Wheat (Triticum aestivum L.) has been the crop with the second-highest global production for most of the past decade, and is the most extensive and productive crop in the UK (FAOSTAT database) 2009}. Wheat grain is the most traded food product in the world} and the major import in developing countries (Dixon et al., 2009). During the 1960's and 1970's the majority of improvement in yield in . wheat came about through selection for raised harvest index (the proportion of above-ground dry matter in grains at harvest; HI). However} since the 1980}s genetic gains in yield potential among winter wheats under optimal growing conditions have been associated with increased above-ground dry matter (AGDM) more closely than biomass partitioning (Clarke et of.) 2012; Shearman et al. 2005). AGDM is determined by radiation interception (f) and radiation-use efficiency (RUE) (Monteith) 1977}. In the UK f can be mainly optimised through agronomy (canopy management)} so raising RUE has been prioritised by breeders (Reynolds et al., 2012; Fischer & Edmeades, 2010).