Wheat (Triticum aestivum L.) is one of the most important sources of plant food for human among the main crops globally. High temperature resulting from delay in planting is one major environmental factor limiting growth and production of wheat, especially in tropical regions. Terminal heat stress during the last phases of wheat development especially in booting, heading, anthesis and grain filling stages of the spring wheat cultivars is considered as one of the major environmental constraints that drastically reduces grain yield and yield components of wheat in Khuzestan province and other warm and dry regions of Iran. Most of the Iranian soils have a high pH and calcareous nature, so absorption of nutrients is limited in these soils. Mineral nutrition of plants plays a critical role in increasing plant resistance to environmental stresses. Zinc is a ubiquitous micronutrient. It is required as a structural and functional component of many enzymes and proteins, and increases the yield and yield components of wheat. Boron is essential for pollen viability, flowering, fruiting and seed production. As a micronutrient, it plays a vital role in nitrogen metabolism, hormonal action, and cell division.
To evaluate the remobilization of photosynthetic materials and their contribution in the formation of grain yield of two bread wheat cultivars affected by terminal heat stress and zinc and boron foliar application, an experiment was conducted as split-split plot based on randomized complete blocks design with 3 replications in Ramhormoz city. The experimental factors were included planting date in two levels optimum (November 21) and late (January 5) to coincide growth terminal stages with high temperatures as the main factor, foliar application in four levels with water (control), zinc, boron and zinc + boron as the sub factor and two cultivars of bread wheat Pishtaz and Chamran 2 as the sub-sub factor.
The results showed that terminal heat stress caused by delay in planting increased significantly traits of stored photosynthetic materials remobilization from stem and spike to seed and relative contribution of stem and spike reserves in grain yield, but decreased significantly traits of current photosynthesis and its relative contribution in yield, and grain yield of two bread wheat cultivars of Pishtaz and Chamran. Zinc and boron foliar application reduced significantly all of the above traits in both optimum and late planting dates except for the traits of current photosynthesis and its relative contribution in yield, and grain yield. Among the wheat cultivars cultivated, Chamran 2 cultivar had a significant advantage in traits of current photosynthesis and current photosynthesis relative contribution in yield and grain yield compared to Pishtaz cultivar, but in other traits this superiority was evident in Pishtaz cultivar.
In general, it can be attributed the main factor increasing grain yield of two bread wheat cultivars of Pishtaz and Chamran to improve current photosynthesis. As well as, it can be used from timely planting date, zinc and boron foliar application and suitable wheat cultivar such as Chamran 2 as three management strategies to reduce the harmful effects of terminal heat stress caused by late planting date in Ramhormoz city.
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