On the Advantages of Earlier Transplanting of Rice in Miyagi Prefecture

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  • 宮城県における水稲早植えの効果について
  • ミヤギケン ニ オケル スイトウ ソウウエ ノ コウカ ニ ツイテ

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Abstract

Traditional planting date of rice seedlings on flooded paddy fields is around 10th of May in Pacific Ocean costal district (Miyagi prefecture) of south Tohoku in Japan, and it has been believed that the transplanting before this date is difficult because air temperature is not yet high enough for rice growth, or even harmful. In this district, however, one of the peak periods of solar radiation intensity appears in the spring. This work was made with the aim to increase the rice yield by advancing the planting date so as to utilize the rich radiation energy in the spring of this district.<br>The temperature conditions in this district were re-examined in 1977 and 1978 for the last decade of April. Several observation spots were spaced over this district (see Fig. 1), and air and water temperatures were measured. Since the numbers of the spots for the water temperature were limited, the spatial distribution of the water temperature was estimated from those of the air by using empirical equations given in Table 1. The averages of the daily maximum, the daily minimum and the daily mean air temperatures over the last decade of April in this district were found to distribute in the ranges of 14-17°C, 2-6°C and 8-12°C, respectively (Figs. 2 and 3), and these temperature conditions were considered not to be sufficient for the rice growth, or even harmful. As is shown in Figs. 2 and 3, however, the averages of the daily maximum, the daily minimum and the daily mean water temperatures over the same period were in the ranges of 24-28°C, 3-9°C and 11-15°C, respectively and considerably higher than those of the air. Since the shoot apical region of rice is in water at the early growth stage of the plant, the rice at this stage is more strongly affected by water temperature than by air temperature. From these facts we concluded that in most part of this district the water temperature at the last decade of April is high enough to permit the rice growth and developments without suffering severe cold damages.<br>The effects of early transplantings on the growth and yield of rice were, then, examined by the field experiments made in 1977 and 1978. The rice cultivar used for the experiments was ‘Sasanishiki’ and 10 experimental fields were distributed over this district (Fig. 1). In each field the rice seedlings (raised in a nursery green house) were transplanted on different dates, which were 0-18 days before the traditional planting date. With advancing the planting date, the plant dry weight and the grain yield, both, linearly increased in both years (Figs. 5 and 6). When the plants were transplanted before the end of April, the yield in both years was above 600kg/10a which was significantly higher than the traditional yield level of this district. The advantage in the yield of the early transplantings was considered to be realized mainly through inceased total sunshine duration that the plants received during the growth, because the sunshine duration increased with advancing the transplanting date and because the yield was approximately linearly proportional to the sunshine duration (Fig. 5). The cumulative water and air temperatures at the grain-filling stage (40 days after the heading) also increased 3 to 7% by the early transplanting of 20th of April.

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