Vegetative Performance of Tropical Highland Maize ( Zea mays L.) in the Field

Introgression of tropical highland germplasm may assist in broadening the genetic base of maize, with the goal of improving early seedling vigour in cool spring weather. To gain acceptance by breeders, this trait must be combined with reliable growth performance at late vegetative growth stages in warm summers. At three locations, to the south and north of the Swiss Alps, five field experiments were carried out with eight adapted varieties or semi‐exotic highland varieties in 1991 and 1992. Temperatures in late spring and early summer were generally above the meteorological average at all locations, 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) decreased steadily during the autotrophic growth phase until anthesis. At the early autotrophic stage a comparatively low leaf area ratio (LAR) of exotic germplasm was mainly responsible for a low RGR, despite high net assimilation rates (NAR). Thus, the early growth advantage was lost by anthesis. Although one variety with just 12.5 % exotic germplasm combined satisfactory early vigour with a good growth potential at later stages, the causes of the heat sensitivity, a disadvantage of tropical highland germplasm, have to be better understood before this germplasm can be directly used for breeding purposes.