The improvement of early vigour is crucial for the adaptation of maize (Zea mays L.) to the climatic conditions of central Europe and the northern Mediterranean, where early sowing is an important strategy for avoiding the effect of summer drought. The objectives of this study were to identify quantitative trait loci (QTL) controlling cold-related traits and to investigate the relationships among them. A set of 168 F-2:4 families of the Lo964 x Lo1016 cross was grown in a sand vermiculite substrate at 15/13degreesC (day/night) until the one-leaf stage. Twenty QTL were identified for the four shoot and two seed traits examined. Analysis of root weight and digital measurements of the length and diameter of primary and seminal roots led to the identification of 40 QTL. The operating efficiency of photosystem II (Phi(PSII)) was related to seedling dry weight at both the phenotypic and genetic level (r=0.46, two matching loci, respectively) but was not related to root traits. Cluster analysis and QTL association revealed that the different root traits were largely independently inherited and that root lengths and diameters were mostly negatively correlated. The major QTL for root traits detected in an earlier study in hydroponics were confirmed in this study. The length of the primary lateral roots was negatively associated with the germination index (r=-0.38, two matching loci). Therefore, we found a large number of independently inherited loci suitable for the improvement of early seedling growth through better seed vigour and/or a higher rate of photosynthesis.