| Among the agronomically important grasses, maize is unusually susceptible to drought at flowering. Drought stimulates a protandrous pattern of development, which lengthens the anthesis-silking interval. Since maize has a short period of flowering and pollen remains viable for small periods of time, each day of delay between pollen shed and silk emergence translates into a reduction in the rate of sexual fertilization, and an increase in barrenness. An ideotype of maize is proposed to overcome this problem. The purpose is to reduce sensitivity to water deficit through a series of morphological and anatomical modifications, including: transference of the site of grain production from the axil of a leaf to the apex of the plant; condensation of the upper internodes of the stem; and reduction of plant height and leaf number. The research efforts were concentrated in two areas: (1) selection to integrate in one plant all desired traits. Genetic diversity was found for all characters of interest. It was possible to identify and select mutants that exhibited in a single phenotype the proposed changes. However, several undesirable characteristics, such as lack of vigor and disease susceptibility, were also associated with the mutants and need to be improved. (2) evaluation of the impact of proposed modifications on the performance of current cultivars.;Since tassel suppression and reduction in plant height are two important characteristics of the ideotype, field experiments were conducted to evaluate their effect on maize tolerance to high plant population and drought stress.;In one trial, four genotypes were sown at two plant populations and submitted to three levels of detasseling. Another experiment evaluated the agronomic performance of contrasting genotypes in plant height and leaf number at four plant populations. Tassel removal did not improve grain yield, regardless of plant density or cultivar. Reduction in plant height decreased grain yield at all plant populations. Favorable environmental conditions and the lack of an improved dwarf genetic background probably prevented positive responses of grain yield to detasseling and dwarfness, respectively. |
