Characterization of Heading Times and Duration of Heading Time of an Individual Using a Wide Range of Variety of Rice (Oryza sativa L.) in One of the Northern-limit Regions of Rice Cultivation, Hokkaido Islands

Aims: Hokkaido islands are one of northern limit regions of rice cultivation. There is about 170-year
history about cultivation and breeding of Hokkaido rice and the history was well described in
Japanese. Hence, cultivation and breeding of rice in Hokkaido islands can be a model case for the
history in high latitudes. However, there is no English references about the history with hard data and
the cultivars studied in the reports written in English have been limited to modern cultivars. Plant
breeders in Hokkaido have mainly genetically improved earliness due to small range of suitable
flowering times. Then, heading times and heading time duration need to be characterized using wide
range of cultivar from introduced to modern cultivars. I provided basic knowledge of total nature of
heading time of Hokkaido cultivars with hard data in this report.
Study Design: The seven land races and six modern cultivars were used. In each genotype, plants
were divided into high-density (H) and low-density (L) condition. Two replicates were prepared. In
each of replicates, genotypes were placed according to plant height to avoid intergenotypic
competition.
Place and Duration of Study: The place of this study is Hokkaido Agricultural Research Center
(HARC) in Sapporo (43°N). Duration of the study was summer season in year 2006.
Methodology: In each of replicates of a genotype, middle three plants were measured in both H and
L conditions. In each plant, the first, second and third heading times were recorded based on daily
observation. After the records, days to the first heading time from germination (DFH), days to the
second heading time from germination (DSH) and days to the third flowering time from germination
(DTH) were calculated. Heading-time duration (HTD) of each plant was conveniently defined as the
equation: HTD = DTH – DFH.
Results: From frequency distribution of DFH, DSH and DTH, DFH showed larger variation of days to
heading time than DSH and DTH, suggesting that genetic control of DFH might differ from that of DSH
and DTH. Analysis of variance revealed that DFH was deterministically dependent on genotype, while
the other flowering time can respond to environment, showing genotype × environment interaction,
suggesting DFH specific genetic controlling. To examine that each of components of HTD, DFH and
DTH, contributes to HTD, regression analyses were performed. As a result, HTD was primarily
determined by DFH. As for HTD and DFH, comparison between land race and modern cultivar was
performed. Modern breeding decreased HTD by genetically modifying the first flowering time from
earliness to lateness, and as a result, HTD decreased due to the negative correlation relationship
between HTD and DFH.
Conclusion: As shown in Results, it was clarified that rice breeders genetically modified rice
population fitted to Hokkaido island as to heading time. This report will be essential reference of heading time of Hokkaido cultivars. Total nature of heading time of Hokkaido cultivars was unknown
because existence of land race has been neglected until the present study.