Abstract
Introduction
Delay in sowing date results in yield production change in wheat. Belated sowing time of wheat cultivars always encounters terminal heat stress, particularly after flowering, in south of Iran. Heat stress is one of the main obstacles influencing crop production worldwide. Currently, producing heat tolerant genotypes is so essential in plant breeding programs. In spite of previous research on physiological aspects of heat stress tolerance, a consensus among scientists on the most important traits controling heat stress tolerance in plants is not available. Phenological traits of wheat genotypes are the major traits influencing heat tolerance and very important in breeding programs.
 
Materials and Methods
In order to investigate the effects of sowing date and terminal heat stress on phenological traits, yield and yield components of bread wheat, a field experiment was conducted in a randomized complete block design with three replications on 30 genotypes including: Pars, Bahar, Sistan, Dez, Shiraz, Bam, Arvand, Chamran 2, Kavir, Neyshabour, Roshan, Pishtaz, Hamoon, Kaz, Arta, Sepahan, Mehregan, Shuosh, Verinak, Arg, Ghods, Hirmand, Chamran, Alvand, Falat, Aflack, Marvdasht, Mantana, Babax and Weebile with different length of phonological stages in experimental fields of Shahid Chamran University of Ahvaz during 2014-2015. The cultivars were planted in normal and late sowing dates (terminal heat stress) and the data were combined and run in a combined analysis of variance.
 
Results and Discussion
In this research, sowing date and consequent heat stress significantly reduced (P≤0.01) the phenological traits. The average of the number of days from sowing to tillage was reduced by 23% under terminal heat stress in comparison with the normal condition. The yield and yield components were also strongly affected by seasonal heat stress and the number of spikelets per spike (16%), number of kernels per spike (18%), thousands kernel weight (11%), biological yield (40%), harvest index (14%) and grain yield (46%) reduced under heat stress. Based on the results, the correlation between phenological traits and yield components was positive in normal conditions and in most cases it was negative under heat stress conditions. The reults of canonical correlation showed that phenological traits in the first canonical correlation could interprete 38% of the variance of yield components in the normal and 47% in the heat stress conditions, which shows the more effective role of phenological traits on yield components under heat stress than normal conditions. Cluster analysis also displayed the important role of phonological features in comparison with other traits in the classification of commercial wheat cultivars of Iran. Generally, under hot climatical conditions such as Khuzestan the length of different phonological stages over the course of plant life significantly influence the grain performance. If the phenology of genotypes is locally engineered based on the optimum length, the promising heat tolerant cultivars will be achieved with ability to produce and maintaine the yield production.
 
Conclusion
Generally, under hot climatical conditions such as Khuzestan the length of different phonological stages over the course of plant life significantly influence the grain performance. If the phenology of genotypes is locally engineered based on the optimum length, the promising heat tolerant cultivars will be achieved with the potential to produce and maintaine the yield production.