Variation in quality of individual seeds within a seed lot of soybean (Glycine max (L.) Merrill)

The research described in this thesis aimed at increasing insight into the sources of variation in quality attributes of individual seeds within a soybean seed lot, into the relations between physical attributes and performance of seeds in seed tests and in controlled seed production conditions, and into the importance of the seed lot quality for crop performance including quality of seeds produced. Cv. IAS-5 (determinate growth habit) was used in all studies. Differences in characteristics within and between plants proved to be important sources of variation in size, weight, shape, viability, and performance of individual seeds in the seed lot produced. Seeds from earlier pods were larger, heavier, more spherical but less viable than from later pods. Seeds from main stems or from the top of the plants were larger and heavier than seeds from the branches or from the bottom of the plants. At these positions also pod set was earlier. Shorter times to emergence and longer periods from first pod set to harvest were associated with larger sizes of seeds produced per plant. Longer periods between first flower and first pod, and shorter periods between first pod and harvest were related to a higher viability of seeds produced per plant, but only under high air temperatures during pod growth and maturation. Within the seed lot studied, different physical seed attributes were associated. Etching and cracking were more frequent in larger and heavier seeds, yellow seeds were larger than greenish seeds. Taking into account these associations was essential to correctly assess the actual contribution of individual attributes to seed performance. Distinct tests assessed different attributes as being important. The tetrazolium test detected etched, cracked, greenish and wrinkled seeds. Conductivity per seed was affected by seed size, weight, cracking and wrinkling. Conductivity on a seed area projection basis (μA/mm 2) was shown to minimize size and weight effects. In this case, also higher conductivity values for greenish seeds were detected. Some individual physical seed attributes affected distinct stages of seedling growth differently. Therefore, conclusions on the importance of physical attributes for seed performance in seedling growth tests may depend on the stage of seedling growth at the time of analysis. Size, weight, shape, etching and wrinkling of seeds planted in soil were not related to emergence under favourable conditions, but fewer seedlings emerged from cracked and from greenish seeds. Within a crop, yield components per plant (number of pods or seeds, total weight of seeds) decreased with increasing size, weight and cracking of the seeds planted. In none of the crops physical attributes of individual seeds planted were associated with quality attributes of the seeds produced by them. Effects of seed lot grading for physical attributes on crop uniformity, and on quality, uniformity and yield of the seeds produced could not be shown.