3.1. Rainfall and irrigation
Figure 2 shows the precipitation, number of irrigation events and cumulative precipitation during the winter jujube growing season from 2020 to 2022. The growing season precipitation was 428 mm, 574 mm, and 745 mm for 2020, 2021, and 2022, respectively. Compared with the mean annual precipitation (487 mm) in the rainy season in this area, the precipitation in 2021 and 2022 was approximately 87 mm and 358 mm greater, respectively, than usual. The amounts of irrigation with saline water were 168 mm, 96 mm and 120 mm in 2020, 2021 and 2022, respectively, as shown in Fig. 2.
3.2. Change characteristics of average soil water and salinity
3.2.1 Soil water
A proper balance of salt and water in the soil is the key to ensuring the yield and quality of winter jujube. The variation pattern of the soil water content in the 0 ~ 100 cm soil layer over the three years was generally consistent, so only the vertical distribution of the soil water content in 2020 is provided (Fig. 3).
The difference lies in the fact that the SWC in the later stage of winter jujube growth in 2021 and 2022 was greater than that in the same period in 2020 due to the impact of rainfall. The SWC was always higher than 12%, which was more than 50% of the total available water. The lower limit of the optimal SWC in the rhizosphere soil at each growth period of winter jujube was determined by the Forest Society of Binzhou city, and there was no water stress in any treatment during the three-year experiment. In the 0 ~ 60 cm layer, the observed SWC fluctuated greatly. The SWC showed a “wavy change” with time; the relatively high points occurred after irrigation and after rain, and the relatively low points occurred before irrigation and before rain. However, it is worth noting that the 20 ~ 60 cm SWC in the different salinity treatments exhibited the following order: T5 > T4 > T3 > T2 > T1. The 3-year average SWC in the 20 ~ 60 cm layer in T5, T4, T3 and T2 was 11.8%, 8.7%, 9.4% and 6.4% greater than that in T1, respectively.
3.2.2 Soil salinity
Figure 4 shows the mean salinity (EC1:1.5) distribution in the soil profile at the beginning and end of each year. The trend of soil salinity for all treatments throughout the soil profile was similar. The mean EC1:1.5 of the 0 ~ 100 cm soil depth for the T1 ~ T5 treatments was 0.52 dS·m− 1, 0.51 dS·m− 1, 0.49 dS·m− 1, 0.51 dS·m− 1, and 0.52 dS·m− 1, respectively. At the end of the 2020 experiment, the mean EC1:1.5 at the 1.0 m soil depth increased significantly with increasing irrigation water salinity. The mean EC1:1.5 increased even with 1.7 dS·m− 1 saline irrigation, which may have been induced by fertigation. At the 0 ~ 100 cm depth, the soil salinity in the T4 and T5 treatments increased significantly from 0.51 dS·m− 1 and 0.52 dS·m− 1 to 0.77 dS·m− 1 and 0.81 dS·m− 1, respectively. However, the mean EC1:1.5 at the 0 ~ 100 cm soil depth decreased slowly at the beginning of the experiment in 2021. Moreover, the mean EC1:1.5 at the end of the experiment greatly decreased and was not positively related to the irrigation salinity. At the beginning of the 2022 experiment, the soil salinity in the soil profile up to 0 ~ 100 cm increased from 0.35 dS·m− 1 (after harvesting in 2021) to 0.40, 0.41, 0.41, 0.45 and 0.51 dS·m− 1 for T1, T2, T3, T4 and T5, respectively. Similar to the situation in 2021, although saline water was irrigated in 2022, the 0 ~ 40 cm soil salinity decreased again due to rainfall. Compared with that in the first soil sample collected in April 2020, the soil salinity in the T1 ~ T5 treatments decreased by 56.4%, 24.7%, 12.4%, 5.6%, and 0.6%, respectively.
3.3. Yield of winter jujube
Figure 5(a) shows the yields of winter jujube under the different treatments over the three years. Compared with those in the T1 treatment, the yields of winter jujube irrigated with water salinity at 3.3, 4.9, 6.5, and 8.1 dS·m− 1 decreased by 6.4%, 2.9%, 14.1%, and 20.3%, respectively, in 2020; decreased by 0.5%, 2.0%, 5.3%, and 12.1%, respectively, in 2021; and decreased by 6.3%, 8.6%, 14.0%, and 19.1%, respectively, in 2022. The yield decreased with increasing irrigation water salinity, and there was a significant negative correlation between the two when the irrigation water salinity exceeded 6.5 dS·m− 1. Furthermore, based on the linear function of the Mass-Hoffman crop salt tolerance model, the response of the winter jujube relative yield to irrigation water salinity is illustrated in Fig. 5(b). The thresholds of irrigation water salinity were 1.79 dS·m− 1, 2.19 dS·m− 1, and 1.72 dS·m− 1 from 2020 to 2022; when the irrigation water salinity exceeded these values, the winter jujube yield decreased by approximately 3.11%, 1.79%, and 2.56%, respectively, per 1 dS·m− 1 increase in 2020, 2021, and 2022. The salt tolerance threshold has not decreased annually, which indicates that a cumulative effect does not exist in terms of the impact of salt on yield.
The response of the winter jujube relative yield to ECe according to the linear function of the Mass-Hoffman crop salt tolerance model is illustrated in Fig. 6. Because there is an evident interannual difference in the relationship between relative yield and ECe, a modified crop salt tolerance discount function was introduced to describe the relationship between the relative yield and ECe in the root zone. According to the linear function, the soil salinity threshold that winter jujube can endure without any yield reduction was 4.51 dS·m− 1. Above this threshold, the relative fruit yield decreased by 3.43% for each additional 1 dS·m− 1. According to the discount function, the relative fruit yield of winter jujube decreased to 50% when the ECe of the rootzone was 12.64 dS·m− 1. Specifically, when the relative yield of winter jujube decreased to 50% and 85%, the average soil conductivity (soil salinity) in the 0 ~ 100 cm soil layer during the growth period was 0.72 dS·m− 1 (0.19%) and 1.03 dS·m− 1 (0.27%), respectively, in this study.
3.4. Quality of winter jujube
Figure 7 shows the changes in winter jujube indicators, such as the fruit shape index, single-fruit weight, fruit moisture content, edible rate, soluble sugar content, total soluble solids, titratable acid content, vitamin C content and flavonoid content, among the three years. The different salinity treatments had no significant effect on the fruit shape index, fruit moisture content or edible rate (P > 0.05). However, salinity treatment had a significant effect on the weight of individual fruits (P < 0.05). Compared with those in the T1 treatment, the single-fruit weights in the T4 and T5 treatments were 10.87% and 20.35% lower in 2020 and 5.76% and 32.58% lower in 2022, respectively. The contents of soluble sugars, total soluble solids, titratable acid, vitamin C and flavonoids increased with increasing irrigation water salinity. The five contents in the saline water treatments were significantly (P < 0.05) greater than those in the T1 treatment in 2020. However, of the five contents soluble sugars and flavonoids were not significantly different in fruits in 2021 (P > 0.05), and vitamin C and total soluble solids were not significantly different in fruits in 2022 (P > 0.05), which may be due to the reduction in salt stress caused by high rainfall in the later stage of growth. Compared with those in the T1 treatment (1.7 dS·m− 1), irrigation with 3.3 dS·m− 1, 4.9 dS·m− 1, 6.5 dS·m− 1, and 8.1 dS·m− 1 of saline water resulted in 2.4%, 9.6%, 17.9% and 4.8% increases in soluble sugar content, respectively, in 2020 and 0.2%, 0.9%, 17.9% and 3.3% increases, respectively, in 2022. Similarly, compared with those in the 1.7 dS·m− 1 treatment, the total soluble solids content in the jujubes increased by 5.5%, 12.3%, 14.8%, and 4.9% in 2020 and by 6.7%, 5.9%, 14.1%, and 20.5% in 2021, respectively, with 3.3 dS·m− 1, 4.9 dS·m− 1, 6.5 dS·m− 1, and 8.1 dS·m− 1 saline irrigation. Moreover, the titratable acid content increased by -6.9%, 10.3%, 10.3% and 17.2% in 2021 and 18.2%, 31.8%, 63.6% and 50.0% in 2022, respectively. Compared with those in T1, irrigation with 3.3 dS·m− 1, 4.9 dS·m− 1, 6.5 dS·m− 1, and 8.1 dS·m− 1 saline water increased the vitamin C content by 3.7%, 10.6%, 29.3%, and 8.8%, respectively, in 2020; by 5.1%, 7.7%, 8.6%, and 4.3%, respectively, in 2021; and increased the flavonoid content by 20.9%, 38.4%, 427%, and 37.0%, respectively, in 2020; and by -3.0%, 2.7%, 8.0%, and 3.8%, respectively, in 2022. Thus, irrigation with saline water mainly enhanced the soluble sugar, total soluble solids, titratable acid, vitamin C and flavonoid contents in the fruit, although it reduced the single-fruit weight.
In addition to irrigation water salinity, the soil salinity in the root zone also impacts crop quality. There were interrelated correlations between electrical conductivity (EC1:5) and winter jujube quality, and correlation analyses were performed among the FMI, SFW, FWC, ER, SS, TSS, TA, VC and flavonoids (Fig. 8). Evidently, EC1:5 had a significant positive correlation with FWC, SS, TSS and flavonoids (0.59, 0.64, 0.86, 0.66). There was a highly significant negative correlation between the SS and TA contents, while the SS content had a distinct positive correlation with the TSS (0.63) and VC (0.84) contents. Moreover, the fruit VC content was strongly negatively correlated with the TA content (-0.65) but positively correlated with the TSS content (0.50).
3.5. Optimal irrigation water quality for high-quality winter jujube
According to Fig. 7, it was difficult to achieve the theoretical maximum value of all quality indicators with saline irrigation. Thus, principal component analysis (PCA) was carried out to evaluate the comprehensive quality of the winter jujube. The following nine indices were selected and calculated: FMI (X1), SFW (X2), FWC (X3), ER (X4), SS (X5), TSS (X6), TA (X7), VC (X8) and flavonoids (X9). With the PCA, two indices (F1 and F2) were determined to represent the eight indices with representativeness of 73.97% and 16.35%, respectively, in 2020, which meant that the original physicochemical information of the winter jujube could be explained by these two principal components (PCs). The two PCs can be expressed as follows:
PC2020 1 = 0.113X1-0.072X2-0.120X3 + 0.144X4 + 0.143X5 + 0.140X6 + 0.135X7 + 0.133X8 + 0.146X9 (5)
PC2020 2 = 0.215X1 + 0.595X2 + 0.397X3-0.037X4 + 0.168X5 + 0.246X6 + 0.151X7 + 0.039X8-0.085X9 (6)
where PCyear1, PCyear2, and PCyear3 are the first and second principal components, respectively.
According to the results of the PCA conducted for 2021, the first three PCs accounted for 94.40% of the total variance, and their contribution rates were 56.96%, 25.52% and 11.92%, respectively. According to the PCA conducted for 2022, the first three PCs accounted for 94.69% of the variance, and their contribution rates were 60.02%, 21.51% and 13.17%, respectively. The three PCs for 2021 and 2022 can be expressed as follows:
PC2021 1 = 0.103X1-0.183X2-0.181X3-0.100X4 + 0.162X5 + 0.186X6 + 0.167X7 + 0.098X8 + 0.100X9 (7)
PC2021 2 = 0.033X2 + 0.148X1 + 0.102X3 + 0.349X4 + 0.128X5-0.043X6-0.041X7 + 0.364X8 + 0.357X9 (8)
PC2021 3 = 0.784X1-0.063X2 + 0.268X3-0.095X4 + 0.050X5 + 0.125X6-0.455X7-0.092X8 + 0.008X9 (9)
PC2022 1 = 0.153X1-0.110X2-0.114X3-0.107X4 + 0.153X5 + 0.164X6 + 0.175X7 + 0.112X8 + 0.177X9 (10)
PC2022 2 = 0.277X2 + 0.372X1 + 0.360X3 + 0.146X4 + 0.078X5-0.098X6-0.006X7 + 0.347X8 + 0.121X9 (11)
PC2022 3 =-0.060X1 + 0.046X2-0.312X3 + 0.640X4-0.327X5 + 0.331X6 + 0.186X7 + 0.260X8-0.107X9 (12)
The comprehensive evaluation function can be established for 2020 to 2022 by the contribution rate of the PCs as follows:
I2020 = 0.82F20201 + 0.18F20202 (13)
I2021 = 0.60F20211 + 0.27F20212 + 0.13F20213 (14)
I2022 = 0.63F20221 + 0.23F20222 + 0.14F20223 (15)
where Iyear is the comprehensive score.
The comprehensive evaluation results are shown in Table 4. The results indicated that the irrigation water salinity in the 6.5 dS·m− 1 treatment had the highest comprehensive score and ranked first among the three years. However, irrigation water salinities of 4.9 dS·m− 1, 8.1 dS·m− 1 and 8.1 dS·m− 1 ranked second in 2020, 2021 and 2022, respectively. The comprehensive score increased with increasing irrigation water salinity from 1.7 dS·m− 1 to 4.9 dS·m− 1. To obtain high-quality winter jujube, irrigation water with a salinity of 4.9 ~ 8.1 dS·m− 1 is recommended based on the present study.
Table 4
Comprehensive score and rank of fruit quality under various salinity treatments
Season | Treatment | Comprehensive score | Rank |
2020 | T1 | 0.790 | 5 |
T2 | 0.826 | 4 |
T3 | 0.902 | 2 |
T4 | 0.928 | 1 |
T5 | 0.855 | 3 |
2021 | T1 | 0.592 | 5 |
T2 | 0.608 | 4 |
T3 | 0.623 | 3 |
T4 | 0.659 | 1 |
T5 | 0.652 | 2 |
2022 | T1 | 0.733 | 5 |
T2 | 0.749 | 4 |
T3 | 0.787 | 3 |
T4 | 0.834 | 1 |
T5 | 0.800 | 2 |