Introduction
Bread wheat is the first grain and the most important annual crop in the world. Like other crops, wheat faces many environmental constraints such as a lack of trace elements during its growth period. Zinc deficiency is a common micronutrient disorder in wheat-growing areas of the world. The growing degree-days (GDD) index is reliable compared to the time calendar due to its stability. These indices are important for comparing genotypes whose physiological developmental stages do not match. Temperature is an important meteorological variable in the study of plant growth and development processes and therefore its study on short time scales leads to a more accurate estimate of thermal need due to the relatively wide range of temperature changes in different phenological stages, setting the planting date. Studying the effect of temperature on wheat genotypes should be based on sufficient time and temperature to complete the phenological stages in which grain yield components are formed.
 
Materials and Methods
To study the phenological stages, cumulative growing degree-days, and grain yield and its components in 64 spring wheat cultivars under optimal and zinc deficiency conditions, a study was conducted in the 2019-2020 cropping season based on a simple lattice design in the research field of Urmia University, Iran. Phenological stages include the number of days to germination, the number of days to booting, the number of days to pollination, the number of days to physiological maturity, and the grain filling period with their GDDs during the growing season in each pot based on 50% of the samples were determined to have reached the relevant vegetative stage. In addition to phenological stages along with the number of grains per spike, thousand-grain weight, and grain yield were examined.
 
Results and Discussion
Analysis of variance revealed that there is a statistically significant difference among all cultivars in terms of all studied traits except the number of days to pollination under optimal and zinc deficiency conditions. According to the results of descriptive statistics, grain yield, the number of grains per spike, and thousand-grain weight decreased and phenological traits increased under zinc deficiency conditions compared to optimal conditions. Among phenological traits, not only the number of days to spike showed relatively high variability but also had a significant relationship with grain yield under zinc deficiency based on the results of correlation and regression analyses. The factor analysis showed that under optimal conditions, the first four components explained 87.81 percent of the total variation, so that the contribution of the first to fourth components were 38.38, 21.52, 15.88, and 12.3 percent. Under zinc deficiency stress conditions, the first five components explained 95.68 percent of the total variation, so that the first to fifth components explained 34.62, 18.48, 17.51, 14.66, and 10.40 percent, respectively. Cluster analysis classified all spring wheat cultivars into three and four clusters under optimal and zinc deficiency stress conditions.
 
Conclusion
Based on the results of cluster analysis under optimal and zinc deficiency stress conditions, Neishabour, Golestan, and Gahar cultivars were identified as desirable and tolerant cultivars that can be used in future breeding programs. The results of cluster analysis were confirmed by results of canonical discriminant function results