Draining the soil just after seeding as a method for improving seedling emergence of rice in a paddy field for submerged direct seeding in Hokkaido

This study was conducted to analyze the factors causing varietal differences in the establishment ability of rice direct-seeded into soil under low temperature conditions, then to develop a seedling emergence improving method by draining just after seeding in Hokkaido. The results are summarized as follows: 1. Analysis of factors influencing the varietal differences in establishment ability under low temperature conditions 1) Determination of the low-temperature-sensitive stage (1) A field experiment was conducted in a paddy field for submerged direct seeding where cool water of about 11degC was continuously irrigated, in order to determine the stage that the seedlings were the most sensitive to low temperature. Cool water treatment was given for 7 and 14 days at 3 growth stages, i.e., just after seeding (JAS), full emergence stage (FES), and second leaf appearing stage (SLA). The percentage of seedling establishment was lowest in FES, followed by SLA. (2) Emerged seedlings were classified depending on the leaf age just before and after the cooling in order to examine the factors that decreased the establishment percentage in each treatment. The seedling establishment was decreased mainly by the decrease of emergence percentage in JAS. On the other hand, it was reduced mainly by the failure of first leaf elongation after emergence in FES and SLA, and also by the failure to survive after the first leaf elongation in SLA. (3) These results indicate that the seedlings were the most sensitive to low temperature just before first leaf elongation, in other words the elongation stage of coleoptile. 2) Relationship between establishment ability and early growth (1) We investigated seedling establishment, in the field, and then examined the relationship between germination at 15degC and early growth of seedlings in 15degC water using 52 varieties. Field experiments were conducted in May and July. The plants were continuously irrigated with 14degC water for 9 days after the appearance of the first leaf in July trial. (2) Highly significant varietal differences in the percentage of seedling establishment (%ES) were observed in both trials. Several foreign cultivars showed higher %ES than 'Iburiwase' which showed the highest %ES among the Hokkaido varieties. (3) The %ES was divided into three components: percentage of seedling emergence (EM), percentage of first leaf elongation in the emerged seedlings (FLE/EM) and percentage of establishment of the first leaf elongated seedlings (ES/FLE). The contribution rate of each component was evaluated by multiple regression analysis. The contribution of EM was the highest in the May trial, while that of ES/FLE was also high in the July trial. (4) Significant positive correlations were observed between the %ES and the early growth characters of seedlings, such as second leaf length, shoot length and maximum root length, while the correlation between germination coefficient and %ES was low. (5) The present results indicate that several foreign cultivars with a high %ES are useful as breeding materials for direct-seeding and also we need to pay more attention to the early growth of seedlings rather than germination. 3) Tolerance to soil reduction and inoculation of Pythium spp. (1) A laboratory experiment was conducted to examine the effects of soil reduction and inoculation of Pythium spp. on seedling emergence and establishment of rice using 8 varieties including foreign varieties. Two types of soil were prepared, i.e., dry oxidized soil (OS) obtained from a paddy field, and reduced soil (RS) with powdered rice straw added. Each soil type was inoculated with Pythium spp. (IP) or not inoculated (NP). (2) Emergence percentage was 75% in RS, while 94% in OS. 'Italica Livorno' and 'Arroz da Terra' showed less reduction of emergence percentage in RS relative to OS, suggesting their tolerance to soil reduction. (3) The inoculation of Pythium spp. to soil significantly reduced the establishment percentage in all varieties; it was 14% in IP and 80% in NP. However, the reduction in 'Dunghan Shali' was relatively small. 2. Development of a method to improve seedling emergence using the draining method 1) Effects of soil reduction and draining just after seeding (1) A laboratory experiment was conducted to examine the effect of draining just after seeding compared with a flooded condition on emergence and seedling establishment using a combination of several treatments, such as temperature (21/13degC and 18/10degC of day/night) and soil reduction (reduced soil by adding rice straw and normal soil). (2) Methylene blue was used to examine soil reduction. The soil near the seeds in draining treatments maintained an oxidative condition during the experiment even in reduced soil, while soil reduction proceeded in flooded treatments. (3) Emergence percentage was inhibited in flooding treatment, especially under combination of lower temperature and reduced soil. Establishment percentage was inhibited in reduced soil, but it was significantly higher in draining than in flooding treatments. (4) The period from emergence to first leaf elongation was 2-3 days in draining treatment, while over 7 days in flooding treatments. This quick elongation of the first leaf was considered to be important for stable establishment. 2) Effects of draining on seedling establishment and grain yield (1) In the field experiment, draining treatment for 12-13 days just after seeding gave better establishment and seedling growth, and resulted in a higher grain yield compared with the flooded treatment. (2) The difference in daily mean soil temperature at one cm under the soil surface between draining and flooding treatments was negligible. These results indicate that draining management is effective for obtaining dense and stable stands of seedlings even in Hokkaido. 3) Seeding depth and sowing machine (1) Recommended seeding depth for better emergence and establishment was 10 to 15 mm in calcium peroxide (CAL)-coated seeds and 5 mm in seeds without CAL. (2) Two types of sowing machines for broadcasting and stripe sowing with a side row fertilizer were used. Both machines were considered to be usable for practical seeding, although further improvement is needed for determining the proper seeding depth and operation efficiency. 4) The optimal date of submerging seeds (1) Two methods were used to estimate the optimal date to submerge the seeds after draining just after seeding using CAL-coated seeds. (2) The first was the accumulated effective temperature (AET) method, in which the accumulated degree was obtained by subtracting basal degree (1 to 10degC) from the daily maximum-minimum average air temperature during the period from the day after seeding to the day the seeds were submerged. The coefficient variation of AET was the smallest when the basal degree was 6degC. The AET at the optimal date was 85.9degC. (3) The second was a developmental index (DVI) method, which was given by integrating the developmental rate (DVR), which was defined by an equation, DVR=1/(1+exp(-A(T-Th)))/G, as a function of daily mean air temperature T. The optimal values of the parameters, A=0.1908, Th=16.06, and G=4.118, were estimated by the Simplex method. (4) Root mean square error of the difference between the measured and estimated dates was 1.4 days in AET method, and 1.2 days in DVI method. (5) Using these two methods, the establishment percentage without CAL was tested. It was 52.2%, significantly lower than that with CAL (70.9%) in standard draining. However, it increased to 60.9% when the draining period was prolonged about three days. The optimal date of submergence without CAL was estimated to be 105degC by the AET method and 1.3 by the DVI method.