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Takashi Ikeda, Kunio Okano, Yuka Sakamoto and Shin-ichi Watanabe

This study was undertaken to investigate the water relations of tomato (Lycopersicon esculentum Mill.) fruit cracking for single-truss tomato plants. The tomato plants were cultured on a closed hydroponic system in greenhouse. Water status of culture solution and plant tissues was measured with psychrometers. Water potential of the culture solution for the stressed plant was changed from -0.06 MPa (control plants) to -0.36 MPa at 24 days after anthesis. Hardness of the fruit skin was not different significantly between the stressed plants and the control plants. Fruit cracking occurred frequently in the control plants, but not in the stressed plants. Water potential gradient between the tissue of fruit flesh and water source for the control plants was bigger than that of the stressed plants. Turgors were increased at the tissues of fruit flesh and fruit skin at the control plants between predawn and morning but not at the stressed plants. These results indicated that the water potential gradient and the increased turgor in these tissues might be a trigger for the occurrence of fruit cracking on single-truss tomato plants.

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Yuji Yamada, Masayoshi Nakayama, Hiromitsu Shibata, Sanae Kishimoto and Takashi Ikeda

During development, the fruit of some paprika (Capsicum annuum L.) cultivars shows a change in color from green to dark purple (e.g., ‘Mavras’) or lilac (e.g., ‘Tequila’). However, this purple coloration is rare among paprika cultivars and disappears in ripened fruit, which are red. Therefore, we investigated the mechanism causing this color change in the cultivars Mavras and Tequila to better understand how purple ripened fruit could be generated. High-performance liquid chromatography (HPLC) analyses of the anthocyanin contents of the fruit indicated that anthocyanin was undetectable in green fruit, accumulated in dark purple or lilac ones, and then decreased again in red ones in both cultivars. Furthermore, expressions of most of the analyzed anthocyanin biosynthesis–related genes and genes for their transcription factors increased in dark purple or lilac fruit and decreased in red ones, i.e., it was synchronized with the changes in anthocyanin contents. Furthermore, anthocyanin degradation activity as a result of peroxidases was detected at all stages but increased when the lilac or dark purple color started to fade. Thus, the development of purple coloration is caused by increased anthocyanin biosynthesis, whereas the fading of this coloration is a result of both a decrease in anthocyanin biosynthesis and an increase in anthocyanin degradation. At the ripening stage, the green pigment (chlorophyll) contents decreased, whereas the red pigment (carotenoid, particularly capsanthin) contents increased. However, these timings did not completely coincide with the timing of anthocyanin degradation, suggesting that the content of each pigment is individually regulated, and so purple, green, and red coloration could be freely expressed in mature paprika fruit.

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Takashi Ikeda, Keisuke Yamazaki, Hiroshi Kumakura and Hiroshi Hamamoto

We demonstrated the effect of cooling of the medium on the fruit set of strawberries (Fragaria × ananassa Duch.) grown on high benches for forcing culture. The cooling by water evaporation promoted by a fan enabled to cool the medium by an average of several degrees compared with no cooling. When runner plants were transplanted in late summer, cooling accelerated flower bud emergence almost 10 days on the primary axillary branch compared with plants grown in uncooled medium. Also, with cooling, fruit was harvested from the inflorescence of the primary axillary branch almost 10 days earlier. We expect that this technique will allow early transplanting around the end of summer and will shorten the time between fruit set on the terminal inflorescence and that on the inflorescence of the primary axillary branch.

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Sachiko Kawamura, Kyoko Ida, Masako Osawa and Takashi Ikeda

We investigated physiological differences in watermelon [Citrullus lanatus (Thunb.) Matsum. et Nakai] fruits among seeded diploid and seedless triploid fruits, N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU)–treated seedless fruits, and soft-X–irradiated pollen-pollinated seedless fruits to investigate the effect of the presence or absence of seeds on water relations and sugar content. We picked fruits at 20 and 40 days after anthesis and sampled flesh at the center, around the seeds, and near the pericarp to measure water status and sugar content. There were no significant differences between seeded and seedless cultivars in sugar contents or in water and osmotic potentials of the flesh, although the latter two were decreased at 40 days. CPPU and soft-X–irradiated pollen eliminated mature seeds, but there were again no significant differences in sugar contents or water status between seeded and seedless fruits. Thus, the presence or absence of seeds did not influence the sugar content or osmotic pressure in watermelon fruit, so sugar accumulation was not related to seeds.

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Takashi Ikeda, Yukihiro Fujime, Satoshi Terabayashi and Shuichi Date

Garlic (Allium sativum L.) calli in vitro were evaluated over a range of salt concentrations and by adding mannitol to culture medium with reduced salt to provide equivalent osmoticum. The water potential of the medium ranged from -0.27 to -0.73 MPa under the various salt and osmotic stress conditions. The percent increase in calli was highest in standard Murashige & Skoog (MS) medium and was reduced when MS salts were reduced but the water potential of medium was adjusted to that of standard MS medium by addition of mannitol. The water potential of callus tissue was similar to that of tissue culture media over a 20-fold range (10% to 200%) of MS concentrations. Turgor of callus tissue was not influenced by any stress conditions. These results indicate that the optimum concentration of salt and water status of medium for formation of garlic calli was provided by standard MS medium.