Differential Rates of Development of Heterotic and Nonheterotic Young Maize Seedlings, I. Correlation of Differential Morphological Development with Physiological Differences in Germinating Seeds.

Heterosis, the evident superiority of a hybrid to its parents, has not been defined in precise genetic terms. The reasons given for heterosis are for the most part rather general and vague. A recent statement is made that heterosis is the phenotypic result of gene interaction and occurs because the heterozygote either masks different detrimental recessive genes which are homozygous in the parents, or is adaptively superior to both parents.' About 30 years ago, East2 suggested that "physiologically active" genes control speed of reactions responsible for heterotic expression. Admittedly, the elucidation of the genetic mechanism(s) responsible for heterotic expression is not as yet possible. Much work needs to be done to describe precisely the required genetic constitution of heterotic individuals, especially when these are compared to the nonheterotic ones. And the time is ripe for such work to be done. It is obvious that an alternative approach to the understanding of heterosis would be through studies of mechanisms or reactions responsible for growth and development in general, since heterosis, for the most part, is a manifestation of a superior growth rate of a hybrid over its parents.3-6 Information on the biochemistry and physiology of heterosis is scant mainly because much of the effort has been directed toward the elucidation of the genetic constitution required for heterotic expression. Cherry and co-workers have shown, however, that a maize hybrid generally had higher quantity of embryonic axis nucleotides than did the inbreds and that RNA was synthesized at a much faster rate in the hybrid, and suggested that hybrid vigor involved the plant as a whole and that reaction speeds were intimately involved.7 However, the study of the reactions per se would not be sufficient. Rather, the agents controlling these reactions should be elucidated. It is felt that then we would be taking a big step in the direction of the answer to the question, "How is heterosis accomplished?" The present paper is one of a series of reports8-'3 intended to elucidate a physiological-biochemical basis for heterosis. It is concerned with early postgermination physiological changes occurring in the endosperm and seedlings of hybrid corn. Observations showing differences in respiration, liberation of reducing sugars, and amylase activity are presented in an attempt to shed light on the mechanism of hybrid superiority in terms of growth. A possible means of control of this mechanism is suggested. Materials and Methods.-Germinating seeds and seedlings of several maize single crosses and their parents were used throughout this study. The strains used were the dent inbreds Ohio 43, Ohio 45, West Virginia 5, and their single crosses W. Va. 5/Ohio 43, and Ohio 43/Ohio 45 (supplied by M. W. Johnson, West Virginia University).