Survey of rice production practices and perception of weedy red rice (Oryza sativa f. spontanea) in Taiwan

Rice (Oryza sativa L.) is the primary staple crop in Taiwan, and it can be grown twice a year. The prevalent subspecies grown in Taiwan is Japonica, and a transplanting system is used for rice production. Although the transplanting system is known for efficient weed control at the seedling stage, weedy red rice (WRR, O. sativa f. spontanea) infestation is progressively being reported. Fieldwork and previous studies have suggested that WRR infestation in Taiwan is probably related to growers’ operating practices and their perception of WRR. However, no data are available for a detailed investigation. The present study aimed to collect data on rice growers’ backgrounds, farming practices, and perceptions of WRR to quantify and characterize the patterns of farming operations for rice growers in Taiwan and to investigate factors contributing to WRR infestation. We collected 408 questionnaires completed by rice growers from 17 counties covering all rice production regions in Taiwan. The growers’ median age was 51 to 60 yr, and 75% of respondents had paddies from 0.25 to 2.75 ha in size, which corresponded with nationwide data for farmers’ backgrounds. In general, growers applied similar farming practices for both cropping seasons. Most respondents did not notice WRR infestation or consider it to be a problem: only 9.8% noticed a moderate to severe infestation of WRR in their fields. The major perceived causes of WRR infestation was seed impurity (55.1%) or cultivar degeneration (18.6%). Correlation analysis and farming patterns estimated with a nonnegative matrix factorization algorithm showed that WRR contamination rate was due to the use of dry or wet tillage. The present study provides the first quantitative and qualitative evidence of rice production practices and growers’ perceptions of WRR infestation in Taiwan.

Outcrossing Potential between U.S. Blackhull Red Rice and Indica Rice Cultivars

Weedy red rice is a major weed pest of rice in the southern United States. Outcrossing between red rice and commercial tropical japonica rice cultivars has resulted in new weed biotypes that further hinder the effectiveness of weed management. In recent years, indica rice has been used increasingly as a germplasm source for breeding and for reduced-input systems in the United States, but little is known about its outcrossing potential with U.S. weedy red rice biotypes. In a 2-yr study, simple sequence repeat marker analysis was used to show that blackhull (BH) red rice (PI 653424) outcrossing to four, late-maturing indica cultivars averaged 0.0086% and ranged from 0.002% for ‘TeQing’ to 0.0173% for ‘4484’ (PI 615022). Rates of outcrossing to a tropical japonica cultivar standard, ‘Kaybonnet’ (0.032%), were substantially greater than for the indica cultivars. These differences in outcrossing were due largely to synchronization of flowering times between rice and red rice, with Kaybonnet and TeQing exhibiting the greatest and least synchronization, respectively. Outcrossing rates also may have been affected by rice–red rice flower density differences within the rice plots. Outcrossing from cultivated rice to the red rice (as pollen recipient), which was taller than all rice cultivars, was undetectable in these studies, and environmental conditions (e.g., temperature, humidity, solar radiation, and rainfall) were not strongly correlated with the outcrossing rates observed. Grain yields of the original BH red rice line were greatest in the Kaybonnet plots, demonstrating that the indica cultivars were superior competitors against this weed. Collectively, these results suggest that red rice biotypes that flower synchronously with rice cultivars are a potential source of pollen for outcrossing and gene flow in rice fields in the southern United States.

Distribution of Weedy Red Rice (Oryza sativa) Resistant to Imidazolinone Herbicides and its Relationship to Rice Cultivars and Wild Oryza Species

Several weedy red rice populations have evolved resistance to imidazolinone herbicides worldwide. The understanding of the factors related to the herbicide resistance in weedy red rice is important to prevent its occurrence in new areas where imidazolinone-resistant rice cultivars are being used, and to manage the new rice cultivars resistant to herbicides with modes of action other than the acetolactate synthase (ALS)-inhibitors that are being developed. The objectives of this study were to analyze the relationship of weedy red rice populations from southern Brazil with rice cultivars and wild Oryza species and to evaluate the occurrence of introgression from rice cultivars and seed migration as the origin of resistance to imidazolinone herbicides in weedy rice. The study was based on 27 weedy red rice populations, seven rice cultivars, and four wild Oryza species that were genotyped with 24 simple sequence repeats and three ALS-specific single-nucleotide polymorphism markers. A large proportion of the genetic variation of the weedy red rice populations was found within (74%) rather than among populations (26%). The weedy red rice populations were more closely related to the newer rice cultivars that are imidazolinone-resistant than to the older cultivars. The South American native Oryza glumaepatula and the other wild Oryza species—Oryza rufipogon, Oryza longistaminata, and Oryza glaberrima—clustered separately from weedy red rice populations, indicating a low likelihood of introgression among weedy red rice and these wild species. Seed migration was an important factor in the genetic structure of the evaluated weedy red rice populations, although gene flow by pollen from resistant cultivars was the principal reason for the spread of herbicide resistance.

Leaf development of cultivated rice and weedy red rice under elevated temperature scenarios

The objective of this study was to simulate leaf development of cultivated rice genotypes and weedy red rice biotypes in climate change scenarios at Santa Maria, RS, Brazil. A leaf appearance (LAR) model adapted for rice was used to simulate the accumulated leaf number, represented by the Haun Stage, from crop emergence to flag leaf appearance (EM-FL). Three cultivated rice genotypes and two weedy red rice biotypes in six emergence dates were used. The LAR model was run for each emergence date using 100 years of synthetic daily weather data and six climate scenarios, including current, +1, +2, +3, +4 and +5 ºC increase in mean air temperature, with symmetric and asymmetric increase in daily minimum and maximum temperature. The increase in air temperature scenarios decreased the duration of the EM-FL phase in the earlier emergence dates (08/20, 09/20, 10/20) whereas in mid (11/20) and late emergence dates (12/20 and 01/20) the longest EM-FL phase was in the symmetric +5 ºC scenario and the shortest duration was in the asymmetric +3 and +4 ºC scenarios. The timing of the onset of flooding irrigation and nitrogen dressing may be altered if global warming takes place and weedy red rice has a potential to enhance its competitiveness with cultivated rice in future climates.