SEED COAT CRACKING IN SOYBEAN

Elliptical cracks in the soybean [Glycine max (L.) Merr.] seed coat have been termed seed coat cracking. This imperfection of the seed coat is an avenue for pathogenic and adverse environmental factors to affect seed quality. To better understand this phenomenon, the time of occurrence and morphological changes involved in seed coat cracking were examined by light and scanning electron microscopy. Physical parameters of normal and cracked seed and the amount of seed coat cracking that occurs in a pod were also determined. Seed coat cracking began in the late R6 growth stage just prior to R7 growth stage (physiological maturity). The initial cracking of the tissues occurred around the hilum; the most common separation was perpendicular to the raphe. Separation also occurred on either side of the embryonic axis. As the seed matured and began to desiccate, the cracks elongated and became perpendicular to the hilum exposing the parenchyma cells. This separation was accompanied by a slight upward rolling of the parenchyma and underlying tissue. The cotyledonary tissue, that was beneath the cracked area was raised above the surrounding tissues, indicating that this area of the cotyledon possibly matured at a different rate than adjacent tissues. Seeds with cracked coats were generally larger than seeds with normal seed coats. There was a significant probability (P < .001) that if one seed coat was cracked in a pod then all seeds in the pod contained cracked seed coats. The results suggest that separation of the palisade and hourglass tissues is caused by parenchyma or underlying tissues that do not contract normally during seed desiccation.