Author : Khin San Nwe

Summary : Two experiments were conducted, one pot experiment at the Crop Science Cluster, the University of the Philippines Los Baños (UPLB) from May to September 2009 and one field experiment at the Central Experimental Station (CES), (UPLB) from January to May 2010 to determine the physiological, morphological and agronomic responses to twelve soybean varieties to flooding stress and to evaluate the sensitivity of biomass accumulation, N2 fixation and yield reduction due to flood duration at different growth stages among three varieties. Three days flooding at vegetative stage reduced the functional leaf area, leaf area ratio, net assimilation rate at flowering stage and total dry matter yield at both flowering and pod setting stages in the pot experiment. Three day flooding at vegetative and reproductive stages also reduced the RGR. However, specific leaf weight and root volume increased at 3 days flooding at vegetative stage. Grain yield plant¯¹ increased when flooded for 1 day at pod setting stage as compared to control. It was reduced when there was flooding for 3 days which was not significantly different from the control. Path coefficient analysis showed that higher soybean grain yield for 1 day flooding than control was due to significant contribution of numbers of pod plant¯¹ and seed pod¯¹. Among the varieties tested, IPB Sy 96-27-21 was the most resistant variety with highest grain yield due to higher hundred seed weight, seed pod¯¹ and high physiological parameters; leaf area, leaf area ratio, net assimilation rate and total dry matter yield. The second most and intermediate resistant varieties were Tiwala 8 and Tiwala 10 respectively. NSIC- Sy-9 was the susceptible variety due to poor grain yield, seed pod¯¹, functional leaf area and filled pods plant¯¹. In the field experiments, 3 day flooding reduced functional leaf area (FLA) and leaf area index (LAI) at pod setting stage. One day flooding had the highest FLA and LAI. There was no reduction in total dry matter yield, nitrogen fixation, grain yield plant¯¹ and grain yield ha¯¹ due to flooding at 1 to 3 days. Among the varieties, Tiwala 8 had the highest total dry matter yield (TDM) at three growth stage while Tiwala 10 had the lowest. NISC- Sy-9 had the highest nitrogen fixation for both flowering and pod setting stages while Tiwala 8 for only vegetative stage. Flooding for 1 day at flowering stage significantly increased grain yield ha¯¹ by 17% compared with the control. At pod setting stage, flooding for 3 days significantly reduced grain yield by 16% compared with the control. However, it was not significantly different with that for control at vegetative and flowering stages. Path coefficient analysis revealed that there were no yield component that positively and significantly contribute to higher grain yield but % nitrogen fixation had positive and high significant correlation with hundred seed weight for 1 day flooding at flowering stage, positive and non significant correlation for control pod setting at pod setting stage and in turn positively and indirectly affected grain yield via its effect on hundred seed weight.