Investigating the Effect of Delayed Sowing of some Wheat Genotypes Under End of Season Heat Stress in Moghan

Abstract
The purpose of the relevant research is to study the effect of delayed planting on the agricultural characteristics of some promising wheat cultivars at high temperatures at the end of the season and to select cultivars that tolerate heat stress with high grain yield. For this purpose, an experiment was carried out in the form of split plots in the research farm of Moghan Faculty of Agriculture and Natural Resources in the form of split-plot randomized complete block design with three replications during crop years 2017-2018. The main plot was three planting dates (15 November, 1 Desember and 15 December) and the sub-plot was 16 advanced wheat genotypes. The results of comparing the average interaction effect of genotype and planting date showed that on the planting date 15th of December, the maximum plant length, spike length and peduncle length corresponded to genotype number G4, and the highest number of spikelets and straw weight corresponded to genotype number G7. The highest number of seeds per spike, weight of seeds per spike, weight of seeds in the whole plant, weight of the whole plant and seed yield (4666.7 kg.ha^-1) belonged to genotype No. G8. Also, under stress, high seed grain had a significant positive correlation with traits of total plant weight and the number and weight of grains per spike. The highest correlation coefficient (0.97) belonged to a trait of total plant weight with seed weight per spike on the planting date (15 December). In other words, the studied genotypes showed different responses in different cultivation dates. G14 and G8 cultivars were able to obtain high average seed yield in both normal and stress conditions on the planting dates of November 15 and December 15, respectively.
Introduction
Wheat (Triticum aestivum L.) is the most important crop on earth and is the major source of calories and protein making up half of the protein and 65% of the daily calorie intake needed for the population of the country. The economic importance of wheat, both in terms of production and nutrition, is more than other agricultural products in the world. Wheat can be produced even in areas where it is not possible to produce other plants due to changing climatic conditions or the dryness of the environment. Global climate changes increasingly affect the production of agricultural products. Critical weather conditions, especially temperature and rainfall anomalies, have a significant effect on the success of the cultivated area of crops. High and unusual temperature is one of the most common forms of abiotic stress, which is considered a great risk for crop production. The flowering stage is the most sensitive stage to heat stress. Complex interactions between phenological stages and the sensitivity of different growth stages to the environment affect the final yield. Heat stress also weakens the pollen and thus weakens pollination. The sensitivity of pollen to high temperature is related to the inability of pollen to make heat shock proteins. Heat stress during flowering and seed filling accelerates the destruction of leaf chlorophyll, resulting in a decrease in both leaf photosynthetic activity and final biomass. The response of wheat to heat stress and dehydration has a complex mechanism that includes molecular changes and its spread to all metabolic activities and its effect on plant morphology and phenology.The traditional approach to estimate the optimal planting date is to conduct direct field experiments with a range of planting dates. Selection for stress tolerance under field conditions is often done by exposing genotypes to high temperatures by changing planting dates. In arid and semi-arid regions, wheat planting cannot be postponed later than December due to the prevention of yield reduction due to late planting and the reduction of the length of the plant's growth period. The purpose of the relevant research is to study the effect of delayed planting on the agricultural characteristics of some promising wheat cultivars at high temperatures at the end of the season and to select cultivars that tolerate heat stress with high grain yield.
Materials and Methods
For this purpose, an experiment was carried out in the form of split plots in the research farm of Moghan Faculty of Agriculture and Natural Resources in the form of split-plot randomized complete block design with three replications during crop years 2017-2018. The main plot was three planting dates (15 November, 1 Desember and 15 December) and the sub-plot was 16 advanced wheat genotypes. Each of the studied plots had six lines, each line with a length of five meters and the distance between the lines was 20 cm. Also, the seed density was 350 seeds per square meter.
Results
The results of comparing the average interaction effect of genotype and planting date showed that on the planting date 15th of December, the maximum plant length, spike length and peduncle length corresponded to genotype number G4, and the highest number of spikelets and straw weight corresponded to genotype number G7. The highest number of seeds per spike, weight of seeds per spike, weight of seeds in the whole plant, weight of the whole plant and seed yield (4666.7 kg.ha^-1) belonged to genotype No. G8. Also, under stress, high seed grain had a significant positive correlation with traits of total plant weight and the number and weight of grains per spike. The highest correlation coefficient (0.97) belonged to a trait of total plant weight with seed weight per spike on the planting date (15 December).
Conclusions
In general, according to the results of the present study, the studied genotypes showed different reactions in different cultivation dates. The cultivars G14 and G8 were able to obtain high average grain yield in both normal and stress conditions on the planting dates of November 24 and December 24, respectively. Also, the simultaneous examination of biochemical and morphological parameters is important for selecting genotypes tolerant to heat stress at the end of the season and is recommended for further studies.