Evaluation of Tropical Japonica Rice Genotypes Performance Under Rainfed Lowland Rice Ecosystem

Japonica rice (Oryza sativa subsp. japonica) is recognized for its high economic value compared to Indica rice (Oryza sativa subsp. indica). It is generally cultivated mostly in the temperate and subtropical zones; later tropical japonica rice was developed by crossing between Japonica and Indica to obtain varieties with similar grain quality to japonica and well-grown in tropical zones. In this study, we evaluated 100 tropical japonica genotypes alongside 30 indica genotypes, and a temperate japonica variety (Nipponbare) using 10 genotypes as a check (5 tropical japonica, and 5 local indica). Field experiments were conducted using a five-block Augmented Randomized Complete Block Design (Augmented-RCBD) from June to December 2022, followed by rice quality test from March to August 2023 at the Cambodian Agricultural Research and Development Institute (CARDI). Results showed that a group of tropical japonica genotypes, on average, flowered in 73 days after sowing, and yielded 363 g/m2, a group of indica genotypes flowered in 79 days and yielded 517 g/m2 whereas Nipponbare flowered in 55 days and yielded 137 g/m2. The average yield of all the tropical japonica genotypes was consistent with that of the tropical japonica checks at 347 g/m2. Among the tested tropical japonica genotypes, 33 genotypes performed relatively well in rainfed lowland ecosystem, with average milled grain size (5.2 mm in length and 2.4 mm in width), and amylose content ranging from 11% to 17%, resembling temperate japonica characteristics. Moreover, five genotypes (J12, J38, J49, J59, and J75) exhibited yields exceeding 450 g/m2 which are like Phka Rumduol (470 g/m2). These 33 tropical japonica genotypes preliminary selected by this study will be used for further evaluation by rice breeders of CARDI.