Yield Analysis of New Bread Wheat Cultivars Based on Agro-Climatic Indices under Different Sowing Dates in Gonbad

Background and objectives: Determining the exact sowing date is an appropriate tool in controlling the length of the phenological stages for optimizing the climate conditions and minimizing the effects of terminal heat and drought stresses. The agro-climatic indices are used to study of plant phenology and its relationship with grain yield. Agro-climatic indices are changed under climate change conditions. The response of crops to climatic conditions can be assessed at the plant phenological stages by monitoring these indices. The aim of this study was to investigate the grain yield of new wheat cultivars related to northern warm and humid agro-climatic zone (Iran) under different sowing dates based on agro-climatic indices in Gonbad area. Materials and methods: This experiment was conducted in Gonbad agricultural research station, Gonbad, Iran, during two cropping seasons (2017-2019) as split plot based on randomized complete block design (RCBD) with four replications. Five sowing dates of 1 November, 16 November, 1 December, 16 December and 31 December were placed in main plots and four spring wheat cultivars including Ehsan, Tirgan, Meraj and Kalateh were placed as subplots. In each phenological stage, agro-climatic indices were calculated which includes total day length (DL), photoperiod (PPD), growth day degree (GDD), photo-thermal unit (PTU), helio-thermal unit (HTU), hydro-thermal unit (HYTU), heat use efficiency (HUE), helio-thermal use efficiency (HTUE), photo-thermal use efficiency (PTUE) and hydro-thermal use efficiency (HYTUE). Results: The results showed that no significant differences were observed in grain yield of second (25 November) and third (10 December) sowing dates (5708.3 and 5626.4 kg ha-1, respectively) and their grain yield was significantly greater than other sowing dates. The grian filling period, number of spike per square meter and number of grain per spike were not significantly different in three primary sowing dates. The third sowing date had the highest values in terms of agro-climatic indices including growing degree days (GDD), day length (DL), photo-thermal units (PTU), hydro-thermal units (HYTU), during grain filling period. While, the second sowing date had maximum heat use efficiency (HUE), helio-thermal use efficiency (HTUE), photo-thermal use efficiency (PTUE) and hydro-thermal use efficiency (HYTUE). Grain yield (5269.8 kg ha-1) and number of spike per square meter (330.17) of Kalateh cultivar were significantly greater than other cultivars (Tirgan, Meraj and Ehsan). Conclusion: The second and third sowing dates provided suitable conditions for plant height and resistance to lodging in different cultivars. Also, the two mentioned sowing dates were provided optimum conditions for different cultivars in terms of climatic parameters including temperature, day length, relative humidity and energy use efficiency, during grain filling period. The grain yield decreased due to severe plant lodging in the first sowing date and exposure to terminal heat and drought stresses and reduced the grain filling period duration in the fourth and fifth sowing dates. The growth period of Kalateh cultivar was shorter than other cultivars (no difference during grain filling period), which caused the plant to escape terminal heat and drought stresses, increasing energy use efficiency and consequently increasing grain yield compared to other cultivars. Therefore, Kalateh cultivar is very suitable and advisable for cultivation in low rainfall areas in northern Golestan province in the second and third sowing dates.