), and between the number of shoots and roots (p ≤ 0.01, r = 0.53). Other studies have also observed that the number of shoots and buds are correlated in asparagus plants (Kim et al., 2016). The number of buds in asparagus is an important factor for spear production in the subsequent season. Bud number was significantly positively correlated with root number (p ≤ 0.01, r = 0.52). Total fresh weight and total dry weight showed positive correlations with the shoot and root fresh and dry weights. Shoot and root fresh weights showed significant positive correlations with the shoot and root dry weights at the p ≤ 0.01 probability level.

Bud Break, Spear Number, and the Ratio of Spear to Bud Number

Bud break in asparagus was induced when the daily average temperature reached approximately 10°C in the rain-shelter house (Fig. 2). Ku et al. (2007) showed that bud break of UC157 was induced at 10°C. Other researchers reported that bud break of Mary Washington and Martha Washington asparagus cultivars started at 4.4°C and 5.8°C, respectively (Culpepper and Moon, 1939; Bouwkamp and McCully, 1975). Shade treatment had no significant effect on the mean number of days to bud break (Table 4). However, the cultivar type significantly affected the mean number of days to bud break. Under the shade treatment, the mean number of days to bud break of Apollo, Grande, UC157, and Walker Deluxe did not differ significantly. In addition, the mean number of days to bud break of UC157 was 1.5 days, which was lower than that of Atlas (4.0 days) under shade treatment. This trend meant that the bud break of UC157 was 2.5 days earlier than that of Atlas. Ku et al. (2007) showed that the mean number of days to bud break for six-month-old UC157 plants was earlier than that for Jersey Giant.

Shade-treated asparagus plants produced more spears than the control plants (Fig. 4A). This effect has resulted from the response of asparagus plants to the shade treatment in the previous season when the numbers of shoots and buds were significantly greater in the shade treatment than in the control (Tables 1 and 2). Wilson et al. (2002a) reported that the growing condition of the fern stage and established harvest period influenced spear number in asparagus. Siomos (2018) reported that during the fern growth and establishment period of asparagus, bud development occurred in the crown of the plants and that most of these buds produced spears in the following season. Number of spears of Grande, UC157, and Walker Deluxe did not differ statistically in both control and shade treatments. Under the control condition, the number of spears was not significantly different between the cultivars (Fig. 4B). However, under the shade treatment, Walker Deluxe, UC157, and Grande produced more spears than the other cultivars. The spear numbers of Walker Deluxe, UC157, and Grande cultivated under the shade treatment were 0.8-fold, 0.6-fold, and 0.8-fold higher than those grown in the control, respectively.

Fig. 4.

The effect of shade treatment on spear number (A), and the spear number of each cultivar (B) in a rain-shelter house. The vertical bars represent the standard deviation (A, n = 25; B, n = 5). Different lowercase letters above bars within the figure indicate significant differences based on Duncan’s multiple range test at p ≤ 0.05.

The shade treatment significantly increased the ratio of spear number to bud number compared with the control condition (Table 4). Cultivar had a significant effect on the ratio of spear numbers to bud numbers. There was an interaction between shade treatment and cultivar, which affected the ratio of spear numbers to bud numbers. In the control condition, this value for UC157 was significantly higher than it was for Atlas and Grande. In the shade treatment, the ratio of spear number to bud number of Grande was 2.7-fold higher than in the control, but not significantly different from those in the shade-treated UC 157 and Apollo.

In conclusion, shade treatment significantly increased the number of shoots, buds, and spears of asparagus compared to the control plants. Under control condition, Grande produced a higher number of shoots, shoots per week, buds, and roots than under shade treatment. However, Walker Deluxe under shade treatment had higher number of shoots, buds, and spears, suggesting that cultivation of Walker Deluxe with 30% shade would be expected to increase spear yield.