Examination of Microorganisms and Deterioration Resistance Mechanisms Associated with Velvetleaf Seed1

Abstract

AbstractAnnual weed seeds persist in cultivated soils due to seed dormancy and possibly to resistance to microbial attack. The relationships between microorganisms and velvetleaf (Abutilon theophrastiMedic.) seeds were investigated to elucidate the deterioration resistance mechanisms in this species. Microorganisms were isolated from velvetleaf seeds matured on the plant and from seeds dispersed on the soil surface. An association of sporulating fungi comprised of Alternaria alternata, Cladosporium cladosporioides, Epicoccum purpurascens, and Fusariumspp. was consistently found on the seed surface. This association persisted on seeds after dispersal to the soil. Seed deterioration was infrequent despite abundant microbial growth on the seed surface. About 80% of the bacteria isolated from within seeds were antagonistic to externally seedborne fungi.The proportion of surface‐sterilized seeds with internal fungi was 10%, indicating the seed coat acts as a barrier to most fungal invasions. When the seed coat was punctured, however, seed deterioration by the seedborne microorganisms readily occurred. Seed coat extracts contained tannin‐like compounds that inhibited in vitro growth of several bacterial and fungal isolates. Histochemical examination of seed sections revealed localization of tannin‐like compounds in the palisade layer of the seed coat and the chalazal area of the embryo. It is suggested that deterioration of velvetleaf seeds by microorganisms is limited by the dense layer of palisade cells in the seed coat, chemical compounds within the seed coat that are inhibitory to certain microorganisms outside the seed, and antagonistic bacteria located within the seed. Resistances to microbial attack may act together with seed dormancy to maintain velvetleaf seed longevity in soil.