The objective of this study was to investigate the influence of a shade treatment on plant growth characteristics, bud break, and spear number of five asparagus (Asparagus officinalis L.) cultivars (Apollo, Atlas, Grande, UC157, and Walker Deluxe) and determine the most appropriate cultivar for cultivation in a shaded rain-shelter house. For the experiment, two-month-old seedlings were grown in the rain-shelter house with 30% shade using polyethylene film on the roof of the rain-shelter house, whereas the control plants were grown in a rain-shelter house without the additional layer of polyethylene. The shade treatment significantly increased the plant height, shoot number, shoot number per week, and bud number, but did not affect the mean number of days to bud break. The number of spears on asparagus grown in the shade was significantly higher than in the control. Under the shade treatment, Walker Deluxe and UC157 showed significantly higher shoot number and shoot number per week than the other cultivars. Compared to other cultivars, Walker Deluxe produced the highest bud number. In the control condition, Grande produced a higher number of shoots, shoots per week, buds, and roots than in the shade treatment. However, the shade treatment increased the ratio of spear numbers to bud numbers of Grande, resulting in a higher number of spears. Walker Deluxe with shade treatment had higher shoots, shoots per week, buds, roots, and spears than the control. These results suggest that Grande is more suited for growth in a rain-shelter house without shade treatment but that Walker Deluxe is more appropriate for cultivating in a rain-shelter house with 30% shade.

This study examined the effects of 30% and 70% shading on the growth characteristics, phenolic content, and antioxidant activity of three male cultivars (Atticus, Avalim, and Herkolim) of asparagus grown in a rain-shelter house. The 70% shading treatment significantly increased the shoot elongation rate of all three male cultivars compared with the control and 30% shade treatments. Compared with the control treatment, 30% shading increased the shoot production rate and bud number of the Atticus and Avalim cultivars. Most of the plant growth characteristics of the Herkolim cultivar were not affected by the 30% shade treatment, except root diameter. Polyphenol contents in all three cultivars were significantly decreased under the 30% shade treatment compared with that in the plants under the control and 70% shade treatments. The Atticus cultivar had a higher polyphenol content than that in the Avalim and Herkolim cultivars under shade treatments. The flavonoid contents in Atticus and Avalim plants under the 30% shade treatment were lower than that in plants under the control treatment. Compared with the control treatment, the 30% shade treatment significantly decreased antioxidant enzyme activities such as superoxide dismutase, catalase, and peroxidase as well as the 2,2'-azino-bis3-ethylbenzothiazoline-6-sulphonic acid (ABTS) free-radical scavenging activity of Atticus and Herkolim. Under the 30% shade treatment, the polyphenol and flavonoid contents in the Atticus cultivar were significantly higher than those in the Avalim cultivar. The results suggest that the 30% shade treatment can be applied in the cultivation of the Atticus cultivar to improve spear production in the following season and minimize decreases in phenolic content.