氏   名
チュン タック ホップ
Truonq Tac Hop
本籍(国籍)
ベトナム
学位の種類
博士(農学)
学位記番号
甲 第110号
学位授与年月日
平成11年3月24日
学位授与の要件
学位規則第4条第1項該当
専  攻
生物生産科学専攻
学位論文題目
Growth and Yield of rice cultivated under nitrogen-free basal dressing
(基肥無窒素栽培水稲の生育と収量)

論文の内容の要旨

   The growth and yield of rice cultivated under the practice, in which no nitrogen (N) was applied at basal dressing followed by the first N top-dressing at the eighth leaf stage and a sparse planting of 13〜17 hills m-2, were not affected so seriously in the extremely cool weather of 1993. This practice is really contrasted to the widely used practice, in which N is commonly applied at basal dressing with a planting density of 22.2 hills m-2 (CONT). In normal weather years, the lower yield under this practice was due to the reduced number of panicles and the number of spikelets per unit area. Therefore, the purpose of the present study was to improve the yield of the above practice through increasing the number of panicles per unit area, the number of spikelets per panicle and the percentage of ripened grains by using two popular rice varieties, Akitakomachi and Hitomebore.

 The experiment was conducted using three main treatments as follows:
(1)N-free basal dressing followed by the first N top-dressing at the eighth leaf stage and common planting density of 22.2 hills m-2 (NFB).

(2) N top-dressing at the neck node differentiation stage and sparse planting of 16.7 hills m-2 under N-free basal dressing (NFBS).

(3) Early N top-dressing at the sixth leaf stage, instead of at the eighth leaf stage, under N-free basal dressing and sparse plating density (NFBS6).

The results obtained from the studies were summarized as below:

1. The common planting density conducted in NFB was thought to prevent the reduced number of panicle per unit area. N content in the leaf blades was lower in NFB at any growth stages, except at heading. The development of roots was more vigorous in NFB at the sixth and eighth leaf stages. The significantly reduced maximum number of stems and the number of panicles per unit area in NFB were due to the low rate of tillering from the lower nodes. The number of spikelets per unit area in NFB was 80〜90% of that in C0NT. It is hardly possible to keep the number of panicles and the number of spikelets per unit area in NFB as high as C0NT. The markedly higher percentage of ripened grins and the l,ooo-grain weight were distinctly observed in NFB. The grain yield in NFB was 630〜670 g m-2, insignificant difference to that in C0NT. The growth analyses were determined at the various growth stages and found that crop growth rate (CGR) and leaf area index (LAI) were lower, but net assimilation rate (NAR) and specific leaf weight (SLW) were higher in NFB at the panicle formation stage. All the values, except LAI, were higher in NFB at the grain-filling stage.

2. In NFBS, N top-dressing at the neck node differentiation stage was aimed to increase the number of spikelets per panicle. Through such treatment, the N content in the leaf blades increased in NFBS. A significantly reduced maximum number of stems, but a higher percentage of fruitful culms were distinctly observed in NFBS. Although the numbers of spikelets per panicle significantly increased in NFBS, the percentage of ripened grains was significantly higher in NFBS. The grain yield in NFBS was 660〜7 lO g m-2, statistically identical to that in C0NT. The NFBS could be a very useful practice to attain high yield through increasing the number of spikelets per panicle and per unit area, and the higher percentage of ripened grains. Although N was top-dressed at the neck node differentiation stage, a luxuriant growth did not appear in NFBS.

3. Although earlier N top-dressing was applied at the sixth leaf stage, the number of panicles from lower nodes, the total number of panicles and spikelets per unit area were not significantly different between NFBS6 and NFBS. This result was probably due to the poor N absorption caused by the undeveloped superficial roots at the sixth leaf stage and low temperature at the top-dressing. The weight of a single panicle of the secondary tillers was significantly greater in both NFBS and NFBS6, but the percentage of ripened grains did not decline. These results indicated that productivity of the secondary tillers was superior in both NFBS and NFBS.

4. The dry matter production was analyzed in NFBS based on per unit area. As compared to C0NT, the total sink size was significantly smaller in NFBS, but the yield was not significantly different between the treatments. However, the sink size and the yield per unit area of the secondary tillers were significantly greater in NFBS. At the grain-filling stage, the increase in total dry weight (ΔW) was significantly greater in NFBS, which probably resulted from the remarkable increase in dry weight of leaf blades (ΔL) in NFBS at the booting stage. At both the early and middle grain-filling stages, CGR, NAR and SLW were greater in NFBS. The canopy in NFBS apparently received a large amount of solar radiation at the grain filling stage.

5. Although the number of spikelets per panicle distinctly increases in NFBS, the percentage of ripened grains was significantly higher in NFBS. Not only the sink size but also the yields per panicle was significantly higher in NFBS. Through the grain-filling, the increase in dry weight of grains (ΔG) and ΔW were significantly greater in NFBS. Through the booting to the late grain-filling stage, the decrease in ΔL, especially ΔL of the secondary tillers was significantly smaller in NFBS. As a result, CGR per panicle was 4l〜7 1 % greater in NFBS. The results indicated that a single panicle in NFBS was more productive than that in CONT.

The results from the present experiment suggested that the practice in NFBS could be useful to apply for rice cultivation under the conditions of Vietnam in some following aspects: (i) it may be applied for some indica rice, which is usually sensitive response to N fertilizer and shows a luxuriant growth due to a heavy N manuring. (ii) It may be useful for the early maturing varieties with short duration of ripening in order to increase the percentage of ripened grains through the improved light-intercepting characteristics as in NFBS. (iii) In case of direct seeding practice, applying NFBS might avoid a luxuriant growth at the early growth stage, which usually causes a dense population at later growth stages.

 (和 訳)

 基肥無窒素―8葉期以降追肥一疎植(13―17株m‐2)の条件で栽培された水稲は,1993年の大冷害にも大きな影響を受けなかった。この栽培法は,現在広く実施されている基肥窒素施用―やや密植(22.2株m‐2)の栽培法とは著しく異なる。しかし,この栽培法では,普通気象の年には収量が若干低くその原因は面積当たり穂数および籾数の減少にあると考えられた。そこで,本研究では基肥無窒素栽培条件における水稲収量の改善を目的として,岩手県下で最もポピュラ-な品種であるあきたこまちとひとめぼれを供試し,面積当たり穂数および1穂当たり籾数の増加と登熟歩合の向上を期待して次の3つの栽培法を試みた。

 (1)基肥無窒素―8葉期以降追肥―22.2株m‐2の普通栽植密度(NFB)

 (2)(1)の栽植密度を16.7株m‐2の疎植に変え,穂首分化期に窒素追肥(NFBS)

 (3)(2)の8葉期追肥の代わりに6葉期に窒素追肥(NFBS6)

得られた結果の大要は次の通りである。

 1.NFBにおいては,栽植密度を普通栽培並とすることで穂数の低下を抑制することを目的とした。出穂期を除く生育期の葉身窒素含有率が低かったが,6葉期および8葉期に調査した根の生長はきわめて活発であった。最高分げつ数および穂数は有意に少なかったが,これは下位節からの分げつが少なかったことに原因があった。その結果,面積当たり籾数は普通栽培(CONT)の80―90%に止まり,この栽培法では穂数および籾数の確保は困難であると考えられた。しかし,登熟歩合および1000粒重は明らかに増加し,収量は630―670gm‐2 の多収となりCONTと大差無かった。物質生産構造を解析するため生長解析を実施した結果,幼穂形成期においては純同化率(NAR)および比葉重(SLW)は大きかったが,個体群生長速度(CGR)および葉面積指数(LAI)は小さかった。しかし,登熟期においてはLAIを除くこれらのパラメーターはすべて大きく登熟歩合の高いことと密接な関係があると考えられた。

 2.NFBSにおいては,穂首分化期の窒素追肥により1穂当たり籾数の増加を意図した。窒素追肥以降葉身窒素含有率は増加した.疎植のため最高分げつ数は著しく低下したが,有効茎歩合が高く穂数の減少は小さかった。1穂当たり籾数が有意に増加しただけでなく登熟歩合も高く,収量は660―710gm-2でCONTと有意差はなかった。この結果から,NFBSは1穂当たり並びに単位面積当たり籾数の増加および登熟歩合の上昇を通じて高収量を実現できる技術として極めて有効なことが明らかになつた。さらに穂首分化期の窒素追肥は過繁茂を誘発しやすいと言われているが,NFBSでは過繁茂の兆候は全く見られなかった。

 3.NFBS6においては,最初の窒素追肥を早めることで下位節からの分げつを促進し穂数の確保を狙った。しかしながら,下位節からの分げつは促進されることなく,面積当たり穂数および籾数はNFBSと大差無いかむしろ低下した。その大きな原因として,6葉期の水稲はうわ根が十分に発達していない上に地温が低いなど追肥窒素の吸収が非効率的なことが考えられた。しかしながら,NFBSおよびNFBS6においては,とくに2次分げつ穂が重くまた登熟歩合が有意に高かった。このことはNFBSおよびNFBS6においては2次分げつにおける生産力が普通栽培に比べて優つていたことを示している。

 4.NFBSにおける生産構造を解析した.CONTに比べて,NFBSでは面積当たりシンクサイズが有意に小さく収量は大差無かったが,2次分げつのシンクサイズおよび収量が著しく大きかった。また,登熟初期において生産量が大きかったが,その原因として穂ばらみ期において葉身重の増加が大きく,また登熟期においてその減少が小さかったことおよびこれらの時期に受光体勢のよかったことが考えられる。実際に登熟初期および中期においてCGR,NARおよびSLWが大きく,光合成活性の高かったことが示唆された。

 5.NFBSでは1穂当たり籾数が増加したにも関わらず登熟歩合は低下しなかった.1穂当たりシンクサイズおよび収量は有意に大きく,また登熟期における粒重および全乾物重増加も大きかった.とくに,2次分げつでは穂ばらみ期から登熟期にかけて葉身重の低下が小さかった,その結果1穂当たりのCGRは41―71%も大きかった。これらの結果からNFBSでは1穂当たりの生産力が大きかったと考えられた。

 以上の実験結果は,現在および将来のベトナムにおける水稲栽培に対し,次の点で有用な栽培技術と考えられる。(i)ベトナムで栽培されている水稲はインデイカ型であり吸収窒素に対する反応が大きく倒伏し易いが,基肥無窒素―疎植栽培は耐倒伏性であり窒素肥料の効率的利用が可能である。(ii)ベトナムの水稲は早生品種の方向にあり,登熟期における生産力が大きいことは登熟期間の短い早生種に有利である。(iii)直播栽培においては基肥窒素の過剰施肥により過繁茂の危険性が非常に高く,基肥無窒素によりその危険性が著しく軽減される。