Metabolism of C14-Bicarbonate, P32-Phosphate, or S35-Sulfate by Lettuce Seed during Germination.

Abstract

Grand Rapids lettuce seed were germinated in the presence of C14-bicarbonate, P32-phosphate, or S35-sulfate. During germination, extracts of seeds were analyzed by chromatography. C14 was fixed into soluble compounds within 1 hour after the beginning of imbibition. Within 3 hours, about one-third of the total fixed activity appeared in malic acid, and 12-15% in glutamine, aspartic, citric, or glutamic acids. Other labeled compounds were asparagine, serine, glycine, alanine, and succinic, fumaric and glyceric acids. The patterns of distribution were similar during all phases of germination, even after radicle protrusion. Thus C14 was fixed by carboxylations into organic acids and the tricarboxylic acid cycle functioned from the earliest phases of germination. No difference in either extent or pattern of Cl4 fixation was noted between seeds germinating under white and those under far-red light. Neither the P32-phosphate nor the S35-sulfate was metabolized by the seeds until after radicle protrusion (approximately 15-18 hours). When the coats surrounding the embryo had been mechanically punctured, the P32 and S35 tracers were metabolized within 3 hours, though the time of radicle protrusion was unaltered. Distributions of P32 and S35, respectively, were similar to those from control seeds extracted after radicle protrusion. The data suggest that the coats surrounding the embryo are impermeable to phosphate and sulfate ions. Most of the P32 was detected in phosphorylcholine and phospholipids, pointing to the importance of lipid metabolism early in the germination process. S35 was detected in many unknowns; the largest fraction of activity corresponded to methionine. Consequently, punctured seeds could esterify phosphate and reduce sulfate to sulfhydryl within 3 hours.