Grain nitrogen supply sources in some rice cultivars in response to heat stress due to planting date changes

Extreme heat-stress events have become more frequent due to climate change often with devastating effects on rice production. Accumulation and translocation of nitrogen (N) in rice organs is an important process that determines rice yield and quality, which will usually change under the influence of environmental stresses including heat. In this order, the research was conducted with the aim of the effect of heat stress induced by planting date and its effect on seed nitrogen sources in rice cultivars (Oryza sativa L.). An experiment was carried out with two factors of planting date and cultivar in split plots arrangement based on randomized complete blocks design in three replications in 2017 and 2018 in the south of Khuzestan province. In order to face high temperatures (heat stress) in field conditions, three planting dates of May 15, June 5, and June 25 respectively with long-term average temperatures of 36.8, 33.4, and 34.2 0C were selected as the main plots. The sowing date with favorable temperature conditions (33.40C) was considered as control and the early sowing date was considered as heat stress (36.80C) so the flowering and seed filling period was faced with heat stress (39.10C). At each planting date, 7 rice cultivars including Anbouri, Champa, Daniyal, Gerdeh, Hamr, and Hoveyzeh (native cultivars) and N22 (International cultivar) were cultivated as sub-plots. The results showed that in all three planting dates in all cultivars, the highest amounts of remobilized nitrogen and nitrogen remobilization efficiency from leaves to grain were obtained in plant leaves. Hoveyzeh and Hamer cultivars, while having higher amounts of direct nitrogen uptake, had higher efficiency and a higher contribution of nitrogen remobilization. In the first planting date, Hamr and Hoveyzeh cultivars clearly had a significant advantage in leaf nitrogen remobilization and total nitrogen remobilization compared to Champa and Anbouri cultivars. However, the contribution of nitrogen remobilization in these cultivars showed a significant decrease compared to Champa and Anbouri cultivars. Among the studied cultivars, the highest nitrogen harvest index was obtained in modified and dwarf Daniyal cultivars with an average of 67.59%, followed by Hoveyzeh and N22 cultivars. In general, it can be concluded that due to heat stress during the reproductive period of the plant, the lowest amounts of nitrogen were absorbed in the grain and also the lowest values ​​of nitrogen remobilization and the contribution of remobilization nitrogen in grain nitrogen supply were obtained. Also, heat-tolerant cultivars (Howeizah, Hamar, and Daniyal) had the highest nitrogen transfer efficiency in all three planting dates and these cultivars can be used in breeding programs.