- Email: [email protected]
Effects of NaCl application to hydroponic nutrient solution on fruit characteristics of tomato (Lycopersicon esculentum Mill.)
Scientia Horticulturae 109 (2006) 248–253 www.elsevier.com/locate/scihorti
Effects of NaCl application to hydroponic nutrient solution on fruit characteristics of tomato (Lycopersicon esculentum Mill.) Suguru Sato *, Sachi Sakaguchi, Hajime Furukawa, Hideo Ikeda College of Agriculture and Biological Science, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Received 16 March 2005; received in revised form 23 December 2005; accepted 3 May 2006
Abstract NaCl was applied to nutrient solution (5 dS m1 versus 1.4 dS m1 in the control) of hydroponically-grown tomato and its effects on taste grading and chemical composition of fruit were investigated. Taste panels indicated NaCl treatment increased sweetness, acidity, umami (i.e. the taste of deliciousness) and overall preference. Hexose concentration of the fruit grown on NaCl treated plants significantly increased; and at the same time, chloric ion, organic and amino acids in general had higher concentrations in NaCl treated plants than the control. Our results showed that (1) consumer grading of the tomato fruit was influenced not only by sugar content but also by the organic and amino acids; (2) increased concentration of soluble solids in the fruit of NaCl treated plants was not the result of simple overall condensation due to the reduction of water transport. The relation of diversified consumer preference, fruit chemical composition, and appropriate evaluation of tomato fruit are also discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Lycopersicon esculentum Mill.; Umami; Sweetness; Consumer preference; Salinity; Nutrient solution; Firmness; Soilless culture
1. Introduction Consumer preference and demand for vegetables are increasingly diversified. Some consumers may weigh priority on how vegetables are produced, i.e. if vegetables are produced environmentally sound and/or organically (Magkos et al., 2003), or on vegetable quality (i.e. lower nitrate content, Escobar-Gutierrez et al., 2002). Consequently, it is essential for growers to understand which segment of the market they are producing for (Carruthers, 2003). Among that diverse preference, there is an increasing consumption and demand for sweeter tomatoes (Aoki, 2003). Some tomatoes are even labeled as ‘dessert tomato’; growers claim these tomatoes are sweet enough to be served as a dessert. For such a specific demand of tomato, growers often apply salt and/or drought stress before the harvest to enhance sweetness of fruit (Ehret and Ho, 1986; Adams and Ho, 1992). Petersen et al. (1998) reported that hydroponically produced tomato with NaCl
* Corresponding author at: Faculty of Horticulture, Chiba University, 648 Matsudo, Matsudo City, Chiba 271-8510, Japan. Tel.: +81 47 308 8806; fax: +81 47 308 8806. E-mail address: [email protected] (S. Sato). 0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.05.003
enriched nutrient solution had higher consumer preference, increased sweetness and flavor, but also made the fruit harder. Salt enrichment in nutrient solution is known to increase ascorbic acid as well, which adds acidic taste to the fruit (Zushi and Matsuzoe, 1998). It has been suggested to use sugar/acid ratio as an index for tomato fruit taste (Adams, 1991). However, contrasting results have been shown too, due to the buffering effects of cations and anions (Kader et al., 1978). Growers often use the Brix value to indicate sugar contents in tomato fruit. However, Brix is not a reflection solely from sugars, it also reflects other soluble solids in the fruit, including organic acids. Therefore, using Brix as a representation of total sugar content can be misleading. Furthermore, sweetness is not only reflecting the sugar concentration because perception of sweetness is different for each sugar; fructose is the sweetest natural carbohydrate and glucose is only 60% as sweet as fructose (Hanover and White, 1993). In addition to sugars and acids, some free amino acids should considerably affect tomato taste. ‘Umami’ has been proposed as a source for the taste of deliciousness (Lindemann, 2001). L-Glutamate, an amino acid, has been known as a major ingredient for ‘umami’, or delicious taste (Ikeda, 1909) and its receptor on the human tongue has been characterized (Chaudhari et al., 2000).
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
Therefore, the relation between physiological/biochemical backgrounds and consumer preference in tomato fruit is not yet fully clarified, and there are few comprehensive reports on the relation between taste of tomato grown with NaCl enriched nutrient solution and chemical composition. In the present study we examine effects of NaCl enrichment of the nutrient solution on chemical and physical properties of tomato fruit in relation to taste panel grading. 2. Materials and methods 2.1. General procedures Seeds of Lycopersicon esculentum Mill. (cv. ‘Momotaro Fight’, Sakata, Kyoto, Japan) were sown on 4 July 2002 in a Petri dish, supplied with de-ionized water, and placed in an incubator maintained at 25 0.6 8C. Germinated seeds were transplanted into plastic pots (5 cm diameter) filled with commercial soil media (Metromix 250, The Scotts Company, Marysville, OH, USA) on 10 July and placed in a glasshouse. The greenhouse did not have any cooling system and its temperature was controlled by ventilation. Average, maximum, and minimum temperatures in the greenhouse were 26.3 0.2, 31.6 0.3 and 22.6 0.2 8C, respectively. Seedlings were transplanted to rockwool cubes (75 mm 75 mm 50 mm) on 3 August, and placed onto rockwool slabs (910 mm 300 mm 100 mm) on 1 September. Basal parts of plants, rockwool slabs and cubes containing root system, were covered with silver plastic sheet. Nutrient solution was supplied by a drip irrigation system three to five times daily depending on plant size and environmental conditions. A total of 50 plants were divided into two groups, 25 plants each to the Control (CT) and the NaCl (NT) treatments. Each plant was handled individually as a replicate. A half strength of commercial nutrient solution with electric conductivity (EC) of 1.4 dS m1 (Otsuka Chemical Co. Ltd., Tokyo, Japan; N, P, K, Ca and Mg = 9.3, 2.6, 4.3, 4.1 and 1.5 mequiv./l in the applied concentration) was applied both to CT throughout the experiment and to NT until 9 October. On 10 October, the EC of the nutrient solution for NT was adjusted to 5 dS m1 by adding NaCl to a half strength of the commercial nutrient solution (described above). The nutrient solution was independently circulated in each treatment, monitored and renewed as needed. Every other day, flowers at anthesis were vibrated manually to stimulate pollination and labeled for later investigation. The experiment continued until 31 December, 2002. Harvested fruit was weighed and subjected to taste paneling and chemical analysis. 2.2. Taste paneling Red stage fruit were cut in six pieces of wedge shape, and served to a taste panel. Sixty-six volunteers were randomly selected at the campus of the College of Agriculture and Biological Science, Osaka Prefecture University, and served as taste panels. The panel consisted 29 males and 34 females, ranged from 21 to 60 years old. Average age of the panel was 28.1 years old. The panel graded juiciness, peel hardness, fruit
249
hardness, sweetness, acidity, umami, aroma, and overall preference as 1–5 (5 as the strongest). Grades obtained were statistically analyzed using Wilcoxon’s signed rank test. 2.3. Physical and chemical investigation Firmness was measured by a penetrometer with the 1 mm diameter plunger (NRM-2002J, Fudoh, Rheo Tech Co. Ltd., Tokyo, Japan). Five fruit from each treatment group were subjected for the measurement. Five red stage fruit each from the CT and NT were randomly selected for Brix reading measurement, cut into six wedge shape pieces. A piece from each fruit was homogenized and centrifuged at 9200 g for 20 min, then supernatant was collected and used for Brix and titratable acidity measurements. Brix was measured with the refractometer (N-20, Atago Co. Ltd., Tokyo, Japan). For titratable acidity measurement, 10 ml of supernatant was dispensed and supplied for titration by 0.1N NaOH until pH 8.1. The amount of NaOH in ml was recorded to calculate titratable acidity in the following equation, which is expressed as the amount of citric acid (mg) in 100 ml of fruit liquid (Moradshahi et al., 1977) T 6:4 100=10 where T is the amount of 0.1N NaOH in ml used in titration, the value 6.4 represents the amount of citric acid neutralized by 1 ml of 0.1N NaOH, the value 100 represents the conversion into 100 ml of fruit liquid and the value 10 represents the amount of fruit liquid used in measurement. For sugars, organic acids, and amino acids analysis, five fruit each from both treatments were selected. For each fruit, fresh fruit weight was measured, 10 g of sample was further cut into smaller pieces and placed into a conical flask with 20 ml of 99.5% ethanol. After heating in boiling water bath for 15 min, flasks were ice-bathed and then the samples were homogenized. Filtrated liquid was collected and brought up to 50 ml with 99.5% ethanol. For sugar analysis, 10 ml ethanol extract liquid, with 1 ml of 1% inositol solution as an internal standard, was dried by rotary evaporator and then brought up to 10 ml with distilled water. Twenty-five microliters of sample filtered by Sep-pak (Waters Corporation, Milford, MA, USA) was subjected to HPLC (LC10A, Shimadzu, Kyoto, Japan). Samples were run on a NH2P-50 (Showa Denko, Tokyo, Japan) column at 0.8 ml/min with a column temperature at 30 8C and the mobile phase was 75% acetonitrile. Concentration of fructose, glucose, and sucrose was detected by RID-6A (Shimadzu, Kyoto, Japan). For measurement of organic acids except ascorbic acids 5 ml ethanol extract, as in sugar analysis, added with 0.5 ml of 0.5% glutaric acid as internal standard was dried by rotary evaporator, then brought up to 5 ml with distilled water. Fifty microliters of the solution sample was filtered and subjected to HPLC analysis (LC10A, JASCO Corporation, Tokyo, Japan) after filtration. Samples were run on a C-811 (Showa Denko, Tokyo, Japan) at 0.3 ml/min with column temperature set at 60 8C. For gradient elution separation, 3 mM perchloric acid
250
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
was used as mobile phase, and 15 mM dibasic sodium phosphate solution containing 0.2 mM Bromothymol Blue was used as reaction phase. Flow rate was set at 0.8 ml/min in both phases. Oxalic, a-keto-glutaric, citric and malic acids were analyzed. In ascorbic acid measurement, the supernatant obtained by same method as Brix and titratable acidity was subjected to ascorbic acid test with RQ Flex Plus (Merck, Darmstadt, Germany). For amino acids measurement, 5 ml of the ethanol extract with 1 ml 0.5% ureic acid as an internal standard was rotary evaporated, and brought up to 5 ml with 0.1N HCl. Ten microliters of the solution was filtrated and subjected to the automatic amino acid analyzer (L-8500, Hitachi High-Tech. Corp., Tokyo, Japan). Ninhydrin solution was prepared with Ninhydrin Reagent-L8500 Set (product code: 142-05051, Wako Pure Chemical Industries Ltd., Osaka, Japan) and the buffer containing lithium acetate dehydrate (2.0 mol/l). For the measurement of chloride ion content, five fruit each from both treatments were randomly selected and cut into six wedge-shapes. One piece from each fruit was subjected for a measurement. A piece of fruit was homogenized and centrifuged at 9200 g for 20 min. Five milliliters of supernatant was mixed with same amount of purified water and a few drops of 8% potassium chromate (K2CrO4), then subjected for the titration by 0.05N silver nitrate (AgNO3). During titration, the following reaction takes place Cl þ AgNO3 ! AgCl þ NO3 Silver chloride is an insoluble solid. When all chloride ions have reacted with silver ions, remaining silver ions react with the chromate in the solution and silver chromate (Ag2CrO4), which is a pink-orange precipitate indicating the end of reaction above. The chloride content in sample solution can be calculated from the amount of titrant given (APHA, 1995). All data, except grades from the taste paneling, were subjected to Student’s t-test after confirmed to be normally distributed. As the number of samples subjected for chemical analysis in the experiment was relatively small, coefficient of variability (CV) was also computed. 3. Results The average fresh weight of CT and NT fruit was 188.4 (10.8% CV) and 113.7 g (14.9% CV), respectively (Table 1). A significant difference between the treatments was observed by t-test at 5% level. Among index graded by taste panel (Fig. 1), juiciness, sweetness, acidity, umami, aroma, and overall preference had
Fig. 1. The effect of NaCl application to nutrient solution on taste of tomato fruit. Sweetness, acidity, umami, aroma, and overall preference were graded between 1 and 5, 5 as the strongest. Grades obtained were statistically analyzed in Wilcoxon’s signed rank test. Asterisk (*) and double asterisk (**) indicate significant differences of 5 and 1% level, respectively, between the treatments by Wilcoxon’s signed rank test. The taste panel was comprised of 66 people.
significantly higher scores for NT than CT fruit. Interestingly, peel hardness graded by the panel was significantly higher in CT than NT, however, physical peel hardness measured by the 1 mm diameter plunger was significantly higher in NT (5% level, 20.6 kg/cm2, 11.9% CV) than in CT (15.7 kg/cm2, 4.1% CV, Table 1). Soluble solids content of CT and NT was 6.12 (2.12% CV) and 7.78% (14.94% CV), respectively. A significant difference between the treatment was found at the 1% level (Table 1). A similar trend was also found in 6 and 12 carbon sugars (Fig. 2). The concentration of fructose and glucose increased significantly by NaCl treatment (1.91 g versus 2.31 g in fructose and 1.88 g versus 2.24 g in glucose). No significant difference was found in sucrose. NaCl enrichment in the nutrient solution increased titratable acidity of tomato fruit (Table 1). CVof CT and NT was 12.5 and 5.6%, respectively and a significant difference was found between the treatments. Among organic acids analyzed (Fig. 3), the concentration of oxalic acid (4.3 and 36.7 mg/ 100 ml juice in CT and NT, respectively), a-keto-glutaric acid (37.1 and 63.3 mg/100 ml juice in CT and NT, respectively), citric acid (246.6 and 413.8 mg/100 ml juice in CT and NT, respectively), and ascorbic acid (23.7 and 34.5 mg/100 ml juice in the CT and the NT, respectively) increased significantly by NaCl treatment.
Table 1 The effect of NaCl application to nutrient solution on the characteristics of tomato fruit Treatment Control NaCl a b c
FWa (g/fruit) c
188.4 a 133.7 b
Firmness (kg/cm2)
SSb (%)
Titratable acid (mg citrate/100 ml)
Chloride concentration (mg/100 ml)
15.7 b 20.6 a
6.12 b 7.78 a
368.1 b 552.2 a
0.55 b 1.52 a
Fresh weight. Soluble solids. Different letters represent significant differences at 5% (FW and firmness) or 1% level (SS, titratable acid, and chloride concentration) by t-test.
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
251
Among the six amino acids studied (Fig. 4), aspartic acid (37.4 and 55.5 mg/100 ml juice in CT and NT, respectively), asparagine (163.1 and 199.2) and glutamic acid (182.7 and 225.3) increased significantly by NaCl application. However, no significant difference was observed in glutamine, GABA (gaminobutyric acid), and proline. The chloride ion content in tomato fruit of CT and NT was 0.55 (12.8% CV) and 1.52 mg/100 ml juice (16.5% CV), respectively, and a significant differences at the 1% level was observed (Table 1). 4. Discussion
Fig. 2. Effect of NaCl application to nutrient solution on 6- and 12-carbon sugar contents of tomato fruit. Asterisk (*) indicates significant difference by t-test at 5% levels. Data are expressed as averages from five observations with standard error bars.
Fig. 3. Effect of NaCl application to nutrient solution on organic acid contents in tomato fruit. Asterisk (*) and double asterisk (**) indicate significant difference between the treatments by t-test at 5 and 1% levels. Data are expressed as averages from five observations with standard error bars.
Fig. 4. Effect of NaCl application to nutrient solution on amino acid content of tomato fruit. Asterisk (*) and double asterisk (**) indicate significant difference between the treatments by t-test at 5 and 1% level. Data are expressed as averages from five observations with standard error bars. Asp = aspartic acid, Asn = asparagine, Glu = glutamic acid, Gln = glutamine, GABA = g aminobutyric acid, Pro = proline.
The taste panel graded the peel of the fruit from CT was harder and overall fruit hardiness not different (Fig. 1). Interestingly, the peel was in fact harder after NaCl application by physical measurement (Table 1). The panel in the current experiment was not trained in sensory evaluation. However, these results may indicate that peel hardiness of fresh market tomato could have been hidden by other factors. As there is no data available if mixed tastes such as sweetness, sourness, and umami could overwhelm the sense of fruit texture, further research of tomato taste grading by professionals, as well as by consumers, is necessary to clarify the characteristics of highly marketable fresh tomato fruit. NT increased the Brix reading 1.27 times of CT (Table 1). Simultaneously hexoses (Fig. 2) were significantly higher in NT than CT (1.2 times in fructose and glucose), however, other soluble solids were higher in NT as well; oxalic acid increased 8.5 times (Fig. 3), a-keto-glutaric acid 1.7 times, citric acid 1.7 times, aspartic acid 1.5 times, asparagine 1.2 times, glutamic acid 1.2 times (Fig. 4), and chloride ion 2.8 times (Table 1). Although fructose, the sweetest sugar, increased significantly by NaCl application, sucrose, the second sweetest sugar next to fructose, did not. Therefore, not only sugars but also other factors must have affected the grades of overall preference and/ or sweetness by the taste panels. This could be explained by the increase of amino acids contents such as glutamic acid. Glutamic acid has been reported to be one of L-amino acids for the taste of umami, i.e. taste of deliciousness (Lindemann, 2001). A molecular receptor of the L-amino acids was isolated in humans and rodents. The receptor isolated, T1R1 + 3, responds not only to L-glutamic acid, but also to various Lamino acids, including asparagine and aspartic acid (Nelson et al., 2002). These findings also support previous discussion that the higher grades of overall preference in the fruit of NaCl treatment came not only from sweetness but also from enhanced umami taste, despite increased acidity. In addition to umami taste from amino acids, overall enhancement of soluble solids might have affected human taste sensory. Buttery et al. (1987) discussed that NaCl would slow the breakdown of volatile compounds by deactivating related enzymes, suggesting tomato flavor of NaCl treatment could have been preserved. Petersen et al. (1998) reported that NaCl treatment improved the sweetness of tomato more than other elements and reasoned that higher Na and Cl contents in the fruit. It has been empirically known that in some case salt
252
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
suppresses bitterness so that sweetness can be enhanced (Pszczola, 2004). Similar effects of NaCl could have occurred in our experiment in which the chloride ion concentration tripled in the fruit of the NT (Table 1). Although sodium concentration was not investigated in our experiment, the ratio of sodium to chloride ion concentration in tomato fruit and leaves grown under long term saline irrigation is almost 1:1 (Maggio et al., 2004); it would be reasonable to speculate that the sodium ion concentration in our experiment was similar to that of chloride ion. Sakamoto et al. (1999) concluded that fruit size reduction under salinity stress was caused by the inhibition of water uptake by the root and therefore the reduction of water transport to the fruit; the concentration of soluble solids increased, but the amount per fruit did not increase or decrease. We observed similar phenomena and significant increase of hexoses but not sucrose (Fig. 2). This may be a sign that soluble solids increase by NaCl treatment was not by simple overall condensation but active osmotic adjustment. Among other soluble solids examined, the reaction to NaCl treatment differed significantly. For instance, citric acid concentration increased 1.8-fold in the NT, however, malic acid did not (Fig. 4). The majority of amino acids are synthesized from three metabolites; pyruvate, oxaloacetate, and a-keto-glutaric acid (2-oxoglutarate). The amino acids we investigated were derived primarily from oxaloacetate (aspartate and asparagine) and 2-oxoglutarate (glutamate, glutamine, GABA, and proline), both in the citric acid cycle. The increase of some amino acids (aspartate, asparagine, and glutamate) might be the result of reduced water transport under saline stress and/or the active physiological reaction of decreasing water potential to cope with the stress. At the same time, some amino acids did not increase, indicating it was not overall condensation by the reduction of water transport. For instance, GABA, which did not increase significantly under the saline stress, might have been re-assimilated back to succinate for energy production in the citric acid cycle since coping with saline stress requires more energy to keep higher membrane potential, i.e. ATPase H+ pump needs to be further activated. The increase of proline has been shown in various plant parts and by various stresses (Hare et al., 1999; Claussen, 2005). For instance, the proline concentration was 10- and 18-fold in shoots and roots when plants were subjected to a nutrient solution containing 100 mM NaCl (Storey and Wyn Jones, 1975). One of the main roles of proline accumulation is to adjust plant water potential to cope with the difficulty of water availability and transport under the stress (Hare et al., 1999). Since none of the previous reports we observed compared proline concentration in fruit with other plant parts, it is unclear to what extent proline takes a role as an osmotic regulator in tomato fruit. It might be reasonable to speculate that proline does not need to accumulate more in fruit under NaCl treatment because the contents of other soluble solids were already high and some increased significantly after the treatment. Further research on proline accumulation in various plant parts under the stress would elucidate the role of proline in relation to other soluble solids.
We have shown that NaCl application to the nutrient solution for tomato hydroponics can obtain higher consumer preference though fruit size reduced. The physiological background of higher grading of NaCl treated fruit was likely supported by the increased contents of not only sugars but also other soluble solids such as amino acids for umami. Our results suggest that the evaluation of tomato fruit quality should include analysis of individual sugars, amino acids and other chemicals, rather than just the Brix measurement. However, due to the requirement of facility and laboratory environment, it is impossible for individual growers to operate such analysis. We need to establish easier alternative analytical methods and protocols in addition to the horticultural procedure for NaCl enriched production for high value added tomato fruit production. Acknowledgement We thank Andrea M.J. Sato, APHIS-USDA, North Carolina State University, for her careful work of proof reading and advice on technical writing of the manuscript. References Adams, P., 1991. Effects of increasing the salinity of the nutrient solution with major nutrients or sodium chloride on the yield, quality and composition of tomatoes grown in rockwool. J. Hort. Sci. 66, 201–207. Adams, P., Ho, L.C., 1992. The susceptibility of modern tomato cultivars to blossom-end rot in relation to salinity. J. Hort. Sci. 67, 827–839. Aoki, H., 2003. Tomato—new cultivars and trends. Noko to Engei 58, 67–87 (in Japanese). APHA (American Public Health Association), 1995. Standard Methods, Method 4500-Cl B, 20th ed. APHA, pp. 4–67. Buttery, R.G., Teranishi, R., Ling, L.C., 1987. Fresh tomato aroma volatiles: a quantitative study. J. Agric. Food Chem. 35, 540–544. Carruthers, S., 2003. Battle of the tomato brands. Pract. Hydroponics Greenhouse 72, 24–29. Chaudhari, N., Landin, A.M., Roper, S.D., 2000. A novel metabotropic glutamate receptor functions as a taste receptor. Nat. Neurosci. 3, 113– 119. Claussen, W., 2005. Proline as a measure of stress in tomato plants. Plant Sci. 168, 241–248. Ehret, D.L., Ho, L.C., 1986. The effects of salinity on dry matter partitioning and fruit growth in tomatoes grown in nutrient film culture. J. Hort. Sci. 61, 361–367. Escobar-Gutierrez, A.J., Burns, I.G., Lee, A., Edmondson, R.N., 2002. Screening lettuce cultivars for low nitrate content during summer and winter production. J. Hort. Sci. Biotechnol. 77, 232–237. Hanover, L.M., White, J.S., 1993. Manufacturing, composition, and applications of fructose. Am. J. Clin. Nutr. 58 (Suppl.), 724S–732S. Hare, P.K., Cress, W.A., van Staden, J., 1999. Proline synthesis and degradation: a model system for elucidating stress-related signal transduction. J. Exp. Bot. 50, 413–434. Ikeda, K., 1909. On a new seasoning. J. Tokyo Chem. Soc. 30, 820–836. Kader, A.A., Morris, L.L., Stevens, M.A., Albright-Holten, M.A., 1978. Composition and flavor quality of fresh market tomatoes as influenced by some postharvest handling procedures. J. Am. Soc. Hort. Sci. 103, 6– 13. Lindemann, B., 2001. Receptors and transduction in taste. Nature 413, 219– 225. Maggio, A., De Pascale, S., Angelino, G., Ruggiero, C., Barbieri, G., 2004. Physiological response of tomato to saline irrigation in long-term salinized soils. Eur. J. Agron. 21, 149–159.
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253 Magkos, F., Arvaniti, F., Zampelas, A., 2003. Organic food: nutritious food or food for thought? A review of the evidence. Int. J. Food Sci. Nutr. 54, 357–371. Moradshahi, A., Vines, H.M., Black, C.C., 1977. CO2 exchange and acidity levels in detached pineapple, Ananas comosus (L.) Merr. leaves during the day at various temperatures, O2 and CO2 concentrations. Plant Physiol. 59, 274–278. Nelson, G., Chandrashekar, J., Hoon, M.A., Feng, L., Zhao, G., Ryba, N.J.P., Zuker, C.S., 2002. An amino-acid taste receptor. Nature 416, 199–202. Petersen, K.K., Willumsen, J., Kaach, K., 1998. Composition and taste of tomato as affected by increased salinity and different salinity sources. J. Hort. Sci. Biotechnol. 73, 205–215.
253
Pszczola, D.E., 2004. Ingredients, CONFECTION: a sweet acronym. Food Technol. 58, 50–65. Sakamoto, Y., Watanabe, S., Nakashima, T., Okano, K., 1999. Effects of salinity at two ripening stages on the fruit quality of single-truss tomato grown in hydroponics. J. Hort. Sci. Biotechnol. 74, 690–693. Storey, R., Wyn Jones, R.G., 1975. Betaine and chlorine levels in plants and their relationship to NaCl stress. Plant Sci. 7, 161–168. Zushi, K., Matsuzoe, N., 1998. Effect of soil water deficit on vitamin C, sugar, organic acid, amino acid and carotene contents of large-fruited tomatoes. J. Jpn. Soc. Hort. Sci. 67, 927–933.
Effects of NaCl application to hydroponic nutrient solution on fruit characteristics of tomato (Lycopersicon esculentum Mill.) Suguru Sato *, Sachi Sakaguchi, Hajime Furukawa, Hideo Ikeda College of Agriculture and Biological Science, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Received 16 March 2005; received in revised form 23 December 2005; accepted 3 May 2006
Abstract NaCl was applied to nutrient solution (5 dS m1 versus 1.4 dS m1 in the control) of hydroponically-grown tomato and its effects on taste grading and chemical composition of fruit were investigated. Taste panels indicated NaCl treatment increased sweetness, acidity, umami (i.e. the taste of deliciousness) and overall preference. Hexose concentration of the fruit grown on NaCl treated plants significantly increased; and at the same time, chloric ion, organic and amino acids in general had higher concentrations in NaCl treated plants than the control. Our results showed that (1) consumer grading of the tomato fruit was influenced not only by sugar content but also by the organic and amino acids; (2) increased concentration of soluble solids in the fruit of NaCl treated plants was not the result of simple overall condensation due to the reduction of water transport. The relation of diversified consumer preference, fruit chemical composition, and appropriate evaluation of tomato fruit are also discussed. # 2006 Elsevier B.V. All rights reserved. Keywords: Lycopersicon esculentum Mill.; Umami; Sweetness; Consumer preference; Salinity; Nutrient solution; Firmness; Soilless culture
1. Introduction Consumer preference and demand for vegetables are increasingly diversified. Some consumers may weigh priority on how vegetables are produced, i.e. if vegetables are produced environmentally sound and/or organically (Magkos et al., 2003), or on vegetable quality (i.e. lower nitrate content, Escobar-Gutierrez et al., 2002). Consequently, it is essential for growers to understand which segment of the market they are producing for (Carruthers, 2003). Among that diverse preference, there is an increasing consumption and demand for sweeter tomatoes (Aoki, 2003). Some tomatoes are even labeled as ‘dessert tomato’; growers claim these tomatoes are sweet enough to be served as a dessert. For such a specific demand of tomato, growers often apply salt and/or drought stress before the harvest to enhance sweetness of fruit (Ehret and Ho, 1986; Adams and Ho, 1992). Petersen et al. (1998) reported that hydroponically produced tomato with NaCl
* Corresponding author at: Faculty of Horticulture, Chiba University, 648 Matsudo, Matsudo City, Chiba 271-8510, Japan. Tel.: +81 47 308 8806; fax: +81 47 308 8806. E-mail address: [email protected] (S. Sato). 0304-4238/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.scienta.2006.05.003
enriched nutrient solution had higher consumer preference, increased sweetness and flavor, but also made the fruit harder. Salt enrichment in nutrient solution is known to increase ascorbic acid as well, which adds acidic taste to the fruit (Zushi and Matsuzoe, 1998). It has been suggested to use sugar/acid ratio as an index for tomato fruit taste (Adams, 1991). However, contrasting results have been shown too, due to the buffering effects of cations and anions (Kader et al., 1978). Growers often use the Brix value to indicate sugar contents in tomato fruit. However, Brix is not a reflection solely from sugars, it also reflects other soluble solids in the fruit, including organic acids. Therefore, using Brix as a representation of total sugar content can be misleading. Furthermore, sweetness is not only reflecting the sugar concentration because perception of sweetness is different for each sugar; fructose is the sweetest natural carbohydrate and glucose is only 60% as sweet as fructose (Hanover and White, 1993). In addition to sugars and acids, some free amino acids should considerably affect tomato taste. ‘Umami’ has been proposed as a source for the taste of deliciousness (Lindemann, 2001). L-Glutamate, an amino acid, has been known as a major ingredient for ‘umami’, or delicious taste (Ikeda, 1909) and its receptor on the human tongue has been characterized (Chaudhari et al., 2000).
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
Therefore, the relation between physiological/biochemical backgrounds and consumer preference in tomato fruit is not yet fully clarified, and there are few comprehensive reports on the relation between taste of tomato grown with NaCl enriched nutrient solution and chemical composition. In the present study we examine effects of NaCl enrichment of the nutrient solution on chemical and physical properties of tomato fruit in relation to taste panel grading. 2. Materials and methods 2.1. General procedures Seeds of Lycopersicon esculentum Mill. (cv. ‘Momotaro Fight’, Sakata, Kyoto, Japan) were sown on 4 July 2002 in a Petri dish, supplied with de-ionized water, and placed in an incubator maintained at 25 0.6 8C. Germinated seeds were transplanted into plastic pots (5 cm diameter) filled with commercial soil media (Metromix 250, The Scotts Company, Marysville, OH, USA) on 10 July and placed in a glasshouse. The greenhouse did not have any cooling system and its temperature was controlled by ventilation. Average, maximum, and minimum temperatures in the greenhouse were 26.3 0.2, 31.6 0.3 and 22.6 0.2 8C, respectively. Seedlings were transplanted to rockwool cubes (75 mm 75 mm 50 mm) on 3 August, and placed onto rockwool slabs (910 mm 300 mm 100 mm) on 1 September. Basal parts of plants, rockwool slabs and cubes containing root system, were covered with silver plastic sheet. Nutrient solution was supplied by a drip irrigation system three to five times daily depending on plant size and environmental conditions. A total of 50 plants were divided into two groups, 25 plants each to the Control (CT) and the NaCl (NT) treatments. Each plant was handled individually as a replicate. A half strength of commercial nutrient solution with electric conductivity (EC) of 1.4 dS m1 (Otsuka Chemical Co. Ltd., Tokyo, Japan; N, P, K, Ca and Mg = 9.3, 2.6, 4.3, 4.1 and 1.5 mequiv./l in the applied concentration) was applied both to CT throughout the experiment and to NT until 9 October. On 10 October, the EC of the nutrient solution for NT was adjusted to 5 dS m1 by adding NaCl to a half strength of the commercial nutrient solution (described above). The nutrient solution was independently circulated in each treatment, monitored and renewed as needed. Every other day, flowers at anthesis were vibrated manually to stimulate pollination and labeled for later investigation. The experiment continued until 31 December, 2002. Harvested fruit was weighed and subjected to taste paneling and chemical analysis. 2.2. Taste paneling Red stage fruit were cut in six pieces of wedge shape, and served to a taste panel. Sixty-six volunteers were randomly selected at the campus of the College of Agriculture and Biological Science, Osaka Prefecture University, and served as taste panels. The panel consisted 29 males and 34 females, ranged from 21 to 60 years old. Average age of the panel was 28.1 years old. The panel graded juiciness, peel hardness, fruit
249
hardness, sweetness, acidity, umami, aroma, and overall preference as 1–5 (5 as the strongest). Grades obtained were statistically analyzed using Wilcoxon’s signed rank test. 2.3. Physical and chemical investigation Firmness was measured by a penetrometer with the 1 mm diameter plunger (NRM-2002J, Fudoh, Rheo Tech Co. Ltd., Tokyo, Japan). Five fruit from each treatment group were subjected for the measurement. Five red stage fruit each from the CT and NT were randomly selected for Brix reading measurement, cut into six wedge shape pieces. A piece from each fruit was homogenized and centrifuged at 9200 g for 20 min, then supernatant was collected and used for Brix and titratable acidity measurements. Brix was measured with the refractometer (N-20, Atago Co. Ltd., Tokyo, Japan). For titratable acidity measurement, 10 ml of supernatant was dispensed and supplied for titration by 0.1N NaOH until pH 8.1. The amount of NaOH in ml was recorded to calculate titratable acidity in the following equation, which is expressed as the amount of citric acid (mg) in 100 ml of fruit liquid (Moradshahi et al., 1977) T 6:4 100=10 where T is the amount of 0.1N NaOH in ml used in titration, the value 6.4 represents the amount of citric acid neutralized by 1 ml of 0.1N NaOH, the value 100 represents the conversion into 100 ml of fruit liquid and the value 10 represents the amount of fruit liquid used in measurement. For sugars, organic acids, and amino acids analysis, five fruit each from both treatments were selected. For each fruit, fresh fruit weight was measured, 10 g of sample was further cut into smaller pieces and placed into a conical flask with 20 ml of 99.5% ethanol. After heating in boiling water bath for 15 min, flasks were ice-bathed and then the samples were homogenized. Filtrated liquid was collected and brought up to 50 ml with 99.5% ethanol. For sugar analysis, 10 ml ethanol extract liquid, with 1 ml of 1% inositol solution as an internal standard, was dried by rotary evaporator and then brought up to 10 ml with distilled water. Twenty-five microliters of sample filtered by Sep-pak (Waters Corporation, Milford, MA, USA) was subjected to HPLC (LC10A, Shimadzu, Kyoto, Japan). Samples were run on a NH2P-50 (Showa Denko, Tokyo, Japan) column at 0.8 ml/min with a column temperature at 30 8C and the mobile phase was 75% acetonitrile. Concentration of fructose, glucose, and sucrose was detected by RID-6A (Shimadzu, Kyoto, Japan). For measurement of organic acids except ascorbic acids 5 ml ethanol extract, as in sugar analysis, added with 0.5 ml of 0.5% glutaric acid as internal standard was dried by rotary evaporator, then brought up to 5 ml with distilled water. Fifty microliters of the solution sample was filtered and subjected to HPLC analysis (LC10A, JASCO Corporation, Tokyo, Japan) after filtration. Samples were run on a C-811 (Showa Denko, Tokyo, Japan) at 0.3 ml/min with column temperature set at 60 8C. For gradient elution separation, 3 mM perchloric acid
250
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
was used as mobile phase, and 15 mM dibasic sodium phosphate solution containing 0.2 mM Bromothymol Blue was used as reaction phase. Flow rate was set at 0.8 ml/min in both phases. Oxalic, a-keto-glutaric, citric and malic acids were analyzed. In ascorbic acid measurement, the supernatant obtained by same method as Brix and titratable acidity was subjected to ascorbic acid test with RQ Flex Plus (Merck, Darmstadt, Germany). For amino acids measurement, 5 ml of the ethanol extract with 1 ml 0.5% ureic acid as an internal standard was rotary evaporated, and brought up to 5 ml with 0.1N HCl. Ten microliters of the solution was filtrated and subjected to the automatic amino acid analyzer (L-8500, Hitachi High-Tech. Corp., Tokyo, Japan). Ninhydrin solution was prepared with Ninhydrin Reagent-L8500 Set (product code: 142-05051, Wako Pure Chemical Industries Ltd., Osaka, Japan) and the buffer containing lithium acetate dehydrate (2.0 mol/l). For the measurement of chloride ion content, five fruit each from both treatments were randomly selected and cut into six wedge-shapes. One piece from each fruit was subjected for a measurement. A piece of fruit was homogenized and centrifuged at 9200 g for 20 min. Five milliliters of supernatant was mixed with same amount of purified water and a few drops of 8% potassium chromate (K2CrO4), then subjected for the titration by 0.05N silver nitrate (AgNO3). During titration, the following reaction takes place Cl þ AgNO3 ! AgCl þ NO3 Silver chloride is an insoluble solid. When all chloride ions have reacted with silver ions, remaining silver ions react with the chromate in the solution and silver chromate (Ag2CrO4), which is a pink-orange precipitate indicating the end of reaction above. The chloride content in sample solution can be calculated from the amount of titrant given (APHA, 1995). All data, except grades from the taste paneling, were subjected to Student’s t-test after confirmed to be normally distributed. As the number of samples subjected for chemical analysis in the experiment was relatively small, coefficient of variability (CV) was also computed. 3. Results The average fresh weight of CT and NT fruit was 188.4 (10.8% CV) and 113.7 g (14.9% CV), respectively (Table 1). A significant difference between the treatments was observed by t-test at 5% level. Among index graded by taste panel (Fig. 1), juiciness, sweetness, acidity, umami, aroma, and overall preference had
Fig. 1. The effect of NaCl application to nutrient solution on taste of tomato fruit. Sweetness, acidity, umami, aroma, and overall preference were graded between 1 and 5, 5 as the strongest. Grades obtained were statistically analyzed in Wilcoxon’s signed rank test. Asterisk (*) and double asterisk (**) indicate significant differences of 5 and 1% level, respectively, between the treatments by Wilcoxon’s signed rank test. The taste panel was comprised of 66 people.
significantly higher scores for NT than CT fruit. Interestingly, peel hardness graded by the panel was significantly higher in CT than NT, however, physical peel hardness measured by the 1 mm diameter plunger was significantly higher in NT (5% level, 20.6 kg/cm2, 11.9% CV) than in CT (15.7 kg/cm2, 4.1% CV, Table 1). Soluble solids content of CT and NT was 6.12 (2.12% CV) and 7.78% (14.94% CV), respectively. A significant difference between the treatment was found at the 1% level (Table 1). A similar trend was also found in 6 and 12 carbon sugars (Fig. 2). The concentration of fructose and glucose increased significantly by NaCl treatment (1.91 g versus 2.31 g in fructose and 1.88 g versus 2.24 g in glucose). No significant difference was found in sucrose. NaCl enrichment in the nutrient solution increased titratable acidity of tomato fruit (Table 1). CVof CT and NT was 12.5 and 5.6%, respectively and a significant difference was found between the treatments. Among organic acids analyzed (Fig. 3), the concentration of oxalic acid (4.3 and 36.7 mg/ 100 ml juice in CT and NT, respectively), a-keto-glutaric acid (37.1 and 63.3 mg/100 ml juice in CT and NT, respectively), citric acid (246.6 and 413.8 mg/100 ml juice in CT and NT, respectively), and ascorbic acid (23.7 and 34.5 mg/100 ml juice in the CT and the NT, respectively) increased significantly by NaCl treatment.
Table 1 The effect of NaCl application to nutrient solution on the characteristics of tomato fruit Treatment Control NaCl a b c
FWa (g/fruit) c
188.4 a 133.7 b
Firmness (kg/cm2)
SSb (%)
Titratable acid (mg citrate/100 ml)
Chloride concentration (mg/100 ml)
15.7 b 20.6 a
6.12 b 7.78 a
368.1 b 552.2 a
0.55 b 1.52 a
Fresh weight. Soluble solids. Different letters represent significant differences at 5% (FW and firmness) or 1% level (SS, titratable acid, and chloride concentration) by t-test.
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
251
Among the six amino acids studied (Fig. 4), aspartic acid (37.4 and 55.5 mg/100 ml juice in CT and NT, respectively), asparagine (163.1 and 199.2) and glutamic acid (182.7 and 225.3) increased significantly by NaCl application. However, no significant difference was observed in glutamine, GABA (gaminobutyric acid), and proline. The chloride ion content in tomato fruit of CT and NT was 0.55 (12.8% CV) and 1.52 mg/100 ml juice (16.5% CV), respectively, and a significant differences at the 1% level was observed (Table 1). 4. Discussion
Fig. 2. Effect of NaCl application to nutrient solution on 6- and 12-carbon sugar contents of tomato fruit. Asterisk (*) indicates significant difference by t-test at 5% levels. Data are expressed as averages from five observations with standard error bars.
Fig. 3. Effect of NaCl application to nutrient solution on organic acid contents in tomato fruit. Asterisk (*) and double asterisk (**) indicate significant difference between the treatments by t-test at 5 and 1% levels. Data are expressed as averages from five observations with standard error bars.
Fig. 4. Effect of NaCl application to nutrient solution on amino acid content of tomato fruit. Asterisk (*) and double asterisk (**) indicate significant difference between the treatments by t-test at 5 and 1% level. Data are expressed as averages from five observations with standard error bars. Asp = aspartic acid, Asn = asparagine, Glu = glutamic acid, Gln = glutamine, GABA = g aminobutyric acid, Pro = proline.
The taste panel graded the peel of the fruit from CT was harder and overall fruit hardiness not different (Fig. 1). Interestingly, the peel was in fact harder after NaCl application by physical measurement (Table 1). The panel in the current experiment was not trained in sensory evaluation. However, these results may indicate that peel hardiness of fresh market tomato could have been hidden by other factors. As there is no data available if mixed tastes such as sweetness, sourness, and umami could overwhelm the sense of fruit texture, further research of tomato taste grading by professionals, as well as by consumers, is necessary to clarify the characteristics of highly marketable fresh tomato fruit. NT increased the Brix reading 1.27 times of CT (Table 1). Simultaneously hexoses (Fig. 2) were significantly higher in NT than CT (1.2 times in fructose and glucose), however, other soluble solids were higher in NT as well; oxalic acid increased 8.5 times (Fig. 3), a-keto-glutaric acid 1.7 times, citric acid 1.7 times, aspartic acid 1.5 times, asparagine 1.2 times, glutamic acid 1.2 times (Fig. 4), and chloride ion 2.8 times (Table 1). Although fructose, the sweetest sugar, increased significantly by NaCl application, sucrose, the second sweetest sugar next to fructose, did not. Therefore, not only sugars but also other factors must have affected the grades of overall preference and/ or sweetness by the taste panels. This could be explained by the increase of amino acids contents such as glutamic acid. Glutamic acid has been reported to be one of L-amino acids for the taste of umami, i.e. taste of deliciousness (Lindemann, 2001). A molecular receptor of the L-amino acids was isolated in humans and rodents. The receptor isolated, T1R1 + 3, responds not only to L-glutamic acid, but also to various Lamino acids, including asparagine and aspartic acid (Nelson et al., 2002). These findings also support previous discussion that the higher grades of overall preference in the fruit of NaCl treatment came not only from sweetness but also from enhanced umami taste, despite increased acidity. In addition to umami taste from amino acids, overall enhancement of soluble solids might have affected human taste sensory. Buttery et al. (1987) discussed that NaCl would slow the breakdown of volatile compounds by deactivating related enzymes, suggesting tomato flavor of NaCl treatment could have been preserved. Petersen et al. (1998) reported that NaCl treatment improved the sweetness of tomato more than other elements and reasoned that higher Na and Cl contents in the fruit. It has been empirically known that in some case salt
252
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253
suppresses bitterness so that sweetness can be enhanced (Pszczola, 2004). Similar effects of NaCl could have occurred in our experiment in which the chloride ion concentration tripled in the fruit of the NT (Table 1). Although sodium concentration was not investigated in our experiment, the ratio of sodium to chloride ion concentration in tomato fruit and leaves grown under long term saline irrigation is almost 1:1 (Maggio et al., 2004); it would be reasonable to speculate that the sodium ion concentration in our experiment was similar to that of chloride ion. Sakamoto et al. (1999) concluded that fruit size reduction under salinity stress was caused by the inhibition of water uptake by the root and therefore the reduction of water transport to the fruit; the concentration of soluble solids increased, but the amount per fruit did not increase or decrease. We observed similar phenomena and significant increase of hexoses but not sucrose (Fig. 2). This may be a sign that soluble solids increase by NaCl treatment was not by simple overall condensation but active osmotic adjustment. Among other soluble solids examined, the reaction to NaCl treatment differed significantly. For instance, citric acid concentration increased 1.8-fold in the NT, however, malic acid did not (Fig. 4). The majority of amino acids are synthesized from three metabolites; pyruvate, oxaloacetate, and a-keto-glutaric acid (2-oxoglutarate). The amino acids we investigated were derived primarily from oxaloacetate (aspartate and asparagine) and 2-oxoglutarate (glutamate, glutamine, GABA, and proline), both in the citric acid cycle. The increase of some amino acids (aspartate, asparagine, and glutamate) might be the result of reduced water transport under saline stress and/or the active physiological reaction of decreasing water potential to cope with the stress. At the same time, some amino acids did not increase, indicating it was not overall condensation by the reduction of water transport. For instance, GABA, which did not increase significantly under the saline stress, might have been re-assimilated back to succinate for energy production in the citric acid cycle since coping with saline stress requires more energy to keep higher membrane potential, i.e. ATPase H+ pump needs to be further activated. The increase of proline has been shown in various plant parts and by various stresses (Hare et al., 1999; Claussen, 2005). For instance, the proline concentration was 10- and 18-fold in shoots and roots when plants were subjected to a nutrient solution containing 100 mM NaCl (Storey and Wyn Jones, 1975). One of the main roles of proline accumulation is to adjust plant water potential to cope with the difficulty of water availability and transport under the stress (Hare et al., 1999). Since none of the previous reports we observed compared proline concentration in fruit with other plant parts, it is unclear to what extent proline takes a role as an osmotic regulator in tomato fruit. It might be reasonable to speculate that proline does not need to accumulate more in fruit under NaCl treatment because the contents of other soluble solids were already high and some increased significantly after the treatment. Further research on proline accumulation in various plant parts under the stress would elucidate the role of proline in relation to other soluble solids.
We have shown that NaCl application to the nutrient solution for tomato hydroponics can obtain higher consumer preference though fruit size reduced. The physiological background of higher grading of NaCl treated fruit was likely supported by the increased contents of not only sugars but also other soluble solids such as amino acids for umami. Our results suggest that the evaluation of tomato fruit quality should include analysis of individual sugars, amino acids and other chemicals, rather than just the Brix measurement. However, due to the requirement of facility and laboratory environment, it is impossible for individual growers to operate such analysis. We need to establish easier alternative analytical methods and protocols in addition to the horticultural procedure for NaCl enriched production for high value added tomato fruit production. Acknowledgement We thank Andrea M.J. Sato, APHIS-USDA, North Carolina State University, for her careful work of proof reading and advice on technical writing of the manuscript. References Adams, P., 1991. Effects of increasing the salinity of the nutrient solution with major nutrients or sodium chloride on the yield, quality and composition of tomatoes grown in rockwool. J. Hort. Sci. 66, 201–207. Adams, P., Ho, L.C., 1992. The susceptibility of modern tomato cultivars to blossom-end rot in relation to salinity. J. Hort. Sci. 67, 827–839. Aoki, H., 2003. Tomato—new cultivars and trends. Noko to Engei 58, 67–87 (in Japanese). APHA (American Public Health Association), 1995. Standard Methods, Method 4500-Cl B, 20th ed. APHA, pp. 4–67. Buttery, R.G., Teranishi, R., Ling, L.C., 1987. Fresh tomato aroma volatiles: a quantitative study. J. Agric. Food Chem. 35, 540–544. Carruthers, S., 2003. Battle of the tomato brands. Pract. Hydroponics Greenhouse 72, 24–29. Chaudhari, N., Landin, A.M., Roper, S.D., 2000. A novel metabotropic glutamate receptor functions as a taste receptor. Nat. Neurosci. 3, 113– 119. Claussen, W., 2005. Proline as a measure of stress in tomato plants. Plant Sci. 168, 241–248. Ehret, D.L., Ho, L.C., 1986. The effects of salinity on dry matter partitioning and fruit growth in tomatoes grown in nutrient film culture. J. Hort. Sci. 61, 361–367. Escobar-Gutierrez, A.J., Burns, I.G., Lee, A., Edmondson, R.N., 2002. Screening lettuce cultivars for low nitrate content during summer and winter production. J. Hort. Sci. Biotechnol. 77, 232–237. Hanover, L.M., White, J.S., 1993. Manufacturing, composition, and applications of fructose. Am. J. Clin. Nutr. 58 (Suppl.), 724S–732S. Hare, P.K., Cress, W.A., van Staden, J., 1999. Proline synthesis and degradation: a model system for elucidating stress-related signal transduction. J. Exp. Bot. 50, 413–434. Ikeda, K., 1909. On a new seasoning. J. Tokyo Chem. Soc. 30, 820–836. Kader, A.A., Morris, L.L., Stevens, M.A., Albright-Holten, M.A., 1978. Composition and flavor quality of fresh market tomatoes as influenced by some postharvest handling procedures. J. Am. Soc. Hort. Sci. 103, 6– 13. Lindemann, B., 2001. Receptors and transduction in taste. Nature 413, 219– 225. Maggio, A., De Pascale, S., Angelino, G., Ruggiero, C., Barbieri, G., 2004. Physiological response of tomato to saline irrigation in long-term salinized soils. Eur. J. Agron. 21, 149–159.
S. Sato et al. / Scientia Horticulturae 109 (2006) 248–253 Magkos, F., Arvaniti, F., Zampelas, A., 2003. Organic food: nutritious food or food for thought? A review of the evidence. Int. J. Food Sci. Nutr. 54, 357–371. Moradshahi, A., Vines, H.M., Black, C.C., 1977. CO2 exchange and acidity levels in detached pineapple, Ananas comosus (L.) Merr. leaves during the day at various temperatures, O2 and CO2 concentrations. Plant Physiol. 59, 274–278. Nelson, G., Chandrashekar, J., Hoon, M.A., Feng, L., Zhao, G., Ryba, N.J.P., Zuker, C.S., 2002. An amino-acid taste receptor. Nature 416, 199–202. Petersen, K.K., Willumsen, J., Kaach, K., 1998. Composition and taste of tomato as affected by increased salinity and different salinity sources. J. Hort. Sci. Biotechnol. 73, 205–215.
253
Pszczola, D.E., 2004. Ingredients, CONFECTION: a sweet acronym. Food Technol. 58, 50–65. Sakamoto, Y., Watanabe, S., Nakashima, T., Okano, K., 1999. Effects of salinity at two ripening stages on the fruit quality of single-truss tomato grown in hydroponics. J. Hort. Sci. Biotechnol. 74, 690–693. Storey, R., Wyn Jones, R.G., 1975. Betaine and chlorine levels in plants and their relationship to NaCl stress. Plant Sci. 7, 161–168. Zushi, K., Matsuzoe, N., 1998. Effect of soil water deficit on vitamin C, sugar, organic acid, amino acid and carotene contents of large-fruited tomatoes. J. Jpn. Soc. Hort. Sci. 67, 927–933.