Premature heading and related factors for early season culture and planting of extremely early rice cultivars in warm south-west Japan

Early season culture using early cultivars has recently been common practice in the coastal areas in Kochi Prefecture in the warm South-West of Japan. Tosapika'. an extremely early rice cultivar bred in Kochi Prefecture was adopted as a recommended cultivar in 1997. Farmers expected much of this cultivar because, apart from that grown in Okinawa Prefecture, it would potentially offer the earliest harvesting and shipping of a new seasons rice in Japan. But premature heading occurred in many farmer's fields in the first year of planting (1988) due to unusual high temperatures during the raising period and the early stages after transplanting. This study was conducted to clarify the characteristics of grain production for early-planted rice and the factors related to premature heading of the cultivar ' Tosapika', and to establish a method of cultivation that would prevent premature heading. I. Characteristics of Dry Matter and Grain Production of the Early Planting Rice in the Early Season Culture To clarify the yield-determining factors in this cropping type, the characteristics of dry matter production and the yielding abilities of the early cultivars,'Tosapika','Natsuhikari' and 'Koshihikari' were compared with a medium cultivar, 'Koganenishiki'. 1. The period from the transplanting to the panicle formation stage of 'Tosapika' was the shortest among the early cultivars and followed by 'Natsuhikari* and 'Koshihikari'. As for the seedling age, the period of the middle seedling at 4.1-4.6 leaf age was shorter than in young seedling at 3.0-3.3 leaf age in all the cultivars. 2 . The yield did not differ between the middle and young seedlings in all the early cultivars. The highest brown rice yield was observed in 'Koshihikari' due to the attaining of higher percentage of ripened grains in spite of higher number of spikelets per square m. 3. Significant differences in the brown rice yield were not observed among the other three cultivars, although 'Tosapika' was comparable with 'Koshihikari' in the number of spikelets, but lower percentage of ripened grains and 1000-grain weight 4. The top dry weights of early cultivars were smaller than that of 'Koganenishiki' through the whole growth duration. However, these cultivars showed the higher crop growth rates due to the higher net assimilation rates and the higher dry matter partitioning ratios to the panicle during ripening period. 5. On the other hand, the sink/source ratio (number of spikelets per square m/leaf area index at full heading stage) of 'Tosapika' was the highest among the cultivars and it might be closely related with the lower degree of grain filling (percentage of ripened grains x 1000-grain weight). 6 . It was suggested that the increase in the degree of grain filling through controlling source size was necessary for attaining the higher stable yield of 'Tosapika', an extremely early cultivar, at the warm south-west Japan. II . Condition of occurrence and related factors of Premature Heading of Extremely Early Rice Cultivar Tosapika' 1. In 1998. premature heading occurred in the farmer's field of the extremely early rice cultivar Tosapika'. 2. According to the survey of interview with the farmers at the middle part of Kochi Prefecture, the occurrence of the premature heading was observed in early May, although the frequency varied among fields. In there fields. 100 - 160g air-dried seeds were sown per box and grown under the vinyl house without heating during hardening period, and 22-34 day-old seedlings were transplanted with a rice transplanter from March 31 to April 16. 3. The temperature before and after transplanting might be one of the factors related to premature heading, because the temperature during the seedling-raising period and after transplanting was unusually high. 4. The prematurely headed plants were morphologically characterized by smaller number (about 4 J of the leaves on the main culm, shorter lengths of culm and panicle, as compared with the normally headed ones. 5. Brown rice yield varied from 206 to 541gmE-2 among the fields, depending on the occurrence frequency of premature heading in the field. 6 . In some of the plants prematurely headed in 2001. panicle internode did not elongate sufficiently and the panicles with bract leaves at the neck nodes appeared from the leaf sheaths. In some plants, young panicles ceased their growth even though the flag leaves expanded normally. 7. In the field with premature heading, the effective cumulative temperature CECT, base temp. 10 deg C) from seeding to premature heading was 469 - 543 deg C days, and that from seeding to transplanting of seedlings (3.4-4.4 leaf age) was 253- 351 deg C days. On the other hand, the ECT from seeding to heading in the normally headed plants were over 80013 days irrespective of cropping years, seedling types and transplanting times. Thus the heading period of normal panicles was longer than that of the prematurely heading panicles. III. Heading Characteristics of the Extremely Early Rice Cultivar 'Tosapika' 1. Effects of temperature on the heading The effect of growing temperature after sowing on the heading of the extremely early rice cultivar 'Tosapika'. bred in Kochi Prefecture was studied comparatively with 49 cultivars and lines bred in various regions from Hokkaido to Kyushu. 1) Pregerminated seeds were sown at a rate of one seed per pot in a stock-raising (kabumaki) pot which had a small hole at the bottom (2.5cm depth) filled with nursery bed soil on March 10. 2000, which is the prevailing seedling time for the eariyseason culture in Kochi Prefecture. After seedlings emerged, kabumaki pots were placed on containers, containing paddy soil with nitrogen at the rate of 1.5gmE-2 or 3.0gmE-2 and placed in the phytotron under natural light conditions with about 12 hours of day and night Day (D) and night (N) temperatures were set at D/N of 30 deg C/20 deg C. 25 deg C/25 deg C or 25 deg C/25 deg C. Total accumulative temperature for the culture period was 1300 deg C days after sowing. 2) The percentage of headed cultivars by at the end of the culture period was highest in the plot exposed to 2513/2513 with 1.5 gmE-2 nitrogen. Heading of the cultivars bred in Hokkaido were accelerated in the plot under 25 deg C/25 deg C and with 1.5gmE-2N. 3) 'Tosapika' headed when the accumulated daily temperature was beyond 1000 deg C days. The heading was scarcely influenced by temperature and nitrogen level Moreover, the accumulated temperature after sowing to flag leaf expansion was also lowest in Tosapika among the headed cultivars, because of a fewer number of final leaves on the main stem in this cultivar. 4) These two types accumulated temperature in 'Tosapika' were insensitive to temperature and nitrogen level, so it was suggested that the heading day of the main stem in this cultivar could be estimated by the accumulated daily temperature after sowing. 2. Effects of daylength and temperature on the differentiation and development of the young panicle To clarify the factors associated with that, the effect of temperatures (constant temperature: 25 and 20 deg C) under natural daylength around the summer solstice (long day:14.15-14.43 hours) and after the autumn equinox (short day : 10.32-11.53 hours) on the differentiation and development of young panicle was examined using a seedlings raised in the kabumaki pot nursery box (each pot size is 1.5x1.5x2.5 cm) at a rate of one seed per pot 1) The panicle differentiation was confirmed in 'Tosapika' and its parent, Kouiku No.27 in the lower accumulated temperatures from the sowing, regardless of the daylength. 2) The effects of daylength and temperature on the panicle differentiation were smaller in the effective cumulative temperature (ECT, base temp. 10 deg C) than in the accumulated temperature in each cultivar. 3) The panicle differentiation and development was scarcely influenced by daylength in the seedling of 'Tosapika' and the panicle formation stage (average length;1mm) was observed at 301-348 deg C days from sowing and at 5.3-5.7 plant age in leaf number of the main culm. And it was about 20 days before heading under 25 deg C condition. 4) Moreover, the ECT after sowing to flag leaf expansion which is closely related with the earliness of cultivars was also lower in 'Tosapika' due to the fewer number of final leaves on the main culm in comparison with the cultivars bred in Hokkaido. From these results, it was concluded that the thermo and photo-sensitivities and basic vegetative growth of 'Tosapika' were comparatively small, and these characteristics were similar to its parent Kouiku No.27. IV. Effects of Premature Heading on Growth, Yield and Brown Rice Quality in Extremely Early Rice Cultivar 'Tosapika* 1. Comparisons of growth and yield performance of rice seedlings with or without differentiated young panicles To clarify the factors responsible for the changes in yield and brown rice quality due to the premature heading in rice, we grew two kinds of seedling of extremely early rice cultivars, 'Tosapika' and 'Kitaake' , under two temperature conditions different in the effective cumulative temperature (base temp. 10 deg C). Then, the seedlings with differentiated young panicles and those without differentiated panicles weretransplanted to a paddy field. 1) Similar results were obtained in the two cultivars. In the plants developed from the seedlings with differentiated young panicles, heading from the main culms was observed 30 - 32 days after transplanting (DAT), and that from the tillers 19-20 days later (denoted as premature-heading group). On the other hand, in the plants developed from the seedlings without differentiated panicles, the main culms headed at 57-58 DAT and the tillers two days later (control group). The heading duration of tillers in the premature-heading group (30-32 days) was longer than that in the control (13 days). 2) In the premature-heading group, the final leaf number on the main culm was about 4 leaves lower than that in the control group (7.6-8.0 leaves) , but the numbers of tillers and panicles were higher. 3) Yield in the premature-heading group was 9-15% lower than that in the control group. In the premature-heading group, although the number of panicles per unit area was higher, the number of spikelets per unit area was lower than in the control group, due to the markedly lower number of spikelets per panicle. The decrease in the number of spikelets, together with the lower percentage of filled grains on the tillers, was considered to be the reason for the lower yield in the prematureheading group. 4) Furthermore, the apparent quality of the brown rice was low in the premature-heading group because of the presence of a lot of green rice kernels. 2. Differences in growth and yield of rice seedlings of differentiated young panicles at different days To clarify the factors responsible for the changes in grain yield due to premature heading in the extremely early rice cultivar 'Tosapika', seedlings raised for 200, 250 and 300 deg C days (ECT. base temp. 10 deg C) from sowing were transplanted at a rate of 20.8 hills mE-2 with 1, 4 and 8 seedlings per hill and the growth and yield performance were measured. 1) Young panicles were not observed in the seedlings at 200 deg C days from sowing, but were at 250 and 300 deg C days, and the floral stage of the young panicles were at the differentiation stages of primary and secondary rachis-branches of the panicle, respectively. 2) In most plants after transplanting, young panicles of main culms ceased their growth even though the flag leaves expanded normally and the percentage of occurrence was 96-100% in the seedlings at 250 and 300 deg C days from sowing in contrast to 51-56% in the seedlings at 200 deg C days. 3) Numbers of stems were the lowest and highest in plots transplanted at 1 and 8 seedlings per hill, respectively. The duration of heading was longest in the plot transplanted at 1 seedling per hill, but the duration was also longer in the plots transplanted at 4 or 8 seedlings at 250 or 300 deg C days from sowing than those at transplanted at 200 deg C days. 4) Numbers of spikelets mE-2 tended to be less in plots transplanted with a higher number of seedlings and with more developed young panicles due to the lesser number of spikelets per panicle, although the number of panicles mE-2 was higher in those plots. These results caused a lower brown rice yield in plots of seedlings transplanted at 300 deg C days from sowing compared with those at 200 and 250 deg C days. In respect to the number of seedlings per hill, the 4 seedling-plots showed the highest brownrice yield irrespective of the degree of young panicle differentiation and development 5) The results noted above indicate that seedlings have to be transplanted before the differentiation stage of the secondary rachis-branch of panicle (young panicle length: less than 0.5mm) at a rate of 4 seedlings per bill when young panicles developed in seedlings during the nursery period. V. Measures to Prevent Premature Heading in the Extremely Early Rice Cultivar, 'Tosapika' 1. Effects of the Nursery Condition on the Differentiation and Development of Young Panicles To clarify the effects of nursery conditions on young panicle development in seedlings of extremely early rice cultivar 'Tosapika', differentiation and development of young panicles were compared under different nursery conditions such as sowing rates, amounts of nitrogen fertilizer and the raising temperature. 1) Differences in young panicle length in plots gradually increased according to the rise in the effective cumulative temperature (ECT, base temp. 10 deg C) from sowing. The young panicle formation stage (average length 1mm) of seedlings occurred at an ECT from 260 to 347 deg C days. 2) Earlier young panicle formation was observed in plots sowed at a lower rate, top-dressed with nitrogen after 200 deg C days from sowing, and raised at a lower temperature, at 20 deg C compared with 25 deg C. 3) Further, nursery conditions that promoted shoot dry weight and shoot/plant length ratio also caused earlier young panicle formation in seedlings. 4) Therefore, it was suggested that nursery management such as not topdressing nitrogen after 200 deg C days from sowing (after 3.5 leaf age) and higher sowing rates for the lowering dry weight/plant height ratio were necessary to retard young panicle differentiation in 'Tosapika' seedlings. 2. Effects of nursery conditions and the leaf cutting treatment on differentiation and development of young panicles and the heading of the main culm To clarify methods of cultivation that would retard premature heading in the extremely early rice cultivar Tosapika', the effects of nursery conditions and leaf cutting treatment on differentiation and development of young panicles and the heading of the main culm were examined. 1) Differentiation and development of young panicles in seedlings tended to be retarded in the plots sowed with 180g seeds and fertilized at 2g nitrogen as the basal dressing per nursery box, due to the lower top dry weight/plant height ratio, compared with plots sowed with 90g seeds and fertilized at lg nitrogen. Moreover, transplanting of seedlings fertilized at 2g nitrogen as a basal dressing per nursery box, i.e., twice the conventional amount at 200 deg C days from sowing increased final leaf numbers on the main culm along with the effective cumulated temperature up to heading, compared with seedlings raised to 250 deg C days from sowing. 2) In contrast young panicle differentiation of seedlings was difficult to retard by leaf cutting treatment in the seedlings transplanted at 300 deg C days from sowing. However, leaf cutting treatment for the seedlings at 150 and 200 deg C days from sowing caused a decreased shoot/plant length ratio and retarded differentiation and development of young panicles in the seedlings. Thus, leaf cutting treatment for seedlings could be used as a method of supplementary management to retard differentiation and development of young panicles for delaying the transplanting date if needed because of situations such as undesirable weather conditions or a larger-scale farmer's work management system. 3) Under conventional nursery conditions, young panicle formation in seedlings was observed at 266 deg C days from sowing, and premature heading occurred in paddy fields where seedlings were transplanted at 232 deg C days from sowing. The results suggested that young panicles were formed in seedlings at 250 deg C days from sowing under conventional nursery conditions, and that the ECT (base temp. 10 deg C) from sowing could be used as an indicator to check young panicle formation in seedlings. Controlling the nursery temperature to a lower level during greening and hardening is very important to avoid young panicle formation in seedlings and so prevent premature heading after transplanting in the 'Tosapika' cultivar. 4) The results noted in this Chapter are shown in Fig. 50 and in Fig 51 for the results in Chapters 3-6. In the conventional nursery period from March to April for early season culture in Kochi Prefecture, premature heading in the extremely early rice cultivar 'Tosapika' could be avoidedby raising seedlings under conditions of a 180g sowing rate, fertilizing at 2g nitrogen per nurserybox, and controlling the nursery temperature to a low level: and then transplanted the seedlings before 200 deg C days from sowing (within 3-leaf age at 20 days after sowing). This study clarified the characteristics of growth and yield ability, along with the factors related to premature heading of the extremely early rice cultivar 'Tosapika' in early season culture in the warm South-West area of Japan. Some preventive measurements to retard premature heading of this cultivar were also proposed. These results should be helpful for breeding and selecting new extremely early rice cultivars for use in Kochi prefecture, and to avoid premature heading and so allow for a stable yields level for these cultivars.