Effects of Residual Mulch Film on the Growth and Fruit

Water Air Soil Pollut (2017) 228:71 DOI 10.1007/s11270-017-3255-2

Effects of Residual Mulch Film on the Growth and Fruit Quality of Tomato (Lycopersicon esculentum Mill.) Xiaoyang Zou & Wenquan Niu & Jingjing Liu & Yuan Li & Bohui Liang & Lili Guo & Yahui Guan

Received: 4 September 2016 / Accepted: 10 January 2017 # Springer International Publishing Switzerland 2017

X. Zou : W. Niu (*) : Y. Guan Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, No. 26 Xinong Road, Yangling, Xianyang, Shaanxi Province 712100, People’s Republic of China e-mail: [email protected]

(CK), 80 kg ha−1 (T1), 160 kg ha−1 (T2), 320 kg ha−1 (T3), 640 kg ha−1 (T4), and 1280 kg ha−1 (T5). Plant height, stem diameter, dry biomass, yield, root length, root surface area, fruit shape index (FSI), soluble sugar content (SSC), organic acid (OA), vitamin C (VC), lycopene, and nitrate content (NC) were measured. Plant height, stem diameter, dry biomass, and yield of tomato had a downward trend as the residual mulch film amount increased. Root length and root surface area were significantly decreased with an increasing amount of residual mulch film, but root volume and root diameter showed an inconspicuous decrease. When the amount of residual mulch film was more than 80 kg ha−1, growth indexes, dry biomass, and yield of tomato showed a sharp decline. FSI, OA, and lycopene decreased as the residual mulch film amount increased, whereas SSC, VC, and NC showed an increase trend. With the increase in residual mulch film amount, the F and membership function values (Xμ) all showed a declining trend in comparison to the CK. Therefore, residual mulch film can aggravate the negative effects on the comprehensive fruit quality of tomato.

X. Zou : Y. Guan University of Chinese Academy of Sciences, Beijing 100049, China

Keywords Residual mulch film . Tomato growth . Root characteristics . Fruit quality

Abstract The quantities of residual mulch film in the soil will further increase with the wide application of agricultural plastic mulch film, and the pollution of residual mulch film, which is a continuous pollutant and the one that is difficult to degrade, is a major limiting factor for the sustainable development of agriculture in China. Residual mulch film in the soil inevitably affects soil hydrodynamic parameters, destroys the homogeneity of the soil texture, seriously impedes the movement of soil water and solutes, and thus greatly influences crop growth and fruit quality. To unravel the effects of residual mulch film on tomato growth and fruit quality, pot experiments in the greenhouse were carried out in 2015 and 2016 in Northwest China. Six levels of residual mulch film were applied: 0 kg ha−1

W. Niu : J. Liu : B. Liang : L. Guo College of Water Resources and Architectural Engineering, Northwest A&F University, Yangling, Xianyang, Shaanxi 712100, China Y. Li Institute of Soil and Water Conservation, Northwest A&F University, Yangling, Xianyang, Shaanxi 712100, China

1 Introduction In modern agriculture, mulch film (Mf) has been utilized in vegetable cropping since the 1950s (Lamont 2005; Lament 1993) and is the preferred intensive farming method for

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most vegetable crops (Fisher 1995; Ramakrishna et al. 2006; Zhang et al. 2013). Mf is used in agriculture to weaken soil degradation and increase surface soil temperature (Wan and El-Swaify 1999; Anikwe et al. 2007; Zhang et al. 2015), which leads to (1) increased crop yield and fruit quality (Shiukhy et al. 2015; Liu et al. 2016), (2) the acceleration of crop germination and growth (Kasirajan and Ngouajio 2012; Li et al. 2013), (3) a decrease in crop farming cost and the inhibition of increasing salt (Chen and Feng 2013; Chen et al. 2015), and (4) the improvement of soil water-fertilizer, gas, and thermal conditions (Tripathi and Katiyar 1984; Marinari et al. 2015). However, Mf has a macromolecular structure with an aromatic ring, which results in difficult degradation and a long residual time (Chiellini et al. 2001; Immirzi et al. 2009). In the northwest of China, plastic mulch film is an effective method for increasing crop productivity, and a total area of more than 2000 ha is covered with 50 varieties of crops planted in film covering (Li et al. 1999; Liu et al. 2014). Moreover, the recovery ratio of Mf used in agriculture is low, and some of it is demolished by humans or nature and remains in the fields as debris (Chen et al. 2013). Zhao et al. (2014) reported that the amount of residual mulch film was up to 317.1 kg ha−1 in Chinese vegetable farming fields. Xie et al. (2007) recently showed that the residual coefficient of peanut and cotton were 9.7 and 14.3% in North China, and the amount of residual mulch film was increasing as the film mulch period was prolonged. These studies suggest that a relatively large amount of residual mulch film may remain in soil and persist in farming fields for long periods (up to 200–400 years) (Liu et al. 2014). The residual mulch film mixed in soil will not only influence soil water movement but also affect the agricultural environment; furthermore, it will endanger food safety and human health (Halley et al. 2001; Tocchetto et al. 2001). The accumulation of residual mulch film, known as Bwhite pollution,^ is a serious concern that can create barren conditions in cultivated land. After interfusing into the soil, the wetting front moving distance and wetting soil area are decreased with increasing residual mulch film. Moreover, the uncertainty of infiltration velocity increases gradually (Niu et al. 2016). Bescansa et al. (2006) have observed an increase in the area where residual mulch film blocks soil pores using computed tomography (CT) scanning technology. Consequently, residual mulch film will disrupt farm land soil permeability and soil quality and can destroy the farmland soil-plant-atmosphere continuum (SPAC) balance. Tomato (Lycopersicon esculentum Mill.) is among the most popular field crops cultivated in

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China and plays an important role in the agricultural economy and the human diet (Li et al. 2016a; Riga 2015). The Food and Agriculture Organization (FAO) estimated 4.7 million cultivated hectare of tomato worldwide in 2012, yielding 161 million metric tons, and this was led by China, who produced 29.8% of this total (Max et al. 2009). Tomato is known as an important source of antioxidants such as vitamin C and lycopene in the human diet, which have been linked with the reduced risk of cancer, prostate, and heart diseases (Toor et al. 2006; Li et al. 2016b). It is one of the most important vegetable crops in terms of production and acreage in both open-field and greenhouse productions in Northwest China. With the development of the social economy and the improvement of people’s living conditions in China, people have placed higher demands on tomato fruit yield (EI-Bassiony et al. 2014). Moreover, consumers’ expectations on tomato quality, organoleptic quality, nutritious value, and taste are getting higher. Therefore, fruit quality should be considered in addition to yield. Tomato fruit quality is a system of the interactions among the different quality attributes mentioned above. The fruit quality is generally classified as organoleptic quality (size, shape, and color), taste (organic acid, sugar acid ratio, soluble sugar content, and total soluble solids content), and nutritional quality (lycopene and vitamin C) (Ripoll et al. 2014). In the literature, there are many studies concerning the effects of other soil mixtures on crop yield and quality. Xie et al. (2010) studied the effects of gravel-sand mulch thickness on watermelon yield and showed that 7–8 cm may be the most appropriate strategy for maintaining high watermelon yield. Xu et al. (2015) found that biochar mixed in the soil improved peanut kernel quality but did not increase peanut yield. Rai et al. (2012) measured the influence of ash-filtered mud on radish yield and fruit quality, and the results showed that the application of this mixture could promote radish growth and metabolism. In addition, the use of adequate amounts of the mixture did not cause obvious metal pollution in radish. However, studies on crop growth and fruit quality of vegetables subjected to residual mulch film are inadequate. In addition, no study has been reported on the sensitivity of tomato plants to residual mulch film at different growth stages, and how this may impact growth and fruit quality.


However, such information would have a practical value under different residual mulch film levels. In the present study, 2-year residual mulch film pollution experiments on greenhouse tomato in Northwest China were conducted. The aim of this study was to identify the adverse effects of residual mulch film on tomato growth and fruit quality. The specific objectives, using the data acquired from the pot experiments, are as follows: (1) to evaluate the response of tomato plant height, stem diameter, dry biomass, yield, and root morphological characteristics at different growth stages to different residual mulch film amounts; (2) to determine the effects of residual mulch film on fruit quality; and (3) to improve our knowledge of residual mulch film pollution and offer a technical basis for the abatement of residual mulch film pollution.

2 Materials and Methods 2.1 Experimental Site and Plant Materials The pot experiments were carried out in the automated glass-covered greenhouse at the Center for Strategic Studies of Agricultural Development in Arid & Semiarid Areas of China, located in Yangling, Shaanxi Province of Northwest China (latitude 34° 20′ N, longitude 108° 04′ E), between October 2015 and March 2016. The site has a warm temperate semi-arid continental monsoon climate with a multi-year mean precipitation of 572.5 mm. There is a significant inter-annual variation in the amount of precipitation, with the amount of precipitation in the flood period (April–September) accounting for 56–62% of the annual amount of precipitation. The mean air temperature is 10.3 °C. There is enough sunlight in this area; the mean sunshine duration is 2163.8 h, the average annual amount of solar radiation is 475.8 kJ m−2, and the frost-free season is 210 days. The greenhouse structure is 8 m long, 3.5 m wide, and 3.8 m in height with an east-west orientation. The greenhouse has no calescence control system. To maintain the interior temperature at night during the winter, straw mats were laid on the surface of the greenhouse glass panes; at the same time, light-compensating lamps were used to enhance night illumination and elevate temperature. The interior ventilation during the tomato growth period was controlled by a high-powered draught fan on the east of the greenhouse. Air temperature and relative humidity during the experimental

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period were measured using an automatic weather station (WS-STD1, Delta-T, UK) at the experimental greenhouse. The air temperature fluctuated between 12.33 and 30.33 °C. Greenhouse humidity fluctuated from 23 to 52.67%. The mean air temperature was 22.45 °C, and the mean humidity was 37.83%. The tested cultivar of tomato (L. esculentum Mill.) was Hai Di (Yang Ling Abundant Agricultural Development Co., Ltd., Northwest A&F University, China), and the preceding crop cultivated in the greenhouse was muskmelon (Cucumis melo L.). The tomato was chosen for this experiment for two primary reasons: (1) it is a widely planted vegetable in the film-covering planting area throughout the world, and (2) it is sensitive to soil moisture and nutrients, which could determine if residual mulch film would have adverse effects on tomato growth and fruit quality. The experimental soil in the pots was a Loessial soil that developed from loess parent material, which was excavated at a 20 cm depth from farmland surface. The soil without obvious profile layers had a uniform property. The soil texture was a sandy clay loam (62.1% sand, 22.6% silt, 15.3% clay) according to the international soil classification system. The pH was 7.23, electrical conductivity (EC) was 0.24 dS m−1 (Hanna HI5522 multi-parameter water quality analyzer), initial soil moisture was 20.16% (oven-drying method), dry bulk density was 1.26 g cm−1 (cutting-ring method), Kjeldahl total N was 0.88 g kg−1, alkali-hydrolyzale N was 32.51 mg kg−1, total K was 22.3 g kg−1, Olsen extractable P was 12.87 mg kg−1, and organic C was 7.47 g kg−1. The thickness of the tested mulch film (Shaanxi Ruifeng Agricultural Development Co., Ltd., China) was 0.008 mm to simulate the frangibility of residual mulch film in the fields. 2.2 Experimental Design and Crop Management In this study, six residual mulch film levels were applied, which were a control soil without a residual mulch film (CK) and five levels of residual mulch film (T1 to T5): 80, 160, 320, 640, and 1280 kg ha−1, respectively. There were three replications per treatment, resulting in a total of 18 pots arranged in a complete random design. The plastic pots had an upper inner diameter of 29 cm, a bottom inner diameter of 22 cm, and a height of 32 cm and were filled with soil to 30 cm to achieve a dry bulk density of 1.26 g cm−3, thus leaving the upper 2 cm for irrigation water. The total volume (V) of soil in the pots

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was 5.3 L calculated as the volume of a truncated right circular cone with a large diameter of 25.2 cm (R), a small diameter of 22 cm (r), and a height of 30 cm (h) using the formula V = [πh(R2 + r2 + Rr)] / 3 (Li et al. 2016c). The weight of soil in each pot was 6.62 kg. Additionally, according to Changrong et al. (2014) who found that residual mulch film mainly remained in the first 30 cm of the soil layer, the mass distribution ratios selected for 0–10, 10–20, and 20–30 cm were 70, 20, and 10%, respectively. To ensure a uniform distribution of residual mulch film mixed in the soil, tested mulch film was loaded in an YJB4 mobile mixer at a 100 r min−1 revolving speed, and the accumulated stir time was 30 min. Next, the treated soil layers were placed in plastic pots, and each 10 cm was compacted by a wooden mallet to obtain the original bulk density and homogeneous soil profile. The packed soil in the pots was allowed to equilibrate for 24 h to obtain a uniform distribution of soil and residual mulch film. To maintain zero evaporation, the soil was covered with a polyethylene sheet. Each treatment was prepared in triplicate. The growth period of tomato plants was 160 days, and when the third true leaf of the seedlings had expanded, seedlings of similar height were selected for transport to the pots on October 2, 2015, and were harvested on March 10, 2016. The entire growth season was divided into three stages: seedling stage (transplant to first fruit set), blossoming and fruit set stage (first fruit set to first harvest), and fruit maturation stage (first harvest to uprooting crops after all fruits were harvested). Before transplant, each plastic pot was irrigated with 3 L of water to make sure the soil was saturated to moisture capacity. Next, 120 t ha−1 of decomposed organic manure (pig and sheep manure), 1500 kg ha−1 of diammonium phosphate (18% N and 46% P2O5), and 400 kg ha−1 of compound fertilizer (18% N, 15% P2O5, and 12% K2O) were broadcast uniformly as the basal fertilizer in the soil. The tomato plants were irrigated based on their water demand with distilled water during the growing period, and the irrigation scheduling and water quantities were equal for all treatments. To minimize the effects of different irrigation quantities on tomato growth, irrigation quantity and irrigation manner for all treatments remained the same. A total of 1.5 L of water was applied per pot with the help of a graduated beaker along one side of the pot every 4 days (Tesfaye et al. 2012). The irrigation did not have any contact with tomato leaves at the time of irrigation. In the meantime,


irrigation water was mixed with 4 g water-soluble fertilizer that contained 21 N-2.2 P-16.6 K (Yang Ling Ling Feng Agricultural Development Co., Ltd., China). Tomato plants were propped up by nylon cord guides to avoid lodging. During the experimental period, other management practices such as fertilization, pest control, pollination, and pruning were the same for all treatments as the local normal levels.

2.3 Measurement Index and Methods 2.3.1 Tomato Yield and Growth Indices of Tomato Plants When the tomato fruits had ripened, the fruit number of each treatment was recorded. The ripened fruits were picked manually from each tomato plant. These were washed with running water and dried thoroughly with absorbent paper before monitoring fresh fruit weight. The procedure was repeated after each picking (total three pickings). Tomato yield was the sum of the fruit mass from the first cluster to the third cluster. To observe the dynamic change of tomato plant growth, the stem diameter of tomatoes was measured at the thickest place of the main stem 10 cm above the ground every week (total weeks = 24 weeks) by a digital Vernier caliper, and the plant height was determined from the soil line to the tip of the tomato plant main stem every week (total weeks = 24 weeks) using a steel ruler. To analyze the difference of tomato growth rates at the different growth stages, the relative growth rate of tomato was introduced into this study (Nagel et al. 1994). The relative growth rate of plant height and stem diameter was then calculated according to Eqs. 1 and 2. V h ¼ ðlnL2 −lnL1 Þ=ðT 2 −T 1 Þ

ð1Þ

V d ¼ ðlnD2 −lnD1 Þ=ðT 2 −T 1 Þ

ð2Þ

where Vh is the relative growth rate of plant height, L1 is the initial plant height (cm), L2 is the measured plant height per week (cm), T1 and T2 are the sampling time (days), Vd is the relative growth rate of stem diameter, D1 is the initial stem diameter (cm), and D2 is the measured plant height per week (cm).


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2.3.2 Root Characteristics of Tomato

where FSI is the fruit shape index, Dy is the vertical diameter (mm), and Dx is the horizontal diameter (mm).

After the aerial parts of the tomatoes were sampled, adequate water was added to pots for scattering soil. Soils were poured into a 2.5-mm sieve to wash with running water, and washed roots were taken to the lab. Three replicates per treatment were analyzed in the same day. The washed tomato roots were scanned by Epson Perfection V600 Photo (Epson American, Inc., Long Beach, CA, USA) at 400 dpi with a pixel size of 0.063 mm, and these scanned images were analyzed using the WinRhizo 2007 (Regent Instruments, Inc., Quebec, Canada) software by following Nakano (2007) to determine the root volume, surface area, length, and diameter. 2.3.3 Dry Biomass Measurements After uprooting, samples were collected from each pot at the fruit maturation stage to measure the content of dry matter. All samples were cut into leaves, stems, and roots. Every part of the tomato plant was put into a 105 °C oven for 1 h and then dried at 75 °C until the parts reached a constant weight; the dry biomass of each part was weighed on an electronic scale.

Taste Quality Three selected tested fruits were added to a blender (MJ-BL25B3 Midea squeezer) to squeeze. The soluble sugar concentration (SSC) was measured using the anthrone sulphuric acid colorimetric method (Leyva et al. 2008). Organic acid (OA) was determined by diluting an aliquot of the blended fruit and titrating the aliquot against a 0.1 mol L−1 NaOH solution using phenolphthalein as an indicator (Abbott 1999). Nutrient Quality Vitamin C (VC) content was determined by the molybdenum blue colorimetry method. This method is based on the reaction of ascorbic acid (VC) with ammonium phosphor-molybdate in the presence of SO42− and PO43−, generating blue molybdenum. This blue molybdenum has a maximum light absorption at a wavelength of 760 nm (Sablani et al. 2006). This vitamin C assay method is accurate, repeatable, and insensitive to the interference in the presence of common reducing sugars (Knutsen et al. 2001). Lycopene was extracted with 2% dichloromethane and petroleum as solvents to enhance the solubility of lycopene, and absorption at 502 nm was subsequently measured (Hyman et al. 2004).

2.3.4 Tomato Fruit Quality Determination Three tomato fruits per plant in the second cluster that were picked for measuring fruit quality were of similar size and ripeness with no surface defects. These fruits were measured for a vertical diameter and a horizontal diameter using a Vernier caliper, the flesh samples (skin and seeds removed) were juiced with a domestic juicer, and the juice was decanted and subjected to a series of tests for the following quality parameters. Tomato fruit quality parameters adopted the average value of three plants for every treatment. The organoleptic quality, nutrient quality, and safety quality were measured using the same methods for all treatments.

Safety Quality Nitrate content (NC) was measured with the UV spectrophotometer method (Li et al. 2015) and was calculated by following equation:

NC ¼

CV W

ð4Þ

where NC is the nitrate content (mg kg−1), C is the nitrate concentration calculated by a regression method (mg L−1), V is the extracting solution volume (ml), and W is the fresh tomato juice weight (g). 2.3.5 Fruits Quality Evaluation Methods

Organoleptic Quality The fruit shape index (FSI) was introduced to describe the organoleptic quality of tomato fruit (Gonzalo et al. 2009). The fruit diameters in two vertical directions were measured by a digital display Vernier caliper. FSI was calculated as follows: FSI ¼

Dy Dx

ð3Þ

In this study, the principal component analysis (PCA) method and the membership function (MF) method were applied to evaluate the effects of residual mulch film on the comprehensive quality of tomato fruit. PCA calculations were performed according to the specific calculation steps described by Wang et al. (2015). The Xμ value can be calculated as follows:

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ðX −X min Þ Xμ ¼ ðX max −X min Þ

ð5Þ

where Xμ is the membership function value of a certain fruit quality parameter, X is the measured value of a certain fruit quality parameter at fruit maturation stage, Xmax is the maximum value among all treatments for a certain fruit quality parameter, and Xmin is the minimum value among all treatments for a certain fruit quality parameter. 2.4 Data Statistics The data shown in the tables are in the form of mean values ± standard deviation error. The PCA was calculated using the SPSS 19.0 software (IBM Crop., Armonk, New York, NY, USA). The MF method was implemented by Microsoft Office Excel 2010. Mean values of the different treatments were compared using Fisher’s least significant difference (LSD) multiple comparison test (P < 0.05) to determine the significant differences using the SPSS 19.0 software. OriginPro 9.0 (OriginLab Corporation, One Roundhouse Plaza, Suite 303, Northampton, MA 01060, USA) was used to generate the figures.

3 Results 3.1 Effects of Residual Mulch Film on Tomato Growth Plant height, stem diameter, dry biomass, and yield of tomato were significantly affected by residual mulch film and varied for different treatments (Fig. 1). Plant height and stem diameter of all treatments were measured by a steel ruler and an electronic digital indicator every week (total weeks = 24 weeks). Plant height and stem diameter decreased with an increase in residual mulch film amount (Fig. 1). In the treatments containing different amounts of residual mulch film, compared to the CK treatment, the decreasing rates of plant height were 1.16, 5.07, 9.14, 15.13, and 23.86% for T1 to T5 at the fruit maturation stage, respectively. Fisher’s LSD multiple comparison test showed that residual mulch film amounts were not significant for plant height and stem diameter at CK and T1. Conversely, the other treatments showed that a residual mulch film amount more than 80 kg ha−1 had a significant declining trend

(P < 0.05). The relative growth rates of plant height and stem diameter for all treatments are shown in Table 1. The relative growth rates of plant height and stem diameter in both residual mulch film treatments were inferior to CK. At the seedling stage of tomato, the relative growth rates of plant height and stem diameter showed a decreasing trend in both residual mulch film treatments. However, the effects of residual mulch film on plant height and stem diameter had no evident regular change (Table 1). The results showed that the stem diameters of tomatoes planted in soils mixed with residual mulch film were less than CK (Fig. 1b), which indicates that the stem diameter of tomato was sensitive to residual mulch film. It was clear that at the T5 treatment, the stem diameters of tomatoes were lower than those of the other treatments. It can also be observed that residual mulch film had no significant impact on stem diameter in the CK and T1 treatments. The declining rate of stem diameter in both residual mulch film treatments ranged from 3.2 to 39.99% at the fruit maturation stage. There were all good linear relationships among plant height (Hp), stem diameter (Ds), and residual mulch film amount (Mr). The linear relationships could be expressed as follows: H p = 118.671 − 0.022M r (R 2 = 0.967) and Ds = 14.634 − 0.005Mr (R2 = 0.733). Figure 1c summarizes the response of dry biomass and yield of tomato to residual mulch film. Dry biomass and yield of tomato showed all decreased trends with the increase of residual mulch film (Fig. 1c). Residual mulch film had a significant effect on dry biomass from T2 to T5, whereas it was not significant at CK and T1 treatments (P > 0.05). There was a significant difference on the yield of tomato from the T2 to T5 treatments (Fig. 1c) (P < 0.05). Yield of tomato was the lowest in treatment T5, whereas yield of tomato reached its maximum in the CK treatment. Compared to CK, the declining rates of dry biomass were 3.75, 17.41, 32.97, 44.88, and 55.10% for T1 to T5, respectively. Moreover, the maximum and minimum decreased rates of yield were 23.35 and 0.79%, respectively. In addition, there was an exponential relationship between dry biomass (DB) and residual mulch film amount (Mr), and the expression is as follows: DB ¼ 126:16 e−0:001M r (R2 = 0.904). However, the yield of tomato (Y) had a linear declining trend with increasing residual mulch film amount. The linear relationship could be expressed as follows: Y = 2413.98 − 0.446Mr (R2 = 0.978).


a

140

120

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b

CK T1 T2 T3 T4 T5

CK T1 T2 T3 T4 T5

18

16

14

Stem diameter(mm)

Plant height(cm)

100

80

60

12

10

8

40 6

20

Seedling stage

2 Blossoming and

Seedling stage

Fruit maturation stage

Blossoming and

fruit -set stage

Growth period of tomato

c

Growth period of tomato

Dry biomass 550

a

a

2600

Yield

b

2400

c 500

d

a a

Dry biomass(g)


fruit-set stage

2200

e

2000

b

450

1800

c

1600

d

400

1400

e

Yield per plant(g)

0

4

1200

350

1000 800

300 CK

T1

T2

T3

T4

T5

Amount of residual mulching film (kg ha-1)

Fig. 1 Plant height (a), stem diameter (b), and dry biomass and yield per plant (c) (n = 3) with different treatments. The plant height was determined from the soil line to the tip of the tomato plant main stem every week (total weeks = 24 weeks) using a steel ruler, and the stem diameter of the tomato plant (n = 3) was measured at the thickest place of main stem 10 cm above the ground every week (total weeks = 24 weeks) by a digital Vernier caliper. The tomato yield was the sum of fruit mass from the first cluster to the third cluster weighed with an electronic scale. Dry biomass is the sum of the

leaves, stem, and roots. Every part of the tomato plant was put into a 105 °C oven for 1 h and dried continuously at 75 °C until they reached a constant weight; the dry biomass of every part was weighed by an electronic scale. Six residual mulch film levels were applied, which were a control soil without residual mulch film (CK) and five levels of residual mulch film (T1 to T5): 80, 160, 320, 640, and 1280 kg ha−1. Data are the means of three replicates, with standard deviations shown by error bars. Different letters above the error bars mean significant differences (P < 0.05) according to the LSD test in c

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Table 1 The relative growth rates of plant height of potted single tomato plants (n = 3) were measured from the soil line to the tip of the tomato plant main stem every week (total weeks = 24 weeks) by a steel ruler, and the stem diameter (n = 3) was measured at the thickest place of the main stem 10 cm above the ground every week (total weeks = 24 weeks) using a digital Vernier caliper for all treatments in the experiment. Six residual mulch film levels were applied, which were a control soil without residual mulch film (CK) and five levels of residual mulch film (T1 to T5): 80,

160, 320, 640, and 1280 kg ha−1. The relative growth rates of plant height or stem diameter were the ratios of plan height (cm) or stem diameter (mm) to growth time (weeks). The seedling stage (transplant to first fruit set) was from October 2, 2015, to December 11, 2015. The blossoming and fruit set stage (first fruit set to first harvest) was from December 11, 2015, to January 29, 2016. In addition, the fruit maturation stage (first harvest to uprooting crops after all fruits were harvested) was from January 29, 2016, to March 10, 2016

Treatments Relative growth rate of plant height (cm week−1) Seedling stage

Blossoming and fruit set stage

Relative growth rate of stem diameter (mm week−1)


Seedling stage

Blossoming and fruit set stage

0.0728 ± 0.0012a 0.0616 ± 0.00153abc


CK

0.1478 ± 0.0015a 0.1078 ± 0.0005a

0.0840 ± 0.0015a

0.0812 ± 0.0009a

T1

0.1443 ± 0.0022a 0.1037 ± 0.0008b

0.0462 ± 0.0008ad 0.0708 ± 0.0010a 0.0560 ± 0.0010b

0.0680 ± 0.0010b

T2

0.1373 ± 0.0019b 0.1051 ± 0.0012b

0.0477 ± 0.0005d

0.0658 ± 0.0016b 0.0574 ± 0.0010c

0.0792 ± 0.0006c

T3

0.1358 ± 0.0005b 0.0764 ± 0.0016c

0.0456 ± 0.0002e

0.0546 ± 0.0021c 0.0574 ± 0.008c

0.0512 ± 0.0008d

T4

0.1254 ± 0.0016c 0.0750 ± 0.0010c

0.0778 ± 0.0009b

0.0498 ± 0.0019d 0.0504 ± 0.0005d

0.0518 ± 0.0009d

T5

0.1162 ± 0.0010d 0.0679 ± 0.0006d

0.0735 ± 0.00010c 0.0434 ± 0.0008e 0.0462 ± 0.0012e

0.0560 ± 0.0006e

The results represent the means of three tomato plants ± standard errors (SD). Values followed by the same letters in the same column and within each experiment are not significantly (P ≤ 0.05) different according to the LSD test

The membership function (MF) method is one common method used in the evaluation of soil pollution hazards to analyze the effects of residual mulch film on tomato growth and to reduce the overlap among the information reflected by the indexes, such as plant height, stem diameter, and the dry biomass of plant tissues. Leaves are one of the main plant components involved in plant biomass production through photosynthesis, which is a process for the production of fruit. Usually, photosynthesis efficiency of tomato has a direct relationship with the amount of the produced dry biomass. Therefore, the dry biomasses of leaves are an indirect index for the evaluation of photosynthesis intensity. The root/shoot ratio could reflect a lodging-resistant capability of tomato plants and the ability of tomato survival in drought-prone and nutrient-poor environments. In consequence, a large root/shoot ratio could be expected to be advantageous in severe environments. The dry biomass of the leaves decreased as the residual mulch film amount increased (Table 2). Among all treatments, the dry biomass of the leaves was the highest in the CK treatment. In contrast, the dry biomass of the leaves was the lowest with 1280 kg ha−1 of residual mulch film. The dry biomass of the leaves had no significant difference when the amount of residual mulch film ranged from 0 to 160 kg ha−1 (P > 0.05). Additionally, at the T3 and T4 treatments, residual mulch film did not significantly affect the dry biomass of the leaves (P > 0.05). However, residual

mulch film can significantly affect the dry biomass of the leaves when the amount of residual mulch film reaches 1280 kg ha−1 (P < 0.05). The root/shoot ratio was the ratio of the dry biomass of the plant aerial portions and the roots and may be used to indicate (1) a favorable environment for root growth and nutrient uptake, (2) the efficiency of the root system for supporting shoot growth and yield, and (3) a heritable trait for increasing yields. According to the calculated results, root/shoot ratios of the different treatments were 0.061, 0.062, 0.058, 0.051, 0.049, and 0.048 for T1 to T5, respectively. It was clear that root/shoot ratios decreased with the increase of residual mulch film amount. In addition, residual mulch film had a significant effect on the root/shoot ratios from T2 to T5. Moreover, Table 2 also shows that the sum of the Xμ values decreased with the increase in residual mulch film. Thus, increasing residual mulch film amounts has an adverse effect on the lodgingresistant capability of tomato plants. 3.2 Effect of Residual Mulch Film on Tomato Root Morphological Characteristics The effects of residual mulch film on root length differed between all treatments; the greatest measured values were with CK and decreased with increasing residual mulch film (Table 3). Residual mulch film had a significant effect on root length (P < 0.05). It appears


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Table 2 Effects of different residual mulch film levels on plant height, stem diameter, dry biomass of leaf, dry biomass of stem, dry biomass of root, and the sum of the Xμ value. The plant height was determined from the soil line to the tip of the tomato plant main stem every week (total weeks = 24 weeks) by a steel ruler, and the stem diameter of the tomato plant (n = 3) was measured at the thickest place of main stem 10 cm above the ground every week (total weeks = 24 weeks) by a digital Vernier caliper. To measure the dry biomass of leaf, stem, and root, tomato plant

samples were cut into leaf, stem, and root. Every part of the tomato plant was put into a 105 °C oven for 1 h and dried continuously at 75 °C until they reached a constant weight, and then the dry biomass of every part was weighed by an electronic scale. The sum of the Xμ value is the sum of the membership function value of a certain fruit quality parameter (plant height, stem diameter, dry biomass of leaf, dry biomass of stem, dry biomass of root) under different treatments

Treatments Plant height (cm)

Stem diameter (mm)

Dry biomass of leaf (g)

Dry biomass of stem (g)

Dry biomass of root (g)

Sum of the Xμ value

CK

120.3 ± 3.9a

15.93 ± 0.34a

221.4 ± 8.6a

225.5 ± 5.4a

27.3 ± 0.7a

5.00

T1

118.9 ± 2.4ab

15.42 ± 0.57a

217.1 ± 4.7a

218.6 ± 1.6a

26.9 ± 0.6a

4.62

T2

114.2 ± 1.2bc

13.51 ± 0.46b

206.6 ± 3.6a

210.5 ± 3.7b

24.1 ± 0.5b

3.56

T3

109.3 ± 2.4c

11.27 ± 0.70c

194.3 ± 5.8b

202.1 ± 2.8c

20.1 ± 1.0c

2.29

T4

102.1 ± 2.1d

10.03 ± 0.15 cd

187.1 ± 4.7b

185.3 ± 3.3d

18.2 ± 0.6d

1.21

T5

91.6 ± 1.7e

9.56 ± 0.96d

164.2 ± 3.2c

175.8 ± 3.7e

16.2 ± 0.8e

0

The results represent the means of three tomato plants ± standard errors. Different letters indicate significant differences in the same column according to the LSD test at 0.05 level

that the residual mulch film had a negative effect on root length. Compared to CK, declining rates of root length were 7.20, 14.10, 20.68, 29.02, and 33.28% for T1 to T5, respectively. It can be seen that a declining rate of root length increased with an increasing amount of residual mulch film. Root volumes for different treatments are listed in Table 3. The results showed that the tomato root volume exhibited a decreasing trend with a decreasing residual mulch film. Among all six treatments, the root volume was highest with no residual mulch film (Table 3). When the amount of residual mulch film was more than 160 kg ha−1, the residual mulch film had an extremely significant impact on the root volume. However, the residual mulch film did not significantly affect the root

volume among CK, T1, and T2, where the maximum declining rate was 5.81%. The data in Table 3 show that root surface area was significantly reduced (P < 0.05). There is no evident regularity of root diameter, and root diameter had a decreasing trend with increasing residual mulch film. Root diameter reduced with increasing residual mulch film, and the declining rate ranged from 8.99 to 46.07%. Under the different amounts of residual mulch film, the root surface area showed a significantly decreasing trend (P < 0.05). The order of root surface area was CK > T1 > T2 > T3 > T4 > T5 when the amount of residual mulch film ranged from 0 to 1280 kg ha−1. The reduce rates of root surface area were 2.64, 10.74, 27.10, 35.44, and 41.93% for T1 to T5, respectively.

Table 3 Effects of residual plastic film on tomato root morphological characteristics. The washed tomato roots (n = 3) were scanned by Epson Perfection V600 Photo at 400 dpi with a pixel

size of 0.063 mm, and these scanned images were analyzed using the WinRhizo 2007 software to determine root volume, surface area, length, and diameter

Treatments

Root length (cm)

Root volume (cm3)

Root diameter (mm)

Root surface area (cm2)

CK

3524.4 ± 10.6a

84.4 ± 1.6a

0.89 ± 0.03a

982.3 ± 7.1a

T1

3270.5 ± 51.6b

82.3 ± 1.6a

0.81 ± 0.04ab

956.4 ± 5.3b

T2

3027.6 ± 8.7c

79.5 ± 1.4a

0.71 ± 0.04b

876.8 ± 6.8c

T3

2795.5 ± 8.2d

70.3 ± 1.8b

0.60 ± 0.04c

716.1 ± 7.6d

T4

2501.6 ± 6.4e

58.6 ± 1.9c

0.51 ± 0.04 cd

634.2 ± 3.6e

T5

2351.4 ± 14.3f

51.1 ± 1.5d

0.48 ± 0.03d

570.4 ± 6.8f

The results represent the means of three tomato plants ± standard errors. Values followed by the same letters in the same column and within each experiment are not significantly (P ≤ 0.05) different according to the LSD test

71


Page 10 of 18

vitamin C (VC), lycopene, and nitrate content (NC) analysis. Figure 2 shows SSC, FSI, OA, VC, NC, and lycopene under different treatments. The SSC mainly refer to the soluble sugars, including monosaccharide, disaccharide, and soluble polysaccharide. The results showed that the SSC was significantly increased with increasing residual mulch film (P < 0.05), whereas the

3.3 Effect of Residual Mulch Film on Tomato Fruit Quality Fruit qualities are considered the most valuable characteristics for tomato and were evaluated at multiple levels, mainly including soluble sugar concentration (SSC), fruit shape index (FSI), organic acid (OA),

a a

a

0.45

1.0

a

ab

bc

c f

0.8

0.6

d 50

0.4

c b a

0.2

c

0.35

FSI OA or VC(mg g-1)

SSC(mg g-1)

e

e

d

0.40

60

40

OA VC

b

FSI

SSC 70

a

a

0.30

b

0.25 0.20 0.15 0.10

a

a

b

bc

c

d

0.05

0.0

30 CK

T1

T2

T3

T4

0.00 CK

T5

T1

T2

T3

T4

T5 -1

-1

Amount of residual mulching film (kg ha )

Amount of residual mulching film (kg ha )

c

Lycopene NC

8.0

a

120

a a

Lycopene(mg g-1)

7.0

100

b ab d

c b

80

c 6.5

e

e

60

c

6.0

NC(mg kg-1)

7.5

40

5.5

20 CK

T1

T2

T3

T4

T5

Amount of residual mulching film (kg ha-1) Fig. 2 The soluble sugar concentration (SSC), fruit shape index (FSI), organic acid (OA), vitamin C (VC) content, lycopene, and nitrate content (NC) under different treatments. The FSI is the ratio of the vertical diameter (mm) to the horizontal diameter (mm). The SSC was measured with the anthrone sulfuric acid colorimetric method. OA was determined by diluting an aliquot of the blended fruit and titrating it against 0.1 mol L−1 NaOH solution using phenolphthalein as an indicator. VC was determined by the

molybdenum blue colorimetry method. Lycopene was extracted with 2% dichloromethane and petroleum as solvents to enhance the solubility of lycopene, and the absorption at 502 nm was subsequently measured. NC was measured with the UV spectrophotometer method. The data are the means of three replicates, with standard deviations shown by error bars. Different letters above the error bars mean significant differences (P < 0.05) according to the LSD test


SSC reached the maximum when the amount of residual mulch film was 1280 kg ha−1. The FSI is an important aspect of the exterior quality of a fruit as well as a commercial indicator that influences the economic value of tomato fruit. Cultivars with large FSI have a greater potential for fresh market. Like most other important traits, the FSI of tomato is not only genetically controlled but also influenced by environmental factors. Figure 2a shows that FSI decreased as the residual mulch film amount increased, and FSI was changed between 0.8 and 1.0. Compared to CK, the decreasing rates of FSI were 0.33, 1.20, 2.74, 4.86, and 6.48% for T1 to T5, respectively (Fig. 2a). The OA is regarded as one of the most important factors in determining fruit favors. As seen in Fig. 2b, the OA of the T5 treatment was significantly lower than that of the other treatments. The reduced rate of the OA changed from 2.82 to 28.17%, which presented an increasing trend. As we know, tomato is well known for its rich VC content. The effects of residual mulch film on VC content are shown in Fig. 2b. VC content was highest in the T5 treatment. As seen in Fig. 2b, VC content increased with increasing residual mulch film from CK to T5 at 0.281 to 0.394 mg g−1. Therefore, residual mulch film had a positive effect on the VC content of fresh tomato fruits. Lycopene is an important intermediate in the biosynthesis of many carotenoids, including beta carotene, which is responsible for yellow, orange, or red pigmentation; photosynthesis; and photoprotection (Dumas et al. 2003; Garcia and Barrett 2006). Figure 2c shows that the residual mulch film had a negative effect on the lycopene content of tomato. In general, the lycopene content showed very small changes between CK and T1 (P > 0.05), but relatively large changes when the residual mulch film amount was more than 80 kg ha−1. In particular, the residual mulch film had no a significant decreasing trend among CK, T1, and T2 (P > 0.05), whereas the lycopene content of the T4 and T5 treatments was significantly lower than that of the other treatments (P < 0.05). However, nitrate is an essential plant nutrient found in soil that is taken in by all plants and used as a primary nitrogen source. Exceeding the nitrate standard can cause the rare blue baby syndrome (Prasad and Chetty 2011; Gómez-López and del Amor 2013). According to Chinese food quality supervision standards, NCs in tomato fruits below the standard content of 600 mg kg−1 would be safe (Wang and Li 2003; Vasconcellos et al. 2016). Figure 2c also shows the variation trend of NC under different residual mulch

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films. Residual mulch film significantly increased the NC of tomato fruits from T2 to T5 (P < 0.05). However, NC was not significantly increased when the amount of residual mulch film was less than 80 kg ha−1 (P > 0.05) (Fig. 2c). As seen in Fig. 2c, the maximum of NC in this study was 113.7 mg kg−1, which was less than the safe standard content of NC. Therefore, tomato fruits produced in areas where the amount of residual mulch film was less than 1280 kg ha−1 would not pose a threat to human health. 3.4 Comprehensive Analysis of Tomato Quality by PCA and MF Method To evaluate the comprehensive quality, the PCA method and the MF method were introduced to evaluate the effects of residual mulch film on tomato fruit quality. PCA was applied to extract fewer independent principal components, which were linear combinations of FSI (X1), SSC (X2), OA (X3), VC (X4), lycopene (X5), and NC (X6). According to the outcome of the PCA method, the contributions of X2 and X6 accounted for 99.65%. Thus, SSC (X2) and NC (X6) were chosen as main factors. In addition, there was a linear relationship between main factors and evaluation factors, and the expressions of SSC (F1) and NC (F2) were given as follows: F 1 ¼ 0:99X 1 −0:994X 2 þ 0:992X 3 þ 0:992X 4 þ 0:991X 5 −0:998X 6

ð6Þ

F 2 ¼ −0:14X 1 þ 0:054X 2 þ 0:129X 3 þ 0:129X 4 −0:1X 5 −0:036X 6

ð7Þ

The comprehensive index (F) can be expressed by F1 and F2, and F was equivalent to the sum of the main factor and weighting factor product. The expression was shown as Eq. 8 F ¼ 0:9853F 1 þ 0:0112F 2

ð8Þ

The comprehensive result and ranking of tomato fruit quality are represented in Table 4. The larger the number F, the better the tomato fruit quality. The value size of F showed a decreasing trend with increasing residual mulch film. Therefore, residual mulch film has an adverse effect on fruit quality of tomato.

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Page 12 of 18

Table 4 Comprehensive results and ranking of tomato fruit quality. The letter F1 and F2 indicate the soluble sugar concentration (SSC) and nitrate content (NC). The comprehensive index F can be expressed by F1 and F2, and F was equivalent to the sum of main factor and weighting factor product Treatments

F2

F1

F

Ranking

CK

−82.9286

−0.7442

−81.7096

1

T1

−86.8381

−0.6777

−85.5605

2

T2

−107.5230

−1.0231

−105.9430

3

T3

−126.5320

−1.0601

−124.6710

4

T4

−141.9800

−1.0784

−139.8910

5

T5

−166.6260

−1.4764

−164.176

6

The measured values of SSC, FSI, OA, VC, lycopene, NC, the sum of the MF value, and the coefficient of variation are given in Table 5, which clearly shows that the sum of the Xμ value evaluated the good and bad levels of tomato fruit quality, which represents a decreasing trend with increasing residual mulch film. This means that tomato fruit quality would deteriorate if cultivated soils were contaminated by residual mulch film (Table 5). The sum of the Xμ value showed an initial increase and then a decrease followed by an increase, in the order of CK > T1 = T3 > T2 > T4 > T5 (Table 5). Compared to CK treatment, the sum of the Xμ value Table 5 Average effects of residual mulch film on tomato quality ± SD. The FSI is the ratio of the vertical diameter (mm) to the horizontal diameter (mm). The SSC was measured with the anthrone sulfuric acid colorimetric method. OA was determined by diluting an aliquot of the blended fruit and titrating it against 0.1 mol L−1 NaOH solution using phenolphthalein as an indicator. VC was determined by the molybdenum blue colorimetry method. Lycopene was extracted with 2% dichloromethane and petroleum

from T1 to T5 treatments was decreased by 2.75, 4, 2.75, 9.75, and 25%, respectively. The coefficient of variation (CV) is often expressed as a percentage and is defined as the ratio of the standard deviation (SD) to the mean. Table 5 also clearly indicates that the CV of FSI, SSC, OA, VC, lycopene, and NC was varied from 5.31 to 17.61%. This means that residual mulch film has an extremely significant effect on tomato fruit quality. In addition, the CVof SSC reached the minimum at 5.31%, whereas the CVof lycopene was highest in all indexes of tomato fruit quality. Based on the above analysis, it can be concluded that the evaluation results of the PCA and MF methods for a comprehensive analysis of tomato quality were consistent. Moreover, the comprehensive quality of tomato fruit at the CK treatment was the best. Accordingly, residual mulch film has a negative effect on tomato fruit quality.

4 Discussion The residual mulch film produced adverse effects on tomato vegetative growth, dry biomass, fruit yield, and fruit quality. Residual mulch film is generally considered to have negative effects on the physical as solvents to enhance the solubility of lycopene, and absorption at 502 nm was subsequently measured. NC was measured with the UV spectrophotometer method. The sum of the Xμ value is the sum of membership function value of a certain fruit quality parameter (plant height, stem diameter, dry biomass of the leaves, dry biomass of stem, dry biomass of root) under different treatments

OA (mg g−1)

VC (mg g−1)

Lycopene (mg g−1)

NC (mg kg−1)

Sum of Xμ value

Treatments

FSI

SSC (mg g−1)

CK

0.934 ± 0.011a

37.68 ± 0.41a 0.071 ± 0.001a

0.281 ± 0.003a 7.42 ± 0.14a

54.21 ± 1.71e

4.00

T1

0.931 ± 0.923a

39.72 ± 0.44b 0.069 ± 0.008a

0.289 ± 0.008a 7.25 ± 0.16a

55.93 ± 1.93e

3.89

T2

0.923 ± 0.007a

43.81 ± 0.39c 0.063 ± 0.002b

0.313 ± 0.005b 7.03 ± 0.23ab 72.37 ± 1.45d

3.84

T3

0.908 ± 0.007ab 50.70 ± 0.62d 0.061 ± 0.002bc 0.344 ± 0.005c 6.85 ± 0.11b

84.39 ± 3.22c

3.89

T4

0.888 ± 0.005bc 55.91 ± 0.20e 0.058 ± 0.002c

0.376 ± 0.006d 6.37 ± 0.24c

94.22 ± 2.00b

3.61

T5

0.873 ± 0.012c

60.71 ± 0.39f 0.051 ± 0.002d

0.394 ± 0.002e 5.96 ± 0.30c

113.72 ± 2.86a 3.00

5.31

9.70

17.08

Coefficient of variation 7.03 (CV)a (%)

16.79

17.61

Values followed by the same letters within columns are not significantly different at P = 0.05 by the LSD test FSI tomato fruit shape index, SSC soluble sugar concentration, OA organic acid, VC vitamin C, NC nitrate content a

Often expressed as a percentage and is defined as the ratio of the standard deviation (SD) to the mean

–


properties of soil such as water retention and flow, soil structure, and texture. Soil texture and structure significantly impact soil water flow and the transport of soil solutes. Due to its homogeneous particle size and low percentages of soil porosity, both water and nutrients are poorly retained. Azooz et al. (1996) reported that waste materials in farmland could alter the bulk density, aggregate stability, total porosity, organic carbon content, and various soil factors affecting the water storage capacity and water transmission properties of the soil. Thereby, residual mulch film will deteriorate the growing environment of crops and impede crop growth. In addition, residual mulch film could decrease the total porosity of the soil. Moreover, soil infiltration rates decreased with the interfusion of residual mulch film, which resulted from the decreased flow of water through macropores (Ehlers 1975) and increased soil surface sealing due to a complete residue cover (Zuzel et al. 1990). Soils behave differently in relation to the amount of residual mulch film. Benjamin (1993) and Roth et al. (1988) reported that soils under residual mulch film treatments had 30–180% lower saturated hydraulic conductivity than soils without residual mulch film. Furthermore, residual mulch film could destroy the homogeneity of surface soil conditions including the presence of macropores, which determines the amount of water entering the soil (Mukhtar et al. 1985). As a consequence, residual mulch film would have negative effects on crop growth and fruit quality. 4.1 Vegetative Growth, Dry Biomass, and Yield of Tomato Our study showed the plant height, stem diameter, dry biomass, and yield were decreased with increasing residual mulch film amount. Soil pollution with residual mulch film deteriorated soil productivity and structure, especially its hydraulic characteristics and water capacity, as well as soil hydrophobicity (Harper et al. 2000). These deteriorations were associated with a decreased trend of plant height and stem diameter. Plant growth is limited by water and nutrient storage in farmland. The residual mulch film promoted the detachment of soil particles that then became available for sealing the soil surface or for transport by flow, increasing the potential to clog water migration pores (Bedaiwy 2008). The mechanism concerning the hindering infiltration of residual mulch film was similar to biochar or straw mulch. When materials mixed in soil were increasing, there was

Page 13 of 18 71

a significant increase in water retention capacity (Soinne et al. 2014). In general, residual mulch film could increase soil water retention of surface soil by clogging water infiltration pores. Therefore, irrigation water stayed in the surface soil. In addition, residual mulch film would increase the roughness of the surface soil (Jordán et al. 2010), enhancing topsoil reflectivity to solar shortwave radiation, delaying the connectivity of the surface soil moisture (Parsons et al. 2015), and accelerating the airflow of surface soil, thereby strengthening the intensity of soil evaporation (Gupta et al. 2015). Finally, the probability of drought stress for tomato plants would increase. Water deficit was the main reason for the reduction of dry biomass and yield (Patanè et al. 2011). However, Jiang et al. (2004) reported that dry matter partitioning from the stem and leaves significantly increases under water deficit conditions compared to well-watered conditions. Chaves (1991) found that a linear increase in the timing of panicle exertion with increasing water stress was imposed just after floral initiation. Nobel et al. (1994) showed that water did not flow from leaves to the fruits when tomato was under water stress. Plants physiologically respond to mild irrigation water stress with stomatal closure, which reduces stomatal conductance over a short response time (Hsiao 1973). Stomatal conductance is affected by light intensity and possibly modified by the severity of water stress (Collatz et al. 1992). Furthermore, Spinelli et al. (2016) showed that there was a significantly decline in stomatal conductance with increasing water stress. Figure 3 shows that soil moisture at 0–10-, 10–20-, and 20–30-cm soil layers in field experiments with similar design and experimental environments was coincident with this study. As the amount of residual mulch film increased, the soil moisture content at 0– 10 cm had a decreasing trend; adversely, the soil moisture at 10–20 and 20–30 cm showed a significant downtrend. Therefore, residual mulch film could enhance the water holding capacity of epipedon. Water retention at epipedon caused a reduction in oxygen availability and root respiration which could result in the death of root cells and decrease cell permeability (Patwardhan et al. 1988). However, soil layers below 10 cm did not store an inadequate amount of available water, which could hamper various physiological processes in plants and tomato yield. When the soil at epipedon became adequate, the oxygen concentration ceased to increase, which could destroy the balance between oxygen

71


Page 14 of 18 T0

T1

T2

22.4

T4

T5

10-20cm 23.5

22.2 22.0

Soil water content(%)

Soil water content(%)

T3

0-10cm

21.8 21.6 21.4 21.2

23.0

22.5

22.0

21.5 21.0 21.0

20.8

Seedling stage

Blossoming and fruit -set stage

Seedling stage


Growing period of tomato

Growing period of tomato 23.5

20-30cm

Soil water content (%)

23.0

22.5

22.0

21.5

21.0

Seedling stage


Growing period of tomato

Fig. 3 Soil water content at 0–10-, 10–20-, and 20–30-cm soil layers during the growth periods of tomato. Soil samples were excavated by a small-bore soil auger with a 4 cm radius (n = 6). The soil sampling points were distributed in the center of tomato plants, sampling at a depth interval of 10 cm down to 30 cm. Soil

water content was measured by the oven-drying method every 3 days. Six residual mulch film levels were applied, which were a control soil without residual mulch film (CK) and five levels of residual mulch film (T1 to T5): 80, 160, 320, 640, and 1280 kg ha−1

requirements and oxygen diffusion of the roots (Brisson et al. 2002). In contrast, root growth was reduced from the lack of oxygen available for root respiration and inhibited root formation and promoted root decay (Bidel et al. 2000). Mild water deficit could occur in the soil layer below 10 cm, which decreased the remobilization of pre-anthesis carbon reserved in the nutritive and reproductive tissues of tomato (Xue et al. 2006).

movement of water and air (Lynch and Brown 2012). These main effects on soil physical properties could hinder crop root extension and reduce the growth and fruit quality of crops (Whitmore and Whalley 2009). The root system played a key role in the root anchorage and absorptive ability for water and nutrients (Manzur et al. 2014). This study found that root volume, root length, root diameter, and root surface area were reduced with increasing residual mulch film, which indicated that tomato root development was inhibited by residual mulch film. Pore size distribution of soil was altered, total porosity was decreased, and there were changes in the movement and content of heat, air, water, and nutrients in the soil mixed with residual mulch film (Shierlaw and Alston 1984). The restricted growth of tomato roots has been variously attributed to all of these properties and to the high mechanical resistance that residual mulch film presents to tomato roots. Soil

4.2 Root Morphological Characteristics Residual mulch film had an increasing trend in Chinese farmland, leading to increased soil compaction and penetration resistance to growing roots and reducing soil connectivity (Batey and McKenzie 2006). In addition, soil pore size distribution was significantly destroyed by a residual mulch film, which increased the proportion of smaller pores that were disadvantageous for the


structure is critical for crop growth and development because the habitat of crop roots should contain enough channels to switch moisture and air (Dexter 1988). As a consequence, residual mulch film lagged in farmland could destroy the balance of field ecosystem and endanger the growth of crops. Root elongation may temporarily stop if pressure caused by residual mulch film was enough. Residual mulch film could also increase soil compaction, which resulted in higher elongation resistance of tomato root. These results were in agreement with those of Lipiec et al. (2016) who found that wheat roots impeded by tough soil grew more difficultly than in sandy soil. Tracy et al. (2013) reported that tomato root growth was significantly influenced by soil bulk density, and the compacted texture of soil caused by foreign matters became very essential in the process of root growth. 4.3 Fruit Quality of Tomato Water plays a crucial role in affecting tomato fruit quality. The changing trend in FSI, SSC, OA, VC, lycopene, and NC observed in our experiment is consistent with the other results concerning deficit irrigation (Giné-Bordonaba and Terry 2016; PérezPérez et al. 2014). The taste of tomato fruits is, in turn, determined mainly by the content in TSS, organic acids, and their ratio (Bedaiwy 2008; Knutsen et al. 2001). In addition, SSC content has an important positive implication for the processing tomato industry, as high values improve processing efficiency (Johnstone et al. 2005). To this regard, the changes of tomato fruit quality in our experiment seem to be sensitive to water stress as also mentioned for other cultivars; Cantore et al. (2016) found that dry matter weight, SSC, and OA showed an increase to the decline of water availability. Nonetheless, Spinelli et al. (2016) found that yield and fruit quality of pistachio trees were not affected by any water stress. These different results can be attributed to the variation in drought tolerance, agronomic practices, and phenological phases. It is well known that tomato plants adapt to changing environment, usually by growth and physiological modifications. Residual mulch film could create different degrees of water deficit, further causing the deterioration of tomato comprehensive fruit quality. The water shortage caused by residual mulch film is

Page 15 of 18 71

considered one of the main factors responsible for poor comprehensive fruit quality.

5 Conclusions This study analyzed the effects of residual mulch film on tomato growth, yield, and fruit quality with the aim to provide a deeper insight on the cultivation of tomato planted in soil mixed with residual mulch film. Therefore, it is beneficial to determine more efficient agronomic measures to increase yield and the comprehensive quality of tomatoes in areas polluted by residual mulch film. The results demonstrated that residual mulch film caused yield decline, as well as declines in plant height, stem diameter, dry biomass, and the comprehensive fruit quality of tomato, while it produced beneficial effects on NC, VC, and SSC. In addition, root volume, root length, root surface area, and root diameter were reduced with increasing residual mulch film. According to our results, the comprehensive quality of tomato fruit with the CK treatment is the best. Moreover, residual mulch film worsened greenhouse tomato plant growth, yield, and nutritional quality. Accordingly, residual mulch film would aggravate the negative effects on comprehensive fruit quality of tomato. In conclusion, this study provides evidence that residual mulch film can successfully retard tomato plant growth, yield, and fruit quality; in addition, it enhances our understanding of the bad role of residual mulch film in crop growth, water movement, and solute transport. Further and systematic research is required, such as the effects of residual mulch film on other crops. Acknowledgements The authors would like to appreciate the reviewers and editors for their selfless suggestions and help. This research was financially supported by the National Twelfth FiveYear Plan for Science and Technology Support Program (2015BAD24B01).

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