Manganese (Mn) is one of the essential mineral nutrients for land plants, and Mn is involved in various physiological reactions as an activator or cofactor of certain enzymes in plant cells. In crop plants, it is well known that Mn deficiency decreases the crop yield by attenuating photosynthetic activity, especially in oxygen-evolving activity in photosystem II. On the other hand, the excess Mn fertilization also suppresses the photosynthetic activities in crop plants to reduce their yield. However, the detailed molecular mechanisms to suppress photosynthetic activities under excess Mn fertilization is remained to be clarified. In this review, we introduce the mechanisms of how excess Mn suppresses photosynthetic activity in land plants based on our recent research using rice plants.

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A thiol-specific oxidant diamide induces the generation of reactive oxygen species and the peroxidation of lipids. In this study, we developed the method to evaluate diamide sensitivity in rice. Our method revealed that there are genetic differences in the diamide sensitivity among rice cultivars. Shoot and root growth of all cultivars tested was remarkably inhibited at the concentration of 2 mM diamide for 7 days treatment. Among cultivars tested, a japonica cultivar Koshihikari showed high tolerance to diamide, and an indica cultivar Nanjing 11 showed a large decrease of root elongation. In two japonica cultivars Akichikara and Hounen-Wase, there was no significant difference in growth inhibition between shoot and root. Furthermore, by using our method, we successfully isolated diamide-sensitive and -tolerant mutants among 328 M₂ lines derived from gamma-irradiated seeds of a japonica cultivar Gimbozu.

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The X-ray CT measuring was carried out in SPring-8 for the judgment of the morphology of the unhulled rice contained in the 12 rice seed aggregates of the early, middle and latter of Yayoi period excavated from the five remains in Nara Prefecture, Tottori Prefecture, and Tokyo. From the analysis of the X-rays CT image, the 12 rice seed aggregates excavated included the spikelet and part of panicle, and the unhulled rice in the 12 aggregates were classified into the temperate japonica type. In many cases, the grain shape of unhulled rice in the aggregates differed among 12 aggregates. The length and width of unhulled rice in the 12 aggregates showed the normal distribution with small variation width as those of modern cultivar. Based on the above-mentioned results, the unhulled rice in each aggregates might originate from single rice variety of the temperate japonica type having different grain shape. The width of unhulled rice in the aggregates of Aoyakamijichi remain were bigger, and these aggregates were classified into the smaller type of length/width ratio as compared with other aggregates.

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Regarding the black-seeded soybean ‘Tambaguro’ for Edamame (green soybean) cv. ‘Murasakizukin 3’ and ‘Murasakizukin 2’ in Kyoto prefecture, it was requested to increase the seeding fields in Kyoto Prefecture due to the lack of seeds. Therefore, in order to gain basic knowledge of seed cultivation in the prefecture, we tried to produce stable seeds with a high germination rate in multiple fields, mainly in the central and northern regions of Kyoto Prefecture. In the case of ‘Murasakizukin 3’, it was suggested from the results of regional trial fields suggested that the combination of late sowing, white-surface black-underside multi and dense planting can contribute to increase of seeds yield of higher germination rate more than the standard value of 75%. On the other hand, it was speculated that the optimum sowing time for seed production of ‘Murasakizukin 2’ would be middle-late July because the germination rate of the obtained grains was higher than 75% of standard value.

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Regarding soybean cultivation in the southwestern warm region, cultivation tests, in which four topdressing treatment plots with different timings and amounts and non-topdressing plot were set up, were conducted in order to examine the effect of nitrogen topdressing using slow release nitrogen fertilizer after beginning of the flowering stage on the soybean growth and yield. The above ground dry matter weight, the above ground nitrogen uptake and the total number of pods increased in the treated plots with topdressing compared to the non-topdressing plot after the pod setting period. The dry grain weight and the amount of grain nitrogen in the mature stage showed large values in the early group for the topdressing timing and in the large group for the topdressing amount. However, the grain nitrogen content showed large values in the late topdressing group. Although the total number of pods was the largest in the contribution to the grain yield, no clear tendency was observed in the total number of pods due to the difference in the topdressing timing and the topdressing amount. From these results, the effect of increasing yield by topdressing using slow release nitrogen fertilizer after the beginning of flowering stage was observed in soybean cultivation in the southwestern warm region. In addition, it was suggested that topdressing at the beginning of flowering stage is effective when the purpose is to secure the dry matter weight, nitrogen uptake and the grain yield, and that topdressing 2 weeks after the beginning of flowering stage was effective when the purpose is to increase the grain nitrogen content.

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The black soybean ‘Tambaguro’ is a specialty product in Kyoto Prefecture, Japan. Recently, it is often reported that the stem and leaf of Tambaguro remain green compared to the ripening progress of pod and the maturation of entire plant is delayed, called green stem disorder. Therefore, we investigated the effect of leaf removal or root cut treatment after the R7 stage on suppressing green stem disorder, yield and grain quality. Depending on the year, the leaf removal and the root cut treatments promoted fading of the color of stem and leaf and decreased the degree of green stem disorder in the R8 stage. Because damaged and wrinkled grains tended to be decreased and increased, respectively by the leaf removal treatment, decrease in the water content of plant and the source to sink in the period of R7-R8 might affect grain quality.

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It is expected to greatly contribute to the subsequent fertilization management and yield prediction, if the above-ground dry matter weight can be estimated accurately in the crop cultivation process. We tested if dry matter weight of soybean could be estimated using a new vegetation index, which combines daily chlorophyll index and daily irradiance (∑(CI×I)). For comparison, the normalized differential vegetation index (NDVI) and the chlorophyll index (CI) were used. Soybean cultivar Sachiyutaka was cultivated in Nara at 2018 and 2019, and Kyoto at 2020, and various soybean populations with different dry matter weights were obtained by changing the planting density or nitrogen fertilizer applications. We measured dry matter weight of soybeans 4 to 5 times and two vegetation indexes, NDVI and CI, 5 to 6 times during the growing period, at about two weeks intervals, as a guide. From the relationship between dry matter weight of soybeans and each vegetation index, we obtained the regression formula and coefficient of determination. As a result, ∑(CI×I) was significantly related to the dry matter weight throughout the three years, and coefficients of determination ranged from 0.90 to 0.97. In addition, in the growing period after the beginning of flowering, there was no significant difference in the slope and Y-axis intercept of the regression formula of the dry matter weight by using ∑(CI×I) among years. It was clarified that ∑(CI×I) could be used to estimate dry matter weight of soybean with one regression formula in the growing period after the beginning of flowering even in different populations. These results indicate that the new vegetation index of ∑(CI×I) is effective as an index for estimating the dry matter weight of soybean.

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Cultivation tests using rice cultivars Fujihikari and Hanahyogo, in which sowing period treatment 2 levels (I: May 12 sowing, II: June 15 sowing) and seedling raising period treatment 2 levels (S: about 20 days, L: about 30 days) were set up, were conducted and the changes in the above-ground dry matter weight, the above-ground nitrogen content during the growing season, the yield and its components were investigated. In addition, the occurrence of tillers at the maturity stage, the number of spikelets and the percentage of ripened grains for each tiller were also investigated. In both Fujihikari and Hanahyogo, the S group showed larger values than the L group during the growing period I and II, and showed a large above-ground dry matter weight, above-ground nitrogen content and yield at the maturation period. Although no significant difference was observed in the four treatments, the dry matter weight, nitrogen content and yield at the maturity stage were all IS＞ ⅡS＞ ⅠL ≒ ⅡL for Fujihikari, while ⅠS ≒ ⅡS ＞ ⅠL ≒ ⅡL for Hanahyogo. In Fujihikari, abnormal heading was observed in 80%, 62%, and 100% of individuals in ⅠL, ⅡS, and ⅡL, respectively, but no abnormal heading was observed in all treatment groups in Hanahyogo. In all the individuals, in which abnormal heading was observed, the occurrence of higher-order bruising was observed, and a decrease in seed fertility was observed. In late transplanting cultivation in the southwestern warm region, Huahyogo was thought to have excellent adaptability, but it was suggested that the yield decreases due to the decrease in the number of spikletes when the seedling raising period is extended

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