Shengzhou nane (Prunus Salicina Var. taoxingli) is a famous local fruit in Zhejiang Province, with extremely rich nutrition, and loved by consumers. Planting Shengzhou nane has become a pillar industry for developing forestry and improving the lives of the people in Jinting, Beizhang, Huangze and other villages and towns in Shengzhou. Compared with other fruits, previous studies on Shengzhou nane are relatively few. At present, in addition to a small amount of research on storability of Shengzhou nane fruit (Mo et al., 2017; He et al., 2017), there is no report on the soil nutrient status, tree nutrient status and fertilizer requirements of fruit trees in Shengzhou nane orchards. Local fruit growers are mostly a household business model, orchard soil and fertilizer management is very extensive, whether fertilizer or fertilizer type selection, are judged by experience, there is a great blindness. Uncoordinated fertilization ratio, unreasonable fertilizer structure, and over or excessive fertilization are very common, resulting in frequent occurrence of low or high yield but small fruit, fruit cracking, fruit drop in production, which seriously affects the improvement of economic benefits.

Shengzhou nane is a kind of perennial plants. Soil nutrient status can directly reflect the supply capacity of soil nutrients (Li et al., 2007). Leaf nutrient analysis is often used as the main method for nutritional diagnosis of fruit trees (Partelli et al., 2018). Therefore, in this study, the contents of the soil nutrients, leaf nutrients during two rapid fruit growth stages of fruit expanding stage and fruit ripening stage and fruit quality indexes of Shengzhou nane with different tree ages in Linge village, Jinting town, Shengzhou, were measured and statistically analyzed. To clarify the fertility of orchards at different tree ages in Shengzhou nane main producing areas, and the effects of soil and leaf nutrients on the quality indexes of Shengzhou nane, to formulate reasonable fertilization solution and provide new technical guidance for improving the yield and quality of Shengzhou nane.

1 Results and Analysis

1.1 Nutrient quality indexes of Shengzhou nane with different tree ages

There were significant differences in fruit quality indexes among different tree ages of Shenzhou nane (Figure 1; Table 1). Except that there was no significant difference in FF of 10~12 year old trees and 21~23, 28~30 year old trees, the MPF, TSS content, SS content, Vc content, the ratios of TSS/TA and SS/TA of 10~12 year old trees were the highest, while the TA content of 10~12 year old trees was the lowest (Table 1). Except for the lowest Vc content in 28~30 year old trees, other nutritional indexes in 38~40 year old trees were the lowest, and MPF, FF, TSS content and TSS/TA were significantly lower than those in other tree age fruit trees. For example, the average MPF of 38~40 year old trees is only 74.9% of 10~12 year old trees. There was no significant difference in MPF, FF, TSS content, SS content, TA content and TSS/TA and SS/TA values between 21~23 and 28~30 year old trees, but the Vc content of 21~23 year old trees was significantly higher than that of 28~30 year old trees. It can be seen that the fruit quality index of Shenzhou nane decreases with the increase of tree age.

1.2 Soil nutrients in Shenzhou nane orchards with different tree ages

Soil nutrient is a prerequisite for the healthy growth of fruit trees. 10~12 and 21~23 year old orchard soil was acid, 28~30 and 38~40 year old orchard soil was strong acid, and the pH value of 38~40 year old orchard soil was significantly lower. That is, with the increase of tree age, the pH value of rhizosphere soil showed a significant downward trend (Figure 2; Table 2). The soil organic matter content of 10~12 and 28~30 year old orchards was significantly higher than that of other orchards, followed by 21~23 year old orchards, and 38~40 year old orchard was the lowest. Soil alkaline N and available Ca contents were highest in 28~30 year old orchard, followed by 10~12, 21~23 and 38~40 year old orchard. And there were significant differences in alkaline N and available Ca among the four types of orchards. The soil available P content of 10~12 year old orchard was the highest, followed by 38~40 year old orchard, 21~23, 38~40 year old orchard was the lowest. The contents of soil available K and available Zn were the highest in 28~30 year old orchards, followed by 21~23, 10~12 and 38~40 year old orchards. And there were significant differences in available P and available Zn contents among the four types of orchards. The available Mg content of 21~23 year old orchard was the highest, followed by 28~30, 10~12 and 38~40 year old orchard. There were significant differences in available Mg content among the four types of orchards. The soil available Fe content was the highest in 21~23 year old orchard, followed by 10~12 and 28~30 year old orchards, and the lowest in 38~40 year old orchards. The soil available Mn content of 21~23 year old orchard was the highest, followed by 10~12 year old orchard, and 28~30 and 38~40 year old orchard was the lowest. Soil available Cu content was highest in 10~12 year old orchard, followed by 21~23 and 28~30 year old orchard, and lowest in 38~40 year old orchard.

1.3 Nutritional status of Shenzhou nane with different tree ages

Nutrient content in leaves of fruit trees with different ages varied greatly. There were significant differences in the same nutrients among different tree ages at fruit expanding stage. In addition to the total nitrogen content of 28~30 and 38~40 year old fruit trees at fruit ripening stage, there were significant differences in other nutrients among different tree ages. However, the variation of nutrient content in different developmental stages was different (Figure 3; Table 3).

The changes of total N and total Cu contents in leaves at fruit expanding stage were consistent, that is, the highest contents were in 28~30 year old fruit trees, followed by 10~12, 21~23 and 38~40 year old fruit trees. Total P and total K contents in leaves at fruit expanding stage were consistent, that is, the highest contents were in 28~30 year old fruit trees, followed by 21~23, 10~12 and 38~40 year old fruit trees. The total Ca content in leaves of 21~23 year old fruit trees was the highest, followed by 10~12, 28~30 and 38~40 year old fruit trees. The total Mg content in leaves of 21~23 year old fruit trees was the highest, followed by 10~12, 38~40 and 28~30 year old fruit trees. The total Mn content in leaves of 21~23 year old fruit trees was the highest, followed by 28~30, 10~12 and 38~40 year old fruit trees. The total Zn content in leaves of 10~12 year old fruit trees was the highest, followed by 21~23, 38~40 and 28~30 year old fruit trees.

The contents of total N, total K, total Ca, total Mg, total Fe, total Cu and total Zn were the highest in the leaves of 10~12 year old fruit trees at fruit ripening stage, and the contents of total P and total Mn were the second.  The content of total Mn was the highest in 21~23 year old fruit trees, followed by the total N, total K, total Fe and total Zn. The total Cu content of 28~30 year old fruit trees was only followed by the 10~12 year old fruit trees. The total P content of 38~40 year old fruit trees was the highest, and the total Ca and Mg content were the second. The contents of total P, total Ca, total Mg, total Fe, total Mn and total Zn of 28~30 year old fruit trees were the lowest. The total P content of 28~30 year old fruit trees was the lowest, the contents of total K and total Cu of 38~40 year old fruit trees were the lowest.

1.4 Relationships between soil and leaf nutrients and the fruit quality of Shenzhou nane

1.4.1 Relationships between soil nutrients and the fruit quality 

Different nutrient contents have different effects on fruit quality. Overall, soil available Cu and Ca content, rhizosphere pH value, organic matter content and available P content had significant indigenous effects on fruit quality, and other nutrient indicators had no significant indigenous effects on fruit quality. Among them, soil available Cu content was significantly positively correlated with MPF, FF, TSS, SS content and TSS/TA, and significantly positively correlated with Vc content and SS/TA value of fruit, but had no effect on TA content. The contents of soil available Ca and organic matter were extremely significantly positively correlated with MPF, FF and TSS, significantly positively correlated with SS content and TSS/TA, and had no significant influence on TA content, Vc content and SS/TA value. Rhizosphere pH was significantly positively correlated with MPF, FF, TSS content and TSS/TA value, significantly positively correlated with SS content, and had no significant influence on TA content, Vc content and SS/TA value. Soil available P content was extremely significantly negatively correlated with TA content in fruit, and significantly positively correlated with Vc content and SS/TA value, but had no significant effect on MPF, FF, TSS content, SS content and TSS/TA value (Table 4).

1.4.2 Relationships between leaf nutrients and the fruit quality at different fruit development stages

The total N content in leaves during fruit expanding stage was extremely significantly positively correlated with the contents of MPF and TSS, and significantly positively correlated with FF. The total K content was significantly positively correlated with MPF. The total Ca content was significantly positively correlated with FF and TSS. The total Fe content was significantly positively correlated with Vc content. The total Zn content was significantly negatively correlated with TA content in fruit, and significantly positively correlated with SS/TA value. Other nutrients had no significant correlation with fruit quality indicators (Table 5; Table 6).

Compared with the fruit expanding stage, nutrient status in leaves during fruit ripening stage had a greater impact on fruit quality. Except that the total Mn content in leaves was not significantly correlated with each nutrient index, other nutrient indexes were significantly or extremely significantly correlated with one or some nutrient indexes. The total N content in leaves was significantly positively correlated with Vc content, TSS/TA and SS/TA values in fruits, significantly positively correlated with MPF, FF, TSS content and SS content in fruits, and significantly negatively correlated with TA content. The total P content in leaves was significantly negatively correlated with MPF and TSS content. The total K content in leaves was extremely significantly positively correlated with the MPF, FF, TSS content and TSS/TA value of fruits, and was significantly positively correlated with the SS content and Vc content of fruits. The contents of total Ca, total Mg, and total Fe in leaves were extremely significantly negatively correlated with TA content in fruits, but extremely significantly positively correlated with Vc content. The total Mg content in leaves was also extremely significantly positively correlated with SS/TA value and significantly positively correlated with TSS/TA value. The total Fe content in leaves was also significantly positively correlated with TSS/TA and SS/TA values. The total Cu content in leaves was extremely significantly positively correlated with MPF, FF, TSS content, SS content, TSS/TA and SS/TA values in fruits. The total Zn content in leaves was extremely significantly positively correlated with Vc content, TSS/TA and SS/TA values, significantly positively correlated with TSS content and SS content, and extremely significantly negatively correlated with TA content.

2 Discussion

This study found that the fruit quality of Shenzhou nane at different ages varied greatly. The MPF, TSS, SS, Vc content, and TSS/TA and SS/TA value of 10~12 year old fruit trees were the highest. The fruit quality of fruit trees with tree age over 20 years decreased significantly, and the tree age over 30 years was worse, that is, the fruit quality index decreased with the increase of tree age. This is consistent with previous studies in other fruits, such as the study found that Kinnow’ mandarin fruit quality is the best when the tree age is moderate (Khalid et al., 2012), when the tree age is more than 20 years of apple fruit quality appear obvious degradation (Treder et al., 2010), single fruit weight and Vc content of Prunus salicina trees aged over 20 years decreased significantly (Wang et al., 2005).

Soil nutrients are very important for the growth and development of fruit trees. Appropriate soil pH is conducive to the absorption of mineral nutrition by fruit trees (Xie et al., 1997). Increasing soil organic matter content can improve soil physical and chemical properties, improve photosynthetic performance of apple (Malus domesica Borkh) trees, and then significantly affect fruit sugar and acid metabolism and fruit quality (Zhao, 2018). Appropriate application of fertilizer K, Ca, Cu, Zn can significantly improve the quality and storage resistance of Actinidia chinensis Planch, Prunus persica var. nectarina, Citrus maxima, and Citrus maxima (Burm.) Merr. cv. Shatian Yu (Wang et al., 2006; Li et al., 2010; Luo, 2010; Zhang et al., 2015). The results of this experiment showed that the soil in the orchards of Shengzhou nane production area was acidic or even strongly acidic, and the soil pH value decreased significantly with the increase of tree age. Similarly, Leng (2015) found that the pH value of Citrus reticulata Blanco cv. Ponkan orchard soil decreased with the increase of tree age. Orchard soil organic matter is lack, alkaline N is insufficient, available P, available Fe, available Mn, available Zn levels are higher. However, the contents of available K, available Ca, available Mg, available Cu and available Zn in rhizosphere soil of 38~40 year old fruit trees were insufficient. The difference of soil nutrient content may be mainly related to farmers' fertilization management. However, from the rhizosphere pH and nutrient content, the rhizosphere pH of 38~40 year old fruit trees decreased significantly, and all kinds of nutrient content were at a very low level, which was not conducive to the growth of fruit trees and was not suitable for continuous cultivation of Shengzhou nane. The results of correlation analysis showed that the fruit quality of Shengzhou nane was mainly affected by five soil nutrient indexes, such as available Cu content, available Ca content, rhizosphere pH value, organic matter content and available P content. Therefore, in order to ensure high yield and high quality of Shengzhou nane, orchard should pay attention to the combination of organic fertilizer and inorganic fertilizer, and appropriately increase the application amount of organic fertilizer. Attention should be paid to maintaining proper application of alkaline fertilizers such as Ca fertilizer to ensure soil Ca content and suitable rhizosphere soil pH, especially in orchards with older tree. Appropriate application of Cu fertilizer and P fertilizer, but it should be noted that the Cu is a kind of trace heavy metal, so it should be applied appropriately in production. Because Cheng (2018) found that excessive Cu in Guanxi pomelo orchard led to the accumulation of Cu in the fruit, which eventually led to the increase of TA content and granulation rate. In addition, the 38~40 year old fruit orchards were lack of nutrients except available P. It is suggested that all kinds of organic and inorganic fertilizers should be applied timely and reasonably.

The fruit development of Shengzhou nane is similar to peach and plum, which can be divided into three stages: fruit expanding stage, fruit growing stage and fruit ripening stage (Ma and Zhang, 2004). The volume and weight of young fruit increased rapidly during the fruit expanding stage, and the fruit grew faster. After the growing stage of core-hardening stage, the fruit enters the second rapid growth period, which is the fruit ripening stage. During this period, the dry weight of fruit grew fastest, and it was the peak period of flesh weight gain. Two fast long-term fruit physiological metabolism is active, nutrient demand is bigger. Therefore, to ensure yield and fruit quality, it is necessary to pay special attention to the nutrient supply of fruit during this period. Leaf is the most sensitive organ to mineral nutrition in the whole tree. Nutrient content in leaves significantly affects tree growth and final fruit yield and quality. Casero et al. (2004) found that the apple acidity was positively correlated with P and K content in leaves, and fruit hardness was positively correlated with Ca content in leaves. P content in leaves of Citrus unshiu was positively correlated with Vc content, and K content in leaves was negatively correlated with soluble solids and soluble sugar during flower bud differentiation (Zhang et al., 2010). Ling et al. (2012) found that the content of soluble solids in navel orange in southern Jiangxi province of China was negatively correlated with K content in leaves, and positively correlated with Mg and Zn content. The titratable acidity was positively correlated with N and Mn content in leaves.

The results of this study showed that the nutrient contents of Shengzhou nane leaves with different tree ages were significantly different from each other. There were significant differences in the same nutrient content of leaves with different tree ages during fruit expanding stage, but the nutrient content of leaves had no obvious tree age advantage. However, the nutrient content in leaves of 10~12 year old fruit trees was the highest, and there were significant differences among different tree ages. The correlation analysis results showed that only the five nutrient contents of total N, total K, total Ca, total Fe and total Zn in the leaves during fruit expanding stage were significantly correlated with one or more of the fruit quality indexes. However, in addition to the total Mn content in leaves had no significant correlation with each nutrient index during fruit ripening stage, the contents of other nutrient contents were positively correlated with one or more of the fruit quality indexes. The contents of total N, total K and total Cu had the greatest impact on fruit quality indicators. It suggested that the correlation between nutrients and fruit quality is not exactly the same at different developmental stages. Wang et al. (2013) also found that the N content in the leaves of Clausena lansium was significantly negatively correlated with the total acid content in the fruits at the flower bud differentiation stage, but there was no correlation between the N content in the leaves and the fruit quality during the fruit expending stage. In this study, compared with the fruit expending stage, nutrient content in leaves during fruit ripening stage had a greater impact on fruit quality. For example, most of the nutrient content in leaves during fruit ripening stage is the highest of 10~12 year old fruit trees, and fruit quality is the best in this age fruit trees. Based on the results of this experiment, it can be seen that the nutrient supply level of leaves during the fruit ripening stage of Shengzhou nane is very important to the fruit quality. In order to ensure the high yield and quality of Shengzhou nane, it is recommended to supplement all kinds of leaf nutrients in time, but the P content of leaves needs to be reasonably controlled.

3 Materials and Methods

3.1 Experimental materials and treatment

The experiment was conducted in Linge village, Jinting town, the main producing area of Shengzhou nane in Shengzhou City in 2019, and four orchard samples with stable yield with different tree ages were selected. The local meteorological conditions are as follows: latitude of 29.56°, longitude of 121.00°, annual average temperature of 16.8°C, precipitation of 1 325.5 mm, average evaporation of 892.6 mm, annual sunshine hours of 1 763.7 h, rainy days of 152.9 d, and average pressure of 1 004.5 P. The soil is weak acid sandy.

The tree ages of experimental fruit trees were 10~12 years (10~12a), 21~23 years (21~23a), 28~30 years (28~30a) and 38~40 years (38~40a), respectively. Three orchards were selected at each tree age stage, and the average spacing of fruit trees was 3 m×5 m. Three sampling plots were randomly selected in each orchard, and three healthy trees with consistent growth were selected in each plot. Soil samples at 0~40 cm soil layer was collected with auger boring at the root of each tree at the full flowering stage on March 25, and passed through a 2 mm sieve. After indoor air drying and grinding, the soil nutrient content was analyzed. The leaves during the two fruit expanding stages were taken on May 15 and June 15. Three healthy and complete functional leaves on the middle and upper healthy vegetative branches of periphery of crown were selected. After the leaves were washed with deionized water, petioles were removed, and de-enzyme at 105°C for 20 min, dried at 80°C to constant weight, crushed and screened, and the nutrient content was determined by sample analysis. When the fruit matures on July 22, 10 fruits with the same size in the middle and upper parts of the periphery of the crown were selected from each tree to analyze and determine the quality indexes of the fruit.

3.2 Items determination

Determination of fruit quality index: The single fruit weight was measured by one thousandth electronic balance, and the fruit hardness was measured by GY-1 fruit hardness meter. The longitudinal and transverse sections of the fruit were taken as samples cutting with stainless steel knife. The content of soluble solids was determined by digital saccharimeter. The titratable acid content was determined by NaOH neutralization titration. The soluble sugar content was determined by anthrone colorimetry (Miao et al., 2013).

Determination of soil nutrient index: Samples were prepared according to the method of Hao (2017). The pH value of soil after air drying was determined by pH meter potentiometric method, and the organic matter content was determined by potassium dichromate-sulfuric acid oxidation method. Alkaline N content was determined by alkali-diffusion method. The content of available P was determined by molybdenum-antimony anti-colorimetric method. The content of available Ca, Mg, K, Fe, Mn, Cu, Zn was determined by ICP-MS method. 

Determination of leaf nutrient content: After the leaves were digested by sulfuric acid-hydrogen peroxide digestion method, the total N content was determined by distillation method, and the total P content was determined by vanadium molybdenum yellow colorimetry method. After HNO₃-HClO₄ digestion, the contents of total K, Ca, Mg, Fe, Mn, Cu and Zn were determined by ICP-MS.

3.3 Data processing

The experimental data were the average of three replicates for each treatment, and SPSS 22.0 statistical software was used for statistical analysis.

Authors’ contributions

GTR was the designer and director of this study, completed data analysis, paper writing and modification. YCT, WQH, and FYT participated in experimental design, experimental operation, experimental results analysis, part of the paper writing. All authors read and approved the final manuscript.

Acknowledgments

This study was supported by the Public Welfare Technology Application Research Project of Shaoxing (2018C20008), Public Welfare Project of Science Technology Department of Zhejiang Province (No.LGN20C150004), Project of Department of Education of Zhejiang Province (Y201839946).

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