Original Paper Environ. Control Biol., 52 (3), 155160, 2014 DOI: 10.2525/ecb.52.155 Effects of the Rare Sugars D-psicose and D-tagatose on the Sugar Content and Incidence of Blossom End Rot in Tomato Grown Hydroponically with Salinity Treatment 1 1 1 1 2 2 Yoshihiro YAMADA ,KazumasaKAKIBUCHI , Ayako KOZUKI , Yutaka ISHIDA ,KenIZUMORI , Shigeyuki TAJIMA , 2 3 3 Kazuya AKIMITSU , Takeo OHKOUCHI and Fumito KASAI 1 Shikoku Research Institute Inc., 21098 Yashima-nishimachi, Takamatsu, Kagawa 7610192, Japan 2 Faculty of Agriculture, Kagawa University, 2393 Ikenobe, Miki, Kida, Kagawa 7610795, Japan 3 Mitsui Chemicals Agro Inc., 1358 Ichimiyake, Yasu, Shiga 5202362, Japan (Received March 4, 2014; Accepted June 12, 2014) Low sugar content of fruits in winter, and blossom end rot in summer, are serious problems in the production of tomato with high soluble solids content. In the present study, we investigated the effects of the rare sugars D-psicose and D-tagatose on the sugar content of fruits and incidence of blossom end rot during several cropping seasons. We determined a highly positive correlation between the sugar content and the cumulative solar radiation during the maturation period. Treatment with D-psicose resulted in an increase in the sugar content over every cropping season. It resulted in a reduction in the in- cidence of blossom end rot, a reduction in stomatal conductance, and an increase in the relative water content in summer. On the other hand, it resulted in an increase in the incidence of blossom end rot in winter and rainy season. Treatment with D-tagatose had no effect on the sugar content, incidence of blossom end rot, water potential, stomatal conductance, or rela- tive water content. Our results indicate that treatment with D-psicose is advantageous for increasing the sugar content in winter and preventing the incidence of blossom end rot in summer. Keywords : D-allulose, fruit quality, monosaccharide, solar radiation, water relations rot, and reduced yield (Ito et al., 1994; Oka et al., 2004). INTRODUCTION Therefore, appropriate regulation of drying stress is re- quired (Saito et al., 2006). Rare sugars are defined by The International Society Kakibuchi et al. (2011) showed that the application of of Rare Sugars as “monosaccharides and their derivatives the rare sugar D-psicose inhibited transpiration and reduced that rarely exist in nature.” The effects of rare sugars have the incidence of blossom end rot in tomato grown recently been investigated. For example, D-allose and D- hydroponically with salt application. Their study was con- psicose have been shown to induce disease resistance in ducted in a warm season, and did not verify the effects of rice and lettuce (Kano et al., 2010; 2011; 2013; Ishida et D-psicose in other seasons. al., 2012). Ishida et al. (2011) demonstrated that D-tagatose In the present study, we investigated the effects of D- reduced the flower-setting node order of morning glories, psicose and another rare sugar, D-tagatose, on the sugar and accelerated flower bud differentiation in strawberries. content and incidence of blossom end rot during production Kakibuchi et al. (2011) reported that some monosacchari- of tomato with high soluble solids content, in warm and des, including rare sugars, especially L-fructose and D- cool seasons. psicose inhibit transpiration. Therefore, these rare sugars may function as antitranspirants for use in various cultiva- MATERIALS AND METHODS tion systems, e.g., production of tomato with high soluble solids content. Plant materials and growth conditions The production of tomato with high soluble solids Tomatoes (Solanum lycopersicum L., cv. Momotaro 8; content is a cultivation method for increasing the sugar Takii Seed Co., Ltd., Japan) were grown in a greenhouse content of fruits by applying a higher level of drying stress with acrylic panels. The temperature was maintained through restriction of irrigation (Ito et al., 1994) or root vol- within the range 1535°C. Tomato plugs were grown in ume (Ishigami et al., 1994), use of a nutrient solution with rockwool cubes (75 mm 75 mm 65 mm, Delta 4G, high electrical conductivity (Noguchi et al., 2012), or appli- Grodan, Netherlands) and transplanted to rockwool slabs cation of a salt solution (Oka et al., 2004; Araki et al., (900 mm195 mm75 mm; Grotop Master 2075 A1W, 2009). However, excessive drying stress results in de- Grodan, Netherlands) when the first truss was visible. The creased fruit weight, increased incidence of blossom end plants were fertilized with a half-strength Otsuka-A solu- Corresponding author : Kazumasa Kakibuchi, fax: 81878870003, e-mail : [email protected] Vol. 52, No. 3 (2014) Y. YAMADA ET AL. tion (Otsuka Chemical, Co., Osaka, Japan), with an EC of weight. We measured the sugar content of the fruits by ex- 1.2 dS m1. The nutrient solution was supplied at a rate of pressing the juice and using a refractometer (PR-1; Atago 960 mL individual1 day1 through a drip irrigation tube, at Co., Japan). 2-h intervals during the daytime (6:0016:00). The main To elucidate the mechanism of action of rare sugars on shoot of each plant was pinched to leave two leaves above the sugar content and incidence of blossom end rot, we the first truss, and all lateral shoots were removed. measured the water potential, stomatal conductance, and The rare sugars D-psicose (D-allulose) and D-tagatose relative water content of the leaves. We made these meas- were supplied by the Rare Sugar Research Center, Kagawa urements in experiments 2 to 4. However, we did not University. D-psicose is the C3-epimer of D-fructose, and measure the water potential in experiment 4, because of dif- D-tagatose is the C4-epimer of D-fructose (Fig. 1). ficulties with the measuring equipment. The water poten- At the day of anthesis, drip irrigation was stopped tial represents the water absorption power, and therefore after two times irrigation in the morning. At 10 : 00 am, to- leaves with low water potential are expected to absorb mato plants in rare sugar treatment were watered to the more water from fruits, thereby increasing the sugar con- plant foot with 350 mL of a nutrient solution containing 50 tent of the fruits. Stomatal conductance corresponds to the mmol L1 rare sugar using a beaker. Tomato plants in con- transpiration rate, and the relative water content is used as trol were similarly watered 350 mL of a nutrient solution an indicator of the drying stress. Increasing leaf transpira- without rare sugar. From the next day of anthesis of the tion inhibits the influx of calcium to fruits, and increases first flower to the end of experiment, 2.98 g L1 of NaCl the incidence of blossom end rot (Adams and Ho, 1993). was added to the nutrient solution, to maintain the same os- Most blossom end rot occurs 3 weeks after anthesis (Saito motic potential (290 kPa) as a high electrical conductivity et al., 2006), and therefore we measured the water poten- nutrient solution (EC8.0dSm1). Fourteen days after tial, stomatal conductance, and relative water content dur- anthesis, rare sugar treatments were repeated at the same ing the daytime (11:0014:00) on sunny days 3 weeks after way as the day of anthesis, but using nutrient solution con- anthesis. We used the unshaded leaflet above the first truss taining 2.98 g L1 . In experiment 1, plants were trans- for all measurements. We measured the water potential by planted on June 14, 2011, and flowered on June 21, 2011. using a pressure chamber (Model 3005; Soil Moisture In experiment 2, plants were transplanted on September 28, Equipment Corp., USA) and the stomatal conductance by 2011, and flowered on October 18, 2011. In experiment 3, using a Porometer (Leaf Porometer; Decagon Devices, Inc., plants were transplanted on May 28, 2012 and flowered on USA). We calculated the relative water content by using May 27, 2012. In experiment 4, plants were transplanted the following equation with fresh weight at sampling time, on July 23, 2012 and flowered on July 30, 2012. The effect maximum weight in the leaf at full turgor, and dry weight. of D-psicose treatment was tested in experiment 1, and the Relative water content (%)100(fresh weight at effects of D-psicose and D-tagatose were tested in experi- sampling time dry weight) / (maximum weight in the ments 24. In each experiment, 6 individual tomato plants leafatfullturgor dry weight) were used. We sampled one leaflet per plant, and immediately de- Measurements termined the fresh weight. The leaflets were then placed on We measured the shading rate of the greenhouse using a plug tray containing water, and shaded at room tempera- irradiance meter (LP9021RAD, Delta OHM, Netherlands). ture. We measured the leaf weight after 5 h as the maxi- We calculated the cumulative solar radiation in the green- mum weight of the leaf at full turgor. We determined the house based on the shading rate and the daily cumulative dry weight after drying the leaf in an oven at 70°C. solar radiation recorded by the Japan Meteorological Agency (http://www.jma.go.jp/jma/menu/report.html). We RESULTS used the sum of the daily cumulative solar radiation during the maturation period for analyses. The tomato fruits were Table 1 shows the cumulative solar radiation during harvested when completely red. We recorded the harvest- the maturation period, the start day of anthesis, the start day ing date, presence or absence of blossom end rot, and fruit of harvest, the average fruit weight, and the average sugar content. We used these data to calculate the correlation co- efficients between environmental factors and fruit quality indexes of control plants (Table 2).