The growth and yield of different rice genotypes, which were nine newly-released rice cultivars or lines(NRC) and three widely-cultivated rice cultivars(WCC), in the practice of nitrogen-free basal dressing accompanied with sparse planting density(BNo) were examined and compared with those in the conventional cultivation(CONT). The objective of the study was to provide some information on the theoretical basics and practical references for the development of the stable and high yielding cultivars or cultivation practices in the Tohoku region of Japan. The results obtained from the study are summarized as follows:

　1.The number of tillers was larger in the cultivars of the panicle-number type than in the cultivars of the panicle-weight type, and was larger in CONT than in BNo. A large number of tilers in CONT, especially the secondary tillers, were nonproductive, and most of those in BNo were productive. The difference between BNo and CONT in the number of panicle m^-2 was larger in the cultivars of the panicle-number type than in those of the panicle-weight type. Grain yield was often higher in the NRC than in the WCC in both BNo and CONT. Grain yield in BNo was often lower than that in CONT, and the yield averaged over years and cultivars was 748g m^-2 in BNo and 772g m ^-2 in CONT(the ratio of yield in BNo to that in CONT was 97%). The difference between BNo and CONT in grain yield varied with the cultivar and the year. Under favorable weather conditions in 2000 and 2001, the grain yield was high in both CONT and BNo, and was higher in CONT than in BNo in most cultivars. However, under the unfavorable weather condition in 1999, grain yield was low both CONT and BNo,and that in BNo was similar to or higher than that in CONT. In all 3 years,the grain yield of Akitakomachi and Fukuhibiki was lower, and that of Ouu316 and Hitomebore tended to be higher in BNo than in CONT.

　2.The number of spikelets per panicle(NSp(^-1),especially in the late-maturing cultivars and in 2001,was larger in BNo than CONT. The number of panicle m^-2 (NPm^-2), however, was significantly smaller in BNo than in CONT,resulting in small number of spikelets m^-2(NSm^-2) in BNo. The difference between BNo and CONT in the NSm^-2 varied with the cultivar and the year, and it was smaller in the late-maturing cultivars than in the early- aand medium-maturing ones, and was also smaller in 2001 than in 1999 or 2000. The percentage of ripened grains(PRG) was higher in BNo than in CONT in all cultivars, and the average PRG of 12 cultivars in BNo was 14.0%, 9.0% and 4.8% higher than that in CONT in 1999,2000 and 2001, respectively. Grain weight(1,000 grain weight) was heavier in BNo than in CONT in most cultivars, and the 1,000-grain weight averaged over cultivars and years was 0.8g(3%) larger in the former than in the latter. High temperature during the grain-filling period significantly and adversely affected the 1,000-grain weight in CONT, but only slightly in BNo.

　3.During the period from transplanting to panicle initiation(PI),dry matter production(DMP) was lower in BNo than in CONT in all 3 years. The DMP during the period from PI to full heading(FH) was also lower in BNo than in CONT, particularly in the high solar radiation year of 1999, because of the small leaf area index in BNo. During the ripening stage, leaf area index in BNo was smaller, but crop growth rate in BNo was similar to or higher than in CONT due to the large net assimilation rate, which resulted from the large flag leaf and 2^nd leaf, and the heavy specific leaf weight in BNo. The DMP per panicle during the period from PI to FH positively and significantly correlated with the number of spikelets per panicle. The PRG was also closely related with the amount of carbohydrates from stems (－△S) and photosynthesis after FH(△W) contributed to a apikelet during the early ripening period(during 20 days after FH). The higher DMP per panicle in BNo compared with CONT during the period from PI to FH,therefore,resulted in the larger number of spikelets panicle per panicle in the former. On the other hand, the large amount of carbohydrate supply per spikelet during the early ripening period could also secure a high and stage PRG in BNo, especially under the unfavorable weather condition.

　4.The number of spikelets per panicle was in the order: the main stems＞primary tillers＞ secondary tillers in all cultivars. The numbers of spikelets per panicle on the main stems, primary and secondary tillers were larger in the cultivars of the panicle-weight type than in the cultivars of the panicle-number type mainly because of the larger number of spikelets on the secondary rachis branch(SRB) in the former cultivars. In comparison with CONT, the number of spikelets per panicle was larger in BNo in most cultivars. The difference between BNo and CONT in the number of spikelets per panicle was larger for the panicle on the secondary tillers than on the main stems and primary tillers, and was also larger in the late-maturing cultivars than in the early- and medium-maturing cultivars. The larger number of spikelets per panicle in BNo compared with CONT was mainly attributed to the increased number of spiketelts on SRB in BNo.

　5.The lodging resistance characteristics were measured and compared among cultivars as well as between BNo and CONT. The lengths of the lower internodes and culms were often shorter in NRC than in WCC. Bending moment by whole plant was not different between NRC and WCC, but the breaking strength at the basal internode(Ⅳ)with leaf sheaths was often larger in NRC than in WCC. As a result, the lodging index was smaller in the former than in the latter.Breaking strength at the basal internode(Ⅳ) without leaf sheaths was also often larger in NRC than in WCC due to a larger cross section modulus or bending stress in NRC. Although the lengths of the upper internodes （Ⅰ＋Ⅱ＋Ⅲ）were not different between BNo and CONT, the lower internodes（Ⅳ＋Ⅴ）were shorter in BNo,resulting in the shortened culms in BNo, especcially in the long-culm cultivars. Breaking strength at the basal internode（Ⅳ）with leaf sheaths was significantly larger in BNo than in CONT, and thus the lodging index was smaller in BNo. The breaking strength at the basal internode（Ⅳ） without leaf sheaths and its two components, cross section modulus and bending stress, were also significantly larger in BNo than in CONT,particularly in the long-culm cultivars.

　The above results suggest that BNo practice is an effective cultivation method for achieving a stable and high yield of rice plants,especially for the late-maturing cultivars and in the unfavorable weather conditions. It could be used for the cultivation of rice not only in the Tohoku region of Japan, but also in the southwest part of Japan and in the tropic, where temperature and low solar radiation adversely affect grain filling, and typhoon as well as heavy rain also often couses severely lodging in rice plants.

和訳
　基肥窒素無施用－８葉期以降追肥の施肥体系と疎植の組合せ栽培は、慣行栽培（以下、CONT区）に較べて 穂孕み期冷温に伴う不稔籾の発生が少なく、冷害による減収程度が小さいことが指摘されている。 本研究では、寒冷地における水稲の普及品種（WCC，３品種）と新規育成品種・系統（NRC,９品種）を供試して、 基肥窒素無施用・疎植栽培（以下、BNo区）が異なる水稲品種の生育特性および収量性に及ぼす影響について検討した。 得られた結果の概要は、以下の通りである。

　１．㎡当たり茎数は、穂重型品種よりも穂数型品種で多く、BNo区よりCONT区で多かった。 しかし、BNo区はCONT区に比べて無効分げつが少ないことから、両区における㎡当たり穂数の相違は小さく、また、 それは穂重型品種に比べて穂数型品種で大きかった。玄米収量は、両区ともWCCよりもNRCの方が高かった。 しかし、３ヶ年・各１２品種の平均収量は、CONT区の772g m^-2に対してBNo区は748g m^-2 (97%)であり、CONT区に比べてBNo区の方がやや低いものの、両区の相違は品種および年次により異なっていた。 すなわち、登熟期が好天に恵まれた２０００年および２００１年の収量は両区とも高く、何れの品種においても CONT区に比べてBNo区の方が低かった。しかし、登熟期の天候が不順であった１９９９年は、両区とも収量は低い ものの、BNo区はCONT区とほぼ同じかやや上回っていた。また、３ヶ年ともCONT区に比べてBNo区の収量が低い品種 （「あきたこまち」と「ふくひびき」）から年次によってはCONT区に比べてBNo区の収量が高い品種（「奥羽316号」と 「ひとめぼれ」）まで、かなり大きな相違がみられた。

　２．BNo区は、CONT区に較べて穂当たり籾数は多いものの、㎡当たり穂数が明らかに少ないことから、㎡当たり 籾数は少なかった。しかし、両区におけるm^-2当たり籾数の相違は、品種および年次によって異なり、 BNo区の㎡当たり籾数は早生および中生品種に比べて晩生品種で少なく、１９９９年および２０００年に比べて ２００１年で少なかった。一方、BNo区の登熟歩合は、何れの品種においてもCONT区よりも高く、全品種の 平均登熟歩合は、年次によって異なるもののそれぞれ14.0%（1999年），9.0%（2000年）および4.8%（2001年）高かった。 また、BNo区の玄米千粒重は、ほとんどの品種においてCONT区より大きく、３カ年・全品種の平均玄米千粒重は、 0.8%(3%)大きかった。更に、登熟期間中における高温は、CONT区の玄米千粒重を明らかに低下させたものの、 BNo区においてはほとんど影響はみられなかった。

　３．移植期以降穎花原基分化期および穂揃い期における各期間の乾物生産量は、３ヵ年ともCONT区に比べて LAIの小さいBNo区の方が小さかった。しかしながら、登熟期においては、CONT区に比べてBNo区のLAIは小さいものの、 後者のCGRはNARが大きいことから前者とほぼ同じか大きかった。BNo区のNARが大きいことには、CONT区に比べて 上位２葉が大きく、且つSLWが大きいことが関与していることが示唆された。また、穂当たり籾数は、穎花原基分化期 以降穂揃期における穂当たり乾物増加量との間に有意な正の相関関係がみられ、穂当たり乾物増加量の大きいBNo区は 穂当たり籾数も大きかった。登熟歩合は、登熟期前半（出穂後２０日間）における１籾当たり乾物増加量と茎葉部からの 転流量の合計値との間に有意な相関関係がみられるとともに、登熟歩合が高いことには登熟期後半における１籾当たり 乾物増加量の相違も密接に関与していた。

　４．両区における穂当たり籾数は、何れの品種においても２次分げつ、１次分げつおよび主茎の順に多かった。 一方、穂当たり籾数は、ほとんどの品種においてCONT区に比べてBNo区の方が多かったものの、両区の相違は主茎 および１次分げつに比べて２次分げつの穂で、また早生および中生品種に比べて晩生品種で大きかった。 CONT区に比べてBNo区の穂当たり籾数が多かったことには、BNo区における２次枝梗着生籾数の多いことが関与していた。

　５．下位節間（Ⅳ）および稈長は、WCCに比べてNRCが短かった。地上部モーメントは両品種間に相違は みられなかったものの、葉鞘付き挫折時モーメントはWCCに比べてNRCの方が大きいことから、倒伏指数は前者に 比べて後者の方が小さかった。上部の節間（Ⅰ～Ⅲ）は、BNo区とCONT区で相違はみられなかったものの、 下部の節間（ⅣとⅤ）はBNo区の方が短く、とくに長稈品種でその傾向が大きかった。下位節間の葉鞘付き挫折時 モーメントはCONT区よりBNo区が明らかに大きいことから、倒伏指数は後者の方が小さかった。 下位節間の稈の挫折時モーメント、断面係数および曲げ応力は、ともにCONT区よりもBNo区で明らかに大きく、 とくに長稈品種でその傾向は大きかった。

　以上のことから、短く、堅い下位節間をもつ品種を育成するほかにも、生育初期の窒素無施用量を抑えた 施肥体系と疎植の組み合わせ栽培法もまた水稲の耐倒伏性を増大する上で有効であることを明らかにした。 以上のことから、基肥窒素無施用の施肥体系と疎植の組合せ栽培法は、寒冷地における晩生品種や不順な 気象条件下での水稲の安定・多収栽培法として有効であると考えられた。また、本栽培法は、寒冷地のみならず、 高温や日照不足による登熟不良および台風や集中豪雨に伴う倒伏が栽培上の大きな問題となっている西南暖地や 低緯度地帯における水稲の安定生産に対しても寄与する可能性が大きいものと推察された。