異なる施肥条件下における水稲品種の窒素利用効率 ―日本晴とタカナリの比較―

The rice cultivar Nipponbare and the high-yielding cultivar Takanari were cultivated in field trials for three years from 2003, and in pot trials (1/2,000a) for two years from 2004. In the field trials, the following three levels of fertilizer were applied：“0N” without fertilizer, “1N” with the standard amount(8kgN 10a–1), and “2N” with twice the standard amount (16kgN 10a–1). In the pot trial, three levels of fertilizer were applied：“0N” without fertilizer, “1.5N” with 1.5times the standard amount (1.5gN pot–1), and “3N” with three times the standard amount (3gN pot–1). In the field trials, yields were higher in Takanari (538 to 843g m–2) than in Nihonbare (423 to 577g m–2), and the increase in yield with fertilizer application was also larger in Takanari. This was related to the larger sink capacity of Takanari and the smaller decrease in the percentage of filled grain with larger sink capacity. The dry matter weight and nitrogen uptake at the panicle initiation stage were higher in the plots with higher fertilizer application, but the differences between the cultivars were small. Dry matter weight and nitrogen uptake at harvest time were higher in Takanari, and nitrogen use efficiency and sink production efficiency were also higher in Takanari, but the differences in nitrogen use efficiency between cultivars became smaller with increasing fertilizer application. The nitrogen use efficiency for dry matter production also decreased with increasing fertilizer application, and was higher in 2005 in Takanari. The leaf photosynthetic rate of Takanari was higher than that of Nipponbare in the pot experiment. The difference in leaf photosynthetic rate was related to the nitrogen use efficiency (photosynthetic rate / leaf nitrogen content), and the difference in leaf nitrogen content between cultivars was small. The nitrogen use efficiency for dry matter was highest in the “0N” and decreased with increasing fertilizer application, and was higher in Takanari than in Nipponbare. This was presumably related to the higher nitrogen use efficiency of photosynthesis. It was found that fertilizer application decreased nitrogen use efficiency and sink production efficiency, but yield increased with increasing sink capacity, and that differences in nitrogen use efficiency among cultivars were related to the amount of nitrogen absorbed up to the panicle initiation stage and sink production efficiency. In order to improve the efficiency of fertilizer application, it is desirable to increase nitrogen absorption, which is expressed as multiplying the number of days to panicle initiation and the rate of nitrogen absorption, and to select cultivars with higher sink production efficiency.