Effect of Sowing Date on Seed Yield of Soybean Genotypes and Determination of Threshold of Temperature Stress in North Khouzestan Conditions

To study the effects of sowing date and heat stress on seed yield of soybean genotypes، a field experiment was conducted in 2005-06 at Safiabad Agricultural Research Center Dezful، Iran. The experiment design was strip block arrangment in complete randomized block Design with three replications. Vertical plots consisted of four levels of planting date (8 June، 22 June، 6 July and 20 July) and horizental plots were twelve soybean genotypes (L14، Sahar، 504، L17، LD8149، Williams، Safiabadi، Katoul، Williams × TN4. 94، 5WCNE، BP and TN5. 95 × Hack). Mean comparison of sowing date showed that the highest (3243 kgha-1) and the lowest (2452 kgha-1) seed yield were achieved in 20 July and 8 June، respectively. The highest (3171 kgha-1) and the lowest (2206 kgha-1) seed yield were obtained from Safiabadi and Williams genotypes، respectively. The threshold of temperature stress was calculated for each genotype and the average of 39. 7°C determined as threshold of temperature stress. Based on the results of the present study، sowing of soybean in late July can be recommended for North Khouzestan areas.