Threshold concentration of plasmon-sensitive polygenes in the expression of quantitative characters of maize (Zea mays L.).

Reciprocal crosses between varieties of maize were developed in such a way that the genome of one variety was introduced into the cytoplasm of the other in fractions of 25, 50, and 75 per cent. Cytoplasmic effects on yield, maturity, plant height, and ear height were measured at each of the three nuclear constitutions. The results indicated that the cytoplasmic effects on quantitative characters were not consistently produced irrespective of the nuclear constitution and some relationship existed between the number of polygenes introduced and the cytoplasmic effects. The hypothesis was put forward that cytoplasmic effects on quantitative characters would be expressed only when the genes of one parent remained below a certain threshold concentration in the hybrid nucleus. Once this level was crossed by increasing the number of genes alien to the cytoplasm the cytoplasmic effects disappeared.