PHYSIOLOGICAL TRAITS IN MAIZE AS AFFECTED BY RECURRENT SELECTION FOR GRAIN-YIELD AND BY INBREEDING

Four cycles of full-sib family recurrent selection for grain yield wer e conducted in an F2 maize (Zea mays L.) population. This research was conducted to study the correlated responses for CO2-exchange rate (CE R), measured during grain filling, and for grain yield per unit leaf a rea, and to study the possible role of inbreeding in the detection of such responses to selection. The source (F2 = S0) and the four selecte d populations were compared in two field trials as S0, S1, S2, and tes tcrosses with the two parental lines. For each trait, the inbreeding d epression rates of the five populations were similar and the average r ate over populations, expressed as a percentage of the estimated value with F = 0, was -10.6% for CER and -52.2% for grain yield per unit le af area. The response per cycle to the selection, first measured withi n each generation, showed for CER an increasing trend from S0 to S2 th ough attaining significance only at level S2, while it was negligible in all instances for the other trait. When all generation mean values were investigated simultaneoulsy according to the model proposed by Sm ith in 1979, a significant genetic gain due to changes in allelic freq uencies at loci with additive effects were detected for both traits. N egative effects due to the heterozygosity loss (in turn caused by sele ction and genetic drift) were significant only for grain yield per uni t leaf area. The genetic gain per cycle, expressed as a percentage of the source population, was 4.4% for CER and 1.8% for grain yield per u nit leaf area.