عنوان مقاله [English]
Introduction: Although jujube (Ziziphus jujuba Mill.) is known as a medicinal plant and is less important than other fruit trees, it has received more attention in recent years due to its significance in traditional Iranian medicine. There is no study on the actual water need for jujube trees and the impact of irrigation on yield and water use efficiency in the country. However, some studies emphasized on the need of regular watering and irrigation to improve the quality and quantity of jujube fruit. Given the importance of jujube in China, extensive researches have been conducted on water requirements, plant strategies to save water, impact of drought stress on plant morpho-physiological behaviors, impact of appropriate water distribution on soil quantity and quality of plant development, root distribution patterns and its impact on the amount of water consumed. In all cases, it has been emphasized that the mechanisms of water consumption in jujube differ by climate, genotype, irrigation method and management. In order to improve the quantity and quality of jujube fruit, it is necessary to balance the soil moisture condition and keep the plant away from stress. However, dehydration in jujube is a serious issue that should be addressed with the aim of saving water and improving fruit quality. In this study, the actual water requirement of the plant, the effect of different levels of deficit irrigation on evapotranspiration (ET) and crop coefficient (Kc) rate, yield and water use efficiency (WUE) in jujube trees were investigated. The moisture stress was applied through all stages of plant growth by deficit irrigation.
Materials and Methods: In this study, the lysimeter experiment site of Yazd (Shahid Sadoghi Desertification Research Station) with 20 weighing drainage lysimeters (170 cm in height and 121 cm in diameter) was used. To measure evaporation from the soil surface, one lysimeter without plant was used. Note that the moisture content in this lysimeter was always maintained at the field capacity. For the measurement of reference ET (ET0), one lysimeter was used and it compared with ET0 calculated by Penman-Monteith-FAO. After preparing the lysimeters and providing the conditions for planting seedlings, we planted one tree per lysimeter. Trees collected from the villages of Alqoor, Flarg and Gyuk (South Khorasan Province). The suckers were two-years-old with the same size and shape. Trees were irrigated with 50 liters water on a weekly basis for six months. At the beginning of autumn of 2018, treatments including complete irrigation (field capacity), 30% and 60% of deficit irrigation were conducted in a completely randomized design with six replications. Soil moisture measurement during the experiment was performed by TDR. Soil moisture was recorded at 4 depths (0-30, 30-60, 60-90 and 90-120 cm) and their mean was considered as an index of soil moisture status to compensate the irrigation fraction. During the experiment and at the end, indices such as different stages of plant growth, ET, ET0, Kc, yield and WUE were determined. The data were analyzed by analysis of variance (ANOVA) using the statistical package SPSS ver. 16.0, and the mean values were also compared using LSD multiple range test (α = 0.05).
Results and Discussion: The results showed that the jujube trees began their vegetative growth from late March (leaves appear) until the end of November (leaves fall) over 2017-2018 agronomic year in Yazd. During this time, which lasted about 235 days, four major and important stages were evident. The steps cited were in FAO's recommendation for deciduous fruit trees (in Issue 122 of the Iranian Irrigation and Drainage National Committee). The results of these studies showed that the length of different growth periods of jujube trees (including early stage, plant development stage, mid stage and end stage) was different with another deciduous fruit trees. ET values in three irrigation regimes showed that the highest ET was observed in July and the lowest in March. Annual ET in control, 30% and 60% of deficit irrigation treatments were 828.06, 514.04 and 386.04 mm, respectively, with 0.45, 0.28 and 0.21, annual Kc, respectively. The results of ET and Kc computed at different growth stages showed that the reproductive growth development period (flowering, fruit set beginning of fruit growth) had the highest ET. In control treatment, the lowest ET (60.81 mm) and Kc (0.27) were observed in early growth period with less than 10% of crown cover. The highest ET (316.22 mm) and Kc (0.60) were found in growth development period with full crown cover. Analysis of variance showed that there was a significant difference (P <0.0001) between the different irrigation treatments in terms of dry matter yield. There was a significant difference for WUE (P <0.001) between different irrigation treatments. Each of different irrigation regimes had a significant difference in yield. The highest yield for each tree was found for the control treatment with 229.36 g and the lowest yield with 57.90 g was observed for 60% deficit irrigation regime. There was no significant difference between control and 30% deficit irrigation treatment in WUE. The value of WUE decreased with increasing the drought stress. In 60% deficit irrigation treatment, WUE was 0.366 g fruit dry weight per liter denoting the significant difference between this irrigation treatment and the others.
Conclusion: The results of this study showed that jujube trees were susceptible to drought stress in all four stages of growth, especially the period of growth coinciding with flowering, fruit set and early fruit growth. Hence, jujube trees yield and WUE seem to decrease under drought.