The jujube industry is highly developed in the Yulin district of northern Shaanxi Province. Because jujube fruit yield is often affected by variation in precipitation throughout the year, accurate irrigation is an important method for increasing the potential of the local jujube industry.
Water demand in jujube trees is most critical during two stages: 1) anthesis and fruit set and 2) fruit development. In this study, the effect of different ecological factors, such as soil water potential (WPs), solar radiation (Rs), air temperature (Ta), and air relative humidity (RH), on jujube trunk diameter (TD) contraction during fruit development was investigated. In addition, the most suitable indicator of TD contraction was determined for use in accurate irrigation management.
Research was conducted in a rainproof shed at the micro-irrigation demonstration station in Mengcha, Yinzhou, Mizhi, Yulin, China. Three-year-old pear-jujube trees were studied using four different WPs treatments: -41 to -51 kPa (T1), -68 to -84 kPa (T2), -96 to -135 kPa (T3), and -311 to -390 kPa (T4). Individual test plots, separated by cement enclosures and containing three jujube trees, were 6 m long × 1 m wide × 1 m high. Each treatment was applied to three test plots. WPs was monitored continuously with an EQ15 equilibrium tensiometer and controlled automatically using a drip irrigation system that supplied water whenever WPs reached a minimum value. TD was continuously measured with a linear variable displacement transducer (LVDT); atmospheric factors were recorded using an automated weather station. Collected data were analyzed using DPS statistics software, and figures and tables were generated with Excel.
The results showed that jujube TD contraction followed a U-shaped curve over the course of a day. In general, trunk diameter shrank during the day, reaching a minimum trunk diameter (MNTD) at 17:00-19:00; at night, trunk diameter expanded, with the maximum diameter (MXTD) occurring at 7:00-9:00. The interval between MNTD and MXTD was about 12 h. TD trended differently depending on growing period, weather, and WPs treatment. During fruit development, when WPs ranged from -41 to -390 kPa, TD increased with increasing WPs. MXTD was significantly different between the treatments, and higher WPs ranges resulted in increased trunk diameter. MXTD was an appropriate water-level diagnostic indicator at this stage. Trunk diameter growth (DG) increased at moderate WPs levels (T2 and T3), but was inhibited when WPs was excessively high (T1) or low (T4). DG was also influenced by weather conditions, increasing greatly on rainy days because of the higher RH. In contrast, the TD daily shrinking range was larger on sunny days, with no increase observed on rainy days. TD had a significant positive correlation with RH (correlation coefficient ca. 0.9) and a significant negative correlation with Ta (correlation coefficient ca. -0.75), but little correlation with Rs. Thus, RH and Ta were the major atmospheric factors influencing TD contraction. Overall, jujube TD contraction was influenced simultaneously by various ecological factors, most significantly RH, Ta, and WPs. At higher WPs ranges, WPs had the greatest direct influence on TD contraction, followed by RH; at lower WPs ranges, RH was the primary factor, followed by WPs. TD was not directly affected by Ta, regardless of WPs level.