気温上昇が水稲品種の玄米外観品質，食味におよぼす影響

The effects of high temperatures on grain quality and palatability of cooked rice of four rice cultivars were examined in a temperature gradient chamber (TGC). Experimental plots going from TG1 (near the air intake side) to TG3 (near the air exhaust side) along the temperature gradient in TGC, corresponding to low and high temperature, and an open field plot (outside of TGC) were arranged. The mean and maximum air-temperatures in TG3 were 3.6℃ and 7.2℃ higher, respectively, than those in TG1. Brown rice yield per m2 in the outside was the highest, and those in the TGC plots were lower than outside in all cultivars. This yield decline was due to the decrease in the percentage of ripened spikelets. The increase in the percentage of sterile spikelets due to high temperatures in Nikomaru and Akimasari was larger than that in Hinohikari and Akebono. The yield difference among TGC plots was not significant. High temperature increased the white berry grains and white back grains in Hinohikari and Akebono, respectively. The overall eating quality (＋3 to －3) in the outside was in the order of Nikomaru (0.56) ＞Akimasari (0.50)＞Hinohikari (0.00)＞Akebono (－0.06). The overall eating quality in TG3 was lower than outside, especially in the quality of appearance. However, the decreasing degrees of appearance quality in Nikomaru and Akimasari were smaller than those in Hinohikari and Akebono. The protein content of milled rice in TG3 (6.0−7.8%) was higher than that in the outside (8.6−10.4%) in all cultivars. The increase in protein content by higher temperature was in the order of Akebono (4.4%)＞Hinohikari (1.9%)＝Akimasari (1.9%)＝Nikomaru (0.8%). In terms of grain quality, the cultivation of new cultivars, Nikomaru and Akimasari are recommended when air temperature exceeds normal years. However, yield reductions in Nikomaru and Akimasari caused by high temperatures were larger than those in the conventional cultivars, Hinohikari and Akebono.

岡山県平野部の主要水稲品種は，中生のヒノヒカリ，晩生のアケボノである．近年，夏期の高温障害が原因とされる乳白米，腹白米などのいわゆる白未熟粒の発生による玄米外観品質の低下が顕著となっており，その回避技術やより高品質，良食味品種の選択など，さまざまな対策が検討されている．このような背景のもと，1996年に九州地方において多収，良食味の観点から「にこまる」，「あきまさり」が育成された．にこまるはヒノヒカリと同等の熟期で極良食味であることに加えて，玄米外観品質が優れ，高温下における白未熟粒の発生が少ない品種である．一方あきまさりは，アケボノと同等の熟期で，ヒノヒカリ並みの食味を有する品種である．すなわち，これら２品種をヒノヒカリ，アケボノに替わる品種として導入することにより，より良食味で高品質米の生産が可能であると考えられる．著者らは，水田および畑圃場に温度勾配チャンバー（TGC）を設置し，温暖化による気温上昇を想定した作物（水稲，ダイズ，冬コムギ）の生産性を評価してきた．水稲（品種：日本晴）では，気温上昇によって不稔籾の増加することで減収となること，白未熟粒の発生によって玄米外観品質が低下することを報告した．また，気温上昇下での米飯の食味についても検討したが，その品種間差については検討を行っていない．本研究では，水田内に設置した TGC を用いて，移植から収穫に至る生育期間全体の気温上昇が，水稲４品種の収量，玄米外観品質，米飯の食味におよぼす影響の品種間差異を検討した．