in most regions where grain crops are produced, and deleterious effects of high temperature on crop yields are well documented. In this context, it is critical identifying genetic and management tools to mitigate the effect of high temperatures on yield. Nitrogen (N) fertilisation is one of the most widely applied management practices in grain crops worldwide. In many regions, crops are frequently well fertilised to maximise productivity. However, there have been limited efforts to elucidate to what degree the level of soil fertility may affect the magnitude of the high temperature effect on crop yield. Analysing the likely interaction may be relevant for designing more appropriate fertilisation strategies to not only increase productivity through better growth conditions but also to mitigate the likely yield penalties imposed by high temperatures. The general objective of this thesis was to assess the genotypic variability in yield components, and the susceptibility of yield determinants to thermal stress and nitrogen availability in maize. The issue was explored throughout 11 field experiments, carried out during 4 years, at two locations of contrasting altitude, under varying N fertilization regimes and a control with up to 12 different maize hybrids of contrasting maturity groups. In 4 of the field experiments different high temperature stresses were imposed, in combination with genotypes and N regimes, at the field by enclosing the designated area for the treatments with transparent polyethylene film (100 μm thickness) mounted in wood structures of 3-3.5 m height. In several of the experiments source-sink manipulations were also imposed to ascertain the origin of the yield penalties imposed by the different treatments. Differences in yield performance among hybrids were not related to the cycle duration, however if the comparison is restricted to the average of all short- and long-cycle hybrids, it can be confirmed that the shorter-cycle hybrids had lower production than the long-cycle hybrids. In parallel but independent set of experiments it was found that long cycle hybrids may be a true option for the high altitude farmers (if they are prepared to assume a higher than usual risk of loses in exceptionally cold autumns), as well as the short-cycle hybrids may be a reasonably productive alternative for farmers in the plain of Lleida (and other similar environments). Overall the range of conditions, yield was more strongly affected by capture, than by partitioning or efficiency of use of resources and was positively related to both of its components similarly (even though grain number was more plastic than grain weight) as well as to grain protein concentration. The negative relationship between yield and Nitrogen Utilisation efficiency (NUtE) found in the context of the wide range of conditions did not preclude the awareness that future hybrids shall be more NU Efficient and that ways to select for improved Nitrogen Use Efficiency (NUE) must be developed for future agricultural systems in which N is expected to be less freely available while yields must keep increased. Then, recently proposed surrogate for phenotyping to improved NUE (the critical specific leaf N, SLNc) was tested for genetic variation. It was proven that large genetic variation exists for SLNc, partly related to genotypic differences in N uptake N uptake . This would imply that SLNc would hardly be a good surrogate to phenotype large populations for improved NUE. It was demonstrated for the first time in maize that the sensitivity of yield to heat stress was increased by N fertilisation. This conclusion is based on field experiments with treatments of a magnitude well within expected variation in realistic conditions. The effect was through affecting the capacity of the plants to set grains and to a lesser extent to allow grain weight to be maximised; and it was independent of any (potentially additional) effects on either uncoupling anthesis and silking or on pollen amount and viability. Heat stress affected grain size by directly affecting the capacity of the grains to grow. This conclusion was reached both due to interpretations on the effects of heat on source-sink relationships of plants as well as from results of manipulations of the source-sink relationships during grain filling. Heat stress reduced grain size even when it increased source-sink ratio (by inducing late abortion of few grains while not affecting much post silking growth), and this direct effect was not worsened by defoliation nor reversed by degraining, and the penalty did not exhibit a clear hierarchical response: it was similar for grains of different potential size.