Comparison of phenolic acids in organically and conventionally grown pac choi (Brassica rapa L. chinensis)

94

0

Research ArticleReceived: 8 October 2008 Revised: 31 December 2008 Accepted: 2 January 2009 Published online in Wiley Interscience: 2 March 2009

(www.interscience.wiley.com) DOI 10.1002/jsfa.3534

Comparison of phenolic acids in organicallyand conventionally grown pac choi (Brassicarapa L. chinensis)Xin Zhao,a† James R Nechols,b Kimberly A Williams,a Weiqun Wangc andEdward E Careyd∗

Abstract

BACKGROUND: Field and greenhouse studies were performed to investigate whether organic production methods influencedlevels of phenolic acid compounds in pac choi (Brassica rapa L. chinensis cv. Mei Qing Choi) compared with conventionalcultivation.

RESULTS: In the field experiment, organic fertilisation (compost + fish emulsion) resulted in significantly higher phenolicconcentrations compared with conventional fertilisation (NPK + CaNO3) under both high tunnel and open field environments.Increased phenolics were accompanied by a significant reduction in plant fresh weight and dry weight, probably dueto nitrogen deficiency. However, the elevated level of phenolics in organically grown pac choi could also have been due toconfounding effects of nitrogen availability, insect attack and pesticide application. A follow-up greenhouse experiment furtherdemonstrated a significant increase in phenolic compounds and a reduction in yield with organic fertiliser (vermicompost +fish fertiliser) relative to conventional treatment (slow release inorganic fertiliser). Preventive insecticide application did notaffect the phenolic levels in pac choi under either organic or conventional fertilisation.

CONCLUSION: Given that higher phenolic content in pac choi was associated with low nitrogen availability and considerableyield reduction, research is needed to determine the extent to which phenolic compounds may differ in organic and conventionalpac choi when nutrient levels are adjusted to produce comparable yields. Additional study is also warranted to determine theextent to which insect attack might contribute to elevated phenolic content in organic pac choi.c© 2009 Society of Chemical Industry

Keywords: total phenolic content; HPLC; fertiliser; high tunnel; open field; greenhouse; insecticide; nitrogen

INTRODUCTIONAmong the most abundant antioxidants present in fruits andvegetables are vitamin C, carotenoids and phenolic compounds,which are believed to play an important protective role in humanhealth.1 Compared with vitamin C, phenolic compounds typicallyexhibit more potent antioxidant activity.2 Olsson et al.3 recentlyreported higher antiproliferative and anticarcinogenic propertiesin extracts from organically grown strawberries, possibly due toincreased levels of phenolics. As plant secondary metabolites,phenolics are accumulated in response to various abiotic andbiotic stresses.4 Therefore cultural practices along with postharvestand processing procedures can significantly influence phenoliccontent in a number of fruit and vegetable crops.1 There isgrowing interest in understanding the potential of productionpractices to enhance phytochemical content.5

Although various authors have reported enhanced phenoliccontent in organic fruits and vegetables,6 – 9 inspection of resultstypically reveals weaknesses in experimental designs. For instance,genotype and soil characteristics might differ between organicand conventional treatments, resulting in confounding effects.To ensure scientifically valid comparisons of the impact oforganic versus conventional cultivation on quality of produce,

all other genetic and environmental factors should be carefullycontrolled.5,10 Goldman et al.11 recommended the use of modelcrops to achieve a more complete understanding of the influenceof diverse agricultural factors on phytochemicals.

Higher levels of abiotic and biotic stresses under organic produc-tion have been suggested as contributors to enhanced phenolic

∗ Correspondence to: Edward E Carey, Kansas State University HorticultureResearch and Extension Center, 35230 W 135th Street, Olathe, KS 66061-9423,USA. E-mail: [email protected]

† Current address: Department of Horticultural Sciences, University of Florida,Gainesville, FL 32611-0690, USA.

a Department of Horticulture, Forestry, and Recreation Resources, Kansas StateUniversity, Manhattan, KS 66506-5506, USA

b Department of Entomology, Kansas State University, Manhattan, KS 66506-4004, USA

c Department of Human Nutrition, Kansas State University, Manhattan, KS66506-1401, USA

d Kansas State University Horticulture Research and Extension Center, 35230 W135th Street, Olathe, KS 66061-9423, USA

J Sci Food Agric 2009; 89: 940–946 www.soci.org c© 2009 Society of Chemical Industry

94

1

Phenolic acids in organically and conventionally grown pac choi www.soci.org

contents in organic crops. Stresses identified include lower nitro-gen bioavailability12 and enhanced pest pressure resulting fromthe absence of pesticides.6,8 Brandt and Mølgaard13 estimated thatorganic vegetables may contain 10–50% more defence-relatedsecondary metabolites than conventionally grown vegetables, buthypothesised that differences might diminish as nutrient suppliesin organic systems increase. According to the carbon/nitrogenbalance hypothesis14 and the growth/differentiation balancehypothesis,15 limited nutrient availability leads to higher phe-nolic content in plants. An inverse relationship between nitrogenavailability and phenolic accumulation in leaves of both tomatoseedlings and mature plants has been reported.16,17 However, theimpact of nitrogen deprivation on phenolic flavonols in tomatofruits diminished during the ripening process, leading Stewartet al.16 to postulate that enhancement of phenolic content bynitrogen deprivation would likely be most important in leafy veg-etables. While higher disease and pest pressure as a result ofrestricted use of pesticides under organic farming might imposeelevated levels of biotic stresses and thus increase the concen-tration of phenolic compounds, it is also possible that phenolicproduction can be induced by pesticide application.18,19

The present study aimed to investigate the effect of organicproduction on phenolic compounds in the leafy vegetable pacchoi (Brassica rapa L. chinensis), a highly nitrogen-responsive cropthat has been used in our laboratory for phytochemical studies inorganic production systems.20 The field trial compared productionunder organic and conventional systems in high tunnels andopen fields, and a greenhouse experiment was conducted tofurther explore the influences of fertiliser source and preventiveinsecticide application on phenolic content in pac choi.

MATERIALS AND METHODSField trialA trial was conducted during summer 2003 on a Kennebec silt loamsoil at Kansas State University Horticulture Research and ExtensionCenter, Olathe. Six 9.8 m×6.1 m high tunnels with 1.5 m sidewalls(Stuppy, North Kansas City, MO, USA) and six adjacent 9.8 m×6.1 mfield plots were employed for the study. The tunnels were coveredwith a single layer of 6 mil (0.153 mm) K-50 polyethylene (Klerk’sPlastic Product Manufacturing, Inc., Richburg, SC, USA). Endwallswere open and sidewalls were rolled up throughout the trial. Theexperimental plots used were established in May 2002, when thesix high tunnels were divided into three groups (blocks) and, withineach block, one high tunnel was assigned to organic managementwhile the other was assigned to conventional management. Asimilar design was used for the field plots. The experimental set-upwas considered a split plot design, with environment (high tunnelversus open field) as main plot factor and cultivation method(organic versus conventional) as subplot factor. Three blocks ineach environment were used as replications. Pac choi cv. Mei QingChoi (Johnny’s Selected Seeds, Winslow, ME, USA) was seededin 200-cell Speedling flats (Speedling Incorporated, Sun City, FL,USA) with Premier Pro-Mix (Premier Horticulture Inc., Quakertown,PA, USA) in the greenhouse on 29 May, then transplanted on 8July and harvested on 5 August. Each plot consisted of ten plantsin a single row at a spacing of 20 cm between plants. Sprinklerirrigation was used and high tunnels were covered with 39% whiteshade cloth (Pak Unlimited, Norcross, GA, USA).

Based on results of soil tests prior to the trial and on arecommended rate of 110 kg total N ha−1, pre-plant fertilisationand fertigation were applied to plots to provide roughly equivalent

amounts of total nitrogen from organic or conventional sources.Organic fertilisation consisted of pre-plant application of Hu-More1-1-1 (composted cattle manure and alfalfa hay; Humalfa, Inc.,Shattuck, OK, USA) followed by fertigation with fish emulsion 5-1-1 (Lilly Miller Brands, Clackamas, OR, USA), while the conventionaltreatment consisted of pre-plant application of N-P2O5-K2O 13-13-13 followed by fertigation with calcium nitrate (N-P2O5-K2O15.5-0-0). Organic and conventional plots in high tunnels andopen fields received pre-plant fertilisation at rates of 17 and 51 kgtotal N ha−1 respectively. All plots were fertigated 10 days beforeharvest at a rate of 6 kg total N ha−1. Organic insecticide pyrethrin(PyGanic, McLaughlin Gormley King Company, Golden Valley, MN,USA) and conventional insecticide permethrin (Pounce 3.0 EC,FMC Corporation, Philadelphia, PA, USA) were sprayed twice in theorganic and conventional plots respectively to control flea beetlesat labelled rates. Both sides of plant leaves were sprayed with careto avoid spray drift.

Greenhouse trialThe experimental design of the greenhouse pot study wasa randomised complete block with five replications. A two-way factorial treatment structure was used consisting of twofertiliser sources (organic and conventional) by three insecticidelevels (organic, conventional and plain water) to evaluate theimpact of fertiliser source and insecticide application on phenoliccompounds in pac choi. Conventional fertilisation was byOsmocote controlled release (3–4 months) fertiliser N-P2O5-K2O19-6-12 (Scotts Company, Marysville, OH, USA) pre-incorporatedinto the potting medium, while organic fertilisation was by pre-plant application of vermicompost (Rising Mist Organic Farm,Belvue, KS, USA) and fertigation weekly with Neptune’s Harvestfish liquid fertiliser N-P2O5-K2O 2-4-1 (Ocean Crest Seafoods, Inc.,Gloucester, MA, USA). Vermicompost composition was analysedfor calculation of nutrient inputs. The amount of total nitrogensupplied by vermicompost in each plot in the organic treatmentwas equivalent to that provided by Osmocote controlled releasefertiliser in the conventional treatment. Epsom salt, dolomitic limeand Micromax micronutrient mix (Scotts Company, Marysville, OH,USA) were also added into the conventional fertiliser treatment tomatch Ca, Mg, S and micronutrient levels in the organic treatmentat the beginning of the experiment, then weekly fertigation withfish hydrolysate at a rate of 150 mg N L−1 began in the organictreatment at day 18 after sowing. Macronutrient inputs for eachplant under organic and conventional fertilisation are shownin Table 1. Insecticide application consisted of organic (PyGanic,McLaughlin Gormley King Company) at a rate of 1.5 mL L−1

distilled water, conventional (Pounce 3.0 EC, FMC Corporation)at a rate of 0.2 mL L−1 distilled water, and a plain distilled water(no insecticide) control, with each plant receiving approximately

Table 1. Total macronutrient inputs (g) per pot under organic andconventional fertilisation in greenhouse study in winter 2004 atManhattan, Kansas

Fertilisation N P K Ca Mg S

Organica 2.40 0.65 0.47 4.19 0.32 0.28

Conventional 2.07 0.29 1.08 4.19 0.32 3.77

a Pre-plant incorporation of vermicompost provided 2.07 g of the totalnitrogen; the remaining nitrogen was supplied by fish fertiliser.

J Sci Food Agric 2009; 89: 940–946 c© 2009 Society of Chemical Industry www.interscience.wiley.com/jsfa

94

2

www.soci.org X Zhao et al.

10 mL of liquid. Insecticides (or water) were sprayed at days30, 34 and 38 after sowing. Root media consisted of 45 : 25:30(v/v/v) peat moss/perlite/vermicompost in the organic fertilisertreatment and 75 : 25 (v/v) peat moss/perlite in the conventionalfertiliser treatment.

Pac choi ‘Mei Qing Choi’ (Johnny’s Selected Seeds) was direct-seeded in 16.5 cm azalea pots on 22 December 2003 and harvestedon 23 February 2004. There were 120 plants, with four plants ineach treatment. Average day/night temperatures were maintainedat 20/18 ◦C. Supplemental light from high-intensity dischargelamps was supplied from 06 : 00 to 18 : 00 starting at week 3 andcontinuing throughout the experiment.

Plant fresh weight, dry weight and nutrient concentrationWhole above-ground shoots from each plot or pot were harvestedand weighed, and per plant fresh weights were obtained. Drymatter content was determined by drying leaf samples (see below)at 70 ◦C in a forced air oven for 48 h. Total plant dry weight wasestimated by multiplying plant fresh weight by dry matter content.Total nitrogen concentration on a dry weight basis was determinedusing a LECO CN 2000 combustion analyser (LECO Corporation,St Joseph, MI, USA). Concentrations of other nutrient elements inthe dried leaf samples were analysed by an inductively coupledplasma spectrometer (SPECTRO Analytical Instruments GmbH,Kleve, Germany) after digestion with nitric acid.

Leaf sample collection and extractionIn the field trial, three fully expanded inner leaves were takenfrom three plants per plot and pooled to form a sample of nineleaves at harvest. The same was done for the samples to be ovendried. Leaf samples for phenolic extraction were immediatelyfrozen in liquid nitrogen and packed on ice in coolers for transportwithin 2 h to Kansas State University for phytochemical analysis.Samples were then stored at −20 ◦C for 1 week until freeze-drying.Freeze-dried samples were stored at −80 ◦C before extraction. Inthe greenhouse study, leaf samples for each treatment werecomposed of six leaves from three plants, with two fully expandedinner leaves taken from each plant.

A 1 g portion of freeze-dried sample was homogenised in 50 mLof 800 mL L−1 aqueous ethanol containing 20 µL L−1 of 980 mLL−1 2-naphthoic acid (Aldrich Chemical Co., Milwaukee, WI, USA)as an internal standard. The mixture was refluxed at 90 ± 1 ◦C for1 h and centrifuged at 241 × g for 4 min. A 20 mL aliquot of thesupernatant was evaporated to dryness using a rotary evaporatorand then redissolved in 8 mL of water. A 2 mL aliquot of theaqueous extract was adsorbed on a reverse phase Accubond ODSC-18 column (Agilent Technologies, Stockport, UK) preconditionedwith 2 mL of methanol and 2 mL of deionised water. After washingthe column with 2 mL of deionised water, the extract was elutedwith 2 mL of methanol. Duplicate extractions of each sample wereconducted.

Total phenolic contentTotal phenolic content was determined using Folin–Ciocalteu’sreagent (Sigma-Aldrich, St Louis, MO, USA) as describedpreviously.21 Chlorogenic acid (Sigma-Aldrich) was used as a stan-dard and results were expressed as mg chlorogenic acid equivalentg−1 dry weight. Duplicate measurements of each sample wereconducted.

High-performance liquid chromatography (HPLC) analysis ofphenolicsA 100 µL aliquot of extract was injected into a Beckman ‘GoldNouveau’ HPLC system equipped with an autosampler and aphotodiode array detector (Beckman Instruments, Fullerton, CA,USA). Phenolic compound separation was done on an Alltima C-18(250 mm × 4.6 mm, i.d. 4 mm) reverse phase column coupled toa guard column (Alltech, Deerfield, IL, USA). Gradient elution wasused with a mobile phase comprising (A) water/acetic acid (10 : 1v/v) and (B) HPLC-grade methanol as follows: start with 100% A toreach 10% B in A within 5 min, maintain for 5 min, then increaseto 40% B in A within 20 min and to 70% B within 10 min. Theflow rate was 0.8 mL min−1 and chromatograms were recorded at280 and 355 nm. Peaks were identified using external standardswhich included chlorogenic acid, ferulic acid and sinapic acid(Sigma-Aldrich). Individual peak areas at 280 nm were calculatedaccording to concentration curves of the standards. Unknownphenolic acids were summed and calculated using chlorogenicacid as the standard. Flavonoids were not reported here becausephenolic acids were predominant in sample extracts as indicatedby our HPLC retention times and previous reports.22

Statistical analysisAnalysis of variance was performed in both field and greenhousestudies using the Proc Mixed program of SAS for Windows Version9.1 (SAS Institute, Cary, NC, USA). Significant differences weredetermined by Fisher’s least significant difference (LSD) testat P ≤ 0.05. Correlation analysis was also conducted in thegreenhouse study.

RESULTS AND DISCUSSIONField trialBoth the fresh and dry weights of pac choi plants were significantlylower under organic production as compared with conventionalproduction. However, plant weights did not differ between shadedhigh tunnel and open field environments. Both organic productionand the open field environment resulted in significantly higherdry matter content of pac choi (Table 2). With restricted growth,organically grown pac choi showed significantly greater levels oftotal phenolics compared with conventionally produced plantsunder both high tunnel and open field conditions (Table 3). Aswith yields, phenolic levels were not differentially affected byhigh tunnel and open field environments. The association ofplant growth and phenolic concentration observed here fits withthe growth/differentiation balance hypothesis, which predictsthat more resources will be diverted into secondary metabolismthan photosynthesis if plant growth is constrained owing toenvironmental factors.15 In addition, the ‘dilution effect’ resultingfrom dry matter accumulation23 might have played a role inthe observed differences in phenolics between organically andconventionally grown pac choi, as the yield of dry matter wassignificantly greater in conventional versus organically producedplants.

HPLC analysis indicated that phenolic acids accounted for amajority of phenolics in pac choi, and chlorogenic acid predom-inated among the phenolic acids identified (Table 3). Organictreatment significantly increased concentrations of chlorogenicacid regardless of growing environment and had a more pro-nounced influence in shaded high tunnels. Sinapic acid was notaffected by either cultivation method or environment, whereas

www.interscience.wiley.com/jsfa c© 2009 Society of Chemical Industry J Sci Food Agric 2009; 89: 940–946

94

3


Table 2. Plant fresh weight, dry matter content and plant dry weight of pac choi (cv. Mei Qing Choi) grown organically (Org.) and conventionally(Con.) in high tunnels and open fields in field summer trial 2003 at Olathe, Kansas

High tunnel Open field

Biomass Org. Con. Org. Con. Productiona effect Environmentb effect P × Ec interaction

Fresh weight (g per plant) 132.87 314.83 133.83 352.67 P < 0.0001 NS NS

Dry matter content (% w/w) 4.50 4.22 6.96 5.22 P < 0.05 P = 0.01 NS

Dry weight (g per plant) 6.05 13.34 9.18 18.55 P < 0.01 NS NS

a Production means organic versus conventional production.b Environment means high tunnel versus open field environment.c P × E means interaction effect of production by environment.NS, non-significant.

Table 3. Concentrations of phenolic compounds (mg g−1 dry weight) in pac choi (cv. Mei Qing Choi) grown organically (Org.) and conventionally(Con.) in high tunnels and open fields in field summer trial 2003 at Olathe, Kansas

High tunnel Open field

Phenolic compound(s) Org. Con. Org. Con. Productiona effect Environmentb effect P × Ec interaction

Total phenolicsd 18.5 12.7 19.2 16.2 P < 0.01 NS NS

Chlorogenic acid 11.3 3.9 8.4 5.5 P < 0.01 NS P < 0.05

Ferulic acid 0.9 0.4 0.6 0.9 NS NS P < 0.05

Sinapic acid 0.5 0.3 0.3 0.5 NS NS NS

Unknown phenolic acidsd 9.4 4.9 9.0 10.2 P < 0.05 P < 0.05 P < 0.01

a Production means organic versus conventional production.b Environment means high tunnel versus open field environment.c P × E means interaction effect of production by environment.d Expressed as mg chlorogenic acid equivalent g−1 dry weight.NS, non-significant.

significantly higher levels of ferulic acid in organically grown pacchoi were observed in shaded high tunnels. Unidentified phenolicacids were substantially increased by organic production in hightunnels but not open field plots, despite the statistically significantmain effects of production method and environment (Table 3).

Low availability of nutrients, especially nitrogen, as well as in-sect attack observed on the plants might have contributed tothe lower yield of pac choi in the organic treatment. As reportedpreviously, the organic insecticide pyrethrin was far less effec-tive than the conventional insecticide permethrin at controllingphytophagous arthropod pests. This led to more noticeable leafdamage on organically grown plants.24 Thus interpretation of ele-vated concentrations of total phenolics in pac choi under organicproduction may have been complicated by confounding effects ofnutrient availability, pest attack and insecticide application. Giventhat the influence of organic cultivation on individual phenolicacids in this study tended to be more evident in the shadedhigh tunnel environments, phenolic comparisons between or-ganic and conventional systems under different environmentsdeserve further study. The field trial took place in the second yearfollowing establishment of organic and conventional productionplots. As differences in soil quality between organic and conven-tionally managed plots are expected to develop over time,25,26

longer-term studies are warranted.Yield decrease in organic farming due to low nutrient inputs has

been reported in long-term comparative studies of vegetables.27,28

However, comparable yields under organic as compared withconventional production have also been demonstrated.29,30 In

studies showing enhancement of phenolics in organically growncrops,7,9,12,31 information about crop productivity unfortunatelywas usually unavailable. Recent work by Sousa et al.32 demon-strated an overall trend of higher total phenolic concentrationsin organically grown tronchuda cabbage, accompanied by lowerplant fresh weight, as compared with conventionally fertilisedsamples. They suggested that the lack of nutrients, particularlyinsufficient nitrogen supply as a result of a low mineralisation rateunder organic production, could have boosted synthesis of pheno-lic compounds while limiting rapid growth of new leaves. However,the relationship between phenolic levels and yield could be morecomplicated. According to Dimberg et al.,33 concentrations of spe-cific phenolics may be either negatively or positively correlatedwith crop yield depending on the type of phenolic compoundand crop. A 10 year study comparing organic and conventionallygrown processing tomatoes demonstrated a consistent increasein flavonoid levels in organic relative to conventional fruits, butthe changes in flavonoids did not correlate with yield.34

The role of biotic stress has sometimes been emphasised inphenolic enhancement of fruits and vegetables under organicfarming, especially in open field production. Out of threestrawberry cultivars investigated, an increase in phenolics underorganic cultivation was shown only in one cultivar, primarily dueto disease pressure.35 In another study, organic apples exhibitedhigher phenolic contents than their conventional counterparts,largely determined by a higher level of pathogen attack on organicapples.36


94

4


Greenhouse studySlow release chemical fertiliser was used in the conventionaltreatment to attempt to more closely match the nutrient supplypattern from organic fertilisation with vermicompost and fishfertiliser. Total nitrogen input was even higher under organicfertilisation (Table 1). However, plant fresh weight and dry weightof organically fertilised pac choi were significantly lower thanwith conventional fertilisation, despite increased dry mattercontent in organically produced plants (Table 4). On the otherhand, compared with the field trial, the yield of organic andconventionally grown pac choi in the greenhouse experiment wasmuch higher. A marked decrease in total nitrogen content perplant in the greenhouse-grown pac choi further demonstratedthe inadequate supply of readily available nitrogen underorganic fertilisation (Fig. 1), although nitrogen concentrationon a dry weight basis did not differ between organically andconventionally fertilised pac choi (Fig. 2). The mineralisationpattern of vermicompost used in this experiment could befurther analysed to verify that nitrogen deficiency occurred in theorganic treatment. On the other hand, analysis of other nutrientelements including phosphorus, potassium, calcium, magnesium,iron, manganese and zinc in pac choi did not seem to indicate anynutrient toxicity that might hinder plant growth under organic orconventional fertilisation (Table 5). Compared with conventionallyfertilised pac choi, organic pac choi had significantly higherlevels of phosphorus and magnesium but lower levels of calcium,manganese and zinc.

Increases in total phenolics as well as individual phenolic acidsoccurred in organically grown plants, which also experienceddramatically impeded growth (Table 6). In contrast, plant growthand levels of phenolic compounds were not affected by preventiveapplication of insecticides under either production regime(Tables 4 and 6). Total phenolic content of pac choi was negativelycorrelated with plant nitrogen content (r = −0.43), but nocorrelation was observed between individual phenolic acids andplant nitrogen content.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Pyrethrin Permethrin Water

N c

onte

nt (

g pe

r pl

ant)

Organic fertiliser Conventional fertiliser

a

bb

aa

b

Figure 1. Total nitrogen (N) contents (means and standard errors of fivereplications) in pac choi (cv. Mei Qing Choi) grown with organic andconventional fertilisers and sprayed with organic insecticide pyrethrin,conventional insecticide permethrin or distilled water in greenhouse studyin winter 2004 at Manhattan, Kansas. Different letters indicate significantdifferences among treatment combinations at the 0.05 level.

The greenhouse results largely supported the field experiment inshowing that lower nitrogen availability under organic productionled to a significant increase in phenolic concentration in pacchoi. An experiment on greenhouse-grown leaf mustard hasdemonstrated a decrease in total phenolic levels in response toincreased supply of nitrogen in the nutrient solution.37 Based onthe study by Stewart et al.,16 the influence of organic fertilisationon phenolics might be more prominent in leafy than in fruitingvegetables. Toor et al.38 found that, compared with mineralfertilisation, soluble phenolics in tomatoes were enhanced byfertilising with chicken manure and grass/clover mulch, but theincrease was not statistically significant. It was also noted intheir study that, although plant shoot biomass was lower in theorganic treatment, the yield of red tomatoes was not affectedby fertiliser type. On the other hand, Mitchell et al.34 attributedsignificantly higher concentrations of flavonoids in organic versus

Table 4. Plant fresh weight, dry matter content and plant dry weight of pac choi (cv. Mei Qing Choi) grown with organic (Org.) and conventional(Con.) fertilisers and sprayed with organic insecticide pyrethrin, conventional insecticide permethrin or distilled water in greenhouse study in winter2004 at Manhattan, Kansas

Pyrethrin Permethrin WaterFertiliser Insecticide F × Ia

Biomass Org. Con. Org. Con. Org. Con. effect effect interaction

Fresh weight (g per plant) 285.14 483.36 282.62 476.02 298.30 480.90 P < 0.0001 NS NS

Dry matter content (% w/w) 5.68 4.88 5.48 4.85 5.47 4.97 P < 0.01 NS NS

Dry weight (g per plant) 19.95 29.46 18.99 29.13 20.06 30.05 P < 0.0001 NS NS

a F × I means interaction effect of fertiliser by insecticide.NS, non-significant.

Table 5. Concentrations of some nutrient elements (dry weight basis) in pac choi (cv. Mei Qing Choi) grown organically and conventionally ingreenhouse study in winter 2004 at Manhattan, Kansas

FertilisationP

(mg g−1)K

(mg g−1)Ca

(mg g−1)Mg

(mg g−1)Cu

(µg g−1)Fe

(µg g−1)Mn

(µg g−1)Zn

(µg g−1)

Organic 5.2 9.9 29.5 4.7 5.1 68.0 27.8 77.0

Conventional 2.8 9.0 26.7 3.2 10.1 62.9 60.5 106.2

Significance P < 0.01 NS NS P < 0.01 P < 0.05 NS P < 0.05 P < 0.01


94

5


Table 6. Concentrations of phenolic compounds (mg g−1 dry weight) in pac choi (cv. Mei Qing Choi) grown with organic (Org.) and conventional(Con.) fertilisers and sprayed with organic insecticide pyrethrin, conventional insecticide permethrin or distilled water in greenhouse study in winter2004 at Manhattan, Kansas

Pyrethrin Permethrin WaterFertiliser Insecticide F × Ia

Phenolic compound(s) Org. Con. Org. Con. Org. Con. effect effect interaction

Total phenolicsb 9.9 8.8 8.8 7.8 10.4 7.6 P < 0.01 NS NS

Chlorogenic acid 1.7 1.2 1.6 1.1 2.0 1.2 P < 0.01 NS NS

Ferulic acid 1.7 0.7 1.9 0.4 2.4 0.3 P < 0.0001 NS NS

Sinapic acid 0.7 0.5 1.1 0.5 1.1 0.4 P < 0.01 NS NS

Unknown phenolic acidsb 11.6 6.8 10.9 6.1 15.0 6.0 P < 0.0001 NS NS

a F × I means interaction effect of fertiliser by insecticide.b Expressed as mg chlorogenic acid equivalent g−1 dry weight.NS, non-significant.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

Pyrethrin Permethrin Water

N c

once

ntra

tion

(mg

g-1 d

ry w

eigh

t)

Organic fertiliser Conventional fertiliser

P > 0.05

Figure 2. Nitrogen (N) concentrations (means and standard errors of fivereplications) in pac choi (cv. Mei Qing Choi) grown with organic andconventional fertilisers and sprayed with organic insecticide pyrenthrin,conventional insecticide permethrin or water in greenhouse study inwinter 2004 at Manhattan, Kansas.

conventional processing tomatoes to the soil nitrogen level andavailability associated with compost application. An increase inphenolic content under organic fertilisation was not observed inlettuce as shown in our previous work.20 It is well recognised thatsymbiotic mycorrhizae can improve plant uptake of phosphorusand nitrogen,39 but B. chinensis does not form mycorrhizal roots.40

The absence of mycorrhizal associations in pac choi possiblycontributed to its distinct response to nitrogen supply. Furtherevaluations of phenolic accumulation in pac choi in relation tonutrient management, particularly nitrogen supply, are warranted.In the present greenhouse study, higher sulfur content underconventional fertilisation appeared not to have an effect (Table 1)on phenolics in pac choi, whereas elevated concentrations of sulfurin nutrient solution have been shown to increase total phenoliclevels in greenhouse-grown leaf mustard.37

Differences in phenolic contents of organically produced cropshave been reported as well. Concentrations of flavonoids andphenolic acids in barley leaves dropped as the fertilisation ratesof organic amendments increased.41 In our study, insufficientnitrogen uptake by plants was believed to be responsible for thephenolic increase in organically fertilised pac choi. The effect ofnitrogen on polyphenols in grape skins has been found to varywith the rate of potassium fertilisation.42 In the present study itwas not clear how the uptake of other macro- and micronutrientsmight have contributed to synthesis and degradation of phenolic

compounds in pac choi. In the case of greenhouse-grownstrawberry, compost in potting media combined with adequatefertilisation led to higher contents of phenolic acids, flavonolsand anthocyanins, which was attributed to the increased nutrientavailability and uptake as well as improved chemical and physicalcharacteristics of root media.43 Undoubtedly, factors in additionto nitrogen availability associated with organic fertilisation needto be assessed as to their roles in modifying phenolic compoundsin a variety of crops.

CONCLUSIONSOrganic production significantly increased levels of phenolics inpac choi under both high tunnel and open field environments.These differences were possibly due to the distinct conditions ofnutrient availability and insect attack in the contrasting organic andconventional systems. If phenolic compounds in leafy vegetables,for example, were induced by stresses truly at the expense ofnew plant growth, the significance of phenolic increase in organicsystems might be compromised when organic and conventionalproduction resulted in similar yields. In other words, while higherphenolic content was associated with considerable yield reduction,it remains unclear to what extent phenolic compounds maydiffer when organic and conventional farming exhibit comparableproductivity. Moreover, future work should consider possiblegenetic variation in pac choi response to production environment,since our work considered only one cultivar. The greenhouse studyfurther identified the contribution of low nitrogen availabilityto elevated phenolic content in organically fertilised pac choi.Preventive application of pyrethrin and permethrin insecticidesdid not affect plant phenolics. Well-designed insect feeding studiesneed to be considered to clarify the impact of biotic stresses onphenolic concentrations in pac choi grown in different productionsystems. Future studies of the influence of organic productionpractices on phenolics in a range of crops across seasons andlocations should lead to further understanding in this area.

REFERENCES1 Kalt W, Effects of production and processing factors on major fruit and

vegetable antioxidants. J Food Sci 70:R11–R19 (2005).2 Eberhardt MV, Lee CY and Liu RH, Antioxidant activity of fresh apples.

Nature 405:903–904 (2000).3 Olsson ME, Andersson CS, Oredsson S, Berglund RH and Gustavs-

son KE, Antioxidant levels and inhibition of cancer cell proliferation


94

6


in vitro by extracts from organically and conventionally cultivatedstrawberries. J Agric Food Chem 54:1248–1255 (2006).

4 Dixon RA and Paiva NL, Stress-induced phenylpropanoid metabolism.Plant Cell 7:1085–1097 (1995).

5 Zhao X, Carey EE, Wang W and Rajashekar CB, Does organicproduction enhance phytochemical content of fruit andvegetables? Current knowledge and prospects for research.HortTechnology 16:449–456 (2006).

6 Asami DK, Hong YJ, Barrett DM and Mitchell AE, Comparison of thetotal phenolic and ascorbic acid content of freeze-dried and air-dried marionberry, strawberry, and corn grown using conventional,organic, and sustainable agricultural practices. J Agric Food Chem51:1237–1241 (2003).

7 Ren HF, Endo H and Hayashi T, Antioxidative and antimutagenicactivitities and polyphenol content of pesticide-free and organicallycultivated green vegetables using water-soluble chitosan as a soilmodifier and leaf surface spray. J Sci Food Agric 81:1426–1432(2001).

8 Tarozzi A, Hrelia S, Angeloni C, Morroni F, Biagi P, Guardigli M, et al.,Antioxidant effectiveness of organically and non-organically grownred oranges in cell culture systems. Eur J Nutr 45:152–158 (2006).

9 Veberic R, Trobec M, Herbinger K, Hofer M, Grill D and Stampar F,Phenolic compounds in some apple (Malus domestica Borkh)cultivars of organic and integrated production. J Sci Food Agric85:1687–1694 (2005).

10 Bourn D and Prescott J, A comparison of the nutritional value,sensory qualities, and food safety of organically and conventionallyproduced foods. Crit Rev Food Sci Nutr 42:1–34 (2002).

11 Goldman IL, Kader AA and Heintz C, Influence of production, handling,and storage on phytonutrient content of foods. Nutr Rev57:S46–S52 (1999).

12 Caris-Veyrat C, Amiot MJ, Tyssandier V, Grasselly D, Buret M,Mikolajczak M, et al., Influence of organic versus conventionalagricultural practice on the antioxidant microconstituent contentof tomatoes and derived purees; consequences on antioxidantplasma status in humans. J Agric Food Chem 52:6503–6509 (2004).

13 Brandt K and Mølgaard JP, Organic agriculture: does it enhance orreduce the nutritional value of plant foods? J Sci Food Agric81:924–931 (2001).

14 Bryant JP, Chapin III FS and Klein DR, Carbon/nutrient balance of borealplants in relation to vertebrate herbivory. Oikos 40:357–368 (1983).

15 Herms DA and Mattson WJ, The dilemma of plants: to grow or defend.Q Rev Biol 67:283–335 (1992).

16 Stewart AJ, Chapman W, Jenkins GI, Graham I, Martin T and Crozier A,The effect of nitrogen and phosphorus deficiency on flavonolaccumulation in plant tissues. Plant Cell Environ 24:1189–1197(2001).

17 Stout MJ, Brovont RA and Duffey SS, Effect of nitrogen availabilityon expression of constitutive and inducible chemical defenses intomato, Lycopersicon esculentum. J Chem Ecol 24:945–963 (1998).

18 Daniel O, Meier MS, Schlatter J and Frischknecht P, Selected phenoliccompounds in cultivated plants: ecologic functions, healthimplications, and modulation by pesticides. Environ Health Perspect107:109–114 (1999).

19 Lydon J and Duke SO, Pesticide effects on secondary metabolism ofhigher plants. Pestic Sci 25:361–373 (1989).

20 Zhao X, Iwamoto T and Carey EE, Antioxidant capacity of leafyvegetables as affected by high tunnel environment, fertilizationand growth stage. J Sci Food Agric 87:2692–2699 (2007).

21 Zhao X, Young JE, Wang W, Iwamoto T and Carey EE, Influences oforganic fertilization, high tunnel environment, and postharveststorage on phenolic compounds in lettuce. HortScience 42:71–76(2007).

22 USDA. USDA Database for the Flavonoid Content of Selected Foods.Release 2.1 [Online]. US Department of Agriculture (2007). Avail-able: http://www.nal.usda.gov/fnic/foodcomp/Data/Flav/Flav02-1.pdf [access date: 5 February 2009].

23 Jarrell WM and Beverly RB, The dilution effect in plant nutrition studies.Adv Agron 34:197–224 (1981).

24 Young JE, Zhao X, Carey EE, Welti R, Yang S and Wang W,Phytochemical phenolics in organically grown vegetables. Mol NutrFood Res 49:1136–1142 (2005).

25 Drinkwater LE, Letourneau DK, Workneh F, Van Bruggen AHC andShennan C, Fundamental difference between conventionaland organic tomato agroecosystems in California. Ecol Appl5:1098–1112 (1995).

26 Werner MR, Soil quality characteristics during conversion to organicorchard management. Appl Soil Ecol 5:151–167 (1997).

27 Fjelkner-Modig S, Bengtsson H, Stegmark R and Nystrom S, Theinfluence of organic and integrated production on nutritional,sensory and agricultural aspects of vegetable raw materials forfood production. Acta Agric Scand B 50:102–113 (2000).

28 Mader P, Fliebach A, Dubois D, Gunst L, Fried P and Niggli U, Soilfertility and biodiversity in organic farming. Science 296:1694–1697(2002).

29 Chiesa A, Frezza D, Moccia S, Oberti A, Fraschina A and Diaz L,Vegetable production technology and postharvest quality. ActaHort 682:565–572 (2005).

30 Warman PR and Havard KA, Yield, vitamin and mineral contents oforganically and conventionally grown carrots and cabbage. AgricEcosyst Environ 61:155–162 (1997).

31 Ferreres F, Valentao P, Llorach R, Pinheiro C, Cardoso L, Pereira JA,et al., Phenolic compounds in external leaves of tronchudacabbage (Brassica oleracea L. var. costata DC). J Agric Food Chem53:2901–2907 (2005).

32 Sousa C, Pereira DM, Pereira JA, Bento A, Rodrigues MA, Dopico-Garcia S, et al., Multivariate analysis of tronchuda cabbage (Brassicaoleracea L. var. costata DC) phenolics: influence of fertilizers. J AgricFood Chem 56:2231–2239 (2008).

33 Dimberg LH, Gissen C and Nilsson J, Phenolic compounds in oat grains(Avena sativa L.) grown in conventional and organic systems. Ambio34:331–337 (2005).

34 Mitchell AE, Hong YJ, Koh E, Barrett DM, Bryant DE, Denison RF, et al.,Ten-year comparison of the influence of organic and conventionalcrop management practices on the content of flavonoids intomatoes. J Agric Food Chem 55:6154–6159 (2007).

35 Hakkinen SH and Torronen AR, Content of flavonols and selectedphenolic acids in strawberries and Vaccinium species: influence ofcultivar, cultivation site and technique. Food Res Int 33:517–524(2000).

36 Lucarini M, Carbonaro M, Nicoli S, Aguzzi A, Cappelloni M, Ruggeri S,et al., Endogenous markers for organic versus conventional plantproducts. Session VI: Quality Assessment. Agri-Food Quality II: QualityManagement of Fruits and Vegetables, pp. 306–310 (2005).

37 Li J, Zhu Z and Gerendaacutes J, Effects of nitrogen and sulfur on totalphenolics and antioxidant activity in two genotypes of leaf mustard.J Plant Nutr 31:1642–1655 (2008).

38 Toor RK, Savage GP and Heeb A, Influence of different types offertilizers on the major antioxidant components of tomatoes. J FoodCompos Anal 19:20–27 (2006).

39 George E, Marschner H and Jakobsen I, Role of arbuscular mycorrhizalfungi in uptake of phosphorus and nitrogen from soil. Crit RevBiotechnol 15:257–270 (1995).

40 Yost RS and Fox RL, Contribution of mycorrhizae to P nutrition of cropsgrowing on an oxisol. Agron J 71:903–908 (1979).

41 Nørbaek R, Aaboer DBF, Bleeg IS, Christensen BT, Kondo T andBrandt K, Flavone C-glycoside, phenolic acid, and nitrogen contentsin leaves of barley subject to organic fertilization treatments. J AgricFood Chem 51:809–813 (2003).

42 Delgado R, Martın P, Alamo M and Gonzalez MR, Changes in thephenolic composition of grape berries during ripening in relationto vineyard nitrogen and potassium fertilisation rates. J Sci FoodAgric 84:623–630 (2004).

43 Wang SY and Lin HS, Compost as a soil supplement increases thelevels of antioxidant compounds and oxygen radical absorbancecapacity in strawberries. J Agric Food Chem 51:6844–6850 (2003).


Documents

Comparison of phenolic acids in organically and conventionally grown pac choi (Brassica rapa L. chinensis)