-Understanding the genetic control of phenotypic traits is essential to increase

Rice (Oryza sativa L.) cold tolerance at the initial stages of development is a highly desirable trait to be incorporated into the state of Rio Grande do Sul, Brazil, cultivars, but selection for this trait must be performed under controlled temperature conditions, which limits the number of lines that can be evaluated. Knowledge of the inheritance of this trait is important to define breeding strategies. So the aim of this paper was to study the genetic basis of rice cold tolerance at the vegetative stage. Six genotypes with constrasting cold tolerance reactions were crossed in a diallel scheme without the reciprocals. The parents and the F 1 and F 2 generations were cultivated in a greenhouse until the V 4 stage, when they were submitted to 10°C for ten days and evaluated for plant survival after seven days of recovery under normal temperature. The results obtained by the diallel analysis of the F 1 generation indicated significance of both additive and non-additive effects, but the general combining ability was more important. The evaluation of the F 2 generation revealed oligogenic inheritance with one or two dominant alleles responsible for cold tolerance in the cold tolerant parents and two complementary genes with recessive alleles segregating in the crosses involving sensitive and intermediate genotypes.Key words: Oryza sativa L., genetics, low temperature, vegetative stage Herança da tolerância do arroz ao frio no estádio de plântula RESUMO: A tolerância ao frio nos estádios iniciais de desenvolvimento do arroz (Oryza sativa L.) é uma característica altamente desejável para incorporar em cultivares do Rio Grande do Sul (RS), porém a seleção para esta característica deve ser feita sob condições de temperatura controlada, o que limita o número de linhas que pode ser avaliada. O conhecimento da herança desta característica é importante para definir estratégias de melhoramento genético, assim o objetivo do presente trabalho é estudar a base genética da tolerância ao frio no estádio vegetativo do arroz. Seis genótipos com reações contrastantes de tolerância ao frio foram cruzados num esquema dialélico sem os recíprocos. Os genitores e as gerações F 1 e F 2 foram cultivados em casa-de-vegetação até o estádio V 4 , quando foram submetidos a 10°C por dez dias e avaliados quanto à sobrevivência de plantas após sete dias de recuperação sob temperatura normal. Os resultados obtidos pela análise dialélica da geração F 1 indicaram significância tanto dos efeitos aditivos quanto não-aditivos, mas a capacidade geral de combinação foi mais importante. A avaliação da geração F 2 revelou uma herança oligogênica com um ou dois alelos dominantes responsáveis pela tolerância ao frio nos genitores tolerantes e dois genes complementares com alelos recessivos segregando nos cruzamentos envolvendo genótipos sensíveis e intermediários.

Aluminum toxicity due to the cation Al +3 is a major factor limiting yields in acid soils. Wide genetic variability to aluminum tolerance is found in oat genotypes. The objectives of this study were to determine the number of genes controlling aluminum tolerance in oats and to verify if any detrimental effects were present of the aluminum tolerance genes on grain yield and grain quality in Al +3 free soils. Aluminum tolerance was estimated as the average regrowth of the main root after exposure to toxic levels of Al +3 in a hydroponic solution under controlled conditions. The number of genes controlling that trait was estimated from the distribution of the average root regrowth frequencies in a population of 333 recombinant inbred lines (RIL's) in generations F 5:6 and F 5:7 . The effects on grain yield and grain quality were assessed in a subpopulation of 162 RIL's chosen based on their aluminum tolerance response. Aluminum tolerance in the evaluated population was controlled by one dominant major gene with the tolerant genotypes carying Al a Al a and the sensitive ones al a al a alleles. No detrimental effects of the Al a allele on grain yield or grain quality were detected.