水稲高温登熟障害の生理生態学的解析 [in Japanese] Eco-Physiological Analysis for High-Temperature Effects on Rice-Grain Ripening [in Japanese]
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近年, 西日本を中心に多発しているイネの高温登熟障害の発生に及ぼす日射量, 施肥, 品種, 高夜温の影響とその要因について解析した。1. 人工気象室で登熟期の気温を平年より3℃高くすると玄米1粒重と玄米外観品質が低下したが, 作期移動など登熟期の気温と日射量がともに上昇する条件では玄米1粒重は低下せず, 良質粒歩合の低下程度は小さかった。高温と低日射が重なると, 普及品種ヒノヒカリでは高温耐性品種にこまるよりも粒重増加速度が低下し, 玄米1粒重と玄米の粒張りが低下した。2. 穂肥の量を増やすと, 高温年では玄米1粒重が増加し外観品質が向上した。穂肥を出穂前後の1ヶ月間に15回に分けて与える施肥法では2回与える慣行法に比べて, また, にこまるではヒノヒカリに比べて, それぞれ未熟粒歩合が低下した。これらの品質向上の要因として, 穂揃い期の茎葉における貯蔵炭水化物の増加が考えられた。3. 玄米1粒重は高夜温 (22/34℃) で低下し, 高昼温 (34/22℃) ではほとんど低下しなかった。穂と茎葉に別々に高夜温処理を与えた実験や粒重増加推移の解析, 胚乳細胞の画像解析などにより, 高夜温条件では粒重増加速度と玄米への乾物分配率の低下および胚乳細胞の成長抑制が玄米1粒重の低下に密接に関与していることが示された。以上, 本研究では高温低日射条件でも登熟が不良になりにくい品種特性や施肥法を示したほか, 高夜温が成熟期の玄米1粒重を低下させるプロセスを明らかにした。
There is a genuine concern that global warming is seriously affecting crop production. Recent high temperatures in Japan have caused a decrease in rice grain weight and quality such as its transparency and roundness. It has been pointed out that the level of damage to rice ripening due to high-temperature changes depends on environmental factors such as solar radiation, nitrogen fertilizer, and cultivars. However, we have not yet found defi nite ways to improve rice ripening under high temperature because the relationship between rice-ripening damage by high temperature and environmental or genetic factors has remained unclear. The influences of solar radiation, nitrogen fertilizer, and cultivars on rice-ripening damage by high temperature were investigated. The effects of eco-physiological factors of high night temperature on rice ripening were also examined because it has been predicted that night temperature will increase more than day temperature in the future. The results of the present study may be summarized as follows. 1. Influences of solar radiation on degree of rice-ripening damage from high temperature. The effects of high temperature during ripening on grain weight and quality were investigated in growth chambers. The results indicated that the grain weight under future predicted 35/26℃ (day/night temperature) air temperature decreases about 5% compared with the present mean temperature of 32/23℃. The rate of whole grain (transparent grain) under 35/26℃ decreased markedly to 20% from 70% under 32/23℃. The influences of air temperature and solar radiation 4 to 20 days after flowering (DAF) on grain weight and quality by shifting the cropping season and moving rice plants grown in pots 400m higher at heading time were also investigated. Neither shifting the cropping season nor moving rice plants altered the grain weight. In contrast, the rate of whole grains increased 6 to 16% when rice plants were moved to higher ground with 4℃ temperature and 20% solar radiation drops. However, the whole grain rate increased less than that of plants in growth chambers with only the temperature drop. These results suggest that the decrease in grain weight under high temperature at ripening is masked and that grain quality under high temperature decreases less under rich solar radiation. Next, the effects of temperature and solar radiation on grain weight and quality with new heat-tolerant cultivars Nikomaru and Hinohikari were investigated using growth chambers. It was demonstrated that the low solar radiation causes further deterioration in grain weight and quality at high temperature, especially in Hinohikari, and that the low solar radiation suppressed the acceleration of grain weight increase under high temperature. In Nikomaru, these deteriorations were reduced by maintaining the rate of grain weight increase under high temperature plus lower radiation. It was also found that the lower grain fullness (roundness) in Hinohikari under high temperature was further deteriorated by low solar radiation using a grain fullness index devised with an image analysis technique in the present study. 2. Influences of nitrogen fertilizer on rice-ripening damage from high temperature. The effects of nitrogen topdressing at the panicle formation stage on ripening were analyzed using the data from three years of field experiments at different temperatures during first half ripening (from heading to 20 DAF). If the nitrogen topdressing is increased when the temperature is 2℃ higher than in a normal year, the grain weight clearly increases and basal immature grain decreases. It was thought that the factor behind these results was maintaining the nitrogen level of rice plants during ripening by means of the topdressing. The ripening of cultivar Nikomaru was also compared that of Hinohikari, and the effect of topdressing methods on ripening was estimated under slightly warm conditions (about 1℃ higher than normal). One topdressing method is the conventional two applications (16 and 6 days before heading). Another method is the continuing application (15 times from 16 days before to 12 days after heading) with the same total amount of nitrogen fertilizer as with the conventional method. The rate of immature grain was found to decrease 10 to 20% in Nikomaru compared to that in Hinohikari and to decrease about 5% in continuing application compared to that in conventional applications. The amount of non structural carbohydrate (NSC) in the culm and sheath at the full heading stage was greater in Nikomaru than in Hinohikari, and greater in the continuing application approach than in the conventional application. Consequently, the cultivar Nikomaru and the continuing application method increase grain quality by increasing storage carbohydrates of the culm and sheath at heading. 3. Different effects of high day and night temperatures on rice ripening and related eco-physiological factors. The grain weight and quality under high night temperature (22/34℃, HNT) and high day temperature (34/22℃, HDT) were compared with those of the control (22/22℃, CONT) as the optimum temperature. The grain weight was clearly lower in HNT than in either HDT or CONT, and the grain quality was diminished by both day and night high temperatures. The effects of exposing either the panicle or vegetative parts to a high night or day temperature on grain weight and the quality were investigated. Exposing the vegetative parts to high night temperatures failed to reduce the average grain weight. Consequently, the main factor causing the decrease in average grain weight under high night temperatures was not thought to be the deficit of carbohydrates in the leaves and culms due to increased respiration loss but rather to the decrease in the partition ratio of dry matter to the grains. This was supported by the fact that there was no signifi cant correlation between the grain weight and the shoot dry weight in the above experiment. Regression analysis of the time course of grain weight increase revealed that the duration of grain growth was reduced by both day and night high temperatures. However, the rate of grain growth was lower in HNT than in HDT. Also, the increase in grain weight and the movement of carbohydrates to the grain occurred mainly in the hours of high temperature, and more carbohydrates moved to grain from the assimilation organs in the day time under HDT than at night under HNT, although both were 34℃ from investigation of the diurnal change of grain weight increase and the ^13C tracer experiment. Measurement of the diurnal change in grain sugar contents, especially in the night time under HNT, revealed that the grain sugar content decreased. These results indicated that the sugar supply from the source organ does not meet the sugar demand at the sink organ because the former is reduced due to the lack of photosynthesis while the latter is accelerated due to rising physiological activity under HNT. This mismatch of sugar supply in relation to demand is the key factor in reduced grain weight increase under HNT. Measurement of the time course of grain water content indicated that its maximal value was lower in HNT than in HDT and both values were obtained at 11 DAF simultaneously. The grain fresh weight at maximal grain water content, which is thought to be an index of endosperm volume, is also lower in HNT than in HDT. The endosperm cell number and cell size, which are closely related to endosperm volume, were examined by image analysis of the endosperm cross section. As a result, the number of cells in endosperm in HNT was found to be similar to that in HDT, and higher than in CONT. The average cell area was smaller in HNT than in either CONT or HDT. The differences in average cell areas between HNT and HDT were greater at 30 to 80% of the distance from the central point of endosperm towards the endosperm surface, which is thought to be the region in which the difference in grain growth rates between HNT and HDT was greater. These results indicate that the high night (rather than high day) temperature affects the grain growth rate at around the middle stage of grain filling and fi nal grain weight through reduced cell enlargement in the region where the cells enlarge at those stages. This study clarified the influences of solar radiation and nitrogen fertilizer on high temperature-induced rice-ripening damage and the resistance characteristics of cultivars to high temperature plus low solar radiation during ripening. It also demonstrated that grain weight is reduced under high night (rather than high day) temperature. It was thus suggested that the mechanism of this reduction in grain weight under high night temperature is not respiration loss. These investigations shed light on the mechanisms of rice-ripening damage at high temperature and will be useful in developing measures to prevent this damage.
- 九州沖縄農業研究センター報告 = Bulletin of the NARO Kyushu Okinawa Agricultural Research Center
九州沖縄農業研究センター報告 = Bulletin of the NARO Kyushu Okinawa Agricultural Research Center (52), 1-78, 2009-08-17
独立行政法人 農業・食品産業技術総合研究機構 九州沖縄農業研究センター