Field Deterioration of Soybean Seed: Role of Oxidative Stresses and Antioxidant Defense Mechanism

Inadequate plant population in the farmer’s field due to planting of poor quality of soybean seed is one of the important factors responsible for poor productivity of soybean in India. Field weathering of soybean seed is by far the most crucial factor responsible for poor quality of seed harvested under tropical conditions followed by faster rate of seed deterioration during storage and mechanical damages during harvesting and post-harvest handling. Extensive research has been carried out in our laboratory on understanding the underlying physiological and biochemical processes responsible for fast deterioration of soybean seed. Our results indicate that there are large genotypic differences among Indian soybean varieties for their response to field weathering. Based on extent of damage to seeds in response to field weathering, these varieties have been categorized into highly tolerant, tolerant, susceptible and highly susceptible classes. Further analysis of highly susceptible and highly resistant varieties has revealed that field weathering triggers rapid accumulation of free radicals in soybean seeds. On the other hand, the availability of antioxidants such as L-ascorbic acid and α-tocopherol, and activity of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and guaiacol peroxidase (POD), which are responsible for quenching of free radicals, is reduced in field weathered seeds. This imbalance between generation and quenching of free radicals results in enhanced lipid peroxidation, loss of membrane integrity and ultimately reduced germination and vigour of field weathered seeds. Seeds that are not exposed to weathering (unweathered seeds), the formation of oxyradicals is much less and the levels of antioxidants and antioxidant enzymes are higher which probably are sufficient to neutralize the low levels of oxyradicals produced. Similar variability among the soybean varieties could also explain the varietal differences observed in seed deterioration due to field weathering.