The drought is the most important factor limiting the upland rice yield in Asia. The selection and incorporation of plant traits adapted to the drought has big possibility for improving the lower yield in upland rice. The water uptake of root systems is not well studied in terms of root morphology and root functions in variable soil environmental conditions. Furthermore, nitrogen uptake, which is affected by the soil water changes, also is critical for determining the yield and closely related to the drought avoidance in upland rice. The following experiments were conducted for clearing up the water and nitrate uptake characteristics in upland rice: 1) to investigate the nodal root traits constructing the deep root systems in relation to the portions of inter-node units and types of tillers in modern upland rice variety grown under different soil water regimes, 2) to evaluate the water uptake of root systems during the soil water stress at panicle initiation stage by characterizing the root morphological traits and changes of root water extraction in different soil layers, 3) to elucidate the effects of lower soil water regimes subjected before water stress and basal phosphorus application on the root development and water uptake under drought, and 4) to examine the water and nitrate take up patterns in different soil depth as affected by the surface soil drying. Obtained results were as follows. 1) The deep root is coming from the lower inter-node units in the main stem in modern upland rice variety. The nodal root is developing deeper in the main and lower order of sub-tiller than those in the higher order sub-tillers. There were varietal differences in adaptability of rooting depth to lower soil water regimes. 2) The poor root length in the layer below 60 cm depth limited water uptake from those layers under soil water stress. Root water extraction responded to soil water potential decreases differently in different soil depths. 3) Lower soil water regimes increased the root length especially lateral root development in the sub-surface layer. Consequently, water uptake from this layer contributed to maintaining the water uptake during the water stress. Phosphorus application in Ultisols enhanced the root development in the layer below 40 cm depth, and water uptake was maintained higher. 4) The water and nitrate uptake patterns differed in different soil layers under drought. In the surface layer both water and nitrate uptake decreased as surface soil dried up. Meanwhile, the water uptake increased from the layers below 20 cm depth as soil water contents decreased in the surface layer, while nitrate uptake decreased in those layers.

(和訳)

　モンスーンアジアにおいて水ストレスは陸稲の収量を制限する最も大きな要因である。 育種技術により、耐乾性と深くかかわる形質を選抜し植物に導入すれば陸稲の耐乾性さらに生産性を 向上することができると考えられる。 根系の吸水は耐乾性と直接関っているが、陸稲の根の形態的特性および機能的特性については明らかでないことが多い。 また土壌の乾燥による影響をうける硝酸の吸収は、耐乾性と深く関係があきらかとなっており、 硝酸の吸収機能特性について知ることは重要である。 本研究では以下の観点から陸稲の吸水および硝酸吸収特性を明らかにした。 1) 陸稲改良品種における節根の深さについて、節位および分げつごとに明らかにする。 また節根ｓ2）幼穂形成期陸稲の、短期水ストレス期間中吸水特性を、土壌深さ別根量および 根長当り吸水の動態から明らかにする。 3) 水ストレス前土壌水分状態およびリン酸元肥施用が根系の発達および水ストレス下の吸水に及ぼす影響を 明らかにする。 4) 土壌表層乾燥が深さごとの吸水および硝酸の吸収に及ぼす影響を明らかにする。 得られた結果は以下のとおりである。 1) 根の深さは節位や分げつの種類により異なった。 2) 60ｃｍ以下の根量が少ないため、陸稲では水ストレス下の吸水能力が小さい。 根長当り吸水能力の土壌水分低下に対する反応は土壌層位ごとに異なった。 3)土壌の物理的、化学的環境は陸稲の根の発達と深くかかわり、結果として水ストレス下の吸水能力に影響を及ぼす。 4) 水ストレス期間中、吸水と硝酸吸収のパターンは土壌層位ごとに異なった。