@article{oai:repository.naro.go.jp:00001943,
 author = {濃沼, 圭一 and KOINUMA, Keiichi},
 journal = {九州沖縄農業研究センター報告, Bulletin of the NARO Kyushu Okinawa Agricultural Research Center},
 month = {Dec},
 note = {To improve the quality of silage maize in Japan, emphasis has been placed on increasing the dry matter ear content. However, genotypic differences in stover digestibility were recently recognized, and breeding programs to improve stover digestibility have been initiated. Development of single-cross hybrids between US dent and Japanese native flint groups has been the basic system of breeding in Japan, because the single-cross hybrids exhibited a significant heterosis effect in favor of productivity. However, the markedly low root lodging resistance of the Japanese flint group has been a constraint in development of root lodging resistant hybrid cultivars in this system. The studies on heterosis between US dent and Japanese flint groups have been aimed primarily at grain yield increase and rarely at improving stover quality and root lodging resistance, even though these traits are significant for silage maize. The present studies were carried out to identify appropriate selection methods for developing hybrids with superior forage quality and root lodging resistance. Attempts were also made to acquire an in-depth understanding of the heterosis expression mechanism in US dent and Japanese flint cross hybrids. For this purpose, genotypic variations and the inheritance of percent Brix of stalk juice were first investigated as indicators of stover digestibility. The development of a non-destructive and quantitative evaluation method for root lodging resistance was then attempted. Finally, the effect of population improvement on the enhancement of root lodging resistance in the breeding materials of Japanese flint was investigated, and the inheritance of root lodging resistance was analyzed to reveal the expression of heterosis and its relationship with the yield. 1. Genotypic variation and inheritance of percent Brix of stalk juice as indicators of stover quality a) The percent Brix of stalk juice varied significantly among the inbred products and also among the hybrids. High-percent Brix was found more frequently in flint inbreds and dent × flint hybrids than in dent inbreds or dent × dent hybrids. A significant positive correlation of the percent Brix between hybrids and mid-parents indicates that the percent Brix is genetically controlled and that inbreds with a high percent Brix show promise for developing hybrids with high stover digestibility. However, a negative correlation between the percent Brix of stalk juice and dry matter ear content was observed, which suggests that a careful selection for higher percent Brix is required to avoid any loss of ear yield (Tables 1 to 3 and Figs. 1 to 2). b) The percent Brix and estimated stalk sugar content (ESSC) increased linearly from the lowest internode up to the one just above the top ear and maintained almost constant values above it. The differences among hybrids reached the maximum for both traits at the internode just above the top ear. The least significant differences for both traits at internodes above the top ear were smaller than at internodes below the top ear. These results confirm that the internode just above the top ear is most suitable for measuring the percent Brix as an indicator of stover digestibility (Tables 4 to 6 and Fig. 3). c) The additive and dominance effect for the percent Brix of stalk juice and ESSC, estimated using a generation mean analysis for two crosses, indicated that the additive effect was approximately three times as large as the dominance effect for both traits. Estimates of the dominance effect were small in both crosses and not significant in one of the two crosses. These results indicate that the percent Brix of F_1 hybrids can be approximately estimated from the mid-parental values. The percent Brix was highly correlated (r = 0.608^** to 0.977^**) with the ESSC in segregating generations, although the correlation was somewhat lower when the percent Brix was higher than 9 to 10%. Therefore, it is desirable for indirect selection of stover digestibility that the selected materials with high percent Brix be initially re-evaluated for ESSC (Tables 7 to 9 and Figs. 4 to 6). 2. Development of a non-destructive and quantitative evaluation method for root lodging resistance a) The horizontal pulling resistance varied widely among 13 inbreds and was highly correlated with the root pulling resistance (r = 0.811^**) and the weight of the root clump (r = 0.863^**). These results suggest that the strength of the root system can be evaluated by the horizontal pulling resistance (Tables 10 to 12 and Fig. 7). b) The horizontal pulling resistance value (HPR value) was calculated to evaluate the root lodging resistance, which is considered to be closely related to the strength of the root system and the size of the above-ground parts ; the following formula was used for the calculation. HPR value = √Stalk legth (cm) × Ear height (cm)/Horizontal pulling resistance (N) The two years' mean HPR value and that for the discriminant function value of the root lodging resistance (DF value) reported by Ishige et al. (1983) were compared for correlation with the three years' mean of the percentages of lodged plants. The correlation coefficients of the HPR values (r = 0.631^** in hybrids, r = 0.564 in inbreds) were higher than those of the DF values (r = 0.277 in hybrids, r = 0.382 in inbreds). However, the correlation was not significant in the inbreds. These results indicate that the HPR value is non-destructively measurable and efficient for a quantitative evaluation of the root lodging resistance of hybrids (Tables 14 to 17). c) The HPR value was applied to evaluate the root lodging resistance of inbreds. Based on the low correlation between the HPR value and the percentage of lodged plants in the 15 inbreds tested, it was considered that HPR values are not always useful for directly evaluating inbreds. However, a significant positive correlation was observed between the HPR value and the percentage of lodged plants for the hybrid mean of each inbred, i.e., the mean of the hybrids in which the inbred was involved as a parent. These results suggest that the root lodging resistance of inbreds can be efficiently evaluated based on the hybrid mean of the HPR value, and that nine crosses in a single-year experiment or four crosses a year in two-year experiments are required for an appropriate evaluation (Tables 18 to 21). 3. Selection efficiency and genetic analysis for root lodging resistance a) A cyclic selection was attempted for two breeding populations, MF and MC, to improve the root lodging resistance of Japanese native flint. The HPR value and DF value indicated that the root lodging resistance of both populations clearly increased with the selection cycle and that the root lodging resistance of the advanced populations reached almost the same level as the US dent population. The changes in the traits related to root lodging resistance differed between the two populations. The MF populations clearly demonstrated increaseing root system strength with the selection cycle. In contrast, the decreased size of the above-ground parts was notable in the MC populations (Tables 22 to 25 and Figs. 12 to 13). b) Diallel analysis was conducted for the percentage of lodged plants and the HPR value. Almost the same results were obtained for both traits. The additive effects (a) were significant (P < 0.01 for the percentage of lodged plants and P < 0.05 for the HPR value). The dominance effects averaged over all hybrids (b_1) and those due to parental inbreds (b_2) were not significant, but the dominance effects due to specific combinations (b_3), i. e. the specific combining ability (SCA) effects, were significant (P < 0.01). The percentages of lodged plants and the HPR values were lower in the dent × flint hybrids than in the dent × dent and flint × flint hybrids. In contrast, the rates of dry matter production were higher in the dent × flint hybrids than in the dent × dent and flint × flint hybrids. These results indicate that the heterosis between the dent and flint inbreds is expressed in the root lodging resistance as well as in the dry matter production and that hybrids with a high yielding ability and high resistance to root lodging can be developed effectively by combining dent inbreds with flint inbreds if the root lodging resistance is adequately improved (Tables 26 and 27 and Fig. 14). c) General and specific combining ability effects for root lodging resistance and the relationship of the root lodging resistance with the whole plant yield were investigated in F_1 hybrids between dent and flint inbreds. Although the general combining ability (GCA) effect was highly significant (P < 0.01), the specific combining ability (SCA) effect was not significant for either the percentage of lodged plants or the HPR value. No definite relationships were detected between the root lodging resistance and the rate of dry matter production in the hybrids. Some hybrids showed a higher root lodging resistance and also a higher yielding ability than the check commercial hybrids. These results indicate that the evaluation of the GCA effect for root lodging resistance of parental inbreds is important in breeding dent × flint hybrids, and also suggest that hybrids with a high root lodging resistance can be developed along with an improvement of the whole plant yield (Tables 29 and 30 and Fig. 15)., デント種・フリント種間での雑種強勢を最大限に利用した高品質・耐倒伏性トウモロコシ品種の効率的な育種法の確立を目的に研究を行った。茎葉消化性の指標形質としての稈汁ブリックス値は品種・系統間差異が大きく, 稈汁ブリックス値の高い系統はフリント種自殖系統とそれらの F_1 組合せに多いことが判明した。稈汁ブリックス値とそれを乾物率で補正した推定糖含量の部位別変動から, 着雌穂節直上の節間部を茎葉消化性評価の際の最適測定部位として決定した。また, 自殖系統間交雑の世代平均分析により, 稈汁ブリックス値と推定糖含量は主として相加的に遺伝することを明らかにし, これらを指標とした高茎葉消化性系統の選抜法について考察した。一方, HPR 値 (引倒し法評価値) を指標とする耐倒伏性の非破壊・計量的簡易検定法を開発し, 本法が F_1 系統の評価に有効であること, 自殖系統の評価には HPR 値の一定親 F_1 平均値による後代検定が有効であることを明らかにした。また, 在来フリント種の耐倒伏性が集団改良によりデント種並の水準に向上していることを確認し, 耐倒伏性が十分に改良されたデント種・フリント種自殖系統間では耐倒伏性と収量のいずれにも雑種強勢が発現し, 耐倒伏性・多収品種を効率的に育成できること, デント種×フリント種 F_1 組合せの耐倒伏性向上には一般組合せ能力の的確な評価が重要であることを明らかにした。},
 pages = {79--125},
 title = {サイレージ用トウモロコシの高品質・耐倒伏性育種に関する研究},
 volume = {39},
 year = {2001},
 yomi = {コイヌマ, ケイイチ}
}