Evaluation of Seed Germination and Seed Vigor of Different Soybean (Glycine max (L.) Merr.) Cultivars Under Different Planting Dates in Gorgan

Introduction\ud Soybean seed germination and seed vigor potential is lower compared with other crops, and it often greatly reduces at the planting time. The occurrence of unfavorable conditions during seed formation in field causes severe deterioration of seeds and reduces the seed quality. Planting date is one of the most important influencing factors on seed quality of soybean in field conditions. Since planting date affect seed quality by changing the environmental conditions including temperature, relative humidity and rainfall during seed development and maturation.\ud Materials and Methods\ud To evaluate the seed germination and seed vigor of soybean seeds wshich were obtained from various planting dates, a field experiment was conducted with a split plot arrangement in a randomized complete block design in three replications in Gorgan University of Agricultural Sciences and Natural Resources in 2011. Main plot included five planting dates April 20, May 13, June 3, June 29 and July 22, and sub plot consisted of three cultivars (Williams, Sahar and DPX). About 50 plants were selected and harvested during R7 (physiological maturity stage) from each plot, and their pods were removed by hand. Then, pods were dried in shade. To avoid mechanical damage, the seeds were removed by hand from the pods. Ultimately, seed quality was assessed by seed germination and seed vigor including accelerated aging, seedling growth rate (SGR) and electrical conductivity tests.\ud Results and Discussion\ud According to obtained results, the use of different planting dates caused the change at the beginning and ending time of seed filling period (R5-R7) of all three cultivars. This event in turn leads to creation different conditions in terms of temperature, rainfall and relative humidity during seed filling period in all three cultivars. Therefore Williams cultivar coincided with high temperatures more than two other cultivars, especially in the early planting dates. While DPX that was serotinous, encountered with lower temperatures and higher relative humidity. Sahar cultivar that in terms of the maturity group is between these two cultivars, in early planting dates encountered with lower temperatures compared to Williams and encountered with higher temperatures compared to DPX. Generally, the air temperature during seed filling period was lower by delay in planting. Planting date had a significant effect on seed germination and seed vigor, too. Seed Germination of two cultivars including Williams and Sahar fixed above 90 percent in four planting dates, but it significantly decreased in the planting date April 20 in Williams and July 22 in Sahar. Planting date affected seed germination of DPX. Accelerated aging and electrical conductivity tests similarly showed that the early spring planting dates reduced seed vigor of three cultivars, because it occurred when the high temperatures during seed filling period. Seed vigor of three cultivars increased with delays in planting, but due to occurrence of very low temperature and high rainfall during seed development in planting date of July 22, seed vigor of Sahar and DPX were reduced. In this study air temperature decreased and relative humidity increased during seed filling period of three cultivars with delay in planting. This issue led to lower seeds vigor in early spring planting dates compared to late planting dates due to collision of seed filling period with high temperatures. Of course germination and vigor of seeds produced in early planting dates in Sahar and DPX has always been higher than the seed germination and vigor of Williams. This was due to the absence of very high temperatures during seed filling period in these two cultivars. Because the seed filling period and maturity in Sahar and DPX occurred later than seed filling period and maturity of Williams. So that the mean maximum temperature during seed filling period of two cultivars Sahar and DPX in all planting dates was always less than Williams. On the other hand, since Williams was precocious, when that long delay occurred in planting date (July 22), matured before the occurrence of cold, wet and high rain conditions in autumn, and escaped from unfavorable conditions at the end of growing season. But Sahar and DPX were forced to deal with such conditions, and therefore their seeds produced in last planting date had a lower quality.\ud Conclusions\ud In summary, this study revealed that planting date with changes in environmental conditions during seed filling period (R5-R7) has a significant impact on soybean seed germination and seed vigor, and it is not considered in soybean fields in Golestan province. It was also noted that the choice of suitable planting date for the production of seeds with high quality, must be implemented according to the cultivar maturity group. Therefore, it is recommended that to produce high-quality seeds in precocious cv. Williams, planting operations must be carried out in July and avoided the early spring planting dates. In the case of serotinous cultivars like Sahar and DPX, it is better that planting operations performed in June, or at the beginning of July, and avoid any the excessive delay in theirs planting