The effect of sowing depth and irrigation frequency on establishment of the xerophyte Zygophyllum xanthoxylum

Zygophyllum xanthoxylum is an extremely xerophytic shrub that is strongly resistant to stress and is important both ecologically and for its feeding value. The two-factor effect of sowing depth (0, 1, 4 cm) and irrigation frequency (0, 1, 2, 3, 4, or 5 times during the study period) on the emergence and establishment of Z. xanthoxylum seedlings was studied in an experimental field in Minqin County, Gansu Province, China. In addition, the overwintering rate, phenological cycle, and growth and development of Z. xanthoxylum were compared between sites in Minqin and Yuzhong Counties. Z. xanthoxylum seedlings had the fastest emergence and establishment when the seeds were planted at 1 cm. At this depth, the rates of emergence and establishment were significantly higher when seedlings were irrigated 2, 3, or 4 times than when they were irrigated 5 times or not at all, with no significant difference among groups when the seedlings were irrigated 2, 3, and 4 times (P > 0.05). Taking water conservation into consideration, the treatment in which the seeds were sown at 1 cm and irrigated twice could be considered the best, with seedling emergence and establishment rates of 69% and 49%, respectively. During the second year of establishment, there was no significant difference between the two test sites regarding the overwintering rate of Z. xanthoxylum seedlings, which was above 90% at both (P > 0.05). At the Minqin test site, regreening, first flowering, peak flowering, and first fruiting took place 4～6 days later than in Yuzhong, and yellowing took place 38 days later, while the fruit ripened 4 days earlier. Furthermore, the duration of flowering in Minqin was 4 days longer than in Yuzhong, and the growth period was 35 days longer. At both sites, the plants reached maximum height at the fruit ripening stage (108 cm in Minqin; 79 cm in Yuzhong). The number of first-level branches was 6.3 in Minqin and 5.7 in Yuzhong, which was not a significant difference ( P > 0.05). However, the number of secondary branches was 23.5 in Minqin vs. 11.8 in Yuzhong, which was a significant difference ( P < 0.05). The pod setting rate (6.3%, 5.1%) and the seed yield per plant (3.2, 1.9 g) were also higher in Minqin than Yuzhong.