Interactive effects of elevated carbon dioxide and nitrogen availability on fruit quality of cucumber (Cucumis sativus L.)

Journal of Integrative Agriculture 2018, 17(11): 2438–2446 Available online at www.sciencedirect.com

ScienceDirect

RESEARCH ARTICLE

Interactive effects of elevated carbon dioxide and nitrogen availability on fruit quality of cucumber (Cucumis sativus L.) DONG Jin-long1, 2, LI Xun1, Nazim Gruda3, DUAN Zeng-qiang1 1

State Key Laboratory of Soil and Sustainable Agriculture/Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, P.R.China

2

University of Chinese Academy of Sciences, Beijing 100049, P.R.China Institute of Plant Sciences and Resource Conservation, Division of Horticultural Sciences, University of Bonn, Bonn 53121, Germany

3

Abstract Elevated CO2 and high N promote the yield of vegetables interactively, whilst their interactive effects on fruit quality of cucumber (Cucumis sativus L.) are unclear. We studied the effects of three CO2 concentrations (400 μmol mol–1 (ambient), 625 μmol mol–1 (moderate) and 1 200 μmol mol–1 (high)) and nitrate levels (2 mmol L–1 (low), 7 mmol L–1 (moderate) and 14 mmol L–1 (high)) on fruit quality of cucumber in open top chambers. Compared with ambient CO2, high CO2 increased the concentrations of fructose and glucose in fruits and maintained the titratable acidity, resulting in the greater ratio of sugar to acid in moderate N, whilst it had no significant effects on these parameters in high N. Moderate and high CO2 had no significant effect on starch concentration and decreased dietary fiber concentration by 13 and 18%, nitrate by 31 and 84% and crude protein by 19 and 20% averagely, without interactions with N levels. The decreases in amino acids under high CO2 were similar, ranging from 10–18%, except for tyrosine (50%). High CO2 also increased the concentrations of P, K, Ca and Mg but decreased the concentrations of Fe and Zn in low N, whilst high CO2 maintained the concentrations of P, K, Ca, Mg, Fe, Mn, Cu and Zn in moderate and high N. In conclusion, high CO2 and moderate N availability can be the best combination for improving the fruit quality of cucumber. The fruit enlargement, carbon transformation and N assimilation are probably the main processes affecting fruit quality under CO2 enrichment. Keywords: amino acid, CO2 enrichment, mineral, nitrate level, soluble sugar, vegetable quality

1. Introduction Received 12 December, 2017 Accepted 3 April, 2018 DONG Jin-long, E-mail: [email protected]; Correspondence DUAN Zeng-qiang, Tel: +86-25-86881562, E-mail: zqduan@ issas.ac.cn © 2018, CAAS. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/) doi: 10.1016/S2095-3119(18)62005-2

Carbon dioxide (CO2), a crucial component of photosynthesis is taken by the plants through the stomata by diffusion, therefore the concentration of CO2 in the greenhouse atmosphere strongly influences CO2 uptake (Gruda and Tanny 2015). Greenhouse is a commonly-used facility to prevent crops from environmental stresses, particularly chilling stress, whilst it inevitably results in a lack of CO2 due to airtight coverings (Kläring et al. 2007; Jin et al. 2009).

DONG Jin-long et al. Journal of Integrative Agriculture 2018, 17(11): 2438–2446

Thus, it is necessary to supplement CO2 in greenhouse vegetable cultivation (Mortensen 1987; Sgherri et al. 2017). On the other hand, during the course of the current century, atmospheric CO2 levels are assumed to progressively rise, and the extent depends on the prevailing scenario, with 935 ppm at the end of the century being the upper limit (Bisbis et al. 2017). Elevated CO2 (eCO2) could promote plant photosynthesis and enhance the plant tolerance to environmental stresses, thus increases plant productivity and yield, referred to CO2 fertilization (Mortensen 1987; Leakey et al. 2009; Huang and Xu 2015). Sánchez-Guerrero et al. (2009) reported that eCO2 (i.e., 700 μmol mol–1) could enhance the yield of cucumber by 19% compared with nonenriched plants. Similarly, eCO2 to 1 200 μmol mol–1 could increase the carbon translocation to fruits and promote cucumber yield to a larger extent, by 73% (Dong et al. 2017). The product quality of grain crops, e.g., wheat or rice has been well-investigated (Taub et al. 2008; Loladze 2014). Elevated CO2 promotes the synthesis of carbohydrates, e.g., soluble sugar and starch, and inhibits N assimilation in plant tissues (Leakey et al. 2009; Bloom et al. 2010), thus it can enhance sugar content and decrease protein content in eatable part of crops (Moretti et al. 2010; Wroblewitz et al. 2014). However, the effects are distinct among different species or cultivars. For example, Jin et al. (2009) found eCO2 decreased nitrate accumulation in celery and leaf lettuce, whereas it increased the nitrate concentration in stem lettuce. Pérez-López et al. (2015) showed that eCO2 had distinct effects on nitrate concentration between two cultivars of pigmented lettuce. On the other hand, nutritional quality of cucumber fruit differs with grain crops, which consists of several attributes, e.g., taste flavor, antioxidants, etc. (Gruda 2005; Slavin and Lloyd 2012). Recently, there are some studies assessing the effects of eCO2 on fruit quality of tomato (Krumbein et al. 2006; Mamatha et al. 2014; Zhang et al. 2014). However, it is impossible to interpret the influence of eCO2 on cucumber quality through exploring other crops or vegetables. Our recent study gives some indications of the effects of eCO2 on fruit quality of cucumber when grown in paddy soil (Dong et al. 2018b). However, the information on cucumber is too little to get a better understanding of how eCO2 affects fruit quality. Nitrogen availability can affect crop productivity under eCO2 (Sanz-Sáez et al. 2010; Aranjuelo et al. 2013), whilst its interactive effects with eCO2 on accumulation of qualityrelated compounds in fruits are hardly investigated. High N availability and eCO2 promote carbohydrate and mineral translocation to sinks (Paul and Foyer 2001; Leakey et al. 2009), thus may promote their accumulation in fruits. By contrast, high N also enlarges seed or fruit size, and promotes crop yield under eCO2 (Islam et al. 1996; Mavrogianopoulos et al. 1999), which may dilute the quality-

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related compounds (Dong et al. 2018a). For instance, eCO2 decreases the protein and mineral concentrations due to its greater yield promotion in several grain crops (Taub et al. 2008; Myers et al. 2014) and vegetable crops (Dong et al. 2018a). Since people are given increasing attention to vegetarian diets and consume more vegetables including cucumber than before (Slavin and Lloyd 2012), it is of vital importance to focus on enhancing product quality. This study aims to evaluate the effect of eCO2 on fruit quality of cucumber, and investigate how to reduce the potential decreases of fruit quality through matching nitrate fertilization. We hypothesize that eCO2 does not reduce fruit quality of cucumber under moderate N availability.

2. Materials and methods 2.1. Experimental design This experiment was defined as a randomized split-plot design with CO2 levels as the main plot, and N supplies as the subplot. CO2 concentrations were created at 400 μmol mol–1 (ambient CO2), 625 μmol mol–1 (moderate CO2) and 1 200 μmol mol–1 (high CO2), in each open top chambers (OTCs), respectively, with one true replicate for each CO2 level. Plants were weekly rotated among and within growth chambers to minimize the chamber effects. Nitrate (NO3–) concentrations in nutrient solution were at 2 mmol L–1 (low N), 7 mmol L–1 (moderate N) and 14 mmol L–1 (high N). Within each chamber, each N level was repeated by six times.

2.2. Plant culture Three OTCs were utilized to create CO2 atmosphere inside the glasshouse in Nanjing, China (32.0596°N, 118.8050°E). The OTCs are made of poly (methyl methacrylate) and transparent to minimize the shading effects. The internal dimensions of the chamber are 2.3 m length×0.8 m width×1.4 m height. Therefore, 36 pots in each chamber were in three rows spaced 0.27 m apart and 0.2 m within rows. CO2 concentrations were monitored by an infrared gas analyzer Ultramat 6 (Siemens, Germany) from 08:00 to 17:00. The experiment was conducted from April to June, 2013. The components of nutrient solution was referred to our previous study (Dong et al. 2017). Cucumber seeds cv. Jinyou 38 (Tianjin Kerun Cucumber Research Institute, China) were sterilized using 10% sodium hypochloride for 15 min, rinsed with ultra-pure water thoroughly, and germinated on a petri dish. The 1-day germinated seeds were sown into a peat-pearlite mixture (2:1, v/v) and cultivated in a controlled environment room

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DONG Jin-long et al. Journal of Integrative Agriculture 2018, 17(11): 2438–2446

with temperature at (25±0.5)°C and humidity at (60±3)%. Sixteen days after sowing, two seedlings with third leaf were washed using deionized water, and transplanted to containers with 1/2 strength nutrient solutions. Twenty-eight days after sowing, the nutrient solutions were changed to full-strength solutions and daily renewed to maintain a sufficient nutrient supply. Growth chambers were opened for air cooling to keep the temperature desirable for plant growth (less than 32°C), while CO2 was not pumped for at most 2 h a day. Solution pH was daily adjusted to 6.5 with 0.1 mol L–1 sodium hydroxide (NaOH) or 0.05 mol L–1 sulfuric acid (H2SO4). The solutions were aerated by air pumps for 30 min per hour. During the entire growth period, the temperature and relative humidity within the OTCs were recorded using an L95-83 recorder (Hangzhou Loggertech Co., China) every 30 min. The average temperature of ambient CO2, moderate CO2 and high CO2 treatments was (23.6±5.0), (24.1±5.0) and (24.1±5.2)°C (means±SD), and the average humidity was (71.4±20.1), (73.4±18.5) and (74.1±18.4)%, respectively. Plants at initial fruit stage were harvested on 66 days after sowing.

2.3. Plant harvest Fruits were harvested once matured, weighed, cut into small pieces and stored at –20°C temporally. After all fruits were harvested, the fruits collected from each time were mixed. A subset of fruits was lyophilized and stored at –20°C before biochemical analysis. Another subset of fruit samples was placed in an oven at 100°C for 15 min to inactivate enzymes and dried to a constant weight at 70°C. This study had not collected enough fruits in low N treatment which were too small to be marketable, therefore most of the fruit quality except mineral concentration had not been measured.

2.4. Fruit quality analysis Oven-dried plant materials were digested using H2SO4H2O2 and the concentration of N was determined using a discrete analyzer Smartchem200 (Alliance, France). Crude protein concentration was calculated by multiplying the N concentration with a factor of 6.25. Another subset of plant samples was digested with HNO3-H2O2 for determing the concentrations of P, K, Ca, Mg, S, Fe, Mn, Zn and Na by Iris-Advantage ICP-AES (Thermo Electron, USA). Lyophilized fruit materials were extracted twice using hot water and used to determine the concentrations of fructose and glucose by HPLC system LC-10AVP (Shimadzu, Japan) using a method developed by Li et al. (2014) As fructose and glucose are the main soluble sugars in cucumber fruits (Handley et al. 1983), the ratio of sugar to acid was calculated as ratio of the concentrations of

(fructose+glucose) to titratable acidity in this study. The left residues were extracted with perchloric acid and the starch concentration of the extracts was determined using the anthrone method. Nitrate (NO3–) in fruits was analyzed after extracted with 2 mol L–1 potassium chloride using the above discrete analyzer (Bungard et al. 1999). The above methods in detail can be referred to Dong et al. (2017). The concentration of crude fiber, also referred to insoluble dietary fiber, was determined following the traditional aciddetergent method. Briefly, 1 g of lyophilized and ground (<1 mm) fruit samples were boiled in 100 mL acid detergent solution (made by mixing 20 g cetyltrimethylammonium bromide (CTAB) into 1 000 mL 0.5 mol L–1 sulfuric acid) for 60 min. The left of the residue was washed using boiled water for three times until a neutral solution pH was achieved, and then the residue was washed with acetone for three times again till the solution was colorless. Finally, the residue was dried at 105°C for 3 h and the dry weight of the residue was recorded to calculate the concentration of dietary fiber. Titratable acidity (lactic acid) was extracted using deionized water and measured by potentiometric titration with 0.1 mol L–1 NaOH (Huang et al. 2009). The fruits were also hydrolyzed with 6.0 mmol L–1 HCl at 110°C for 22 h, then concentrations of amino acids in the digested solution were measured using an amino acid analyzer L-8900 (Hitachi, Japan). All the measurements above were on dry weight basis.

2.5. Statistical analysis The data were analyzed using GenStat 17.1 (VSN International, UK). The chamber effects were not considered here because the plant rotation minimizes the defect of this experiment without true replications of CO2 chamber (Hocking and Meyer 1991; Porter et al. 2015). The data were subjected to a two-way analysis of variance (ANOVA) for split-plot design to determine the main effect of CO2, N and their interaction. Least significant difference tests (P≤0.05) were used to indicate the significant differences between any two means.

3. Results 3.1. Carbohydrates Compared to the ambient CO2, moderate CO2 had no significant effects on the concentrations of both fructose and glucose (Fig. 1 and Table 1). By contrast, high CO2 increased the concentrations of fructose and glucose by 75 and 73% in moderate N, but had no impact on these parameters in high N, resulting in a significant main effect

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DONG Jin-long et al. Journal of Integrative Agriculture 2018, 17(11): 2438–2446

400 µmol mol–1

b

b

50 7

Titratable acidity (%)

3 2.5

a

a

a

a

a

a

2 1.5 1 0.5 7

Starch (mg g–1)

25

a

20

14

ab

b

10 5 0

7

a

a ab

b

1 000 500 0

20 16

H

c

c

7 14 Nitrate concentration (mmol L–1)

b

b

b

b

50

12

7

b

a

14 a

b

ab

b

8 4 7

14

25 20

a

a

ab

b b

b

15 10 5 0

14

2 500

1 500

b

100

D

F

ab

ab

ab

15

2 000

150

0

0 E

200

0

14

Dietary fiber (%)

C

Nitrate (mg kg–1)

b

100 0

G

b

b

a

250

Sugar/Acid ratio

200

Glucose (mg g–1)

a

250 150

1 200 µmol mol–1

B 300

Crude protein (%)

Fructose (mg g–1)

A 300

625 µmol mol–1

7

14

25 20

ab

a bc

c

b

b

15 10 5 0

7

14

Nitrate concentration (mmol L–1)

Fig. 1 Fructose (A), glucose (B), titratable acidity (C), ratio of soluble sugar to organic acid (sugar/acid, D), starch (E), dietary fiber (F), nitrate (G) and crude protein (H) concentrations in fruit of cucumber (Cucumis sativus L.) plants grown under various CO2 (400, 625 and 1 200 µmol mol–1) and N levels (nitrate, 7 and 14 mmol L–1) at the initial fruit stage (66 days from sowing). Values are mean±SD (n=4) and columns with different letters are significantly different (P≤0.05) among treatments according to least significant difference test.

of CO2 and CO2×N interaction. There were no main effect of

dietary fiber concentration by 13 and 18%, respectively, with

moderate and high CO2 nor CO2×N interaction on titratable

no interaction with N availability. Compared to moderate N,

acidity. Both moderate and high CO2 increased sugar/acid

high N decreased fructose, glucose and sugar/acid as main

in moderate N without significant effect in high N, leading to

effects of the N levels.

a significant CO2×N interaction. Although moderate and high CO2 had no significant impact on the starch concentration

3.2. Nitrogenous compounds

averagely, there was a trend of decrease. Similarly, as a main effect of CO2, moderate and high CO2 decreased

Moderate CO2 decreased the nitrate concentration by 31%,

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Table 1 Analysis of variance (ANOVA) of the data of cucumber (Cucumis sativus L.) plants in this experiment Factor CO2 N CO2×N

Fructose

Glucose

**

*

**

**

**

**

Titratable acidity ns ns ns

Sugar/Acid ** ** **

Starch ns ns ns

Dietary fibre

Nitrate

Crude protein

**

***

***

ns ns

ns ns

ns

***

ns, *, ** and *** denote levels of significance at P>0.05, P≤0.05, P≤0.01 and P≤0.001, respectively.

whilst high CO2 decreased nitrate concentration by 84%, from 1 461 to 239 mg kg–1 DW averagely, without significant interactions with N levels (Fig. 1-G and Table 1). Moderate and high CO2 decreased the protein concentration in both N treatments by 19 and 20% on average with no interactions with N levels. Similar with protein, high CO2 decreased all measured amino acid concentrations averagely, except glutamic acid (Table 2). Among them, the largest decrease was tyrosine (50%), whilst the decrease of other amino acids ranged from 10 to 18%. Compared to high N, the concentrations of protein, glutamic acid, serine, arginine, glycine and alanine in moderate N decreased on average.

3.3. Minerals High CO2 increased the concentrations of P, K, Ca and Mg by 74, 67, 500 and 100% in low N, whilst it has no significant effect on these parameters in moderate and high N, resulting in significant CO2×N interactions (Table 3). High CO2 also decreased the concentrations of Fe and Zn by 57 and 63%, respectively in low N, indicated as main effects of CO2. In moderate and high N, high CO2 decreased Na concentration by 25 and 18% respectively, whereas it had no impact on the concentrations of S, Fe, Mn and Cu.

4. Discussion As Bisbis et al. (2017) summarized for several vegetable crops, the effect of increased CO2 on vegetables is mostly beneficial for production, but may alter their nutritional and internal product quality. The situation could further be deteriorated when other environmental factors, e.g., light, nutrients or water, are scarce. This is the first report on the interactive effect of eCO2 and three N levels on product quality of greenhouse cucumber conducted in hydroponics, and it gives practical information to improve the cucumber quality for soilless horticulture.

4.1. Carbohydrates High CO2 together with moderate N availability enhanced the sugar accumulation to a greater extent than organic acid. The greater increases in sugar accumulation than fruit acidity in our study (Fig. 1-A–D) is confirmed in several

other vegetables although the effects on sugar or acid alone differ among species (Wang and Bunce 2004; Azam et al. 2013; Zhang et al. 2014). As sugars from photosynthesis are precursor for many compounds, the synthesis and accumulation of organic acid may be distracted by other reaction paths. Furthermore, sugar concentration was not enhanced by high CO2 in high N (Fig. 1-A and B). The soluble sugar may be allocated to fruits to a lesser extent under relative high CO2 levels or be diluted by fruit enlargement to some extent (Bénard et al. 2009; Dong et al. 2018b). However, both moderate and high CO2 tended to inhibit the formation of dietary fiber (Fig. 1-F). Current results indicate that eCO2 could promote formation of soluble sugar rather than increase the carbohydrate transformation to dietary fiber. Elevated CO2 potentially deteriorates the fruit quality as dietary fiber is advisable for losing weight, and controlling diabetes or cardiovascular diseases (Lattimer and Haub 2010; Wanders et al. 2011). However, eCO2 can increase either sugar or fiber concentration in the root vegetables, i.e., carrot, radish and turnip (Azam et al. 2013), and cucumber cv. Jinmei 3 (Dong et al. 2018b) which is inconsistent with our study. The discrepancy may be attributed to the distinct genetic background, e.g., the strengthened carbon allocation to belowground as roots of root vegetables are also for carbon storage or the environmental variations.

4.2. Nitrogenous compounds In our experiment, moderate and high CO2 largely inhibited the nitrate accumulation in cucumber fruits (Fig. 1-G). Most of the studies found eCO2 decreases nitrate concentration (Donnelly et al. 2001; Jin et al. 2009; Dong et al. 2018b), which confirms our results. However, eCO2 has also been demonstrated to increase nitrate concentration in stem lettuce (Jin et al. 2009) and pigmented lettuce (Pérez-López et al. 2015). In a general, nitrate concentration depends on the difference between the stimulation of nitrate reduction and nitrate uptake under eCO2 (Gruda 2005), which varies among different species and environmental factors. In our study, it is more likely that high CO2 promoted nitrate assimilation more than nitrate uptake as cucumber is a vegetable with less nitrate accumulation (Santamaria 2006).

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Table 2 Amino acid concentrations in fruit of cucumber (Cucumis sativus L.) grown under two CO2 concentrations (400 and 1 200 μmol mol–1) and two nitrate supplies (7 and 14 mmol L–1) at initial fruit stage (66 days from sowing) Amino acids (% DW)1) Non-essential Glutamic acid Aspartic acid Proline Serine Glycine Alanine Essential for children Cysteine Tyrosine Semi-essential Histidine Arginine Essential Threonine Valine Methionine Isoleucine Leucine Phenylalanine Lysine

400 (μmol mol–1) 14 (mmol L–1) 7 (mmol L–1)

1 200 (μmol mol–1) 7 (mmol L–1) 14 (mmol L–1)

N

CO2×N

ns

**

**

ns ns

ns ns ns ns ns ns

LSD

CO2

4.07±0.79 1.08±0.09 0.43±0.01 0.64±0.05 0.66±0.02 0.60±0.04

4.81±0.26 1.16±0.02 0.42±0.01 0.72±0.02 0.74±0.04 0.66±0.01

3.29±0.20 0.96±0.05 0.36±0.03 0.56±0.03 0.59±0.02 0.54±0.03

4.68±0.10 1.02±0.04 0.36±0.02 0.60±0.03 0.65±0.03 0.57±0.03

0.81 0.11 0.04 0.07 0.05 0.06

0.10±0.01 0.31±0.02

0.10±0.01 0.32±0.01

0.09±0.00 0.15±0.02

0.09±0.00 0.16±0.00

0.01 0.04

0.25±0.02 0.58±0.01

0.27±0.01 0.73±0.07

0.22±0.02 0.54±0.02

0.23±0.01 0.59±0.03

0.03 0.08

0.47±0.03 0.56±0.05 0.18±0.03 0.45±0.04 0.82±0.08 0.51±0.04 0.78±0.06

0.52±0.03 0.60±0.01 0.18±0.01 0.47±0.01 0.88±0.02 0.52±0.02 0.78±0.01

0.42±0.03 0.51±0.03 0.15±0.01 0.41±0.03 0.73±0.06 0.45±0.04 0.68±0.05

0.45±0.02 0.53±0.02 0.15±0.01 0.41±0.02 0.75±0.03 0.44±0.02 0.68±0.03

0.05 0.06 0.03 0.06 0.10 0.06 0.07

*** **

*

***

**

**

*

**

ns ns

ns ns

**

ns

**

**

ns ns

**

ns ns ns ns ns ns ns

***

* * ** ** ** **

ns ns ns ns ns ns ns

1)

DW, dry weight. Values given are mean±SD (n=3). Least significant difference tests (LSD, P≤0.05) are used to compare any two means. ns, *, denote levels of significance at P>0.05, P≤0.05, P≤0.01, and P≤0.001, respectively.

**

and

***

Table 3 The effects of CO2 enrichments (400 and 1 200 μmol mol–1) and different N treatments (nitrate, 2, 7 and 14 mmol L–1) on fruit mineral concentrations of cucumber plants (Cucumis sativus L.) at the initial fruit stage (66 days from sowing) N (mmol L–1) 2 7 14 LSD CO2 N CO2×N

CO2 (μmol mol–1) 400 1 200 400 1 200 400 1 200

P (% DW) 0.57 0.99 0.64 0.60 0.62 0.59 0.13

K (% DW) 3.91 6.54 3.78 3.27 3.84 3.54 0.94

Ca (% DW) 0.41 2.46 0.57 0.63 0.59 0.64 0.16

Mg (% DW) 0.55 1.10 0.65 0.58 0.61 0.55 0.13

S (% DW) 0.78 0.77 0.63 0.61 0.45 0.43 0.10 ns

Fe (mg kg–1) 251 109 342 229 240 233 115 *

**

*

***

**

***

***

***

***

***

*

***

***

***

***

ns

ns

Mn (mg kg–1) 30.5 31.8 34.7 28.7 23.9 25.5 11.7 ns ns ns

Zn (mg kg–1) 43.7 16.2 52.0 44.4 69.9 52.2 22.3 * **

***

ns

ns

Values are means (n=4). Least significant difference tests (LSD, P≤0.05) are used to compare any two means. ns, *, levels of significance at P>0.05, P≤0.05, P≤0.01, and P≤0.001, respectively.

A 30–80% decrease of nitrate concentration will allow using of such products, e.g., for baby food. With a lesser extent relative to nitrate, both CO 2 treatments inhibited protein concentration in fruits and high N cannot counteract the effect of eCO2 (Fig. 1-H). An FACE study on winter wheat or winter barley gave the same results as ours (Erbs et al. 2010). The decrease of protein concentration has been attributed to N deficiency in soils (Taub et al. 2008). However, as high N failed to recover the decrease of protein concentration, some other factors must play a role in the decreased protein concentration

Na (mg kg–1) 194 232 1 151 858 915 746 158 ns

**

and

***

denote

besides N availability. Bloom et al. (2010) found that CO2 enrichment could inhibit nitrate reduction in wheat and Arabidopsis physiologically. More specifically, as a result of the decreased protein concentration, high CO2 decreased almost all amino acid concentration (Table 2). The extent of decrease in amino acid was similar, ca. 10–18%, except for tyrosine, which was greater than the results in wheat (Högy et al. 2013) and oilseed rape (Högy et al. 2010) but much less than some of the amino acids in carrot and turnip (Azam et al. 2013). Högy et al. (2009) found that CO2 enrichment resulted in greater reduction in non-essential

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DONG Jin-long et al. Journal of Integrative Agriculture 2018, 17(11): 2438–2446

than essential amino acids, which is inconsistent with our results. The differences could be attributed to the higher CO2 concentration in our study, different growth systems or species variations (Fernando et al. 2014).

4.3. Minerals Finally, high CO 2 did not shift most of the mineral concentration in marketable fruit (Table 3). Because of dilution effect, most of the mineral concentration has been reported to be decreased under eCO2 (Taub and Wang 2008; McGrath and Lobell 2013; Myers et al. 2014). However, the results vary among different species and different environmental factors. For example, high CO2 decreased the Na concentration of marketable fruit in our study, whilst CO2 enrichment had no impact on the Na concentration in grains of wheat (Högy et al. 2013) and oilseed rape (Högy et al. 2010). CO2 enrichment had no impact on Na concentration in pigmented lettuce with low Na supply, whereas it decreased the Na concentration with high Na supply (Pérez-López et al. 2015). It is interesting to notice that high CO2 boosted P, K, Ca, and Mg concentrations in fruit with low N supply where the fruit enlargement was inhibited. It has an implication that relative low N supply probably improves some of the mineral accumulation in fruits, which provides a practice to alleviate mineral deficiency in cucumber fruit under eCO2 when yield penalty can be acceptable to some extent. However, there is no similar interaction on mineral concentration in wheat grains (Erbs et al. 2010), which also indicates the environmental and species variations on mineral concentration under eCO2.

5. Conclusion High CO2 promoted the accumulation of fructose and glucose in moderate N to a greater extent than moderate CO2, whilst it decreased the nitrate accumulation, and maintained most of the mineral concentration in marketable fruits. High CO2 also decreased the protein concentration by 20% which cannot be alleviated by high N availability. More specifically, the effects of high CO2 on different amino acids were similar, excluding tyrosine. High CO2 strengthened the soluble sugar accumulation more than dietary fiber in moderate N, and it boosted the concentrations of P, K, Ca, and Mg in low N. Thus, high CO2 can improve some of the fruit quality in conditions where fruit enlargement was inhibited. High CO2 can be therefore recommended to match moderate N to increase fruit quality of cucumber. Fruit enlargement, carbon transformation and N assimilation are probably the main processes related to the fruit quality under CO2 enrichment, which still needs to be investigated from physiological and molecular aspects.

Acknowledgements The authors appreciate the funding supports from the National Key Technologies R&D Program of China during the 12th Five-Year Plan period (2014BAD14B04), the Strategic Priority Research Program of the Chinese Academy of Science (XDB15030300), and the Frontier Project of Knowledge Innovation Program of Institute of Soil Science, Chinese Academy of Sciences (ISSASIP1635).

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