Field sites and experimental design
The experiment was conducted in the greenhouse at the Experimental Station, China Agricultural University, Beijing (39˚54N’, 116˚23E’), China, during June 2015 to October 2016. The experiment design was complete random block design with three replicates. Four irrigation treatments were furrow irrigation (FI), micro-sprinkler irrigation (MS), furrow irrigation under plastic film mulching (PF) and micro-sprinkler irrigation under the plastic film mulching (MSP) (Fig. 1). Moisture transducer (Shengshixintong Technology, Beijing, China) was used to monitor soil moisture. The irrigation started when soil moisture lowered to 60% of field capacity, and the total irrigation in four treatments are shown in Table 1. The irrigation water given each time was assumed to increase the soil moisture to 80% of field capacity. Thus, the irrigation amount for each time was 15.21 m3 667 m− 2 in AC and 16.87 m3 667 m− 2 in SC, calculated with the following formula [15]:
Table 1
Total irrigation in four treatments
treatment | AC (m3 667 m− 2) | SC (m3 667 m− 2) |
FI | 197.33 | 247.37 |
MS | 182.12 | 230.51 |
PF | 151.69 | 196.78 |
MSP | 151.69 | 179.91 |
Note: AC: Autumn season crop, SC: Spring season crop. |
I = r × p × h × θf × s×(q1-q2)/η, (1)
where r is the soil bulk density (g cm− 3), p is the soil wetting ratio with a value of 100%, h is the planned wetting layer (0.3 m), θf is the field capacity (%), q1 and q2 are 80%, 60%, respectively, s is the area of 667 m2, η is the water use coefficient, taken as a value of 1 in this experiment.
The celery (variety ‘Ventura’) was germinated in greenhouse use 128-plug tray (one plug with one seed) with vegetable seedling substrate (charcoal, vermiculite, slag and perlite are mixed by volume ratio 20: 20: 50: 10) on June 15, 2015 for AC and on January 20, 2016 for SC. The four-leaf seedlings with similar morphological were transplanted on August 1, 2015 for AC and on March 2, 2016 for SC in a plot with size of 3.6 m2 (3.60 m in length × 1.0 m in width), which consisted of four rows with a 20 cm × 25 cm row space in each furrow (Fig. 1). Plastic film inserted vertically 50 cm into the soil profile between adjacent plots to prevent water runoff to adjacent plots. 3000 kg 667 m− 2 decomposed chicken manure (the total content of N, P and K ≥ 4%, total organic matter content ≥ 30%) and 150 kg compound fertilizer 667 m− 2 (the contents of N, P2O5, K2O were 15%, 10% and 15% respectively) were applied before planting. The celery was harvested on November 11, 2015 and on May 31, 2016. The detailed soil physical and chemical characteristics are shown in Table 2.
Table 2
Soil physical and chemical properties in the greenhouse.
| organic matter (%) | total N (g kg− 1) | available N (mg kg− 1) | available P (mg kg− 1) | available K (mg kg− 1) | bulk density (g cm− 3) | field capacity (cm3 cm− 3) |
AC | 2.47 | 1.66 | 228.75 | 101.46 | 260.96 | 1.21 | 31.43 |
SC | 2.39 | 1.48 | 206.32 | 78.89 | 240.33 | 1.12 | 37.63 |
Micro-spraying irrigation technique
In the treatments with micro-spraying irrigation technique, main rigid pipes were with a diameter of 50 mm and micro-sprinkler pipes were with a diameter of 28.6 mm, a hole with 0.1 mm diameter (Figure ). In the MSP treatment, the same pipes were used as in the MS treatment and placed under the plastic film. The plastic film used in PF and MSP treatments was an ordinary black agricultural plastic film with a thickness of 0.01 mm (Figure 1).
The soil and plant sampling and analysis
Soil samples were taken by soil-drilling from 0–10, 10–20 and 20–30 cm soil layers before planting (July 30, 2015 for AC and on March 1, 2016 for SC), during the growing season (September 16, 2015 for AC and on April 12, 2016 for SC) and after harvest (November 10, 2015 for AC and on May 30, 2016 for SC). Five soil cores on raised bed were taken in each plot using the soil-drilling in an S-shape. Soil was thoroughly mixed, and about 1 kg of soil was sieved through a 1-mm mesh and afterwards air-dried [34,35].
Plant samples were collected using the five-spot method . Plant height (from ground to the highest point of plant) was measured by ruler (accuracy is 1 mm). Ten individual plants were randomly selected for each plot to determine the plant height every 10 days (from August 10, 2015 to October 29, 2015 in AC, and from March 3, 2016 to May 21, 2016). At harvest (November 10, 2015 for AC and on May 30, 2016 for SC), ten plants for each plot were sampled both aboveground and belowground for plant weight. The fresh weight was measured using electronic balance ALC-210.2 (Puyi Electronic Technology, Shanghai, China). The plant samples were oven-dried at 105 oC for 30 min and then at 80 oC for 36 hours to constant weight.
The soluble sugar contents were analyzed by anthrone colorimetry method [37]. 0.5 g of fresh celery leaves were cut into pieces and put into a test tube, to which 5 mL distilled water was added and mixed. After 30 min in a boiling water bath, the supernatant was collected, this step was repeated twice. Extra distilled water was added to fill the solution to 10 mL, then analyzed with a spectrophotometer UV-2102C (Honglang Company, ZhengZhou, China) at 630 nm wavelengths.
For determination of vitamin C [38], 10 g fresh celery leaves were grinded 60 mL of mixture of metaphosphoric and acetic acid (3% HPO3 + 8% CH3COOH) was added instantly to avoid vitamin C breakdown in air. The dilutions were shaken in shaker reciprocating 190–200 rpm min-1 for one hour using. After that, 2 mL of starch indicator was added to it and the solution was immediately titrated with standard solution of iodine I2 until the solution turns the color from initial reddish-brown color to greyish-blue.
Fiber content was measured by ANKOM A200i semi-automatic fiber analyzer [39]: 0.5 g of petiole dry sample was put into the filter bag. The filter bag was washed with the configured neutral detergent solution in the ANKOM A200i semi-automatic fiber analyzer (ANKOM Technology, New York, America) for 75 min. The filter bag was taken out and rinsed with acetone for 5 min. After being dried naturally, it was put into the oven at 100 oC for 4 hours. The sample was taken out from the previous step to a dryer and cool to room temperature, then recorded the weight of the sample and add acidic washing solution for 60 min, take out the filter bag and rinse with acetone for 5 min, dry naturally. The filter bag was put into the oven at 100 oC for 4 hours, then cooled to room temperature in a dryer and weigh it. The filter bag was immersed in 72% sulfuric acid solution for 3 hours, rinse the filter bag to pH neutral with water, then rinse with acetone for 3 min, dry naturally. The filter bag was put in a 100 oC oven for 4 hours, dry and weighed it after cooling to room temperature.
The root system was measured by scanning method [40] at the end of harvest (November 10, 2015 for AC and on May 30, 2016 for SC). The whole root system of a celery was collected and rinsed with water, in three replications, then scanned the root system in the EPSOM EXPRESSION 4990 (Seiko Epson Corporation, ChangYe, Germany). The data of root length (RL), root surface area (RSA), root volume (RV) were analyzed by WinRHIZO (Regent Instrument Inc, Ville de Québec, QC Canada).
Calculation of WUE
Economic and biological of WUE (WUEe and WUEb, respectively) were calculated with the following formulas:
WUEe = EY/I, (2)
WUEb = BY/I, (3)
where EY is the economic yield (kg), including the fresh weight of the stems and leaves in celery, BY is the biological yield (kg), including the fresh weight of the roots, stems, and leaves in celery, and I is the irrigation content (m3).
Data analysis
The data were analyzed by Excel and SPSS 13.0, and data from each sampling event were analyzed separately. The data were subjected to analysis of variance (ANOVA) and Duncan's least significant difference (LSD) test to determine the significance, and the significance was set as p < 0.05. The homogeneity of variances was checked by Bartlett tests, and the normality of the variables was tested by the Shapiro-Wilk test.