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In Japan, sugarcane cultivation is limited to its Southwest Islands, at the southernmost end of the Japanese Archipelago. These islands have a mostly subtropical climate, but the natural environmental conditions are harsh for sugarcane cultivation because of frequent typhoons, droughts in summer, poor soil fertility, and low temperatures in winter. Although the sugarcane industry has played an essential role in food security in Japan and the local economy of the Southwest Islands, cane production, production area, average unit yields, and number of farmers involved have been decreasing in recent years due to aging farmers, relatively slow mechanization, and adverse natural conditions. To maintain and develop the sugarcane industry and its sustainability, a network of sugarcane researchers is engaged in collaborative efforts to improve sugarcane production in the region. This paper briefly reviews the history and current status of sugarcane production and breeding in Japan, challenges the research teams are addressing, and the methodological approaches applied, including introgression-based, biotechnological, and molecular genetic breeding techniques.

Smut disease of sugarcane causes considerable yield losses and the use of resistant varieties is the best control practice. Our group identified a Japanese wild sugarcane with highly smut disease resistance named ‘Iriomote8’. In this study, we conducted QTL analysis for smut disease resistance using a mapping population derived from a resistant variety ‘Yaenoushie’, in which resistance is inherited from ‘Iriomote8’. We identified 4813 non-redundant markers using GRAS-Di technology and developed a linkage map of mapping parents. We evaluated smut disease resistance of the mapping population by the inoculation test. Consequently, a large number of clones did not show the disease symptoms and the distribution of smut disease incidence tended to be “L shaped”. Composite interval mapping detected an identical QTL for indices of smut disease incidence with a markedly high LOD score (26.6~45.6) at the end of linkage group 8 of ‘Yaenoushie’. This QTL explained approximately 50% of the cases of smut disease incidence. In the mapping population, there were no correlations between the indices of smut disease incidence and other agronomic traits. In conclusion, this QTL could be used for marker-assisted selection to significantly improve smut disease resistance without negative effects on other agronomic traits.

Direct planting – in which small storage roots are planted instead of transplanting stem cuttings – has been studied as a labor-saving system in sweet potato. Early vigor is important for cultivars used in direct planting because it affects their ability to compete with weeds. In this study, the genotypic difference in early vigor was evaluated. Twelve genotypes were tested in 2018 and 2019. Days to emergence, shoot dry matter yield, plant height, and NDVI were investigated as indicators of early vigor. Significant differences in days to emergence and shoot dry matter yields were observed among genotypes in both years. ‘Kyushu No.198ʹ was outstanding in terms of early vigor. A strong relationship between plant height and shoot dry matter yield was observed; this relationship was also observed with NDVI. Growth traits that can be monitored in a non-destructive way, such as plant height, are reliable predictors of shoot dry matter yield during early growth.

As a step toward breeding cultivars with tolerance to chilling stress, phenological changes in chlorophyll content (SPAD index) and its fluorescence (Fv/Fm) were investigated under over-wintering conditions using different sugarcane clones. A leading commercial cultivar in Japan, NiF8, and two interspecific lines, KY11-6114 and KY08-1200, were tested in this study. Daily minimum temperature decreased gradually with the advance of winter, and the lowest temperature of 1.2 °C was recorded in January. Although the temperature increased toward spring season, it remained below 10 °C until the end of experiment. Stem elongations in KY11-6114 and KY08-1200 during winter were significantly greater than in NiF8. The decrease in SPAD index was calculated from the difference between the beginning of winter and each measuring time. KY08-1200 and KY11-6114, with longer stem elongation, had a smaller decrease in SPAD index than NiF8 under over-wintering conditions. The Fv/Fm of all three clones decreased in January when the lowest temperature was recorded and recovered toward spring. Among three clones, KY08-1200 was less affected by chilling stress and presented a higher value of Fv/Fm up to the end of experiment. Considering these results, we concluded that the decreased chlorophyll content measured by the SPAD index was a simple indicator for screening chilling-tolerant clones.

To provide relevant information on utilizing fodder-sugarcane for dairy cows, we investigated the chemical composition, and long-term effect of fodder-sugarcane feeding on physiological aspects in non-lactating cows and milk production in lactating cows. The CP content of fodder-sugarcane silage (KRFo93-1; harvested 4 months after regrowth) was at the lowest level, whereas NDFom content peaked in roughage fed to lactating cows on conventional dairy farms. When 6 non-lactating Holstein cows were fed fodder-sugarcane silage with soybean meal and steam-rolled corn or sudangrass hay for 106 days, there were few changes in BW, hematological parameters, and blood metabolites throughout the experimental period. Seven lactating Holstein cows were fed TMR (TDN, 69%DM; CP, 15%DM) containing 0, 3, 5, 10, 15, or 20% fodder-sugarcane silage along with commercial concentrate (1.8 kg DM/cow/day). Milk yield and milk composition were unaffected by the proportion of fodder-sugarcane silage in the TMR. Conversely, the percentage of large particles (>19 mm) in TMR and in orts rose with an increasing proportion of fodder-sugarcane silage in TMR. The BUN concentration increased when cows were fed TMR containing over 15% fodder-sugarcane silage. These results suggest that fodder-sugarcane can be fed to cows without adversely affecting their physiological condition. However, for practical feeding to lactating cows, it is necessary to observe sorting against fodder-sugarcane silage.

To generate drought-tolerant sugarcane, we obtained hybrid lines by the intergeneric hybridization of a Saccharum spp. hybrid with Erianthus arundinaceus. A test using 5S rDNA as a marker identified five intergeneric hybrid lines with a production efficiency of approximately 3 %. There was difficulty in maintaining these hybrid lines because they were weak, and one line only survived. We examined the amount of roots and polyphenol content of the roots, which are related to drought tolerance, in the surviving line. Although the shoot–root ratio, an indicator of the amount of roots, was high in sugarcane and low in E. arundinaceus, the shoot–root ratio of the hybrid line was intermediate between those of the parents. Regarding the polyphenol content of the roots, only the surface of a root sample was stained by NaNO3 in sugarcane, whereas the entire cross section of a root sample was stained in E. arundinaceus. For the hybrid line, the entire cross section of the root was faintly stained. From these results, we concluded that the roots characteristics were introduced by the intergeneric hybridization between a Saccharum spp. hybrid and E. arundinaceus.

Sugarcane production in Japan dropped to a record low of 1,186,000 tons in 2004. Severe drought and tropical storms (typhoons) often reduce sugarcane yields. In order to avoid these environmental stresses, an early harvest system and early maturing cultivars are required. The maturing characteristics of one early maturing clone (“KY96T-547”) were thus investigated in breeding experiments to clarify effective methods of identifying early maturing clones. The results of spring planting crops in a regular breeding program and autumn planting crops intended for early harvest were also analyzed. Among the spring planting crops, the early maturing clone exhibited earlier increases in brix value at the top position of the stem, with higher orders of brix for this clone also being observed at the top. The coefficient of variance for the top brix among approximately 50 clones was higher than that for other stem positions. The coefficient of variance was higher at the beginning of October, when the brix values of early maturing clones increased. This is the best time for screening at the research site . Among the autumn planting crops, the early maturing clone matured October or November at the same level as in the regular harvest season.

Sugarcane is an important crop and also a potential feed resource in the Southwestern Islands of Japan. We investigated chemical composition and in situ degradability of two Japanese sugarcane varieties, a common variety, NiF 8 and a new variety developed for cattle feed, KRFo 93-1. We also evaluated these sugarcanes as alternatives to sugarcane top and imported oaten hay, i.e. commonly used roughages in the Southwestern Islands. The crude protein (CP) contents of both sugarcane varieties were usually lower than those of sugarcane top and oaten hay, showing a decrease with advancing maturity after ratooning. On the other hand, the non-fiber carbohydrate contents of both varieties and the effective degradability of dry matter in NiF 8 increased with advancing maturity, possibly due to sucrose accumulation. The effective degradability of neutral detergent fiber for KRFo 93-1 decreased with advancing maturity, being lower than that for sugarcane top and close to that of oaten hay. Overall, highest nutritional benefits would be obtained at a later stage harvest for NiF 8 and at an earlier stage harvest for KRFo 93-1. CP and/or energy supplements are needed when either of the sugarcane varieties is used as an alternative to sugarcane top or imported oaten hay.

In Japan a high dependency on imported food, particularly livestock feed, has resulted in the accumulation of large amounts of nutrients, especially nitrogen, in the form of livestock manure. Cultivating feed crops that can take up a large amount of N would not only mitigate the water pollution caused by the nitrate leached from the livestock sector, but would also improve food self sufficiency. Sugarcane (Saccharum spp.) is considered to be a promising candidate in this respect. Two field experiments were conducted in 2006 and 2007 in Kagawa to evaluate the dry matter (DM) yield, N content and nitrate-N concentration of sugarcane under high N input. The first experiment was designed as a factorial experiment with two factors, varieties/lines/clones and N rates set at two levels, 30 g m−2 year−1 and 60 g m−1 year−1, with three replicates. In the second experiment, 30 varieties/lines/clones of sugarcane and related genera as well as control crops were planted (n = 2) and a single N rate of 45 g m−...

ポットおよび水田で栽培した水稲品種コシヒカリを用いて, 追肥窒素が水稲に吸収される様相を把握するため, 追肥後における茎基部からの出液速度と出液の窒素濃度から出液中の窒素量を推定し, その経時的な推移について検討した. ポット, 圃場試験ともに, 出液の窒素濃度は硫安追肥後6時間目以降に上昇が認められ, 24時間目に最大となった後は漸減した. 出液の窒素濃度の上昇までに6時間以上が経過しているが, これは追肥が土壌中で拡散して根に吸収されて木部導管に入るまでに要する時間と考えられた. 圃場試験の追肥後168時間目では, 追肥区の出液の窒素濃度が追肥を施用しない対照区と同じレベルまで低下しており, 追肥の吸収は終了したと考えられた. なお, 出液の窒素分析の結果から, 圃場試験の追肥後168時間目での追肥硫安の利用率を推定すると約55％であった. 一方, 出液速度は, ポット, 圃場試験ともに, 出液の窒素濃度の上昇から12時間以上が経過した後に, 追肥区で対照区よりも高くなった. この時間的なズレが生じた原因は, 導管液中の窒素化合物の増加が直接的に根の吸水を促進したというよりは, 追肥による根の生理機能の向上により, 間接的に根の能動的吸水が促進されたためと考えられた. 葉色の反応はさらに遅れて, 追肥後48時間目に初めて追肥区が対照区よりも高くなった. 以上のように, 出液の窒素分析から, 追肥が吸収される様相を詳細かつ迅速に把握できることが明らかとなり, さらに追肥窒素の利用率や土壌中での挙動についても把握できる可能性が示された.