Ratio of Spike to Stem in Wheat Cultivars: Associations with Agro-Phenological Traits under Different Environmental Conditions

The ratio of spike to stem measured at anthesis (spike:stem ratio) could be considered as a promising trait in physiological studies and breeding programs of wheat. This study was performed to investigate the genotypic variations for spike:stem ratio among Iranian wheat cultivars, and to study the possible associations between this trait and the phenological and agronomical characteristics. Eighty-one wheat cultivars released between 1930 and 2006 were examined in 2010–2011 with normal sowing conditions and in 2013–2014 with late sowing conditions. Considerable variations were found for spike:stem ratio among wheat cultivars. Spike:stem ratio was, on average, 0.60 and 0.97 in 2010–2011 and 2013–2014 growing seasons, respectively. Linear regression analysis of spike:stem ratio against the year of cultivar release indicated that this trait significantly increased from old to modern cultivars. Improvements in spike:stem ratio were 0.002 and 0.005 per year during 2010–2011 and 2013–2014, respectively. Generally, spike:stem ratio correlated negatively with thermal time (TT) from sowing to anthesis. No clear relationship was found between spike:stem ratio and TT from jointing to anthesis. Spike:stem ratio correlated positively and significantly with grain number obtained either from square meter or from main stem spike.