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We investigated the effects of fertilizer-free and fertilizer-applied cultivation on growth, yield and nitrogen (N) utilization of rice cultivars in our Kurashiki paddy fields (Institute of Plant Science and Resources, Okayama Univ.), which have been cultivated without fertilizer since 1970, and also in our Okayama paddy fields, which are conventionally cultivated. In 2001, the cultivars Nipponbare (NIP) and Nourin 18 (N18) were cultivated in the Kurashiki fields, with a “0N plot” (no fertilizer application), a “1N plot” (standard fertilizer application), and a “2N plot” (double fertilizer application). In 2002, five cultivars were grown without fertilizer in the Kurashiki fields, and 51cultivars were tested in 0N and 1N plots in the Okayama fields. Yield (2001) in the Kurashiki fields was higher in the 0N plot for N18 (379g m–2), which had a higher number of spikelets per m2, than NIP (300 g m–2), while in the 1N and 2N plots it was higher for NIP, which had a higher percentage of ripening, and N18 had high yield potential even without fertilizer application, but low fertilizer tolerance. The differences in yield were related to N-uptake (NU), and the differences in N use efficiency (NUE, yield/NU) between cultivars were small. The pot experiment showed that the yield of 0N plot was higher for N18 than NIP grown in Kurashiki soil because of the higher number of spikelets per hill, and the yield in the Okayama soil was higher than that in the Kurashiki soil. Long-term non-fertilized soils are of poor soil fertility, which also decreases the NUE, and the NUE of N18 is higher than that of NIP under isolated conditions. The difference in yields is closely related to sink capacity (SC). In 2002, yields in the Kurashiki fields were highest in Takanari (TAK, 494g m–2) and lowest in NIP (350g m–2), and differences in yields were closely related to SC. NUE was highest in TAK (68.6) and lowest in Akebono (48.1). TAK had high NUE and high sink production efficiency (SPE, SC/NU), while N18 had low NUE but high SC due to higher NU, ensuring high yield even under unfertilized cultivation. Yields in the 0N and 1N plots cultivated in 2002 varied between 244–631g m–2 and 199–769g m–2, respectively. A close positive correlation was observed between yield and SC, and between NU and SC, suggesting that the SC through NU is involved in determining yield. A positive correlation was also observed between NUE and yield. It was found that yield increased with an increase in NUE, and that NUE decreased although yield increased with fertilizer application. Through selection of cultivars with high SPE, it is expected that it will be possible to breed low-input, high-yielding cultivars with high NUE in the future.
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