Introduction

Rice is the major stable food source in Asia. Rice is divided into Indica, Japonica and Javanica type depending on ecotype. Japonica rice is round in shape and rice is sticky when it is cooked. However Indica rice has more long shape and the rice is less sticky than Japonica rice. Javanica rice shows a medium quality between Japonica and Indica rice (Hoshkikawa, 1989). In temperate region such as Korea and Japan, Japonica rice is mainly consumed. But the 90% of rice demanded in worldwide is Indica rice type. Japonica rice is recognized as the premium rice compared to Indica rice in subtropical region especially in Philippines. Therefore development of adaptive Japonica rice variety in subtropical region has been actively conducting to increase income of local farmers in the subtropical regions (RDA, 2010).

Each plant has preferred flowering time since it has ability to detect of seasonal changes (Thomas B. et al., 1997). Rice is a typical short day length plant. Heading of rice is promoted by short day length and high temperature conditions. From sowing to heading, the rice growth period is divided as three periods. In basic vegetative phase, day length do not effect on flowering. However, day length and temperature greatly influence on flowering during reproductive growth stage especially on sensitivity stage (Andaya and Mackill, 2003; Jagadish et al., 2007; Fu et al., 2008). After that, the young spikelet appears and grows within shoot then flowers. Typical Japonica rice type shows earlier flowering when it is cultivated in subtropical region before getting enough growth due to short day length and high temperature condition. Since early heading of rice leads to the difficulty of securing the yield, the less sensitive of heading on day length and temperature can be a one of the major trait to develop more suitable rice in subtropical region. Therefore rice which is adaptable in subtropical region should have the long basic vegetative phase and low sensitivity of heading on temperature and day length to have enough growth before heading.

Here we tried to investigate the basic vegetative phase and temperature sensitivity of heading during sensitive and reproductive stage under short day length condition among Korean major cultivars. Through this research, we try to recommend the Korean rice cultivars which show the suitable heading characters in subtropical region then help to develop adaptive rice cultivars in subtropical region.

Material and method

Plant materials and growth conditions

We used the 37 kinds of Korean varieties (Oryza sativa sp. Japonica) as materials (Table 1). After disinfecting at 30°C for one day, each seed of material was cultivated at 24°C condition for two days then germinated at 30 °C to equalize the degree of germination. After germination, three seeds of each cultivar were sowed in 1/5000a Wagner pot in phyto-tron. To confirm basic vegetative phase (BVP), we created the phyto-tron condition as high temperature (mean temperature - 28°C, ±4°C) and short day length (12 h) condition. Reproductive stage in rice is usually regarded as 35 days, therefore BVP is calculated by subtracting 35 days from sowing to heading (Vergara et al, 1965; Ikeda, 1975). In this experiment we checked heading date when the half of first spikelet was appeared from the flag leaf, so we deducted 30 days to calculate BVP. After checking the days from sowing to heading in each material, we deducted 30 days as reproductive growth stage to calculate BVP. After confirming BVP, each seed was germinated, sowed then cultivated under high temperature (mean temperature - 28°C, ±4°C) and short day length (12 h) conditions. Each pot was moved to the three different condition(mean temperature - 22, 24, 26°C) two days before reaching BVP to check heading delay by different temperature conditions during sensitivity and reproductive stage. Daily temperature range was 8°C and photoperiod was fixed as 12 hours in each condition. Heading date was checked when first spikelet was appeared.

Results and Discussion

Basic vegetative phase of each material

To confirm basic vegetative phase (BVP), we cultivated each material under high temperature and short day length condition since sowing to heading (Fig. 1). According to maturing type, BVP was different. In early maturing type, mean BVP was 29 days. It was decreased as 24.6 and 20.1 days in medium maturing and mid-late maturing type, respectively. However BVP was also different among the same maturing type. In early maturing type, BVP was from 24 to 33 days. BVP was from 14 to 31 and 16 to 27 days in medium maturing and mid-late maturing type, respectively. Among 37 kinds of materials, 18.9 % of materials had longer BVP than 30 days. The 37.8 % of materials had 26 to 30 days of BVP. Among materials, Manchu was showed the longest BVP as 39 days. Gopum was showed the shortest BVP as 14 days. We calculated the accumulated temperature during BVP by maturing types and classified distribution ratio according to the accumulated temperature to analyzing difference of BVP by maturing types (Fig. 2). Accumulated temperature range during BVP was from 300 to 1000°C in materials. Accumulated temperature during BVP in early maturing was 600~1000°C and distribution range was the narrowest in heading types. In medium maturing types, accumulated temperature during BVP showed big difference in cultivar, accumulated temperature range was 300~900°C. The 22% of cultivars required 301~400°C, 501~600°C of accumulated temperature during BVP and 22, 33 and 22% of cultivars required 601~700, 701~800, 801~900°C of accumulated temperature during BVP in medium maturing types. However accumulated temperature during BVP was concentrated at 401~600°C in mid-late maturing type.

Temperature effect on sensitive and reproductive stage

We checked the heading delay by treating different temperature after BVP on the short day length condition. We conducted temperature treatment two days before final date of BVP to ensure that the temperature treatment was done more reliably during the sensitive and reproductive stage. At first, we analyzed delayed days of heading (DDH) under each temperature condition compared to 28°C condition according to heading types (Fig. 3, A). DDH was longer in 22°C condition than other temperature conditions in all maturing types. In 22°C condition, DDH was 14.6, 16.1 and 13 days in early maturing, mid maturing and mid late maturing type, respectively. In 24°C condition, DDH was 11.1, 11.2 and 8.5 days in early maturing, mid maturing and mid late maturing type, respectively. In 26°C condition, DDH was 3.6, 2.9 and 2.8 days in early maturing, mid maturing and mid late maturing type, respectively (Fig. 3, A). With those date, we could confirm that heading date was more delayed at lower temperatures. However we could not confirm which temperature was more critically effected on heading delay in each maturing type. In order to confirm the effect each temperature on the heading delay, we calculated the delay days of heading for each temperature condition by calculating the difference in heading delay days between temperature conditions. For example, the number of days of heading delay according to the 24°C condition was calculated by calculating the difference in the number of heading delayed days between the 24 and 26°C condition. Then we tried to analyze the delayed rate of heading date (%) in each temperature condition by calculating the ratio of the number of heading delayed days in each temperature condition to the total heading delayed days (Fig. 3, B). Among different temperature conditions, the 24°C condition was more seriously affected on heading delay than other temperature conditions in all heading types. In the 24°C condition, delayed rate of heading date was 49.5, 50 and 44 % in early maturing, mid maturing and mid late maturing type, respectively. The 22 and 26°C conditions were similarly effected on heading delay in early maturing and mid maturing type. However the 22°C condition was more seriously affected on heading delay in mid late maturing type. We confirmed that the 24°C condition could be the critical temperature for heading delay.

Selection of materials showing stable heading habit in subtropical regions

In subtropical region, day length is short and temperature is high. In those conditions, rice with high sensitive on day length and temperature do not have stable growth by early flowering. The long BVP and less sensitivity of heading on temperature could be a good heading trait of rice in subtropical region. Rice needs three to seven days for taking enough roots then starts to make a tiller and usually has the maximum tiller number stage 35 to 40 days after transplanting in general cultivation method (Matsubayashi, M., 1963; Yoshida, S. 1981). However maximum tiller number stage can be accelerated depending on the time of the transplanting. In late transplanting, maximum tiller number stage is known to come within 30 days after the transplanting. Analyzing BVP among materials, the one cultivar has the BVP over 35days, and six cultivars have the BVP over 30days. The 56% of cultivars among materials have BVP over 25days. With these data, we considered it was difficult to set the suitable BVP as 30days or more. So we determined that BVP should be above 25 days to ensure normal growth before heading in subtropical region. Therefore we firstly selected the materials having longer BVP than 25 days.

Then we tried to calculate the effect of one degree of temperature treatment on heading delay in different temperature groups (Fig. 4). The effect of the one degree of temperature on heading delay is considered to be different depending on temperature range. So we divided the temperature group as three groups according to different temperature range such as 22~24°C, 24~26°C and 26~28°C since we treated 22, 24 and 26°C on sensitive and reproductive stage. We analyzed the delay of heading days in each group by dividing the number of heading delayed days with the temperature range in a group. The number of heading delayed days by one degree of temperature was different among temperature treatment groups. Heading was delayed from 0 to 7, 1 to 9.5 and 0 to 5.5 days by one degree of temperature in 22~24°C, 24~26°C and 26~28°C group, respectively. An average heading delayed days by one degree of temperature was 2 days in the 22 to 24°C group. The 67 % and 50% of materials showed heading delay within 2 days and 1 day by one degree of temperature, respectively in such a group. In the 24 to 26°C group, an average days of heading delay was 3.4 days by one degree of temperature. The 47% of materials showed delayed heading more than 4 days and the 28% of cultivars showed delayed heading within 2 days by one degree of temperature. However, an average heading delayed days was 1.56 days in the 26 to 28°C group. All cultivar showed heading delayed days within 4 days and the 77% of materials showed delayed heading days within 2 days in the 26 to 28°C group.

As a result of the those research, we can select and recommend the cultivars that are suitable for cultivating in subtropical region, which is short in day length and high in temperature (Fig. 5). Among 37 Korean rice cultivars, Joun showed the most stable heading habit. The BVP of Joun was 33 days and heading delayed days by one degree of temperature was less than 2 days when the growing temperature was between 22 to 28°C condition after BVP. Although the BVP was shorter and temperature sensitivity on heading was higher than Joun, Manho, Joan and Cheonga also showed stable heading habit even in 22~24°C condition. Those cultivar showed heading delay within 2 days between 22 to 28°C condition after BVP. Keumo2, Mannan and Jinsumi showed low temperature sensitivity on heading in 22~24°C condition, but temperature sensitivity on heading of those cultivars increased in 26~28°C condition. Therefore heading date could be delayed if the temperature is slightly lower than 28°C in Keumo2, Mannan and Jinsumi. Manchu and Mikwang showed high temperature sensitivity on heading in 22~24°C but temperature sensitivity on heading in those cultivars was decreased between 24 to 28°C condition. If the growing temperature do not go below 24°C, Manchu and Mikwang could show stable heading. However the expecting stable heading date is difficult for these cultivar because the temperature of less than 24°C appears during the rice cultivation period even in subtropical region (AccuWeather). Geumyoung, Samkwang, Unkwang and Goun showed heading delay within 4 days between 22 to 28°C condition but heading delayed days of those cultivars was more than 2 day by one degree of temperature between 22 to 26°C condition. The other cultivar in materials showed higher temperature sensitivity on heading after BVP. So we can suggest Joun, Manho, Joan and Cheonga as the most suitable rice cultivar which could show good growth in the subtropical region.

적 요

고온 단일조건인 아열대지역에서 안정적인 출수기를 가진 품종을 선발하기 위해 국내 육성 벼 품종 37점을 대상으로 기 본영양생장성을 측정하고 생식생장기 온도에 따른 출수지연 정도를 분석한 결과는 다음과 같다.

시험재료의 기본영양생장기 분석결과 같은 생태형 안에서 도 품종별 차이가 큰 것으로 보였으나, 생태형별 조생종 29 일, 중생종 24.6, 중만생종 20.1의 기본영양생장기를 보여 조생종이 만생종보다 기본영양생장기가 긴 경향이었으며 기 본영양생장기동안의 적산온도를 분석한 결과 적산온도의 분 포범위는 조생종에서 가장 좁았으며 만생종에서 가장 넓은 것으로 나타났다.

생식생장기 중 생식생장기에 온도처리를 하여 출수변화를 분석한 결과 28°C에 대비하여 26°C, 24°C, 22°C로 온도가 점점 낮아지면서 출수지연정도는 증가하는 경향이었으며 온도 별 출수지연에 미치는 영향을 분석한 결과 24°C가 출수지연 에 가장 큰 영향을 미치는 것으로 나타났다.

기본영양생장일수가 25 일 이상으로 고온단일 조건에서 생육확보가 유리하며 온도에 따른 출수지연이 적어 안정적 인 출수를 보이는 품종을 선발한 결과 조운, 만호, 조안, 청 아가 선발되었다.