BROWNING OF LENTIL SEEDS, CONCOMITANT LOSS OF VIABILITY, AND THE POSSIBLE ROLE OF SOLUBLE TANNINS IN BOTH PHENOMENA

Abstract

Under normal storage conditions, lentil seeds, Lens culinaris Medic. (Fabaceae), were observed to discolor gradually over a 2-yr period from the olive green of freshly harvested seeds to a light yellow and finally a deep brown. Browning was experimentally accelerated by exposure to a warm, humid environment and conversely, retarded by exposure to a cold or dry atmosphere. Browned seeds were judged less viable than green seeds on the basis of (1) a fainter reaction to the tetrazolium test, (2) lower percent germination, and (3) a higher electrical conductivity of the imbibition medium indicative of a loss of membrane integrity and resultant loss of cell solutes. Coats of both green and browned seeds contained a high amount of proanthocyanidins (condensed tannins), both procyanidin and prodelphinidin, but browned seeds exuded much lower amounts of low molecular weight proanthocyanidins (soluble tannins) into the imbibition medium than did green seeds. It is concluded that (1) browning was a result of polymerization of soluble tannins to brown-colored high MW polymers (condensed tannins), and (2) under experimentally provided storage conditions of high relative humidity, embryo death was hastened in the presence of the seed coat, possibly by the interaction of soluble tannins or their precursors with embryo membranes.Key words: Lentils, Lens, seed browning, seed ageing, proanthocyanidins, soluble tannins