optimizing grain filling period of spring wheat in the warm and humid agro-climatic zone of northern iran

To optimize the grain filling period of commercial wheat cultivars, an experiment was conducted at two agricultural research stations in the warm and humid agro-climatic zone of northern iran. the experiment was conducted as a split plot with four replications. the main plots consisted of five sowing dates: 1 november (sd1), 16 november (sd2), 1 december (sd3), 16 december (sd4), and 31 december (sd5), and four spring wheat cultivars, namely ehsan, tirgan, meraj, and kalateh, were the subplots. the results indicated that the highest yield was obtained from sd3 (5892.2 kg ha-1). despite the one-month difference between sd1 and sd3, the grain filling period remained unaffected, providing suitable conditions for plant height and resistance to lodging across the different cultivars. at sd3, optimal conditions were established for the wheat cultivars regarding temperature, day length (dl), relative humidity (rh), and energy use efficiency during the grain filling period. both sd2 and sd3 exhibited the highest energy use efficiency. the grain yield at sd1 decreased due to severe plant lodging, while exposure to terminal heat and drought stresses, and shortened grain filling period reduced yield at sd4 and sd5. for the early anthesis cultivar, i.e., kalateh, the grain yield was significantly higher than that of the other cultivars. the grain filling period for the early anthesis cultivar was not significantly different from the others, allowing the plant to escape terminal heat and drought stresses, thereby increasing energy use efficiency and, consequently the grain yield. therefore, the early anthesis cultivar is highly suitable and recommended for cultivation in the studied zones where terminal heat and drought stresses are prevalent at optimal sowing dates.