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Estimation of Potential Area for Upland Rice Production in Kenyaケニアにおける畑地利用による米増産の可能性
... Like other sub-Saharan African countries, Kenya is experiencing a rapid increase in rice consumption, and increase in its national domestic rice production is a key challenge (Onyango, 2014). On the condition that New Rice for Africa (NERICA) varieties, i.e. interspecific rice varieties between Oryza sativa and Oryza glaberrima originally developed by the Africa Rice Center (Jones, Dingkuhn et al., 1997;Jones, Mande et al., 1997), need rainfall greater than 360 mm in 90 days and air temperature higher than 19°C during the initial reproduction stage, Saito et al. (2015) estimated that the area suitable for NERICA cultivation was 2,600,000 ha in Kenya. Moreover, if these varieties can withstand temperature as low as 17°C during the initial reproduction stage, the area would increase by 640,000 ha (Saito et al., 2015). ...
... On the condition that New Rice for Africa (NERICA) varieties, i.e. interspecific rice varieties between Oryza sativa and Oryza glaberrima originally developed by the Africa Rice Center (Jones, Dingkuhn et al., 1997;Jones, Mande et al., 1997), need rainfall greater than 360 mm in 90 days and air temperature higher than 19°C during the initial reproduction stage, Saito et al. (2015) estimated that the area suitable for NERICA cultivation was 2,600,000 ha in Kenya. Moreover, if these varieties can withstand temperature as low as 17°C during the initial reproduction stage, the area would increase by 640,000 ha (Saito et al., 2015). However, breeding of coldtolerant varieties has not been actively undertaken in Kenya so far, partly owing to the absence of a screening method for cold-tolerant rice varieties in the country. ...
Cold tolerance of New Rice for Africa (NERICA) and their parent varieties was evaluated under natural air temperature conditions of Kenyan tropical highlands using the multiple-sown field evaluation method. Rice varieties were cultivated 10 times to expose them to a shift in temperature during 20-day periods before heading (critical period), measuring the average daily mean temperature during the critical period (Tmean) for each variety and filled grain ratio, which was used as an indicator of cold tolerance. Tmean of each variety was low in 6 out of 10 experiments. In the remaining four high-temperature experiments, there were no significant varietal differences in filled grain ratio. In the six low-temperature experiments, all varieties experienced 20.1−21.5°C Tmean. After comparison under virtually the same cold stress, NERICA 1, 3, 4, 7−10, 12, 17, 18, and WAB 56–104 were judged as cold-tolerant; NERICA 2, 5, 11, 13, and 14, and WAB 56–50 and 181–18 were moderately tolerant, and NERICA 6, 15, and 16 were cold-susceptible varieties. These evaluations generally corresponded with previous experimental results under temperature-controlled environments, showing the practicality of multiple-sown field cold tolerance evaluation in Kenya. It also indicated that NERICA varieties, which recorded different levels of cold tolerance and whose average number of days to heading in all experiments were 66−78, are suitable as a set of standard check varieties in this region to evaluate cold tolerance of rice varieties with this range of days to heading.
Growth, yield, and yield components of five upland rice varieties (MWUR 1, MWUR 4, NERICA 1, NERICA 4, and IRAT109) were evaluated under four different soil N conditions (0, 26, 52, and 78 kg N/ha) to identify the factors contributing to their adaptability to low soil-fertility. The results showed that MWUR 1, MWUR 4, and NERICA 4 had greater adaptability to low N conditions. Specifically, MWUR 1 showed the highest adaptability to low soil fertility. The greater low soil-fertility adaptability of these varieties was attributed to their ability to maintain dry matter production. Furthermore, their greater dry matter production under low N conditions could be attributed to the increased root length, which allowed improved soil nutrient absorption. Our findings suggest that rice grain yield was mainly restricted by sink size, particularly panicle number per plant under low N conditions. The higher grain yield of MWUR 1 under low N conditions could be attributed to greater tillering ability. Thus, MWUR 1 could be a good candidate for cultivation under nutrient-poor soil conditions.
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