Effect of shading on dry-matter production, yield and grain appearance quality of Vietnamese rice cultivars （Oryza sativa L.） in the paddy field

Light intensity is one of the most important environmental factors to determine the growth and yield of rice. However, little information is known about the response of indica cultivars, especially Vietnamese cultivars to shading conditions. In this study, a field experiment was conducted in 2015 to determine the response of Vietnamese cultivars to shading condition during the grain filling period. Fourteen Vietnamese cultivars were covered by black cloth (50% shading treatment) after the heading stage in the field conditions. Grain yield of fourteen Vietnamese cultivars in shading condition fell to about 32.2–65.0% of the natural light condition. The most sensitive cultivars to shading are ‘Jasmine 85’ and ‘OM2395’, of which the grain yield decreased to 65.0 and 59.8% of natural light condition, respectively. The shaded yield decreased corresponding to the reduction in crop growth rate (CGR) and yield components, especially in the percentage of filled grain due to the increase in spikelet sterility. Shading increased the percentage of immature grains in all cultivars. Shading induced an increase in the percentage of chalky and green grains, which was largest in ‘OM2517’ and ‘OM5451’, respectively. Clearly, shading depressed both grain yield and the appearance quality of brown rice in Vietnamese cultivars, similarly reported in previous studies with Japanese cultivars.

　光強度は，水稲の生育・収量を決定する最も重要な環境要因である．しかし，光強度の減少，すなわち遮光処理がインド型水稲品種，特にベトナム産品種の生育・収量に及ぼす影響に関する知見は限られている．本研究では，登熟期間の遮光がベトナム産水稲品種の乾物生産，収量と収量構成要素，玄米の外観品質に及ぼす影響を2015年に調査した．ベトナム産水稲14品種を圃場栽培を行い，最初に出穂が認められた品種の出穂期以降収穫期まで，群落上層を黒色寒冷紗（遮光率50 ）を用いて被覆する，遮光処理を行った．ベトナム産14品種の収量は，遮光に伴い自然光条件に比較して32.2～65.0 の低下がみられた．遮光処理の影響が大きかった品種は‘Jasmine 85’ と‘OM2395’ で，それぞれ65.0，59.8 の収量低下がみられた．収量の低下には，登熟期の個体群成長速度，収量構成要素では穎花不稔の増加による登熟歩合の低下に起因していた．遮光処理はすべての品種で玄米の未熟粒割合を増加させた．遮光処理により，白未熟粒と青未熟粒の増加がみられ，その程度は白未熟粒が‘OM2517’ で，青未熟粒が‘OM5415’ で最も著しかった．以上の結果，遮光処理はベトナム産水稲14品種の収量と玄米の外観品質を低下させたが，その程度には品種間差があることが明らかになり，これらの傾向は日本産水稲品種で得られた結果と同様であった．

原著論文 (Original paper)