Photoperiodic Response of Soybean Varieties in Paraguay as Revealed by Different Sowing Times

Sixteen soybean varieties were sown on 8 different sowing times from Sept. 30 to Feb.5 through 1981 to 1982 in the field of Regional Center of Agricultural Research. Paraguay. The mean day length during the period from sowing to flowering was the longest when the plants were sown in Nov. The largest number of stem nodes was realized also by sowing in Nov. The increasing rates of the node number owing to the photoperiod were estimated by means of the exponential regression to compare the varietal difference in the photosensitivity for flowering. Two varieties which were adaptable to early sowing exhibited the lower photosensitivity than the others. The later was the sowing time, the shorter became the period from flowering to maturity due to decrease of the mean day length during maturation. The limits of optimum photoperiod for maturation as well as the retardation rates of maturation with excessive photoperiod were estimated for each of the varieties by means of the linear regression. A tendency was observed that later varieties had longer limit of the optimum photoperiod and higher rate of the retardation of maturation with the excessive photoperiod. When the plants were sown early, the pod setting of late varieties was delayed probably due to long photoperiod at the time of flowering.