Current trends in population growth suggest that global food production is unlikely to satisfy future demand under predicted climate change scenarios. Therefore, further improvements on wheat yield potential will be essential to meet future food demand. Thus, to further raise wheat grain yield it is critical to understand the physiological basis of grain number per m2 determination. Yield responsiveness to resource availability is usually related to grain number per m2 in most of grain crops, such as durum wheat. Under Mediterranean conditions, wheat yields vary widely, mainly in response to erratic rainfall, but it has been proposed that part of this variability may be reduced through nitrogen (N) management. Wheat yield responses to N fertilisation are usually related to those of grains per m2, which in turn is the consequence of processes related to floret development (floret initiation followed by floret death/survival) during stem elongation. However, there are almost no studies published relating floret primordia generation and survival in response to increases in N availabilities in wheat. Understanding the dynamics of floret primordia development as a physiological determinant of grain setting can be relevant to further increase grain yield in wheat. The focus of this thesis was to study the effects of N availability and the differences among modern cultivars (and, in some cases, combined with availability of other resources) on dynamics of floret development and survival determining the fate of these primordia and the generation of grain number in durum wheat. Six experiments were conducted (4 semi-controlled and 2 field conditions) during four consecutive growing seasons at Catalonia, NE Spain, with a combination of different N and water availabilities and contrasting modern durum wheat cultivars. Across all experiments, yield responsiveness to N was, in general, related to grain number increases as an indirect response to N through its effect on increasing growth. This is not only based on the fact that the relationship of grain number with spike dry matter was not improved if spike N were used instead, but also on the fact that detillering did increased grain number through the same relationships to spike dry matter. Also, improved spike fertility was due to both producing more fertile florets (in response to N and water or to the removal of competing shoots) and to reduce the percentage of failure of fertile florets in becoming grains. Responsiveness of the number of fertile florets was determined by a developmental response of floret primordia, which under high resource availability conditions continued developing normally in some distal florets of the spikelets, at any position of the spike, whilst in more stressful conditions their development stopped. Genotypic variation in fruiting efficiency was found for modern durum wheat cultivars and there was a tendency for some cultivars to have higher values of fruiting efficiency in most of the environments. Differences in fruiting efficiency were responsible for genotypic differences in grain number. A functional negative relationship was found between grain weight and fruiting efficiency, revealing a trade-off related to spike growth per floret developing normally.