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Food Research International

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Minimal nitrogen and water use in horticulture: Effects on qualityand content of selected nutrients

Dario Stefanelli a,*, Ian Goodwin b, Rod Jones a

a Knoxfield Centre, Department of Primary Industries, Victoria, Private Bag 15, Ferntree Gully DC, VIC 3180, Australiab Tatura Centre, Department of Primary Industries, Victoria, Private Bag 1, Tatura, VIC 3616, Australia

a r t i c l e i n f o

Article history:Received 17 February 2010Accepted 21 April 2010Available online xxxx

Keywords:Sustainable horticultureNutritional qualityPolyphenolsAntioxidantsAscorbic acidHuman health

0963-9969/$ - see front matter Crown Copyright � 2doi:10.1016/j.foodres.2010.04.022

* Corresponding author. Tel.: +61 3 9210 9225; faxE-mail address: dario.stefanelli@dpi.vic.gov.au (D.

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a b s t r a c t

Horticultural production has primarily focused on increasing productivity through intensification of fer-tilizers and water, resulting in high environmental costs. Recent research indicates that high nitrogenapplications can have significantly negative effects on produce quality as well as secondary plant metab-olite and vitamin content within fruits and vegetables. Consumer awareness of both sustainable produc-tion practices and nutritional content within fruits and vegetables has risen dramatically in recent years,indicating that there is a likely market for produce grown with minimal nitrogen and water inputs. Thisreview investigates the effects on produce quality of nitrogen and water applications in fruit and vege-table crops, with specific emphasis on the content of flavonoids, carotenoids, glucosinolates and ascorbicacid. The link between the nutritional quality of horticultural crops and the environmental and social sus-tainability of reducing nitrogen and water is considered and discussed.

Crown Copyright � 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Agricultural research and development in the past 50 years hasprimarily focused on increasing productivity through intensifica-tion of input use to firstly maximise grower income and, secondlyminimize hunger and food shortages (Andow et al., 2009;Benbrook, 2009). The full environmental costs of this policy suchas water and soil pollution, soil and resources impoverishment(Tegtmeier & Duffy, 2004), only recently become apparent, and thisawareness has led to the beginning of a change in philosophy awayfrom purely production-oriented practices towards those that aremore environmentally sustainable (Alexanian, Metera, & Schuler,2009). In addition, agriculture and its interactions with the naturalenvironment have begun to emerge more strongly in political, eco-nomic and social agendas reflecting the increasing importance ofsustainability to consumers (Poincelot, Francis, & Bird, 2006).Western consumers are now demanding foods that are of highquality, locally produced, regionally specialized, organic, fairlytraded, and are humanely and ethically produced (Burt et al.,2009a; Kirschenmann & Bird, 2006; Peck, Andrews, Reganold, &Fellman, 2006; Rosen & Allan, 2007). Achieving all, or some, ofthese goals with fewer resources is now the objective of modernsustainable agriculture (Mason, 2003) and success will largely de-pend on the type of production technology applied (Van Ittersum &Rabbinge, 1997).

010 Published by Elsevier Ltd. All r

: +61 3 9800 3521.Stefanelli).

et al. Minimal nitrogen and watj.foodres.2010.04.022

Modern agriculture is generally divided into two main produc-tion systems: production systems directed by the demand for‘‘conventional” products resulting in high volumes of standardquality; and ‘‘alternative” systems directed by the supply of spe-cific products, identified by their origin or their production systemse.g. regional or organic production (Loyat, 2006). In Europe theprincipal alternative agrosystems are: organic farming, integratedproduction, conservation agriculture and agriculture under guar-anteed quality (Kahiluoto & Roetter, 2009). Consumer demand isincreasingly leading producers towards environmentally sustain-able production systems, with no reduction infood quality, safety,and with an increasing focus on the relationship between dietand health to combat serious Western diseases, such as diabetes,cancer and heart disease (Coulston, 1999; EC, 2005). Trout, Francis,& Barbuto (2006) and Andow et al. (2009) state that social out-comes are now a desired goal for agricultural production and thepractices adopted to achieve them are defined as ‘sustainable’. Itcan therefore be argued that by improving or maintaining producequality with reduced production inputs the industry can move to-wards the sustainability goals now demanded by consumers of re-duced environmental impacts and increased consumer health andwellbeing.

These consumer demands can be met by: reducing the environ-mental impact by minimizing inputs, particularly chemicals usedin nutrition and disease management; improving the nutritionalcomposition of food for health purposes; and developing andapplying methods to remove anti-nutrients, allergens and toxinsand, environmental sustainability can be reached by reducing

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fertilizers, chemicals, water, and fuel while minimizing reductionsin yield (Andow et al., 2009; Azais-Braesco, Goffi, & Labouze, 2006;Roberfroid, 2002; Weibel, Treutter, Graf, & Häseli, 2004). Researchcan facilitate improvements in the nutritional quality of food bymore clearly linking plant physiology during production with thedevelopment of plant foods with higher levels of important macroand micronutrients (essential fatty acids, oils, vitamins, aminoacids, antioxidants, fibers), resulting in improved managementpractices, better taste and quality, improved post-harvest qualityand storage performance; and high pest and disease resistance toreduce the risk of residues (Atkinson, Nestbyb, Forda, & Doddsa,2005; Hoefkens et al., 2009; Luby & Shaw, 2009; Marcelle, 1995).

It is clear that growing produce primarily for yield has had a sig-nificant detrimental effect not only on the environment, but alsoon particular aspects of produce quality. Current methods of bulkproduction are reducing nutritional quality by diluting mineralcontent due to the high applications of inputs (especially fertiliz-ers) to achieve faster growth (Benbrook, 2009; Davis, 2009;Ganeshamurthy, Reddy, Anjaneyulu, & Kotur, 2004). According toMason (2003) and Strapatsa, Nanos, and Tsatsarelis (2006), envi-ronmental sustainability can be reached by reducing fertilizers,chemicals, water, and fuel while minimizing reductions in yield.This review therefore focuses on the connection between high vol-ume horticultural production and the consumer-driven qualityrequirements of sustainable production and healthy produce withthe use of production practices that reduce inputs and maintaintraditional quality and yield. We will investigate whether it is pos-sible to reduce the quantity of two major inputs in agriculture,water and nitrogen (N) fertilization without reducing producequality, while keeping yield reduction to a minimum. By describingthe role that these two agricultural inputs play in determining thequality and stability of horticultural produce, we aim to show thatreducing N and water inputs is not only beneficial for the environ-ment, but can have positive impacts on certain quality parameters,especially human health-related compounds, which is a highlydesirable social outcome. In this review we would like to focuson the effect of N on fruit and vegetable nutritional quality, specif-ically on ascorbic acid (vitamin C) and selected secondary metabo-lites: flavonoids, carotenoids and glucosinolates. There isincreasing evidence that secondary metabolites, such as polyphe-nolic compounds, carotenoids and glucosinolates contained withinfruits and vegetables, may protect against serious diseases, includ-ing cardiovascular disease and certain cancers if consumed regu-larly (Hu, 2003; Duthie, Duthie, & Kyle, 2000; Barillari, Rollin, &Iori, 2002; Birt, Hendrich, & Wang, 2001). In the Western diet fruitsand vegetables are a major dietary source of polyphenols, particu-larly flavonoids (Nijveldt et al., 2001; Rao & Rao, 2007; Tsao &Akhtar, 2005), moreover consumption of fruit and vegetables thatare high in phytochemicals and vitamins would also enhance thehealth of underdeveloped nations (FAO, 2006).

2. Nitrogen

The past 60 years have seen the use of Nitrogen/Phosphorous/Potassium (NPK)-based fertilizers increase dramatically aroundthe world, particularly in North America and Europe (Burt et al.,2009b). While use in Europe is now decreasing, N usage in NorthAmerica is still on the rise, albeit at a slower rate (Burt et al.,2009a). Leaching of N and phosphorous (P) due to excessive appli-cations in agriculture are widely considered the main cause ofeutrophication in fresh and salt water supplies throughout theworld (Burt et al., 2009b; Erhart, Feichtinger, & Hart, 2007). Severalpractices have recently been implemented to try to reduce groundwater pollution by N, such as split applications (Drake, Raese, &Smith, 2002), target timing (Tagliavini & Marangoni, 2002; Wade,

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Holzapfel, Degaris, Williams, & Keller, 2004), integrated production(Ganeshamurthy et al., 2004; Peck et al., 2006), fertigation(Neilsen, Neilsen, & Herbert, 2009; Raina, Thakur, Shashi, & Spehia,2005), best management practices (Alva, Paramasivam, Fares,Obreza, & Schumann, 2006; Alva, Paramasivam, Obreza, &Schumann, 2006), foliar applications (Dong, Neilsen, Neilsen, &Fuchigami, 2005; Reay, Fletcher, & Thomas 1998), and varyingthe type of N sources (El-Boray, Mostafa, Iraqi, & Mohamed,2006; Erhart et al., 2007; Racskó et al., 2008; Rosen & Allan,2007). While not all of the above mentioned practices reducedthe total amount of N applied; all were effective in limiting pollu-tion by increasing plant N use efficiency.

Fertilizers are extremely important factors in determining hor-ticultural crop yield, quality and nutritional content (Martinez-Ballestra et al., 2008). While many decades of research have helpedelucidate ideal NPK applications (and other minor nutrients) formost horticultural crops, these studies have tended to concentrateprimarily on crop yield (Benbrook, 2009). The effects of fertilizerapplications on nutrient and phytonutrient content in horticulturalcrops, however, have not been widely studied and are not yet fullyunderstood. The organic movement has tried for many years to re-late their production and fertilization methods to improved nutri-tional quality and there is now strong debate on the merits oforganic vs. conventional production systems with regards to levelsof minerals, phytonutrients and vitamins. This review does notpropose to enter this debate but only to comment that there areseveral papers stating that organic production has the capabilityto increase vitamin C, polyphenols, anthocyanins, antioxidants,and mineral content in fruits and vegetables (Benbrook, 2009; Bru-ulsema, 2002; Peck et al., 2006; Weibel et al., 2004; Worthington,2001) and several that found no effect (Carbonaro, Mattera, Nicoli,Bergamo, & Cappelloni, 2002; Dangour et al., 2009; Hoefkens et al.,2009).

2.1. Applied N and quality

All major quality attributes in horticultural crops, including vi-sual quality and taste, are directly influenced by N availability(Locascio, Wiltbank, Gull, & Maynard, 1984). Sufficient N is essen-tial for normal plant growth and development, being an integralpart of protein development and chloroplast structure and function(Barker & Bryson, 2007). While N deficiency symptoms have beenwell characterized for all common horticultural crops (Mengel,Kirkby, Kosegarten, & Appel, 2001), the effects of over-supply ofN on quality are not as widely studied. Furthermore, most pub-lished studies consider the effects of much increased or decreasedN compared to standard commercial application levels, making itdifficult to determine the more subtle effects of slight reductionsin N applications on quality and yield.

In vegetable crops, excessive N generally leads to an over-emphasis on vegetative growth, often to the detriment of root orfruit development (Mengel et al., 2001), but has little impact on vi-sual quality (Locascio et al., 1984). In root crops, high N can lead toreduced sensory quality, for example sugar content can be reducedin potato and sugar beet if excessive N is applied late in the grow-ing season (Mengel et al., 2001). Nitrogen has an inverse relation-ship with specific gravity, starch content and % dry matter inpotatoes (Locascio et al., 1984), and minimal N applications areideal for potatoes best suited for processing. In vegetable cropsarising from fruiting, high N can also lead to poor fruit set, suchas in glasshouse-grown tomatoes, as well as reduced sugars andresultant poor taste (Dorais & Papadopoulos, 2001). High N canalso indirectly delay tomato fruit development due to increasedshading caused by high vegetative growth (Locascio et al., 1984).In leafy vegetable crops, such as lettuce, nitrate accumulation canbecome a significant problem when grown with high N availability

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and low light (Demsar, Osvald, & Vodnik, 2004), as nitrates can beconverted to deleterious nitrites post-harvest (Maynard & Barker,1979). Furthermore, other lettuce quality indices (dry matter, su-gar and vitamin C content) declined in Crisphead lettuce grown un-der 200 kg ha�1 applied N (Poulsen, Johansen, & Sorensen, 1995).Butterhead, romaine and oak leaf lettuce quality, as perceived bya sensory panel, was maximized by as little as 80 kg ha�1, signifi-cantly less than the normal recommended nitrogen applicationrate for field-grown lettuce in Italy (D’Antuono & Neri, 2001).Head-forming leafy vegetables, such as cabbage and broccoli canform split heads under high N (Locascio et al., 1984). Rapid vegeta-tive growth under high N conditions can also lead to ‘succulent’leafy crops that are more prone to disease attack and have shorterpost-harvest shelf-life (Locascio et al., 1984; B. Tomkins, pers.comm.). High N application can also stimulate fungal diseaseseverity (Mengel et al., 2001).

Similarly, correlations between N applications and fruit qualityhave focused primarily on internal (firmness, sugar, acid content,ripening and storability) and external (color and form) fruit qual-ity. Several studies have consistently reported the negative effectshigh N applications had on reducing red color or increasing thegreen background of apples (Marcelle, 1995; Neilsen et al., 2009;Tahir, Johansson, & Olsson, 2008) due to a decrease in both chloro-phyll degradation and anthocyanin synthesis in the skin (Wang &Cheng, 2009). High N also reduced storage performance in apple(Fallahi, Fallahi, & Seyedbagheri, 2006) and kiwi fruits (Vizzotto,Lain, & Costa, 1999). Susceptibility to a range of physiological dis-

Table 1Optimal nitrogen (N) application to maximize the concentration of the specific phytocexperimental conditions. The most relevant reference is cited.

Phytochemical considered Reference

PhenolicsApple Reay et al. (1998)Basil Nguyen and Niemeyer (2008)Bilberry Åkerström, Forsum, Rumpunen, Jäderlund,Broccoli Jones et al. (2007)Capsicum Wang, He, Chen, Geng, and Tong (2009)Chinese cabbage Zhu et al. (2009)Grapes (wine) Hilbert et al. (2003)

Keller and Hrazdina (1998)Olive Fernandez-Escobar et al. (2006)

Morales-Sillero et al. (2008)Onion Mogren et al. (2007)Pak choi Zhao et al. (2007)Tomato Bernard et al., 2009Strawberry Anttonen et al., 2006

CarotenoidsApple Reay et al. (1998)Cabbage Rosen et al. (2005)Carrot Hochmuth et al. (1999)Lettuce Coria-Cayupan et al. (2009)Kale Kopsell et al. (2007b)Parsley Chenard, Kopsell and Kopsell (2005)Tomato Simonne et al. (2007)Watercress Kopsell et al. (2007a)

GlucosinolatesBroccoli Jones et al. (2007)

Omirou et al. (2009)Pak choi Shattuck and Wang (1994)Watercress Kopsell et al. (2007a, 2007b)

Ascorbic acidCabbage Sorensen (1984)Cauliflower Lisiewska and Kmiecik (1996)Lettuce Chiesa et al. (2009)

Poulsen et al. (1995)Spinach Mozafar (1996)Tomato Benard et al. (2009)

Simonne et al. (2007)

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orders increased with high N fruit content, such as apple watercore(Fallahi, Fallahi, Retamales, Valdés, & Tabatabaei, 2006; Marcelle,1995); apple fruit softening (Marcelle, 1995; Neilsen et al., 2009;Sams, 1999); melon fruit softening (Cabello, Castellanos, Romojaro,Martínez-Madrid, & Ribas, 2009) and decreased apple acidity(Raese, Drake, & Curry, 2007). In addition, high N increased suscep-tibility to cactus pear fungal diseases (Ochoa, Leguizamón, & Uhart,2006), and reduced pear tree cold tolerance (Raese, 1997) whileQuaggio, Mattos, Cantarella, Almeida, and Cardoso (2002) reporteda decrease in essential oil content of lemon fruits.

Several authors, however, reported that there were no signifi-cant effects of N fertilization levels on internal and external fruitquality indices in blackberry (Alleyne & Clark, 1997), peach (Olie-nyk et al., 1997), apple (Cmelik, Tojnko, & Unuk, 2006), apricot(Raina et al., 2005), wine grapes (Neilsen, Stevenson, & Gehringer,1987), and cherry (Neilsen, Neilsen, Kappel, & Toivonen, 2005). Allof these studies, however, were consistent in concluding that N fer-tilization in fruit production should be reduced to a minimum levelwithout affecting yield, independent of the positive or neutral ef-fects on internal and external quality.

2.2. Effects of N fertilization on secondary metabolites and ascorbicacid in fruit and vegetables

2.2.1. Flavonoids and other phenolicsThe effects of N application rates on flavonoids are summarized

in Table 1, showing the range of applied N and the rate at which

hemical. The optimal and nitrogen range reported are specific to the specie and

N range Optimal N

0–1.0% urea 0% urea0.1–5.0 mM 0.1 mM

and Bång (2009) 12.5–50 kg ha�1 No effect0–250 kg ha�1 0 kg ha�1

96–288 kg ha�1 96 kg ha�1

0–300 mg kg�1 0 mg kg�1

1.4–7.2 mM 1.4 mM0–90 kg ha�1 0–60 kg ha�1

0–1.5 kg/tree 0 kg/tree0.2–0.6 kg/tree No effect72–152 kg ha�1 No effect0–156 kg ha�1 0 kg ha�1

4–12 mM 4 mM0.6–2.4 mS cm�1 0.6 mS cm�1

0–1.0% urea 1% urea125–250 kg ha�1 125 kg ha�1

150–180 kg ha�1 160 kg ha�1

0–1.6 g/100 g 0.46 g/100 g6–105 mg L�1 105 mg L�1

6–105 mg L�1 105 mg L�1

0–392 kg ha�1 0–78 kg ha�1

6–106 mg L�1 56 mg L�1

0–250 kg ha�1 30 kg ha�1

50–600 kg ha�1 250 kg ha�1

14–224 mg L�1 14 mg L�1

6–106 mg L�1 56 mg L�1

0–600 kg ha�1 0 kg ha�1

80–120 kg ha�1 80 kg ha�1

0–150 kg ha�1 0 kg ha�1

50–200 kg ha�1 50 kg ha�1

1.5–15 mmol L�1 1.5 mmol L�1

4–12 mmol L�1 4 mmol L�1

0–392 kg ha�1 0–78 kg ha�1

er use in horticulture: Effects on quality and content of selected nutrients.

Increased Carbohydrate Concentration

Increased C-based Compounds (eg 2o

metabolites)

Decreased Photosynthesis

Low N Availability

Decreased N Absorption

Decreased N Concentration

Decreased Growth

Decreased N-based Compounds(eg alkaloids)

Normal Light & CO2

Fig. 1. Effect of low N availability on carbon–nutrient relationships of plants and onconcentrations of nitrogen- and carbon-based compounds (after Bryant et al.,1983). Arrows between boxes indicate a positive effect, unless crossed with arectangle, which indicates a negative effect. Thickness of arrows indicates magni-tude of effect.

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optimal flavonoid content was reached. This table is a summary ofpublished results; studies that confirm results presented in Table 1are presented in the text below.

It is notable that the lowest N application rate resulted in high-est flavonoid content in 11 out of 13 studies, the exceptions beingno effect in bilberries (Åkerström, Forsum, Rumpunen, Jäderlund, &Bång, 2009), onions (Mogren, Olsson, & Gertsson, 2007) and olives(Morales-Sillero, Jimenez, Fernandez, Troncoso, & Rejano, 2008)(Table 1). Low N applications in ‘Merlot’, ‘Grignolino’, and ‘Shiraz’grapes resulted in an increase in total anthocyanins, comparedwith plants fed high N (Table 1) (Hilbert et al., 2003; Malusà, Laur-enti, Ghibaudi, & Rolle, 2004; Wade et al., 2004). Furthermore, highN availability also affected wine quality by reducing total phenolic,anthocyanin and flavonol contents (Table 1) (Keller & Hrazdina,1998). Significant N stress (zero application), or N applications thatsignificantly depressed yield resulted in the highest content of phe-nolic compounds in Chinese cabbage (Zhu, Lin, Jin, Zhang, & Fang,2009), broccoli heads (Jones, Imsic, Franz, & Tomkins, 2007), apple(Reay et al., 1998), basil (Nguyen & Niemeyer, 2008), pak choi(Zhao, Carey, Young, Wang, & Iwamoto, 2007), lettuce (Coria-Cayu-pan, Sanchez de Pinto, & Nazareno, 2009), tomato (Be’nard et al.,2009)., olives (Fernandez-Escobar et al., 2006), and strawberries(Anttonen, Hoppula, Nestby, Verheul, & Karjalainen, 2006) (Ta-ble 1). Polyphenol concentration declined as N availability in-creased in olives, diminishing the resulting oil quality (Table 1)(Fernandez-Escobar et al., 2006; Morales-Sillero et al., 2008). To-mato fruit antioxidant capacity and vitamin C content was highestin plants supplied with N as chicken manure or grass/clover mulchcompared with nitrate (NO�3 )/ammonium (NHþ4 ) fertilizers (Toor &Savage, 2006), while fruit dry matter and Total Soluble Solids wereno different. While all the above studies indicate that increased Ngenerally resulted in lower flavonoid content, no studies have beenfound attempting to identify an optimal N application rate thatstrikes a ‘balance’ where yield is maintained and flavonoid contentis optimized.

Flavonol content did not differ significantly in strawberriesgrown under conventional or organic production systems (Häkki-nen & Törrönen, 2000). Similarly, Zhao et al. (2007) found no differ-ences in phenolic content in lettuce grown conventionally or underorganic guidelines. Organic production was reported to improvephenolic content in peaches and pears (Carbonaro et al., 2002; Fau-riel, Bellon, Plenet, & Amiot, 2007), and in two tomato varieties, butnot in peppers (Chassy, Bui, Renaud, Van Horn, & Mitchell, 2006).

Flavonoid accumulation can be induced by a number of envi-ronmental conditions, including UV light (Winkel-Shirley, 2002),pathogen attack (Dixon & Paiva, 1995) and nutrient deficiencies(Bongue-Bartelsman & Phillips, 1995; Stewart et al., 2001). In-creased flavonoid accumulation under low N availability has beenwidely reported, for example in tomato leaves (Stewart et al.,2001), broccoli (Jones et al., 2007) and basil leaves (Nguyen &Niemeyer, 2008). N depletion is known to stimulate activation ofthe flavonoid pathway via a number of genes and enzymes(Bongue-Bartelsman & Phillips, 1995), but does not have a pro-nounced effect on the shikimate pathway which produces phenyl-alanine (Lillo, Lea, & Ruoff, 2008). The link between primary andsecondary metabolic pathways in plants is postulated to be phen-ylalanine ammonia-lyase (PAL) as increases in PAL activity parallelhigh levels of flavonoids (Lillo et al., 2008). Furthermore PAL re-leases nitrogen from phenylalanine, thereby providing N for redis-tribution when plants are under N stress, making PAL a keyenzyme in survival under N deficiency (Olsen et al., 2009). It couldalso be expected that N deficiency to the point of stress withinplants could increase phenolic compounds, particularly flavonoidsas they may be involved in the hydrogen peroxide-scavenging cas-cade (Takahama & Oniki, 1997). Manipulation of N availabilitytherefore could be a powerful tool for stimulating secondary plant

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metabolite production, particularly in glasshouse-grown crops(Lillo et al., 2008). However, Unuk, Tojnko, Cmelik, and Stopar(2006) reported that N fertilization did not affect the major regula-tory factors in polyphenolic formation in apples.

Flavonoid accumulation under N stress can be at least partly ex-plained by the carbon/nutrient balance (CNB) hypothesis (Bryant,Chapin, & Klein, 1983) (Fig. 1), which postulated that limited Navailability results in lower N uptake, causing a reduction ingrowth and photosynthesis, which leads to both a decrease in N-based secondary metabolites (eg alkaloids) and an increase in theavailability of carbon which leads to increased production of car-bon-based secondary metabolites eg flavonoids. Conversely, in-creased N availability causes an increase in photosynthesis andprimary plant growth at the expense of the synthesis of carbon-based secondary metabolites (Fig. 1). While initially postulated toexplain increased plant chemical protection against herbivores,this theory is now more relevant as the carbon-based compoundsthat increase with restricted N are not only herbivore deterrents,but have also been recently implicated as having a putative rolein human health, eg flavonoids and glucosinolates. Limited N couldalso result in higher phenylalanine availability for flavonoid syn-thesis, as the decline in protein synthesis resulting from a decreasein cell growth and metabolism (Sato, Nakayama, & Shigeta, 1996)would free up amino acid availability. In addition, N-stressedplants have lower chlorophyll levels and starch accumulation inphotosynthetic membranes, leading to greater sensitivity to light,which could be compensated for by higher flavonol production inphotosynthetic tissues (Guidi et al., 1998; Reay et al., 1998).

2.2.2. CarotenoidsThe specific effects of N nutrition on carotenoid synthesis are

better understood (Table 1), particularly in tomato fruits. AppliedN at the highest rate generally resulted in higher carotenoids in ap-ples (Reay et al., 1998), carrot (Hochmuth, Brecht, & Bassett, 1999),kale (Kopsell, Kopsell, & Curran-Celentaro, 2007), while mid-rangeapplications were most effective in tomatoes (Dorais, 2007; Simo-nne, Fuzere, Simonne, Hochmuth, & Marshall, 2007), lettuce (Coria-Cayupan et al., 2009) and watercress (Kopsell, Barickman, Sams, &McElroy, 2007) (Table 1). A decrease in NO�3 from 12 to 4 mM/plantresulted in a slight reduction in tomato yield (7.5%), and an in-crease in fruit eating quality as determined by lower fruit acidityand higher soluble sugars (Be’nard et al., 2009), but there was no

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effect of reduced N on carotenoids. Increasing N applications tofield-grown tomatoes from 50 to 250 kg ha�1 increased total fruityield but had no significant effect on marketable quality (Parisi,Giordano, Pentangelo, D’Onofrio, & Villari, 2006). Tomato colorwas unaffected by decreased N, but high N (392 kg ha�1) resultedin lower fruit acidity (Table 1) (Simonne et al., 2007). While resultsvary, it is generally recognized that lycopene content increased intomatoes with lower N (Dumas, Dadomo, Di Lucca, & Grolier,2003), but increased P or K resulted in higher carotenoid contentas well. These results indicate a reduced N application rate is pos-sible in glasshouse or field-grown tomatoes without significant de-cline in yield or fruit quality.

2.2.3. GlucosinolatesThe influence of N and sulphur (S) on glucosinolate content in

broccoli has been widely studied, but similar studies are lackingfor cabbage and cauliflower (Table 1), with the exception of Rosen,Gardener, Hecht, Carmella, and Kenney (2005). Glucosinolate syn-thesis in broccoli requires optimal (i.e. high) S (Rangkadilok et al.,2004). Once S supply is adequate, however, minimal N applicationsresulted in highest glucoraphanin content in broccoli (Table 1)(Jones et al., 2007), but N application rates that optimized gluco-raphanin content in broccoli resulted in a significant decline inhead weight (Jones et al., 2007) In a study with higher N applica-tion rates (50–600 kg ha�1), Omirou et al. (2009) found glucoraph-anin content in broccoli florets was maximized with a mid-range Napplication of 250 kg ha�1. Growth stress caused by limited N alsoresulted in enhanced glucosinolate synthesis in pak choi (Shattuck& Wang, 1994), watercress (Kopsell et al., 2007a, 2007b), and cab-bage (Rosen et al., 2005) (Table 1).

Jones et al. (2007) postulated that poor N availability in broccoliflorets may indirectly result in an increased availability of methio-nine for glucosinolate production, as suggested by Dick-Hennesand Buning-Pfaue (1992). While it is still not completely clear whateffect limited N has on methionine and tryptophan availability forglucosinolate synthesis, it is again possibly partly explained by theCBN hypothesis (Fig. 1) (Bryant et al., 1983), which is more com-monly used to explain an increase in flavonoids with N stress(see Section 2.2.1). In glucosinolate production, the CNB theorypredicts that a decrease in N supply causes a restriction in plantgrowth thereby reducing the demand for amino acids needed forprotein synthesis. Amino acids, such as phenylalanine, are thenshunted into the flavonoid biosynthesis pathway, and it is possibleexcess methionine, at least, could be shunted to glucosinolate syn-thesis (Jones et al., 2007). Glucosinolates may also play a role inintermediate metabolic storage (Fismes, Vong, Guckert, & Frossard,2000), and could be used by the plant in the synthesis of primarycompounds when S is limited, thereby reducing their content (Sch-nug & Haneklaus, 2000).

2.2.4. Ascorbic acidIn general decreased N application rates resulted in higher vita-

min C content in fruits and vegetables (Table 1; for review see Lee& Kader, 2000), but responses were variable depending on genus,climate and other factors. N applied at low rates resulted in higherascorbic acid content in cabbage (Sorensen, 1984), cauliflower (Lis-iewska & Kmiecik, 1996), lettuce (Chiesa, Mayorga, & Leon, 2009),spinach (Mozafar, 1996) and tomato (Simonne et al., 2007) (Ta-ble 1). Increased N also resulted in lower ascorbic acid in tomatofruits (Dumas et al., 2003), while a decrease in NO�3 applicationscaused a significant increase in ascorbic acid content (Be’nardet al., 2009) (Table 1). However, when N was applied at sub-opti-mal levels ascorbic acid content tended to drop slightly, indicatingsufficient N was required to maintain ascorbic acid synthesis (Ta-ble 1) (Mozafar, 1994).

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Increased N applications result in increased vegetative growthand larger fruits, so that the decline in ascorbic acid could, in part,be due to a dilution effect. Smaller cabbages, for example, also con-tained higher levels of ascorbic acid (Murneek & Wittwer, 1948).On the other hand, plants supplied with 600 kg ha�1 N producedcabbages 3� larger than plants supplied with 0 kg ha�1, but ascor-bic acid content was 34% lower (Table 1) (Sorensen, 1984). In-creased vegetative growth can also result in increased shadingeffects of fruit, with reduced direct light resulting in lower ascorbicacid in shaded fruits, such as tomatoes and apples (Mozafar, 1994).

A major disadvantage of high levels of applied N to leafy vege-tables in particular is the resultant high nitrate content. Transientnitrogen deprivation in spinach close to harvest reduced nitratelevels and increased ascorbic acid (Table 1) (Mozafar, 1996). Thishas also been demonstrated in hydroponic lettuce plants that weretransferred to low N solutions for 2–7 days before harvest(Shinohara & Suzuki, 1988). Longer periods in low N (14 days) alsoreduced nitrates and increased ascorbic acid in lettuce and spinach,but significantly reduced yield (Table 1) (Mozafar, 1994). There isalso evidence that applied N as NHþ4 caused a stronger decline inascorbic acid compared to NO�3 applications, but this is a species-specific effect (Mozafar, 1994).

In general it can be deduced that it is possible to directly influ-ence nutrient concentrations in the edible parts of fruit and vege-tables by careful manipulation of N availability and thatfertilization management should not only focus on maximizingyield.

3. Water

Water is becoming an increasingly scarce resource in horticul-tural production areas of the world, especially in arid and semiaridregions, such as China, Spain, south eastern Australia and westernUSA (Rouphael, Cardarelli, Colla, & Rea, 2008). Minimizing wateruse in horticulture, while maintaining produce quality and yield,has become a critical issue in some areas due to a dramatic declinein rainfall over the past decade, possibly due to climate change.Avoiding excessive water applications is also a necessary step inreducing the contamination of water tables with excess nutrients,nitrates and pesticides (Burt et al., 2009b). The increased demandof fruit and vegetables outside their normal production periodhas increased crop value and the profitability of using irrigation(Costa, Ortuño, & Chaves, 2007). However, a major challenge forhorticultural producers is the increasing competition for waterfrom urban and industrial users (Turral, Svendsen, & Faures,2010). Due to continuing high population growth, particularly indeveloping countries, it is critical to adopt agricultural practicesthat will reduce usage of limited water resources, but at the sametime maintain yield and quality (FAO, 2006). Therefore it is becom-ing increasingly important to adopt strategies to increase wateruse efficiency in agriculture (Kirda et al., 2007; Turral et al.,2010). Reduced water availability, however, can significantly re-duce yield and quality, so it is necessary to identify optimal waterneeds on a crop by crop basis.

3.1. Reduced water effects on quality

Deficit irrigation (i.e. irrigation below optimal crop waterrequirements) research to improve productivity of horticulturalcrops began in the 1970’s with the aim to control excessive vege-tative vigor in high density orchards. Tree physiology was inten-sively studied to examine timing of water deficits that wouldminimize the impact on fruit growth but maximize effects on shootgrowth. This strategy later became known as regulated deficit irri-gation (RDI). Further studies on peach (Williamson & Coston,

er use in horticulture: Effects on quality and content of selected nutrients.

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1990), European pear (Brun, Raese, & Stahly 1985a, 1985b;Chalmers, Burge, Jerie, & Mitchell, 1986; Mitchel, Chalmers, Jerie& Burge, 1986; Mitchell, Jerie, & Chalmers, 1984; Mitchell, van deEnde, Jerie, & Chalmers, 1989) and Asian pear (Caspari,Behboudian, & Chalmers, 1994) supported the initial findings thatsignificant reductions in water use can be made with minimal ef-fects on yield and quality. RDI was not effective, however, in allfruit crops. For example, studies on apples have shown loss in fruitsize and yield from deficit irrigation (Ebel, Proebsting, & Evans,2001;Landsberg & Jones, 1981; Mpelasoka, Behboudian, & Mills,2001).

In recent years there has been resurgence in the application ofRDI to horticultural crops due to changes in climate resulting in se-vere drought. However, focus has now switched from controllingexcessive vigor to investigating opportunities to stimulateimprovements in fruit quality so that any yield loss can be com-pensated by an increase in crop value.

The effects of RDI on sweetness, acidity, color and firmness havenow been investigated in a range of crops. The majority of studieson deficit irrigation have shown an increase in fruit soluble solids;kiwifruit (Miller, Smith, Boldingh, & Johansson, 1998), watermelon(Leskovar et al., 2007), peach (Crisosto, Johnson, Luza, & Crisosto,1994; Gelly et al., 2004; Li, Huguet, Schoch, & Orlando, 1989), plum(Naor, Peres, Greenblat, Gal, & Ben Arie, 2004), apple (Leib, Caspari,

Table 2Effect of irrigation reduction (water deficit, regulated deficit irrigation, or partial root zonerelevant reference is cited.

Phytochemicalconsidered

Reference Variety – cv. Irrigattype

PhenolicsWine grapes Esteban et al. (2001) Tempranillo Water

deficitMathews & Anderson (1988) Cabernet Franc WaterMathews et al. (1990) Cabernet Franc WaterRoby et al. (2004) Cabernet

SauvignonWater

Castellarin et al. (2007a,2007b)

CabernetSauvignon

Water

Tea Hernandéz et al. (2006) WaterOlives Tognetti et al. (2007) Frantoio and

LeccinoWater

d’Andria et al. (2004) Several Water

Pattumi et al. (1999) Several WaterPattumi et al. (2002) Kalamata WaterGómez-Rico et al. (2007) Cornicabra RDId

Apple Stefanelli et al. (2009) Royal Gala Water

CarotenoidsTomato Matsuzoe et al. (1998) Several Water

Zushi and Matsuzoe (1998) Several WaterDumas et al. (2003) Several Water

Water melon Leskovar et al. (2007) Several Water

GlucosinolatesMustard (B.rapa) Mailer and Cornish (1987) Bunyip RDIRape (B.napus) Jensen et al. (1996) Global RDIPack choi Jones unpublished data Sumo RDI

Ascorbic acidLeeks Sørensen et al. (1995) Imperial RDITomato Dumas et al. (2003) Several Water

Zushi and Matsuzoe (1998) Several WaterRudich et al. (1977) VF 317 RDI

Table grape Du et al. (2008) Rizamat PRDe

Strawberry Dodds et al. (2007) Elsanta RDI an

a Comprehends various aspects of the quality such as soluble sugars, acidity, aminoconditions.

b Effect on the specific phytochemical.c Reduction of water to specific volumes but applied during the whole growing seasod regulated deficit irrigation is the application of various volumes of water at specifice Partial Root Drying is the application of water only to a part of the root system instf Data not available.

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Redulla, Andrews, & Jabro, 2006; Mills, Behboudian, & Clothier,1996; Mpelasoka & Behboudian, 2002), citrus (Navarro,Pérez-Pérez, Romero, & Botía, 2010; Verreynne, Rabe, & Theron,2001), grapes (Du, Kang, Zhang, Li, & Yan, 2008), berry crops(Prange & DeEll, 1997) and tomato (Pulupol, Behboudian, & Fisher,1996). Detailed studies of the effects of RDI on specific sugars haveshown an increase in sorbitol and fructose concentration (Behbou-dian & Lawes, 1994; Mills et al., 1996) and total soluble sugar con-centration (Failla, Zocchi, Treccani, & Cocucci, 1992) particularlywhen RDI was applied early in the season.

In most cases the increases in fruit soluble solids can be attrib-uted to a lower fruit water content (solute concentration), how-ever, some studies reported increases that cannot be explainedsolely by a concentration effect in which an increased activesynthesis was involved (Failla et al., 1992; Mills et al., 1996; Roby,Harbertson, Adams, & Matthews, 2004; Yakushiji, Morinaga, &Nonami, 1998). Both Failla et al. (1992) and Roby et al. (2004) con-cluded that there was an active physiological mechanism in play inapples and grapes that increased soluble sugars when water defi-cits were imposed during the cell enlargement phase but thismechanism fails under severe water stress.

The effects of RDI on acidity, firmness and color vary widely.There was no affect on peach fruit acidity (Crisosto et al., 1994;Gelly et al., 2003; Li et al., 1989). In apple fruit, Mills, Behboudian,

drying) on fresh produce quality, productivity and specific phytochemicals. The most

ion Productivity Qualitya effect Phytob effect

cDecreased Increased Increased

deficit Decreased No effect Increaseddeficit NAf No effect Increaseddeficit NA Increased Increased

deficit Decreased Increased Increased

deficit NA Increased Increaseddeficit Decreased No effect No effect

deficit No effect or decreased No effect No effect ordecreased

deficit No effect or decreased No effect Increasedeficit No effect or decreased No effect Increased

No effect Increased Increaseddeficit Decreased Increased Increased

deficit NA No effect or increased Increaseddeficit NA No effect or increased Increaseddeficit NA NA Increaseddeficit Decreased Increased No effect

Decreased Decreased IncreasedDecreased No effect or decreased IncreasedDecreased No effect Increased

Decreased No effect or decreased Increaseddeficit NA NA Increaseddeficit NA No effect or increased Increased

Decreased Decreased DecreasedIncreased Increased Increased

d PRD No effect No effect Increased

acids, color, various acids, or visual depending on the species and experimental

n.times during the growing season.

ead of the whole.

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Tan, and Clothier (1994) reported an increase in fruit aciditywhereas Drake, Proebsting, Mahan, and Thompson (1981) reporteda decrease. Water deficits during the harvest period increasedstrawberry fruit firmness (Prange & DeEll, 1997). Similarly, RDItended to increase fruit firmness in plum (Naor et al., 2004) and ap-ple (Mpelasoka & Behboudian, 2002). However, there was no affecton fruit firmness from RDI in peach (Crisosto et al., 1994; Gellyet al., 2003; Li et al., 1989) and kiwifruit (Miller et al., 1998). Higherfruit color from RDI has been observed in peach (Gelly et al., 2003),blackberry (Prange & DeEll, 1997), plum (Naor et al., 2004) and to-mato (Pulupol et al., 1996). Other studies have also reported areduction in red color from RDI in peach (Goldhamer et al., 2002)or no affect in peach (Crisosto et al., 1994; Li et al., 1989) and Clem-entine mandarin (Verreynne et al., 2001).

3.2. Effects of water deficit on secondary metabolites and ascorbic acidin fruit and vegetables

3.2.1. Flavonoids and other phenolicsThe flavonoid biosynthesis pathway appears to be stimulated

by water stress, particularly in wine or table grapes. The majorityof studies in red wine grapes report an increase in berry and wineanthocyanin, phenolic and tannin concentration from water defi-cits irrespective of the timing and level of water stress (Table 2)(Dry, Loveys, Mccarthy, & Stoll, 2001; Esteban, Villanueva, &Lissarrague, 2001; Goodwin, 2002; Matthews & Anderson, 1988;Matthews, Ishii, Anderson, & O’Mahony, 1990). In all these studiesthere was a concomitant reduction in yield and berry size. More re-cent studies by Roby et al. (2004) showed that the increase in skinanthocyanin and tannin content after RDI was greater than couldbe attributed solely to reduction in berry size and hence ruledout a concentration effect (Table 2). In support, Castellarin,Matthews, Di Gaspero, & Gambetta (2007) and Castellarin et al.(2007b) found that chalcone synthatase and flavanone 3-hydroxy-lase, key regulatory enzymes in the flavonoid pathway, were up-regulated in water stressed grapes, resulting in an increase in totalanthocyanin content between 37% and 57% (Table 2). Flavan-3-ols

Table 3Effect of irrigation volumes on quality aspects of ‘Royal Gala’ apple.

Irrigationwatervolume(mm)

Yield(kg tree�1)

Avg. fruitweight (g)

Soluble solidscontenta

(� Brix)

Total Polyphenolsa

(mg GAEb g�1 FWc)

160 (38%) 56.7 ad 124.7 b 16.4 a 1.45 a210 (50%) 65.7 a 128.0 b 15.9 b 1.45 a315 (74%) 64.7 a 142.1 a 15.5 c 1.25 c410 (100%) 59.6 a 138.5 a 15.3 cd 1.36 b620 (162%) 60.2 a 142.5 a 15.0 d 1.35 b

a Analysed considering average fruit weight as covariance to avoid concentrationeffects.

b Gallic Acid Equivalents.c Fresh weight.d Different letters means significant difference between treatments at p 6 0.05.

Table 4The effect of periods of water stress on Fresh Weight (FW), gluconapin (major glucosinoisorhamnetin (mg/g DW) in the leaves of immature pak choi ‘Sumo’. Seedlings were transpla3 and 4, weeks 5 and 6, or for all 6 weeks after transplanting. All analyses were conducted awater was added to replace the weight lost from the previous day. Control (0 water stresssignificant difference (Lsd) at p = 0.05.

Drought Frequency 0 Week2 and 3

FW (g) 53.9 14.2Gluconapin (lmol/g DWa) 3.04 4.26Quercetin (mg/g DW) 18.6 58.8Kaempferol (mg/g DW) 187.1 243.1Isorhamnetin (mg/g DW) 71.9 390.6

a Dry weight.

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and proanthocyanidins were also induced by water stress in tealeaves (Table 2) (Hernandez, Allegre, & Munne-Bosch, 2006), butstudies on other important horticultural crops are lacking. Preli-minary results on ‘Royal Gala’ apple indicate that despite a mar-ginal fruit size decrease, a 50% or more reduction of irrigationvolumes increased fruit phenolic content without diminishing to-tal yield (Table 3) (Stefanelli, Goodwin, & Jones, 2009). Similarly,a 2 week period of water stress applied between 2 and 3 weeksafter transplanting (seedling age = 8–10 weeks) induced significantincreases in flavonol content in immature pak choi leaves (Table 4),but similar periods of stress had no effect when applied to olderseedlings. Interestingly continuous water stress applied for thelength of the experiment (6 weeks) had no significant effect on fla-vonol content, despite significantly depressing fresh weight indi-cating that immature pak choi leaves may have adapted to longterm stress (Table 4).

Research on olive trees has shown an increase in the polyunsat-urated /monounsaturated fatty acid ratios in the oil with waterstress (when applied at the slow fruit growth period pre-veraison)but no impact was found on polyphenol content (Table 2) (Tognettiet al., 2007). However, other studies have shown an increase in oilpolyphenol content when whole of season water deficits wereapplied (Table 2) (d’Andria et al., 2004; Gómez-Rico et al., 2007;Patumi et al., 1999, 2002).

3.2.2. CarotenoidsThe effects of reduced water availability on carotenoid accumu-

lation have not been widely studied, with the exception of tomatofruit. In tomato fruits drought stress consistently increased lyco-pene and total carotenoids (Table 2) (Matsuzoe, Zushi, & Johjima,1998; Dorais & Papadopulos, 2001).Water stress, however, hadno effect on b-carotene or xanthophyll content in tomatoes, whilelycopene significantly increased (Table 2) (Zushi & Matsuzoe,1998). However deficit irrigation of watermelon did not increaselycopene concentration (Table 2) (Leskovar et al., 2007).

3.2.3. GlucosinolatesDrought stress is known to negatively affect yield in brassicas

(Hill, 1990; Mahmud, Atherton, Wright, Ramlan, & Ahmad, 1999)and the few specific studies conducted indicated that droughtstress also resulted in an increase in specific glucosinolate contentin the field crops rape (Brassica napus) and mustard (Brassica rapa)seeds (Table 2) (Jensen et al., 1996; Mailer & Cornish, 1987).Brassica seeds are high in glucosinolates and as such could providea useful source for glucosinolate extraction for pharmaceuticalpurposes (Rochfort & Jones, 2010). However, while there are noknown studies linking drought with increased glucosinolate con-tents in other Brassicas, specifically Brassica oleracea types. Inimmature pak choi leaves (Table 4), gluconapin content was in-creased by drought stress, but only when applied for a 2 week per-iod 2–3 weeks after seedling transplant, indicating that timing andduration of the stress is critical.

late) content (lmol/g DW), and content of the flavonols quercetin, kaempferol andnted into pots at 6 weeks of age and water stress was applied at weeks 2 and 3, weekst the end of the experiment. Stressed seedlings were weighed each day and sufficient) and unstressed plants received excess water (approximately 100 ml per day). Least

Week 3 and 4 Week 5 and 6 All 6 Lsd

39.4 39.1 17.8 3.63.07 3.23 2.98 0.55

20.9 21.7 27.5 10.1192.3 182.6 181.9 45.9

80.3 83.0 122.7 38.0

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3.2.4. Ascorbic acidThere are few published studies investigating the effects of low

water availability on ascorbic acid content. Leeks grown underinfrequent irrigation had higher ascorbic acid content (Table 2)(Sorensen, Johansen, & Kaack, 1995). Although results do vary be-tween varieties it appears in general that water deficit during toma-to fruit development causes an increase in vitamin C, dry matter andsoluble solids (Table 2) (Dumas et al., 2003). In large tomato fruits,water stress had a variable effect on vitamin C (Table 2) (Zushi &Matsuzoe, 1998). However, an earlier study indicated that lowwater tension in the soil during tomato fruit development resultedin lower vitamin C and soluble solids content and higher yield (Ta-ble 2) (Rudich, Kalmar, Geizenberg, & Harel, 1977). Partial Root zoneDrying (PRD), a system in which only part of the root system receiveirrigation while the rest remains dry to reduce total water usage, in-creased vitamin C content in table grapes by 15–42% (Du et al.,2008) and the concentration of ascorbic acid and ellagic acid instrawberry (Table 2) (Dodds, Taylor, Else, Atkinson, & Davies, 2007).

4. Conclusion

It can be concluded that reducing water and nitrogen inputs inagriculture is highly desirable from the considered aspects of sus-tainability, both environmental and social (improved human dietand health). By manipulating field growing conditions, phytonutri-ents content can be maximized improving human health whilelimiting production and environmental losses (Wang, 2006). Goodquality horticultural produce may be obtained only when waterand nutrient inputs are applied to maximize dry matter accumula-tion in each plant growth stage, avoiding any excessive vegetativegrowth, and allowing an effective translocation into the edible partwhen it is the dominant sink (Battilani, 2008).We agree with sev-eral colleagues (Ganeshamurthy et al., 2004; Kirschenmann & Bird,2006; Neilsen & Neilsen, 2002; Trout et al., 2006) that research onthe effects and limits of the reduction of water and N should beencouraged and extended to other inputs as well; the knowledgetransfer to all sectors of society involved should be increased.The effects of reducing N and water on human health compoundsin fruit and vegetables is highly species and often variety depen-dent, furthermore in fruit variety/rootstock combination may havean effect as well. In addition to a precise management of water andnutrients, a holistic approach to the complexity of horticulturalsystem as well as an interdisciplinary management is required toimprove any aspect of product quality (Battilani, 2008). Thereforeresearch should consider measuring human health compounds infruit and vegetables in addition to yield and the current qualitymeasurements. The social gains from human health should alwaysbe considered when investigating agricultural production systems.

Acknowledgements

This paper is a publication from Vital Vegetables (http://vital-vegetables.com.au), a Trans Tasman research project jointly fundedand supported by Horticulture Australia Ltd., New Zealand Insti-tute for Crop and Food Research Ltd., the New Zealand Foundationfor Research, Science and Technology, the Australian Vegetable andPotato Growers Federation Inc., New Zealand Vegetable and PotatoGrowers Federation Inc. and the Victorian Department of PrimaryIndustries.

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