Flooding, burial depth, and duration influence on seed mortality and induced dormancy of overwintering weedy rice ( Oryza spp.) biotypes in California

Abstract

Resurgence of weedy rice ( Oryza spp.) poses a growing threat to California rice production due to its high genetic diversity, seed dormancy, seedbank persistence, and lack of registered herbicides for in-season control. Cultural methods like fall tillage and winter flooding may offer control. Therefore, outdoor experiments were conducted in Colusa County, CA, from 2019 to 2021 to evaluate the effects of winter flooding (no flood vs. 10-cm flood), burial depth (2-cm shallow vs. 6-cm deep), and duration (30, 60, 90 d) on seed mortality and dormancy of four weedy rice biotypes (B1, B2, B3, and B5) in comparison with a commercial rice cultivar (‘M-206’). Flooding had no significant main effect in 2019 but increased overall mortality in 2020 to 2021 (24% no flood vs. 32% flood). Shallow burial (41%) produced greater mortality than deep burial (28%) in 2019; this pattern reversed in 2020 to 2021 (deep 30%, shallow 26%). Burial duration increased mortality from 19% (30 d) to 46% (90 d) in 2019 and from 13% to 44% in 2020 to 2021. Flooding increased M-206 (38% to 62%) and B2 (25% to 34%) mortality in 2020 to 2021, reduced B1 mortality (13% to 7%, indicating anaerobic protection), and reversed direction for B3 and B5 between year strata. Under no-flood conditions, B1 (65%) and B5 (82% in 2019) mortality peaked at 90 d under shallow burial; flooding suppressed this depth effect. Year effects were substantial, with 2019 producing consistently higher mortality coinciding with subzero minimum air temperatures (−0.5 to −2.8 C) within the first three burial days and above-average precipitation. Flooding-induced secondary dormancy was restricted to B1 (deep burial) in 2019; in 2020 to 2021, M-206, B1, B2, and B3 showed the same response, but B5 did not. These biotype-specific responses confirm that winter management strategies must be tailored to weed population composition.