MDPI AG Acta Medica Okayama 2073-4395 16 16 2026 Genetic Dissection and Fine-Mapping of QTL for Salinity Tolerance at the Reproductive Stage on Chromosome 2 of Rice Variety Nona Bokra 1518 EN Farjana Rauf Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Tran Thi Thu Hien Graduate School of Environmental and Life Science, Okayama University Nguyen Thi Thu Thuy Graduate School of Environmental and Life Science, Okayama University Asami Tomita Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Yoshihiko Hirai Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Soil salinity is a major constraint limiting rice productivity, particularly at the reproductive stage. To elucidate the genetic basis of reproductive stage salinity tolerance, this study validated and fine-mapped quantitative trait loci (QTL) for yield-related traits using a salinity-tolerant line SL506, identified through screening of Nona Bokra–CSSLs in a Koshihikari background. In 2016, the F2 population derived from SL506/Koshihikari was evaluated under long-term salt stress; three QTLs associated with plant dry weight, panicle number, and grain weight were detected on chromosome 2. In 2023, validation analysis using F3 individuals confirmed the presence of these QTLs. Subsequent fine-mapping using F4 near-isogenic lines (NILs) delimited the QTL to a 1.7 Mb interval and high-resolution mapping using an F5 recombinant population progressively refined it to a 473 kb genomic region containing 69 annotated genes. Variant Effect Predictor analysis identified 15 deleterious nonsynonymous variants (SIFT < 0.05) in six candidate genes. Based on annotated gene functions, predicted variant effects, and their membership in stress-related gene families, OsPP2C24, OsFbox102, and OsWAK14 were suggested as the most promising candidate genes underlying qPDW2. These findings provide insights into the genetic basis of reproductive-stage salinity tolerance from valuable resources for the future improvement of salt tolerance and yield stability in rice. No potential conflict of interest relevant to this article was reported.

MDPI AG Acta Medica Okayama 2673-7655 6 2 2026 Effects of Different Heading Dates on Agronomic and Yield-Related Traits Under Salt Stress in Rice 28 EN Sadia Afrin Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Sayuri Ito School of Agriculture, Okayama University M M Emam Ahmed Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Shuto Ogino School of Agriculture, Okayama University Asami Tomita Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Yoshihiko Hirai Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Salinity is a major abiotic stress limiting rice production worldwide. This study aims to elucidate the effects of heading date on salt tolerance in rice. Five near-isogenic lines (NILs) developed from the SL2038/Koshihikari backcross population were grown with or without salt stress. SL2038 is a salt-tolerant line with delayed heading (~18 days) compared to the salt-sensitive background Koshihikari. The results showed that late-heading NILs produced significantly higher plant dry weight, panicle weight, percentage of filled grains, and grain weight (p < 0.05) under long-term salt stress. In Koshihikari, which exhibited delayed heading due to long-day treatment, the percentage of white heads was low, and panicle and grain weights were significantly higher under salt stress. Experiments with different sowing times indicated that late heading, such as sowing in June, resulted in higher grain weights. This is the first report to assess the impact of heading date on agronomic and yield-related traits under salt stress. In conclusion, even with a prolonged salt treatment period, heading during periods of low temperature and solar radiation results in higher grain weight under salt stress. This is proposed as one of the strategies for salt escape. These findings can be used to improve rice yield and implement crop management in salt-affected regions. No potential conflict of interest relevant to this article was reported.

Springer Science and Business Media LLC Acta Medica Okayama 0971-5894 2026 Suppression of salt-enhanced apoplastic flow by salicylic acid in rice EN Md. Asadulla Al Galib Graduate School of Environmental and Life Science, Okayama University Maoxiang Zhao Graduate School of Environmental and Life Science, Okayama University Toshiyuki Nakamura Graduate School of Environmental and Life Science, Okayama University Yoshimasa Nakamura Graduate School of Environmental and Life Science, Okayama University Yoshihiko Hirai Graduate School of Environmental and Life Science, Okayama University Yoshitaka Nakashima Graduate School of Environmental and Life Science, Okayama University Shintaro Munemasa Graduate School of Environmental and Life Science, Okayama University Izumi C. Mori Institute of Plant Science and Resources, Okayama University Yoshiyuki Murata Graduate School of Environmental and Life Science, Okayama University Salinity enhances apoplastic flow, resulting in an increment of Na+ uptake and a lower K+/Na+ ratio. Salicylic acid (SA) plays an important role in improving salinity tolerance in plants. The effect of exogenous SA on apoplastic flow in salt-treated rice seedlings was studied using an apoplastic tracer, 8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS) in light. Application of NaCl at 25 mM to the hydroponic solution significantly increased PTS uptake, while 25 mM NaCl did not affect seedling growth. Application of 25 mM NaNO3 increased PTS uptake to the same degree. Salinity significantly increased sodium (Na+) content but had no significant effect on potassium (K+) content, resulting in a lower K+/Na+ ratio. The application of SA at 0.05 mM and 0.1 mM to the hydroponic solution reduced Na-enhanced PTS uptake. Salicylic acid at 0.05 mM and 0.1 mM significantly reduced Na+ content and slightly increased K+ content in the shoots of rice seedlings, resulting in a higher K+/Na+ ratio. However, SA at up to 0.1 mM did not increase SA contents in shoots under salt stress. These results suggest that exogenous SA reduces Na+ uptake by suppressing Na+-enhanced apoplastic flow in rice seedlings. These findings provide insight into modulation of Na+ transport pathways from roots to shoots by SA and may allow us to utilize brackish water for rice cultivation and to improve salt-tolerant rice through suppression of salt-enhanced apoplastic flow by chemicals such as salicylic acid. No potential conflict of interest relevant to this article was reported.

Springer Science and Business Media LLC Acta Medica Okayama 0718-9508 2025 Suppression of Na+ Uptake Via Apoplastic Flow by Chitosan in Rice EN Maoxiang Zhao Graduate School of Environmental and Life Science, Okayama University Md. Asadulla Al Galib Graduate School of Environmental and Life Science, Okayama University Toshiyuki Nakamura Graduate School of Environmental and Life Science, Okayama University Yoshimasa Nakamura Graduate School of Environmental and Life Science, Okayama University Yoshihiko Hirai Graduate School of Environmental and Life Science, Okayama University Yoshitaka Nakashima Shintaro Munemasa Graduate School of Environmental and Life Science, Okayama University Izumi C. Mori Institute of Plant Science and Resources, Okayama University Yoshiyuki Murata Graduate School of Environmental and Life Science, Okayama University Purpose: Chitosan enhances tolerance to salinity in rice. Apoplastic flow plays a crucial role in the accumulation of sodium (Na+) in rice under salinity. This study investigated the effects of exogenous chitosan on apoplastic flow and Na+ uptake in NaCl-treated rice seedlings. Methods: We employed an apoplastic tracer, trisodium salt of 8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), in order to evaluate apoplastic flow in rice (Oryza sativa L., cv. Nipponbare) seedlings that were hydroponically grown in the solution containing NaCl (0 and 25 mM), and chitosan (0 mg L− 1, 10 mg L− 1, and 50 mg L− 1). Results: Application of 25 mM NaCl significantly increased PTS uptake and Na+ content in shoots but did not affect K+ content, resulting in a lower K+/Na+ ratio although 25 mM NaCl did not affect the seedling growth. The application of chitosan suppressed Na+-enhanced PTS uptake and Na+ accumulation in shoots without affecting the K+ content, which led to a higher K+/Na+ ratio. Moreover, chitosan did not affect the reducing sugar content or electrical conductivity in the solution containing NaCl. Conclusions: These results suggest that application of chitosan suppressed Na+-enhanced apoplastic flow to reduce Na+ uptake in rice seedlings. No potential conflict of interest relevant to this article was reported.

岡山大学農学部 Acta Medica Okayama 2186-7755 114 2025 Effects of dark respiration on dry matter production of various crop species 1 10 EN Kuniyuki Saitoh Graduate School of Environmental, Life, Natural Science and Technology, Okayama University Tomohiro Murakami Graduate School of Natural Science and Technology, Okayama University Yumi Nakamura School of Agriculture, Okayama University Misa Nishibori School of Agriculture, Okayama University Yuki Takagoshi Graduate School of Natural Science and Technology, Okayama University Yoshihiko Hirai Graduate School of Environmental, Life, Natural Science and Technology, Okayama University 　Eleven crops were cultivated: maize, sunflower, soybean, groundnuts, sesame, kenaf, barley, wheat, rice, potato, and sweet potato. The crop growth rate (CGR) and specific dark-respiration rate (Rs) were measured, and growth efficiency GE =CGR/(CGR+R) (R, respiratory loss) was calculated. In each crop, whole-plant Rs reached a maximum in the earlier stages of growth, declined rapidly until the early reproductive growth, and remained almost constant during the ripening period. The Rs of leaves was higher than that of stems during the reproductive growth period, except for maize and potato. The Rs of storage organs was highest in the earlier stages, followed by a rapid decline to similar or lower values than those of leaves and stems during the ripening period. The GE in whole plant was higher than 60% in wheat, maize, barley, sunflower, rice, kenaf, sesame, but lower in soybean, sweet potato and groundnuts, and lowest in potato, which was affected by the higher respiratory loss. The GE in whole plant during the reproductive growth period was significantly lower, which we attributed to increased maintenance costs due to the increase of non-assimilative organs, and decrease in the dry weight of vegetative organs. A positive correlation was observed between the carbohydrate content of storage organs and GE, indicating that a crop with higher carbohydrate content in storage organs tended to have a higher GE. Crops with higher protein and crude fat content in storage organs tended to have lower GE. The GE over the growing season was low for kenaf, a fiber crop which contains high molecular weight compounds such as lignin and cellulose, and lower for sesame, groundnuts, and soybean, which contain high oil and protein and have high respiration costs for the synthesis of storage materials, suggesting that these higher respiration costs are related to lower dry matter production and hence lower yields. No potential conflict of interest relevant to this article was reported.

MDPI Acta Medica Okayama 2077-0472 11 8 2021 Identification and Validation of QTLs for Yield and Yield Components under Long-Term Salt Stress Using IR64 CSSLs in the Genetic Background of Koshihikari and Their Backcross Progenies 777 EN Mai Nguyen Sao Graduate School of Environmental and Life Science, Okayama University Hanh Dao Duy Graduate School of Environmental and Life Science, Okayama University Mai Nakashima Faculty of Agriculture, Okayama University Kotaro Kumamoto Faculty of Agriculture, Okayama University Thuy Nguyen Thi Thu Graduate School of Environmental and Life Science, Okayama University Tohru Kobata Faculty of Life and Environmental Science, Shimane University Kuniyuki Saitoh Graduate School of Environmental and Life Science, Okayama University Yoshihiko Hirai Graduate School of Environmental and Life Science, Okayama University Unraveling the complex genetic bases and mechanisms underlying salt tolerance is of great importance for developing salt-tolerant varieties. In this study, we evaluated 42 chromosome segment substitution lines (CSSLs) carrying chromosome segments from IR64 on the genetic background of Koshihikari under salt stress. Two CSSLs, SL2007 and SL2038, produced higher plant dry weight and grain yield than did Koshihikari under the stress condition. These CSSLs also showed lower Na+ and Cl- accumulation in the leaf and whole plant at the full heading stage, which might be related to the higher grain yield and yield components. To understand the genetic control of its grain yield and yield components, a SL2007/Koshihikari F-2 population was generated for quantitative trait locus (QTL) analysis. Six QTLs for grain yield and yield-related traits were detected on chromosome 2. Using near-isogenic lines (NILs) from a SL2007/Koshihikari F-5 population, qSTGY2.2 was delimited to a 2.5 Mb region and novel qSTPN2 was delimited to a 0.6 Mb region. We also detected a novel QTL, qSTGF2, for grain filling, which was considered an important contributor to grain yield under salt stress in this CSSL. Our results provide insights into mechanisms conferring grain yield under salinity stress and new genetic resources for cloning and breeding. No potential conflict of interest relevant to this article was reported.

岡山大学農学部 Acta Medica Okayama 2186-7755 101 2012 根損傷の軽減が塩土壌における移植イネの生育と乾物重におよぼす影響 7 12 EN Makoto Tsuda Yukie Ueda Yoshihiko Hirai Rice root has an important role in water absorption and exclusion of sodium ion in saline soil. However, it is injured during transplanting to paddy field. The objective of this study was to examine whether prevention of such root injury reduces salinity damage in rice. Extremely early rice cultivar "Akihikari" that is not salt tolerant was grown in plastic bags filled with soil. Roots of seedlings were clipped leaving 5mm~10mm, then transplanted to 5L pots filled with soil applied with 0g, 7g and 10g of sodium chloride (NaCl). The growth and dry weight at the mature stage was compared to those of plants transplanted with intact roots. Time of transplanting was 12, 19, 26 and 33 days after sowing. Increase of number of stems was suppressed due to root clipping and salinity, but there was no interaction between the two factors. Dry weight of total above ground part and panicle decreased due to salinity though root clipping did not change them. The reduction of panicle dry weight was caused by the reduction of mean number of spikelets per panicle, thus mean panicle dry weight, as number of panicles and mean spikelet dry weight showed no changes. It was concluded that prevention of the root injury may not effectively alleviate salinity damage in less salt tolerant rice cultivars. No potential conflict of interest relevant to this article was reported.

岡山大学農学部 Acta Medica Okayama 0474-0254 85 1 1996 作物生理の動態解析に関する研究 109 113 EN Yoshihiko Hirai In order to realeze a high-efficient growth control for higher yielding, it is important to clarify the relationships between the physiolofical dynamics and the growth of a particular crop. Therefore, an assimilation chamber for simultaneous measurement of photosynthesis, shoot and root respiration,transpiration and ion uptake of a whole plant was developed. The relationship between plant growth and constructive respiration coefficient which was calculated by using McCree's equation, was investigated. The coefficient increased with the ratio of the assimilation of nitrogen and carbohydrate(�儂/�僂) during measurement. The higher ratio of �儂/�僂 is closely related to an increase in dry-matter partitioning of the leaf blade.In this measurement,light intensity was daily lowered to estimate the constructive respiration coefficient and plant gowth might be influenced under these conditions. Then measurements at condtant daylight intensity were done under different humidity and nutrient conditions, using several varieties. Consequently,shoot respiration rate was estimated by relative shoot growth rate and leaf blade/sheath ratio of dry matter increase.Respiratory enegy costs for organogenesis differed among the plant parts, and that of the leaf blade was higher than that of the leaf sheath. Root respiration rate was explained by the combination of relative root growth rate and nitrogen uptake rate. The respiratory energy cost for nitrogn uptake did not change with the transpiration rate, the nitrogen concentration in the culture solution nor with other varieties. Dry matter partitioning rate to shoot was found to have two main componets,i.e.initial nitrogen content of shoot and ratio of assimilated nitrogen and carbon in the first day. Dry-matter increase and dry matter partitioning of intact plants could be discovered by measuring CO2 exchange, nitrogen uptake and shoot nitrogen content. No potential conflict of interest relevant to this article was reported.

岡山大学農学部 Acta Medica Okayama 0474-0254 89 1 2000 イネの根におけるナトリウムイオン排除率と蒸散量の変化が地上部ナトリウム含有率に及ぼす影響 31 37 EN Daigo Makihara Makoto Tsuda Yoshihiko Hirai The relationship between sodium contents of tops and transpiration rates was studied in two rice varieties with different sodium exclusion rates in roots; Kala-Rata1-24(KR1;low exclusion rate) and IR28 (high exclusion rate). Seedlings at 7th leaf stage grown in culture solution were subjected to saline water(100mM sodium chloride) and transpired for 12 hours. Various transpiration rates were obtained under different humidity, light intensity and temperature conditions. Transpiration stream concentration factor of Na+ (TSCFNa+), which denotes the sodium exclusion rate in the root, decreased with increase in the transpiration rate under different humidity and light intensity conditions. On the other hand, TSCFNa+ was lower in KR1 than in IR28 under different temperature conditions. There were no different in the sodium exclusion rates at high transpiration rates. Sodium contents of tops initially increased with the transpiration rates but afterwards decreased with the transpiration rates. Sodium contents of tops were higher in KR1 than in IR28 at low transpiration rates under high humidity and low light intensity conditions, but it was higher in IR28 under low temperature conditions. There were no varietal differences in the sodium contents of tops at high transpiration rates. These results indicated that the varietal differences in sodium exclusion rates were detectable at low transpiration rates and affected the sodium contents of tops, but there were no differences in the sodium contents of tops at high transpiration rates. No potential conflict of interest relevant to this article was reported.

岡山大学農学部 Acta Medica Okayama 0474-0254 92 1 2003 酸素同位体分別を利用したイネ葉身におけるシアン耐性呼吸の測定 47 52 EN Yoshihiko Hirai Takayuki Masaki Makoto Miyoshi Toshi Yamada Takenori Saitou Kiyohiro Nakai Makoto Tsuda A reduction in the dark respiration of rice may improve dry-matter production and yield. To explore the possibility of controlling cyanide-resistant alternative respiration, of which energy production efficiency is low, we constructed a measuring system for cyanide-resistant respiration in leaf blades of rice with the use of oxgen isotope discrimination. Dark respiration was lowest at 10mM KCN, the inhibitor of cytochrome pathway, and it was lowest at 30mM salicylhydroxamic acid (SHAM), the inhibitor of the alternative pathway. Oxygen isotope discrimination was highest when leaf blade had HCN applied for 1 hour after being soaked in 10mM KCM for 3 minutes, and it was lowest when lowest when leaf blade was soaked in 30mM SHAM regardless of the soaking method. The discrimination factors of the alternative oxidase and the cytochrome oxidase of leaf blade were estimination factors of the alternative oxidase and the cytochrome oxidase of leaf blade were estimated as 20.3‰ and 13.9‰, respectively, and the ratio of cyanide-resistant respiration to dark respiration of leaf blade was 38%. No potential conflict of interest relevant to this article was reported.

岡山大学農学部 Acta Medica Okayama 0474-0254　 96 1 2007 Reduction of Dry Matter Production Related to Electrical Conductivity of Stagnant Water in Rice Subjected to Increasing Salt Stress 49 54 EN Makoto Tsuda Yoshihiko Hirai 　Knowledge about the relationships between the intensity of salinity of rooting medium and dry matter production may contribute to the improvement of rice yield under saline conditions. A rice cultivar Koshihikari was grown in pots with soils under submerged soil conditions. The stagnant water was salinized with NaCl from 31, 45 and 59 days after sowing, and for each of those, the level of salinity was increased every 6 days to maturity at three different rates of increase. Electrical conductivity (EC) of stagnant water was measured. Final dry weight of above ground parts and panicles decreased as initiation of salinization took place earlier and the rate of increase of EC was higher. Reduction in panicle dry weight was correlated with the occurrence of white heads. The values of EC at which plants died increased with the plant development and the rate of increase of EC, while these depended mainly on the rate of increase of EC but not on the time of salinization. In the plants which survived after heading, the final dry weight decreased in proportion to accumulated EC (sum of EC values for each day until harvest). Therefore, except for strong salinity where plants may die before heading, accumulated EC should be abe to predict dry matter production under salinity as an integrated indicator of level and duration of salinity. No potential conflict of interest relevant to this article was reported.

岡山大学農学部 Acta Medica Okayama 0474-0254 95 1 2006 塩条件下で発生するイネ（Oryza sativa L.) 高節位分げつの成長とナトリウム蓄積 41 47 EN Chiharu Sone Makoto Tsuda Yoshihiko Hirai The mechanism of late-grown tillers which come out of the upper culm nodes in rice (Oryza sativa L.) under saline conditions was examined. A rice cultivar, Akihikari was grown in pots under flooding conditions. The panicles were excised below the neck node of the panicle at the heading stage. Salinity treatment consisted of adding 100mM sodium chloride to water of pot at the excision of panicle (Na0), and 14 days after the excision (Na2). Tillers on the upper clum nodes grew in NaO and Na2. Dry weight of tillers increased with decrease of dry weight of stock shoots that produced the tillers. So the sum of dry weights of the stock shoots and tillers did not change, indicating that tiller growth apparently depended on reserve of carbohydrate in the stock shotts. Sodium was accumulated gradually in the plant, while sodium content of internode and tiller were kept lower than those of leaa blade and leaf sheath. In the leaf blade and leaf sheath, the upper organs accumulated less sodium than the lower organs. The tillers on the upper nodes grow faster and had low sodium content in the early stages of tiller development in NaO. Therefore, it was concluded that tillers which come out of the upper culm nodes could grow under saline conditions because the sodium content of tiller, internode and the upper leaf sheath were kept at a relatively low level. No potential conflict of interest relevant to this article was reported.

Acta Medica Okayama 2003 暗呼吸の制御によるイネの生産効率の向上に関する研究 EN Yoshihiko Hirai No potential conflict of interest relevant to this article was reported.