Studies on the genetics and breeding of maize (Zea mays L.) for cold tolerance and early maturity
1. Thirty two double crosses (made from cold tolerant 'Cambridge' material) were evaluated for their germinability at 6°C constant temperature. The five earliest germinating seeds from each double cross were selected, grown in the glasshouse, and evaluated for flowering and maturity, based on method of accumulated heat-unit degrees. All S₀ plants were selfed. S₁ progeny evaluation was carried out in the glasshouse and S₂ seeds were obtained. Germination tests and field evaluations were carried out for all the double crosses, 48 S₁ and 22 ,S₂ (selected and non selected) families and the following results obtained: a) good response and variability for germination at low temperatures were found among the 32 double crosses and their response was found to be better than that for the single crosses from which they were obtained, b) S₁ and S₂ families, developed from the selected seeds, germinated at 6°C as well as the double crosses or better for some families, c) the procedure used for selection for early flowering and early maturity under the glasshouse conditions, which based on less heat-unit degrees to maturity was found to be effective to distinguish the early maturing families, d) selection for early maturity did not alter the cold tolerance and the other agronomic characters in these genotypes, e) most of the variability for cold tolerance, and the flowering and maturity stages was due to additive genetic effects. No important G x E interaction was found in the field, f) highly significant correlation among the emergence traits and seedling vigour traits was found, suggesting that the same genetic system controlled these traits. Similarity between the germination test in the laboratory and the emergence in the field was also observed, g) no significant differences were found between two heat-unit degrees methods (Gilmore-Rogers 1958 and the Ontario of Brown, 1975) in the evaluation of the flowering and maturity stages for all generations, h) selecting of the earliest S₁ and S₂ families resulted in a positive selection differential and positive expected gain from selection for most traits studied. Promising families for further improvement were identified. No negative effect on the yield and other agronomic characters resulted from selection. 2. Five selected inbred lines from the Cambridge material were mated with 5 USA lines in a North Carolina mating design 2. Thirty nine F₁s and their F₂s were obtained. All materials were evaluated under controlled and uncontrolled conditions. The NC2 analysis was used to study the genetic variability and the general combining ability (GCA) and the specific combining ability (SCA) for both sets of inbred lines for two seasons (two years), and the following conclusions were obtained: a) for any cold-tolerance study for these genotypes, measurements and analysis of the germination, emergence and seedling growth traits are required. Different behaviour was observed for some genotypes in the germination, emergence and seedling growth tests. The USA inbred lines were more vigourous in their seedling growth rate than the Cambridge lines. b) highly positive heterosis for most of the traits studied was observed, and additive and non-additive genetic effects found to be important for most of the traits studied, c) results for flowering and Maturity stages in both years were similar for both the GCA and SCA for the two sets of inbred lines. Both GCA and SCA were important, d) some promising hybrids for early maturity with less range of spread of maturity were detected, e) the production of new genotypes, combining together the early maturing characteristics of the Cambridge lines with the good agronomic traits of the USA lines, is worthwhile and very promising for the establishment of grain-maize genotypes ideal for cold environments, f) it was confirmed that the NC2 mating design is very satisfactory mating system to be followed to study genetic variation in hybrids produced from unrelated sets of inbred lines.