Vegetative performance of tropical highland mazie (Zea mays L.) in the field

Introgression of tropical highland germplasm may assist in broadening the g enetic base of maize, with the goal of improving early seedling vigour in c ool spring weather. To gain acceptance by breeders, this trait must be comb ined with reliable growth performance at late vegetative growth stages in w arm summers. At three locations, to the south and north of the Swiss Alps, five field experiments were carried out with eight adapted varieties or sem i-exotic highland varieties in 1991 and 1992. Temperatures in late spring a nd early summer were generally above the meteorological average at all loca tions, so the data are mostly described on an average basis. In comparison to the means of all varieties, exotic germplasm improved the early seedling vigour at the heterotrophic phase, showing a greater relative advantage in cool spring weather, whereas the ranking of relative growth rates (RGR) de creased steadily during the autotrophic growth phase until anthesis. At the early autotrophic stage a comparatively low leaf area ratio (LAR) of exoti c germplasm was mainly responsible for a low RGR, despite high net assimila tion rates (NAR). Thus, the early growth advantage was lost by anthesis. Al though one variety with just 12.5 % exotic germplasm combined satisfactory early vigour with a good growth potential at later stages, the causes of th e heat sensitivity, a disadvantage of tropical highland germplasm, have to be better understood before this germplasm can be directly used for breedin g purposes.