Gustavo Ariel Slafer1,2, Roxana Savin1
1Department of Crop and Forest Sciences and Centre for Research in Agro-technology, University of Lleida, Spain
2ICREA (Catalonian Institution for Research and Advanced Studies)

Improvements in crop performance in terms of both productivity and efficiency of resource-use will remain to be a major objective of agronomy and naturally, a focal aim of any breeding or agronomic decision-making program. The eminently empirical approach of trial-and-error used has been successful in the past to increase yields. There is a growing consensus that to increase the likelihood to regain the rate of genetic gains achieved by cereal breeders during the second half of the 20th century it may be needed to reach an analytical approach for understanding the fundamental physiology of the crop and apply elements from this discipline in breeding. In addition, improving our knowledge of the physiological bases of crop performance under field conditions would also help in facilitating the use of molecular biology to improve yield. In general, wheat yield has been improved through increases in the number of grains per m2 associated with a reduction in stem length. Although further reducing height seems not appropriate to keep increasing yield, it seems that in general wheat yield is still source-limited during grain filling and therefore further increases in grain number is required. This is not only true for high-yielding conditions but also for most stressful environments, as shown by experiments manipulating source-sink balance during post-anthesis as well as by the frequently reported relationship between yield potential and actual yield under low yielding environments. As modern wheat cultivars already optimized plant height and time to anthesis we must find out alternative physiological traits to continue increasing grain number without further reducing height or modifying time to flowering. As grain number is, to a large degree, the consequence of the survival of spikelet/floret survival and this is tightly linked with the growth of the juvenile spike before flowering, the alternatives must either further increase spike growth before flowering (through increasing crop growth during stem elongation without reducing spike partitioning) or the fruiting efficiency (grains set per unit spike dry weight at flowering). Increasing crop growth during stem elongation might be achieved through lengthening the duration of stem elongation or through increasing rate of growth during that phase. Increasing fruiting efficiency seems also possible from empiric approaches highlighting the relevance of this trait to establish differences in grain number among modern, well adapted cultivars. In this presentation we will discuss at the light of physiological and genetic studies the likelihood of actually increasing yield in both stressful and stress-free environments through the manipulation of these traits.