Trends in important agronomic traits, grain yield and its components in bread wheat cultivars released in northern warm and humid climate of Iran, 1968–2018

Investigating the relationship between grain yield and main agronomic traits and understanding the trend of changes in traits during breeding history are important for breeding strategies. In order to determine the trend of changes in improved wheat cultivars and calculate the genetic gain of yield and related traits, 20 spring wheat cultivars released for the northern climate of the Iran during the years 1968 to 2018 were evaluated. Cultivars were cultivated in a randomized complete block design with three replications in Gorgan, Gonbad, Sari and Moghan in three consecutive years (2015–18). Morphological and phenological traits and grain yield and related traits were assessed in each year. Linear regression analysis between investigated characteristics and year of cultivar release showed that the grain yield, biological yield, harvest index, number of spikes per square meter, thousand-kernel weight and grain filling rate were significantly increased. No significant changes, however, were observed during breeding programs in maturity-related traits such as days to heading, days to anthesis, days to maturity, duration of grain filling, plant height, peduncle length, spike length and number of grains per spike. According to the positive and significant correlation between biomass, harvest index, number of spikes per square meter and thousand-kernel weight with grain yield, it seems that breeders in the northern climate of the country have been selected these traits to increase grain yield during the process of breeding cultivars