SOYBEAN SEED COAT STRUCTURES - RELATIONSHIP TO WEATHERING RESISTANCE AND INFECTION BY THE FUNGUS PHOMOPSIS-PHASEOLI

The fungus Phomopsis phaseoli (Desmaz.) Sacc. (syn. P. sojae Lehman) incites a serious disease of soybean [Glycine max (L.) Merr.] seeds. This study was conducted to determine if an impermeable seed coat reduces infection by this fungus. Seeds of four soybean cultivars found in blotter tests to be infected by P. phaseoli in very low amounts (0-5%) and seeds of four other cultivars with high amounts of infection by this fungus (65-83%) were studied under scanning electron microscopy. Seed coats of cultivars with low P. phaseoli infection were found to lack pores, whereas those with high infection had multiple pores. In addition, seeds of the four cultivars with low infection had closed micropyles, whereas those with high infection had relatively open micropyles. This was reflected in their measurements: closed micropyles had an average area of 0.17 mm2, compared with 0.44 mm2 for the open ones. The percentage of impermeable seed coats in the eight cultivars was determined by soaking seeds in distilled water for 1 h. The average percentage of seeds with an impermeable seed coat was 75% for the four cultivars with low infection and 5% for those with high infection. In a field study, in which pairs of cultivars with either normal or impermeable seed coats were compared, there were significant differences between the two seed coat types in amount of P. phaseoli infection at maturity and after field weathering. This study supports the theory that the resistance of certain soybean cultivars to infection by P. phaseoli is due to the physical characteristics of impermeable seed coats.