Comparative analysis of yield formation and grain quality of Japanese rice cultivars under upland cultivations in Hawai‘i and Japan

Interest in local food production in Hawai‘i is increasing to address food insecurity. Rice is a promising candidate crop; however, no commercial rice cultivation exists in the state, and quantitative evaluations of yield formation and grain quality under Hawai‘i conditions are lacking. Here, we evaluated the agronomic performance of two elite Japanese rice cultivars, Koshihikari and Hitomebore, grown under direct-seeded upland conditions in both Hawai‘i and Japan in the 2025 summer growing season. The grain yield of Koshihikari and Hitomebore in Hawai‘i reached 3.79–4.01 t ha−1, which was comparable to the yields obtained in Japan (3.89–4.16 t ha−1), despite a markedly shorter growth duration. Although the spikelet number per unit area was lower in Hawai‘i, this reduction was compensated for by a higher grain-filling rate and greater thousand-grain weight. In addition, rice produced in Hawai‘i exhibited a higher whole grain ratio and a lower incidence of white immature grains compared with rice grown in Japan. These advantages were associated with higher solar radiation and moderate temperatures during the post-heading period. These findings demonstrate the agronomic feasibility of cultivating Japanese japonica rice cultivars under upland conditions in Hawai‘i.