Effect of Exogenous Tryptophan on Primary Metabolism and Oxidative Stress and Their Relationship with Seedling Germination and Vigor of Glycine Max L.

Abstract

Treatments that increase the germination potential and vigor of Glycine max seedlings are continuously being stimulated, with the aim of achieving a higher percentage of emergence and better performance in the field. Considering the relationship of tryptophan with germination-associated phytohormones, this study tested the hypothesis that exogenous supply of tryptophan to soybean seeds can affect germination, physiological vigor, and the accumulation of primary and oxidative metabolism molecules in seedlings. Soybean seeds were exposed to soaking solutions containing different concentrations of the amino acid (0, 25, 50, 100, and 200 µM), and the seedlings were evaluated at three time periods, at 8 h after sowing (HAS), and 5 and 14 days after sowing (DAS). Treated seeds showed better germination fitness and seedlings showed greater vigor, and these parameters increased with increasing concentrations of tryptophan. In the initial hours and days of germination process evaluation (14 HAS and 5 DAS), the activities of starch metabolism enzymes (α- and β-amylase) tended to be higher, resulting in increased contents of sucrose, reducing sugars, and total soluble solids at 8 DAS, constituting an important metabolic effect for seedling growth. On the other hand, the induction of germination and vigor promoted by exogenous tryptophan in soybean seedlings occurred by stimulating the metabolic pathways of oxidative stress, resulting in increased concentrations of H2O2, malondialdehyde, and proline in the tissues. Additionally, it led to increased activities of the antioxidant enzymes superoxide dismutase and ascorbate peroxidase. These parameters were responsive to increasing supplied concentrations of tryptophan. Thus, the metabolic stress in soybean seeds induced by auxin seems to be an important inductive pathway for germination and vigor of G. max seeds.