Genetic progress for floral morphology under different nitrogen of rice cultivars in Japan

Reproductive development of rice (Oryza sativa L.) is critical for obtaining high yield by promoting fertilization at the flowering stage. However, limited information is available about historical changes in anther and stigma morphology during breeding programs; these characteristics contribute strongly to a cultivar's abiotic stress resistance. We tested the hypothesis that genetic progress for culm length during breeding has affected the floral morphology of rice cultivars released in Japan. We used field trials in 2 years in northern Japan to quantify anther and stigma morphology and culm length for rice from 147 cultivars (2020) and 284 cultivars (2021) that had been released from 1902 to 2011 (109 years) in Japan under two nitrogen (N) regimes (no-N and high-N). Culm length significantly decreased over time at a rate of 1.9–3.1 cm per 10 years at low N. However, anther length, anther width, and stigma length showed no consistent trend during the period. High N significantly increased culm length of most cultivars by an average of 21%–26% but tended to shorten anther length by 2%–3% and anther width by 1%–2% on average and increased stigma length by only 1%, with large variation among cultivars. Our results suggested no evidence on positive breeding efforts targeting on floral morphology to improve abiotic stress tolerance and yield in contract with culm length.