Euphytica 81: 85-92, 1995. 85 ~) 1995 KluwerAcademic Publishers. Printed in the Netherlands. Genotype × environment interaction for root traits in maize, as analysed with factorial regression models Yannick Htbert, Christophe Plomion & Nathalie Harzic INRA - Station d'Amdlioration des Plantes Fourrag~res, F-86600 Lusignan, France Received 30 August 1994;accepted13 October1994 Key words: maize, climatic factor, genotype × environment interaction, root trait, statistical model, Zea mays Summary The efficiency of genotypic and climatic characteristics in accounting for the interaction between genotypes and environments has been assessed in a three-year trial involving a set of genotypes presenting a range of root morphology characteristics (number and size). Climatic information on rainfall and temperature was recorded during the experiments, together with extra data on the growth and development of the genotypes. Their effects have been tested in factorial regression models. Climatic covariates were very powerful in accounting for the genotype by year interaction as well as the year main effect alone. For the number of adventitious roots on internode 7, the main effect of year could be described as a linear function of the average temperature and precipitation that occurred during the period of root initiation and growth. For internode 6, no clear conclusion was possible. For the root traits studied, 74 to 98% of the interaction could be explained by one climatic covariate. The regression coefficients can be considered as measures of genotypic stability. The genotypic covariates describing aerial development performed rather poorly, compared with environmental ones, even though the physiological and functional relationships between root and shoot are well known. Neither genotype main effect nor genotype by year interaction could be described sufficiently by factorial regression. Still, the genotypic covariates performing best clearly differed between root counts and size. Also the best genotypic covariates differed for main effect and interaction. Introdu~ion In many trials in which a set of genotypes is grown in a range of environments, the genotypes do not evidence the same responses under all conditions. This phe- nomenon, known as genotype x environment (GE) interaction, leads to serious problems in comparing the performance of genotypes in several environments and affects the importance of genetic progress through selection. From the plant breeder's point of view, it is essential to find out what factors could explain the greatest part of GE interaction. A comprehensive approach only would allow an efficient breeding strat- egy for genetic stability to be developed. Much work has been done on this subject, both by statisticians interested in non-additivity and by agronomists, breeders and geneticists particularly con- cerned with GE interaction. Many fixed models have been used for detecting and characterizing GE inter- action. They have been reviewed by Freeman (1973), Westcott (1986) and Denis & Vincourt (1982). The influence of climatic factors on aerial and root growth traits has already been observed by Htbert et al. (1992) in a study of genetic variability of root lodging resistance-related traits within a set of maize lines. In addition, GE interactions have been demonstrated by these authors, in many experiments on lines and hybrids. The study of GE interaction is quite usual for yield data but not common for the components of aerial growth or for root growth traits. Investigation of GE interaction is a complementary step towards under- standing the genetic pattern of root lodging resistance in maize. It is likely to bring about advances in the