Yield is a complex trait which determined by high number of genes and effected by many other traits; yield components, traits relate to the seeds, and agronomic traits. One of the most important yield component traits is productive tiller number. Productive tiller number is a quantitative trait which controlled by large number of genes, one of these genes that controlled the productive tiller number recently identified by Noruaka et al 2011 is (QTn.mst-6B). Since the The high tiller allele (QTn.mst-6B) was tightly linked with gwm88 markers LOD was 4.4 (Noruaka et al 2011) the opportunity to have Chromosome rearrangement was < 0.0001, therefore all effects should return to the environmental factors and the interaction between populations and environmental factors. The nine locations divided into four major groups according to the growing seasons, Bozeman which had the longest growing season, Pullman, Huntley, and Lind (Table 5). Long growing season means long vegetative growth period, long seed filling period, the high tiller allele effects yield significantly with an acceptable temperature, while high temperature during seed filling period would reduce the filling period and enhanced seeds maturation
The high tiller allele (QTn.mst-6B) affected significantly on early tiller number over all locations table 3 Fisher 1985, Rahman, and Chikushi 2009 found on their study that high temperature and water stress inhibited the initiation and survival of tiller on wheat plant .Stern and Kirby 1979, reported that the tiller initiation tiller process in wheat plant, they found under unfavorable environment, the growth of tiller will be slower or stops, while affected significantly on productive tiller number on some locations and other locations were not affected. Productive tiller number as quantitative trait, affected by many other genes and the interaction with the environment, high temperature, and water stress, which reduce the final number of tiller per plant by increasing tiller mortality and decreasing tiller production (Fisher 19773). It was due to survival of only a few productive tiller numbers on high temperature and water stress. Several studies suggested that the ability to producing tillers and their survival depends on genotype, spacing, nutrition, temperature, and water stress (Kirby et al 1985, Longnecker et al 1993, Rahman 2009). Several reports explained plant ability to produce productive tillers of wheat plant and survival depends on genotype, spacing, and nutritional management practices and also on environmental factors especially air temperature (Rahman 2009).
Seeds number per head, is a quantitative traits which controlled by large number of genes affected by genotypes, environmental conditions, and the interactions between genotype and environmental conditions. Stone and Nicolas 1995, Gibson and Paulsen 1999 were reported that seed number per head response to high temperature, which affected the pollen viability and fertilization which reduced in seed number per head. Seed number per head is one of three yield component traits, therefore reducing the seed number per head, causing reducing grain yield.
The third yield components in spring wheat are individual seed weight, seed weight also quantitative traits which is also significantly influenced by temperature. High temperature during filling seeds enhanced maturation causing reduction seed growth duration (Rahman et al 2005), which ultimately resulted in smaller seeds.
Even the high tiller allele (QTn.mst-6B) affected significantly initiating early tiller, unfavorable environmental conditions increasing early tiller mortality, and only few of them will survival and produce heads. Unfavorable environmental conditions during flowering time would +-educe seeds number by killing the pollen; while high temperature during seeds filling would enhance maturation reducing seed growth period would reduce seeds weight. On some locations like Huntley, Lind, Pullman 2012 and Huntley 2013, large seed number per head, association with large individual seed weight. Usually these two yield components have negative relationships, the large seed number association with small seed weight.
Pleiotropic effects were found for PTN and roots weight, seed weight and diameter, protein and test weight tables 6, 7, 8.