Population improvement has been used as an alternative to the conventional methods used by plant breeders to develop irrigated rice in Brazil. The principal objective is to improve genetically divergent populations through recurrent selection, permitting the extraction of lines with yield and other agronomic traits superior to those of commercial cultivars used by farmers. Moreover, the programme seeks to expand the genetic base of Brazilian irrigated rice cultivars to make possible the development of alternatives for changing currently established limits in yield and minimizing risks of insect pest and disease epidemics in the crop.
The programme was launched in 1990 with the management of population CNA-IRAT 4, formed by crossing 10 cultivars or lines from the indica group with CNA-IRAT P, made up of genotypes from both indica and japonica groups. Later, new populations were incorporated into the programme: CNA-1 in 1991, CNA-5 in 1993 and CNA-11 in 1996 (Rangel and Neves, 1997; Rangel et al., 2002). All these populations possessed the male-sterility gene from the mutant IR36 (Singh and Ikehashi, 1981). This mutant has a recessive allele (ms) that, in homozygosis (msms), produces sterility in pollen grains, making recombination possible in the field without need for manual crossing. In 2002, population CNA-12, synthesized specifically to extract lines with stable resistance to blast and developed without the male-sterility gene, was submitted to the first cycle of recurrent selection.
In 2002, cultivar SCSBRS 113-TioTaka, the first irrigated rice to originate from recurrent selection in Brazil and in the world generally, was released for irrigated conditions in the State of Santa Catarina. The principal traits of this cultivar, recommended for the pre-germinated cropping system, are high yield, short height, resistance to lodging, high tillering capacity, high industrial grain yield and good culinary qualities. It also has a high capacity for regrowth after the main harvest, making ratoon cropping possible.
The expression recurrent selection was introduced by Hull (1945) to mean to precode re-selection, generation after generation, with crosses of the selected families and with the goal of promoting gene recombination. This method was previously used to improve maize. Although the rice programme is only 12 years old, the launching of a cultivar in so little time shows, as with maize, the efficiency of population improvement.
During these years, in addition to the routine processes of improving and extracting lines from the populations, technical information was accumulated and fed back to the population improvement programme to make it more efficient. Several research projects were developed, including:
Morais (1997), who recommended that the effective number of individuals to select is equal to at least 50 for recombination between successive selection cycles
Zimmermann (1997), who established the experimental and statistical procedures such as plot size, experimental design and number of replications
Rangel et al. (1998) and Rodriguez et al. (1998), who evaluated the genetic potential of populations CNA-IRAT 4 and CNA-1, respectively
Geraldi and Souza (2000), who recommended evaluating, for yield trials, a minimum of 200 families to maintain the populations genetic properties.
This chapter discusses the technological innovations developed in the context of the population improvement programme in Brazil during 2000/02. These are: