Effects of water and fertilizer deficit regulation with drip irrigation at different growth stages on fruit quality improvement of kiwifruit in seasonal arid areas of Southwest China

ZHA Yu-xuan,CHEN Fei,WANG Zhi-hui,JlANG Shou-zheng,CUl Ning-bo

State Key Laboratory of Hydraulics and Mountain River Engineering,College of Water Resource and Hydropower,Sichuan University,Chengdu 610065,P.R.China

Abstract For a long time,seasonal drought occurs frequently in Southwest China,and the management of water and fertilizer in kiwifruit orchards has no quantitative standards,which seriously affects the yield and quality of kiwifruit. Therefore,the effects of water and fertilizer deficit regulation with drip irrigation (WFDRDI) on the quality of kiwifruit at different growth stages were explored to achieve water and fertilizer saving,and green and efficient production of kiwifruit. We select‘Jin Yan’ kiwifruit and set two water deficit levels (WD20% and WD40%) and three fertilizer deficit levels (FD15%,FD30% and FD45%) at bud burst to leafing stage (stage I),flowering to fruit set stage (stage II),fruit expansion stage (stage III) and fruit maturation stage (stage IV),respectively,with a full irrigation and fertilization as the control treatment (CK) in 2017 and 2018. Results showed that the WFDRDI at stage II and III had significant effect on fruit physical quality of kiwifruit,specifically,the III-WD40%FD30% and III-WD20%FD45% treatments significantly increased fruit firmness by 13.62 and 15.59%(P<0.05),respectively;the II-WD40%FD15% and III-WD40%FD15% treatments significantly increased dry matter by 8.19 and 6.47% (P<0.05),respectively;the III-WD20%FD15% treatment significantly increased single fruit weight and fruit volume by 9.33 and 12.65% (P<0.05),respectively;the II-WD20%FD15% treatment significantly increased fruit water content by 1.99%(P<0.05). The WFDRDI had an obvious effect on fruit chemical quality of kiwifruit. The III-WD20%FD45%,IV-WD40%FD15%and IV-WD20%FD30% treatments significantly increased vitamin C (Vc) content by 69.96,36.96 and 34.31% (P<0.05),respectively;the III-WD40%FD15% and IV-WD40%FD15% treatments significantly increased total soluble solid (TSS) content by 3.79 and 17.05% (P<0.05),respectively,and significantly increased soluble sugar content by 28.61 and 34.79%(P<0.05),respectively;the contents of fructose,glucose and sucrose also had a significantly increasing trend,which was increased significantly by 5.58–19.63%,40.55–60.36% and 54.03–54.92% in the III-WD40%FD15% and IV-WD40%FD15%treatments (P<0.05),respectively;sugar–acid ratio was increased significantly in the IV-WD40%FD15% treatment by 64.65%(P<0.05). The degree and duration of water and fertilizer deficit had a comprehensive effect on fruit quality of kiwifruit.The WFDRDI at stage II and III contribute to improving fruit physical quality,and the threshold of water and fertilizer deficit were 20 and 15%,respectively;stage III and IV are the critical periods for improving fruit chemical quality by water and fertilizer coupling effect,and the threshold of water and fertilizer deficit were 40 and 15%,respectively.Therefore,aiming at precise water and fertilizer saving,the I-WD20%FD30%,II-WD40%FD15%,III-WD40%FD15% and IV-WD40%FD15%treatments under WFDRDI during the whole growth period of kiwifruit were the best mode to improve quality and production of kiwifruit.

Keywords: water deficit,fertilizer deficit,water and fertilizer coupling,fruit physical quality,fruit chemical quality

1.Introduction

Kiwifruit is one of the major fruits in the world with high nutritional value and delicious taste. China is one of the most important producers and consumers of kiwifruit,the total planting area was 182.6 thousand ha in 2019(Zhonget al.2021). The fruit expansion stage (from July to August) is the rainy season,thus leading frequent waterlogging that wilted kiwifruit leaves,and inhibiting the accumulation of fruit nutrient and the formation of fruit quality (He 2015;Wu 2021). Furthermore,the loss of soil nutrient was increased,resulting in fertilizer waste,surface and groundwater environment polluted (Gao 2016),seriously affecting the sustainable development of kiwifruit production. Therefore,appropriate water and fertilizer coupling strategies need to be explored to save irrigation and fertilization,improve the quality and nutritional value of kiwifruit in seasonal arid areas of Southwest China.

Water deficit with drip irrigation can timely regulate irrigation according to the nutritional characteristics of fruit at different growth stages. The vegetative growth of fruit from fruit setting to pit hardening and flowering requires sufficient water (Levinet al.2017;Huesoet al.2021). Fruit expansion stage is an important period of fruit water requirement,the total soil evaporation reached 88.5–101.8 mm (Cui 2021). Severe drought stress during this stage may reduce fruit volume and damaged fruit physical property of kiwifruit (Calderón-Orellanaet al.2021). However,45–55% water deficit enhanced the water-use efficiency of apples (Huet al.2022),the net photosynthetic rate was also increased(Liu 2021). Moderate water also promoted fruit maturity of sweet oranges (Silveiraet al.2020),and increased the accumulation of vitamin C (Vc) and total soluble solid(TSS) of citrus (Chenet al.2022).

Excessive application of chemical fertilizers may lead a large surplus of soil nutrients,increase the accumulation of NO3–-N (Gao 2016),cause plant uptake of heavy metals and potentially toxic elements (Stoleruet al.2020).It may inhibit the absorption and utilization of nutrients and hinder fruits growth and quality formation. Fertilizer deficit is an effective measure to improve fruit quality,reduce production costs and environmental risks. Kiwifruit has a great demand for nitrogen during fruit bud bursting stage (Wuet al.2020). Moderate nitrogen deficit at this stage could improve the enzyme activity,microorganism abundance and root activity in rhizosphere soil (Guet al.2019),thus promoting nitrogen use efficiency and nutrient absorption capacity to satisfy the vegetative growth of fruit (Ameset al.2020). Fruit gradually strengthen the absorption of phosphorus and potassium from flowering to fruit maturation stage (Wuet al.2020). Basal application of phosphate fertilizer at early growth stage and topdressing of potassium fertilizer at late growth stage can significantly improve plant nutrient absorption and utilization ability (Liuet al.2020;Yuanet al.2022). In addition,fertilizer deficit had a good effect on improving fruit physical quality of citrus (Ran 2020),and increased Vc,TSS and soluble sugar contents of tomato and mango fruits (Liu Jet al.2021;Sunet al.2022).

Water and fertilizer deficit regulation with drip irrigation(WFDRDI) is a new technology of water and fertilizer saving. It can accurately and homogeneously transport water and nutrient to the root zone of plant,to synchronize the water and nutrient supply with plant demand (Kumaret al.2021;Liet al.2021). The WFDRDIgreatly increased fertilizer use efficiency,reduced phosphorus surplus in soil (Wang S Het al.2022),and improved soil microbial metabolic activities (Chenet al.2017),thus promoting the growth of fruit trees. The water and fertilizer coupling effect could improve the photosynthesis and water use efficiency of citrus (Chenet al.2018),promote the accumulation of photosynthetic products,and increase the nutrient absorption and utilization of fruit.

In summary,seasonal drought occurs frequently and uneven temporal and spatial distribution of annual rainfall in main kiwifruit orchards of Southwest China.However,the lack of precise water and fertilizer regulation mode seriously affects the yield and quality of kiwifruit.Exploring the effect of WFDRDIunder rain-shelter cultivation on fruit quality of kiwifruit is the key to realize water and fertilizer saving,quality-improving,green and efficient development in kiwifruit orchards. Therefore,the main purpose of this study is to: (1) explore the effect of different water and fertilizer deficit regulation at different growth stages on fruit physical and chemical quality of kiwifruit;(2) analyze the effect of water,fertilizer and water and fertilizer coupling at different growth stages on fruit physical and chemical quality of kiwifruit;(3) identify the water and fertilizer deficit threshold of water saving,fertilizer saving and quality improvement of kiwifruit at different growth stages,to propose an appropriate strategy of WFDRDI.

2.Materials and methods

2.1.Experimental site

The experiment was conducted in the experimental kiwifruit base in Fuxing Township,Pujiang County,Chengdu,China (30°19′N,103.25′E,approximately 534 m a.s.l) from March 2017 to October 2018. This region is classified as a subtropical humid monsoon climate,with an average annual temperature of 16.3°C,precipitation of 1 248.9 mm,relative humidity of 84%,sunshine duration of 1 122 h yr–1,drought index of 0.68.Soil in the experimental site is mainly yellow soil,a detailed description of its properties provided in Heet al.(2023). The field moisture capacity,saturated moisture and available water content are 28.24,34.76 and 16.81%,respectively.

2.2.Experimental design

Seven-year-old kiwifruit trees (Actinidiachinensis‘Jin Yan’) were selected as the experimental trees. The main growth period of the kiwifruit trees in the experiment was from late March to late October every year. According to the growth characteristics of kiwifruit,the whole growth period was divided into four growth stages,including bud burst to leafing stage (stage I,late March to middle April),flowering to fruit set stage (stage II,middle April to early May),fruit expansion stage (stage III,early May to late June) and fruit maturation stage (stage IV,late June to late October). The kiwifruit trees with a height of 170–200 cm and a diameter of 8–10 cm,were equally spaced with the planting spacing of 5.0 m and row spacing of 5.0 m,branched in a “Y” shape along the column,and unilateral extension of 2.8–3.3 m. Three kiwifruit trees were selected for each treatment according to the completely randomized blocks with three replications.

Two deficit irrigation levels (20% water deficit(WD20%) and 40% water deficit (WD40%)) and three deficit fertilization levels (15% fertilizer deficit (FD15%),30% fertilizer deficit (FD30%) and 45% fertilizer deficit(FD45%)) were set at the growth stages of I,II,III and IV,respectively,as WD40%FD45%,WD40%FD30%,WD40%FD15%,WD20%FD45%,WD20%FD30%,WD20%FD15%,with a full irrigation and fertilization as the control treatment (CK). The irrigation quantity was 229.5,327,468 and 396 m3ha–1at the growth stage I,II,III and IV in CK treatment,the fertilization quantity was 346.5,273,300 and 270 kg ha–1,respectively. The specific experimental design is shown in Table 1. There was a total of 25 treatments. The main component of formulated fertilizer is nitrogen,phosphorus and potassium,fertilizers were operated by the fertigation equipment and then applied by fertigation,the specific application amount is shown in Table 2. The drip irrigation with DripNet PC pressure compensated drip tape. Each drip irrigation belt was evenly distributed with 7 pressurecompensated drip emitters at a distance of 0.4 m,with the flow rate of 1.6 L h–1,and along both sides of fruit trees for 2.8 m. The experiment was carried out in rain-shelter with a height of 5.5 m to avoid the disturbance of rainfall,which is open around to ensure ventilation,and laying plastic films with the light transmittances of 92%. Irrigation when the soil moisture content in CK treatment reaches the lower limit of irrigation,the irrigation amount of different water and fertilizer treatments is measured and controlled by flow meter and valve.

2.3.Measurements of kiwifruit qualities

The specific experimental methods refer to Chenet al.(2022) and Heet al.(2023). The physical quality of kiwifruit mainly includes firmness,dry matter,single fruit weight,fruit volume and fruit water content. Fruit firmness was measured at the upper,middle and lower parts of the fruit using GY-4 (GY-4,Top Cloud-agri Technology Co.,Ltd.,China),and the firmness of these three parts were averaged as the actual kiwifruit firmness. Dry matter was measured by drying method,dried at 55°C for 24 h. Single fruit weight of the harvested kiwifruit was determined. Fruit volume was measured by vernier caliper in the pedicel,navel and middle of the harvested kiwifruit,and calculated by the ellipsoid volume calculation formula. Fruit water content determined by drying method,fresh fruits were dried at 105°C for 30 min and then dried at 70°C until constant weight.

The chemical quality of kiwifruit mainly includes Vc,TSS,titratable acid (TA),soluble sugar,fructose,glucose and sucrose. Vc was determined by the 2,6-dichloroindophenol titrimetric method (Sunet al.2022). TSS was determined by a digital refractometer(Pal-1,Atago,Japan) with automatic temperature compensation. Soluble sugar was determined by the anthrone colorimetric method. TA was determined by titration with 0.1 mol L–1NaOH. The contents offructose,glucose and sucrose were determined by high performance liquid chromatography. The sugar–acid ratio was calculated by the ratio of soluble sugar to TA.

Table 1 Scheme of the water and fertilizer deficit regulation with drip irrigation (WFDRDI) at different growth stages for kiwifruit trees

2.4.Statistical analysis

Two-way analysis of variance (ANOVA) was used to evaluate the effects of drip irrigation and fertilizer integration on kiwifruit quality at different growth stages.Duncan’s multiple-range test was used to assess the significance of differences among all test parameters(P<0.05). SPSS 20.0 (IBM SPSS Statistics,USA),Microsoft office Excel 2021 and Origin 2019 were used for all statistical analysis and data plotting.

3.Results

3.1.Effect of water and fertilizer deficit regulation with drip irrigation at bud burst to leafing stage (l)on kiwifruit quality

The WFDRDIat stage I had no significant effect on kiwifruit physical quality,as shown in Table 3. Compared to CK,fruit firmness and dry matter showed a decreasing trend under WFDRDIat stage I,among which firmness were significantly decreased by 12.19–21.97% in the I-WD20%FD15%treatment in two years (P<0.05),dry matter were significantly decreased by 6.41,2.81,6.19 and 5.10% in I-WD40%FD45%,I-WD40%FD30%,I-WD20%FD45%and I-WD20%FD30%treatments in 2018 (P<0.05),respectively.In addition,the degree of fertilizer deficit had extremely significant effect on firmness and dry matter content of kiwifruit (P<0.01),the coupling effect of water and fertilizer had no significant effect (P>0.05).

Table 3 Effects of water and fertilizer deficit regulation with drip irrigation during bud burst to leafing stage (stage I) on the physical qualities of kiwifruit

The WFDRDIat stage I had a slight effect on kiwifruit chemical quality,as shown in Table 4. Compared to CK,Vc content of kiwifruit showed an increasing trend under WFDRDIat stage I,the I-WD40%FD45%and I-WD40%FD15%treatments significantly increased Vc by 11.41–65.01%and 11.17–69.91% in two years (P<0.05),respectively.TSS content was increased under mild water deficit at stage I,the increment was 12.47 and 7.53% in I-WD20%FD45%and I-WD20%FD30%treatments in 2017,respectively,and reached significant levels (P<0.05),but it was significantly decreased by 5.80–10.58%in I-WD40%FD45%treatment in two years (P<0.05). The WFDRDIat stage I showed no significant effects on TA content of kiwifruit (P>0.05). The I-WD20%FD30%treatment significantly increased soluble sugar,fructose,glucose and sucrose by 75.76,64.35,91.37 and 12.84% in 2017(P<0.05),respectively. Sugar–acid ratio showed a slightly decreasing trend,and the I-WD40%FD45%and I-WD20%FD45%treatments significantly decreased it by 36.20 and 38.64%in 2018 (P<0.05),respectively. In addition,the degree of water deficit had an extremely significant effect on the TSS of kiwifruit (P<0.01),the degree of fertilizer deficit had a significant effect on Vc,TSS,soluble sugar content and sugar–acid ratio (P<0.05),and the coupling effect of water and fertilizer had an extremely significant effect on fructose,glucose and sucrose content (P<0.01).

Overall,considering the effects of water and fertilizer deficit on fruit physical and chemical quality,the I-WD20%FD30%treatment is the recommended WFDRDImode at stage I to improve fruit quality of kiwifruit,which significantly increased the dry matter,TSS and sugar contents of kiwifruit,and no negative effect on other quality indicators.

3.2.Effect of water and fertilizer deficit regulation with drip irrigation at flowering to fruit set stage (ll)on kiwifruit quality

The WFDRDIat stage II had a certain effect on kiwifruit physical quality,as shown in Table 5. Compared to CK,the WFDRDIat stage II had a positive effect on fruit dry matter in two years,among which the II-WD40%FD15%treatment significantly increased it by 8.19% in 2017(P<0.05). Fruit firmness,single fruit weight and fruit volume of kiwifruit showed a decreasing trend under WFDRDIat stage II in two years,among which firmness was significantly decreased in the II-WD20%FD15%treatment by 11.29% (P<0.05);single fruit weight and fruit volume was significantly decreased in the II-WD40%FD45%treatment by 15.61 and 15.81% in 2018 (P<0.05),respectively,and no obvious negative effects were observed in other treatments (P>0.05). The II-WD20%FD15%treatment significantly increased fruit water content by 1.99%(P<0.05). In addition,the coupling effect of water and fertilizer had significant extremely effect on firmness (P<0.01),had significant effect on dry matter and water content of kiwifruit (P<0.05).

The WFDRDIat stage II had a certain effect on kiwifruit chemical quality,as shown in Table 6. Compared to CK,the WFDRDIat stage II had a positive effect on Vc content of kiwifruit,showed the II-WD40%FD45%,II-WD40%FD15%and IIWD20%FD30%treatments significantly increased Vc by 6.64,10.78 and 18.78%in 2017 (P<0.05),and the II-WD40%FD30%and II-WD20%FD15%treatments significantly increased it by 51.11 and 57.68% in 2018(P<0.05),respectively. TSS content was increased significantly in the IIWD40%FD15%and II-WD20%FD45%treatments by 3.97–12.47% and 2.25–6.67% in two years (P<0.05),respectively. There was no significant effect of WFDRDIat stage II on TA content of kiwifruit (P>0.05). Sugar content showed an increasing trend under WFDRDIat stage II,of which soluble sugar content was significantly increased in the II-WD40%FD30%,II-WD40%FD15%and IIWD20%FD30%treatments by 4.24–31.86%,7.01–39.45% and 3.20–21.32% in two years (P<0.05),respectively;fructose and sucrose content was significantly increased in the II-WD40%FD15%treatment by 8.79–33.81% and 17.48–21.31%(P<0.05),respectively;glucose content was significantly increased in the IIWD40%FD15%,I I-WD20%FD45%a n d I IWD20%FD30%treatments by 67.64,103.49 and 46.89% (P<0.05),respectively.Sugar–acid ratio of kiwifruit was increased significantly under WFDRDIat stage II in 2017,whose increment was 28.46–59.74%,and reached significant levels (P<0.05). In addition,the coupling effect of water and fertilizer had significant extremely effect on the content of TSS,sugar and sugar–acid ratio of kiwifruit (P<0.01),had significant effect on Vc content (P<0.05).

Overall,considering the effects of water and fertilizer deficit on fruit physical and chemical quality,the IIWD40%FD15%treatment under WFDRDIatstage II significantly increased the physical quality and the chemical quality of kiwifruit,which was the best treatment to improve the quality of kiwifruit.

Table 5 Effect of water and fertilizer deficit regulation with drip irrigation during flowering to fruit set stage (stage II) on the physical qualities of kiwifruit

3.3.Effect of water and fertilizer deficit regulation with drip irrigation at fruit expansion stage (lll) on kiwifruit quality

The WFDRDIat stage III had significant effect on kiwifruit physical qualities,as shown in Table 7. Compared to CK,the WFDRDIat stage III had a positive effect on fruit firmness and dry matter of kiwifruit,which showed that the firmness was significantly increased in the IIIWD40%FD30%and III-WD20%FD45%treatments by 13.62 and 22.76% in 2018 (P<0.05),respectively;dry matter was significantly increased in the III-WD40%FD15%treatment by 4.70–6.47% in two years (P<0.05). The single fruit weight and fruit volume were significantly increased in III-WD20%FD15%treatment by 7.40–11.26% and 8.46–14.96% (P<0.05),respectively. Fruit water content showed a decreasing trend,which was significantly decreased in the III-WD40%FD30%,III-WD40%FD15%and IIIWD20%FD15%treatments by 1.00,1.37 and 0.86% in 2018(P<0.05),respectively,and other treatments had nonsignificant changes (P>0.05). In addition,the coupling effect of water and fertilizer had a significant effect on firmness and single fruit weight (P<0.05).

The WFDRDIat stage III had a significant effect on kiwifruit chemical quality,as shown in Table 8.Compared to CK,the WFDRDIat stage III had an obviously positive effect on Vc content of kiwifruit,of which the III-WD40%FD30%and III-WD20%FD45%treatments significantly increased Vc by 11.67 and 18.63% in 2017 (P<0.05),respectively,the III-WD40%FD15%and III-WD20%FD45%treatments significantly increased it by 63.67 and 104.98% in 2018 (P<0.05),respectively.Except for III-WD40%FD45%treatment,fruit TSS content showed an increasing trend,and the III-WD40%FD15%treatment significantly increased it by 3.57–4.00% in two years (P<0.05). The III-WD40%FD45%,III-WD40%FD30%and III-WD20%FD15%treatments significantly increased TA content by 20.00,13.33 and 14.44% in 2018 (P<0.05),respectively. The WFDRDIat stage III significantly increased sugar content of kiwifruit,showed the IIWD40%FD30%,III-WD40%FD15%and III-WD20%FD15%treatments significantly increased soluble sugar content by 9.16–26.94%,11.22–46.00% and 15.37–29.19% in two years (P<0.05),respectively;the III-WD40%FD30%and IIIWD40%FD15%treatments significantly increased glucose content by 9.08–97.43% and 12.97–107.75% (P<0.05),respectively;the III-WD40%FD15%,III-WD20%FD45%and IIIWD20%FD30%treatments significantly increased sucrose content by 16.64–91.41%,25.46–96.74%and 68.95–94.26% (P<0.05),respectively;the III-WD40%FD15%treatment significantly increased fructose content by 5.58% in 2018 (P<0.05). Sugar–acid ratio of kiwifruit increased obviously under WFDRDIat stage III,which was significantly increased by 6.03,7.83 and 6.68% in the III-WD40%FD15%,IIIWD20%FD30%and III-WD20%FD15%treatments in 2018 (P<0.05),respectively. In addition,the coupling effect of water and fertilizer had an extremely significant effect on Vc,TSS and sugar content of kiwifruit (P<0.01).

Overall,considering the effects of water and fertilizer deficit on fruit physical and chemical quality,the III-WD40%FD15%treatment at stage III had the best effect on improving fruit quality,which can maintain the physical quality and promote the accumulation of Vc,TSS and sugar of kiwifruit.

3.4.Effect of water and fertilizer deficit regulation with drip irrigation at fruit maturation stage (lV) on kiwifruit quality

The WFDRDIat stage IV had a certain effect on kiwifruit physical quality,as shown in Table 9. Compared to CK,fruit firmness,dry matter,single fruit weight and fruit volume of kiwifruit showed an increasing trend under WFDRDIat stage IV,of which the IVWD40%FD45%,IV-WD40%FD30%and IV-WD40%FD15%treatments significantly increased fruit firmness by 15.59,9.14 and 7.53% in 2018(P<0.05),respectively;the IV-WD40%FD15%and IV-WD20%FD15%treatments significantly increased dry matter by 6.47–8.65%and 5.27–8.42% in two years (P<0.05),respectively;the IV-WD20%FD15%treatment significantly increased single fruit weight and fruit volume by 11.22–15.27% and 9.56–13.94% (P<0.05),respectively. However,fruit water content decreased under WFDRDIat stage IV,which was significantly decreased by 1.01–1.90% in the IVWD20%FD45%treatment in two years (P<0.05),and no significant effect in other treatments(P>0.05). In addition,the degree of water deficit had an extremely significant effect on firmness of kiwifruit (P<0.01),and the degree of fertilizer deficit had a significant effect on dry matter and single fruit weight (P<0.05).

Table 7 Effects of water and fertilizer deficit regulation with drip irrigation during fruit expansion stage (stage III) on the physical qualities of kiwifruit

The WFDRDIat stage IV had an obvious effect on kiwifruit chemical quality,as shown in Table 10. Compared to CK,Vc content of kiwifruit increased obviously under WFDRDIat stage IV,which was significantly increased by 7.50–66.42% and 13.38–55.24% in the IV-WD40%FD15%and IV-WD20%FD30%treatments in two years (P<0.05),respectively. TSS content of kiwifruit increased under WFDRDIat stage IV in 2018,whose increment was 3.57–16.79%,and reached significant levels (P<0.05).TA content of kiwifruit showed a decreasing trend,and the IV-WD40%FD45%and IV-WD40%FD15%treatments significantly decreased it by 37.50 and 26.92% in 2017(P<0.05),respectively. Sugar content of kiwifruit showed a significantly increasing trend under WFDRDIat stage IV,among which the IV-WD40%FD15%,IV-WD20%FD45%and IV-WD20%FD15%treatments significantly increased soluble sugar content by 33.35–36.22%,9.30–57.54% and 9.88–41.80% in two years (P<0.05),respectively;the IV-WD40%FD15%and IV-WD20%FD15%treatments significantly increased glucose content by 31.63–49.47% and 39.50–40.06% (P<0.05),respectively;the IV-WD40%FD15%treatment significantly increased sucrose content by 51.96–57.88%(P<0.05);the IV-WD40%FD15%and IV-WD20%FD45%treatments significantly increased fructose content by 19.63 and 29.14% in 2017 (P<0.05),respectively. Sugar–acid ratio was also increased significantly in the IV-WD40%FD15%treatment by 22.72–86.14% in two years (P<0.05). In addition,the coupling effect of water and fertilizer had extremely significant effect on Vc,TSS and sugar content of kiwifruit (P<0.01).

Overall,considering the effects of water and fertilizer deficit on fruit physical and chemical quality,the recommended model to improve fruit quality was the IV-WD40%FD15%treatment under WFDRDIat stage IV,which had the best effect on improving the physical and chemical quality of kiwifruit.

3.5.The water and fertilizer deficit threshold affecting fruit quality of kiwifruit under WFDRDl

The effect of WFDRDIon the physical quality of kiwifruit is shown in Fig.1. Compared to CK,fruit firmness increased significantly under WFDRDIat stage III(Fig.1-A),and reached the highest of 6.85 N cm–2in the III-WD20%FD45%treatment. Fruit dry matter,single fruit weight and fruit volume of kiwifruit were higher under WFDRDIat stage III and IV,of which dry matter (Fig.1-B)and single fruit weight (Fig.1-C) reached the highest of 19.03% and 123.73 g in the IV-WD20%FD15%treatment,whose increment was 8.99 and 13.33%,respectively.Fruit volume (Fig.1-D) reached the highest of 103.07 cm3in the III-WD20%FD15%treatment,whose increment was 12.65%. Fruit water content was higher under WFDRDIat stage I and II (Fig.1-E),and reached the highest of 84.18% in the IIWD20%FD15%treatment,whose increment was 1.99%,while was decreased significantly under WFDRDIat stage IV. Therefore,the WFDRDIat middle growth stages had a certain effect on fruit physical quality of kiwifruit,the threshold for water and fertilizer deficit being 20 and 15%,respectively.

The effect of WFDRDIon the chemical quality of kiwifruit is shown in Fig.2.Compared to CK,Vc content of kiwifruit was higher under WFDRDIat stage III (Fig.2-A),and reached the highest of 132.58 mg 100 g–1in the III-WD20%FD45%treatment,whose increment was 69.96%. TSS content of kiwifruit was higher under WFDRDIat stage IV(Fig.2-B),and reached the highest of 17.67% in the IV-WD40%FD15%treatment,whose increment was 17.05%. TA content of kiwifruit decreased obviously under WFDRDIat stage III and IV (Fig.2-C),while soluble sugar content increased significantly(Fig.2-D). Soluble sugar content increased with the increase of fertilization amount,and reached higher of 9 352.15 and 9 944.90 mg 100 g–1in the IV-WD40%FD15%and IV-WD20%FD15%treatments,whose increment was 68.71 and 79.40%,respectively.Sugar–acid ratio also increased significantly under WFDRDIat stage IV (Fig.2-E),and reached the highest of 10.34 in the IVWD40%FD15%treatment,whose increment was 64.65%. Therefore,the WFDRDIat stage III and IV had an obvious improvement on the chemical quality of kiwifruit,the threshold for water and fertilizer deficit being 40 and 15%,respectively.

Fig.2 Effects of the water and fertilizer deficit regulation with drip irrigation (WFDRDI) on fruit chemical qualities of kiwifruit at different growth stages. A–E,effects of WFDRDI on vitamin C (Vc),total soluble solid (TSS),titratable acid (TA),soluble sugar and sugar–acid ratio of kiwifruit,respectively. 80 and 60% refer to soil water threshold;85,70 and 55% refer to fertilizer threshold.

4.Discussion

4.1.The water and fertilizer deficit regulation with drip irrigation at middle growth stages had an obviously effect on fruit physical quality of kiwifruit

In this study,WFDRDIat stage II and III showed good improvement effects on the physical quality of kiwifruit. Similarly,15–45% water deficit improved the physical quality of citrus under drip irrigation at theearly growth stage (Chenet al.2022). We found that the fruit firmness was significantly improved at these stages and reached the highest under the III-WD20%FD45%treatment,which was mainly related to fruit physical properties,chemical composition and degree of cell wall degradation (Heet al.2023). Fruit expanding stage(stage III) is a critical period for the development of fruit pulp cells. A mild water deficit can enhance the activity of fruit cell enzymes (Zhonget al.2019),promote the decomposition of cell wall polymers and increase the osmotic potential of fruit (García-Tejeroet al.2010).On the other hand,WFDRDIat stage III decreased fruit water content,which was reported to reduce water loss,enhance swell capacity,avoid fruit softening,thus improving fruit firmness and benefit fruit storage and transportation (Burdonet al.2014). Fruit dry matter under WFDRDIat stage II presented an increasing but not significantly changed trend. This may due to the hypothesis that moderate water deficit increase the accumulation of dry matter in kiwifruit through keeping flowers closed at flowering stage,increasing ovary and ovule starch content at fruit set stage (Rapoportet al.2012),and promoting the transformation of starch to sugar in fruit.

Table 9 Effects of water and fertilizer deficit regulation with drip irrigation during fruit maturation stage (stage IV) on the physical qualities of kiwifruit

Fruit dry matter,single fruit weight and fruit volume of kiwifruit were significantly increased under WFDRDIat stage III,with even higher increase under the IIIWD20%FD15%and III-WD40%FD15%treatments. This may result from the fact that mild water and fertilizer deficit increased the activity of catalase and glutathione reductase in fruit cells (Alikhani-Koupaeiet al.2018),thus enhancing the ability of fruit to adapt stress environment,improving the intrinsic water-use efficiency of plant during water deficit periods (Zhanget al.2016). Meanwhile,the growth,division,cell expansion and nutrient absorb ability of fruit cells were promoted under WFDRDI(Zhonget al.2019;Heet al.2023),thus the fruit volume was increased. In addition,mild water deficit increased the stomatal density and conductance of plant and the ability of nutrient uptake (Chtoukiet al.2022). Moderate nitrogen deficit increased the nitrogen accumulation and the photosynthetic rate of plant (Sunet al.2018),thus enhancing the photosynthesis of fruit trees,and promoting photosynthetic products formation and dry matter accumulation. Zhouet al.(2021) also reported a similar conclusion in apple trees that moderate water and fertilizer coupling under drip irrigation increased the accumulation of fruit dry matter. On the other hand,about 66–70% of dry matter in plant is allocated to fruit growth after fruit setting under moderate water and nitrogen deficit (Zhanget al.2018),which promotes the natural drop of flowers and fruits at flowering to fruit set stage(Huesoet al.2021),resulting in a single fruit allocated with more photosynthetic products and the accumulation and movement of effective nutrients in rhizosphere soil. These physiological changes further altered the distribution of dry matter to improve their accumulation in fruits (Zhanget al.2018),and eventually lead to single fruit weight increase.

4.2.The water and fertilizer deficit regulation with drip irrigation at fruit expanding and maturation stages significantly improved fruit chemical quality of kiwifruit

The result demonstrated that Vc content of kiwifruit displayed a significant increasing trend under WFDRDI. The variation in Vc content is probably due to the difference in Vc synthetases expression level and activity at fruit early growth stages (Liuet al.2022).The expression ofAceGME,AceGMPandAceGGPis induced by water and fertilizer regulation,and is positively correlated with Vc accumulation (Wang W Jet al.2022). In addition,Vc biosynthesis can be enhanced by regulating Vc accumulation pathways (such as L-galactose pathway)during development and physiological processes in higher plants (Liuet al.2022).Fruit expanding stage is critical for fruit vegetative growth and quality formation. In our study,Vc content of kiwifruit reached the highest of 132.58 mg 100 g–1under the III-WD20%FD45%treatment. The main reason is that the mild water and fertilizer deficit at stage III can satisfy the need of fruit trees for water and nutrient,promote the plant growthviapromoting rhizobacteria to roots to establish mutually beneficial associations and increase the abundance and metabolic capacity of rhizosphere bacteria (Reidet al.2021). It then subsequently improved soil rhizosphere environment (Renet al.2021)and enhanced soil microbial metabolic activity and the carbon utilization (Chenet al.2017). In summary,the transport and utilization of soil nutrient were improved under WFDRDI,with more photosynthetic products transferred to reproductive organs,increased the content of Vc synthesis substrate based on carbohydrate,and promotion of Vc synthesis and accumulation in kiwifruit.

The WFDRDIat stage III and IV significantly increased TSS content of kiwifruit,which reached the highest of 17.67% under the IV-WD40%FD15%treatment. The main reason may be that the outer pericarp tissue from ripe kiwifruit had lower water mobility and a greater capacity to swell (Burdonet al.2014),and the high sink activity of kiwifruit under moderate soil water deficit resulted in solute accumulation in plant (Heet al.2023). Meanwhile,the water flow of soil and the water and nutrient requirement of fruit were reduced. However,excessive irrigation and fertilization inhibited the transport of nutrient and the accumulation of photosynthetic products,resulting in the accumulation retardation of TSS. Fruit water content was also increased under full irrigation,which caused the dilution of TSS (Wu 2021). On the other hand,the decrease in TSS under high nitrogen application may be due to a sink competition between shoot development and ripening fruit (Ameset al.2020),which delays fruit ripening and inhibit TSS accumulation.

Sugar content of kiwifruit was significantly increased under WFDRDIat stage IV. Positive accumulation of sugar gradually increased with the increase of fertilizer application,and reached higher under the IV-WD40%FD15%and IV-WD20%FD15%treatments. This may be because water deficit at maturation stage increases the distribution of photosynthesis products to reproductive organs (Liu X Get al.2021),and promotes the conversion of starch to soluble sugar during post-maturation after photosynthetic products from leaves to fruit (Guoet al.2019),which further promotes sugar accumulation of kiwifruit by changing the distribution of photosynthate from leaf to fruit. In addition,the content of fructose,glucose and sucrose also showed an increasing trend. This may be because that mild water and fertilizer deficit could satisfy nutrient requirements for photosynthesis and sugar metabolism,improve the activities of neutral invertase and sucrose synthase during sugar metabolism in fruits (Gaoet al.2021),as well as improve the osmotic regulation of fruit to promote the active accumulation of sugar in kiwifruit (Heet al.2023).Therefore,the generous accumulation of sucrose under WFDRDIat late growth stage may also be the main reason for the increase of soluble sugar in kiwifruit.

4.3.The degree and duration of water and fertilizer deficit had a comprehensive effect on fruit quality of kiwifruit

WFDRDIshowed a good effect of water and fertilizer interaction. Appropriate water deficit can improve nitrogen use efficiency of fruit (Zhou 2015),and increase the water production efficiency by about 21–71% (Calderón-Orellanaet al.2021). Drip fertigation can increase the vertical migration depth of nitrogen and phosphorus nutrients in orchard soil,and reduce the accumulation of nitrogen and phosphorus in surface soil,thus increasing the yield of pear fruit (Songet al.2023). Importantly,many previous studies have shown that water and fertilizer coupling have a significant interaction effect on fruit physical and chemical qualities,which could maintain the accumulation of dry matter of tomatoes (Luoet al.2015),increase fruit firmness and single fruit weight of apples (Yanget al.2021),increase total sugar by 3.7–8.5% and carotenoid by 1.5–13.1% of tomatoes (Liu Jet al.2021),and promote the accumulation of Vc,TSS,organic acid and soluble protein in fruits (Caiet al.2020;Rasoolet al.2020;Huanget al.2022). In this study,the changes of kiwifruit quality under WFDRDIwere affected by the degree and duration of water and fertilizer deficit.WFDRDIat middle growth stage had a certain effect on fruit physical quality of kiwifruit,the II-WD20%FD15%treatment significant increased fruit water content,the III-WD20%FD15%treatment significantly increased single fruit weight and fruit volume,thus enhancing the storage and appearance quality of kiwifruit. WFDRDIsignificantly improved fruit chemical quality of kiwifruit. The coupling effects of water and fertilizer under WFDRDIat stage II,III and IV had an extremely significant effect on Vc,TSS and sugar content of kiwifruit (P<0.01). Vc is an important antioxidant and acts as a redox buffer in almost all organisms that has important nutritional values for humans (Maet al.2022). Vc content significantly increased in II-WD40%FD15%,III-WD40%FD15%and IV-WD40%FD15%treatments. Fruit TSS and soluble sugar contents were higher under WFDRDIat stage IV,which showed higher in the IV-WD40%FD15%treatment,and significantly increased with the increase of fertilizer application. Meanwhile,the contents of fructose,glucose and sucrose also increased significantly under WFDRDIat stage IV. Furthermore,TA content decreased significantly under WFDRDIat stage III and IV,and sugar–acid ratio increased significantly.

In summary,the WFDRDIin kiwifruit orchards can combine drip irrigation system with water deficit and nutrient deficit,to strengthen the cooperative effect of water and fertilizer coupling,and achieve the purpose of saving water and fertilizer. Meanwhile,on this basis,timely water and nutrient regulation was carried out according to the nutritional characteristics of kiwifruit at different growth stages. Therefore,considering the degree and duration of water and fertilizer deficit,the I-WD20%FD30%,II-WD40%FD15%,III-WD40%FD15%and IVWD40%FD15%treatments under WFDRDIensured the realization of water saving,quality improvement and green efficiency of kiwifruit. It also provides a suitable water and fertilizer supply strategy for future kiwifruit production in seasonal arid areas of Southwest China.Moreover,future research should pay attention to soil fertility while reducing irrigation and fertilization,ensure the balance of soil nutrients in kiwifruit orchards,and achieve long-term productivity and the sustainable development of kiwifruit production.

5.Conclusion

The degree and duration of water and fertilizer deficit had a comprehensive effect on fruit quality. WFDRDIat middle growth stages improved fruit physical quality of kiwifruit,which at stage III significantly increased fruit firmness,dry matter,single fruit weight and fruit volume,and at stage II significantly increased fruit water content. WFDRDIat stage III and IV had an obvious effect on fruit chemical quality of kiwifruit,among which Vc content significantly increased in the III-WD20%FD45%treatment;TSS,soluble sugar content and sugar–acid ratio were higher under WFDRDIat stage IV,specifically,the IV-WD40%FD15%treatment had the most significant improvement effect. In addition,the thresholds of water and fertilizer deficit affecting fruit physical quality were 20 and 15%,and the thresholds of that affecting fruit chemical quality were 40 and 15%,respectively.Therefore,the WFDRDIof I-WD20%FD30%,II-WD40%FD15%,IIIWD40%FD15%and IV-WD40%FD15%was the best mode in kiwifruit orchards to achieve the water and fertilizer saving,quality improvement and efficiency increase in seasonal arid areas of Southwest China.

Acknowledgements

The study was funded by the National Natural Science Foundation of China (51779161 and 52279041),the National Funds for Distinguished Young Scientists of China (51922072),the Sichuan Science and Technology Program,China (2023YFN0024 and 2023NZZJ0015),and the Key Development Project of the Chengdu Science and Technology Plan,China (2022-YF05-01008-SN). The authors acknowledge all the personnel who worked on the experiments.

Declaration of competing interest

The authors declare that they have no conflict of interest.