Abstract
Nutrient deficiency leads to a high fruit abscission rate and low yields of Sapindus mukorossi Gaertn. (Soapberry), which is one of the most widely cultivated biodiesel feedstock forests in China. Exogenous sucrose can provide a solution to nutrient deficiency and fruit abscission leading to low yields; therefore, it was applied to whole trees at two stages, 20 days before blooming (DBB stage) and before fruit abscission [days before fruit abscission (DBFA) stage]. Six sucrose concentrations, 0%, 1%, 1.5%, 3%, 5%, and 7%, were sprayed three times using a completely randomized block design with five replications and six treatments. 13CO2 labeling experiments were performed after the three sprayings. The results indicated that the 3% treatment had the highest yield, reaching 15.9 kg/tree. During the DBB stage, the 3% treatment significantly increased the inflorescence fructose and glucose contents 1- to 1.2-times and resulted in the highest fruit gibberellic acid, leaf indole acetic acid (IAA), fruit IAA, and fruit zeatin contents; however, it decreased the inflorescence abscisic acid (ABA) from 16 μg/g to 4 μg/g. The 1.5% and 3% treatments significantly increased the carbohydrate content and decreased the fruit ABA content to 30% to 50% of the control level during the DBFA stage. High-concentration sugar treatment (>3%) increased the nitrogen, phosphorus, and potassium contents, which decreased the calcium and magnesium contents. The 13C-dispatching ability of the inflorescence was three-times greater than that of leaves under the high-concentration sugar treatment during the DBB stage. Supplying 1.5% sucrose nearly doubled the allocation capacity during the DBFA stage. The source-sink nutrient migration pathway showed that leaf and fruit sugars were directly correlated with phosphorus. Fruit fructose and glucose contents affected the leaf mineral element contents.
Soapberry (Sapindus mukorossi Gaertn.) is widely cultivated as a biodiesel tree in southern China, which is demonstrated in the 2013 Planning of National Forestry Biomass Energy Development. Because of its high seed oil content (42.7%) and multifunctionality (saponin material and bioenergy and medical applications), it has formed an industrial chain (Gao Y. et al., 2015; Jia and Sun, 2012) and has been planted on nearly 26,000 ha in China. However, the high fruit abscission rate of soapberry results in poor yields and has become the bottleneck to the sustainable development of this plant for industrial uses. There are three fruit abscission stages during the growing season: the first is the initial fruiting stage, with a drop rate of 49.02%; the second is the fruit core-hardening stage, with a drop rate reaching 72.74%; and the last is the mature period, with a drop rate of 17.69% (Gao et al., 2015). The first fruitlet abscission stage (June) is caused by incomplete pollination. The second fruit abscission stage, which is called the physiological fruit-falling period (August–September), is hypothesized to result from a shortage of carbohydrates and nutrients. The third stage occurs before fruit maturation and is caused by a high ethylene content (Janssen et al., 2008; Mitra et al., 2005; Sánchez-Torres and González-Candelas, 2003).
Fruit set is one of the most important factors that determines the net production potential of a fruit crop (Gill et al., 2012). It is imperative to improve fruit set by reducing flower and fruitlet drop with the use of cross-pollination, plant growth regulators (PGRs), nutrients, and chemicals (Mostafa et al., 2001). The first and second abscission stages, with high abscission rates, need to be addressed and regulated regarding soapberry. Sucrose supplementation could result in lower abscission rates by an increase in carbon availability for developing fruitlets (Iglesias et al., 2003). The main reason for spraying sugar onto plants before blooming is to boost carbohydrates, which are the main energy source for most insects (Hagen et al., 1984). Carbohydrates can attract bees and ensure complete pollination (Du et al., 2017; Rogers and Potter, 2004); therefore, sucrose application significantly improved flower production and increased bud size and fresh weight, which increased the fruit set rate during the first abscission stage. Carbohydrates, mineral nutrients, and hormonal growth regulators have important roles in the physiological regulation of reproductive growth. In particular, with adequate supplies of sugar, woody plants maintain floral induction, fruit set, and high yield (Giulia et al., 2013; Ulrich, 2007) indicated that the physiological fruit-falling period (second abscission stage) of apple results from the deficiency of sugar signals. Nonstructural carbohydrates (TNCs) are the main type of energy transferred during the fruit growth stage (Ainsworth and Bush, 2010; Turgeon, 2010). Exogenous sources of sugar are greatest for plants, and appropriate nutrient supplementation can further improve plant starch production (Xu et al., 2019). Furthermore, sugar metabolism is closely related to the ripening and senescence of fruit, which are directly regulated by ethylene (Deng et al., 2013), especially when light conditions are poor or light intensity is low. Berrie (1960) and Wojcik et al. (2013) demonstrated that spraying sucrose onto cherry during the growing season guaranteed an optimal soluble sugar content of fruit under conditions of low solar radiation, a high canopy density, or overcropping. Most of the soapberry feedstock forests in China occur in low-light density areas, which have a high canopy density and narrow canopy gaps (Gao Yuan et al., 2018); therefore, the sugar supply is important for nutrition transfer in these forests. Amiri et al. (2012) showed that the application of sucrose before the initiation of the June fruit drop resulted in a 19.5% reduction in abscission and a 13% increase in yield. Sugar application also significantly increases root and leaf yields in radishes (Smoleń and Sady, 2012).
The TNCs include starch and soluble sugars (sucrose, glucose, fructose, and sorbitol). These carbohydrates have an essential role in supporting new growth by providing structural components and energy (Cheng et al., 2004). The application of exogenous sucrose may result in a larger flow of carbon compounds from glycolysis to organic acids and increased amino acids (Morcuende et al., 1998). Previous research proved that sucrose and sorbitol are the main forms of assimilated carbohydrates (photosynthetic products) that are transported from leaves to fruits (Loescher et al., 1982). Leaves are regarded as a source, and fruit growth depends largely on the carbohydrates synthesized by leaves on the same branch that bears the fruit (Kozlowski, 1992). Sugar metabolism in leaves is very fast, dynamic, and complex, and it can strongly alter the leaf carbohydrate profile (Rolland et al., 2006). Fruits or seeds are regarded as sinks, and strong sinks can absorb large amounts of TNCs from the branches, stems, and leaves of trees (Cannell, 1997). The capacity of fruits to mobilize carbohydrates varies seasonally and between treatments in accordance with differences in the rates of fruit growth. Changes in the availability of assimilates will also trigger changes in the plant hormonal balance (Domingos et al., 2015); at the same time, plant hormones not only control fruit growth and development but also regulate the transport and distribution of carbohydrates from “source” to “sink” (Cakmak et al., 1989; Sekimoto et al., 1997). Sucrose and fructose have significant roles in the cold response and response to ABA (Liu et al., 2013), and ABA has a clear effect on the activity of sucrose synthase. Additionally, IAA has no significant effects on the glucose content in grains, but it has effects on invertase and leads to an increase in the conversion of sucrose to fructose (Ghorbani Javid et al., 2011). Sufficient evidence has revealed that different phytohormones can serve as modulators of specific rate-limiting components in photoassimilate metabolism (Brenner and Cheikh, 1995). Minerals are recycled from the xylem to the phloem in leaves; then, they are transported to developing fruits together with carbohydrates. Minerals are also essential during flowering (Brunkhorst, 2001; Gosnell, 1973); for example, potassium (K) is involved in the transport of sucrose to the apical region (Bell and Leigh, 1996). Calcium (Ca) affects the transport of sugar within the plant and the development of flowers (Endres et al., 2015). Nitrogen (N) application can decrease plant concentrations of glucose, fructose, and sucrose (Bi et al., 2004).
Previous research of nutrient transfer has mainly focused on branches or nursery stock. There has been less research of nutrient transfer in whole fruit trees. Most studies have focused on the changes in single nutrients after treatments or increases in the fruit set rate after nutrients are supplied, and few studies have analyzed the correlations between all nutrients. Supplying sucrose to whole plants will cause different sugar metabolism responses and hormone and mineral element changes compared with those observed in simplified models using leaf segments (Lobo et al., 2015). Supplying exogenous sucrose before blooming and at the fruit expansion stage will influence the relative transfer of all nutrients, especially in soapberry, which requires a large amount of carbohydrates to grow for many years under low-intensity light conditions. Our research addresses which sucrose concentration contributes to increasing TNC and endogenous hormone and mineral element contents during the blooming and fruiting stages. It also addresses how the carbohydrate allocation and transfer abilities of leaves (source) and fruits (sink) change after sucrose application. Finally, it addresses the direct and indirect relationships between TNCs, endogenous hormones, and mineral elements. The answers to these questions will reveal the sink transport ability, elucidate the relevant nutrient pathways, and provide theoretical and technical support for decreasing the fruit abscission rate. These results provide a foundation for further study of the relevant mechanisms and signaling pathways and guidance for the practical application of sugar.
Material and Methods
Experimental site description and experimental materials.
The research site was located in Jianning County, Fujian Province (lat. 26°30′–27°06′N, long. 116°35′–117°04′E), where the elevation ranges from 280 to 858 m, the annual average temperature ranges from 16.5 to 17.5 °C, the average rainfall is 1800 mm to 2100 mm, the average sunshine duration is 1721 h per year, and the relative humidity is 84% (Gao Yuan et al., 2018; Gao Y. et al., 2015). The soil parent material is feldspar sandstone, and the soil texture is mostly heavy loam (Cartesian) to light clay (soil layer thickness >100 cm) (Ding, 2014; Li, 2013).
The experimental forest (≈5200 ha) was planted by Yuan Hua Forestry Biotechnology Co. in 2008. The experimental plot was located in the Fengyuan Demonstration Zone. The study trees were planted in 2009 using 2-year-old nursery stock, which came from Tiantai, Zhejiang Province, and level terraces, and the planting density was 675 to 750 plants/ha. Management measures included pruning once per year, disease and insect pest prevention three to five times per year, and fertilization once per year. Fertilizer was applied in March every year. The fertilizer was compound fertilizer with a mass ratio of N, phosphorus (P), and K was 20:5:20, and the amount of fertilizer was 2 kg/tree.
Thirty healthy trees that had three scaffold branches with an angle of 60° and 16 to 18 fruiting branches per unit area that had grown for 8 years were selected. The planting holes were 50 × 40 × 40 cm, the plant spacing was 4 × 4 m, the tree height was 5.9 ± 0.64 m, the tree basal diameter was 16.14 ± 0.51 cm, and the tree crown width was 4.2 ± 0.66 m.
Spraying treatments, 13C labeling, and sampling.
This experiment was conducted to evaluate the effects of exogenous sucrose application at two stages: the DBB stage (20 d before blooming) and the DBFA stage (second fruit abscission stage). The dates of these two stages were defined according to the research Gao et al. (2015) from 2013 to 2015 The blooming stage began on 18 May (±2 d), and the abscission stage began on 28 Aug (±2 d) (Fig. 1). Sucrose solutions of six different concentrations, 0% (control), 1%, 1.5%, 3%, 5%, and 7%, were applied by spraying three times every 7 d (Awasthi et al., 1984; Gill et al., 2012) until 5 d before blooming and abscission for a total of six times during these two different stages. The experiment was conducted with a completely randomized block design with five replications (n = 1 tree per replicate) and six treatments. Each tree was sprayed with 5 L of water containing dissolved sucrose, which made the surface wet to the point of dripping. The experiment was performed under temperatures of 27 and 21 °C (maximum and minimum, respectively), and the light ratio percentage under the canopy was in the range of ≈30% to 40%. The samples were randomly selected; eight soapberry inflorescences and infructescences from outer branches growing in different directions at the middle canopy height were selected from each treatment after 7 d of spraying. The leaves were selected from the fourth-eighth leaf positions from the shoot tip. The yield per tree of each treatment was investigated at the end of October when the fruit was ripening.
Major phenological stages of Sapindus mukorossi. The top three lines depict the important phenological stages throughout an entire year. FAR indicates the three fruit abscission rates stages measured from 2013 to 2015. The bottom six pictures show the six different sucrose sprayings. DBB refers to 20, 13, and 5 d before the blooming stage; DBFA refers to 20, 13, and 5 d before the fruit abscission stage.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Major phenological stages of Sapindus mukorossi. The top three lines depict the important phenological stages throughout an entire year. FAR indicates the three fruit abscission rates stages measured from 2013 to 2015. The bottom six pictures show the six different sucrose sprayings. DBB refers to 20, 13, and 5 d before the blooming stage; DBFA refers to 20, 13, and 5 d before the fruit abscission stage.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Major phenological stages of Sapindus mukorossi. The top three lines depict the important phenological stages throughout an entire year. FAR indicates the three fruit abscission rates stages measured from 2013 to 2015. The bottom six pictures show the six different sucrose sprayings. DBB refers to 20, 13, and 5 d before the blooming stage; DBFA refers to 20, 13, and 5 d before the fruit abscission stage.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
13CO2 labeling experiments were performed under sunny conditions from 11:00 to 13:00 after three sprayings during the two stages. Four inflorescences and infructescences were selected from branches with different orientations for each treatment group at the middle canopy height for 13C labeling. Transparent polyethylene plastic bags (40 × 50 cm) were used to cover the inflorescences and infructescences.
Then, 20 mL of 13CO2 (99 atom%) (purchased from SRICI) was injected into the bags every hour for a total of four times to maintain the 13CO2 density at 500 μmol/mol. The samples were taken 24, 48, 72, and 96 h after 13C isotope labeling. Five soapberry leaves, flowers, or fruits were randomly selected from the labeled inflorescences and infructescences. The consumption trends of these four samples were compared for the same time periods, and there was no significant consumption at 96 h; therefore, 24 and 48 h were chosen for analysis of the results.
Determination of sucrose, glucose, and fructose.
One-gram samples were dissolved in 5 mL of ultrapure water and subjected to ultrasonic bath extraction for 10 min. The samples were centrifuged for 10 min at 12,000 rpm and 4 °C; then filtered through 0.45-μm hydrophobic membranes.
The concentrations of sucrose, glucose, and fructose were quantified by high-performance liquid chromatography (HPLC). An Agilent Zorbax carbohydrate column (4.6 × 150 mm, 5 μm) was used for the analysis and maintained at 35 °C. The mobile phase was 80% acetonitrile plus 20% ultrapure water. The sample injection volume was 10 μL, the flow velocity was 1 mL/min, and the injection time was 17 min. Before determining TNCs in the sample, standard solutions of sucrose, fructose, and glucose were prepared. Calibration curves were made for each of the sugars using these solutions (Ghorbani Javid et al., 2011; Ulger et al., 2004; Yuan et al., 2014).
Determination of endogenous hormones.
During this study, extraction of plant hormones was performed by HPLC as previously described (Pan et al., 2008; Zeng et al., 2006), with some modifications detailed here. Approximately 1 g of fresh leaves was frozen in liquid N and ground into powder. Then, 5 mL of 80% ice-methanol (4 °C, maintained in the dark) was used to soak the samples for 10 h. Then, the samples were centrifuged at 12,000 rpm at 4 °C for 30 min and transferred to a bottle, to which one to two drops of ammonium hydroxide were added. The bottles were rotary evaporated at 38 °C until reaching one-third of their original volume. Then, 2 mL of distilled water was used to wash the samples in 10-mL tubes; the samples were centrifuged at 12,000 rpm at 4 °C for 30 min again and adjusted to a pH of 2.5 to 3.0 [for IAA, gibberellic acid (GA3), and ABA] or 7.5 to 8.0 [for zeatin (ZT)]. IAA, GA3, and ABA were extracted by ethyl acetate. ZT was extracted by water-saturated n-butyl alcohol. The extractions were repeated three times. The samples were rotary evaporated to dryness and finally dissolved in 1 mL of methanol–0.1 mol/L acetic acid (1:4, v/v), except for ZT, which was dissolved in 3% methanol-97% ultrapure water with a pH of 7. Before HPLC analysis, the solution was filtered using a 0.45-µm microfilter. A 4.6- × 250-mm 5-µm Zorbax Eclipse × DB-C18 analytical column was used for the analysis, along with a sample injection volume of 5 μL, a flow velocity of 1 mL/min, and a temperature of 30 °C. The mobile phase for IAA, GA3, and ABA was 20% methanol pus 80% 1 N acetic acid, and that for ZT was 40% methanol + 60% ultrapure water with a pH of 7.
Determination of mineral elements.
According to the principle of plant nutrition, leaves are the most active part of the fruit tree’s anabolic and metabolic functions, and the changes in the nutrient supply are more sensitive to leaves; therefore, leaves can represent the tree’s nutrient level and serve as the organ of nutritional diagnosis (Chapman, 1980). Therefore, we only analyzed the changes in the mineral content of leaves. The leaves were cleaned, separated, and dried in a 70 °C oven, and the dry plant samples were ground and analyzed to determine their nutrient contents (N, P, and K). H2SO4–H2O2 was used for digestion; then, the N (total nitrogen), P (total phosphorus), and K (total potassium) concentrations were measured using a Kjeldahl nitrogen meter (Beijing, China), the vanadium molybdenum yellow colorimetric method, and flame photometry, respectively (Bao, 2007). Samples were wet-digested in a mixture of nitric acid perchloric acid [HNO3:HClO4 (4:1)], and the K, Ca, and magnesium (Mg) concentrations were quantified by atomic absorption spectrophotometry (Varian Model Spectra-400 Plus) (Kacar, 1972).
Data analysis.
The data were statistically analyzed using Student’s t test (P < 0.05). Five biological replicates were used in the TNC, endogenous hormone, and mineral element analyses, and data were analyzed with SPSS Statistics 20.0 software (SPSS Inc., Chicago, IL) at a significance level of P ≤ 0.05 using Duncan’s multiple range test. To identify significant differences, repeated measurements were statistically compared between the treatments using a one-way analysis of variance. Correlation and linear regression analyses were performed to identify and evaluate the relationships among the lengths of shoots, inflorescences, and infructescences and the 13C contents of leaves, fruits, and seeds.
The relationships between fruit and leaf nutrients were Investigated using a canonical correlation analysis. Five groups were created [leaf sugar group: leaf sucrose (LS), leaf glucose (LG), and leaf fructose (LF); fruit sugar group: fruit sucrose (FS), fruit glucose (FG), and fruit fructose (FF); leaf hormones group: leaf (L)-GA3, leaf (L)-IAA, leaf (L)-ABA, and leaf (L)-ZT; fruit hormones group: fruit (F)-GA3, fruit (F)-IAA, fruit (F)-ABA, and fruit (F)-ZT; and leaf mineral elements group: leaf nitrogen (LN), leaf phosphorus (LP), leaf potassium (LK), leaf calcium (LCa), and leaf magnesium (LMg)] to examine the correlation between a linear combination of the fruit nutrient variables (X-set), designated canonical variable U, and a linear combination of the leaf nutrient variables (Y-set), designated canonical variable V. All data were standardized. Ten repeated comparisons were used in the analysis. All computations used to examine the relationships between the two sets of traits were performed with R software (Cankaya et al., 2010; Gunderson and Muirhead, 1997). Excel 2016 and Origin 2017 were used to create the charts.
Results
Comparison of yield among different treatments.
As shown in Fig. 2, the yields of exogenous sucrose applications were higher than that of the control group. The yields of 1.5% and 3% exogenous sucrose applications were three- to five-times higher than that of the control. The 3% application had the highest yield, reaching 15.9 kg/tree.
Comparison of yield with the six exogenous sugar application treatments. Vertical bars indicate ses. Different letters above bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of yield with the six exogenous sugar application treatments. Vertical bars indicate ses. Different letters above bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of yield with the six exogenous sugar application treatments. Vertical bars indicate ses. Different letters above bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of nonstructural carbohydrates among different treatments.
As shown in Fig. 3, the fructose, glucose, and sucrose contents on the day before blooming were significantly higher than those at the fruit abscission stage, and the fruit content was 20% to 70% higher than the leaf content.
Comparison of nonstructural carbohydrates after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. (A) Amount of fructose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. (B) Amount of glucose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. (C) Amount of sucrose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. Vertical bars indicate ses. Different letters above bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05. DW = dry weight.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of nonstructural carbohydrates after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. (A) Amount of fructose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. (B) Amount of glucose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. (C) Amount of sucrose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. Vertical bars indicate ses. Different letters above bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05. DW = dry weight.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of nonstructural carbohydrates after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. (A) Amount of fructose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. (B) Amount of glucose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. (C) Amount of sucrose in leaves and inflorescences/fruits with the six exogenous sugar application treatments. Vertical bars indicate ses. Different letters above bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05. DW = dry weight.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Spraying did not significantly impact leaf fructose (Fig. 3A). In inflorescences and fruits, the fructose content of the treatment groups was higher than that of the control. The 3% and 5% exogenous sucrose applications before the blooming stage significantly increased the inflorescence fructose content, which was 0.5- to 1-times greater than that of the other treatment groups with the first two sprayings. After the third spraying, the inflorescence fructose content of the 3% sucrose treatment group was 1- to 1.2-times higher than that of the other treatment groups. Although the fructose content in the inflorescence was higher at DBB20 and DBB13 with only the 3% and 5% treatments, it was higher at DBB5 with only the 3% treatment compared with the control. Before the fruit abscission stage, the 1.5% sucrose treatment resulted in a significantly (0.5–1 mg/g dry weight) higher fructose content than that observed for the other treatments after three sprayings.
The leaf glucose content (Fig. 3B) showed no significant differences, and the 0% treatment (control) group maintained a decreasing trend for the inflorescence glucose content. Clearly, the 3% and 5% treatments resulted in glucose contents that were at least 30% higher than those of the other groups in fruits at DBB20 and DBB13, respectively. Until the third spraying, the inflorescence glucose content with the 5% treatment was decreased to half that with the 3% treatment. It was higher at DBB5 with only the 3% treatment compared with the control. During the fruiting stage, after the first spraying, the high sugar concentration of 7% resulted in high glucose levels; however, thereafter, the 1.5% and 3% treatments significantly increased the glucose levels.
The amount of LS showed no significant differences during the six sprayings (Fig. 3C). After the first spraying, the inflorescence sucrose level significantly increased with the 1.5% to 5% treatments, but there was no difference from the control levels after the DBB13 spraying. After DBB5, the 3% treatment resulted in sucrose levels 20% to 40% higher than those achieved with the other treatments. The sucrose content in the fruit was higher at DBB20 with only the 3% treatment. The sucrose content was higher at DBB5 with the 1.5%, 3%, and 5% treatments compared with the control.
Comparison of endogenous hormones among different treatments.
Figure 4A shows that the GA3 content at DBB20 to DBB5 was higher than that at DBFA20 to DBFA5. After the second spraying, inflorescence GA3 significantly increased (four- to eight- times) under the high-concentration treatment compared with the control, but leaf GA3 decreased as the fruit GA3 increased. Furthermore, the 1.5% treatment at DBB20 increased the GA3 level in the leaf, whereas all treatments at DBB13 decreased GA3 levels compared with the control. After the third spraying, the 3% and 5% treatments significantly increased the inflorescence GA3 to levels more than 900 μg/g at DBB5. At the DBFA stage, a significant change in fruit GA3 appeared at DBFA5, especially with the 5% treatment, for which the fruit GA3 was one-fold higher than the control level.
Comparison of endogenous hormone contents after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. (A) Amount of gibberellic acid (GA3) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (B) Amount of indole acetic acid (IAA) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (C) Amount of zeatin (ZT) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (D) Amount of abscisic acid (ABA) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of endogenous hormone contents after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. (A) Amount of gibberellic acid (GA3) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (B) Amount of indole acetic acid (IAA) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (C) Amount of zeatin (ZT) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (D) Amount of abscisic acid (ABA) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of endogenous hormone contents after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. (A) Amount of gibberellic acid (GA3) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (B) Amount of indole acetic acid (IAA) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (C) Amount of zeatin (ZT) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. (D) Amount of abscisic acid (ABA) in leaves and inflorescences/fruits with the six exogenous sucrose treatments. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Compared with the changes in GA3, those in IAA were completely different. The L-IAA level at the DBFA stage was higher than that at the DBB stage (Fig. 4B), whereas F-IAA showed the opposite trend. The L-IAA of the control maintained a decreasing trend in the DBB stage compared with that with the 1% to 3% treatments, which increased after the second spraying. Until DBB5, the 3% treatment group maintained the largest amounts of L-IAA, and the 3% and 5% treatment groups maintained the largest amounts of F-IAA. During the DBFA stage, the L-IAA of the control was higher than that of all the treatment groups. The F-IAA significantly increased with the high-concentration treatments after the third spraying.
As shown in Fig. 4C, the ZT content was significantly higher during the blooming stage than during the fruit abscission stage. The high-concentration treatments resulted in more stable L-ZT levels during the DBB stage, and the 1.5% to 5% treatments resulted in inflorescence ZT contents at least 50% higher than that of the control after the third spraying. The ZT content in the inflorescence was higher at DBB13 and DBB5 with only the 1% treatment compared with the control. At the DBFA stage, the 3% treatment increased the L-ZT at DBFA5, but the F-ZT showed no significant change compared with that of the control.
L-ABA showed a decreasing trend in the DBB20 stage (Fig. 4D). In contrast, it maintained an increasing trend during the DBFA20 stage. Exogenous sucrose decreased the ABA amount in flowers and fruits, and the smallest amounts at the two stages appeared with the 5% and 7% treatments. After the third spraying during the DBB stage, the F-ABA content reached only 30% to 50% of the control level. The ABA content in the inflorescence was lower with only the 3% to 7% treatments during the DBB stage, and it was lower during the DBFA stage with only the 1.5% treatment compared with the control.
Comparison of mineral elements among different treatments.
During the comparison of the levels of five mineral elements (Fig. 5), before the blooming stage, the N content was found to have the highest percentage (>3%). During the DBFA stage, the Ca levels were ≈6% (±1.5%) higher than the levels of the other mineral elements, and the P content was the lowest during these two stages.
Comparison of mineral elements after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. The amounts of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in leaves with six exogenous sucrose treatments are shown, respectively. The last panel compares these five mineral elements. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of mineral elements after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. The amounts of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in leaves with six exogenous sucrose treatments are shown, respectively. The last panel compares these five mineral elements. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of mineral elements after exogenous sucrose applications at 20, 13, and 5 d before the blooming stage, and at 20, 13, and 5 d before the fruit abscission stage. The amounts of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in leaves with six exogenous sucrose treatments are shown, respectively. The last panel compares these five mineral elements. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
A slightly increased N content was observed during the DBB stage after high-concentration treatment spraying. During the DBFA stage, there was a significant increase with the 1.5% and 3% treatments, and the highest-concentration treatment of 7% sucrose led to a lower leaf N content. The phosphorus content was highest with the 3% treatment during the first and second sprayings. During the DBFA stage, treatment with different concentrations had no significant influences on the P content, but it was clear that the P content significantly decreased in the control after the third spraying. It can be concluded that without exogenous sucrose, P decreased quickly during the 20 d before the fruit abscission stage. The K content during the DBFA stage was higher than that during the DBB stage. During the DBB stage, the 3% treatment resulted in a significantly higher K content than that observed with the other treatments after the third spraying; during the DBFA stage, the K content with the 7% treatment was higher than that with the other treatments. The Ca content was higher with the high-concentration treatment than that with the other treatments during the DBB stage, but there was almost no difference among treatments after the third spraying. The Ca content was significantly higher during the fruiting stage than during the flowering stage, but the spraying treatments resulted in Ca contents lower than those of the control during the DBFA stage. After the 1% and 1.5% sucrose treatments, the Mg content was nearly 0.6%, which was the highest content observed during these two stages.
Comparison of 13C isotope labeling among different treatments.
As shown in Fig. 6A, no correlation was detected between shoot length and leaf 13C content. The inflorescence length was negatively correlated [r2 = 0.322 (24 h); r2 = 0.453 (48 h)] with the flower 13C content (Fig. 6b). A weak negative correlation was observed between inflorescence length and fruit 13C content [r2 = 0.157 (24 h); r2 = 0.225 (48 h)], but the former was significantly positively correlated [r2 = 0.567 (24 h); r2 = 0.608 (48 h)] with seed 13C content.
The relationship between 13C values and the lengths of shoots, inflorescences, and infructescences after 24 h and 48 h of isotope labeling. (A) The relationship between the length of shoots and the leaf 13C content. (B) The relationship between the length of inflorescences and the flower 13C content. (C) The relationship between the length of infructescences and the fruit 13C content. (D) The relationship between the length of infructescences and the seed 13C content.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
The relationship between 13C values and the lengths of shoots, inflorescences, and infructescences after 24 h and 48 h of isotope labeling. (A) The relationship between the length of shoots and the leaf 13C content. (B) The relationship between the length of inflorescences and the flower 13C content. (C) The relationship between the length of infructescences and the fruit 13C content. (D) The relationship between the length of infructescences and the seed 13C content.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
The relationship between 13C values and the lengths of shoots, inflorescences, and infructescences after 24 h and 48 h of isotope labeling. (A) The relationship between the length of shoots and the leaf 13C content. (B) The relationship between the length of inflorescences and the flower 13C content. (C) The relationship between the length of infructescences and the fruit 13C content. (D) The relationship between the length of infructescences and the seed 13C content.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
The opposite trend was observed between leaves and inflorescences, which indicated an exchange from source to sink (Fig. 7). With the blank control and low concentration treatment, flowers contained 37% more 13C than leaves, and when the 3% sucrose treatment was applied, the flowers contained more than three-times the 13C observed in leaves, which indicated that higher sucrose contributed to increases in leaf-to-flower allocation and transport ability. Similar to the findings during the DBFA stage, with the treatments with 1.5% to 5% sucrose, the ability of plants to transfer 13C from leaves to fruit increased from 85.7% to 200%, especially with the 1.5% treatment. It was clear that there was a large gap between the leaf and fruit 13C contents. The highest seed 13C content appeared with the 5% treatment, which indicated that a high concentration of sucrose resulted in an increased absorption ability of seeds.
Comparison of 13C amounts in different organs with six exogenous sucrose treatments after labeling for 24 h to 96 h. (A) The black column indicates the leaf 13C value from 24 h to 96 h at 5 d before blooming (DBB5). The grey column indicates the inflorescence 13C value from 24 h to 96 h at DBB5. (B) The black column indicates the leaf 13C value from 24 h to 48 h at 5 d before fruit abscission (DBFA5). The black column at the top indicates the fruit 13C value, and grey represents the seed 13C content. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of 13C amounts in different organs with six exogenous sucrose treatments after labeling for 24 h to 96 h. (A) The black column indicates the leaf 13C value from 24 h to 96 h at 5 d before blooming (DBB5). The grey column indicates the inflorescence 13C value from 24 h to 96 h at DBB5. (B) The black column indicates the leaf 13C value from 24 h to 48 h at 5 d before fruit abscission (DBFA5). The black column at the top indicates the fruit 13C value, and grey represents the seed 13C content. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Comparison of 13C amounts in different organs with six exogenous sucrose treatments after labeling for 24 h to 96 h. (A) The black column indicates the leaf 13C value from 24 h to 96 h at 5 d before blooming (DBB5). The grey column indicates the inflorescence 13C value from 24 h to 96 h at DBB5. (B) The black column indicates the leaf 13C value from 24 h to 48 h at 5 d before fruit abscission (DBFA5). The black column at the top indicates the fruit 13C value, and grey represents the seed 13C content. Vertical bars indicate ses. Different letters above the bars indicate significant differences between treatments based on Duncan’s multiple range test at P < 0.05.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Correlations among carbohydrates, endogenous hormones, and mineral elements.
Figure 8A shows that fruit sugar levels are closely related to LF, and that leaf sugar is also closely related to FS and FF, which are also correlated with L-ZT and L-ABA (Fig. 8B). Leaf hormones are directly related to LF and LG. Figure 8C indicates that leaf sugar is also related to F-ABA, and that fruit hormones are related to LF. Comparing the leaf sugar content with leaf mineral element content, the former was correlated with LN and LP, and the total mineral element content was correlated with LS (Fig. 8D). Figure 8E shows no correlations between fruit sugar and leaf hormones, but fruit sugar was directly correlated with F-GA3, F-ZT, and F-IAA, and the total fruit hormone content was also related to FS (Fig. 8F). Fruit sugar was also related to LP (Fig. 8G), leaf hormones were related to F-ABA, and fruit hormones were related to L-ZT (Fig. 8H). Leaf mineral elements were related to L-IAA, and fruit hormones were related to LN.
Canonical correlation analysis (CCA) of leaf sugar, fruit sugar, leaf hormones, fruit hormones, and leaf mineral elements. (A) CCA of leaf sugar and fruit sugar; comx indicates total leaf sugar and comy indicates total fruit sugar. (B) CCA of leaf sugar and leaf hormones; comx indicates total leaf sugar and comy indicates all leaf hormones. (C) CCA of leaf sugar and fruit hormones; comx indicates total leaf sugar and comy indicates all the fruit hormones. (D) CCA of leaf sugar and leaf mineral elements; comx indicates total leaf sugar and comy indicates all leaf mineral elements. (E) CCA of fruit sugar and leaf hormones; comx indicates total fruit sugar and comy indicates all leaf hormones. (F) CCA of fruit sugar and fruit hormones; comx indicates total fruit sugar and comy indicates all fruit hormones. (G) CCA of fruit sugar and leaf mineral elements; comx indicates total fruit sugar and comy indicates all leaf mineral elements. (H) CCA of fruit hormones and leaf hormones; comx indicates total fruit hormones and comy indicates all leaf hormones. (I) CCA of leaf hormones and leaf mineral elements; comx indicates total leaf hormones and comy indicates total leaf mineral elements. (J) CCA of fruit hormones and leaf mineral elements; comx indicates total fruit hormones and comy indicates total leaf mineral elements.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Canonical correlation analysis (CCA) of leaf sugar, fruit sugar, leaf hormones, fruit hormones, and leaf mineral elements. (A) CCA of leaf sugar and fruit sugar; comx indicates total leaf sugar and comy indicates total fruit sugar. (B) CCA of leaf sugar and leaf hormones; comx indicates total leaf sugar and comy indicates all leaf hormones. (C) CCA of leaf sugar and fruit hormones; comx indicates total leaf sugar and comy indicates all the fruit hormones. (D) CCA of leaf sugar and leaf mineral elements; comx indicates total leaf sugar and comy indicates all leaf mineral elements. (E) CCA of fruit sugar and leaf hormones; comx indicates total fruit sugar and comy indicates all leaf hormones. (F) CCA of fruit sugar and fruit hormones; comx indicates total fruit sugar and comy indicates all fruit hormones. (G) CCA of fruit sugar and leaf mineral elements; comx indicates total fruit sugar and comy indicates all leaf mineral elements. (H) CCA of fruit hormones and leaf hormones; comx indicates total fruit hormones and comy indicates all leaf hormones. (I) CCA of leaf hormones and leaf mineral elements; comx indicates total leaf hormones and comy indicates total leaf mineral elements. (J) CCA of fruit hormones and leaf mineral elements; comx indicates total fruit hormones and comy indicates total leaf mineral elements.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Canonical correlation analysis (CCA) of leaf sugar, fruit sugar, leaf hormones, fruit hormones, and leaf mineral elements. (A) CCA of leaf sugar and fruit sugar; comx indicates total leaf sugar and comy indicates total fruit sugar. (B) CCA of leaf sugar and leaf hormones; comx indicates total leaf sugar and comy indicates all leaf hormones. (C) CCA of leaf sugar and fruit hormones; comx indicates total leaf sugar and comy indicates all the fruit hormones. (D) CCA of leaf sugar and leaf mineral elements; comx indicates total leaf sugar and comy indicates all leaf mineral elements. (E) CCA of fruit sugar and leaf hormones; comx indicates total fruit sugar and comy indicates all leaf hormones. (F) CCA of fruit sugar and fruit hormones; comx indicates total fruit sugar and comy indicates all fruit hormones. (G) CCA of fruit sugar and leaf mineral elements; comx indicates total fruit sugar and comy indicates all leaf mineral elements. (H) CCA of fruit hormones and leaf hormones; comx indicates total fruit hormones and comy indicates all leaf hormones. (I) CCA of leaf hormones and leaf mineral elements; comx indicates total leaf hormones and comy indicates total leaf mineral elements. (J) CCA of fruit hormones and leaf mineral elements; comx indicates total fruit hormones and comy indicates total leaf mineral elements.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Based on the correlation analysis and previous research, the pathway diagram shown in Fig. 9 was constructed from the aforementioned correlations to show the clear relationships between nutrients in fruits (source) and leaves (sink). Leaf sugar and fruit sugar are directly correlated through P; therefore, P is the main factor affecting the source-to-sink transfer of carbohydrates. Fruit sugars and fruit growth hormones (gibberellins, cytokinins and auxins) are positively correlated. Leaf sugar is positively correlated with leaf ZT, leaf N, and ABA. Leaf total sugar accumulates when the sink (fruit) absorption ability decreases, which results in an increase in fruit ABA. The FS has a direct relationship with fruit endogenous hormones, and the FF and FG contents affect leaf mineral elements. Additionally, leaf mineral elements affect L-IAA and LS directly, whereas LN is directly correlated with fruit endogenous hormones and leaf total sugar. Finally, fruit total sugar and fruit endogenous hormones are correlated with LF.
The influence of main nutrition pathways on leaves and fruits. Brackets indicate combined functions. Lines with arrows indicate the correlations between two factors.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
The influence of main nutrition pathways on leaves and fruits. Brackets indicate combined functions. Lines with arrows indicate the correlations between two factors.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
The influence of main nutrition pathways on leaves and fruits. Brackets indicate combined functions. Lines with arrows indicate the correlations between two factors.
Citation: HortScience 57, 11; 10.21273/HORTSCI16626-22
Discussion
Yield response to exogenous sucrose.
The period from flowering to maturity is the key period for the formation of fruit yield. The material sources during this period are the photoassimilates that are directly transported to the grains by photosynthesis and the assimilates that are stored in the reservoir organs (Dordas, 2009). Sucrose is considered to be the main product of photosynthesis in higher plants, the main form of carbon transport, and the substrate of sink metabolism. Exogenous sucrose treatments significantly increased the yield during this study. The 3% application had the highest yield, reaching 15.9 kg/tree.
Nonstructural carbohydrates, endogenous hormones, and mineral elements responses to exogenous sucrose.
Exogenous sucrose had a significant influence on carbohydrates, endogenous hormones, and mineral elements of fruits in this study. These results show that the leaf nonstructural carbohydrate content cannot be influenced by exogenous sucrose because the leaves can produce carbohydrates by photosynthesis and do not absorb exogenous sugar, whereas the fruits (flowers) comprise the sink that absorbs most of the energy from exogenous sucrose (Saadati et al., 2013). The FG and FF are present in approximately equal amounts that are larger than the amount of sucrose, which agrees with previous research findings (Nagy et al., 2010). Spray application of sucrose during the blooming stage significantly increased fruit sugar, especially after the first and second sprayings at medium (3%) and high concentrations (5%). However, the 5% and 7% sucrose treatments decreased the fruit sugar content after the third spraying. These findings indicate that a high concentration of exogenous sucrose decreased the fruit absorption ability, whereas a medium concentration (3%) resulted in the maintenance of a high carbohydrate content with the three sprayings during the blooming stage. The blank control maintained a decreasing trend in fructose during the DBFA stage compared with that of the 1.5% treatment, which resulted in increased carbohydrate levels after the third spraying during this stage.
GA3 was increased in flowers and fruits with the 3% and 5% sucrose treatments. Previous research indicated that GA3 has a role in fruit growth promotion rather than abscission, but the levels of GA3 are strongly reduced when sucrose is deficient (Gómez-Cadenas et al., 2000). These results also proved that GA3 increased after exogenous sucrose application. Ülger et al. (2018) showed that GA3 does not affect the IAA level significantly, but that increasing GA3 will result in a decrease in ABA, which indicates that the trend of changes is different between these two hormones. The control treatment resulted in a higher leaf IAA content than the other treatments during these two stages. After exogenous sucrose application, the F-IAA increased because the sugar signal induced the differentiation and expansion of fruits (Giulia et al., 2013). L-ZT maintained a stable trend under the high-concentration treatment when compared with the control during the DBB stage because it can maintain the nutrition pathway balance when leaves rapidly differentiate, thus increasing the flower’s ZT to guarantee sufficient hormone stimulation for flower budbreak. The natural drop of apple fruitlets occurs after increases in ABA and ethylene production, thus leading to negative feedback in fruit development (Botton et al., 2010). A decrease in the ABA content in fruitlets can decrease the rate of flower drop and increase fruit set (Glozer et al., 2006); it is clear that F-ABA was significantly lower after treatment spraying than it was in the control during the DBB stage. The F-ABA under treatment spraying maintained an increasing trend in the DBFA stage when compared with that of the control, which exhibited a decreasing trend because of an increase in the ABA content in mature fruit, which promotes the unloading of sugar into fruit (Gibson, 2004), combined with TNCs, which were more abundant after spraying. These observations are in agreement with the hypothesis of Talón et al. (1997), who proposed that a carbohydrate shortage will reduce hormonal stimulators of growth, such as GA3, and increase stress-sensitive signals, such as ABA levels. The high-concentration treatments (5% and 7%) lowered the ABA content, which can also indirectly enhance fruit set compared with that determined by other chemicals (Gill et al., 2012).
The leaf N, P, and K contents will be reduced with the increasing leaf age. In contrast, Ca has the opposite trend, because it will gradually increase with the increasing leaf age; however, Mg increases first and then decreases (Lin, 2005). The N content continued to decrease from the blooming stage to the fruiting stage, and the Ca content continued to increase. The N content was highest among all the mineral elements in apple leaves, followed by Ca, K, Mg, and P (Li et al., 1987). Soapberry leaves had the same order of mineral nutrients; therefore, the foliar spraying of apple trees can be a model for soapberry trees. Previous research indicated that N contents have minimal value during the fruiting stage (Fernández-Escobar et al., 1999). After a spray application of 3% sugar, the N content showed a significant increase when compared with that of the control, which indicated that exogenous sucrose can solve the problem of a low N content during the fruit abscission stage. The P content loss also decreased after the spray application of sucrose, whereas a rapidly decreasing trend was maintained with the control treatment. K can promote starch transformation to sugar in fruit, and the K content during the DBFA stage was higher than that during the DBB stage, which could increase the sugar content and yield; therefore, the K content will be increased after exogenous sucrose application (Huang et al., 2000; Qu et al., 2000). The abscission rate of apples decreased after the spray treatment with Ca, which reduced the ethylene content (Guan et al., 1991). After the spray application of sugar during the DBFA stage, the leaf Ca content was lower than that of the blank control, which indicated that fruits (sink) absorbed much more Ca from leaves (source) after exogenous sucrose application than before.
Leaf (source) and fruit (sink) exchange in response to different treatments.
Fruit shoot length was not correlated with the ability of leaves to absorb 13C, and previous research also demonstrated that the distance to the source leaves had no effect on fruit growth in the path of transport (Garcia-Luis et al., 2002). It is clear that shoot length cannot influence plant source–sink allocation and transport abilities. This suggests that the shoot-cutting and training method can be used for soapberry pruning, not for nutrient transfer but instead to regulate apical dominance and light distribution, especially in most soapberry feedstock forests in light-limited areas (Gao et al., 2018).
There was a weak negative relationship between inflorescence length and the ability of flowers to absorb 13C. Furthermore, shorter and more compact inflorescences had a greater ability to allocate and transport nutrients. Infructescence length was weakly correlated with the fruit absorption ability and exhibited a significant negative relationship with the kernel absorption ability, which demonstrated that the source–sink transport capacity was determined by kernels. This also demonstrated by the fact that Sapindus delavayi has short infructescences, which have a high kernel oil content and high yield (Sun et al., 2017).
Sucrose is the major photoassimilate transported from photosynthetic leaves to developing fruit, where it is converted into hexose (Walker and Ho, 1977). Walker reported that the rate of sucrose import is regulated by the sucrose concentration gradient between leaves (source) and fruits (sink), and that there is an inverse relationship between import rates and sucrose levels in the fruit. After exogenous sucrose application, the import rates changed, and the sink absorption ability increased, which prevented the formation of a fruit abscission layer.
Endogenous hormones are the signal for carbohydrate transfer from source to sink, especially during the DBFA stage, and the balance between IAA and ABA in leaves and fruit is very important. When carbohydrates decrease, ABA will increase rapidly (Gómez-Cadenas et al., 2000). Positive hormones, such as IAA, ZT, and GA3, increase in fruits, whereas ABA decreases, which solves the problem of fruit abscission (Gao, 2006). This was demonstrated during this study by ABA in leaves and fruits decreasing and IAA increasing after the spray application of 5% sucrose.
Direct and indirect pathways among nonstructural carbohydrates, endogenous hormones, and mineral elements.
The pathways outlined in this study clearly show the positive relationships between fruit sugar and fruit hormones (gibberellins, cytokinins, and auxins), and the results of this study are the same as those of previous research indicating that exogenous cytokinins (Asthir et al., 1998) and IAA (Cole and Patrick, 1998) promoted sucrose accumulation in developing wheat grains. Leaf total sugar was also positively related to leaf cytokinins, LN, and fruit ABA. This result indicates that if the fruit (sink) absorption ability decreases, which will cause the accumulation of leaf total sugar; then, fruit ABA will increase, directly leading to fruit drop. Therefore, maintaining a carbohydrate difference between the source and sink can reduce the leaf sugar content and increase the fruit setting rate. The FS is directly related to fruit endogenous hormones. The FF and FG contents will affect leaf mineral elements, and leaf mineral elements directly affect leaf auxins and sucrose. Leaf total sugar and fruit total sugar are related to LP, and previous research has indicated that P is involved in the transport of carbohydrates, which have an energy transfer role in the process of sugar alienation (Vance et al., 2003; Wang, 2012) and participate in the photosynthetic pigment synthesis pathway. P deficiency is related to a reduction in chloroplast carbon fixation as a consequence of photosynthetic potential (Chrysargyris et al., 2016). Therefore, P is the main element involved in light acquisition and carbohydrate metabolism and transport (Xiao and Wang, 2004). This research demonstrated that the energy transfer between the source and sink mainly relies on P, making this element a key factor for transmission. The LN has a direct correlation with fruit endogenous hormones and leaf total sugar. The LN is an important component of endogenous hormones and is necessary for cell division, which promotes photosynthesis and carbohydrate synthesis, thus explaining why LN has a direct impact on leaf sugar. LF and LG are correlated with leaf endogenous hormones. ABA stimulates sorbitol oxidase activity to enhance sugar accumulation in the fruit (Kobashi et al., 1999). Fruit total sugar and fruit endogenous hormones are also correlated with LF; therefore, and this mechanism is worthy of further in-depth studies.
This study included only the DBB and DBFA stages in 2016; therefore, it cannot provide sufficient data to create a pathway for all of the phenological stages. However, these two stages are most important during the flowering and fruiting stages, and the pathway depicts a clear correlation explaining nutrition transfer from source to sink. Ongoing studies should focus on all phenological stages and the changes in more nutrients during consecutive years. Future research should use a molecular genetics approach to define the role of sucrose synthase in fruit development. Long-term studies should be performed to construct a pathway from genetic to physiological control.
Conclusions
The yields of 1.5% and 3% exogenous sucrose applications were three- to five-times higher than that of the control. The 3% application had the highest yield, reaching 15.9 kg/tree. Fructose, glucose, sucrose, and ZT (cytokinin) levels on the day before blooming were significantly higher than those during the fruit abscission stage. After three exogenous sucrose applications before the blooming stage, there was no significant variation in the leaf TNC content. The carbohydrate content of fruits was 20% to 70% higher than that of leaves, which increased the potential of the sink and ensured transport capacity. The 3% sucrose treatment resulted in a significant (1- to 1.2-fold) increase in the inflorescence fructose and glucose levels compared with those achieved with the other treatments, and sucrose was 20% to 40% higher than that achieved with the other treatments. Treatment with 3% and 5% sucrose significantly increased fruit GA3 to more than 900 μg/g during the DBB stage, and treatment with 3% sucrose also maintained the largest amounts of L-IAA, F-IAA, and F-ZT. The ABA content in leaves decreased from 15 μg/g to 2 μg/g after sucrose application, and the content in the inflorescences decreased from 16 μg/g to 4 μg/g, which successfully reduced the stimulation of the flower abscission layer. After three exogenous sucrose applications before the fruit abscission stage, the fructose content with the 1.5% sucrose treatment was significantly (0.5–1 mg/g dry weight) higher than that with the other treatments, and the 1.5% and 3% treatments significantly increased the glucose and sucrose levels. The GA3 level with the 5% sucrose treatment increased one-fold greater than that of the control during the DBFA stage, and the control L-IAA level, which was negatively correlated with F-IAA, was much higher than that achieved with all the spraying treatments. Exogenous sucrose decreased the ABA amount in flowers and fruits, reducing F-ABA to 30% to 50% of the control level.
Before the blooming stage, N was detected at the highest percentage (>3%). During the DBFA stage, Ca was present at a higher percentage than the other mineral elements, with a percentage of ≈6% (±1.5%). P showed the lowest content during these two stages. High-concentration sucrose treatment (>3%) increased the N, P, and K contents, which decreased the Ca and Mg contents. Spray application of sucrose is beneficial for increasing the seed absorption capacity; when the sprayed sucrose concentration was more than 3% during the DBB stage, the C dispatching ability of the inflorescence was three-times greater than that of the leaves. During the DBFA stage, the application of 1.5% sucrose can guarantee nearly two-times the allocation capacity from sink to source.
A pathway of source–sink nutrition migration with P induction was constructed. It showed that leaf sugar and fruit sugar are directly correlated through P, meaning that P is the main factor affecting the source-to-sink transfer of carbohydrates. Fruit sugar and fruit growth hormones (gibberellins, cytokinins, and auxins) are positively correlated. Leaf sugar is positively correlated with leaf ZT, leaf N, and ABA. The FF and FG contents affect leaf mineral element contents, and leaf mineral elements affect leaf IAA and sucrose directly. Additionally, fruit total sugar and fruit endogenous hormones are correlated with LF.
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