IRGA 431 CL, new rice variety for red rice management in Southern Brazil

IRGA 431 CL is an early rice variety, and its main characteristic is the resistance to Imidazolinones herbicides. The variety stands out for its excellent plant architecture, strong stems, high yield potential, good grain quality, and resistance to blast and high levels of toxic iron in the soil. It is recommended for exploitation in the rice fields of Rio Grande do Sul, Brazil.


INTRODUCTION
The irrigated rice acreage is of high expression in the state of Rio Grande do Sul (RS), Brazil, where more than one million hectares are cultivated annually (IRGA 2018), distributed in the southern region of the State.
In the past years, a notable progress occurred in red rice management with the delivery of new technologies used in different cropping systems in southern Brazil. The adoption of advanced management practices and high yielding varieties have revolutionized the rice production in the state of Rio Grande do Sul. However, some limitations must be overcome for the of rice varieties to achieve the yield potential. These field limitations are caused mostly by red rice or weed rice, which has currently damaged this crop in the state of RS. Breeding has contributed significantly by developing new varieties with characteristics that confer good adaptation to different environments and rice cropping systems.
Regarding the management of red rice, the breeding program of Instituto Riograndense do Arroz (IRGA) has pioneered the development of herbicide-resistant rice varieties in the state of RS as a genetic tool for red rice control. In recent years other breeding programs have also developed rice varieties herbicide-resistant, according to the work carried out by Rangel et al. 2018. This milestone was achieved with the IRGA 422CL variety, released in 2002 (Lopes et al. 2003). This variety carries the technology developed by the Louisiana State University Agricultural Center (LSUAC), which means the first mutation in the ALS gene and confers resistance to the herbicide (Croughan et al. 1996). The IRGA 422CL variety has had a substantial impact on rice production in the state of RS by increasing the yield and improving grain quality of harvested rice, considering the reduction of red grains in the milled rice. Moreover, farmers recovered some crop lands that had become unprofitable due to the high incidence of red rice.

MCB Lopes and SIG Lopes
The breeding activities at IRGA were successfully accomplished in the last decade with the delivery of IRGA 428 and IRGA 424 RI, both resistant varieties in the years of 2011 and 2013, respectively (Lopes et al. 2011). Within this same technology, IRGA, in partnership with Metropolitana Incorporações e Locações de Bens LTDA, also delivered two high yielding hybrid rice varieties (QM 1010 CL -intermediate cycle and Prime CL -early cycle).
In this context, the new variety IRGA 431 CL, developed by the IRGA's breeding program, is recommended for exploitation, especially in red rice-contaminated crop lands of the state of RS and in the southern region of Brazil. The main characteristic of this new variety is its resistance to Imidazolinones herbicides. Also, it stands out for the high yield potential, superior grain quality, and resistance to blast and toxic iron in the soil.

APPLIED BREEDING METHODS
IRGA 431 CL derivates from the inbreed line IRGA 176 CL-6-9-4-1-5. This inbreed line was developed by pedigree selection in a segregating population derived from the cross IRGA 2913-18-4-I-3Pg * 1 / IRGA 420CL, performed at the Rice Experiment Station (RES), IRGA, Cachoeirinha, RS, Brazil. The initial cross-breeding between IRGA 2913-18-4-I-3Pg and IRGA 420CL lines was carried out in the 2006/2007 crop season, and the first backcross was conducted in the 2007/2008 season. The IRGA 420CL line carries the resistance gene to Imidazolinones herbicides. This resistance was transferred by backcrossing the herbicide-resistant line PCW16. The PCW16 breeding line was introduced to Brazil by IRGA; it came from the United States in 2000 for research purposes and has a mutation in the gene encoding the ALS enzyme (Ser653Asp or S653D).
The RC 1 F 1 generation was cultivated in the seedling transplant system, and the generations RC 1 F 2 , RC 1 F 3 , RC 1 F 4 , RC 1 F 5 , and RC 1 F 6 (Observation Plots) were grown in rows in a conventional soil preparation system. In each generation, individual plants were selected according to the pedigree method, considering the following characteristics: plant reproductive cycle; size and shape of panicles and grains; leaf and tiller's angle; shattering resistance; biotic and abiotic diseases and stresses; grain milling and cooking quality; among others. In all generations, herbicide resistance selection was performed by spraying plants at the vegetative stage (V3/V4) with the herbicides Onlyâ (1.0 L ha -1 ) or Kifixâ (150 g ha -1 ) and the Dash adjuvant (at a dose of 500 mL/100 L).

PERFORMANCE CHARACTERISTICS
IRGA 431 CL is an early cycle variety; it carries the mutated gene (S653D) that confers resistance to Imidazolinone herbicides. This mutation event belongs to the second generation since the herbicide resistance degree is higher than that of IRGA 422 CL. This variety has excellent plant architecture, high tillering capacity, and strong stem, which confer lodging resistance in dry soil seeding systems. IRGA 431 CL is resistant to soil iron toxicity (soluble iron in flooded rice -Fe 2+ ) and leaf and panicle blast disease (Pyricularia oryzae); moderately resistant to grain spotting (Bipolaris sp. and Phoma sp.) and sheath blight (Rhizoctonia solani); moderately susceptible to brown spot (Bipolaris oryzae); and moderately resistant to leaf scald (Microdochium oryzae) (see notes in Table 1). Some morphological and agronomic characteristics of this new variety are presented in Box 1. Figure 1 shows a photograph of a genetic seed production plot in the Rice Experiment Station.
The mean paddy yield in VCU trials was 10.627 kg ha -1 , surpassing the best check variety (Table 2). Table 3 shows that this new variety has a long slender grain and low levels of chalkiness (0.3). Milled grains are almost completely translucent, with an excellent visual appearance (Figure 2). The total milled grain was 70%, and head rice yield

MCB Lopes and SIG Lopes
was 65%, in an average from VCU trials. According to a field study carried out in the 2016/2017 crop season, the total panicle dry mass at the maturation stage was 2.61 g (Figure 3). In the same study, grain harvesting should begin at 30 days after full flowering when moisture is between 24 and 20% w. b. At this time, the head rice yield is 65%, depending on the environment. Grains must be harvested in a maximum period of three weeks since this variety is not so tolerant to late harvest.
The cooking evaluation was carried out at the IRGA's Quality Laboratory and showed that the grains have 29% of amylose content and low gelatinization temperature. These indirect cooking characteristics indicate that the grains have cooked entirely and remain loose and soft, even after cooling. These results corroborated with the sensorial evaluations performed at IRGA, which showed that cooking quality meets the consumption patterns of Brazilian consumers. The texture, aroma, and flavor of cooked grains are similar to and compatible with those of all commercial varieties cultivated in Brazil. IRGA 431 CL is adapted to flooded rice systems and is recommended to all rice regions of the extreme southern state of Rio Grande do Sul, according to the Agroclimatic Zoning Risk, Agriculture Department of Brazilian Federal Government.   1 According to official patterns to be classified as long and slender milled grain, the dimensions must be: length ≥ 6 mm; thickness ≤ 1.89 mm; and C/L ratio ≥ 2.75 mm (MAPA 2018)

FOUNDATION SEED PRODUCTION
IRGA 431 CL is registered by the Ministério da Agricultura, Pecuária e Abastecimento (Ministry of Agriculture, Livestock and Supply) under the number 37568. Genetic seeds were produced in the 2017/18 crop season at the Rice Experiment Station, in Cachoeirinha. Foundation and certified seeds will be produced in the next seasons, in partnership with rice farmers, who must be licensed in advance by IRGA.