Assessment of outcrossing potential between cultivated and weedy rice under alternate wetting and drying irrigation management

Authors: Rohila, Jai; GEALY, DAVID - Retired ARS Employee; Jackson, Aaron; ZISKA, LEWIS - Retired ARS Employee

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2024
Publication Date: 5/14/2024
Citation: Rohila, J.S., Gealy, D.R., Jackson, A.K., Ziska, L.H. 2024. Assessment of outcrossing potential between cultivated and weedy rice under alternate wetting and drying irrigation management. Agronomy Journal. https://doi.org/10.1002/agj2.21594.
DOI: https://doi.org/10.1002/agj2.21594

Interpretive Summary: Weedy red rice is a close relative of cultivated rice that deteriorates yield and quality of rice crop. Producers routinely use Clearfield and Provisia varieties with herbicide resistance (HR) to control weedy rice in their fields because weedy rice is susceptible to such herbicides. This chemical management could jeopardize if outcrossing between HR rice cultivars and weedy red rice occurs. The resulting hybrids become diverse new HR weedy red rice genotypes and complicate the chemical (herbicide) management of this highly competitive and difficult-to-control weed in rice fields. Recently, diminishing water supplies in large portions of the rice-producing areas of the USA has spurred innovative attempts to reduce irrigation water usage by adopting alternate wetting and drying (AWD) irrigation system, while maintaining high rice yields. Under AWD soil is flooded, allowed to dry down naturally, and re-flooded only intermittently, which can stress the plants. Very little is known about the outcrossing risk between rice cultivars and weedy red rice, in such water conserving irrigation systems. Thus, it is important to know whether AWD stress can increase or decrease outcrossing or have no effect compared to conventional, continuously flood-irrigated systems (FLD). To address this issue, a two-year field study was conducted at Stuttgart, Arkansas, to measure outcrossing frequencies between two HR rice varieties (CL142AR and CL261), and two major weedy rice genotypes (blackhull, BH; and strawhull, SH) grown together in plots irrigated under a FLD system or an AWD system. Outcrossing in both rice and weedy rice plants occurred in all four combinations of plant types (CL142AR or CL261 rice cultivars with BH or SH weedy rice) and in both FLD and AWD irrigation management. Interestingly, the AWD-stress reduced outcrossing in SH weedy rice substantially, while having little effect on the BH genotype. In CL261 plots, outcrossing averaged about 6.5 times lower than in CL142AR plots. This study also identified other factors playing role in reducing outcrossing. Several factors such as: temporal alignment of the rice and weedy rice flowering dates and lesser vertical distance between seed heads [e.g., tall weedy rice and CL142AR (a taller variety) compared with CL261 (a shorter variety)]. These results suggest that AWD reduces outcrossing risk for SH, which is a predominant group of weedy rice genotypes in the southern USA, and thus is a potential added benefit of this conservation irrigation system.

Technical Abstract: In the United States, rice growers often control weedy rice (also known as red rice, Oryza spp.) by cultivating varieties (e.g., Clearfield or Provisia) that possess imidazolinones or quizalofop herbicide resistance. However, an ongoing concern with such technology is that over time hybridization or outcrossing between herbicide resistance rice cultivars and weedy rice. Such outcrossing may result in transfer of herbicide resistance to weedy rice, which promotes new, herbicide resistant weedy rice ecotypes and reduce the efficacy of herbicide management. In recent years, reduced irrigation input systems such as alternate wetting and drying (AWD) are being developed as an alternative to conventional continuous flood-irrigated systems (FLD), but little is known regarding if AWD could affect weedy rice outcrossing potentials. Our objective was to compare and quantify outcrossing rates between two Clearfield rice varieties (CL142AR and CL261) and two common weedy rice genotypes, strawhull (SH) and blackhull (BH) as a function of FLD and AWD management. The study showed that although outcrossing occurred in all four genotype combinations (i.e., CL142AR or CL261 cultivated rice and SH or BH weedy rice) in both FLD and AWD, outcrossing rates with CL261 averaged about 6.5 times lesser than those with the CL142AR. Additional analysis revealed that the difference in outcrossing rates was associated with better synchrony of flowering times and closer vertical proximity of panicles of weedy rice and CL142AR compared with the CL261. AWD irrigation reduced outcrossing in SH weedy rice substantially, suggesting that reduced outcrossing of SH weedy rice might be an additional benefit from AWD irrigation management in rice production.