The cultivation of early maturing rice in the Honam plain area of Korea is increasing to diversify the cropping systems. The cropping systems of this rice are usually classified as early, ordinary, and late cultivations based on transplanting time. The characteristics of varieties vary depending on the cultivations. To evaluate the performance of varieties and interpret the relationships between genotype and environment, nine yield and 17 panicle-related traits of six early maturing rice varieties (Jopyeong, Odae, Unkwang, Haedamssal, Jinkwang, and Haedeul) were characterized on early, ordinary, and late cultivations. Heading date was longer in order of early, ordinary, and late cultivations. The cumulative mean temperature of growth stage was similar for all cultivations. The variation in the number of spikelets per panicle (NS) was mainly due to the variety and the traits related with secondary rachis-branch were affected more by variety than the traits related to primary rachis-branch. The varieties with the highest yield were Haedamssal on early maturing cultivation and Unkwang on ordinary and late cultivations. Haedamssal displayed a panicle-number type plant architecture with relatively higher number of panicles per hill (PN) and average NS. Unkwang exhibited panicle-weight type with many NS and less PN. Additive main effects and multiplicative interaction analysis revealed that, NS and HD were mostly affected by genotype and environment, respectively. Among yield-related traits, NS contributed the most to enhanced yield of varieties in all cultivations. NS could be the target trait of breeding programs intended to improve the yield potential of early maturing rice adaptable to the Honam plain area. However, proper PN should be considered because PN, which was negatively correlated with NS, also affected the yield.

‘Haepum’ is a bacterial blight-resistant, medium maturing rice cultivar with high grain quality. It was derived from a cross between ‘Iksan493’ (cultivar name ‘Jinbaek’) and the F₁ cross between ‘Iksan495’ (‘Dacheong’) and ‘Iksan496’ (‘Jungmo1005’). Of these three cultivars, ‘Jinbaek’ is a bacterial blight-resistant mid-late maturing rice cultivar with high grain quality, ‘Dacheong’ is a mid-late maturing rice cultivar with multiple resistance to disease and insects, and ‘Jungmo1005’ is a mid-late maturing rice cultivar with lodging tolerance. To develop fixed lines, the anther culture method was applied to F₁ plants. The cultivar ‘Haepum’ was selected using the pedigree method, yield trials, and local adaptability tests. The heading date of ‘Haepum’ was August 11^th, three days earlier than that of ‘Nampyeong’ cultivar. Haepum’ is a cultivar tolerant to lodging and it has short culms. Due to its low rate of viviparous germination, ‘Haepum’ could be useful for preventing pre-harvest sprouting in cultivation of medium maturing rice in the southern plain area of Korea. ‘Haepum’ carries two bacterial blight resistance genes (Xa3 and xa5), and in our study, it exhibited high-level and broad-spectrum resistance against bacterial blight, including K3a, the most virulent race in Korea. ‘Haepum’ is also resistant to the rice stripe virus and moderately resistant to rice blast. The yield of ‘Haepum’ was similar to that of ‘Nampyeong’. ‘Haepum’ showed excellent grain appearance and good taste of cooked rice, and therefore it could contribute to the development of bacterial blight-resistant rice cultivars of improved quality. ‘Haepum’ would be suitable for cultivation in the southern plain area of Korea as well as in bacterial blight-prone areas. (Registration No. 6068)

Rice panicle architecture is an important factor affecting yield potential. Korean rice cultivars have a narrow genetic background for panicle architecture. To enhance the yield potential of Korean rice cultivars, we developed and characterized rice lines with new panicle architecture. Rice with improved panicle architecture has clustered spikelets and dense panicles (CD type). CD rice was derived from a cross between “Binhae Col.#1” carrying dense panicles, and “ARC10319” that has the clustered spikelets gene (Cl). CD rice lines had short and semi-erect panicles with two to five high density spikelets clustered at the tips of primary and secondary rachis branches. CD rice lines had dramatically increased numbers of spikelets; almost twice as many as those of Korean rice cultivars. The increase in spikelet number was mainly caused by the increased spikelets and branches on secondary rachises compared to those on primary rachises. The increase in spikelet number was expected to enhance the yield of CD rice by expanding sink capacity. However, the yield of selected lines; CD9, CD27, CD34, and CD39, did not reach the level of the Korean high-yielding cultivars “Boramchan” and “Hanareum2,” due to the reduction in panicle number and grain weight, and poor ripening. Although no substantial yield increase was observed in CD rice, the panicle architecture of CD rice, clustered spikelets, and dense panicles could be new genetic resources as breeding material for diversifying panicle architecture and enhancing yield potential.

To enhance rice yield and diversify grain quality of Korean japonica rice, we developed japonica rice line with long and spindle-shaped grain. Korean japonica rice cultivars have narrow genetic background of grain size and shape. Most of cultivars show medium-short and semi-round grain. To diversify the genetic background for grain, we developed Jeonju625, japonica rice line with long and spindle-shaped grain, derived from a cross between DSG79, the breeding material with extra-long and spindle-shaped grain, and Boramchan, japonica super high-yielding cultivar with medium-short and semi-round grain. Jeonju625 had GW2gs3qSW5+qGL3 allele type for grain-related genes, which conferred extra-long and spindle-shaped grain. The grain length and ratio of length to width of brown rice of Jeonju625 was 7.06 mm and 2.72, respectively. Jeonju625 was improved the deteriorated traits of DGS79, very late heading, long culm, long awn, droopy flag leaves, and susceptibility to lodging, by strong selection pressure focused on field breeding. The milled rice yield of Jeonju625 was 559 kg/10a, which was similar level of Boramchan (552 kg/10a) and 29% enhanced yield compared to DSG79. Jeonju625 had suitable characteristics for cooked rice. Jeongju625 showed lower protein contents and better glossiness and palatability of cooked rice than Boramchan and Hanareum2. Elite japonica rice line, Jeonju625, with extra-long and spindle-shaped grain showed good characteristics for rice yield and grain quality. Jeonju625 could be utilized practical cultivar and breeding material for enhancing rice yield and diversifying grain quality.

To diversify the grain size and shape of japonica rice, we developed the Breeding Materials with Diverse Grain Size and Shape (BM_DGS) and characterized the grain and yield-related traits. We used the donor parents Jizi1560 and Jizi1581, japonica germplasm with extremely large grain size. Four cross combinations between the each donor parents and Korean high yielding japonica rice cultivars, Deuraechan and Boramchan, were constructed and anther culture method was applied. Among 290 doubled haploid lines, we selected 91 elite lines with diverse grain size and shape and designated to the BM_DGS. The grain size and shape of BM_DGS exhibited beyond the characteristics of previously developed Korean rice cultivars. The alleles of major grain-related genes, GW2, GS3, and qSW5, in BM_DGS showed two types, wild type or loss-of-function mutant type. The loss-of-function mutant alleles, gw2, gs3, and qSw5, had an effect on increasing grain size. The phenotypic variation of grain length was mostly controlled by GS3 alleles, and grain width and thickness were influenced by the combinations of GW2 and qSW5 alleles. 1,000-grain weight was determined by the combinations of GW2, GS3, and qSW5. The grain-related genes influenced the phenotypic variation of yield-related traits. The result of this study could be useful to elucidate the relationship between the grain-related genes and agronomic traits. And the BM_DGS are being utilized in the breeding programs to diversify the grain size and shape in japonica rice.